Note: Descriptions are shown in the official language in which they were submitted.
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HANDOVER MECHANISM IN CELLULAR NETWORKS
FIELD
[0001] This disclosure relates to handover procedures in cellular
wireless
networks, and more particularly, in heterogeneous networks.
BACKGROUND
[0002] Wireless communication systems can include a network of one or
more
base stations to communicate with one or more user equipment (UE) such as
fixed and
mobile wireless communication devices, mobile phones, or laptop computers with
wireless communication cards. Base stations are spatially distributed to
provide radio
coverage in a geographic service area that is divided into cells. A UE that is
located
within a base station's cell of coverage area is generally registered with the
base
station. The UE and the base station communicate with each other via radio
signal.
The base station is called the serving base station of the UE and the cell
associated
with the base station is called the serving cell of the UE.
[0003] In some wireless networks, cells of different coverage sizes may be
deployed to improve cell coverage or to offload traffic. For example, in an
Evolved
Universal Terrestrial Radio Access Network (E-UTRAN), small cells (e.g., pico
cells,
relay cells, or femto cells) may be deployed with overlaid macro cells. A
network
including large cells (e.g., macro cells) as well as small cells (e.g., pico
cells, relay
cells, femto cells) may be referred to as a heterogeneous network. A UE in the
heterogeneous network may move in a large geographical area which may trigger
a
handover procedure and result in changing of the UE's serving cells.
BRIEF DESCRIPTION OF DRAWINGS
[0004] For a more complete understanding of this disclosure, reference
is now
made to the following brief description of the drawings, taken in connection
with the
accompanying drawings and detailed description, wherein like reference
numerals
represent like parts.
[0005] FIG. 1 is a schematic representation of an example
heterogeneous
wireless communications network.
[0006] FIG. 2 is a schematic block diagram illustrating various layers of
access
nodes and user equipment in a wireless communication network.
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[0007] FIG. 3 is a schematic block diagram illustrating an access node
device.
[0008] FIG. 4 is a schematic block diagram illustrating a user
equipment
device.
[0009] FIG. 5 is a schematic presentation of an example deployment of
a
heterogeneous network.
[0010] FIG. 6a is a schematic state diagram illustrating a handover
mechanism
involving an intermediate handover (IHO) state.
[0011] FIG. 6b is a schematic flow chart illustrating a method may be
performed by a serving cell for IHO candidate cell selection.
[0012] FIG. 7 is a flow chart illustrating an example method may be
performed
by a serving cell of a UE in a handover mechanism.
[0013] FIG. 8 is a flow chart illustrating an example method may be
performed
by a candidate cell in a handover mechanism.
[0014] FIG. 9 is a flow chart illustrating an example method may be
performed
by a UE in a handover mechanism.
[0015] FIG. 10 is a schematic flow diagram illustrating an example
handover
algorithm with the IHO state.
[0016] FIG. 11 is a schematic plot illustrating different state
regions with
respect to different signal quality of the serving cell and a target cell.
DETAILED DESCRIPTION
[0017] The present disclosure is directed to systems, methods, and
apparatuses
for handover in wireless communications networks, especially in heterogeneous
wireless communication networks. Heterogeneous networks may include cells of
various coverage sizes resulting at least in part from different transmission
power
levels of base stations, e.g., macro cell, femto cell, pico cell, relay cell,
etc. As the UE
moves across cell boundaries, a handover procedure may be performed to ensure
that
the UE is connected or camped on a serving cell with good coverage for the UE.
[0018] Since the heterogeneous network may contain various types of
cells,
there may be overlaps between coverage areas of multiple cells, especially in
unplanned clustered cell deployments where a large number of small cells may
be
situated within a macro cell's coverage area. When a UE traverses between
adjacent
cells with overlapped coverage area, there might be multiple handovers. The UE
may
only stay with one cell for a short time before it switches to another cell.
Frequently
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switching a UE among multiple cells may incur significant signalling overhead,
delay,
data interruptions, and/or quality of service (QoS) degradation.
[0019] To improve the QoS, an intermediate handover (IHO) state can be
introduced to reduce unnecessary and unwanted handovers. The UE may be in the
IHO
state before it is handed over to a target cell completely. During the IHO
state, the UE
can be connected to the serving cell as well as one or more neighbouring
cells. The
neighbouring cells that are connected to the UE during the IHO state are
referred to as
IHO candidate cells. One cell that actively transmits data to the UE is
referred to as the
Anchor cell. The IHO state can be transparent to the core network. Therefore,
the IHO
state can also be referred to as a network agnostic mobility management (NA-
MM)
state.
[0020] To enable the IHO state, a method can be performed at a serving
base
station of a UE in a wireless communications network. The method includes
receiving
a downlink (DL) signal quality indicator from the UE; determining, from the
signal
quality indicator, whether a condition for an intermediate handover (IHO)
state is
satisfied; and if the condition for the IHO state is satisfied, initiating the
IHO state.
Specifically, the signal quality indicator can indicate that UE is proximate
to a
neighbouring base station and can receive data packets from the neighbouring
base
station.
[0021] In addition, a method can be performed at an IHO candidate base
station in the wireless communications network. The method includes receiving
an
indication from the serving base station that an IHO state has been
initialized;
receiving an indication from the serving base station that the IHO candidate
base
station has anchor functionality for the UE; and determining whether to inform
the
serving base station to handover the UE to the target base station.
[0022] Furthermore, a method can be performed at the UE in the
wireless
communications network. The method includes sending a receive signal quality
report
of the serving base station and one or more neighbouring base stations; and
receiving a
message from the serving base station initiating the IHO state.
[0023] During the IHO state, the UE can transition to a HO state where the
UE
is handed over to a target cell, or to a STAY state where the UE stays with
the serving
cell. Conditions for the IHO, HO, and STAY states can be defined based on the
signal
quality indicator. If the condition for the HO state or the STAY state is
satisfied,
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corresponding state transition from the IHO state can be initiated by the
serving base
station or the UE.
[0024] Certain aspects of the method performed at a serving base
station of a
wireless communications network, the serving base station serving a user
equipment
(UE). The method may include receiving a downlink (DL) signal quality
indicator
from the UE, the signal quality indicator indicating that UE is proximate to a
neighboring base station and can receive data packets from the neighboring
base
station. It can be determined, from the signal quality indicator, whether a
condition for
an intermediate handover (IHO) state is satisfied. Responsive to the
determining, the
intermediate handover state can be initiated.
[0025] Certain aspects are directed to a base station of a wireless
communications network , the base station serving a user equipment (UE), the
base
station may be configured to receive a downlink (DL) signal quality indicator
from the
UE, the signal quality indicator indicating that UE is proximate to a
neighboring base
station and can receive data packets from the neighboring base station. The
base
station can determine, from the signal quality indicator, whether a condition
for an
intermediate handover (IHO) state is satisfied. Responsive to the
determination that
the condition for the intermediate handover state is satisfied, the base
station can
initiate the intermediate handover state.
[0026] Certain aspects of the disclosure are directed to systems,
apparatuses,
and methods performed at a IHO candidate base station of a wireless
communications
network, the wireless communications network comprising a serving base station
serving a user equipment (UE), the UE in communication with the serving base
station
and the IHO candidate base station. An indication can be received from the
serving
base station that a IHO state has been initialized. An indication can be
received from
the serving base station that the IHO candidate base station has anchor
functionality
for the UE. It can be determined whether to inform the serving base station to
handover the UE to a target base station.
[0027] Certain aspects of the implementations are directed systems,
apparatuses, and methods performed at a user equipment (UE), the UE served by
a
base station of a wireless communications network. A measurement report may be
sent to the serving base station. A message may be received from the serving
base
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station initiating an IHO state. The UE may operate in the IHO state with the
serving
base station and at least one neighbor base station.
[0028] In certain implementations, the receive signal quality
comprised at least
one of the following: reference signal receive quality, reference signal
receive power,
signal to interference plus noise ratio, or average packet delay.
[0029] In certain implementations, the signal quality indicator
indicates that
the UE is proximate to a plurality of neighboring base stations and can
receive data
packets from at least a subset of the plurality of neighboring base stations.
[0030] Certain aspects of the implementations may include selecting
one or
more neighboring base station from a plurality of neighboring base stations
when the
DL signal quality indicator received from the UE is above a predefined
threshold,
sending an IHO request message to the one or more neighboring base stations,
receiving an IHO response from the one or more neighboring base stations, and
evaluating the IHO response to determine the IHO candidate base stations.
[0031] In certain implementations, the predefined threshold is a UE
specific
parameter and selected to satisfy the promised Quality of Service (QoS) to the
UE.
[0032] In certain implementations, evaluating the IHO responses may
include
selecting a neighboring base station as a potential IHO candidate base station
when the
neighboring base station responds positively to the IHO request, choosing the
IHO
candidate base stations when the number of potential IHO candidate base
stations is
above a maximum number, N, of allowed IHO candidate base stations, and sending
IHO cancellation messages to the neighboring base stations which accepted the
IHO
request but are not included in the subset of IHO candidate base stations.
[0033] In certain implementations, choosing the IHO candidate base
stations
may include ordering the IHO candidate base stations in decreasing order
signal
quality indicator and selecting the IHO candidate base stations starting from
the first in
the order list and up to the maximum allowed IHO candidate base stations.
[0034] In certain implementations, the maximum number of IHO candidate
base stations, N, is a network configuration dependent parameter.
[0035] In certain implementations, choosing the IHO candidate base stations
comprises selecting N base stations having a highest signal quality indicator.
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[0036] Certain aspects of the implementations may also include
starting an
IHO timer, sending an IHO initiation message to the UE initiating an IHO
state, and
transmitting data to the UE.
[0037] In certain implementations, the value of the IHO timer is
included in the
IHO initiation message to the UE.
[0038] In certain implementations, the value of IHO timer is included
in the
IHO request message to the neighboring base stations.
[0039] In certain implementations, the serving base station is a
default anchor
base station after the IHO state is initiated. The anchor base station in IHO
state is
configured to assign DL resources and grant UL resources to the UE and
send/receive
data packets to/from the UE.
[0040] In certain aspects, during the IHO state, the implementations
may
include transferring anchor base station functionality to one of the IHO
candidate base
stations when the DL signal quality from the IHO candidate base station as
indicated
in the DL signal quality indicator is better than the DL signal quality from
the serving
base station.
[0041] In certain aspects, during the IHO state, initiating a handover
to one of
the IHO candidate base stations when the DL signal quality from the IHO
candidate
base station and from the serving base station meets HO criteria.
[0042] In certain implementations, meeting the HO criteria may include that
the DL signal quality from the serving base station is inferior to the DL
signal quality
from the IHO candidate base station by a predefined threshold.
[0043] In certain aspects, during the IHO state, the implementations
may
include initiating a stay state when the DL signal quality, as indicated in
the DL signal
quality indicator, from all the IHO candidate base stations and serving base
station
meets criteria for stay state.
[0044] In certain implementations, the criteria for stay state may
include the
DL signal quality from the IHO candidate base station is inferior to the DL
signal
quality from the serving base station by a predefined threshold.
[0045] In certain implementations, during the IHO state,a handover to a non-
IHO candidate base station may be initiated when the DL signal quality from
the non-
IHO candidate base station and serving base station meet HO criteria.
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[0046] In certain implementations, the HO criteria may also include
that the
DL signal quality from the serving base station is inferior to the DL signal
quality from
the non-IHO candidate base station by a predefined threshold.
[0047] In certain implementations, the IHO state is determined to be
satisfied if
the signal quality between the UE and the base stations is above a
predetermined
threshold.
[0048] In certain implementations, the IHO state is enabled when the
difference of the signal quality between the UE and the serving base station,
and the
signal quality between the UE and at least another neighboring base station is
below a
predefined threshold.
[0049] In certain implementations, if the condition for the IHO state
is
satisfied, the implementations may include initializing a timer associated
with the IHO
state.
[0050] Certain aspects of the implementations may include cancelling
the IHO
state at the expiration of the IHO state timer.
[0051] In certain aspects, after initializing the IHO state timer, the
implementations may also include receiving from the UE a signal quality
indicator
indicating that the signal quality between the UE and the serving base station
is better
than the signal quality between the UE and the candidate base stations and
decrementing the IHO state timer.
[0052] In certain implementations, the received signal quality
indicator
indicates that a signal quality between the UE and at least one of the other
base
stations is better than a signal quality between the UE and the serving base
station.
Certain aspects may include, if the condition for the IHO state is satisfied,
initializing a
handover procedure to handover the UE to the target base station.
[0053] In certain implementations, the IHO state initiation is
transparent to the
wireless communications network.
[0054] In certain implementations, the signal quality indicator
comprises an
indication of a signal quality between the UE and the neighboring base
station.
[0055] In certain implementations, in the IHO state, the method further
comprises transmitting data packets destined for the UE to at least one of the
IHO
candidate base stations over a backhaul communications link.
[0056] In certain implementations, the data packets are PDCP packets.
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[0057] In certain implementations, in the IHO state, the method
further
comprises transmitting one or both of a radio resource control message or a
non-access
stratum (NAS) message to at least one of the IHO candidate base stations.
[0058] In certain implementations, determining, from the signal
quality
indicator, whether a condition for an IHO state is satisfied comprises
receiving an
acknowledgement signal from the UE.
[0059] In certain aspects, in the IHO state, the implementations may
include
receiving a request from the UE for a handover from the serving base station
to one of
the IHO candidate base stations and initializing a handover procedure to
handover the
UE to the IHO candidate base station.
[0060] Certain aspects of the implementations may include canceling
the IHO
state.
[0061] In certain aspects, in the IHO state, the implementations may
include
receiving a request from the UE to stay with the serving base station and
sending IHO
cancellation message to the IHO candidate base stations.
[0062] In certain aspects, in the IHO state, the implementations may
also
include receiving a signal quality indicator indicates that a signal quality
between the
UE and an IHO candidate base station is better than a signal quality between
the UE
and the serving base station, and transferring control of communications for
the UE to
the IHO candidate base stations.
[0063] Certain implementations may include receiving control of
communications for the UE from the IHO candidate base station.
[0064] Certain implementations may include receiving a message of
handover
from the candidate base station, the candidate base station acting as an
anchor base
station for the UE and cancelling the intermediate handover state.
[0065] In certain implementations, the message indicates a handover to
another
base station.
[0066] In certain implementations, the message indicates the UE to
stay with
the anchor base station.
[0067] In certain implementations, the receive signal quality comprised at
least
one of the following: reference signal receive quality, reference signal
receive power,
signal to interference plus noise ratio, or average packet delay.
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[0068] In certain implementations, the signal quality indicator
indicates that
the UE is proximate to a plurality of neighboring base stations and can
receive data
packets from at least a subset of the plurality of neighboring base stations.
[0069] In certain aspects of the implementations, the base station may
be
configured to select one or more neighboring base station from a plurality of
neighboring base stations when the DL signal quality indicator received from
the UE is
above a predefined threshold. An IHO request message may be sent to the one or
more neighboring base stations. An IHO response may be received from the one
or
more neighboring base stations. The IHO response may be evaluated to determine
the
IHO candidate base stations.
[0070] In certain implementations, the predefined threshold is a UE
specific
parameter and selected to satisfy the promised Quality of Service (QoS) to the
UE.
[0071] In certain implementations, evaluating the IHO responses may
include
selecting a neighboring base station as a potential IHO candidate base station
when the
neighboring base station responds positively to the IHO request. The IHO
candidate
base stations may be chosen when the number of potential IHO candidate base
stations
is above a maximum number, N, of allowed IHO candidate base stations. IHO
cancellation messages may be transmitted to the neighboring base stations
which
accepted the IHO request but are not included in the subset of IHO candidate
base
stations.
[0072] In certain implementations, choosing the IHO candidate base
stations
may include ordering the IHO candidate base stations in decreasing order
signal
quality indicator and selecting the IHO candidate base stations starting from
the first in
the order list and up to the maximum allowed IHO candidate base stations.
[0073] In certain implementations, the maximum number of IHO candidate
base stations, N, is a network configuration dependent parameter.
[0074] In certain aspects of the implementations, choosing the IHO
candidate
base stations comprises selecting N base stations having a highest signal
quality
indicator.
[0075] In certain aspects of the implementations, the base station may be
configured to start an IHO timer. An IHO initiation message may be sent to the
UE
initiating an IHO state. Data may be transmitted to the UE.
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[0076] In certain implementations, the value of the IHO timer is
included in the
IHO initiation message to the UE.
[0077] In certain implementations, the value of IHO timer is included
in the
IHO request message to the neighboring base stations.
[0078] In certain implementations, the serving base station is a default
anchor
base station after the IHO state is initiated, and the anchor base station in
IHO state is
configured to assign DL resources and grant UL resources to the UE and
send/receive
data packets to/from the UE.
[0079] In certain aspects of the implementations, the base station may
be
configured to, during the IHO state, transfer anchor base station
functionality to one of
the IHO candidate base stations when the DL signal quality from the IHO
candidate
base station as indicated in the DL signal quality indicator is better than
the DL signal
quality from the serving base station.
[0080] In certain aspects of the implementations, the base station may
be
configured to, during the IHO state, initiate a handoyer to one of the IHO
candidate
base stations when the DL signal quality from the IHO candidate base station
and from
the serving base station meets HO criteria.
[0081] In certain implementations, meeting the HO criteria comprises
the DL
signal quality from the serving base station to be inferior to the DL signal
quality from
the IHO candidate base station by a predefined threshold.
[0082] In certain aspects of the implementations, the base station may
be
configured to, during the IHO state, initiate a stay state when the DL signal
quality, as
indicated in the DL signal quality indicator, from all the IHO candidate base
stations
and serving base station meets criteria for stay state.
[0083] In certain implementations, the criteria for stay state comprises
the DL
signal quality from the IHO candidate base station is inferior to the DL
signal quality
from the serving base station by a predefined threshold.
[0084] In certain aspects of the implementations, the base station may
be
configured to, during the IHO state, initiate a handoyer to a non-IHO
candidate base
station when the DL signal quality from the non-IHO candidate base station and
serving base station meet HO criteria.
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[0085] In certain implementations, the HO criteria may include the DL
signal
quality from the serving base station is inferior to the DL signal quality
from the non-
IHO candidate base station by a predefined threshold.
[0086] In certain implementations, the IHO state is determined to be
satisfied if
the signal quality between the UE and the base stations is above a
predetermined
threshold.
[0087] In certain implementations, the IHO state is enabled when the
difference of the signal quality between the UE and the serving base station,
and the
signal quality between the UE and at least another neighboring base station is
below a
predefined threshold.
[0088] In certain aspects of the implementations, the base station is
configured
to, if the condition for the IHO state is satisfied, initializing a timer
associated with the
IHO state.
[0089] In certain aspects of the implementations, the base station may
be
configured to cancel the IHO state at the expiration of the IHO state timer.
[0090] In certain aspects of the implementations, the base station may
be
configured to, after initializing the IHO state timer, receiving from the UE a
signal
quality indicator indicating that the signal quality between the UE and the
serving base
station is better than the signal quality between the UE and the candidate
base stations
and decrementing the IHO state timer.
[0091] In certain implementations, the received signal quality
indicator
indicates that a signal quality between the UE and at least one of the other
base
stations is better than a signal quality between the UE and the serving base
station.
The base station may be further configured to, if the condition for the IHO
state is
satisfied, initialize a handover procedure to handover the UE to the target
base station.
[0092] In certain implementations, the IHO state initiation is
transparent to the
wireless communications network.
[0093] In certain implementations, the signal quality indicator
comprises an
indication of a signal quality between the UE and the neighboring base
station.
[0094] In certain implementations, in the IHO state, the method further
comprises transmitting data packets destined for the UE to at least one of the
IHO
candidate base stations over a backhaul communications link.
[0095] In certain implementations, the data packets are PDCP packets.
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[0096] In certain implementations, in the IHO state, the method
further
comprises transmitting one or both of a radio resource control message or a
non-access
stratum (NAS) message to at least one of the IHO candidate base stations.
[0097] In certain implementations, determining, from the signal
quality
indicator, whether a condition for an IHO state is satisfied comprises
receiving an
acknowledgement signal from the UE.
[0098] In certain aspects of the implementations, the base station may
be
configured to, in the IHO state, receive a request from the UE for a handover
from the
serving base station to one of the IHO candidate base stations and initialize
a handover
procedure to handover the UE to the IHO candidate base station.
[0099] In certain aspects of the implementations, the base station may
also be
configured to cancel the IHO state.
[00100] In certain aspects of the implementations, the base station may
be
configured to, in the IHO state, receive a request from the UE to stay with
the serving
base station and send IHO cancellation message to the IHO candidate base
stations.
[00101] In certain aspects of the implementations, the base station may
be
configured to, in the IHO state, receive a signal quality indicator indicates
that a signal
quality between the UE and an IHO candidate base station is better than a
signal
quality between the UE and the serving base station and transfer control of
communications for the UE to the IHO candidate base stations.
[00102] The base station may be configured to receive control of
communications for the UE from the IHO candidate base station.
[00103] Certain aspects of the implementations may include the base
station
receiving a message of handover from the candidate base station, the candidate
base
station acting as an anchor base station for the UE cancelling the
intermediate
handover state.
[00104] In certain implementations, the message indicates a handover to
another
base station.
[00105] In certain implementations, the message indicates the UE to
stay with
the anchor base station.
[00106] In certain implementations, the target base station is another
IHO
candidate base station or another non-IHO base station or the anchor base
station.
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[00107] Certain aspects of the implementations may include receiving
from the
UE a signal quality indicator indicating that the signal quality between the
UE and a
second base station is better than the signal quality between the UE and the
candidate
base stations and transferring anchor functionality to the second base
station.
[00108] Certain aspects of the implementations may also include
initializing a
timer associated with the IHO state.
[00109] Certain aspects of the implementations may also include
cancelling the
IHO state at the expiration of the IHO state timer.
[00110] In certain implementations, the measurement report sent by the
UE
includes the receive signal quality with respect to the serving base station
and at least
one other neighbor base station.
[00111] In certain implementations, the receive signal quality
comprised at least
one of the following: reference signal receive quality, reference signal
receive power,
signal to interference plus noise ratio, or average packet delay.
[00112] In certain implementations, the anchor base station is the serving
base
station.
[00113] In certain implementations, the message initiating the IHO
state
includes at least one of the following:
[00114] an IHO timer value;
[00115] a list of IHO candidate base stations;
[00116] a descriptor of a handover algorithm; or
[00117] representative parameter values of a handover algorithm.
[00118] In certain implementations, the UE initiates the IHO state by
initiating a
IHO timer and determining an anchor base station.
[00119] In certain implementations, the UE initiates the IHO state by
acquiring
DL and UL synchronization with respect to the candidate base stations.
[00120] In certain implementations, the UE initiates the IHO state by
acquiring
system information parameters of the candidate base stations.
[00121] In certain implementations, when the UE is in the IHO state,
the signal
quality can be monitored with respect to the serving base station and the
candidate
base stations. The signal quality report can be sent to the anchor base
station. A
message can be received from the anchor base station indicating that the
anchor
functionality is to be transferred to a different base station, wherein upon
transferring
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the anchor functionality, the different base station becomes the anchor base
station.
Radio resource assignment and grant messages can be received from the current
anchor base station. Data packets can be transmitted to and received from the
current
anchor base station.
[00122] In certain implementations, when the UE is in the IHO state, the
signal
quality with respect to the serving base station and the at least one neighbor
base
station can be monitored. A measurement report can be sent to the anchor base
station.
A message can be received from the anchor base station, wherein the message
comprises an indication to handover to a target base station. The IHO state
can be
cancelled and the UE can move to the target base station.
[00123] In
certain implementations, the target base station is a IHO candidate
base station.
[00124] In
certain implementations, the target base station is the anchor base
station.
[00125] In certain aspects, when the UE is in the IHO state, the signal
quality
with respect to the serving base station and one or more candidate base
stations can be
monitored. The UE can send an indication of its desired candidate base
station(s) to
the anchor base station, wherein the indication includes an indication to
handover to a
target base station. A message may be received from the anchor base station.
The
IHO state can be cancelled. The UE can transfer to the candidate base station.
[00126] In
certain implementations, the target base station is the anchor base
station.
[00127] In
certain implementations, the target base station is a IHO candidate
base station.
[00128] FIG. 1 is schematic representation of an example heterogeneous
wireless communication network 100. The term
"heterogeneous wireless
communication network" or "heterogeneous network" may also be referred to as a
"Hetnet." The illustrated heterogeneous network 100 includes a core network
110 and
a macro cell or overlay cell 120. The term "cell" or "wireless cell" generally
refers to
an area of coverage of wireless transmission by a network or network
component, such
as an access node. The core network 110 can be connected to the Internet 160.
In the
illustrated implementation, the macro cell 120 can include at least one base
station.
The term "base station" can be interchangeably used with a network node, an
access
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node, or a network component. Two or more base stations may operate on the
same
radio frequency or on different radio frequencies. In this disclosure, the
term "base
station" is sometimes interchangeably used with the term "cell," where the
base station
provides the coverage of wireless transmission of the cell.
[00129] The base station can be an overlay access node 121 connected to the
core network 110 via a backhaul link 111a, including optical fiber or cable.
The term
"overlay access node" generally refers to a network element or component that
at least
partly serves to form a wireless cell. In one implementation in which the
network 100
is an LTE network, the overlay access node 121 can be a Universal Terrestrial
Radio
Access Network (UTRAN) node B or "eNB" which is part of an evolved Universal
Terrestrial Radio Access Network (E-UTRAN). An eNB that forms an overlay
access
node of a macro cell can be generally referred to as a "macro eNB." The term
"eNB"
may be interchangeably used with an "evolved node B." The eNBs may cooperate
to
conduct a handover procedure for User Equipment (UE) in the network 100. To
conduct the handover procedure, the eNBs may exchange control information via
the
backhaul link 111a or 111b or 111c or 111d.
[00130] The network 100 can also include one or more underlay cells,
for
example, a pico cell 130 and a femto cell 140. The underlay cells can have a
coverage
at least partially overlapping with the coverage of the macro cell 120. While
the term
"underlay cell" is described herein in the context of the long term evolution
(LTE)
standard, other wireless standards can also have components similar to
underlay cells.
The implementations described herein can be adapted for such standards without
departing from the scope of this disclosure. Although FIG. 1 illustrates only
one pico
cell and only one femto cell, the network 100 can include more or less cells.
The
underlay cells 130, 140 have a smaller coverage than the overlay cell 120. For
example, in a suburban environment, the overlay cell 120 may have a coverage
radius
of 0.5 kilometer, while the underlay cells 130, 140 may have a coverage radius
of 0.2
kilometer. Access nodes 131, 141 forming the underlay cells 130, 140 can use a
lower
transmission power than that of the overlay access node 121. The underlay
cells 130,
140 may further include a range expansion area used for increasing the
coverage area
for the cells having a smaller coverage.
[00131] The pico cell 130 can include a pico eNB 131 connected to the
core
network 110 via a backhaul link 111b and to the macro eNB 121 via a backhaul
link
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111c. The backhaul links 111b and 111c may include cable, fiber, wireless
links, or
others. In some implementations, the pico eNB 131 can have a transmission
power
that is, for example, about 30 dBm, which is about 13 dB lower than that of
the macro
eNB 121.
[00132] The femto cell 140 can include a femto eNB 141 connected to the
core
network 110 via the Internet 160 via a wired or wireless connection. The term
"femto
eNB" can also be referred to as a "home eNB (HeNB)." The femto cell 140 is a
subscription based cell. Three access modes can be defined for HeNBs: closed
access
mode, hybrid access mode and open access mode. In closed access mode, HeNB
provides services only to its associated closed subscription group (CSG)
members. The
term "closed subscription group (CSG)" can be interchangeably used with closed
subscriber group. Hybrid access mode allows HeNB to provide services to its
associated CSG members and to non-CSG members. In some implementations, the
CSG members are prioritized to non-CSG members. An open access mode HeNB
appears as a normal eNB.
[00133] The network 100 can also include a relay node 150 which serves
to
wirelessly relay data and/or control information between the macro eNB 121 and
user
equipment 170. The macro eNB 121 and the relay node 150 can be connected to
each
other via a wireless backhaul link 111d. In such an instance, the macro eNB
121 can
be referred to as a donor eNB. In some implementations, the relay node 150 can
have
a transmission power that is, for example, about 30 or 37 dBm, which is about
13 dB
or 6 dB lower than that of the macro eNB 121. The term "underlay access node"
generally refers to pico eNBs, femto eNBs, or relay nodes.
[00134] The user equipment 170 can communicate wirelessly with any one
of
the overlay access nodes 121 or the underlay access nodes 131, 141, 150,
depending
on the location or the existence of subscription in the case of the femto cell
140. The
term "user equipment" ("UE") can refer to various devices with
telecommunications
capabilities, such as mobile devices and network appliances. The UE 170 may
switch
from the coverage of one cell to another cell, for example, from the coverage
of the
pico cell 130 to the coverage of the macro cell 120, i.e., a pico-to-macro
cell change,
or from the coverage of a macro cell 120 to the coverage of the pico cell 130,
i.e., a
macro-to-pico cell change. A handover procedure may be conducted to ensure
that the
UE does not lose connection with the network while switching between cells.
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[00135] Examples
of user equipment include, but are not limited to, a mobile
phone, a smart phone, a telephone, a television, a remote controller, a set-
top box, a
computer monitor, a computer (including a tablet computer such as BlackBerry
Playbook tablet, a desktop computer, a handheld or laptop computer, a netbook
computer), a personal digital assistant (PDA), a microwave, a refrigerator, a
stereo
system, a cassette recorder or player, a DVD player or recorder, a CD player
or
recorder, a VCR, an MP3 player, a radio, a camcorder, a camera, a digital
camera, a
portable memory chip, a washer, a dryer, a washer/dryer, a copier, a facsimile
machine, a scanner, a multi-functional peripheral device, a wrist watch, a
clock, a
game device, etc. The UE 170 may include a device and a removable memory
module, such as a Universal Integrated Circuit Card (UICC) that includes a
Subscriber
Identity Module (SIM) application, a Universal Subscriber Identity Module
(USIM)
application, or a Removable User Identity Module (R-UIM) application. In some
implementations, the UE 170 may include the device without such a module. The
term "UE" can also refer to any hardware or software component that can
terminate a
communication session for a user. In addition, the terms "user equipment,"
"UE,"
"user equipment device," "user agent," "UA," "user device," and "mobile
device" can
be used synonymously herein.
[00136] FIG. 2
is a schematic block diagram 200 illustrating various layers of
access nodes and user equipment in an example wireless communication network.
The
illustrated system 200 includes a macro eNB 215, a pico eNB 225, a macro UE
205,
and a pico UE 235. Here macro UE 205 and Pico UE 235 are UEs which are either
actively communicating or camping on macro eNB 215 and pico eNB 225
respectively. The macro eNB 215 and the pico eNB 225 can be collectively
referred to
as a "network," "network components," "network elements," "access nodes," or
"access devices." FIG. 2 shows only these four devices (also referred to as
"apparatuses" or "entities") for illustrative purposes, and the system 200 can
further
include one or more of these devices without departing from the scope of this
disclosure. The macro eNB 215 can communicate wirelessly with the macro UE
205.
The pico eNB 225 can communicate wirelessly with the pico UE 235. The macro
eNB
215 can communicate with the pico eNB 225 via a backhaul link, for example, an
X2
backhaul link, a wireless connection, or a combination thereof In some
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implementations, the macro eNB 215 and pico eNB 225 may exchange handover
control information via the backhaul link.
[00137] Each of the devices 205, 215, 225 and 235 includes a protocol
stack for
communications with other devices via wireless or wired connection. The macro
eNB
215 can include a physical (PHY) layer 216, a medium access control (MAC)
layer
218, a radio link control (RLC) layer 220, a packet data convergence protocol
(PDCP)
layer 222, and a radio resource control (RRC) layer 224. In the case of user
plane
communications for data traffic, RRC layer is not involved. The macro eNB 215
can
also include one or more transmit and receive antennas 226 coupled to the PHY
layer
216. In the illustrated implementation, a "PHY layer" can also be referred to
as "layer
1 (L1)." A MAC layer can also be referred to as "layer 2 (L2)." The other
layers
(RLC layer, PDCP layer, RRC layer and above) can be collectively referred to
as a
"higher layer(s)."
[00138] Similarly, the pico eNB 225 includes a PHY layer 228, a MAC
layer
230, a RLC layer 232, a PDCP layer 234, and an RRC layer 236. The pico eNB 225
can also include one or more antennas 238 coupled to the PHY layer 228.
[00139] The macro UE 205 can include a PHY layer 202, a MAC layer 204,
a
RLC layer 206, a PDCP layer 208, an RRC layer 210, and a non-access stratum
(NAS)
layer 212. The macro UE 205 can also include one or more transmit and receive
antennas 214 coupled to the PHY layer 202. Similarly, the pico UE 235 can
include a
PHY layer 240, a MAC layer 242, a RLC layer 244, a PDCP layer 246, an RRC
layer
248, and a NAS layer 250. The pico UE 235 can also include one or more
transmit
and receive antennas 252 coupled to the PHY layer 240.
[00140] Communications between the devices, such as between the macro
eNB
215 and the macro UE 205, generally occur within the same protocol layer
between the
two devices. Thus, for example, communications from the RRC layer 224 at the
macro eNB 215 travel through the PDCP layer 222, the RLC layer 220, the MAC
layer
218, and the PHY layer 216, and are sent over the PHY layer 216 and the
antenna 226
to the macro UE 205. When received at the antenna 214 of the macro UE 205, the
communications travel through the PHY layer 202, the MAC layer 204, the RLC
layer
206, the PDCP layer 208 to the RRC layer 210 of the macro UE 205. Such
communications are generally done utilizing a communications sub-system and a
processor, as described in more detail below.
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[00141] Some typical functionality of different protocol layers is
briefly
described below. The NAS protocol, which runs between a core network and the
UE,
can serve for control purposes such as authentication, session management, and
UE
mobility management. The RRC layer in the eNB may be capable to make handover
decisions based on neighbor cell measurements sent by the UE, broadcasts
system
information, controls UE measurement and allocate cell-level temporary
identifiers to
active UEs. The functionality of PDCP layer includes, among other things,
encryption
of user data stream and header compression and decompression. The RLC layer
can be
used to format and transport traffic between the UE and the eNB. The MAC layer
is
responsible for, among other things, control of random access procedure,
scheduling of
data packets, and mapping of logical channels to transport channels. The PHY
layer
may involve modulation and demodulation, error protection of data package by
utilizing coding, radio frequency (RF) processing, radio characteristics
measurements
and indications to higher layers, and support for multiple input multiple
output
(MIMO) if multiple antennas are equipped with the eNB or the UE.
[00142] In the implementations described in this disclosure, various
steps and
actions of the macro eNB, macro UE, pico eNB, and pico UE can be performed by
one
or more of the layers described above in connection with FIG. 2. For example,
handover procedure for the macro UE 205 can be performed by one or more of the
layers 202-212 of the macro UE 205. Handover procedure by the pico UE 235 can
be
performed by one or more of the layers 240-250 of the pico UE 235. Channel
quality
measurement may be performed by the PHY layer and MAC layer of the macro UE
205 and pico UE 235. For another example, handover of UE may be initiated by
the
RRC layer 224 of the macro eNB 215 and the RRC layer 236 of the pico eNB 225.
[00143] FIG. 3 is a schematic block diagram 300 illustrating an access node
device. The illustrated device 300 includes a processing module 302, a wired
communication subsystem 304, and a wireless communication subsystem 306. The
wireless communication subsystem 306 can receive data traffic and control
traffic from
the UE. The wired communication subsystem 304 can be configured to transmit
and
receive control information between other access node devices via backhaul
connections. The processing module 302 can include one or more processing
components (also referred to as "processors" or "central processing units"
(CPUs))
capable of executing instructions related to one or more of the processes,
steps, or
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actions described above in connection with one or more of the implementations
disclosed herein. The processing module 302 can also include other auxiliary
components, such as random access memory (RAM), read only memory (ROM),
secondary storage (for example, a hard disk drive or flash memory). The
processing
module 302 can form at least part of the layers described above in connection
with
FIG. 2. In particular, the processing module 302 may be configured to receive
signal
quality indicators from the UE. The processing module 302 may also be
configured to
determine a handover or an intermediate handover based on the received signal
quality
indicators, and to transmit a handover or an intermediate handover command.
The
processing module 302 can execute certain instructions and commands to provide
wireless or wired communication, using the wired communication subsystem 304
or a
wireless communication subsystem 306. A skilled artisan will readily
appreciate that
various other components can also be included in the device 300.
[00144] FIG. 4 is a schematic block diagram 400 illustrating user
equipment
device. The illustrated device 400 includes a processing unit 402, a computer
readable
storage medium 404 (for example, ROM or flash memory), a wireless
communication
subsystem 406, a user interface 408, and an I/O interface 410.
[00145] Similar to the processing module 302 of FIG. 3, the processing
unit 402
can include one or more processing components (also referred to as
"processors" or
"central processing units" (CPUs)) configured to execute instructions related
to one or
more of the processes, steps, or actions described above in connection with
one or
more of the implementations disclosed herein. In particular, the processing
module
402 may be configured to estimate signal quality associated different cell and
transmit
signal quality indicators to an access node. The processing module 402 may
also be
configured to receive signaling from access nodes and perform operations
accordingly,
such as transitions between a handover state and an intermediate handover
state. The
processing module 402 can form at least part of the layers described above in
connection with FIG. 2. The processing unit 402 can also include other
auxiliary
components, such as random access memory (RAM) and read only memory (ROM).
The computer readable storage medium 404 can store an operating system (OS) of
the
device 400 and various other computer executable software programs for
performing
one or more of the processes, steps, or actions described above.
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[00146] The
wireless communication subsystem 406 is configured to provide
wireless communication for data and/or control information provided by the
processing unit 402. The wireless communication subsystem 406 can include, for
example, one or more antennas, a receiver, a transmitter, a local oscillator,
a mixer,
and a digital signal processing (DSP) unit. In some implementations, the
subsystem
406 can support multiple input multiple output (MIMO) transmissions.
[00147] The user
interface 408 can include, for example, one or more of a
screen or touch screen (for example, a liquid crystal display (LCD), a light
emitting
display (LED), an organic light emitting display (OLED), a
microelectromechanical
system (MEMS) display), a keyboard or keypad, a trackball, a speaker, and a
microphone. The I/O interface 410 can include, for example, a universal serial
bus
(USB) interface. A skilled artisan will readily appreciate that various
other
components can also be included in the device 400.
[00148] FIG. 5
is a schematic presentation 500 of an example deployment of a
heterogeneous network. As shown in FIG. 5, a macro eNB 510 provides a macro
coverage area 512. Pico cells 520a and 520b and a femto cell cluster 530a-c
may be
situated within the coverage of a macro cell 512. The pico cell eNBs 521a and
521b
and macro cells eNB 510 are connected to EPC (Evolved Packet Core) network
through the MME (Mobility Management Entity)/S-GW (Serving Gateway) 515 via
backhaul connections 540a-c. The backhaul connection can be, for example, an
51
interface. Femto cell eNBs (HeNBs) 53 la-c are connected to an intermediate
gateway
HeNB-GW 550 through backhaul links 560a-c such as 51 interfaces. The HeNB-GW
550 can be connected with the MME/S-GW 515 via an 51 interface 540d as well.
Backhaul connections may exist between different types of eNBs. For example,
the
macro eNB 510 and the pico eNBs 52l a-b can be connected through an X2
interference (not shown). The femto eNBs 53 la-c may be connected with each
other
via the X2 interface 572a-b. Moreover, an X2 interference 570 can be also
introduced
between the macro eNB 510 and HeNB-GW 550 in order to facilitate
communications
and coordination between the macro cell 512 and the femto cells 53 la-c and
provide
seamless service coverage for UEs in this area.
[00149] When a
UE moves around in the area of 512, it may traverse different
cells and trigger multiple handovers. In one example, a UE 580a may move along
a
trajectory 590a where it starts from the pico cell 520a, gets exposed to the
coverage
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area of the macro cell 512 when it arrives at cell edge of the pico cell 520a,
and then
enters another pico cell 520b. During this trajectory, two handovers may
occur: a first
one from the pico cell 520a to the macro cell 512 and a second one from macro
cell
512 to the next pico cell 520b. In another example, if UE 580b moves along a
trajectory 490b, similarly, there can be multiple handovers between the macro
cell 512
and the femto cells 530a-c. Frequent handovers between multiple cells can
result in
increases of signalling overhead and delay, prolonged data interruptions, and
degradation of the QoS of UEs.
[00150] In heterogeneous communication networks, especially under a
small
cell cluster deployment as shown in FIG. 5, large overlaps in coverage between
macro
and femto/ pico cells are generally expected. Cell boundaries between the
macro and
femto/ pico cells can have acceptable coverage for control signaling receipt.
Furthermore, some type of interference cancellation and/ or coordination
methods is
generally used in this type of deployments. Therefore a UE may receive control
signaling from multiple cells and collaborate with multiple cells for handover
operations accordingly. The UE may maintain downlink (DL) and uplink (UL)
transmissions synchronization within the cluster deployment. The UE may
control its
transmit power and timing on the uplink based on the receive point at any
given time.
In an alternate embodiment, the UE may be capable of maintaining separate UL
and
DL synchronization with the multiple neighboring cells within an acceptable
range
simultaneously.
[00151] To restrict the handover and reduce unnecessary and unwanted
data
interruptions, an intermediate handover (IHO) state can be introduced. With an
enablement of the IHO state, the number of handovers can be reduced to one for
both
trajectories 590a-b mentioned above. For example, for the trajectory 590a
where the
UE 580a is traversing between the pico cells 520a and 520b, the handover
to/from
macro cell 512 can be avoided by keeping the UE 580a to the pico cell 520a
until the
UE 580a completely enters the coverage area of pico cell 520b. Then, the UE
580a can
only be handed over once from the pico cell 520a to the pico cell 520b.
Implementations of the IHO state will be described in further details below.
[00152] FIG. 6a is a schematic state diagram 600 illustrating a
handover
mechanism involving the IHO state. In general, when a UE is registered with a
serving
cell, it has RRC (Radio Resource Control) connection with the serving cell and
can
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actively communicate with the core network. A UE in "STAY/ ACTIVE" 610 is in
RRC ACTIVE state with the serving cell and can transmit and receive, for
example,
Packet Data Control Protocol (PDCP) packets, from the serving eNB. The UE may
send a signal quality indicator to the serving base station. The signal
quality indicator
can be a measurement feedback, such as Channel Quality Information (CQI)
reports of
a target cell. The serving cell may send an RRC message so that the UE may
transition
to "HO" state 620 or "IHO" state 630 based on the measurement reports from the
UE.
From the "IHO" state 630, the UE may transition back to RRC ACTIVE state 610
with the serving cell or to a "HO" state. This transition may happen at the
request of
the serving cell or can be triggered autonomously. Once the HO is performed
the UE
goes into "STAY/ ACTIVE" state with a target cell.
[00153] In the IHO state 630, the UE is not handed over to any of the
target
cells completely. These target cells are referred to as IHO candidate cells
during the
IHO state. An example method to select IHO candidate cells will be discussed
in
further details below. The PDCP packets from the serving eNB are routed to the
IHO
candidate eNB(s) over a backhaul link, such as the X2 interface, which
connects eNBs.
If the QoS of a candidate cell is expected to be better than that of the
serving cell, the
PDCP packets are scheduled and transmitted by that candidate cell to the UE.
Recall
that the PDCP processing can provide encryption of the data packets for
security and
identity protection. During the IHO state, the encryption of the data packets
may
remain unchanged and still be conducted by the serving cell. Therefore, the
data
rerouting from the serving cell to the candidate cell is completely
transparent to the
EPC network. Moreover, most control signaling of the RRC and NAS may also
originate from the serving eNB and be rerouted to the candidate eNB(s), for
example,
through backhaul links. Therefore, the IHO state in this disclosure can also
be referred
as a network agnostic mobility management (NA-MM) state, which means the state
is
transparent to the core network. When the expected QoS difference with respect
to
serving cell and IHO candidate cell is larger than a threshold, the UE may be
instructed to handover completely to one of the candidate cells (i.e. exits
the
intermediate handover state).
[00154] In the intermediate handover state, the UE may transmit/
receive
packets to/ from either the target cell or the serving cell. The cell which
actively
transmits the data to the UE is referred to as the Anchor cell. The switching
of
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transmission/ receipt between the cells may be decided by the Anchor cell. The
packet
transmission/ receipt cell may be indicated in an RRC message transmitted by
the
anchor cell.
[00155] Normally the switching of anchor cell can occur at the start of
a new IP
packet/ PDCP SDU transmission. For DL, the anchor eNB is aware of the
transmission
of a new PDCP SDU. In the case of UL, UE may be aware of this condition and
inform the completion of the IP packet/ PDCP SDU so that new resources are
assigned
by the new anchor cell. The IP packet segmentation is done independently at
each
candidate cell. Normally the switching between the candidate cells is not
expected to
be very frequent. The switching times are typically dependent on the
application type.
For example, for Gaming applications, the IP packets tend to be small. In this
case the
switching between the cells may be faster (if the signal quality with respect
to each cell
varies very rapidly).
[00156] The IHO state may be time limited. Because each candidate cell
participating in the IHO state may reserve resource for the UE, configuring a
timer
associate with the IHO state can avoid excessive system resource reserved for
one UE
whereas qualify of service of other UEs in the network may be affected. The
value of
the timer Tilio can be implementation specific, for example, depending on a
deployment scenario. The network operator can have the freedom to configure
the time
limit for the IHO state to optimize the system performance. In some
implementations,
the serving eNB may send the value of the timer to the candidate cells during
the IHO
request.
[00157] The system operator can determine under what scenarios the IHO
state
can be enabled. For example, it might be set that IHO state can be only
enabled if one
or more of the neighboring cells are low power cells, i.e. pico/ femto/ relay
cells/
nodes. The IHO state may be enabled or disabled by the operator through OAM
(Operations, Administration, and Maintenance) settings.
[00158] FIG. 6b is a schematic flow chart 640 illustrating a method may
be
performed by a serving cell of a UE for IHO candidate cell selection. In a
heterogeneous network, a UE may receive and measure downlink (DL) signal
quality
with respect to the serving cell, as well as a plurality of neighboring cells.
A
neighboring cell can be, for example, a macro cell, a pico cell, or a femto
cell. The UE
may send a DL signal quality indicator to the serving cell. The DL signal
indicator
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may indicate the serving cell that the UE is proximate to one or more
neighboring base
stations and can receive data packets from the neighboring base stations. In
some
implementations, the DL signal indicator can be signal quality measurement
feedback,
such as Channel Quality Information (CQI) reports of the neighboring cells, or
any
other channel quality parameters. For example, the receive signal quality can
include
at least one of the following: reference signal receive quality, reference
signal receive
power, signal to interference plus noise ratio, or average packet delay.
[00159] Upon the receipt of the signal quality indicator at step 642,
the serving
cell may determine whether a condition for an IHO state is satisfied based on
the
signal quality indicator from the UE at step 644. Given the condition
satisfied, in step
646, the serving cell may select one or more neighboring base stations whose
DL
signal quality report is above a predefined threshold. The predefined
threshold can be a
UE specific parameter and selected to guarantee the promised Quality of
Service
(QoS) to the UE. Then, at step 648, the serving cell can send IHO request
messages to
the one or more neighboring base stations. The neighboring base stations can
determine whether to participate in the IHO state based on several factors ,
such as,
whether the base station has enough resource to allocate to the UE, and/or
whether the
UE is a subscription group (CSG) member if neighboring base station is a HeNB
with
closed access or hybrid access mode.
[00160] If a neighboring base station agrees to join the IHO state, it may
reserve
DL resource for the UE. The neighboring base stations may inform the serving
cell
their respective decisions via the IHO responses. After receiving the IHO
responses
from the neighboring cells in step 650, the serving cell can further select
one or more
potential IHO candidate base stations in step 652 out of the neighboring base
stations
that respond positively to the IHO request. Thus a group of potential IHO
candidate
base stations is formed.
[00161] In some implementations, the serving base station may compare
the
number of potential IHO candidate base stations with a maximum allowed number
of
IHO candidate base stations in step 654. The maximum allowed number can be a
network configuration dependent parameter and be set by the network operator.
If the
number of potential IHO candidate base stations is above the maximum allowed
number of IHO candidate base stations, the serving base station may remove one
or
more base stations from the group of potential IHO candidate base stations in
step 656
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and send cancellation messages to those base stations in step 658. If the
number of
potential IHO candidate base stations does not exceed the maximum allowed
number
of IHO candidate base stations, the base station can start an IHO timer and
send a
control information to the UE to initiate the IHO state. In some
implementations, the
control information can be sent via a radio resource control (RRC) message.
[00162] FIG. 7 is a flow chart 700 illustrating an example method may
be
performed by a serving cell of a UE during a handover mechanism. The handover
mechanism can involve transitions among STAY/ACTIVE state 610, HO state 620,
and IHO state 630. The method may contain three procedures: initializing IHO
705,
procedures of the serving cell as Anchor cell during IHO 715, and procedures
of the
serving cell as Non-Anchor cell during IHO 725. The detailed procedures are
described below.
[00163] During initializing IHO procedure 705, a serving cell of a UE,
say, a
serving cell-j of UE-i, at step 702, may receive signal quality indicators Qik
that
represents the quality of the received signal at the receiver of UE-i from
neighbouring
cell-k, for k = 0, , N ¨ 1, as mentioned in step 642 of FIG. 6b. The signal
quality
indicators can be, for example, RSRP (Reference Signal Received Power), RSRQ
(Reference Signal Received Quality), Channel Quality Information (CQI), or any
other
channel quality parameters. Those indicators can have associated values
representative
of the quality of the DL channel, signal, etc. These values can serve as
quantitative
measurements of quality of service (QoS) and can be used to compare measured
signal
against promised quality of service (QoS) quantitatively.
[00164] If the quality indicator satisfies the condition for IHO state
704, the
serving cell-j can initiate IHO with one or more the neighbouring cells at
step 706. The
conditions for initiating IHO state will be described in further details below
with an
example HO algorithm. The serving cell can follow the method described in
steps 644-
660 of FIG. 6b to select a group of IHO candidate cells. Specifically, the IHO
initiation to one or more neighbouring cells may be performed either in
parallel (e.g.,
the X2AP (application protocol) messages requesting for IHO are scheduled to
the
neighbouring cells, for example, via broadcasting or multicasting) or serial
(e.g., the
X2AP messages requesting for IHO are scheduled in sequence). The serving cell
may
then wait for their respective responses of the neighbouring cells.
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[00165] If one or more neighbouring cells acknowledge in participating
the IHO
state in step 708, the serving cell may start a timer Tilio for the IHO state
in step 710.
The value of the timer Tilio is dependent on deployment scenarios and is
normally set
by the network operator. In some implementations, the value of the IHO timer
is
included in the RRC message transmitted to the UE. In other implementations,
the
serving eNB may send the value of this parameter to the neighbouring cells
during the
IHO request.
[00166] If the serving cell does not receive any positive response from
the
neighbouring cells 708, the serving cell may keep the UE with it 712. The UE
may
remain in STAY state 610 with the serving cell. Then the serving cell may go
back to
step 702 waiting for the measurement reports from the UE containing the signal
quality indicator with respect to the neighbouring cells.
[00167] Returning now to step 704, if the conditions for initiating IHO
state is
not met, the serving cell proceeds to step 714 in determining whether the
condition for
initiating HO state is satisfied. Given the HO conditions satisfied, the
serving cell can
then initiate a handover with one of the target cells 716. When the HO
condition is not
met in step 714, the serving cell goes back to step 702 waiting for the
measurement
reports from the UE with respect to neighbouring cells.
[00168] At the start of the IHO state, the default anchor cell can be
the UE's
serving cell. Following the step 710, the serving cell is in Anchor cell
status 720 and
can start procedures of the serving cell as Anchor cell during IHO 715. The
functionality of the anchor base station can include assigning DL resources
and
granting UL resources for UE; and receiving /transmitting data packets to/from
the
UE.
[00169] In some embodiments, a "HO" condition may be satisfied with respect
to one of the IHO candidate cells in step 722, the serving base station may
initiate a
handover to the IHO candidate base station and cancel the IHO state 724. The
HO
condition can include, for instance, that the reported DL signal quality
indicator with
respect to the serving base station has a value less than that of the DL
signal quality
indicator with respect to an IHO candidate base station by a predefined
threshold.
[00170] In some other implementations, a "HO cancellation (STAY)"
condition
may be satisfied 726. The HO condition can include, for instance, that the DL
signal
quality from the IHO candidate base station has a value less than that of the
DL signal
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quality from the serving base station by a predefined threshold. The UE may
stay with
the serving cell. The serving base station may cancel the IHO state with IHO
candidate
cells 728 by sending the appropriate cancel message to the candidate cells. It
may
trigger the UE to send measurement reports to the serving cell 730 and the
serving cell
goes back to step 702 waiting for the signal quality indicator.
[00171] During the IHO state, the decision of HO or HO cancellation
(STAY)
can be UE controlled. In some implementations, these decisions can be left to
the
network. In some UE-controlled embodiments, based on the quality indicator
that the
UE has with respective to the neighbouring cells, the UE can perform certain
algorithm to determine whether a handover is needed and choose a target cell
for the
handover. For example, the decisional steps 722 and 726 can be whether the UE
requests a HO to one of the IHO candidate cells, or whether the UE requests
STAY
with the serving cell, respectively. In some network-controlled embodiments,
the
anchor base station can determine whether the condition for HO or STAY is
satisfied
or not based on the signal quality indicators sent from the UE. More details
about
criteria/conditions for HO, STAY, and IHO states will be described with an
exemplary
HO algorithm below.
[00172] In some embodiments, the anchor cell may relinquish its anchor
cell
status based on the UE reported signal quality indicator during the IHO state.
For
example, one of the candidate cells may be assigned as the anchor cell if the
measurement report from the UE indicates that the signal reception quality
from the
candidate cells is superior. In some implementations, the anchor base station
can
monitor the acknowledgement (ACK)/ negative acknowledgement (NACK) of its
UL/DL packets to make decision about whether to relinquish the anchor state.
In the
illustrated example in FIG. 7, if the received signal quality with respect to
one of the
candidate cells is better than that with the Anchor cell (i.e., the serving
cell in this
case) 732, the anchor cell may relinquish the anchor state and transfer the
anchor
control to the candidate cell in step 734.
[00173] In some implementations, a non-candidate base station may
provide a
better signal quality to the UE than the serving cell and the candidate cells.
Therefore
if the HO condition is satisfied with respect to the non-candidate cell in
step 733, the
non-candidate base station can be regarded as a target base station and the
serving base
station can initiate a handover to the target base station in step 735.
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[00174] If no state transition is needed or requested, the serving cell
may check
whether the IHO timer is expired in step 736. Given that the IHO timer is
still running,
the serving cell can decrement the timer in step 738 can return to 720 as the
anchor cell
in the IHO state. If the IHO timer is found to be expired in step 736, the
serving cell
can go back to step 702 via step 730 to re-initiate the IHO with the same or a
different
set of candidate cells.
[00175] The procedures of the serving cell as Non-Anchor cell during
IHO 725
can start after the serving cell transfers the Anchor control to another
candidate cell in
step 734. The serving cell enters Non-Anchor state 740 and may first decrement
the
IHO timer in step 742. The serving cell then checks whether the IHO timer is
expired
or not. Where the IHO timer expires, the serving cell can inform the Anchor
cell about
the expiry of Tilio and prepare to reclaim resources reserved for the UE 746.
In this
case, the serving cell may then exit the IHO state and the UE may come back to
the
serving cell.
[00176] If the IHO timer is still running 744, the serving base station may
listen
to control channels to see if any state transition is needed. For example, the
serving
base station may receive a transfer of Anchor control of communications for UE
from
the current Anchor cell in step 748. In this case, the serving cell can return
to the
Anchor status 750 (or equivalently 720), resume active data communications
with the
UE, and then further proceed from step 720.
[00177] In some implementations, the serving base station may receive a
message of handover from the anchor base station indicating that the UE needs
to
handover to a target cell 752. The target base station can be either a
candidate base
station or a non-candidate base station. Then the serving base station may
cancel the
IHO state and prepare to hand over the UE to the target base station in step
754.
[00178] In other implementations, the serving base station may receive
an
indication from the current Anchor base station that the UE needs to STAY with
the
current Anchor base station 756. Accordingly, the serving base station may
cancel the
IHO state and wait for a handover initiation from the Anchor base station 758.
[00179] In some aspects of implementation, the network may override the HO
or STAY request from the UE based on the availability of the radio resources
at the
candidate cell or for any other reason. When no indication of state transition
is
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received, the serving cell remains in Non-Anchor status 740 and can repeat the
above
procedures of the serving cell as Non-Anchor cell during IHO 725.
[00180] FIG. 8 is a flow chart 800 illustrating an example method may
be
performed by a candidate cell in a handover mechanism. The candidate cell may
first
receive an indication from a serving cell of a UE that an IHO state has been
initialized.When a candidate cell participates in the IHO state, it may first
start an IHO
timer in step 802. In the beginning, it may be in Non-Anchor status 804 and
wait for
anchor assignment in step 806. If no transfer of Anchor control from a current
anchor
cell is received, the candidate cell may proceed to check if the IHO time is
expired
808. Upon the expiry of the IHO timer, the candidate cell can inform the
serving cell
that the time limit for this IHO state is reached and prepare to reclaim the
resources
reserved for the UE in the step 810. If the IHO timer is running 808, the
candidate cell
can decrement the IHO timer 812 and go back to step 804 remaining in Non-
Anchor
status.
[00181] If the IHO candidate cell receives an indication from a current
anchor
base station that the IHO candidate base station can have anchor functionality
for the
UE in step 806, it can enter Anchor status 820 and have anchor control. As an
anchor
cell, the candidate cell may allocate DL resource, grant UL resource for the
UE, and
actively communicate with the UE. Similar to the procedures of a serving cell
in an
Anchor status as illustrated in FIG. 7, the candidate cell may determine if
any state
transition of the UE is needed. During the IHO state, the decision of HO or HO
cancellation (STAY) can be UE controlled or network controlled.
[00182] In some embodiments, a "HO" condition may be satisfied with
respect
to a target base station in step 822. The target base station can be the
serving base
station of the UE, another candidate base station, or a non-candidate base
station. If the
target base station is the serving base station at step 824, the candidate
base station can
inform the serving base station that the target base station is the serving
base station
and cancel the IHO state in step 826. When the target base station is not the
serving
base station, then the candidate base station may inform the serving base
station that
the UE needs to be handed over to the target cell 828. Then the serving base
station
may communicate with the target base station for handover procedures.
[00183] In some embodiments, the candidate cell may satisfy a "STAY"
condition 829, which means the UE can be handed over to the candidate base
station
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that is the current anchor base station. In this case, the candidate base
station may send
a handover initiation to the serving base station of the UE in step 830. The
candidate
base station and the serving base station may collaborate in the handover
procedures.
[00184] In some implementations, neither the "HO" condition in step 822
nor
the "STAY" condition in step 829 is satisfied. The candidate cell, as the
Anchor cell,
can check the quality measurements reported from the UE in step 832, and
further
proceeds to step 834 or step 840 to check if the IHO timer is expired or not.
Given the
expiry of the IHO timer, the candidate cell may inform the serving cell that
the expiry
of IHO state, cancel the IHO state, and reclaim the resources reserved for the
UE in
step 836. If the IHO timer is still running 834 and no other cell is expected
to have
better signal quality than the candidate cell 832, the candidate cell may
decrement the
timer 838, remain in the Anchor status 820, and repeat the above mentioned
process
from step 822. When the IHO timer is still running 840 and there is another
candidate
cell has better signal quality than the current anchor cell 832, the anchor
cell may
transfer Anchor control to the candidate cell 842.. After decrementing the IHO
timer
844, the candidate cell may exit from the Anchor status and go to Non-Anchor
status
804.
[00185] FIG. 9 is a flow chart 900 illustrating an example method may
be
performed by a UE in a handover mechanism with IHO state. The UE can first
measure DL signal quality with respect to its serving cell and neighboring
cells and
send the corresponding measurement reports to the serving cell in step 902.
The
measurement report sent by the UE can include the receive signal quality with
respect
to the serving base station and at least one other neighbor base station,
wherein the
receive signal quality comprised at least one of the following: reference
signal receive
quality, reference signal receive power, signal to interference plus noise
ratio, or
average packet delay.
[00186] The UE can listen to the control channel to see if the serving
cell
initiates an IHO state 904 or a HO state 906. If the serving cell indicates
that the UE
may hand over to a target cell in step 906, for example, by sending a HO
initiation
message, the UE can then prepare corresponding handover procedures with
respect to
the target cell in step 908. In some implementations, the serving cell may
indicate the
UE to enter the IHO state in step 904 by sending an IHO initiation message.
The
message can include one or more of, for example, an IHO timer value, a list of
IHO
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candidate base stations, a descriptor of a handover algorithm, or
representative
parameter values of a handover algorithm (such as a promised QoS that the
network
promises to the UE and various QoS thresholds). Upon receipt of the IHO
initiation
message, the UE can start an IHO timer in step 910. The UE may determine an
anchor
base station of the IHO state. In the illustrated case shown in FIG. 9, the UE
enters the
IHO state in step 910 and continues the active communications with the serving
cell
because the serving cell is the default anchor cell during the initiation of
the IHO state.
The UE may continue to measure the DL signal quality with respect to the
serving cell
and neighboring cells in step 912 and report the signal quality to the anchor
cell in step
914. In some implementations, the UE can initiate the IHO state by acquiring
DL and
UL synchronization with respect to the candidate base stations. In some
implementations, the UE can initiate the IHO state by acquiring system
information
parameters of the candidate base stations.
[00187] If no request for HO or IHO state is received from the serving
cell at
step 904, the UE may go back to step 902 and continue to measure signal
quality and
report the signal quality to the serving cell.
[00188] With the measured signal quality, the UE can optionally perform
some
algorithm 915 to determine whether to exit the IHO state by handing over to
one of the
neighboring cells, or by staying with the serving cell. This process can
correspond to
the UE-controlled scenarios described above. Specifically, the UE can evaluate
the
measured signal quality with respect to all neighboring cells to check if a HO
condition or a STAY condition is satisfied in decisional steps 911 and 913,
respectively. Given the HO condition satisfied with one of the neighboring
cells, the
UE can then send a request to the serving cell to initiate a handover to the
neighboring
cell 917. In some other scenarios, the STAY condition may be satisfied, for
example,
when the serving cell is expected to have a superior signal quality than the
other cells.
Then the UE may request to stay with the serving cell and cancel the IHO state
919. If
neither the HO condition nor the STAY condition is satisfied, the UE may
remain in
IHO state with the serving cell as the anchor cell, and proceed to the step
914.
[00189] During the IHO state, the UE may receive a notification from the
current anchor base station (a first anchor base station) that the anchor base
station is
changed to a new base station (a second anchor base station) in step 916. In
this case,
the UE may receive radio resource assignment and grant from the second anchor
base
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station to establish resource access and synchronization to the second anchor
base
station; and start transmitting/ receiving data packets to/from the second
anchor base
station. It may also start monitoring the DL signal quality with respect to
the second
anchor base station 918.
[00190] In some implementations, the UE may receive from the anchor cell a
handover request to a target cell. The target cell can be an IHO candidate
cell 920, or a
non-candidate cell 930. The UE can then prepare corresponding procedures for
handover to the IHO candidate cell and the non-candidate cell in steps 922 and
932,
respectively.
[00191] In some other implementations, the UE may receive from an
indication
to STAY with the current anchor cell 924. In cases that the serving cell is
the current
anchor cell, the UE may cancel IHO state and stay with the serving cell 926.
[00192] If no above state change request is received, the UE can check
the IHO
timer 932. If the timer is expired, the UE may cancel the IHO state and stay
with the
serving cell 934. If the timing is still running, the UE may decrement the
timer 936 and
go back to the step 912 continuing signal quality measurement and report.
[00193] The UE steps 920-936 are illustrated with an embodiment that
the
anchor cell is the serving cell in FIG. 9. In fact, these steps can be applied
to scenarios
where the anchor cell is not the serving cell with a few modifications. For
example,
when the UE receives a STAY request from the current anchor cell that is not
the
serving cell, the UE may prepare handover from the serving cell to the current
anchor
cell and cancel the IHO state.
[00194] FIG. 10 is a flow chart 1000 illustrating an example handover
algorithm
with an IHO state. The algorithm can be performed, for example, by a serving
base
station of a UE. In this example algorithm, Qs and QT represent QoS
measurements
measured by the UE with respect to the serving cell and a target cell
respectively. Qp
represents the QoS that the network promises to the UE. a, p and 77 are
implementation
specific thresholds which share the same unit of the quality indicators and
can be set
by operators to optimize the HO performance. The QoS measurement may be
averaged
over a suitable time period. The QoS measurements can also be referred as
signal
quality indicators. These measurements such as Qs, Qp, and QT, as well as the
threshold such as a, p and 77 have associated values that representative of
the quality
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of the channel, signal, etc. These values can be used for quantitative
comparisons
between each other.
[00195] As mentioned above, there can be associated conditions for HO,
IHO
and STAY states. The conditions can be defined based on the signal quality
indicators
with some predetermined thresholds. The example HO algorithm below will
provide
more details on some conditions of HO, IHO, and STAY states. Other conditions
for
HO, IHO and STAY states can also be defined without departing from the scope
of
this disclosure.
[00196] When the base station received the QoS measurement or Quality
indicator (Q) from the UE at step 1002, the base station can first check, in
step 1004,
whether the expected QoS of the serving cell Qs is greater than the promised
QoS
value Qp by a factor a. If so, the base station may proceed further to step
1006 to
compare the QoS of the target cell with the Qos of the serving cell. If the
QoS of the
target cell is even better than the Qos of the serving cell by a factor 77,
the QoS of the
target cell is well beyond the promised QoS to the UE. The base station may
decide to
perform handover to the target cell for better QoS expected by the target cell
for the
UE 1008. In some implementations, if the QoS of the target cell is not larger
than
(Qs + 77 ), the base station may restrict handover and stay with the current
serving cell
1010.
[00197] In some implementations, the QoS with respect to the serving cell
may
not exceed the promised QoS by a factor a at step 1004, the base station can
move to
step 1012 to determine if the QoS of the serving cell is above than the
promised QoS
less Ig. If the QoS of the serving cell satisfies Qp ¨ ig < Qs Qp + a and the
QoS
difference between the serving cell and the target cell is below a threshold
77 (step
1014), the IHO state 1016 may be triggered to reduce unnecessary handovers. On
the
other hand, given Qp ¨ ig < Qs Qp + a but the QoS difference between the
serving cell
and the target cell is larger than the threshold 77 (step 1014), the base
station may
proceed to step 1018 to determine which one has a larger value between the Qs
and the
QT. If QT is above Qs by 77 at the decisional step1018, the QoS with respect
to the target
cell is better than the QoS associated with the serving cell to the extent
such that a
handover to the target cell is triggered 1020. In some implementations, if Qs
is above
QT by II, the UE may stay with the serving cell and proceed to step 1022 in
determining if Qs has a value larger than the promised QoS Q. If so, the
serving cell
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can provide a promised QoS to the UE where the UE is in STAY state with the
serving
cell 1024. However, if Qs is below Qp 1022, the UE may have to stay with the
serving
cell with degraded QoS 1026 where no candidate cells expect a better QoS than
Qs in
this case.
[00198] In some implementations, the serving base station may decide that
the
expected QoS associated with the serving cell is less than (Qp ¨ p) at step
1012. Under
such a scenario, the serving base station may conduct a handover 1030 if the
QoS of
the target cell is better than that of the serving cell 1028. Otherwise, the
serving base
station may drop the UE in step1032 given that the promised QoS cannot be
guaranteed by either the serving cell or the target cell 1028.
[00199] The above example algorithm may be evaluated by the serving
base
station with respect to all the neighboring cells. In some implementations,
the above
algorithm can be evaluated, at least in part, by a UE. For example, in a UE-
controlled
scenario, during an IHO state, the UE can follow the steps 1018-1026 of the
algorithm
in FIG. 10 to determine if a HO condition or a STAY condition is satisfied.
Because
the UE has the signal quality measurements, the UE can make the decision and
inform
the decision to the anchor cell, without sending signal measurements to the
anchor
base stations. In this way, signaling overhead can be reduced.
[00200] FIG. 11 is a schematic plot 1100 illustrating different state
regions with
respect to different signal quality of the serving cell and a target cell. The
plot is based
on the algorithm described above in FIG. 10. In this example, only one target
cell is
considered. If more than one target cells are considered, a multi-dimensional
figure
can be expected. As shown in FIG. 11, the horizontal axis 1105 is Qs, the QoS
with
respect to the serving cell, while the vertical axis 1115 is QT, the QoS with
respect to
the target cell. A 45-degree dashed line 1125 corresponds to the points that
Qs and QT
are equal. Region above the line 1125 is where the QoS of the target cell is
better than
the QoS of the serving cell whereas the area below the line 1125 represents
the case
where the QoS of the target cell is worse than the QoS of the serving cell.
Between the
two parallel lines 1135 and 1145, the difference between Qs and QT is within
the
threshold 77 1102. The promised QoS Qp is shown as lines 1155 and 1165 with
respective to QT and Qs, respectively. Similarly, lines 1150 and 1170
correspond to
Qp ¨ )6' with respect to Qs and QT; lines 1160 and 1180 correspond to Qp + a
with
respect to Qs and QT, respectively. Parameters a 1104, 16 1106, and 77 1102,
collaborate
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with the promised QoS Qp in determining different handover regions, such as
Drop
region 1108, IHO state region 1110, Stay region 1112, Stay with degraded QoS
region
1114 and HO state region 1116. In particular, the IHO state can occur in
region 1110
where the QoSs of the serving cell and the target cell are around the promised
QoS Q.
Applying IHO state in this region can reduce unnecessary handovers because Qs
and
QT do not differ too much (the difference is within 77); potential QoS gain
(from a
slightly higher QT) of applying a complete handover might be outweighed by QoS
degradation due to overhead, delay, data interruption of the handover
procedure.
Preferably, applying the IHO state in this case can restrict handover and
avoid
unnecessary data interruptions. During the IHO state, the UE can obtain better
QoS by
receiving data packets from either the serving cell or the target cell. In
other words, UE
obtains advantage of switched diversity receipt. While some degradation is
expected
because of the slowness of the switching process, the trade-off is that the
handoff
procedure is restricted and therefore the likelihood of the negative effects
of frequent
handoff (such as dropped calls) is minimized. The resource sharing between the
serving and the target cell, if properly managed, can also result in better
overall
spectral efficiency.
[00201] The parameters a 1104, 16 1106, and q 1102 can be configurable
and
implementation specific. The method and algorithm mentioned above enable the
network operators to design these parameters to optimize the handover
performance
depending on different deployment scenarios.
[00202] While several implementations have been provided in the present
disclosure, it should be understood that the disclosed systems and methods may
be
embodied in many other specific forms without departing from the scope of the
present
disclosure. The present examples are to be considered as illustrative and not
restrictive, and the intention is not to be limited to the details given
herein. For
example, the various elements or components may be combined or integrated in
another system or certain features may be omitted, or not implemented.
Variations,
modifications, and enhancements to the described examples and implementations
and
other implementations can be made based on what is disclosed.
[00203] Also, techniques, systems, subsystems and methods described and
illustrated in the various implementations as discrete or separate may be
combined or
integrated with other systems, modules, techniques, or methods without
departing from
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the scope of the present disclosure. Other items shown or discussed as coupled
or
directly coupled or communicating with each other may be indirectly coupled or
communicating through some interface, device, or intermediate component,
whether
electrically, mechanically, or otherwise. Other examples of changes,
substitutions, and
alterations are ascertainable by one skilled in the art and could be made
without
departing from the spirit and scope disclosed herein.
[00204] While the above detailed description has shown, described, and
pointed
out the fundamental novel features of the disclosure as applied to various
implementations, it will be understood that various omissions and
substitutions and
changes in the form and details of the system illustrated may be made by those
skilled
in the art, without departing from the intent of the disclosure.
37
