Note: Descriptions are shown in the official language in which they were submitted.
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CELL LOAD SHARING IN A
MOBILE-CONTROLLED CELL
SELECTION ENVIRONMENT
BACKGROUND OF T'I-lE INVENTION
Technical Field of the Invention
The present invention relates in general to the radio telecommunications
field and, in particular, to a method and system for enabling cell load
sharing in a
mobile-controlled cell selection environment, such as, for example, in a radio
packet data system.
Description of Related Art
In the new generation of wireless data communication services, such as, for
example, the General Packet Radio Service (GPRS) in the Global System for
Mobile Communications (GSM), data packets are transferred across a radio air
interface between a base: transceiver station (BTS) and a mobile station (MS).
The
radio system uses a known cellular structure (e.g., as in the GSM), wherein
each
BTS is assigned to cover a certain geographical service area (cell). As such,
each
BTS is responsible for communicating with all of the MSs that are active in
its cell.
Typically (but not always), the cell of a BTS (e.g., BTS A) includes the
2 o geographical area in which, out of all of the BTSs in the system, that BTS
provides
the best radio link quality. The border at which an MS changes communication
from BTS A to BTS B is called the cell border between cell A and cell B.
In a cellular data. communication system, the traff c load generated by the
users (MSs) varies in tune. This fact implies that the radio network needs to
be
2 5 over-dimensioned in order to compensate for traffic peaks. At any given
time,
there will typically be cells in which the existing load is too high to
provide
adequate service quality to the MSs in those cells, while at the same time,
neighboring cells can have surplus capacity. In such situations, it would be
advantageous to allow some of the MSs in a congested cell to connect to
30 neighboring cells that are experiencing less load. This approach, which is
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commonly referred to as "cell-load sharing," is used in conventional cellular
speech
communication systems (e.g., in the GSM).
Essentially, there are two ways to administer cell selection in cellular radio
systems: MS-controlled cell selection; and network-controlled cell selection.
For
MS-controlled cell selection, it is the MS's responsibility to determine to
which
BTS it is most suitable to connect. As such, the MS considers whatever
information it has at hand (radio conditions, received signal strengths,
service
capabilities, and possibly, additional information received in system
information
messages, etc.) as inputs) to a pre-determined cell selection algorithm. 'The
output
l0 from the cell selection algorithm denotes the cell to which the MS should
connect.
For network-controlled cell selection, the network commands the MSs to
connect to a certain BTS (cell). A necessary condition for network-controlled
cell
selection is that every MS consistently provides the network with measurement
reports. Essentially, the measurement reports inform the network about how the
reporting MS perceives the candidate BTSs or cells, in terms of their signal
strength and radio link quality.
Both cell selection approaches have their merits and drawbacks. For
example, the main advantage of network-controlled cell selection (also
referred to
as "locating") is that a relatively intelligent cell selection algorithm can
be used.
2 0 As such, the network can use the wealth of information it has available,
riot only
about the radio link conditions, but also about the load in each cell,
buffered data
packets, quality of service contracts with users, preferred home zones, etc.
Network-controlled cell selection is used, for example, in the circuit-
switched
GSM.
The biggest disadvantages of network-controlled cell selection are the
increased complexity for the network involved, and that each MS has to provide
measurement reports regularly to the network. Unfortunately, the measurement
reports use uplink bandwidth and, therefore, actually decrease the system's
capacity. The increased complexity results from the network having to
administer
3 0 a locating procedure for each active MS.
On the other hand, for MS-controlled cell selection, each MS is responsible
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for its own cell selection procedures. In a GPRS configuration, the system can
use
either MS-controlled or network-controlled cell selection. However, for
reasons
described below, it is highly likely that MS-controlled cell selection will be
the
method of choice for the majority of future GPRS networks.
The pertinent prior art focuses on cell load sharing in systems using
network-controlled cell selection approaches. In such systems, the process
whereby an active MS selects another cell is always initiated by a cell re-
selection
(i.e., "handover") comman.d issued from the network. Using such a cell load
sharing approach, during periods of congestion, the network can identify
candidate
l0 MSs in a congested cell, and direct these MSs to connect to less congested
cells by
issuing re-selection commands for dedicated cells (i.e., the less congested
cells).
As such, the details of which MSs to move to another cell, or where and when
to
move them, are essentially matters of implementation.
The main shortcoming of the above-described prior art approaches to cell
load sharing is that they cannot be used in systems with MS-controlled cell
selection. Consequently" in such systems as GPRS, where either network-
controlled or MS-controlled cell selection can be used, the prior art cell
load
sharing approaches cannot be used in conjunction with those systems' MS-
controlled cell selection part. This shortcoming exists because the network
has no
information about each MS's individual radio environment (no measurement
reports are sent to the network). Furthermore, in a system with strict MS-
controlled cell selection, the decision to select a new cell is the MS's
responsibility,
not the network's. On the other hand, the MSs know nothing about the global
load
situation, current congestion levels, or lengths of queues in the system.
2 5 Consequently, the MSs cannot administer cell load sharing by themselves.
Additionally, this shortcoming imposes a severe limitation in such dual-
mode cell selection systems as GPRS. However, because of the drawbacks of the
network-controlled cell selection approaches as described earlier, the MS-
controlled cell selection schemes will still likely be the methods of choice
in most
3 0 GPRS networks.
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SUMMARY OF THE aNVENTION
In accordance with a preferred embodiment of the present invention, a
method and system are provided for implementing cell load sharing in an MS-
controlled cell selection environment, such as, for example, in a radio packet
data
system, wherein dynamic system information (e.g., SI messages) is used to
enable
an MS to perceive cell borders that depend on the system load. Essentially,
cell
selection parameters in the system information messages are associated with
the
distribution of the load in various cells. In this way, the MSs (which are
responsible for their own cell selection) in overloaded cells can be prompted
to
select alternative, less loaded cells.
An important technical advantage of the present invention is that a method
is provided for using cell load sharing in a system in which MS-controlled
cell
selection is also being used.
Another important technical advantage of the present invention is that it
enables systems with only MS-controlled cell selection to also use cell load
sharing.
Still another important technical advantage of the present invention is that
in systems that can choose between a network-controlled and MS-controlled cell
selection mode (e.g., GF'RS), cell load sharing can be used while the system
is
2 0 operating in the MS-controlled cell selection mode.
Yet another important technical advantage of the present invention is that it
enables the use of load sharing in radio packet data systems (e.g., GPRS} to
increase capacity and avoid congestion, without overloading the uplink with
measurement report traffic.
2 5 BRIEF DESCRIPTION OF THE DRAWINGS
A more complete: understanding of the method and apparatus of the present
invention may be had by reference to the following detailed description when
taken
in conjunction with the accompanying drawings wherein:
FIGURE 1 is a diagram that illustrates an exemplary system that can be
3 0 used to implement the present invention;
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FIGURE 2 is a diagram that further illustrates how the exemplary system
shown in FIGURE I can be used to implement the present invention; and
FIGURE 3 is a diagram that illustrates how system information messages
can be used to implement a preferred embodiment of the present invention.
DETAILED DESCR1PTION OF THE DRAWINGS
The preferred embodiment of the present invention and its advantages are best
understood by referring to FIGUREs I -3 of the drawings, like numerals being
used for
like and corresponding parts of the various drawings.
Essentially, in accordance with a preferred embodiment of the present
I O invention, a method and system are provided for implementing cell load
sharing in a
radio packet data system, wherein dynamic system information (e.g., in the
form of
SI messages) is used to enable an MS to perceive cell borders that depend on
the
system load. For MS-controlled cell selection, the network determines the cell
borders
by including pertinent cell selection parameters in the system information
messages.
The system information messages can be either broadcast to all MSs in the
network,
or sent dedicated to a selected set of one or more MSs in an overloaded cell.
The cell
selection parameters contain information, for example, about offsets that can
be
applied to the measured signal strengths, threshold values, cell priorities,
etc. For the
GPRS, such cell selection parameters can be found in the GSM Technical
2 0 Specification GSM 05.08, version 6Ø0. As such, for this exemplary
embodiment, the
cell selection parameters in the system information messages are associated
with the
distribution of the load in various cells. In this way, the MSs (which are
responsible
for their own cell selection) in overloaded cells can be prompted to select
alternative,
less loaded cells. As mentioned above, the system information messages can be
sent
2 5 on the broadcast channel, or transmitted dedicated to a selected subset of
MSs in the
overloaded cell.
Specifically, FIGURE 1 is a diagram that illustrates an exemplary system 100
that can be used to implement the present invention. System 100 is preferably
a
system that employs MS-controlled cell selection, and can be a radio packet
data
3 0 system or packet-switched system such as the GPRS. The exemplary radio
packet data
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system 100 can be operating in conjunction with a circuit-switched system,
such as the
GSM, the Digital-Advanced Mobile Phone System (D-AMPS), or any other system
capable of supporting a radio packet data system or service. System 100
includes a
plurality of base stations or BTSs, such as, for example, BTS A ( 102) and
neighboring
BTS B (104). BTS A defines one cell (A), and BTS B defines a second cell (B).
The
dotted line 106 represents a cell border between cells A and B where (to a
mobile
terminal or MS) the signal strength of cell A is perceived to be equal to the
signal
strength of cell B (i.e., SSA=SSH). For the GSM and similar systems, it can be
assumed that a base station controller (BSC) is connected to, and controls the
functions of, the two BT'Ss A 102 and B 104 shown.
For a relatively low traffic load scenario, the cell selection parameters
contained in the system information messages (e.g., SI messages in the GSM and
GPRS) broadcast or transmitted to the mobile terminals 110-118 shown, define
an
offset of 2 dB which is added to the MS-measured signal strength from BTS A
102
(i.e., SSA+2dB=SSB) to form the (solid line) cell border 108. As shown for
this
exemplary scenario, three of the mobile terminals (110, 112, 114) have
initially
connected to BTS A 102, while two mobile terminals (116, 118) have coru~ected
to
BTS B 104.
FIGURE 2 is illustratively similar to FIGURE l, except that the scenario
2 0 depicted in FIGURE 2 has more mobile terminals connected to BTS B 104 than
to
BTS A 102. For example, three mobile terminals (110, 112, I 14) have connected
to
BTS A 102, while seven mobile terminals (116-128) have connected to :BTS B
104.
As such, it can be assumed for this scenario, that the load in cell B is such
that a
sufficient quality of service (e.g., QoS in GPRS) can no longer be offered to
the packet
data system users in cell B. However, at the same time, it can be assumed that
there
are more than enough radio resources available for adequate service in
neighboring
cell A.
FIGURE 3 is a diagram that illustrates how system information messages can
be used to implement a preferred embodiment of the present invention. For the
3 0 scenario depicted in FIGURE 3, the network of system I 00 sends system
information
messages (via a BSC and one or more of the BTSs A and B) which contain
appropriate
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cell parameters such that the affected mobile terminals add an offset with a 4
dB
increase to the perceived signal strength of cell B (i.e., SSA+4dB=SSB).
Effectively,
as a result of the offset, for the affected mobile terminals, the cell border
(108)
between cells A and B appears to be shifted towards cell B, and thereby, cell
A
effectively covers a larger number of the mobile terminals than before. For
example,
because of the 4 dB off.~et received in a system information message, five of
the
mobile terminals (110-118) now appear (to the mobile terminals) to be covered
by cell
A, and the five other mobile terminals ( 120-128) appear to be covered by cell
B.
An example of a system information message that can be used to convey the
above-described offset to one or more mobile terminals in an MS-controlled
cell
selection environment is a GPRS cell selection parameters information element
as
defined at Section 11.2.19 of the GSM Technical Specification GSM 04.60,
version
6Ø0, June 1998. The information element can be delivered to all, or a subset
of, the
mobile terminals in cell A., either on the Packet Broadcast Control Channel
(PBCCH)
in cell A, or on one or more Packet Associated Control Channels (PACCHs) in
cell A.
For each cell defined as a neighboring cell to cell A, the information element
can
contain the 5 bit information element GPRS RESELECT OFFSET that defines a
bias, or offset, to be used in the "C31/C32" criterion which is used in the
GPRS for
MS-controlled cell reselection. This bias or offset value can be different far
different
2 0 cells and thus can be used to shift the MS-perceived boundary between two
cells (e.g.,
between A and B). For example, for mobile terminals camping on cell A, an
offset
value of 4 dB can be added to the ranking criterion, C32, of cell B, by
setting the five
corresponding bits in the system information element described above to
"01100" (+4
dB). When the system information element is transmitted on the Broadcast
channel
2 5 in cell A (or when transmitted on the one or more Associated control
channels in cell
A), the receiving mobile terminals perceive the cell border between cell A and
cell B
(108 in this example) as shifted by 4 dB relative to the equal-signal-strength
border
(106). In order to avoid a situation in which the mobile terminals that have
selected
cell B from cell A immediately return to cell A, the corresponding system
information
3 0 element in cell B (i.e., the element controlling the cell border between
cell A and cell
B, as viewed from cell B) can be set accordingly.
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Yet another example of a system information message that can be used to
convey the above-described offset is found in a GSM/GPRS system using no
PBCCH.
In such systems, the MS-controlled cell selection ofGSM mobile terminals in
the Idle
mode, and the GPRS mo'~bile terminals functioning in any mode, can use the GSM
SI
message Cell Selection Parameters information element as defined in Section
10.5.2.34 of the GSM Technical Specification GSM 04.08, version 6.1:.1, August
1998. The SI 3 Rest Octcas information element is a type 5 information element
that
contains a "CELL-RESEL.ECT-OFFSET" information field. The CELL-RESELECT-
OFFSET is a bias or offset value (in dB) that is included in the so-called
"C2" criterion
for a cell. This bias or offset value can be different for different cells and
thus can be
used to shift the MS-perceived boundary between two cells (e.g., between A and
B).
For example, a CELL-RF;SELECT-OFFSET value of 4 dB can be added as an offset
in the C2 criterion for cc;ll A, by transmitting the corresponding bit pattern
in the
CELL RESELECT OFFSET information element in the System Information 3 Rest
Octets transmitted on the 13CCH in cell A. However, this exemplary scenario
assumes
that the CELL RESELE(~'r_OFFSET field in the System Information 3 Rest Octets
transmitted on the BCCH in cell B corresponds to a 0 dB bias or offset in cell
B.
Consequently, in both examples above, and in accordance with the preferred
embodiment of the present invention, the offset caused by the cell parameters
in the
2 0 transmitted system infom~ation messages causes the mobile terminals in
cell B which
are close to the border of cell A (e.g., mobile terminals 116 and 118) to rank
cell A as
a "better" cell to be connected to than cell B. Therefore, these two mobile
terminals
effect a change (e.g., via a conventional handover procedure) from cell B to
cell A.
The result is that the load (mobile terminal traffic) is moved from cell B to
cell A in
2 5 such a way that a sufficient quality of service can be offered to the
users in both cell
A and cell B (as illustrated. in FIGURE 3). If the load in cell A and/or cell
B changes,
the cell parameters in the ;system information messages can be altered and the
offset
changed accordingly, in order to compensate for the change in load. By using
the
contents of the system information messages in such a dynamic fashion, the
network
3 0 can move neighboring cell borders in order to adjust to changes in the
distribution of
the load. As such, the present invention increases the cellular system's
ability to
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maintain a sufficient quality of service during periods of local congestion in
the
network.
As such, there are a number of load conditions that can "trigger" a change in
transmitted system information to cause an MS-perceived shift of a cell
border, in
accordance with the teachings of the present invention. For example, a
pertinent load
can be related to ( 1 ) the total number of users in a cell, (2) the total
number of active
users in a cell, (3) the ratio of the total number of active users in a cell
to the number
of channels in the cell, (4) the ratio of the total number of queued data
packets in a cell
to the number of channels in the cell, (5) a measure of the QoS that can be
offered in
1 o a cell (e.g., bandwidth per user, fraction of QoS contracts to users that
can be fulfilled,
etc.), and so on. Furthermore, the cell boundary shifts that can result are
not limited
to boundaries between so-called macro-cells, but could include boundaries
between
disparate types of cells, such as, for example, micro-cells and macro-cells,
pico-cells
and micro-cells, etc.
In a different aspect of the present invention, the system information in all
cells
in a defined network area is dynamic information. Using this dynamic
information at
any given time, the nehvork can (with any given temporary load distribution)
optimize
the quality of service offered to the packet data service users in the whole
area. In
order to achieve this capability, in accordance with the present invention,
the system
2 0 information (to be broadcast or transmitted from the network) can be made
dependent
on the number of mobile terminals in each cell, the number of scheduled data
packets
in each cell, by the users' perceived quality of service in each cell, the
packet delays
in each cell, and so on. As such, a network algorithm (e.g., being executed by
a
processor at a base station controller, BSC, or a mobile services switching
center,
2 5 MSC) can be used to derive an appropriate system information message with
an offset
value included, and direct the message to dedicated MSs or broadcast it to all
pertinent
MSs.
Although the present invention has been described above in the exemplary
context of a GSM packet data service (e.g., GPRS), the present invention can
be
3 0 applied to any cellular radio system that can use system information as
input to an MS-
controlled cell selection algorithm. Furthermore, the present invention can be
applied
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to any cellular radio system that can operate in an MS-controlled cell
selection mode.
Such a system can include, for example, GPRS in GSM, GPRS in D-AMPS, an IS-95
system, cellular radio satellite systems, as well as the Universal Mobile
Telephone
System (UMTS), and both present and future spread spectrum and Wideband Code
Division Multiple Access (WCDMA) systems.
Although a preferred embodiment of the method and apparatus of the present
invention has been illustrated in the accompanying Drawings and described in
the
foregoing Detailed Description, it will be understood that the invention is
not limited
to the embodiment disclosed, but is capable of numerous rearrangements,
modifications and substitutions without departing from the spirit of the
invention as
set forth and defined by the following claims.
