Note: Descriptions are shown in the official language in which they were submitted.
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A BROADCAST CONTROL CHANNEL
STRUCTURE FOR WIDEBAND TDMA
The present invention relates generally to mobile communication systems.
More. particularly, the present invention relates to synchronization,
handover, and cell
relations in a cellular mobile communication system.
BACKGROUND OF THE INVENTION
In known wireless communication systems, radio resource management
algorithms such as power control and dynamic channel allocation are provided
to
optimize the use of system resources (spectrum, power, etc.).
Many such algorithms can benefit from knowing the relations between cells -
for example, how much disturbance exists from other cells in a system. Cell
relationship information allows more centralized algorithms to be used, and
can
improve algorithm performance.
One way of obtaining cell relations is to let the mobile stations (MSs) in the
system monitor the control channels transmitted from different base stations
(BSs). By
registering the strength and identity of received base station control
channels, the cell
relations can be deduced. Base station identification is typically performed
by
determining the base station identity codes (BSIC) that are transmitted by the
base
stations on their respective control channels.
In the GSM system, mobile stations are capable of identifying neighboring base
stations by decoding the encoded BSIC. Each BSIC is a non-unique code
transmitted
by each base station on time slot 0 of the control channel frequency, every
10th TDMA
frame.
Because the mobile stations are not synchronized with neighboring cells, each
mobile station must listen for eight time slots of the control channel, to
make sure that
time slot 0 (and thus BSIC information) is decoded. This is made possible by
introducing idle frames. FIGS. lA and 1B show exemplary successive idle
frames, 13
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frames apart in a GSM system, where mobile stations transmitting on time slots
ts0-ts7
transmit control channel information C, idle slots I (i.e., no transmission),
anti
signaling information S. It will be appreciated that time slots ts0 and tsl
are typically
reserved for control channel information C, and that no mobile station uses
those time
slots for transmission of data or signaling information S. In the GSM system,
not all
mobile stations in a cell have their idle frames at the same time. During a
mobile
station's idle frame, the mobile station is silent - that is, the mobile
station does not
transmit information. Because different mobile stations have different idle
frames,
other mobile stations may transmit during one mobile station's idle frame. For
IO example, in FIG. lA, mobile stations communicating on ts2, ts4, and ts6 are
idle,
while mobile stations on ts3, ts5, and ts7 send signaling information.
As BSIC information is sent only once every 10th TDMA frame, the GSM
control channel multi-frames and trafftc channel multiframes are designed so
that the
idle frame will be adjusted to correspond with different types of control
channels, and
eventually correspond to the BSIC information (sliding multiframes). This
complicated
procedure requires up to 10 seconds for a mobile station to decode the BSICs
of the six
best neighbors.
In GSM, the reliable determination of cell relations is limited for at least
three
reasons.
A first reason is that the coding of the BSIC is relatively weak in GSM. To be
decoded by a base station, the carrier-to-interference ratio (C/1) for the
control channel
signal must be relatively high. While this is often the case for neighboring
base
stations, for base stations further away the carrier strength (C) is usually
lower and the
interference (1) is usually higher. The worst case is when two co-channel
BSICs are
received which are relatively equal in signal strength. Then the CII ratio on
this
channel is around 0 dB, and it is impossible to decode either of the two
BSICs.
A second reason is that in GSM, problems arise when a mobile station tries to
decode the BSIC of a co-channel cell (a cell that is using the same control
channel as
the mobile station's serving cell}. Then, the serving base station can become
a source
of interference. The reason for this is that while one mobile station is idle
in this
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frame, other mobile stations in the same cell may receive control channel
information
or signaling information in this frame, creating interference on these slots.
A third reason is that the BSIC identities used in GSM are only 6 bits long;
therefore, the same combination of control channel frequency and BSIC are used
by
multiple cells in a system. Thus, a BSIC decoded on a certain control channel
frequency cannot automatically be associated with a specific cell.
While other solutions for obtaining path loss cell relations exist, they are
complex, can only decode a very limited number of identities, or require
central control
of the network.
Therefore, it would be desirable to provide a communication method and system
which allow cell relations in a relatively large area to be quickly, easily,
and reliably
determined.
The present invention provides far a communication system and method for fast
identity decoding. The present invention allows cell relations to be obtained
for a
relatively large area in an efficient way.
The present invention makes use of one or more of the following: strongly
coded base station identities, substantially synchronized idle frames to
reduce
interference from traffic channels; and increasing the "reuse distance" for
control
channels by placing the control channels on designated time slots in different
co-
channel cells.
The present invention is preferably implemented in a slot and frame
synchronized system. A method for automatically obtaining approximate, local
synchronization is also provided.
BRIEF DESCRIPTION OF HE D AWlrlr:c
The present invention can be more fully understood upon reading the following
Detailed Description in conjunction with the accompanying drawings, in which
like
reference indicia are used to designate like elements, and in which:
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FIGs. lA-1B are exemplary consecutive idle flames in a GSM communication
system;
FIG. 2 is a representation of three base stations communicating with three
mobile
stations over a common fiequency;
FIG. 3 is a representation of broadcast channel allocation according to one
embodiment of the present invention;
FIG. 4 is a diagram of an area of cells using the broadcast channel allocation
scheme of FIG. 3;
FIGs. SA-SB are diagrams showing an ideal case and a more practical case,
respectively, of control channel allocation;
FIG. 6 is a exemplary identity burst transmitted from a base station over a
control
channel according to an aspect of the present invention; and
FIG. 7 is a diagram showing a mobile station determining cell relation
information and providing synchronization information according to one
embodiment of
the present invention.
The present invention will be described assuming a mobile TDMA system
where the base stations transmit only control channel or signaling information
(occupying one or more time slots) during idle frames. The system can either
be a
packet switched system or a circuit switched system where all mobile stations
in a cell
have substantially synchronized idle frames. It will be appreciated, however,
that the
invention can be applied to other types of communication systems. It is also
assumed
that the BS identities are unique within at least some area which includes non-
neighboring cells.
According to one aspect of the present invention, base station identities are
encoded stronger than in a conventional (e.g., GSM} system. This makes it
possible to
decode the identities at relatively low C/I levels. Even with increased
coding, it is still
desirable to reduce interference, and thereby increase the CII ratio.
Therefore,
according to a second aspect of the present invention, the interference from
traffic
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channels is reduced by substantially synchronizing the idle frames for all
base stations
in an area including non-neighboring cells. If perfect synchronization and
zero
propagation delays could be achieved with this area, interference would be
eliminated.
As a practical matter, local slot and frame synchronization between base
stations within
5 an area can substantially decrease interference.
It is also desirable to place the broadcast control channels for different co-
channel cells on designated time slots to substantially reduce interference
from the
broadcast channel of a serving cell. This is exemplified in FIG. 2, where
three co-
channel base stations, A, B and C use the same control channel frequency, but
the
broadcast information is transmitted on different time slots. This means that
if the
mobile station in cell C tries to decode the identities of A and B, it will
not be
disturbed by base station C's control channel information. Furthermore, it is
possible
to decode the identities of both base stations A and B in the same idle frame,
as they
transmit their respective identities at different times. The only interference
comes from
other cells using the same control channel frequency and time slot as A and B.
Due to
the high reuse that is obtained by placing the broadcast channels on
designated time
slots, this interference will be significantly weaker than interference in
conventional
systems.
FIGs. 3 and 4 give an example of the broadcast channel allocation in an area
of
cells which include non neighboring cells. In this example, the traffic
channel reuse is
three, and the broadcast channel is spread over five time slots. This means
that the
broadcast channel is not reused within five traffic channel clusters, so the
broadcast
channel reuse is 15. The five base stations using the same frequency fx are co-
channel
cells in different clusters. These base stations transmit on different
broadcast channel
time slots. It will be appreciated that the broadcast channel reuse is
increased five
times when the broadcast channel is spread over five designated time slots.
FIG. 3
shows the broadcast channel allocation, where each of the different patterns
represents
one of the five clusters in FIG 4. The three base stations in a cluster can
use the same
broadcast channel time slot, since they use different frequeacies. The
different
frequencies fx, fy, and f2 can be adjacent or non-adjacent frequencies. If the
frequencies
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are adjacent,' different time slots can be used within the cluster to decrease
adjacent
channel interference.
During an idle frame, each mobile station chooses one frequency and decodes as
many base station identity codes on it as possible. In the next idle frame, a
new
frequency is chosen. The frequency choice can be based on intermediate
interference
measurements, or, if the number of frequencies is not very large, the mobile
stations
can sequentially scan through all frequencies.
The invention is particularly useful in a wideband TDMA system with a
relatively small number of carrier frequencies and a relatively large number
of time
slots on each carrier. In such a system, the mobile station can decode a
relatively large
number of identities in each idle frame. Because the number of frequencies is
limited,
the mobile station can decode all identities relatively quickiy.
The results (the base station identities and signal strengths with which each
base
station identity code was received) are reported to the serving base station.
The
measurements from each base station can be reported to a central entity where
the cell
relations can be calculated and stored. The stored cell relations can be used
for
performing handovers in a quick and efficient manner. For example, the stored
cell
relations can improve automatic frequency planning and can be used to
determine
which potential new base stations can be measured for handover purposes. The
information can also reduce the need for certain handover measurements.
An exemplary implementation of the present invention will now be described,
assuming a wideband TDMA system, where each frequency carrier is divided into
64
time slots each of which is 72 ms long.
If perfect synchronization could be achieved with zero transmission delay, the
mobile stations could decode an identity in each time slot, that is 64
identities per idle
frame.
More practically, the control channels can be placed on, for example, every
second time slot, as shown in FIGS. SA-B, which represent the case where a
mobile
station attempts to decode the identities of two base stations, A and B, that
have the
same broadcast channel frequency f, but different time slots. The time shift
between
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the transmissions from base stations A and B represented in FIG. SB is due to
imperfect synchronization and propagation delay.
FIG. SA is an ideal case in which the mobile station is able to decode both
identities. In FIG. SB, imperfect synchronization and different propagation
times from
S the two base stations result in a time shift between the transmissions from
base stations
A and B, and also result in interference between the two broadcast channels.
If one
base station is relatively close and the other is relatively distant (i.e. the
synchronization aad propagation time difference is mainly due to the
difference in
transmission delay), the mobile station can decode the identity of the closer
base
station. Alternatively, if interference is caused by poor synchronization, the
signals
from different base stations may be of approximately equal strength, such that
a mobile
station will be unable to accurately decode any of the identities.
In this example, the synchronization and propagation time differences do not
add up to more than one time slot. If the burst size is 72 ms, half of a 1
slot shift
results from transmission delay difference, and the other half of the shift
results from
imperfect synchronization, there may be difficulties in decoding the
identities of base
stations further away than 11 km.
Depending oa the degree of synchronization, the number of time slots on each
carrier frequency used for control channel information can be varied. If, for
example,
control channels are placed on every eighth time slot in this 64. slot
example, both the
synchronization and the transmission delay difference requirements become less
critical. The broadcast channel reuse is still high and the mobile stations
can decode
eight identities per idle frame. This is sufficient for handover purposes.
An exemplary identity burst for transmission on the broadcast channel slots is
shown in FIG. 6. The identity burst contains a training sequence part and an
information part (e.g., including BSIC and current time slot inform tion). The
identity
burst can preferably be received and decoded in an environment with
considerably
lower C/N than what is possible in e.g. the GSM case, since the broadcast
control
channel structure according to the invention reduces most of the interference,
but does
not reduce noise. In other words, to decode BSIC from cells which are beyond
the
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neighboring cells, a more robust channel is desirable. The burst Iength in
FIG. 6 is the
same as a normal burst. There is no additional guard time needed, since there
are
guard slots) between the broadcast control channel slots.
The training sequence in the identity burst is relatively long, in order to
allow
capture of the identity burst and synchronization in low C/N environments.
There can
be either only one possible training sequence, or different training sequences
in the
TDMA slots. Different training sequences can be used for synchronization
purposes,
where the training sequence designates a slot number. In this case, the
information
part of the identity burst need not include current time slot information.
The information part of the identity burst preferably includes an encoded
BSIC,
where the code is a relatively low rate convolutional code or block code. For
low C/N
operation, a code rate of approximately 1/4 or less is desirable. A CRC code
can also
be encoded into the burst to allow for error detection. The receiver can use
the CRC
together with soft receiver metrics (e.g, decoder path metrics and CII
measures) to
distinguish between correctly and incorrectly received bursts.
The identity burst is transmitted for each idle slot. If a mobile station can
synchronize to a burst without successfully decoding it, the received signal
can be
stored and a second synchronization can be made at the next idle slot. The
received
signal from two or more idle slots can then be soft combined before a second
decoding
attempt is made. This allows for successful reception of identity bursts
received at an
even lower CIN ratio.
Referring now to FIG. 7, a mobile station is shown determining cell relations
and providing synchronization information based on received identity bursts.
The
mobile station in FIG. 7 is served by a base station B, and decodes an
identity burst
from base station A, including the base station identity code and
synchronization
information for base station A. Since the mobile station knows the current
time slot for
the serving base station B, it can calculate an Observed Time Difference, OTD,
for
transmissions from base stations A and B, and forward the OTD to serving base
station
B. The relation between the OTD and the RTD (Real Time Difference) is
RTD = OTD + tB - t,,
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where t~ and tB are the propagation delays from BSs A and B, respectively.
is and OTD are now known in base station B, and t,, can be estimated from
geometrical considerations. Hence an estimate of RTD can be calculated in base
station B.
S If OTD is reported by the mobile station, and RTD calculated by the base
station or a central processor each time an identity is decoded by a base
station, large
statistics on synchronization differences between different base stations can
be obtained
over time. It should be appreciated that the further away base station A is
located from
the mobile station, the less accurate will t,, be. Therefore, it is preferable
to
synchronize to closer base stations. It will be appreciated that this
synchronization
method can be used to achieve synchronization for any number of purposes, and
can be
used with or without the broadcast control channel scheme described above. To
achieve synchronization, the mobile station receives and decodes identity and
synchronization information from multiple base stations, and transmits this
information
1S to abase station for determining synchronization based on the decoded
information
(using, e.g., the OTD technique described above). If synchronization
processing is
performed locally in a base station, the base station can use the decoded
information to
adjust its own synchronization. If the synchronization processing is performed
at a
central location for multiple base stations, the base stations will be
instructed from the
central location as necessary to adjust synchronization.
If RTD values discussed above are needed for synchronization of neighboring
base stations, special synch & identity bursts can be transmitted that contain
both BSIC
and a frame number. These can be transmitted at regular intervals in the TDMA
frame
structure. Since more bits (BSIC + frame number) are now transmitted, less
coding is
2S possible. This means that the synch & identity burst can be detected within
a shorter
range than the identity burst.
It will be appreciated that the invention provides a fast and efficient way to
decode a relatively large number of base station identities in an
approximately
synchronized wideband TDMA system. This can be used for establishing cell
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relations, to be used by long-term algorithms, as well as for improving the
performance
of short-term algorithms such as handover.
The broadcast control channel structure can significantly reduce interference
on
the broadcast channels if carefully constructed identity bursts are
transmitted by the
5 base stations. If synchronization information is included in the identity
bursts, the
system can autonomously perform approximate synchronization.
While the foregoing description includes numerous details and specificities,
it is
to be understood that many modifications can be made without departing from
the spirit
and scope of the invention, as defined by the following claims and their legal
10 equivalents.
