Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND SYSTEM FOR FACILITATING TIMING
OF BASE STATIONS IN AN ASYNCHRONOUS
CDMA MOBILE COMMUNICATIONS SYSTEM
This is a divisional application of Canadian Patent Application Serial
No. 2,320,996 filed on Feb. 9, 1999.
BACKGROUND OF THE INVENTION
Technical Field of the Invention
The present invention relates in general to the mobile communications field
and, in particular, to a method and system for facilitating the timing of base
stations
in an asynchronous Code-Division Multiple Access (CDMA) mobile
communications system. It should be understood that the expression "the
invention" and the like encompasses the subject-matter of both the parent and
the divisional applications.
Description of Related Art
Direct-Sequence CDMA (DS-CDMA) mobile communications systems can
be either inter-cell synchronous or inter-cell asynchronous systems. In other
words,
the base stations (BSs) in an inter-cell synchronous system are accurately
synchronized with one another, and the BSs in an inter-cell asynchronous
system
are not. More specifically, asynchronous BSs do not share a common time
reference, and their transmissions, therefore, have arbitrary, not
predetermined
timing relative to each other. An example of an inter-cell synchronous system
is
the North American IS-95 system. Examples of inter-cell asynchronous systems
are the Wideband CDMA (WCDMA) systems proposed in the CODIT, ETSI
SMG2 Group Alpha, and ARIB technical specifications.
The main disadvantage of inter-cell synchronous systems is that the BSs
have to be very accurately synchronized (down to the ps level). This high
level of
accuracy is typically provided through the use of highly accurate time
references
co-located with the BSs, such as Global Positioning System (GPS) receivers.
However, because of the line-of-sight nature of satellite signal propagation,
the use
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of such co-located references are likely not feasible for BSs located
underground,
in buildings or tunnels. Another related disadvantage is that the GPS system
is
controlled by a govemment agency. Consequently, the use of GPS receivers for
BS
network synchronization may be undesirable in some national regions. These
disadvantages are the main reasons why inter-cell asynchronous systems are now
being considered.
For inter-cell asynchronous systems to work properly, there are two crucial
functional issues that need to be addressed: (I) Soft Handovers (SOHOs); and
(2)
Cell-Searches. In a state of SOHO, a mobile station (MS) is in communication
with more than one BS at the same time. To facilitate the SOHOs, the MS
constantly scans for other BSs in the vicinity. The MS can thereby monitor the
received signal quality from the multiple BSs and determine the time delay of
the
BSs. For a SOHO to occur, the MS being handed over has to be able to receive
the
"target" BS's signal at approximately the same time as the "source" BS's
signal, in
order to minimize buffering requirements (i.e., a smaller time difference
between
BS signals requires less buffer area than larger time differences). Also, the
target
BS has to be able to find the MS's signal without an unreasonable expenditure
of
processing resources.
These SOHO issues are resolved for asynchronous systems by a "per-call"
synchronization technique, which is disclosed in "A Design Study for a CDMA-
Based Third-Generation Mobile Radio System," by A. Baier et aI., IEEE.ISAC,
Vol. 12, pp. 733-743, May 1994. Using this technique, the MS involved in the
SOHO calculates and reports to the network the time difference between the
target
BS and source BS. The network notifies the target BS via the Base Station
Controller (BSC) or Radio Network Controller (RNC) about the time difference.
The target BS can then adjust its receive and transmit timing for the signal
intended
for the MS involved, to compensate for the difference.
A similar known SOHO technique is used in which the MS reports the
timing difference between the target BS's transmission and its own
'transmission,
rather than the difference between the target BS's transmission and the source
BS's
transmission. However, since the MS's transmit/receive timing relationship is
always fixed, the two above-described SOHO techniques are substantially
equivalent.
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These techniques are referred to as mobile assisted handover (MAHO). In other
words, the MS assists the target BS in compensating for the difference in
timing
between the target BS and source BS.
A cell-search generally refers to a procedure whereby an MS accomplishes
chip-, slot- and frame-synchronization with a BS, and detects the BS's
downlink
scrambling code. This procedure is used both during power on (initial
synchronization) and continuously thereafter during the idle or active modes
while
the MS is searching for SOHO candidate BSs. In a synchronous system, the cell-
search can be performed efficiently (i.e., with a relatively low level of
complexity)
because the same scrambling code can be used by all BSs. As such, the MS can
perform the complete search for BSs using only a single matched filter (or a
similar
functionality). However, this same technique cannot be readily used in an
asynchronous system because of the different scrambling codes used by the
different BSs. Consequently, a need has arisen for a low-complexity, rapid
cell-
search procedure for asynchronous CDMA systems.
A rapid, multi-step cell-searcb procedure for asynchronous CDMA systems
has been proposed, whereby each BS transmits one unmodulated symbol. This
transmitted symbol is spread by a globally-known short code, without a
scrambling
code, in each slot of each frame. In one such proposal, this symbol is denoted
as a
"Perch I Long Code Masked Symbol (LCMS)". In a second proposal, this symbol
is denoted as a "Primary Synchronization Channel" or Primary (SCH). With the
proposed multi-step procedure, an MS can thus find the chip- and slot-timing
of a
BS, using a single matched filter which is matched to the Primary SCH.
Subsequently, the MS still has to find the BS's frame-timing and downlink
scrambling code (which spans one frame in the proposed multi-step procedure).
The MS can find the BS's fra.me-timing by detecting a second regularly
transmitted
symbol, which is denoted as a "Perch 2 LCMS" or "Secondary SCH".
This second symbol is transmitted in parallel with the first symbol, but the
second symbol is spread by a second short code (again without a scrambling
code).
The second symbol may also have a unique repetitive modulation pattern per
frame, and by detecting this pattern, the MS can determine the BS's frame-
timing.
The spreading code used for the second symbol indicates to the MS which group
of
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possible scrambling codes an actually-used scrambling code belongs to_ The MS
can then find the scrambling code used, by correlating with the scrambling
codes
belonging to the indicated group, at the above-identified frame-timing (or at
different possible frame-timings). However, a problem with the proposed multi-
step procedure is that the level of complexity of the cell-search is still
relatively
high, especially in the case of a SOHO candidate search (which the MS has to
perform on a regular basis).
Another problem with inter-cell asynchronous systems is that the timing
difference between BSs makes it difficult to determine the position of the
MSs.
Mobile conununications systems capable of determining the position of MSs in
the
system are becoming increasingly desirable. Currently, mobile positioning is
generally performed by the use of external systems, such as a GPS system.
Preferably, however, mobile positioning would be performed by the cellular
system
itsqlf without the need for such external systems. To perform such cellular
positioning, a method is needed to accurately detennine the absolute or
relative
distances between an MS and each of several different BSs. The distances can
be
calculated using propagation time, time of arrival (TO), or time difference of
arrival
(TDOA) measurements on the signals transmitted between the MSs and each of
several different BSs. Once these measurements are available, a number of
algorithms exist to calculate the geographical position of the MS. For
example,
according to the TOA method, the distance from an MS to each of the BSs is
obtained using TOA measurements. Each of these distances can be conceptualized
as the radius of a circle with the respective BS in the center. In other
words, the
TOA measurement can be used to determine the radial distance of the MS from a
particular BS, but the direction cannot be determined based on a single TOA
measurement; thus, the MS might lie anywhere on the circle defined by the
calculated radius. By determining the intersection of the circles associated
with
each of several different BSs, however, the position of the MS can be
determined.
The TDOA method, on the other hand, uses the difference in TOA between two
BSs to determine a TDOA between those two BSs. The position of the MS can
then be estimated to be along a curve, namely a hyperbola, in accordance with
the
TDOA calculation. By using three or more BSs, more than one such curve can be
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obtained. The intersection of these curves gives the approximate position of
the
MS.
In the simplest mobile positioning technique, a SOHO is made to a number
of BSs. During each of these handovers, the propagation time between each BS
and the MS can be measured. The location of the MS can then be determined by
triangulating the position of the mobile. This positioning method is the
simplest to
implement because it involves very little change in the mobile radio design.
In
addition, the BSs do not need an absolute time reference; i.e., this method
may be
used in an asynchronous cellular system. However, because of the geographical
separation between BSs, handover to two other geographically located BSs is
only
possible in a small number of cases. In other words, when the MS is in close
proximity to one BS, a SOHO with other BSs will often not be possible. This is
because the "hearability" of signals between the MS and multiple BSs will
normally be unsatisfactory.
Another possible solution is to use an antenna array at the BS. When the
BS has an antenna array, the position of the MS can be calculated by
estimating the
direction from which uplink signals are propagating and by measuring the round-
trip delay of the communications signal. In this method, the MS only needs to
be
in communication with one BS to calculate the position. However, widespread
use
of antenna arrays for positioning purposes is expensive. Furthermore, the
effects of
multipath propagation characteristics of the uplink and downlink signals often
make an antenna array undesirable, particularly in cities, where signals
frequently
reflect off buildings and other structures.
As mentioned above, it is also possible that a GPS can be incorporated into
the mobile without using an extra radio receiver. This method, however,
requires
excessive computational and receiver complexity in the MS.
Another solution is to measure the propagation time, TOA, or TDOA of
signals transmitted by the BSs to the MS or by the MS to the BSs. For example,
a
downlink solution can be used wherein, in the case of CDM.A, the MS measures
the
TOA of pilot channel data that is transnutted by several different BSs.
Alternatively, an uplink solution can be used wherein several BSs each measure
the
TOA of a signal transmitted by the mobile to the multiple BSs. However, both
of
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these methods require an absolute or accurate relative time reference in, or
synchronization of, the BSs. Therefore, both downlink and uplink solutions
normally require extra hardware (e.g., a GPS receiver located in the BSs to
obtain
timing of the BSs) in an asynchronous network.
A system and method are needed for reducing the complexity of and the
processing resources used during the cell search and mobile positioning
processes
in asynchronous networks. In particular, it would be advantageous to utilize
as
much a priori search information as possible to help reduce the level of
complexity
and increase the search rate for cell-searches and to enable simplified mobile
positioning solutions. As described in detail below, the present invention
successfully resolves the above-described problems.
SUMMARY OF THE INVENTION
A method and system are provided for facilitating the timing of base
stations in asynchronous CDMA mobile communications systems, whereby a
source BSC (or RNC) sends to an MS (e.g., in a neighbor cell list message)
estimates of the Relative Time Difference (RTD) between the source BS and each
of the BSs on the neighboring cell list. For SOHO purposes, a plurality of MSs
can
report to the network the estimated RTDs along with signal quality information
for
the neighboring BSs. Each BS can maintain an RTD estimate table, which can be
updated continuously from the RTD reports received from the MSs. Subsequently,
the BSs can send entries from this RTD estimate table to the MS in the
neighboring
cell list message, along with corresponding scrambling codes. Using this novel
technique, the BSs have known relative timing differences. Consequently, when
the MS initiates a cell-search for a potential target BS, the MS already has
an
estimate of the timing of that BS as compared to its source BS. As such, the
resulting cell-search procedure used in an asynchronous CDMA system has a
lower
level of complexity and thus can be accomplished much quicker than with prior
procedures.
In another aspect of the invention, the accuracy of the estimated RTD's can
be greatly improved by accounting for propagation delays between the MS and
the
BSs that are used to estimate the RTD. These improved RTDs can be used to
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further improve timing estimates for performing cell-searches. The improved
RTDs can also be used to calculate the position of MSs in the mobile
communications system. Once highly accurate RTDs are known, distances
between an MS and several BSs can easily be determined using the propagation
times, TOAs, or TDOAs of signals traveling between the MS and the several BSs.
An important teclmical advantage of the present invention is that
neighboring BSs in an asynchronous CDMA mobile communications system have
known relative timing differences.
Another important technical advantage of the present invention is that the
hardware and software complexity of MSs in an asynchronous CDMA mobile
communications system is reduced.
Yet another important technical advantage of the present invention is that
the overall level of complexity of the cell-search procedure in an
asynchronous
CDMA mobile communications system is significantly reduced.
Still another important technical advantage of the present invention is that
the speed of the cell-searches performed in asynchronous CDMA mobile
communications systems is significantly increased as compared to prior
procedures.
Another important technical advantage of the present invention is that
mobile positioning can be detennined in an asynchronous mobile communications
system by performing simple calculations on easily obtainable data and without
the
need for an extemal system.
According to an aspect of the present invention there is provided a method for
estimating a relative timing of a plurality of base stations in an
asynchronous mobile
conununications system, comprising the steps of receiving at a first mobile
station a first
downlink signal transmitted by a first one of the base stations and a second
downlink
signal transmitted by a second one of the base stations, transmitting an
uplink signal
from the first mobile station to the first and second base stations, and
calculating an
estimated relative time difference between the time base of the first base
station and the
time base of the second base station using receive times at the first and
second base
stations of the uplink signal, transmit times of the first downlink signal and
the second
downlink signal, and a time difference at the first mobile station between a
receive time
of the second downlink signal and the transmit time of the uplink signal,
wherein the
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receive times of the uplink signal and the transmit times of the first and
second downlink
signals are in the time base of the base station transmitting or receiving the
respective
signal, the calculation accounting for propagation delays between the first
mobile station
and the first and second base stations.
According to another aspect of the present invention there is provided a
method
for facilitating timing between mobile stations and base stations in an
asynchronous
mobile telecommunications network, comprising the steps of receiving relative
timing
difference data from each of a plurality of mobile stations, the relative
timing difference
data from each mobile station including a measured difference between the time
bases of
at least two base stations as measured by the mobile stations, determining a
relative
timing difference estimate based on the received relative timing difference
data, the
relative timing difference estimate representing an estimate of a difference
between the
time bases of at least two base stations, accounting for propagation delays
between a
measuring mobile station and the at least two base stations of which the time
base
difference is measured in the relative timing difference estimate, storing the
relative
timing difference estimate in a relative timing difference table, transmitting
the relative
timing difference estimate to a receiving mobile station.
According to a further aspect of the present invention there is provided an
asynchronous mobile teleconununications system, comprising a plurality of base
stations for transmitting data to and receiving data from a plurality of
mobile stations,
the plurality of base stations individually receiving relative timing
difference data, the
relative timing difference data from each mobile station comprising a measured
difference between the time bases of two of the plurality of base stations as
measured by
the mobile station, a register storing a relative timing difference table,
each of a plurality
of entries in the table comprising a relative timing difference estimate
calculated from
the relative timing difference data and the register storing error data for
each relative
timing difference estimate, and wherein a first one of the plurality of base
stations
transmits a relative timing difference estimate to a receiving mobile station
to facilitate
timing of communications between the mobile station and a second one of the
plurality
of base stations.
According to a further aspect of the present invention there is provided a
method
for facilitating the timing of a plurality of base stations in an asynchronous
mobile
communications system, comprising the steps of at least one of the plurality
of base
stations sending at least one estimated relative time difference value to a
mobile station,
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the at least one estimated relative time difference value comprising an
estimated timing
difference between the at least one of the plurality of base stations and a
neighbor base
station, the mobile station receiving the at least one estimated relative time
difference
value, the mobile station correlating the at least one estimated relative time
difference
value with a matched filter output signal, and initiating a cell search based
on a result of
the correlating step.
According to a further aspect of the present invention there is provided a
method
for facilitating the timing of a plurality of base stations in an asynchronous
mobile
communications system, comprising the steps of at least one of the plurality
of base
stations sending at least one estimated relative time difference value to a
mobile station,
the at least one estimated relative time difference value comprising an
estimated timing
difference between the at least one of the plurality of base stations and a
neighbor base
station, the mobile station receiving the at least one estimated relative time
difference
value, and determining an approximate position of the mobile station using the
at least
one estimated relative time difference value.
According to a further aspect of the present invention there is provided a
system
for synchronizing a plurality of base stations in a mobile communications
system,
comprising a first base station of the plurality of base stations, the first
base station
operable to broadcast or transmit at least one estimated relative time
difference value,
the at least one estimated relative time difference value comprising an
estimated timing
difference between the first base station and a neighbor base station, a
mobile station for
receiving the at least one estimated relative time difference value, and a
processor for
determining an approximate position of the mobile station using the at least
one
estimated relative time difference value.
According to a further aspect of the present invention there is provided a
system
for synchronizing a plurality of base stations in a mobile communications
system,
comprising a first base station of the plurality of base stations, the first
base station
operable to broadcast or transmit at least one estimated relative time
difference value
and at least one uncertainty value associated with the at least one estimated
relative time
difference value, the at least one estimated relative time difference value
comprising an
estimated timing difference between the first base station and a neighbor base
station,
and a mobile station for receiving the at least one estimated relative time
difference
value and the at least one uncertainty value.
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According to an aspect of the present invention there is provided a method for
estimating a relative timing of a plurality of base stations in an
asynchronous mobile
communications system, comprising the steps of:
receiving at a first mobile station a first downlink signal transmitted by a
first one of
said base stations and a second downlink signal transmitted by a second one of
said base
stations;
transmitting an uplink signal from the first mobile station to the first and
second base
stations; and
calculating an estimated relative time difference between the time base of
said first
base station and the time base of said second base station using receive times
at the first
and second base stations of said uplink signal, transmit times of the first
downlink signal
and the second downlink signal, and a time difference at the first mobile
station between
a receive time of the second downlink signal and the transmit time of said
uplink signal,
wherein the receive times of the uplink signal and the transmit times of the
first and
second downlink signals are in the time base of the base station transmitting
or receiving
the respective signal, said calculation accounting for propagation delays
between said
first mobile station and said first and second base stations.
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 flow diagram that illustrates an exemplary method that can
be used for facilitating the timing of base stations in an asynchronous CDMA
mobile communications system, in accordance with a preferred embodiment of the
present invention;
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FIGURE 2 is a simplified block schematic diagram of an exemplary mobile
communications system that can be used to implement the method shown in
FIGURE 1, in accordance with the preferred embodiment of the present
invention.
FIGURE 3 is a simplified block schematic diagram of an MS that is in or is
about to enter a SOHO and that can be used for facilitating improved timing
calculations of BSs in an asynchronous CDMA mobile communications system, in
accordance with a preferred embodiment of the present invention;
FIGURE 4 is a diagram of the relative timing of signals involved in the
SOHO scenario depicted in FIGURE 3; and
FIGURE 5 is a flow diagram that illustrates an exemplary method that can
be used to determine the position of an MS, in accordance with one embodiment
of
the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
The preferred embodiment of the present invention and its advantages are best
understood by referring to FIGURES 1-5 of the drawings, like numerals being
used
for like and corresponding parts of the various drawings.
In an asynchronous CDMA system, a BSC "knows" the downlink
scrambling codes for all of its BSs. Typically, a list of neighboring cells is
broadcast
in each cell (for MSs operating in the idle mode), or transmitted on a
dedicated control
channel (for MSs operating in the active mode). When an MS receives the
neighboring cell information, it determines the scrambling codes of the listed
neighboring cells that are potential SOHO candidate cells. Having such a
priori
knowledge of this scrambling code information for the candidate SOHO cells
enables
the MS to reduce the total SOHO cell-search time (or complexity level),
because the
number of possible scrambling codes is reduced in comparison with the number
for
initial synchronization (power on). However, even if the set of scrambling
codes to
be searched by the MS is relatively small, the MS still does not know the
timing of
these codes. This lack of timing information is the main reason why current
proposals
for an asynchronous system cell-search take more time (and are more complex)
than
a synchronous system cell-search.
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The present invention solves this lack of timing information problem by having
the source BS send to the MS (along with the neighboring cell list) an
estimated RTD
between the source BS and each of the BSs on the neighboring cell list. In
other
words, instead of sending only the scrambling codes of the neighboring BSs to
the
MS, the source BS also transmits each of their estimated RTDs. For SOHO
purposes,
the MSs can report (on a regular basis, triggered by some event, or on demand
from
the BSC) to the network the estimated RTDs along with signal quality
information
(e.g_, signal strength, signal-to-interference ratio or SIR, etc.) for the
neighboring BSs.
Consequently, each BSC can maintain an RTD estimate table, which can be
updated
continuously from the RTD reports received from the MSs. In a preferred
embodiment of the present invention, the RTD estirriate table is maintained in
a
database at the BSC.
Subsequently, the BSCs can send entries from this RTD estimate table to the
MS in the neighboring cell list message, along with the corresponding
scrambling
codes (with the BSC keeping track of the estimated RTD information it has
already
sent in previous messages to the MS). Using this novel technique, the BSs have
known relative timing differences. Consequently, in an exemplary embodiment,
when
an MS initiates a search for a potential target BS, the MS already has an
estimate of
the timing of that BS (i.e., from the RTD information) as compared to its
source BS.
As such, the resulting cell-search procedure used in an asynchronous CDMA
system
can be accomplished much quicker ihan with prior procedures. When the MS has
synchronized with the potential target BS, the MS has an improved estimate of
the
RTD, which in tum, the MS can report back to the source BS (preferably along
with
quality information for the potential target BS). The source BS (or its
associated BSC)
can then update this entry in the RTD estimate table.
More specifically, FIGURE 1 is a flow diagram that illustrates an exemplary
method 100 that can be used for facilitating the riming ofBSs and increasing
the speed
of hand-over candidate cell-searches in an asynchronous CDMA mobile
communications system, in accordance with a preferred embodiment of the
present
invention. At step 104 of the exemplary method shown in FIGURE 1, a BSC
prepares
a neighbor cell list (e.g., "neighbor set" in an IS-95 system) with respective
scrambling
codes, along with a plurality of RTD estimates between a source BS and the
respective
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hand-over candidate BSs from an RTD estimate table (preferably maintained in a
database at the BSC). At step 106, the source BS broadcasts or transmits the
neighbor
cell list with scrambling codes and RTD estimates in a "neighbor list message"
to the
MS involved. In actuality, the BSC keeps track of the estimated RTDs it sends
to the
MS, in order not to unnecessarily duplicate RTD infonmation the MS may already
have. At this point, the MS has now received a list of the BSs it can
synchronize with
(and also report quality information for). The received neighbor list message
can also
include an uncertainty estimate (described in more detail below). The MS
stores the
neighbor cell list information in local memory.
At step 108, with the a priori neighbor cell RTD estimate (timing) information
readily at hand, along with the other corresponding neighbor cell infonnation,
the MS
can initiate a primary cell-search using a conventional matched filter
arrangement.
The MS's utilization of the primary cell-search matched filter produces signal
peaks
that correspond to the BSs that the MS can receive with sufficient quality to
qualify
as hand-over candidate cells. At step 110, the MS correlates the RTD estimates
with
the produced matched filter signal peaks to detennine which peaks are most
likely to
correspond to which scrambling codes in the neighbor cell list (step 112). At
step 114,
based on the correlations produced at step 112, the MS can select the
scrambling codes
for the most likely hand-over candidate cells from the neighbor cell list. The
MS can
then initiate the cell-search (step 116).
Theoretically, if the above-described RTD estimates are perfectly accurate,
then the MS could (up-front) discard all of the matched filter output signal
peaks not
corresponding to the RTD "estimate" information. In this hypothetical
situation, the
scrambling code correlation procedure (e.g., step 112) could be omitted
altogether.
However, in any event, in accordance with the present invention, the MS's
utilization
of the RTD estimates to determine the most likely hand-over candidate cells
from the
neighbor cell list enables the MS to disregard a significant number of the
matched
filter peaks, and/or associate certain of those peaks with corresponding
scrambling
codes, which significantly reduces the complexity of the cell-search procedure
and
substantially increases the speed of the search.
By using the above-described inventive method, each BS (cell), with the
assistance of the MSs connected to it, has a known relative timing difference
with
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respect to its neighbor BSs (cells). If, for some reason, there are no MSs
connected
to a particular BS, the RTD estimate table corresponding to that BS is not
updated.
Consequently, since the relative timing between the neighboring BSs may be
continually shifting, the uncertainty (or variance) of the RTD estimate table
entries for
this BS will increase. In general, the uncertainty of the RTD estimate may
increase
with time, but this uncertainty is typically niinimal immediately after an
update has
been completed (e.g., based on an RTD report received from an MS).
Consequently,
in order for the conununications system to be more robust during periods of MS
inactivity (e.g., at night, or during holidays in private indoor systems), as
mentioned
earlier, an RTD uncertainty estimate can be broadcast or transmitted from the
BS
along with the RTD estimate, in the neighbor list message. The MS can then,
for
example, set (e.g., increase) its time-search window accordingly to allow for
the
additional level of uncertainty. The MS can thus cope with those BSs having a
relatively uncertain knowledge of its RTDs, and also minimize its complexity
level
when relatively certain RTD estimates have been provided.
An additional method for further mitigating the uncertainty problem
encountered when there are too few active MSs for relatively long periods is
to place
"dummy" MSs at fixed locations throughout the system. These "dummy" MSs can
have a limited functionality, and can be called upon by BSs having relatively
high
uncertainty RTD estimate table entries to provide more current RTD updates.
Such
"dummy" MSs can be thus advantageously located where they can be reached by a
plurality of BSs (e.g., near the cell borders).
FIGURE 2 is a simplified block schematic diagram of an exemplary mobile
communications system 200 that can be used to implement the method 100 (FIGURE
1) for facilitating the timing (e.g., the known relative timing differences)
of BSs and
increasing the speed of cell-searches, in accordance with the preferred
embodiment of
the present invention. System 200 is preferably an asynchronous CDMA mobile
communications system that includes, for illustrative purposes, three BSs and
three
MSs. However, it should be understood that the number of BSs and MSs shown is
for
illustrative purposes only, and that a typical system can include more than
three BSs
and three MSs. For this example, MS I is operating in the active mode and
connected
via air interface link 202 to BSI. In accordance with step 106 of method 100
CA 02603277 2007-09-28
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(FIGURE 1), MSI has received a neighbor list message preferably including
respective RTD estimates and, optionally, associated uncertainty estimates on
a
dedicated control channel from the BSC 204 (via BSI once it is "connected" to
MSI).
At least two of the neighbors (cells) listed as entries in the RTD estimate
table are BS2
and BS3. On a periodic basis (or on demand), MS I monitors and reports the
quality
(signal strength, SIR, signal-to-noise ratio or SNR, Bit-Error-Rate or BER,
etc.) of
those BSs to the BSC 204 (via BSI). Since MSI has received RTD estimates from
BSC 204 (via BSI), MS1 can synchronize itself relatively rapidly with BS2 and
BS3,
at least during the first occasion when MSI searches for BS2 and BS3. When MSI
has synchronized with BS2 (or BS3), it can be assumed that MS1 has a "good"
RTD
estimate for that BS. On a periodic basis, or on demand, MS I can report the
estimated
signal quality of at least one of the entries in the neighbor cell list to BSC
204 (via
BS1). In addition to the quality estimates, MSI can also report the current
RTD
estimate to BSC 204.
The cell-search situation for MS2 is similar to that of MSl, except for the
example shown, MS2 is involved in a SOHO with both BSI and BS2, and monitors
only one other BS (e.g., BS3 via air interface link 214). For this example,
MS3 is
operating in the idle mode (has no connection set up), but it can still
monitor the BSs
according to the neighboring-cell list received on the broadcast channel of
the BS the
MS3 considered the "best one" to listen to (e.g., in this case BS3 via air
interface link
218). As such, MS3 can also monitor BS1 (via air interface link 208) and BS2
(via
air interface link 212). Again, theRTD estimates broadcast by BS3 assists MS3
in
synchronizing more rapidly with BSI and BS2, or at least the first time the
synchronization procedure occurs_ The complexity of the cell-searches are thus
reduced, and the speed of the cell-searches is thereby significantly
increased.
Preferably, each MS operating in the mobile communications system 200 will
transmit its measured RTD estimate on a periodic basis, or on demand, to the
BSC 204
(via BSI). The BSC 204 stores the RTD estimates received from the MSs in an
RTD
estimate table. Alternatively, each entry stored in the RTD estimate table
(i.e.,
representing an estimated difference between a pair of base stations) can be
calculated
based on estimates received from a plurality of different MSs. For example,
the stored
estimate can constitute an average of the previous x received estimates, or of
the
CA 02603277 2007-09-28
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estimates received in the precedingy minutes. The values in the table can be
updated
by replacing previous estimates or by recalculating particular estimates based
on
newly received data. The values stored in the table are then sent to other
MSs, as
described above, along with the neighbor cell list, to assist in synchronizing
those MSs
with neighboring BSs, as necessary. In addition, it will be appreciated by
those skilled
in the art that the RTD estimate table does not have to be stored in the BSC
204;
rather, the table can be stored in one or more databases located virtually
anywhere in
the network (e.g., in a register associated with the MSC or in an entirely
separate
database).
In another aspect of the invention, the RTD estimates can be used to determine
the position of the MS. Positioning calculations, however, require more
accurate RTD
estimates than in the case of cell searches. This is because the mobile
positioning
concept relies upon a determination of the propagation delay between the
MS and each of a plurality of BSs or upon TOA or TDOA measurements among the
various BSs. In most cases, the speed of the cell search can be significantly
improved
without having to account for propagation delays. Thus, it is normally
sufficient to
base the RTD estimates on the time difference between two BSs as measured by
one
or more mobiles without considering the effect of propagation delays of the
downlink
signals received by an MS from each of the BSs. To perform mobile positioning,
on
the other hand, a more accurate estimate of the RTD is needed.
The present invention solves this problem by calculating an improved RTD
that accounts for the propagation delays of uplink and downlink signals.
The improved RTD is the difference between the time at which a first BS begins
transmitting its downlink signal and the time at which a second BS begins
transmitting
its downlink signal. This improved RID estimate can be calculated using: (1)
the
local receive and transmit times of the uplink and downlink signals in the BSs
of
interest, as measured at each of the respective BSs, and (2) the TOA
difference at the
MS of the downlink signals from the BSs, as reported by the mobile. This
improved
RTD information can then be used by other mobiles for positioning purposes.
In a preferred embodiment, the improved RTD estimates are stored in a
database table at the BSC or the MSC. Subsequently, a position determination
for a
second MS is desired (on a regular basis, triggered by some event, or upon
request by
CA 02603277 2007-09-28
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the BSC or the MS). The second MS measures the time differences between the
BSs
based on the receive time at the second MS of the downlink signal from each BS
and
reports the measured time differences to the BSC. The BSC then compares the
stored
improved RTD estimate between a particular pair of BSs with the measured time
difference between the same pair of BSs as reported by the MS. Based on this
comparison, the propagation delays between each of several BSs and the MS can
be
calculated, and an accurate determination of the MS's location caii be made.
Again,
TOA or TDOA measurements can also be used to determine the location.
Regardless
of which positioning method is used, however, the positioning calculations
rely upon the existence of propagation delays in the mobile environment.
Generally, the detennination of each RTD estimate by an MS involves only
two BSs, even if a three-part SOHO (i.e., a SOHO involving three different
BSs) can
occur. By repeating the RTD determination during multiple different SOHO
procedures, an improved, estimated RTD between a substantial number of
possible
pairs of BSs can be determined. The improved RTD estimates are normally then
used
by other MSs (i.e_, MSs that were not involved in the estimated RTD
calculations) to
determine the position of those other MSs. it will be appreciated, however,
that the
position of an MS that was involved in the estimated RTD calculations can also
be
determined using the improved RTD estimates. In any event, the positioning
procedure preferably utilizes as many BSs as possible in order to improve the
accuracy
of the estimated location.
Referring now to FIGURE 3, there is shown a schematic illustration of an MS
that is in or is about to enter a SOHO. A first base station BS 1 transmits a
frame of
a downlink signal 302 (either a pilot frame or a traffic data frame) at a time
T,,, as
measured in the time base of the first base station BSI. An uplink signal 304
from a
Mobile station MS 1 is received by the first base station BS l at a time Tr,,
also
measured in the time base of the first base station BSI. Similarly, a second
base
station BS2 transmits a downlink signal 306 at a time Ta and receives the
uplink signal
304 from the mobile station MS1 at a time Ti, measured in the time base of the
second
base station BS2. Generally, the time base of the two base stations in an
asynchronous
network will have a relative time difference (RTD) A. In other words, if an
event
CA 02603277 2007-09-28
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(such as the transmission of a pilot frame) occurs at a time T, in the first
base station
BS 1, a corresponding event will occur at a time
T2=T,+A (1)
in the second base station BS2. Once the RTD A is known, it can be used by
other
MSs for mobile positioning.
In addition, each downlink signal has an offset t; relative to the transmit
time
of the pilot channel frame. Thus, the traffic channel data from the first base
station
BS1 is transmitted at a time
Tti = ToI + ti, (2)
where TP, is the transmit time of the pilot channel frame from the first base
station
BS1. Similarly, the traffic channel data of the downlink signal from BS2 is
transmitted at time
T12 =Tp2+t2. (3)
When the SOHO is initialized, the mobile station MS1 simply listens to the
pilot and
t2 = 0. Later, when the mobile station MS I is in SOHO, the second base
station BS2
will transmit data and the offset t2 of the signals will be adjusted so that
the data from
the first base station BS I and the second base station BS2 will arrive at the
mobile at
approximately the same time. In the following discussion, a generic scenario
can be
considered in which it is assumed that the offsets t, and tZ are known. This
scenario
covers both the cases of SOHO initialization and an already set-up SOHO.
Referring now to FIGURE 4, there is illustrated a relative timing diagram of
the various SOHO signals that are transmitted and received in the system of
FIGURE
3. All of the times in the figure are illustrated in a common, arbitrary time
base. For
purposes of making RTD calculations in accordance with the present invention,
however, the time of each event is reported in the local time base of the
station (i.e.,
the MS or the BS) associated with that event.
At a time Tr,, as measured in the time base of the first base station BS 1, a
pilot
or traffic frame 402 is transmitted by the first base station BSI. The frame
402 is
received at the mobile station MSI at a time T.,,, which is measured in the
time base
of the mobile station MS1. The time T,,,,, is delayed after the transmission
time T,, by
a propagation delay time T,, which is the time required for the signal to
travel from
the first base station BS1 to the mobile station MS] and vice versa. The
mobile
CA 02603277 2007-09-28
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station MSI transmits its uplink signal 404 at time T,,,,. For simplicity and
without loss
of general applicability, it can be assumed that the mobile station MSI
transmits its
uplink signal 404 at the same time it receives the downlink signal 402 from
the first
base station BS 1. Thus,
Tm, = Tmrl , and (4)
the uplink signal 404 is received in the first base station BSI at time
T,, = T;, + 2i,. (5)
The uplink signal 404 from the mobile station MSI is received in the second
base
station BS2 at a time T,Z and is delayed after the transmission time Tm, by a
propagation delay time iZ.
The second base station BS2 also transmits a traffic or pilot frame 406 at a
time
T,Z. After the propagation delay time T2, the downlink signal 404 is received
by the
mobile station MS1 at a time Tm,2.
To calculate the RTD A, the mobile station MS 1 reports the time difference
tdiff
between the reception time T,,,,2 of the downlink signal 406 from the second
base
station BS2 and the transmit time Tm, of the uplink signal 404 from the mobile
station
MSI. Thus,
td;= TA,, - T,A.z= (6)
It should be noted that in FIGURE 4, the time difference tdt6r is relatively
large, which
is typically indicative of an initial acquisition scenario.
Using the above notations, we can then formulate the following expression for
the receive time T,2 of the uplink signal in the second base station BS2:
Trz = 2t2 + tdff + T,Z- (7)
Finally, we can formulate the following expression for tdB
te~B=Tf,-Trz+TI -ti2+A, (8)
which is obtained by subtracting the arrival time Tui2 of the downlink signal
406 from
the second base station BS2 (either the pilot frame at SOHO initialization or
the traffic
data during SOHO) from the transmission time Tm, of the uplink signal 404 from
the
mobile station MS1, all measured in the time base of the second base station
BS2.
Thus, as will be understood by persons of ordinary skill in the art, in the
time base of
the second base station:
Tm,, = Tr, - t2, and (9)
CA 02603277 2007-09-28
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T,õ, = Tn + A+. i,. (l 0)
Now there are three equations: (5), (7), and (8), and three unknowns: 1) the
propagation delay time tti between the mobile station MSI and the first base
station
BS1; 2) the propagation delay time Tz between the mobile station MS I and the
second base station BS2; and 3) the time difference A between the first base
station
BS 1 and the second base station BS2. It is easy to solve for A to get
0='/z(td;-Tn-TH +Ta+Tr2), (11)
which provides a solution for the desired RTD 0 between the base stations BSI
&
BS2.
According to a preferred embodiment of the invention, the mobile station
MSI reports the time difference tdiff and each of the base stations BS 1& BS2
report
their respective transmit and receive times to the network. The computation of
the
RTD A is then made in the BSC or the MSC. In the alternative, the computation
can be performed in the mobile station MSl or in a base station once the
necessary
timing data is provided.
By calculating improved RTD estimates between various pairs of BSs in an
asynchronous mobile communications system, an uplink solution or a downlink
solution can be used to determine the position of MSs in the system without
the
need for an absolute time reference. For example, FIGURE 5 is a flow diagram
that illustrates one possible method 500 for facilitating the timing of BSs
and
determining the position of a selected MS in an asynchronous CDMA mobile
communications system, in accordance with one embodiment of the present
invention. As will be appreciated by those of ordinary skill in the art,
numerous
other positioning methods, such as TOA or TDOA, can also be used in connection
with the improved RTD estimate to facilitate positioning in accordance with
the
invention. At step 504, a BSC calculates a plurality of improved RTD estimates
between various pairs of BSs that are controlled by the BSC or that are listed
in the
neighbor cell list. This calculation is made by using data provided by other
MSs in
the mobile communications system. Accordingly, the effect of propagation
delays
are taken into account so as to calculate highly accurate RTD estimates.
Preferably, a table of these improved RTD estimates is maintained in a
database at
the BSC. At step 506, the selected MS monitors the BSs in neighboring cells.
For
CA 02603277 2007-09-28
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purposes of the present positioning method 500, this involves monitoring, for
example, a known sequence that is periodically transmitted by the BSs. This
monitoring procedure can include the ordinary monitoring of BSs for potential
handover candidates. It should be noted tliat monitoring of a known sequence
from
a BS can usually be performed even in cases where limited "hearability"
prevents a
SOHO with that BS.
At step 508, the MS measures the TOA of downlink signals transmitted by
several different BSs. Each TOA measurement can be measured in the time base
of
the MS or as a relative value to the source BS or to some other BS. The TOA
measurements are temporarily stored in local memory along with information
identifying the BS that corresponds to each TOA measurement. This data is then
sent to the BSC for further processing. The measurements of step 508 can be
made
on the pilot channel data or the traffic channel data. Because the BSs
generally
"know" the offsets t;, the time difference between the BSs ().e_, the time
difference
between the pilot frame transmissions of the BSs) is known even when traffic
channel is used. At step 510, the BSC adjusts the TOA measurements to account
for the RTDs between the various BSs by adding the RTD estimates to the TOA
measurements. At step 512, a propagation delay time is calculated for each
downlink signal using the adjusted TOA measurements, and the location of the
MS
is estimated at step 514 using the calculated propagation delay times. The
positioning information can then, for instance, be transmitted to the MS,
stored at
the BSC, or sent to the Home Location Register (HLR). In an alternative
embodiment, the calculations of steps 510, 512, and 514 can also be made in
the
MS, MSC, or some other location in the network.
The method 500 illustrated in FIGURE 5 provides positioning estimates
based upon measurements made at the MS of the TOA of a downlink signal. In
another alternative embodiment, mobile positioning is determined using an
uplink
signal. The uplink solution is substantially the same as the downlink solution
except
that, instead of measuring the TOA of downlink signals at step 508, TOA
measurements are made at multiple BSs on an uplink signal that is transmitted
by
the MS. These uplink signal measurements are then provided to the BSC or the
CA 02603277 2007-09-28
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MSC, and adjusted TOA measurements and propagation delay times are calculated,
as in steps 510 and 512 of the downlink solution method 500.
As discussed above in connection with the standard RTD estimates, the
uncertainty in the improved RTD estimates will also increase with time if the
RTD
estimate table for a particular BS is not updated. For positioning purposes,
however, the required accuracy of the RTD estimates is much greater than in
the
context of cell-searches. Thus, the improved RTD estimates obtained during
SOHO should be recent enough so that the clocks in the BSs have not drifted
compared to each other. Otherwise, it will be difficult, if not impossible, to
perform accurate mobile positioning determinations. Many of the same methods
described above for addressing the uncertainty problem in the standard RTD
estimate context can be used in a similar manner to address uncertainty in the
improved RTD estimate context.
The method of obtaining an improved RTD estimate can also be used to
further improve the cell-search process described in connection with FIGURES 1
and 2. In one preferred embodiment of the cell-search method 100 (see FIGURE
1), the time difference reported by the MS in SOHO is directly used to
calculate the
RTD estimates; no information from the BS is required. Thus, referring again
to
FIGURE 4, the standard RTD estimate is equal to an MS's receive time T.ri of a
downlink signal from a first base station BS 1, subtracted from the MS's
receive
time Tiiir2 of a downlink signal from a second base station BS2. In FIGURE 4,
this
value is illustrated by the time differential td,8 The direct use of the time
difference
as reported by the MS offers a significant improvement over prior cell-search
procedures and, in most cases, sufficiently reduces the complexity of the cell-
search process to overcome the problems found in other potential positioning
methods.
If greater accuracy is required, however, the timing uncertainty in the MSs
that do SOHO searches can be further reduced by as much as fifty percent by
using
an improved RTD estimate that takes propagation delays into account. By using
improved RTD estimates, the set of time delays that the MS has to search
during
the cell-search process is considerably decreased, especially when compared to
prior art cell-search methods. The cell-search uncertainty interval will then
depend
CA 02603277 2007-09-28
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upon the size of the cell and the amount of sectorization of the cell. For
example,
in a non-sectorized cell system having a cell radius of approximately 30
kilometers,
the uncertainty is less than 300 microseconds, assuming that the position of
the
mobile can be estimated to within 3 cell radii. The normal search window for
prior
cell-search methods, in contrast, is about 10 milliseconds. Thus, the use of
an
improved estimated RTD provides a two-orders-of-magnitude improvement in the
search complexity. The results are even better in cellular systems with
smaller
cells or with sectorized cells. It is also possible to decrease the
uncertainty interval
for the cell-search even further by estimating the round-trip delay between a
target
BS and the mobile that is performing the cell-search, especially in the case
of
sectorized cells. An estimated round-trip delay can easily be calculated from
the
data available when making RTD calculations or if the approximate location of
the
mobile is known.
Although several preferred embodiments of the method and apparatus of the
present invention have 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 embodiments disclosed. For example,
measurements
of the relative timing of BSs, made in accordance with the present invention,
could
also be used for the pseudo-synchronization of BSs. Thus, the invention 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.
