Note: Descriptions are shown in the official language in which they were submitted.
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MOBILE STATION ASSISTED TIMING
SYNCHRONIZATION IN A CDMA COMMUNICATION SYSTEM
This is a divisional of Application Serial
No. 2,302,404, filed September 18, 1998.
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communication
systems. More particularly, the present invention relates
to a novel and improved method and apparatus for
synchronizing a base station by means of signals transmitted
from a mobile station which is concurrently in communication
with a synchronized base station.
II. Description of the Related Art
The use of code division multiple access (CDMA)
modulation techniques is but one of several techniques for
facilitating communications in which a large number of
system users are present. Although other techniques, such
as time division multiple access (TDMA), frequency division
multiple access (FDMA) and AM modulation schemes such as
amplitude companded single sideband (ACSSB) are known, CDMA
has significant advantages over these other modulation
techniques. The use of CDMA techniques in a multiple access
communication system is disclosed in U.S. Patent
No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS
COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL
REPEATERS" and U.S. Patent No. 5,103,459, entitled "SYSTEM
AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA
CELLULAR TELEPHONE SYSTEM", both of which are assigned to
the assignee of the present invention. The method for
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providing CDMA mobile communications was standardized in the
United States by the Telecommunications Industry Association
in TIA/EIA/IS-95-A entitled "Mobile Station-Base Station
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Compatibility Standard for Dual-Mode Wideband Spread
Spectrum Cellular System", referred to herein as IS-95.
In the just mentioned patents, a multiple access
technique is disclosed in which a large number of mobile
station users, each having a transceiver, communicate
through satellite repeaters or terrestrial base stations
(also known as cell base stations or cell-sites) using code
division multiple access (CDMA) spread spectrum
communication signals. By using CDMA communications, the
frequency spectrum can be reused multiple times thus
permitting an increase in system user capacity. The use of
CDMA techniques result in much higher spectral efficiency
than can be achieved using other multiple access techniques.
A method for simultaneously demodulating data that
has traveled along different propagation paths from one base
station and for simultaneously demodulating data redundantly
' provided from more than one base= station is disclosed in
U.S. Patent No. 5,109,390 (the '390 patent), entitled
"DIVERSITY RECEIVER IN A CDMA CELLULAR COMMUNICATION
SYSTEM", assigned to the assignee of the present invention.
In the '390 patent, the separately demodulated signals are
combined to provide an estimate of the transmitted data
which has higher reliability than the data demodulated by
any one path or from any one base station.
Handsoff can generally be divided into two
categories- hard handoffs and soft handoffs. In a hard
handoff, when a mobile station leaves an origination base
station and enters a destination base station, the mobile
station breaks its communication link with the origination
base station and thereafter establishes a new communication
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link with the destination base station. In soft handoff,
the mobile station completes a communication link with the
destination base station prior to breaking its communication
link with the origination base station. Thus, in soft
handoff, the mobile station is redundantly in communication
with both the origination base station and the destination
base station for some period of time.
Soft handoffs are far less likely to drop calls
than hard handoffs. In addition, when a mobile station
travels near the coverage boundary of a base station, it may
make repeated handoff requests in response to small changes
in the environment. This problem, referred to as ping-
ponging, is also greatly lessened by soft handoff. The
process for performing soft handoff is described in detail
in U.S. Patent No. 5,101,501, entitled "METHOD AND SYSTEM
FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA
CELLULAR TELEPHONE SYSTEM" assigned to the assignee of the
present invention.
An improved soft handoff technique is disclosed in
U.S. Patent No. 5,267,261, entitled "MOBILE STATION ASSISTED
SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATION SYSTEM", which
is assigned to the assignee of the present invention. In
the system of the '261 patent, the soft handoff process is
improved by measuring the strength of "pilot" signals
transmitted by each base station at the mobile station.
These pilot strength measurements are of assistance in
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the soft handoff process by facilitating identification of viable base station
handoff candidates.
The base station candidates can be divided into four sets. The first set,
referred to as the Active Set, comprises base stations which are currently in
communication with the mobile station. The second set, referred to as the
Candidate Set, comprises base stations whose signals have been determined
to be of sufficient strength to be of use to the mobile station but are not
currently being used. Base stations are added to the candidate set when their
measured pilot energy exceeds a predetermined threshold TADD. The third
set is the set of base stations which are in the vicinity of the mobile
station
(and which are not included in the Active Set or the Candidate Set). And
the fourth set is the Remaining Set which consists of all other base stations.
In IS-95, a base station candidate is characterized by the phase offset of
the pseudonoise (PN) sequence of its pilot channel. When the mobile
station searches to determine the strength of the pilot signal from a
candidate base station it performs a correlation operation wherein the
filtered received signal is correlated to a set of PN offset hypotheses. The
method and apparatus for performing the correlation operation is described
in detail in U.S. Patent No. 5,644,591.
The propagation delay between the base station and the mobile station
is not known. This unknown delay produces and unknown shift in the PN
codes. The searching process attempts to determine the unknown shift in
the PN codes. To do this, the mobile station shifts in time the output of its
searcher PN code generators. The range of the search shift is called the
search window. The search window is centered about a PN shift hypothesis.
A base station transmits to the mobile station a message indicating the PN
offsets of base station pilots in its physical proximity. The mobile station
will center its search window around the PN offset hypothesis.
The appropriate size of the search window depends on several factors
including the priority of the pilot, the speed of the searching processors,
and
the anticipated delay spread of the multipath arrivals. The CDMA standards
(IS-95) define three search window parameters. The searching of pilots in
both the active and candidate sets is governed by Search Window "A".
Neighbor Set pilots are searched over window "N" and Remaining Set
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pilots over window ."R". The searcher window sizes are provided below in
Table 1; where a chip is1
1.2288 MHz
SRCH_WIN_A Window Size SRCH_WIN_A Window Size
SRCH_WIN_N (PN chips) SRCH_WIN_N (PN chips)
SRCH_WIN_R SRCH_WIN_R
0 4 8 60
1 6 9 80
2 8 10 100
= 3 10 11 130
4 14 12 160
20 13 226
6 = 28 14 320
7 40 15 452 =
5 TABLE 1
Window sizing is a trade-off between search speed and the probability of
missing a strong path lying outside the search window.
The base station transmits to the mobile station a message which
specifies the PN hypotheses that the mobile station should search relative to
its own PN offset. For example, the originating base station may instruct the
mobile station to search for a pilot 128 PN chips ahead of its own PN offset.
The mobile station in response sets its searcher demodulator 128 chips ahead
in the output chip cycle and searches for the pilot using a search window
centered about the specified offset. Once the mobile is instructed to a search
a PN hypothesis to determine the resources available for performing a
handoff, it is critical that the PN offset of the destination base station
pilot is
very close in time to the directed offset. The speed of searching is of
critical
importance near base station boundaries because delays in completing the
necessary searches can result in dropped calls.
In CDMA systems in the United States, this base station
synchronization is achieved by providing each base station with a Global
Positioning Satellite (GPS) receiver. However, there are cases where a base
station may not be able to receive the GPS signal. For example, within
subways and tunnels the GPS signal is attenuated to a degree that prohibits
their use for timing synchronization of base stations or micro base stations.
The present invention provides a method and system for providing timing
synchronization in these circumstances where a fraction of the network is
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capable of receiving a centralized timing signal and achieving timing
therefrom and a portion of the base stations are not capable of receiving the
centralized timing signal.
SUMMARY OF THE INVENTION
The present invention is a novel and improved method and apparatus
for tim.e synchronizing a base station which is not capable of receiving a
centrA1i7ed timing signal in a network where some of= the base stations are
capable' of receiving the centralized timing signal. The reference base
station
has timing synchronization through receipt of a centralized timing signal or
other means. In the exemplary embodiment, the reference base station
synchronizes using a global positioning satellite (GPS) receiver. = The slave
base station lacks the capacity to synchronize, because for example of an
inability to receive the centralized timing signal.
In the present invention, the slave base station attains synchronization
with the reference base station through messages transmitted from and
received by a mobile station in the soft hartdoff region between the reference
base station and the slave base station. First, the round trip delay between
the
mobile station and the reference base station is measured by the reference
base station. Next, the slave base station searches until it acquires the
signal
transmitted by the mobile station, referred to as the reverse link signal. In
response to the acquisition of the reverse link signal, the slave base station
adjusts its dining so that the mobile station can acquire its signal, referred
to
as a forward link signal. This step may be unnecessary if the timing error in
the slave base station is not severe.
Once the mobile station acquires the signal from the slave base station,
it measures and reports the difference between the am.ourit of time it takes a
signal to travel from the reference base station to it and the amount of time
it
takes a signal to travel from the slave base station to it. The last
measurement
necessary is a measurement by the slave base station of the time difference
between the time it received the reverse link signal from the mobile station
arid the time it transmitted a signal to the mobile station.
A series of computations described in detail herein are performed.
upon the measured time values to determine the time difference between the
slave base station and an adjustment of the slave base station timing is
performed in accordance therewith. It should be noted that all of the
measurements mentioned are performed during the nonnal operation of an.
15-95 CDMA communication system..
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According to one aspect of the present invention, there is provided a
method for synchronizing a base station with a wireless communication system
upon
the base station's power up, comprising: disabling a transmission from the
base
station; obtaining initial timing at the base station; receiving at the base
station
signals transmitted from a mobile station; providing the mobile station with
an identity
of the base station; transmitting signals at successively increasing power
levels from
the base station in accordance with an adjusted timing until the mobile
station detects
the transmitted signals; and synchronizing timing of the base station with
another
base station communicating with the mobile station, comprising: measuring a
round
trip delay interval of transmissions from the another base station to the
mobile station
and back from the mobile station to the another base station; receiving at the
base
station communications transmitted by the mobile station and noting the time
of
reception; determining an estimate of a delay which occurs between
transmission by
the mobile station and reception by the base station; and computing a timing
correction value in accordance with the estimate of the delay, the noted time
of
reception, and the measured round trip delay interval.
According to another aspect of the present invention, there is provided
an apparatus for synchronizing a base station with a wireless communication
system
upon the base station's power up, comprising: a transmitter; a processor
communicatively coupled to said transmitter; a receiver, communicatively
coupled to
the processor, configured to receive signals transmitted from a mobile
station; and a
storage medium coupled to said processor and containing a set of instructions
executable by said processor to: disable said transmitter; obtain initial
timing; and
synchronize timing of the base station with another base station communicating
with
the mobile station, comprising: initiating a communication between the base
station
and the mobile station; measuring a first round trip delay interval of
transmissions
from the base station to the mobile station in communication with the base
station
and back from the mobile station to the base station; measuring a second round
trip
delay interval of transmissions from the another base station communicating
with the
mobile station and back from the mobile station to the another base station
communicating with the mobile station; and computing a timing correction value
in
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accordance with the first round trip delay interval and the second round trip
delay
interval.
According to still another aspect of the present invention, there is
provided an apparatus for synchronizing a base station with a wireless
communication system upon the base station's power up, comprising: means for
disabling a transmission from the base station; means for obtaining initial
timing at
the base station; means for receiving at the base station signals transmitted
from a
mobile station; means for providing the mobile station with an identity of the
base
station; means for transmitting signals at successively increasing power
levels from
the base station in accordance with an adjusted timing until the mobile
station detects
the transmitted signals; and means for synchronizing timing of the base
station with
another base station communicating with the mobile station, comprising: means
for
measuring a round trip delay interval of transmissions from the another base
station
to the mobile station and back from the mobile station to the another base
station;
means for receiving at the base station communications transmitted by the
mobile
station and noting the time of reception; means for determining an estimate of
a delay
which occurs between transmission by the mobile station and reception by the
base
station; and means for computing a timing correction value in accordance with
the
estimate of the delay, the noted time of reception, and the measured round
trip delay
interval.
According to yet another aspect of the present invention, there is
provided a processor-readable non-transitory medium having one or more
instructions stored thereon which when executed by one or more processors
causes
the one or more processors to: disable a transmission from the base station;
obtain
initial timing at the base station; receive at the base station signals
transmitted from a
mobile station; provide the mobile station with an identity of the base
station; transmit
signals at successively increasing power levels from the base station in
accordance
with an adjusted timing until the mobile station detects the transmitted
signals; and
synchronize timing of the base station with another base station communicating
with
the mobile station, by: measuring a round trip delay interval of transmissions
from the
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another base station to the mobile station and back from the mobile station to
the
another base station; receiving at the base station communications transmitted
by the
mobile station and noting the time of reception; determining an estimate of a
delay
which occurs between transmission by the mobile station and reception by the
base
station; and computing a timing correction value in accordance with the
estimate of
the delay, the noted time of reception, and the measured round trip delay
interval.
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BRIEF DESCRIPTION OF THE DRAWINGS
The features, objects, and advantages of the present invention will =
become more apparent from the detailed description set forth below when
taken in conjunction with the drawings in which= like reference characters
identify correspondingly throughout and wherein:
FIG. 1 is a block diagram illustrating the network configuration of a
wireless communication system comprising a reference base station and a
slave base station;
FIG. 2 is a diagram illustrating the various transmissions between the
mobile station, the synchronous base station and the asynchronous base
station and the corresponding time intervals;
FIG. 3 is a flowchart illustrating the method for synchronizing a base
station which is incapable of receiving a centralized timing signal;
= FIG. 4 is a block diagram of the mobile station of the present
invention;
FIG. 5 is a block diagram of the searcher in the mobile station of the
present invention;
FIG. 6 is a block diagram of the traffic channel modulator of the
mobile station of the present invention;
FIG. 7 is a block diagram of the base station of the present invention;
FIG. 8 is a block diagram of transmission system of the base station of
the present invention; and
FIG. 9 is a block diagram of receiver system of the base station of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS
Overview of Timing Error Computation
Referring to FIG. 1, mobile station 60 is in communication with base
station 62, while it is roughly within the coverage area delineated by base
station coverage boundary 61. Base station 62 is synchronized to the rest of
the network by means of a central timing system such as the global
positioning system (GPS). In contrast, base station 64 is not synchronized to
the central timing system. Base station controller 66 routes calls from the
PSTN to a base station 62 or 64 by means of a T1 line or other means. In
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addition, frequency- synchronization is provided to base station 64 through
T1 lines.
For short time periods, frequency synchronization can be provided
with an acceptable degree of accuracy through T1 lines by methods well.
known in the art. However, glitches are common in these schemes for
providing frequency information. These glitches result in timing errors
which can be corrected by use of the present invention. Because of the
relationship between phase and frequency, the present invention's
intermittent correction of phase will permit the utilization of a less
accurate
frequency sources when necessary.
Referring to FIG. 2, an illustration of the transmission and
corresponding time intervals used to synchronize the timing of slave base
station 64 with the synchronized timing of reference base station 62. Signal
path 500 illustrates the transmission of a forward link signal from reference
base station 62 to mobile station 60. The time interval over which this
transmission occurs is designated as T1. At mobile station 60, the start of
frame transmissions on the reverse link are time aligned with the start of
frame arrivals on the forward link. This time alignment is standardized in
IS-95 and incorporated in hardware designed in conformance therewith
= 20 such that methods and apparatus for performing this alignment are well
known in the art.
Transmission 502 depicts the transmission of a reverse link frame
from mobile station 60 to reference base station 62. The time for a signal 500
to travel from base station 62 to mobile station 60 (T1) is equal to the time
for
signal 502 to travel from base station 62 to mobile station 60 (also T).
Because base station 62 knows the time at which it transmitted signal 500
and knows the time at which it received signal 502, base station 62 can
compute the round trip delay time (RTD1), which is the first value necessary
in the computation of the time error (V-To).
Signal path 504 is the reverse link signal transmission from mobile
station 60 traveling along a different propagation path to slave base
station 64. The time which it takes signal 504 to travel from mobile
station 60 to slave base station 64 is designated as T2. The time at which the
reverse link signal 504 reaches base station 64 is designated as T2. The time
it
takes a forward link signal 506 traveling from base station 64 to mobile
station 60 is also equal to T2. In addition, slave base station 64 can measure
the time difference between the time it received the reverse link signal from
mobile station 60 and the time it transmitted its forward link signal to
mobile station 60. This time difference is designated as IZTD2. Knowing
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these =times allows for the computation of the time error (T0'-T0). The
method for computing the time error 't o' is described below.
First it can be observed from FIG. 2 that:
T2= 'Li+ T2, and
(1)
(2)
By manipulating the terms of equations (1) and (2), the following is
observed:
T2+ T = To' +2 'T2
=(3)
2' T2 = T2 - To' +
(4)
To simplify the notation, a new variable RTD2 is defined as:
RTD2 = T2 - To'
= (5)
It can be seen that =
=2 RTD2 AT 2
(6)
T2 = T 1 +
(7)
Therefore,
T2- To = + T2, and
(8)
RTD2 = 2 = T2 - AT
By substitution, it can be seen that the time error (To' - To) is equal to:
To' - To = - T2+ AT
(9)
RTD AT2 2
= (10)
To' - To = RTD RTD2 AT + ¨
(11)
2 2 2
=
To' - To = RTD +AT - RTD2 2
(12)
Once base station 64 knows the amount of its timing error (To' - To), it
adjusts its timing so as to synchronize it to the timing of= base station 62.
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These measurements are subject to error, so, in a preferred embodiment,
many of the measurements are redundantly made to assure the accuracy of
the timing correction.
The method and apparatus for measuring each of the necessary time
values in equation (12) is now described.
11. Measurement of Round Trip Delay (RTD1)
FIG. 3 is a flow diagram illustrating the method of the present
invention for synchronizing slave base station 64 to the timing of reference
base station 62. In step 300, the synchronizatiOn method commences, with
mobile station 60 in communication with reference base station 62 and
within range to conduct communications with slave base station 64. In step
302, the round trip delay (RTD1) time for a signal to travel from reference
base station 62 to mobile station 60 and back from mobile station 60 to
reference base station 62 is measured. This is done by aligning the frame
boundaries of frames being received by mobile station 60 with the frame
boundaries of frames being transmitted by mobile station 60. The method
and apparatus for providing this alignment is well known in the art. Thus,
the round-trip delay (RTD1) is measured as the time difference between the
start of frames transmitted by reference base station 62 and the start of
frames received by reference base station 62 from mobile station 60.
Referring to FIG. 4, forward link frames of data from reference base
station 62 are received at antenna 2 and provided through duplexer 3 to
receiver (RCVR) 4. Receiver 4 downconverts, filters and amplifies the
received signal and provides it to searcher 50 and traffic demodulators
(TRAFFIC DEMODS) 54. Searcher 50 searches for pilot channels in
accordance with a neighbor list provided by reference base station 62. The
neighbor list is provided as signaling data on the traffic channel from
reference base station 62. A signal indicating the start of received frames
from reference base station 62 is provided to control processor 55. Control
processor 55 generates and provides a time alignment signal to traffic
modulator 58 which aligns the start of frames transmitted from mobile
station 60 with the start of frames received at mobile station 60.
Frames of data from the user of mobile station 60 are provided to
traffic modulator 58 which in response to the timing signal from control
processor 55 time aligns the frames transmitted through transmitter
(TM'TR) 56 with the frames received by mobile station 60 from reference base
station 62. The reverse link frames are upconverted, filtered and amplified
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by transmitter 56 then provided through duplexer 3 for transmission
through antenna 2.
IR Acquisition of Mobile Station by Slave Base Station
FIG. 6 illustrates the traffic channel modulator 58 of mobile station 60.
Frames of data are provided tc. frame formatter 200. In =the exemplary
embodiment, frame formatter 200 generates and appends a set of cyclic
redundancy (CRC) check bits and generates a set of tail bits. In the exemplary
embodiment, frame formatter 200 follows the frame format protocol
standardized in IS-95 and described in detail in U.S. Patent No. 5,600,754,
entitled "METHOD AND SYSTEM FOR THE ARRANGEMENT OF
VOCODER DATA FOR THE MASKING OF TRANSMISSION CHANNEL
INDUCED ERRORS", which is assigned to the assignee of the present
invention.
The formatted data frame is provided to encoder 202 which encodes
the data for error correction and detection. In the exemplary embodiment,
encoder 202 is a convolutional encoder. The encoded data symbols are
provided to interleaver 204 which reorders the symbols in accordance with a
predetermined interleaving format. The reordered symbols are provided to
Walsh mapper 206. In the exemplary embodiment, Walsh mapper 206
receives eight coded symbols and maps that set of symbols to a 64 chip
Walsh sequence. The Walsh symbols are provided to spreading _means 208
which spreads the Walsh symbols in accordance with a long spreading code.
Long PN code generator 210 generates a pseudonoise (PN) sequence that
spreads the data and differentiates the data from the reverse link transmitted
data from other mobile stations in the vicinity.
In the exemplary embodiment, the data is transmitted in accordance
with a quaternary phase shift keying (QPSK) modulation format wherein
the I and Q channels are spread in accordance with a short PN sequence.
The spread data is provided to spreading means 214 and 216 which perform
a second spreading operation on the data in accordance with a short PN
sequence provided by PN generators (PNI and PNQ) 212 and 218 respectively.
In step 304, slave base station 64 acquires the reverse link signal
transmitted by mobile station 60. Base station controller 66 sends a signal to
slave base station 64 indicating the PN code offset which mobile station 62 is
using to spread its reverse link signal. In response to this signal from base -
station controller 66, slave base station 64 performs a search for the mobile
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station 60 centered about the PN offset indicated by the
signal from base station controller 66.
In the exemplary embodiment, slave base station 64
bank loads its searchers long code PN generator 106 and its
short code PN generators 108 and 110 (illustrated in FIG. 9)
in accordance with a signal from base station controller 66.
The searcher process of slave base station 64 is described
in detail further herein.
FIG. 7 illustrates the apparatus of slave base
station 64. In slave base station 64, a signal from base
station controller 60 indicating the PN of mobile station 60
is received. This message is provided to by control
processor 100. In response thereto, control processor 100
computes the window search range centered at the specified
PN offset. Control processor 100 provides the search
parameters to searcher 101 and in response to those
parameters slave base station 64 conducts a search for the
signal transmitted by mobile station 60. The signal
received by antenna 102 of slave base station 64 is provided
to receiver 104 which downconverts, filters and amplifies
the received signal and provides it to searcher 101. In
addition, the received signal is provided to traffic
demodulators 105 which demodulate the reverse link traffic
data and provide that data to base station controller 66.
Base station controller 66, in turn provides it to a public
switched telephone network (PSTN).
FIG. 9 illustrates searcher 101 in greater detail.
The demodulation of the reverse link signal is described in
detail in U.S. Patent No. 5,654,979. An estimate of the PN
offset of mobile station 60 is provided to control processor
100 from base station controller 66. In response to the PN
offset estimation provided by base station controller 66,
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control processor 100 generates an initial long PN sequence
hypothesis and an initial short PN sequence hypothesis for
the search to be performed by slave base station 64. In the
exemplary embodiment, control processor 100 bank loads the
shift registers of PN generators 106, 108 and 110.
The signal received by antenna 102 is
downconverted, filtered and amplified and passed to
correlator 116. Correlator 116 correlates the received
signal to the combined long and short PN sequence
hypothesis. In the exemplary embodiment, the PN sequence
hypothesis is generated by multiplying the short PN
hypotheses generated by PN generators 108 and 110 by the
long PN sequence generated by PN generator 106. One of the
combined PN sequence hypotheses is used to despread the I
channel and the other is used to despread the Q channel of
the received QPSK signal.
The two PN despread signals are provided to fast
Hadamard transform (FHT) processors 118 and 120. The design
and operation of fast Hadamard transform processors is
described in detail in U.S. Patent No. 5,561,618. FHT
processors 118 and 120 correlate the despread signals with
all possible Walsh symbols to provide a matrix of the
resultant amplitudes to energy computation means (I2+Q2) 122.
Energy computation means 122 computes the energy of the
amplitude matrix elements and provides the energy values to
max detector 124 which selects the maximum energy
correlation. The maximum correlation energies are provided
to accumulator 126 which accumulates the energies for a
plurality of Walsh symbols and based upon these accumulated
energies, a decision is made as to whether the mobile
station 60 can be acquired at that PN offset.
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IV. Initial Timing Adjustment by Slave Base Station
Once mobile station 60 is acquired, then, in block
306, slave base station 64 adjusts its timing so that mobile
station 60 will be able to successfully acquire its forward
link transmissions. Slave base station 64 computes an
initial timing adjustment by determining the difference
between the PN offset at which it acquired the reverse link
signal from mobile station 60 and the PN offset which
reference base station 62 used for reception of the reverse
link signal from mobile station 60. Using this PN offset
difference, slave base station 64 adjusts the timing of its
pilot signal in such a way that when mobile station 60
searches for its pilot signal it will be within the search
window of mobile station 60.
V. Acquisition of the Slave Base Station by the Mobile Station
In searching for the mobile station signal, it is
necessary for slave base station 64 to have some indication
of time. In the preferred embodiment,
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the time error of slave base station 64 is kept at or below 1 ms by means of
an
alternative synchronization scheme. There are schemes which enable slave
base station 64 which is incapable of receiving a GPS signal to keep time to a
level of lesser precision. One possible method of obtaining a degree of
initial
synchronization is to manually set the time of slave base station 64 at
certain
intervals. A second method is to set the time using a WWV receiver, the
implementation of which is well known in the art. Unlike the GPS signal,
the WWV centralized timing signal is transmitted at very low frequency
and is able to penetrate into tunnels and subways. However, W W V =
receivers are not capable of providing the degree of time synchronization
necessary for providing CDMA communications.
In the exemplary embodiment, slave base station 64 adjusts its timing
in accordance with the assumption that mobile station 60 is located directly
adjacent to slave base station 64. Thus, the initial timing. adjustment is
made under the hypothesis that there will be no propagation delay between
slave base station 64 and mobile station 60. Thereafter, slave base station 64
adjusts its PN sequence generators 72 and 74 forward in time which accounts
for greater and greater propagation delay times between slave base station 64
and mobile station 60. Once mobile station 60 has acquired the pilot channel
of slave base station 64, using normal procedures the final adjustment of
timing for slave base station 64 can be performed in accordance with the
computations described above.
As is known in the art and standardized in IS-95, pilot channels of
different base stations are distinguished from one another by the phase of
their PN generators. Reference base station 62 instructs mobile station 60 to
search for slave base station 64 via the neighbor list. Reference base station
62 indicates by means of the signaling data that the pilot of slave base
station
64 can be acquired at a PN phase offset which is described relative to the
received PN offset of reference base station 62. This message is demodulated
and decoded by traffic demodulators 54 and provided to searcher 50. In
response, searcher 50 performs a search centered on a PN phase offset about
the PN phase indicated in the signal from reference base station 62.
The pilot signal is typically generated by a linear feedback shift
register, the implementation of which is described in detail in the
aforementioned patents. In order to acquire the pilot signal from slave base
station 64, mobile station 60 must synchronize to the received signals from
slave base station 64 in both phase, 0, and in frequency, co. The object of
the
searcher operation is to find the phase of the received signal, 4) . As
described
earlier, a relatively accurate frequency synchronization can. be supplies to
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slave base station 64 by means of a T1 link from base station controller 66 as
is known in the art. The method by which a mobile finds the phase of the
received signal is by testing a set of phase hypotheses, referred to as a
search
window and determining if one of the offset hypotheses is correct.
FIG. 5 illustrates mobile station searcher 50 in greater detail. A spread
spectrum signal is received at antenna 2. The objective of the apparatus is to
gain synchronization between pseudorandom noise (PN) sequences
generated by PN sequence generator 20 and the received spread spectrum
signal which is spread by identical PN sequences of unknown phase
transmitted by slave base station 64. In the exemplary embodiment, both
. pilot signal generator 76 (of FIG. 7) and PN generator 20 are maximal length
shift registers which generate the PN code sequences for spreading and
despreading the pilot signals respectively. Thus, the operation of obtaining
synchronization between the codes used to despread the received pilot signal
and the PN spreading code of the received pilot signal involves determining
the time offset of the shift register.
The spread spectrum signal is provided by antenna 2 to receiver 4.
Receiver 4 downconverts, filters and amplifies the signal and provides the
signal to despreading element 6. Despreading element 6 multiplies the
received signal by the PN code generated by PN generator 20. Due to the
random noise like nature of the PN codes, the product of the PN code and
the received signal should be essentially zero except at the point of
synchronization.
Searcher controller 18 provides an offset hypothesis to PN generator
20. The offset hypothesis is determined in accordance with a signal
transmitted to mobile station 60 by reference base station 62. In the
exemplary embodiment, the received signal is modulated by quaternary
phase shift keying (QPSK), so PN generator 20 provides a PN sequence for
the I modulation component and a separate sequence for the Q modulation
component to despreading element 6. Despreading element 6 multiplies the
PN sequence by its corresponding modulation component and provides the
two output component products to coherent accumulators 8 and 10.
Coherent accumulators 8 and 10 sum the product over the length of
the product sequence. Coherent accumulators 8 and 10 are responsive to
signals from searcher controller 18 for resetting, latching and setting the
summation period. The sums of the products are provided from summers 8
and 10 to squaring means 14. Squaring means 14 squares each of the sums
and adds the squares together.
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The sum of the squares is provided by squaring means 12 to non-
coherent combiner 14. Noncoherent combiner 14 determines an energy
value from the output of squaring means 12. Noncoherent accumulator 14
serves to counteract the effects of a frequency discrepancy between the base
station transmit clocks and the mobile station receive clock and aids in the
detection statistic in a fading environment. Noncoherent accumulator 14
provides the energy signal to comparison means 16. Comparison means 16
compares the energy value to predetermined thresholds supplied by
searcher controller means 18. The results of each of the comparisons is then
feedback to searcher controller 18. The results fedback to searcher controller
18 include both the energy of the correlation and the PN offset that resulted
in the measurement.
In the present invention, searcher controller 18 outputs the PN phase
at which it synchronized to base station 64. This offset is used to compute
the time error as described further herein.
In the exemplary embodiment, when mobile station 60 acquires slave
base station 64 it computes the difference between the time it received the
signal from slave base station 64 and the time it received the signal from
reference base station 62. This value is provided to message generator 52
which generates a message indicative of the difference value. The message
is transmitted as signaling data on the reverse link to reference base station
62 and slave base station 64 which send the message back to base station
controller 66.
VI. Measurement of Delay Between Transmission of Forward Link Signal
From Slave Base Station and Receipt of Reverse link Signal at Slave
Base Station
In step 311, slave base station 64 measures the time difference between
it received the reverse link signal from mobile station 60 (T2) and the time
it
transmitted its forward link signal to mobile station 60 (T1). Slave base
station 64 stores the PN offset at the time it transmits its forward link
signal
and upon detection of the reverse link signal from mobile station 60
computes the time difference RTD2. In the exemplary embodiment, this
computed time difference is provided by slave base station 64 to base station
controller 66 and the computation of the timing adjustment is conducted at
base station 66. It will be understood by one skilled in the art that the
present invention is easily extended to the case wherein the computations
are performed at the base stations or mobile stations.
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VII. Timing Adjustment of Slave Base Station
Base station controller 66, in response, performs the computation
described in equation (12) and sends an indication of the necessary timing
adjustment to slave base station 64. Referring back to FIG. 7, the timing
adjustment signal is received by slave base station 64 at control processor
100. Control processor 100 generates and provides a control signal to timing
adjustment processor 99. Timing adjustment processor 99 generates a signal
which changes the time of timing source 98 by the amount indicated in the
signal from base station controller 66.
