U.S. patent number RE38,523 [Application Number 09/559,771] was granted by the patent office on 2004-06-01 for spreading code sequence acquisition system and method that allows fast acquisition in code division multiple access (cdma) systems.
This patent grant is currently assigned to InterDigital Technology Corporation. Invention is credited to Fatih Ozluturk.
United States Patent |
RE38,523 |
Ozluturk |
June 1, 2004 |
Spreading code sequence acquisition system and method that allows
fast acquisition in code division multiple access (CDMA)
systems
Abstract
Methods for generating code sequences that have rapid
acquisition properties and apparatus which implement the methods by
processing spreading codes on in-phase and quadrature channels. A
first method combines two or more short codes to produce a long
code. This method may use many types of code sequences, one or more
of which are rapid acquisition sequences of length L that have
average acquisition phase searches r=log2L. Two or more separate
code sequences are transmitted over the complex channels. If the
sequences have different phases, an acquisition may be done by
acquisition circuits in parallel over the different code sequences
when the relative phase shift between the two or more code channels
is known. When the received length L codes or the length L
correlation codes used to find the phase of the received codes have
a mutual phase delay of L/2, the average number of tests to find
the code phase of the received code is L/4. The codes sent on each
channel may be the same code, with the code phase in one channel
being delayed with respect to the other channel, or they may be
different code sequences.
Inventors: |
Ozluturk; Fatih (Port
Washington, NY) |
Assignee: |
InterDigital Technology
Corporation (Wilmington, DE)
|
Family
ID: |
32329682 |
Appl.
No.: |
09/559,771 |
Filed: |
April 27, 2000 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
669776 |
Jun 27, 1996 |
05748687 |
May 5, 1998 |
|
|
Current U.S.
Class: |
375/367; 370/342;
375/142; 375/145; 375/149; 375/150 |
Current CPC
Class: |
H04B
1/70753 (20130101); H04B 1/70754 (20130101); H04B
1/708 (20130101); H04L 25/0212 (20130101); H04L
27/206 (20130101); H04L 27/2332 (20130101); H04W
52/143 (20130101); H04W 52/146 (20130101); H04W
52/265 (20130101); H04W 52/267 (20130101) |
Current International
Class: |
H04L
27/233 (20060101); H04B 1/707 (20060101); H04L
27/20 (20060101); H04L 25/02 (20060101); H04B
001/707 (); H04L 007/00 () |
Field of
Search: |
;375/140,141,142,144,145,149,150,367 ;370/320,335,342,441 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0631397 |
|
Dec 1994 |
|
EP |
|
2280575 |
|
Jan 1995 |
|
GB |
|
9428640 |
|
Dec 1994 |
|
WO |
|
9507578 |
|
Mar 1995 |
|
WO |
|
Other References
Braun et al., "An All-Digital Receiver for Satellite Audio
Broadcasting Signals Using Trellis Coded Quasi-Orthogonal
Code-Division Muliplexing," European Transactions on
Telecommunications and Related Technologies, No. 1, pp. 23-32,
Jan./Feb. 1993. .
Giannetti et al., "Design of an All-Digital Receiver for Narrowband
Continuous-Phase Asynchronous CDMA Systems," IEEE, vol. 3, pp.
468-472, May 1993. .
Pahlavan et al., "Performance of Adaptive Matched Filter Receivers
Over Fading Multipath Channels," IEEE Transactions on
Communications, No. 12, pp. 2106-2113, Dec. 1993. .
Rick et al. "Noncoherent Parallel Acquisition in CDMA Spread
Spectrum Systems," IEEE, pp. 1422-1426, May 1994. .
Valerio Bernasconi, "Receiver Architectures for the Down-link in a
DS-CDMA Mobile System," IEEE Transactions on Communications, pp.
51-55, Sep. 1994. .
Zhao Liu et al., "Sir-Based Call Admission Control For DS-CDMA
Cellular Systems" IEEE Journal on Selected Areas in Communications,
May 1994, 639640..
|
Primary Examiner: Vo; Don N.
Attorney, Agent or Firm: Volpe and Koenig, P.C.
Parent Case Text
This application claims the benefit of U.S. Provisional Application
60/000,775 filed Jun. 30, 1995.
Claims
The invention claimed is:
1. A fast acquisition apparatus for quickly synchronizing a
spreading code phase of a spread-spectrum communication system to a
transmitted code signal having a transmitted in-phase (I) code
signal and a transmitted quadrature (Q) code signal, said
transmitted I-code signal including a first spreading code sequence
and said transmitted Q-code signal including a second spreading
code sequence; the transmitted I-code signal and the transmitted
Q-code signal having a predetermined mutual code sequence phase
offset value, the fast acquisition apparatus comprising: receiving
means for receiving the transmitted code signal and for separating,
from the received code signal, the transmitted I-code signal and
the transmitted Q-code signal; correlating means for correlating
code sequences with the transmitted code signal and comprising an
I-code signal correlator and a Q-code signal correlator; a local
code sequence generator responsive to a code control signal value
to generate a local portion of the I-code sequence having an I-code
phase value and a local portion of the Q-code sequence having a
Q-code phase value; and controller means for determining, obtaining
and maintaining code sequence lock said controller means coupled to
the I-code signal correlator, the Q-code signal correlator, and the
local code sequence generator, said I-code signal correlator
correlating said local portion of the I-code sequence with said
transmitted I-code signal and generating an I-high value provided
to said controller means when the I-code phase value of the local
portion of the I-code sequence and a code phase value of the
transmitted I-code signal have matching code phase values and said
Q-code signal correlator correlating said local portion of the
Q-code sequence with said transmitted Q-code signal and generating
a Q-high value provided to said controller means when the Q-code
phase value of the local portion of the Q-code sequence and a code
phase value of the transmitted Q-code signal have matching code
phase values; wherein said controller means using said
predetermined mutual code sequence phase offset value, generates
the code control signal value to lock the I-code phase value of the
local portion of the I-code sequence responsive to the I-high value
and to set the Q-code phase value of the local portion of the
Q-code sequence, and generates the code control signal value to
lock the Q-code phase value of the local portion of the Q-code
sequence responsive to the Q-high value and to set the I-code phase
value of the local portion of the I-code sequence; and said
controller means is responsive to the absence of the I-high value
and the Q-high value to generate the code control signal value
which adjusts the I-code phase value and the Q-code phase
value.
2. The fast acquisition apparatus of claim 1, wherein the first
spreading code sequence is equivalent to the second spreading code
sequence, and the transmitted I-code signal and the transmitted Q
code signal have the predetermined mutual code sequence phase
relationship such that the respective code phases are not
identical.
3. The fast acquisition apparatus of claim 1, wherein the first
spreading code sequence and the second spreading code sequence are
each chosen from a plurality of fast acquisition sequences of
length L code periods; each of said fast acquisition sequences
including a short code portion having length of N code periods and
a long code portion having length of M code periods and having a
mean search value of log 2L phases wherein said short code portion
occurs repetitively, where L, M and N are integers, wherein: said
local portion of the I-code sequence includes an I-sequence
equivalent to the short code portion of the respective fast
acquisition sequence, and said local portion of the Q-code sequence
includes a Q-sequence equivalent to the short code portion of the
respective fast acquisition sequence; said I-code signal correlator
further includes means for generating an I-middle value when the
I-code phase value of the local portion of the I-code sequence and
the code phase of the transmitted I-code signal have code phase
values which correspond to the I-sequence being in phase with one
occurrence of the respective short code sequence of the first
spreading code sequence; said Q-code signal correlator further
includes means for generating a Q-middle value when the Q-code
phase of the local portion of the Q-code sequence and the code
phase of the transmitted Q-code signal have code phase values which
correspond to the Q-sequence being in phase with one occurrence of
the respective short code sequence of the second spreading code
sequence; and said controller is responsive to the I-middle value
and to the absence of the I-high value and the Q-high value for
generating the code control signal having a value which adjusts the
I-code phase value and the Q-code phase value to maintain the
respective local short code sequence portion of the local portion
of the I-code sequence in phase with each respective occurrence of
the short code sequence of the first spreading code sequence; and
being responsive to the Q-middle value and the absence of the
I-high value and the Q-high value for generating the code control
signal value for adjusting the I-code phase value and the Q-code
phase value to maintain the respective Q-sequence of the local
portion of the Q-code sequence in phase with each respective
occurrence of the short code sequence of the second spreading code
sequence.
4. The fast acquisition apparatus of claim 3, wherein N is an even
integer and M is an odd integer.
5. The fast acquisition apparatus of claim 3, wherein N is an odd
integer and M is an even integer.
6. The fast acquisition apparatus of claim 3, wherein L is equal to
M multiplied by N.
7. The fast acquisition apparatus of claim 3, wherein L is equal to
the least common multiple of M and N.
8. The fast acquisition apparatus of claim 3, wherein the first
spreading code sequence and the second spreading code sequence are
shifted in phase by L/2 code sequences relative to each
other..Iadd.
9. A fast code acquisition detector for a code division multiple
access receiver wherein the code sequence of the signal to be
received has I-code and Q-code signal components which have a known
phase relationship comprising: an I-code despreader for despreading
an I-code signal component with a despreading sequence at a
selected phase value and outputting the result; a Q-code despreader
for despreading a Q-code signal component with a despreading
sequence at a selected phase value and outputting the result; and a
controller for controlling the selected phase values of said I-code
and Q-code despreaders in response to a phase acquisition
correlation of each of the outputs of said I-code and Q-code
despreaders such that: said I-code despreader is provided an
initial I-code phase value and said Q-code despreader is provided
with an initial Q-code phase value which is off-set a predetermined
amount from said I-code initial phase value; if the correlation of
the output of neither said I-code or Q-code despreaders indicates
signal phase acquisition, said controller selectively increments
the selected phase value of said I-code and Q-code despreaders; and
if the correlation of the output of one of said I-code and Q-code
despreaders indicates phase acquisition, said controller
selectively increments the selected phase value of the other
despreader based on the known phase relationship so that both
despreaders output a phase correct despread
signal..Iaddend..Iadd.
10. A fast code acquisition detector according to claim 9 further
comprising: a first demodulator having a received signal input and
a filtered I-code signal output coupled to said I-code despreader;
and a second demodulator having a received signal input and a
filtered Q-code signal output coupled to said Q-code
despreader..Iaddend..Iadd.
11. A fast code acquisition detector according to claim 9 wherein:
said I-code despreader includes a phase adjustable spreading
sequence generator which generates an I-despreading sequence at
said selected phase value as controlled by said controller; and
said Q-code despreader includes a phase adjustable spreading
sequence generator which generates a Q-despreading sequence at said
selected phase value as controlled by said
controller..Iaddend..Iadd.
12. A fast code acquisition detector according to claim 9 wherein:
each of the I-code and Q-code signal components has a code sequence
period of length L consisting of a plurality of subsequences having
a period of length N, where L and N are integers such that L>N;
and said controller controls the selected phase values of said
I-code and Q-code despreaders in response to a correlation of each
of the outputs of said I-code and Q-code despreaders such that said
controller increments the selected phase value of said I-code and
Q-code despreaders by N is the correlation of either said I-code or
Q-code despreaders indicates phase acquisition of the signal
N-period subsequences and the correlation of the output of neither
said I-code or Q-code despreaders indicates signal phase
acquisition..Iaddend..Iadd.
13. A fast code acquisition detector according to claim 12 further
comprising: a first correlator associated with said controller
having an I-code despreader energy output detector which utilizes a
first threshold for detection of despread N-period subsequences or
a higher second threshold; and a second correlator associated with
said controller having a Q-code despreader energy output detector
which utilizes a first threshold for detection of despread N-period
subsequences or a higher second threshold; and said correlators
using said second higher threshold after either correlator detects
acquisition of despread N-period subsequences..Iaddend..Iadd.
14. A fast code acquisition detector according to claim 9 wherein
each of the I-code and Q-code signal components has a code sequence
period of length L and wherein the phase relationship between the
I-code and Q-code signal components is a phase shift of L/2 whereby
said controller selects a correct phase value for said despreaders
within L/4 iterations of phase acquisition
correlations..Iaddend..Iadd.
15. A fast code acquisition detector for a code division multiple
access receiver, wherein the code sequence of the signal to be
received has a period of length L consisting of a plurality of
subsequences having a period of length N, where L and N are
integers such that L>N, comprising: a despreader for despreading
a signal with a despreading sequence at a selected phase value and
outputting the result; and a controller for controlling the
selected phase value of said despreader in response to a phase
acquisition correlation of the output of said despreader such that:
said despreader is provided an initial phase value; if the
correlation of the output of said despreader does not indicate
signal phase acquisition or phase acquisition of the signal
N-period subsequences, said controller increments the selected
phase value of said despreader by one; and if the correlation of
the output of said despreader indicates phase acquisition of the
signal N-period subsequences, but not signal phase acquisition,
said controller increments the selected phase value of said
despreading by N..Iaddend..Iadd.
16. A fast code acquisition detector according to claim 15 further
comprising: a correlator associated with said controller having a
despreader energy output detector which utilizes a first threshold
for detection of despread N-period subsequences or a higher second
threshold; and said correlator using said second higher threshold
after said correlator detects acquisition of despread N-period
subsequences..Iaddend..Iadd.
17. A fast code acquisition detection method for a code division
multiple access receiver wherein the code sequence of the signal to
be received has I-code and an Q-code signal components which have a
known phase relationship comprising: despreading an I-code signal
component with a despreading sequence at a selected phase value to
produce a despread I signal; despreading a Q-code signal component
with a despreading sequence at a selected phase value to produce a
despread Q signal; controlling the selected phase values of said
I-code and Q-code despreading in response to a phase acquisition
correlation of the despread I and Q signals; said I-code
despreading being performed at an initial I-code phase value and
said Q-code despreading being performed at an initial Q-code phase
value which is off-set a predetermined amount from said I-code
initial phase value; if the correlation of neither the despread I
or Q signals indicates signal phase acquisition, selectively
incrementing the selected phase value of said I-code and Q-code
despreading; and if the correlation of one of the despread I or Q
signals indicates phase acquisition, selectively incrementing the
selected phase value of the other despreading based on the known
phase relationship so that both said I-code and Q-code despreading
produce phase correct despread signals..Iaddend..Iadd.
18. A fast code acquisition detection method according to claim 17
further comprising: demodulating and filtering a received signal
input to produce a filtered I-code signal component for said I-code
despreading; and demodulating and filtering a received signal input
to produce a filtered Q-code signal component for said Q-code
despreading..Iaddend..Iadd.
19. A fast code acquisition detection method according to claim 17
wherein each of the I-code and Q-code signal components has a code
sequence period of length L consisting of a plurality of
subsequences having a period of length N, where L and N are
integers such that L>N, and wherein the controlling of the
selected phase values of said I-code and Q-code despreading in
response to a correlation of said I and Q signals is such that the
incrementing of the selected phase value of said I-code and Q-code
despreading is by N when the correlation of either said I or Q
signal indicates phase acquisition of the signal N-period
subsequences and the correlation of the output of neither said I or
Q signal indicates signal phase acquisition..Iaddend..Iadd.
20. A fast code acquisition detection method according to claim 19
further comprising: correlating said I signal based on energy
detection at a first threshold for detecting despread N-period
subsequences or at a higher second threshold; correlating said Q
signal based on energy detection at a first threshold for detecting
of despread N-period subsequences or at a higher second threshold;
and said correlating being at said second higher threshold after
either I signal or Q signal correlating detects acquisition of
despread N-period subsequences..Iaddend..Iadd.
21. A fast code acquisition detection method for a code division
multiple access receiver, wherein the code sequence of the signal
to be received has a period of length L consisting of a plurality
of subsequences having a period of length N, where L and N are
integers such that L>N, comprising: despreading a signal with a
despreading sequence at a selected phase value to produce a
despread signal; and controlling the selected phase value of said
despreading in response to a correlation of the despread signal
against a predetermined threshold; initially despreading at an
initial phase value; and if the correlation of the despread signal
does not indicate signal phase acquisition or phase acquisition of
the signal N-period subsequences, incrementing the selected phase
vale of said despreading by one; and if the correlation of the
despread signal indicates phase acquisition of the signal N-period
subsequences and the correlation does not indicate signal phase
acquisition, incrementing the selected phase value of said
despreading by N..Iaddend..Iadd.
22. A fast code acquisition detection method according to claim 21
further comprising: correlating said despreading based on energy
detection at a first threshold for detection of despread N-period
subsequences or at a higher second threshold; and said correlating
being at said second higher threshold after the correlating detects
acquisition of despread N-period subsequences..Iaddend.
Description
BACKGROUND OF THE INVENTION
The present invention generally pertains to Code Division Multiple
Access (CDMA) communications, also known as spread-spectrum
communications. More particularly, the present invention pertains
to a new system and method employing a new code sequence design for
providing fast acquisition of a received spreading code phase in a
CDMA communications system.
DESCRIPTION OF THE RELEVANT ART
Recent advances in wireless communications have used spread
spectrum modulation techniques to provide simultaneous
communication by multiple users. Spread spectrum modulation refers
to modulating a information signal with a spreading code signal;
the spreading code signal being generated by a code generator where
the period Tc of the spreading code is substantially less than the
period of the information data bit or symbol signal. The code may
modulate the carrier frequency upon which the information has been
sent, called frequency-hopped spreading, or may directly modulate
the signal by multiplying the spreading code with the information
data signal, called direct-sequence (DS) spreading. Spread-spectrum
modulation produces a signal with bandwidth substantially greater
than that required to transmit the information signal. The original
information is recovered at the receiver by synchronously
demodulating and despreading the signal. The synchronous
demodulator uses a reference signal to synchronize the despreading
circuits to the input spread-spectrum modulated signal in order to
recover the carrier and information signals. The reference signal
may be a spreading code which is not modulated by an information
signal. Such use of a synchronous spread-spectrum modulation and
demodulation for wireless communication is described in U.S. Pat.
No. 5,228,056 entitled SYNCHRONOUS SPREAD-SPECTRUM COMMUNICATIONS
SYSTEM AND METHOD by Donald L. Schilling, which techniques are
incorporated herein by reference.
One area in which spread-spectrum techniques are used is in the
field of mobile cellular communications to provide personal
communication services (PCS). Such systems desirably support large
numbers of users, control Doppler shift and fade, and provide high
speed digital data signals with low bit error rates. These systems
employ a family of orthogonal or quasi-orthogonal spreading codes,
with a pilot spreading code sequence synchronized to the family of
codes. Each user is assigned one of the spreading codes as a
spreading function. Related problems of such a system include:
handling multipath fading effects. Solutions to such problems
include diversity combining of multipath signals. Such problems
associated with spread spectrum communications, and methods to
increase capacity of a multiple access, spread-spectrum system are
described in U.S. Pat. No. 4.901,307 entitled SPREAD SPECTRUM
MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL
REPEATERS by Gilhousen et al. which is incorporated herein by
reference.
The problems associated with the prior art systems focus around
reliable reception and synchronization of the receiver despreading
circuits to the received signal. The presence of multipath fading
introduces a particular problem with spread spectrum receivers in
that a receiver must somehow track the multipath components to
maintain code-phase lock of the receiver's despreading means with
the input signal. Prior art receivers generally track only one or
two of the multipath signals, but this method may not be
satisfactory because the combined group of low power multipath
signal components may actually contain far more power than the one
or two strongest multipath components. The prior art receivers
track and combine the strongest components to maintain a
predetermined Bit Error Rate (BER) of the receiver. Such a receiver
is described, for example, in U.S. Pat. No. 5,109,390 entitled
DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM by Gilhousen
et al. A receiver that combines all multipath components, however,
is able to maintain the desired BER with a signal power that is
lower than that of prior art systems because more signal power is
available to the receiver. Consequently, there is a need for a
spread spectrum communication system employing a receiver that
tracks substantially all of the multipath signal components, so
that substantially all multipath signals may be combined in the
receiver, and hence reduce the required transmit power of the
signal for a given BER.
Providing quality telecommunication services to user groups which
are classified as remote. Such as rural telephone systems and
telephone systems in underdeveloped countries, has proved to be a
challenge in recent years. These needs have been partially
satisfied by wireless radio services, such as fixed or mobile
frequency division multiplex (FDM), frequency division multiple
access (FDMA), time division multiplex (TDM), time division
multiple access (TDMA) systems, combination frequency and time
division systems (FD/TDMA), and other land mobile radio systems.
Usually, these remote services are faced with more potential users
than can be supported simultaneously by their frequency or spectral
bandwidth capacity.
The problems associated with the prior art systems focus around
reliable reception and synchronization of the receiver despreading
circuits to the received signal. Since spreading code sequences in
a communications system which supports a relatively large number of
users may be very long with a corresponding long code period, one
particular problem associated with prior spread spectrum receivers
is to rapidly determine the correct code phase of a received spread
spectrum signal. One solution of fast acquisition of the correct
spreading code phase is to form spreading code sequences with
specific characteristics which a receiver can derive from a
particular received code phase.
For example, prior art systems employ a method in which a code
generator produces a pseudorandom code of length N, divides the
code in half to generate two new codes with code period N/2, and
multiplies the data with each code for transmission over an
In-phase and Quadrature channel. The receiver only searches for the
occurrence of the short code period on the I or Q channel. The
advantage of the system is that the number of users supportable
with codes of length N can be transmitted with a bandwidth
necessary to support codes of length N/2. Such a system is
described in U.S. Pat. No. 5,442,662 entitled CODE-DIVISION
MULTIPLE-ACCESS COMMUNICATIONS SYSTEM PROVIDING ENHANCED CAPACITY
WITHIN LIMITED BANDWIDTH to Fakasawa et al. with is incorporated
herein by reference.
Another method and apparatus for producing a composite code for
fast acquisition in a CDMA system may employ a code that is made to
appear more complex by the use of one or more masking codes. The
composite code generator comprises a plurality of component code
generators. The composite codes are used to modulate in-phase and
quadrature channels. A receiver has enhanced speed of acquisition
because of the shorter time needed to search for composite codes in
the quadrature channel, and the plurality of component codes of the
in-phase channel are derived from the codes used in the quadrature
channel. Such a system is described in U.S. Pat. No. 5,022,049
entitled MULTIPLE ACCESS CODE ACQUISITION SYSTEM to Abrahamson et
al. which is incorporated herein by reference.
In related CDMA systems, a two-tier ciphering method ensures
security by cycling code masks. A pseudorandomly generated code key
is used to select one of a plurality of scrambling masks. A variant
of this method uses orthogonal code hopping or random code hopping.
A CDMA system can be viewed as encoding an information signal into
blocks of L code symbols, and each block is then encoded with a
scrambling mask of length L. A system of this type is described in
U.S. Pat. No. 5,353,352, entitled CALLING CHANNEL IN CDMA
COMMUNICATIONS SYSTEM to Dent et al. which is incorporated herein
by reference.
SUMMARY OF THE INVENTION
Rapid acquisition of the correct code phase by a spread-spectrum
receiver is improved by designing spreading codes which are faster
to detect. The present embodiment of the invention includes a new
method of generating code sequences that have rapid acquisition
properties by using one or more of the following methods. First, a
long code may be constructed from two or more short codes. The new
implementation uses many code sequences, one or more of which are
rapid acquisition sequences of length L that have average
acquisition phase searches r=log2L. Sequences with such properties
are well known to those practiced in the art. The average number of
acquisition test phases of the resulting long sequence is a
multiple of r=log2L rather than half of the number of phases of the
long sequence.
Second, a method of transmitting complex valued spreading code
sequences (In-phase (I) and Quadrature (Q) sequences) in a pilot
spreading code signal may be used rather than transmitting real
valued sequences. Two or more separate code sequences may be
transmitted over the complex channels. If the sequences have
different phases, an acquisition may be done by acquisition
circuits in parallel over the different code sequences when the
relative phase shift between the two or more code channels is
known. For example, one of two sequences may he sent on an In phase
(I) channel while the other is sent on the Quadrature (Q) channel.
To search the code sequences, the acquisition detection means
searches the two channels, but begins the (Q) channel with an
offset equal to one-half of the length of the spreading code
sequence. With a code sequence length of N, the acquisition means
starts the search at N/2 on the (Q) channel. The average number of
tests to find acquisition is N/2 for a single code search, but
searching the (I) and phase delayed (Q) channel in parallel reduces
the average number of tests to N/4. The codes sent on each channel
may be the same code, with the code phase in one channel being
delayed with respect to the other channel, or they may be different
code sequences.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a typical code division multiple
access communication system spreading code acquisition detector of
the prior art.
FIG. 2 is a block diagram of the spreading code acquisition
detector of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
In a CDMA communication system where they are a number of users,
each user's signal is coded using a unique code sequence.
Consequently, a receiver can detect the signal coming from a
particular user. The first step in establishing a communication
link with a user is to acquire the received spreading code phase.
Typically, this process includes determining the phase (shift) of
the observed sequence.
Communication is not possible until the proper spreading code phase
has been determined. The invention described here in a new method
of designing the code sequences such that a receiver can rapidly
determine the received code sequence phase.
Generally, in a spread spectrum communication system, the receiver
does not initially know the received spreading code phase. A
particular system may "guess" at a spreading code phase and attempt
to despread the received signal. If the despread signal is
despread, the receiver declares synchronization, but if the signal
is not despread, the receiver adjusts the locally generated code
phase to a new value ("guess") and repeats the test. An exemplary
acquisition system is shown in FIG. 1.
Referring to FIG. 1, the received signal r(t) is applied to a
multiplier 101 and multiplied by the locally generated code
sequence signal c(t) to attempt to despread the received signal
r(t). The signal produced after the despreading either corresponds
the received signal power plus noise power is the locally generated
code phase is synchorinzed to the received spreading code phase, or
corresponds to noise power only if the locally generated code phase
is not synchronized to the received spreading code phase. Since the
despread signal is a narrowband signal compared to the bandwidth of
the spreading code, the output signal of the despreader is applied
to the filter 102, which can be a bandpass or matched filter. The
output despread signal is applied to the energy detector 103, which
is used to measure the despread signal power.
The decision logic 104 compares the despread signal power to a
predetermined threshold value V.sub..tau. to decide whether the
locally generated code phase is synchronized to the received
signal. The decision logic provides a decision value to the control
logic which determines whether synchronization is achieved. If
there is synchronization, the search is stopped, but if
synchronization has not been achieved, the control logic (105)
adjusts the code phase of the locally generated signal c(t) by
sending the appropriate code phase adjustment signal s(t) to the
spreading code waveform generator (106).
The appropriate code phase adjustment signal s(t) is determined by
search technique implemented in the control logic (105). Existing
systems typically employ serial search techniques, which are well
known in the art. Using these techniques, each code phase is
searched one at a time in sequence. Other search techniques may be
used, such as a Z-search method by which each code phase before and
after a chosen code phase is searched alternatively, each test
increasing the phase shift of the tested code phase from the
initial chosen code phase. This technique is commonly used to
resynchronize a system which has temporarily lost code phase
synchronization.
the method of one embodiment of the present invention uses a
transmitted spreading code sequence (a long sequence) which is
generated using two short sequences. The long sequence (the new
code sequence) is formed by repeating one of the short sequences
according to a predetermined method defined by the second short
sequence. For example, if the first short sequences is 0110 and the
second short sequence is 1100, and if the predetermined method is
such that the first sequence is repeated as it is for each 1 in the
second sequence and inverted for each 0 in the second sequence, the
long sequence is 0110 0110 1001 1001. In another method the first
short sequence is repeated as it is when the bit value in the
second sequence does not change, and the first sequence is inverted
when the bit value in the second sequence changes from 1 to 0 or 0
to 1. In this example, the long sequence would be 0110 0110 1001
0110.
A specific embodiment of the applicant's invention uses maximal
length sequences (m-sequences). The m-sequences are generated using
shift register circuits as is well known in the art. These
sequences have the important property that if a shift register of
length r is used, the period of the m-sequence is N=2'-1, and so
r-bit portion of the m-sequence repeats in a period (each r bit
section occurs only once in a period). The implication of this
property is that, when the second short sequence described above is
an m-sequence, the acquisition circuit needs to search only
r=log.sub.2 N phases of the sequence instead of N phases, which
makes acquisition much faster.
For example, if the first short sequence has a length of 511 code
periods or chips, and the second short sequence (m-sequence) has a
length of 1023 code periods. Then the long sequence (the final
sequence is of length 511.times.1023=522753. Since 1023=2.sup.10
-1, the acquisition circuit will acquire the code in at most
511.times.10=5110 phases instead of 522753 phases. Therefore the
worst-case acquisition is over one hundred times faster.
It may be desirable for the short code phase to have boundaries
which are aligned with information symbols that are transmitted
through the channel. Because symbols are typically represented by
2.sup.n bits, symbol boundaries will occur on even-numbered bit
boundaries. As described above, the short code has a length of 511
code periods. In order for the short code to be aligned with symbol
boundaries when the symbols each include 2.sup.n bits, it may be
desirable to concatenate another bit, either 1 or 0 onto the
.[.551.]. .Iadd.511 .Iaddend.length first short sequence to form a
512 short code. In this instance, the length of the long sequence
would be 512.times.1023=523776 code periods. Alternatively, the
second short sequence may be extended to be an even number of code
periods. For example, if the second short sequence were extended to
1024 bits, the length of the long sequence would be
511.times.1024=523264 code periods.
To further decrease the acquisition time, one embodiment of the
invention transmits complex valued spreading code sequences
(In-phase (I) and Quadrature (Q) sequences) in a pilot spreading
code signal, rather than transmitting real valued sequences. Two or
more separate code sequences may be transmitted over the complex
channels. If there is a known phase shift between the codes, an
acquisition may be done in parallel over the different code
sequences.
In this embodiment, one sequence is used to modulate the .[.In
phase.]. .Iadd.In-phase .Iaddend.(I) carrier while the other phase
modulates the Quadrature (Q) carrier. To search the code sequences,
the acquisition detection means searches the two channels
simultaneously. If there is no phase shift between the two code
phases, the acquisition means begins the search on the (I) channel
at the beginning of the code sequence, but begins the (Q) channel
with an offset equal to one-half of the spreading code sequence
length. For this example, the acquisition means may search either
channel beginning at any particular phase, as long as the search of
the other channel begins by offsetting the search by a
predetermined code sub-period. For example, with a code sequence
length of N, the acquisition means start the search at N/2 on the
(Q) channel. The average number of tests to find acquisition is N/2
for a single code search, but searching the (I) and phase delayed
(Q) channel in parallel with an initial offset of N/2 code periods,
reduces the average number of tests to N/4. The codes sent on each
channel may be the same code, the same code sequence but delayed in
one channel, or different code sequences.
An exemplary embodiment of a receiver which uses the fast
acquisition sequences of the present invention is shown in FIG. 2.
The received signal r(t) is demodulated by the synchronous In-phase
demodulator 201 and by the synchronous Quadrature modulator 202 to
produce .[.in phase.]. .Iadd.in-phase .Iaddend.channel signal
r.sub.r (t) and quadrature channel signal r.sub.Q (t).
For the .[.in phase.]. .Iadd.in-phase .Iaddend.channel signal
r.sub.r (t), the locally generated code sequence begins searching
the received .[.in phase.]. .Iadd.in-phase .Iaddend.channel with
the long code spreading code sequence using a predetermined initial
code phase. After despreading in multiplier 203, the in-phase
signal is applied to a bandpass, envelope or matched filter 207 to
produce a despread. Next, the energy detector 209 generates a
measure of the signal power in the in-phase channels and applies
this measure to decision logic 211. The decision logic 211 compares
the despread signal energy with the predetermined threshold
V.sub..pi. with three possible outcomes. First, the measured energy
level may indicate that the code phase of the locally generated
despreading code sequence from the .[.quadrature.]. .Iadd.in-phase
.Iaddend.channel spreading code generator 205 corresponds to
acquisition of the correct code phase of the long code sequence. In
this instance, the control logic 215 provides long code
synchronization signals to spreading code generators 205 and 206 to
lock the code phase of the generator 205 and to adjust the
generator 206 to the offset code phase. Second, the measured energy
level may indicate that the locally generated code phase
corresponds to acquisition of the short code phase, in which case
the control logic 215 provides short code synchronization signals
to the spreading code generators 205 and 206, and initiates the
next series of tests. These tests adjust the locally generated code
sequence signal phases by the length of the short code instead of
by the period of one code sequence value until synchronization of
the long code is found. Third, the measured energy level may
indicate that the locally generated code phase does not correspond
to synchronization of either the long or short code, in which case
the control logic continues the serial search by adjusting the
phases of the locally generated code sequences by one code sequence
period for each successive test.
The system operates in the same way for the quadrature channel
signal r.sub.Q (t). The locally generated code sequence has a phase
which is offset by one-half of a code period of the locally
generated code sequence used to despread the in phase channel
signal r.sub.r (t). After despreading in multiplier 204, bandpass,
envelope or matched filtering in the filter 208, and measuring the
despread quadrature signal power in the energy detector 210, the
decision logic 212 compares the signal to a predetermined threshold
V.sub.rQ to determine one of three possibilities. First, whether
the code phase of the locally generated despreading code sequence
from the quadrature channel spreading code generator 206
corresponds to acquisition of the correct code phase of the long
code sequence, in which case the control logic 215 provides the
long code synchronization signals to spreading code generators 206
and 205 to lock and adjust their respective code phases. Second,
whether the locally generated code phase corresponds to acquisition
of the short code phase. As with the in-phase channel, in this
instance, the control logic 215 provides short code synchorinzation
signals to the spreading code generators 205 and 206, and performs
the next series of tests by adjusting the locally generated code
sequence signal phases by the length of the short .Iadd.code
.Iaddend.until synchronization of the long code is found. Third,
whether the locally generated code phase doe s not correspond to
synchronization of either the long or short code in which case the
control logic continues the serial search by adjusting the locally
generated code sequences phases by one code sequence period for
each successive test.
Further, the control logic 215 may adjust the threshold values
V.sub..pi. and V.sub.rQ to greater values when the short code is
detected on either the in-phase or quadrature channels to increase
the probability of detection and decrease probability of false
detection.
While the invention has been described in terms of an exemplary
embodiment, it is contemplated that it may be practiced as outlined
above with modifications that are within the scope of the following
claims.
* * * * *