Multipath equalization using taps derived from a parallel correlator

A communication system (10) includes a central terminal (12) and a subscriber terminal (14) that communicate information through an air interface (16). A receiver (22) suitable for deployment in either the central terminal (12) or the subscriber terminal (14) includes a parallel correlator (120), a weighting module (140), and a summer (180) that provide acquisition, equalization, and tracking functions.

TECHNICAL FIELD OF THE INVENTION 
This invention relates in general to communications systems and more 
particularly to multipath equalization using taps derived from a parallel 
correlator. 
BACKGROUND OF THE INVENTION 
A wireless communication system includes a transmitter for encoding or 
modulating user data for transmission over an air interface to a receiver. 
In addition to the direct or line-of-sight transmission, the receiver may 
also detect multipath transmissions caused by reflections from terrain 
features and man-made objects. For effective communication, the 
demodulator in the receiver resolves the additive combination of these 
delayed and attenuated versions of the direct transmission. The 
degradation of the transmitted signal due to multipath effects may 
severely limit the performance of a wireless communication system. With 
increased bandwidth requirements in communications and development of new 
and more complex modulation techniques, such as quadrature amplitude 
modulation (QAM), the reduction or elimination of multipath interference 
becomes more important. Directional antennae placed on the transmitter, 
receiver, or both may geometrically reduce the number of potential 
multipath transmissions between the transmitter and the receiver. Also, 
traditional tapped delay lines or rakes may perform some level of channel 
equalization to accurately recover the transmitted signal. However, many 
of these techniques require excess signal to noise levels to resolve 
multipath interference, which reduces the total available information 
bandwidth in a wireless communication system. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an equalization technique is 
provided that addresses problems associated with previously developed 
techniques. In particular, the present invention discloses a technique for 
multipath equalization using taps derived from a parallel correlator. 
In one embodiment of the present invention, a communications system 
includes a transmitter that generates a transmitted signal. A receiver 
receives the transmitted signal and includes a code module that generates 
a number of codes. A number of correlators are coupled to the code 
modulator and combine the transmitted signal and the codes to generate a 
number of correlator outputs. Each correlator has an associated code to 
generate an associated correlator output. A weighting module is coupled to 
the correlators and multiplies the correlator outputs by a number of 
weights to generate a number of tap values. Each correlator has an 
associated weight. A summer is coupled to the weighting module and sums 
the tap values to generate an estimate. 
Technical advantages of the present invention include an equalization 
technique that incorporates a parallel correlator having a number of 
correlators arranged in parallel. Specifically, each correlator combines a 
version of the transmitted signal with a correlation code to produce a 
correlator output. This correlator output is then multiplied by a weight 
associated with the correlator to generate a tap value. A summer combines 
the tap values from the parallel correlator to generate an estimate of the 
transmitted signal. 
Another important technical advantage of the present invention includes the 
adaptation of the parallel correlator to a spread spectrum communication 
system that includes a central terminal servicing a number of associated 
subscriber terminals. Receivers incorporating the equalization techniques 
of the present invention may reside at the central terminal and subscriber 
terminals. In a particular embodiment each receiver in such a system 
includes a number of correlators associated with codes that comprise at 
least a portion of a multiple access spreading sequence associated with 
the receiver. Each code reflects a different phase adjustment to the 
multiple access spreading sequence to provide a tapped delay line 
configuration. This equalization technique can process in-phase (I) and 
quadrature (Q) signals in a complex environment to support a variety of 
modulation techniques, including quadrature amplitude modulation (QAM). 
Still another technical advantage of the present invention includes the use 
of a parallel correlator to perform other functions in the receiver. For 
example, the parallel correlator may operate in a first mode to acquire 
the signal, and in a second mode to provide both channel equalization and 
tracking. In a particular embodiment, the parallel correlator includes 
many gates to reduce signal acquisition time, and implements traditional 
early, late, and on-time gates for signal tracking. Other technical 
advantages of the present invention are apparent to one skilled in the art 
in view of the attached figures, description, and claims.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a wireless communication system 10 that includes a 
central terminal 12 and a number of subscriber terminals 14 that 
communicate information over an air interface 16. Generally, central 
terminal 12 and subscriber terminals 14 include receivers having a 
parallel correlator that performs channel equalization to reduce the 
effects of multipath interference introduced by air interface 16. 
Central terminal 12 includes a receiver bank 20 having a number of 
individual receivers 22. Central terminal 12 also includes a transmitter 
module 24 that transmits signals to subscriber terminals 14. In a 
particular embodiment, each receiver 22 receives and processes signals 
from an assigned subscriber terminal 14, whereas transmitter 24 combines 
and transmits signals destined for many subscriber terminals 14 serviced 
by central terminal 12. Central terminal 12 may include receivers 22 and 
transmitter 24 in any number and arrangement of components to accomplish 
communication with subscriber terminals 14. 
An interface 26 couples to receiver bank 20 using receive line 28 and to 
transmitter 24 using transmit line 30. Interface 26 performs multiplexing 
or demultiplexing functions, data encoding or decoding functions, protocol 
conversions, device or network interfacing, or any other appropriate 
processing to communicate information between receive line 28, transmit 
line 30, and network line 32. Network line 32 may be any bidirectional 
communication link that communicates information between central terminal 
12 and other components of a communications network, such as the public 
switched telephone network (PSTN), other switched or dedicated networks, a 
local area network (LAN), a wide area network (WAN), or any other 
communication facilities. Communication system 10 supports the 
transmission of any form of information whether originally in analog or 
digital form, including voice, video, data, or other form of information. 
Control module 34 is coupled to receiver bank 20, transmitter 24, and 
interface 26. Control module 34 includes one or more processors or 
computers that execute program instructions to manage the overall 
operation of central terminal 12. Control module 34 may include a link 36 
to other central terminals 12 to provide control, management, diagnostic, 
and troubleshooting functions to a network of central terminals 12 
included in communication system 10. 
Each subscriber terminal 14 in communication system 10 includes a receiver 
22, similar in design and function as receivers 22 in receive bank 20 of 
central terminal 12. A transmitter 40 provides similar encoding, 
modulation, and transmission capabilities as transmitter 24 in central 
terminal 12, but transmits signals associated with subscriber terminal 14. 
An interface 42 performs multiplexing and demultiplexing functions, data 
encoding or decoding functions, protocol conversions, device or network 
interfacing, or any other appropriate processing to communicate 
information between transmit line 44, receive line 46, and subscriber line 
48. Subscriber line 48 is a bidirectional communication link between 
subscriber terminal 14 and equipment at the subscriber premises, such as a 
telephone handset, computer, or other form of communications equipment. A 
control module 50 is coupled to receiver 22, transmitter 40, and interface 
42 and includes one or more processors or computers that execute program 
instructions to manage the overall operation of subscriber terminal 14. 
Air interface 16 between central terminal 12 and its associated subscriber 
terminals 14 may include a number of natural features 60 and man-made 
objects 62 that cause multipath transmission of signals. For example, a 
transmitted signal from subscriber terminal 14 to an associated receiver 
22 at central terminal 12 may include a direct or line-of-sight 
transmission 64 as well as a multipath transmission 66 that reflects off 
of man-made object 62. Similarly, a transmitted signal transmitted from 
central terminal 12 to subscriber terminal 14 may include a direct 
transmission 68, as well as multipath transmissions 70 and 72 caused by 
reflection off of natural features 60. Generally, a transmitted signal 
passing through air interface 16 includes a direct transmission, a 
multipath transmission, or any combination of a direct transmission and 
one or more multipath transmissions. 
Receivers 22 at central terminal 12 and subscriber terminals 14 include an 
equalization function described below with reference to FIGS. 2-4 that 
reduces or eliminates the effect of multipath transmissions, thereby 
increasing the capacity, reliability, and performance of communication 
system 10. In a particular embodiment, antenna 38 of central terminal 12 
and antenna 52 of subscriber terminal 14 are directional to define a 
maximum cone or geometric spread of transmission or reception that can 
further reduce the potential multipath transmissions in air interface 16. 
In operation, central terminal 12 receives information to transmit to 
subscriber terminal 14 using network line 32. Interface 26 processes 
information received on network line 32 to place on transmit line 30 for 
delivery to transmitter 24. Transmitter 24 receives information destined 
for a number of subscriber terminals 14 and combines, encodes, modulates, 
mixes, and/or amplifies this information to generate a single composite 
signal for transmission using antenna 38. In a particular embodiment, the 
transmitted signal that includes information for subscriber terminals 14 
serviced by central terminal 12 arrives at subscriber terminal 14 along 
direct transmission 68 as well as multipath transmissions 70 and 72. 
Receiver 22 performs an equalization function to reduce or eliminate the 
effects of multipath transmissions 70 and 72 and retrieves user data in 
the transmitted signal associated with subscriber terminal 14. Receiver 22 
then passes user data to subscriber line 48 using receive line 46 and 
interface 42. 
Similarly, subscriber terminal 14 receives information to transmit to 
central terminal 12 using subscriber line 48. Interface 42 processes the 
information, if appropriate, and passes it to transmitter 40 using 
transmit line 44. Transmitter 40 encodes, modulates, mixes, and/or 
amplifies the signal for transmission using antenna 52. The transmitted 
signal arrives at receiver 22 in central terminal 12 associated with 
subscriber terminal 14 as a direct transmission 64 and multipath 
transmission 66. Receiver 22 performs an equalization function to reduce 
or eliminate the effects of multipath transmission 66 and passes user data 
to network line 32 using receive line 28 and interface 26. 
In a particular embodiment of the present invention, communication system 
10 communicates information in air interface 16 using code division 
multiple access (CDMA) technology. Each subscriber terminal 14 maintains a 
distinct code that allows central terminal 12 to communicate 
simultaneously with a number of subscriber terminals 14. The codes may 
include one or a combination of Walsh codes, Gold codes, pseudorandom 
noise (PN) codes, or other suitable sequences. The codes may also comprise 
or be combined with suitable spreading sequences, as well. 
Central terminal 12 transmits a composite signal that includes a number of 
different information signals, each information signal coded for a 
different subscriber terminal 14. Each subscriber terminal 14 receives the 
composite signal and extracts its associated information signal by 
combining its distinct code with the composite signal. Subscriber 
terminals 14 can simultaneously transmit to produce a composite signal at 
antenna 38 of central terminal 12. Each receiver 22 in receiver bank 20 
has a distinct code used by its associated subscriber terminal 14, and 
uses this code to extract information for communication to network line 32 
using receive line 28 and interface 26. 
FIG. 2 illustrates a block diagram of receiver 22 used in central terminal 
12 and subscriber terminals 14. Receiver 22 in central terminal 12 may 
differ in design or component structure from receiver 22 in subscriber 
terminal 14 due to cost, sizing, programmability, reliability, or other 
considerations. However, receiver 22 described below provides the 
functions and overall architecture applicable to either embodiment in 
central terminal 12 and subscriber terminal 14. 
Receiver 22 includes a radio frequency (RF) module 100 and an intermediate 
frequency (IF) module 102 that transforms transmitted signal 104 into 
in-phase (I) 106 and quadrature (Q) 108 baseband components. Receiver 22 
applies these components 106 and 108 to a correlator bank 120, a weighting 
module 140, and a summer 180 to produce estimates 182 (I) and 184 (Q) of 
transmitted signal 104. Receiver 22 also includes a code module 122, an 
error estimator 192, and a mapper 186 to transform estimates 182 and 184 
into user data 188. 
Baseband I 106 and baseband Q 108 are applied to a number of individual 
correlators 110, 112, 114, and 116 (generally referred to as correlators 
110) in parallel correlator 120. Code module 122 is coupled to parallel 
correlator 120, and provides different codes 124, 126, 128, and 130 
(referred to generally as codes 124) to respective correlators 110. In a 
particular embodiment, codes 124 comprise at least a portion of a multiple 
access sequence associated with receiver 22, and each code 124 reflects a 
different phase adjustment to the multiple access sequence. For example, 
code 124 may be shifted by a bit, chip, fraction of a bit or chip, or 
other suitable amount from code 126. Likewise, code 126 is shifted in 
relation to code 128, and code 128 is shifted in relation to code 130. 
Described in more detail with reference to FIG. 3, parallel correlator 120 
performs a phase adjusted correlation of baseband I 106 and baseband Q 
108. This correlation may be done using an appropriate technique, such as 
an integrate and dump, to produce a measure of the correlation between 
transmitted signal 104 and an associated code 124. In a particular 
embodiment, the correlation performed by parallel correlator 120 may be 
done on a symbol-by-symbol basis, where codes 124 may be the full length 
or a portion of the length of the symbol processed by receiver 22. 
An important technical advantage of the present invention is the use of 
parallel correlator 120 in a tapped delay line configuration. This 
configuration includes weighting module 140 having an adjust module 142 
and multipliers 144, 146, 148, and 150 (referred to generally as 
multipliers 144) to apply weights 154, 156, 158, and 160 (referred to 
generally as weights 154) to outputs of correlators 110. In a particular 
embodiment, multipliers 144 and weights 154 operate with complex values. 
Correlator 110 using code 124 generates an I correlator output 170 and a Q 
correlator output 172. Correlator outputs 170 and 172 are multiplied by 
complex weight 154 using complex multiplier 144 to generate an I tap value 
174 and a Q tap value 176. This correlation and weighting function is 
performed for all correlators 110 in parallel correlator 120 to produce 
tap values 174 and 176 for presentation to summer 180. 
Summer 180 sums all tap values 174 and 176 to produce an I estimate 182 and 
a Q estimate 184 for presentation to a mapper 186. Estimates 182 and 184 
represent an information signal extracted from transmitted signal 104 
using a multiple access sequence associated with receiver 22 and equalized 
by the tapped delay line configuration of parallel correlator 120, 
weighting module 140, and summer 180. 
Mapper 186 applies estimates 182 and 184 to a mapping function to generate 
user data 188. For example, a mapping function may comprise a quadrature 
amplitude modulation (QAM) constellation that translates the phase and 
magnitude of a received signal into a series of bits. Mapper 186 may 
employ any suitable frequency shift keying (FSK), phase shift keying 
(PSK), QAM or any combination of these modulation techniques to translate 
estimates 182 and 184 into user data 188. Moreover, mapper 186 may operate 
on estimates 182 and 184 directly as in-phase and quadrature components or 
translate estimates 182 and 184 into a phase and magnitude representation. 
User data 188 output from mapper 186 may be further processed using 
forward error correction (FEC) techniques, protocol conversions, or other 
digital bit stream processing technique. 
Parallel correlator 120 may operate in a first mode to acquire transmitted 
signal 104, and in a second mode to provide both channel equalization and 
tracking in receiver 22. In a particular embodiment, parallel correlator 
120 uses many correlators 110 to reduce signal acquisition time, and at 
least a portion of correlators 110 to implement traditional early, late, 
and on-time signal tracking. Summer 180 generates a tracking signal 190 to 
identify one or more correlators 110 most closely aligned with transmitted 
signal 104. The absence or reduced value of tracking signal 190 may 
indicate that receiver 22 has not acquired transmitted signal 104. In 
response to tracking signal 190, code module 122 may adjust phases of 
codes 124 provided to correlators 110 to track and center transmitted 
signal 104. Since parallel correlator 120 already includes numerous 
correlators 110, receiver 22 also provides quicker, parallel acquisition 
capabilities using the same components that provide the tracking and 
equalization function. Upon acquiring a signal as indicated by large 
correlator outputs 170 and 172, track signal 190 identifies the on-time or 
tracking correlator 110 to provide additional fine tuning in tracking and 
centering. Receiver 22 contemplates a variety of acquisition, 
equalization, and tracking functions performed simultaneously or in 
sequence by parallel correlator 120. 
Receiver 22 may also include an error estimator 192 that generates an error 
194 representing the estimated error of estimates 182 and 184 produced by 
summer 180. Adjust module 142 in weighting module 140 uses error 194 to 
adjust weights 154. In a particular example, receiver 22 initializes 
weights for an ideal or known channel response, and then adjusts the 
weights as needed using error estimator 192 and adjust module 142. The 
adjustment of weights 154 may be performed using any appropriate technique 
or algorithm, including but not limited to least mean squares (LMS), 
recursive least squares (RLS), or property restoral algorithms such as 
constant modulus or average modulus. 
FIG. 3 illustrates the processing sequence over time of receiver 22. The 
sequence illustrates a symbol 200 received in transmitted signal 104. 
Symbol 200 may comprise any arrangement or sequence of digital information 
presented to parallel correlator 120 in one or more components. Symbol 200 
includes a start point 202 and a stop point 204. A code length 206 
represents the full or partial length of symbol 200. Each correlator 110 
uses its associated code 124 of length 206 to correlate transmitted signal 
104. 
Phase-adjusted code sequences 208 directly below symbol 200 represent the 
different correlators 110 and associated codes 124 in parallel correlator 
120. For example, correlator 110 uses code 124 having a delay time 210 
with respect to start time 202 of symbol 200. Likewise, correlator 116 
uses code 130 having an advance time 212 with respect to start time 202 of 
symbol 200. In this example, correlator 114 using code 128 has a zero or 
near-zero phase adjustment with respect to start time 202, and represents 
the on-time or tracking correlator as represented by star 214. Parallel 
correlator 120 may include any number of correlators, preferably having a 
variety of delay times 210 and advance times 212 with reference to start 
time 202 of symbol 200. 
Towards the end of a prior symbol 216 in transmitted signal 104, the most 
advanced correlator 116 begins to process transmitted signal 104. After 
correlating over code length 206, correlator 116 provides a correlator 
output as indicated by arrow 220 which may include I correlator output 170 
and Q correlator output 172. Other correlators 110 process transmitted 
signal 104 in a similar manner, but at a different time depending upon 
their associated phase adjustment. Finally, the last processing correlator 
110 provides its correlator outputs 170 and 172, as indicated by arrow 
222, to complete a full set of outputs 224. 
Next, weighting function 140 and summer 180 produce estimates 182 and 184, 
as represented by arrow 230. If appropriate, error estimator 192 also 
produces error 194 as represented by arrow 232. To conclude the symbol 
processing sequence, adjust module 142 adjusts weights 154 in response to 
error 194, as represented by arrows 234. Processing of symbol 200 by 
receiver 22 occurs between process start time 236 and process end time 
238. The processing of a next symbol 240 begins after process stop time 
238 as indicated by the next round of phase-adjusted code sequences 242. 
FIG. 4 illustrates a flow chart of a method of operation of receiver 22 in 
communication system 10. The method begins at step 300 and 302 where 
receiver 22 initializes codes 124 maintained in code module 122 and 
weights 154 maintained in weighting module 140, respectively. Receiver 22 
next receives a portion of transmitted signal 104 such as symbol 200 at 
step 304, and converts transmitted signal 104 into baseband I 106 and 
baseband Q 108 at step 306 using RF module 100 and IF module 102. 
Receiver 22 next generates correlator outputs 170 and 172 at steps 308 to 
318. Receiver 22 selects first correlator 110 at step 308, and generates 
correlator outputs 170 and 172 at step 310. In one embodiment, correlator 
110 uses code 124 to perform an integrate and dump function on a portion 
of symbol 200. Receiver 22 then generates tap values 174 and 176 using 
weighting module 140 at step 312. For example, weighting module 140 may 
multiply correlator outputs 170 and 172 by weight 154 using complex 
multiplier 144. Summer 180 adds tap values 174 and 176 generated by 
weighting module 140 to the current value of estimates 182 and 184 at step 
314. If there are more correlators 110 to process transmitted signal 104 
at step 316, then the process selects the next correlator at step 318 and 
repeats steps 310 through 314 for each additional correlator 110 in 
parallel correlator 120. 
If appropriate, summer 180 generates track signal 190 in response to tap 
values 174 and 176 received from correlators 110 at step 320. If track 
signal 190 indicates that receiver 22 has not acquired transmitted signal 
104 at step 322, then code module 122 adjusts codes at step 324 and 
continues to step 304 to receive the next transmitted signal 104 or the 
next symbol 240 at step 304. If receiver 22 has acquired transmitted 
signal 104 at step 322 but not properly tracked or centered transmitted 
signal 104 in parallel correlator 120 at step 326, then code module 122 
may make further adjustments to codes 124 at step 328. Steps 320 to 328 
contemplate any suitable acquisition, tracking, or centering operation 
using parallel correlator 120. 
Summer 180 provides the final value of estimates 182 and 184 at step 330. 
Using estimates 182 and 184, error estimator 192 generates error 194 at 
step 332. Adjust module 142 selects the first weight 154 to adjust at step 
334, and adjusts weight 154 in response to error 194 at step 336. The 
adjustment of weights 154 at step 336 may be performed using any 
appropriate technique or algorithm, including but not limited to least 
mean squares (LMS), recursive least squares (RLS), or property restoral 
algorithms such as constant modulus or average modulus. If receiver 22 
needs to adjust more weights 154 at step 338, then the next weight is 
adjusted at step 340, and the process repeated. 
Before, after, or simultaneous with the adjustment of weights in steps 334 
to 340, mapper 186 applies a mapping function to estimates 182 and 184 at 
step 342. This mapping function may translate I and Q components of 
estimates 182 and 184 into user data 188, with or without an intermediate 
translation into phase and magnitude. If receiver 22 is not done 
processing transmitted signal 104 at step 344, the process continues to 
receive the next symbol 240 or transmitted signal 104 at step 304. 
Although the present invention has been described with several embodiments, 
a myriad of changes, variations, alterations, transformations, and 
modifications may be suggested that one skilled in the art, and it is 
intended that the present invention encompass such changes, variations, 
alterations, transformations, and modifications as fall within the spirit 
and scope of the appended claims.