Method and apparatus of adaptive maximum likelihood sequence estimation using filtered correlation synchronization

A method and apparatus are provided for maximum likelihood sequence estimation. The method and apparatus includes a first maximum likelihood sequence estimator signal path for flat fading and an at least second maximum likelihood sequence estimator signal path for other than flat fading. The method and apparatus for further includes selecting the signal path with a least relative magnitude mean square error.

FIELD OF THE INVENTION 
The field of the invention relates to decoding of radio signals and, in 
particular, to maximum likelihood sequence estimation. 
BACKGROUND OF THE INVENTION 
The effects of a radio channel upon a radio signal transmitted on the radio 
channel are well known. Well-known effects include poor signal quality due 
to low signal to noise ratio (SNR), adjacent and co-channel interference 
and multi-path propagation. Where extreme distance is a factor a poor SNR 
may be due to thermal noise. Where distance is slight a poor SNR may be 
due to competing signals on the same or an adjacent channel. 
Multi-path propagation, on the other hand, produces an effect on the signal 
characterized by multiple copies of the signal being presented to a 
receiver at slightly different times and with slightly different phases. 
In extreme cases multiple copies of a signal may arrive at a receiver 
offset over a time interval comparable to a symbol transmission rate. 
The problem of multi-path propagation results in a summation of signals 
being presented to a receiver that may bear little resemblance to the 
originally transmitted signal. Where either the transmitter or receiver is 
moving (e.g., a radiotelephone in an automobile) the problem of multi-path 
propagation may be further aggravated in that the effects on the signal 
may also vary with physical location. 
Past efforts to improve decoding of signals subject to low SNR and the 
effects of multi-path propagation have included adding a training 
(synchronization) sequence to the beginning of data transmission within a 
frame of information and cross-correlating the received signal against the 
known training sequence. The results of the cross-correlation are then 
used to characterize and compensate for the affects of the transmission 
channel. 
While characterizing the transmission channel is effective for short 
periods, such characterization may not be effective for frames having 
durations of several milliseconds. For frames of longer duration the 
transmitter and receiver may change physical locations thereby changing 
the transmission channel and altering transmission characteristics. 
Past efforts to improve performance under such conditions have included the 
systems discussed in IEEE Transactions On Information Theory, January 
1973, pgs. 120-124, F. R. Magee Jr. and J. G. Proakis: "Adaptive Maximum 
Likelihood Sequence Estimation for Digital Signaling in the Presence of 
Intersymbol Interference". The Magee and Proakis article teaches of a 
system having an adaptive filter used in conjunction with a viterbi 
decoder. The values of the adaptive filter are determined upon detection 
of a training sequence and subsequently modified based upon each new 
symbol output from the viterbi decoder. 
While the Magee and Proakis system has been effective, the effectiveness of 
the adaptive filter is dependent upon detection and timing of the training 
sequence. Where the training sequence is corrupted or subject to 
superposition of multiple copies of the training sequence then the 
effectiveness of the adaptive filter declines because of synchronization 
deficiencies and dispersion of signal energy. Because of the importance of 
maximum likelihood sequence estimators a need exists for a better method 
of synchronization with, and optimization of signal energy within the 
training sequence. 
SUMMARY OF THE INVENTION 
A method and means is provided for maximum likelihood sequence estimation. 
The method and means for includes a first maximum likelihood sequence 
estimator signal path for flat fading and an at least second maximum 
likelihood sequence estimator signal path for other than flat fading. The 
method and means further includes selecting the signal path with a least 
relative magnitude mean square error.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The solution to the problem of synchronization of maximum likelihood 
sequence estimators lies, conceptually, in the use of filtered correlation 
synchronization. Filtered correlation synchronization provides a method of 
optimizing timing of decoding of a channel over a variety of time-delayed 
signal conditions. 
FIG. 8 illustrates the method of adaptive maximum likelihood sequence 
estimation according to a preferred embodiment of the present invention. 
Reference may be had to FIG. 8 throughout the following detailed 
description of the preferred embodiments. 
The overall communications channel can be viewed as a convolutional encoder 
that convolves the information data with a set of time varying channel 
coefficients (h.sub.i). The channel can therefore be viewed as imposing a 
fixed pattern (in the short term) on the transmitted data. The resultant 
signal pattern is further corrupted by additive white Gaussian noise. The 
decoder must determine which data sequence, when convolved with the 
channel coefficients, produces a pattern which is most likely to be close 
to the received pattern. If the transmitted data consists of N symbols 
then there are M.sup.N possible data sequences, with each considered 
equally likely (M is a number of possible symbols in a constellation of 
symbols). 
Given that the constellation of possible data sequences (a(i))include 
values from i=1, . . . , K=M.sup.N, a maximum likelihood sequence 
estimator (MLSE) chooses a sequence a(m) as the most likely if the 
expression, P(r.vertline.a(m))&gt;P(r.vertline.a(i)), is true for the chosen 
sequence (a(m)) over all other possible sequences. Such a determination is 
based upon a minimal total error of the chosen sequence over all other 
sequences (minimal Euclidean distance through a viterbi trellis). 
FIG. 1 is a block diagram of a radio transmission system (10). A 
transmitter (11) generates digital symbols s(n) from digital data and 
transmits such symbols for the benefit of a receiver (12). The signal 
received at the receiver (12) is filtered and sampled to produce a 
received digital signal y(j) which is sent to a channel equalizer (13). 
The equalizer (13) delivers, with a certain time delay, estimated signals 
s(j-L), which constitute an estimation of the transmitted signals s(n). 
(The designation (j) denotes a sampling timepoint and the designation 
(j-L) indicates that the estimated symbols are delayed by L sampling 
intervals. 
The double signal paths shown in FIG. 1 indicate that the channel between 
the transmitter (11) and receiver introduces a time dispersion into the 
signal received at the receiver (12). Shown in FIG. 1 is a signal "A" 
which indicates a disturbance signal on the same channel as that used 
between transmitter (11) and receiver (12). Fading and noise also disturbs 
the transmission. 
The radio transmission system (10) is time sharing with separate time slots 
1 to j in accordance with FIG. 2 (T.sub.o indicates time). A signal 
sequence (SS) includes a synchronizing sequence (SO) and a data sequence 
(DO) within each time slot "f". The signal sequence (SS) contains binary 
signals encoded, for instance, under a quadrature phase shift keying 
(QPSK) format. 
FIG. 3 is a block diagram of an adaptive maximum likelihood sequence 
estimator (AMLSE) (14) in accordance with one embodiment of the invention. 
Within the AMLSE (14) the synchronization (training) sequence (which 
includes the SO field and some symbols on either side of this field due to 
timing uncertainty) of a received signal y(j) is correlated with a stored 
copy of the synchronization word within a synch word correlator (21) (101, 
FIG. 8). This is done to provide a correlated output sequence, an initial 
channel estimate h.sub.o, and a detected (synch point). The detected 
synchronization point is used within a decimator (20) to decimate the 
oversampled received signal y(j) to an information bandwidth consistent 
with the transmitted signal. 
The viterbi decoder (22), processing the decimated signal, may by 
functionally equivalent to the viterbi equalizer described in the 
aforesaid article by F. R. Magee, Jr. and J. G. Proakis. The viterbi 
decoder (22) (102, FIG. 8) receives the decimated signal and delivers the 
estimated symbols yHD(j-D), which are estimated in a known manner with the 
delay of D sampling steps, to a least mean square (LMS) channel estimator 
(25). The LMS channel estimator (25) receives the estimated signals 
yHD(j-D) and filters them with a filter representing the current estimate 
of the channel impulse response in order to regenerate or estimate the 
channel impaired signal (y(j-D)). An error signal e(j-D) is generated 
based on the difference (24) between the decimated signal (y(j-D)) and the 
estimated received signal (y(j-D)). The error signal (e(j-D) generated by 
the difference is returned (dotted line 27) to the LMS channel estimator 
(25) and is used to update a current channel impulse response estimate 
(channel estimate). 
Upon determination of a current channel estimate (h(j-D)) (based either 
upon an initial channel estimate (h.sub.o) or upon an update through use 
of feedback error (e(j-D)) a channel prediction estimate (h(j)) is 
determined within a channel predictor (26). The channel prediction 
estimate (h(j)) is determined based upon changes in the current channel 
estimate over previous values and upon trends in the current channel 
estimate. 
Performance of the AMLSE (14) is optimized when the decimated signal of the 
signal data y(j) are sampled near their maximum signal to noise ratio 
(SNR) and when the current channel estimate is a close reflection of 
actual channel conditions. The accuracy of the current channel estimate is 
closely related to a selected synchronization point. 
The selection of a synchronization point, on the other hand, is complicated 
by delay spread of the sampled signal (y(j)). Delay spread, under one 
embodiment of the invention, is accommodated through use of a number of 
delay spread sensitive filters (e.g., delay spread detection (DSD) 
filters) and selection of the delay spread sensitive filter providing the 
largest filter peak. A set of synchronization point location (SPL) filter 
coefficients are selected based upon the identity of the selected DSD 
filter. Application of the SPL filter coefficients to the correlated 
output provides a synchronization point and initial channel estimate that 
optimizes AMLSE performance within a varying delay spread environment 
provided by the sampled signal (y(j)). 
By way of example, FIG. 4 is a block diagram of a sync word correlator 
(21), in accordance with one embodiment of the invention, using two DSD 
filters and assuming a sampling rate (T.sub.s) of 8 samples per symbol 
interval (T). Of the two DSD filters a first DSD filter (31) has indicated 
filter values ((1, 0, 0, 0, 0, 0, 0, 0, 1)/2) for a medium, to large, 
delay spread (for other than flat fading). A second DSD filter (32) has 
indicated filter values ((2, 0, 0, 0, 1, 0, 0, 0, 2)/5) for zero to medium 
delay spread (for flat fading). As above described where the first DSD 
filter provides the largest DSD detection filter peak, the first DSD 
filter (31) is selected and where the second DSD filter (32) provides the 
largest DSD detection filter peak, the second DSD filter (32) is selected. 
Coefficients for the SPL filter (33) where the first DSD filter (31) is 
selected are as follows: (1, 0, 0, 0, 0, 0, 0, 0, 1). Coefficients for the 
SPL filter (33) where the second DSD filter is selected is as follows: 
(32, 16, 4, 0, 0, 0, 4, 16, 32). 
Within the sync word correlator (21) a sampled data synchronization field 
(which includes the SO field and some symbols on either side of this field 
due to timing uncertainty) is cross-correlated with a stored 
synchronization word to provide a correlated output (c(n)). The correlated 
output (c(n)) is filtered using the medium-large DSD filter (31) and the 
small-medium DSD (32) (104, FIG. 8) filter. The magnitude of the outputs 
of each filter (the delay spread correlation peaks) are then compared. 
Based upon the identify of the largest delay spread correlation peak a set 
of coefficients for the SPL filter (33) (105, FIG. 8) are selected. The 
selected coefficients are then applied to the correlated output (c(n)) to 
provide a synchronization point and initial channel response (34) based on 
the location of the peak value of the filter output (sync point). The 
cross correlation complex value at the sync point and the complex value 
T/T.sub.s samples, on either side of the sync point represent the initial 
channel impulse response T-spaced tap estimates. 
FIG. 5 is an example of an AMLSE using two parallel processing paths in 
accordance with another embodiment of the invention. The first processing 
path (41, 43, and 45) is functionally equivalent to the above described 
AMLSE (14, FIGS. 3 and 4) with block 41 corresponding to block 21, block 
43 corresponding to block 20 and block 45 corresponding to blocks 22, 23, 
24, 25, and 26. Within the second processing path (48, 42, and 44), block 
42 is functionally equivalent to block 20 and block 44 corresponds to 
blocks 22, 23, 24, 25, and 26. 
In the second processing path (48, 42, and 44) the synch word correlator 
(21) is replaced with a max peak correlator (48). Contained within the max 
peak correlator (48) is a sync word correlator (50) (FIG. 6) and an 
alternate type of delay spread sensitive filter (SPL filter (51)). The 
filter coefficients of the SPL filter (48) have been selected (with tap 
values of (1, 0, 0, 0, 0, 0, 0, 0, 0)) to provide a maximum output upon 
conditions of flat delay. 
In operation the max peak correlator (48) correlates the sampled data 
synchronization field with the stored synchronization word to provide a 
correlated output. The correlated output (103, FIG. 8) is then convolved 
with the SPL filter (51) to provide a synchronization point and an initial 
channel response (106, FIG. 8). The synchronization point is then used as 
described above to decimate the data synchronization field. The decimated 
data synchronization field is then subjected to maximum likelihood 
decoding as described above using the initial channel response (106, FIG. 
8). 
Under condition of flat fading it has been determined that an SPL filter 
(51) used in conjunction with a sync word correlator (50) provide superior 
results. When the SPL filter (51) is used in the form of a second 
processing path with the above described AMLSE (14) the combination 
further improves overall bit error rate (BER) within a communication 
system. 
The outputs of the two signaling paths (FIG. 5) are supplied to a bit 
decoder (46) and an AMLSE switch control (47). The AMLSE switch control 
(47) compares mean square error estimates of each signal path and selects 
the path providing the least error. Upon selection of the signal path the 
AMLSE switch control (47) activates the bit decoder (46) to decode the 
signal from the path providing the least error (108, FIG. 8). 
In another embodiment of the invention a constrained search window (63) 
(FIG. 7) is used within the sync word correlator (41) to further improve 
the performance of the AMLSE (40). Under such an embodiment a delay spread 
correlation peak is selected by repeated filtering and a constrained 
window defined by a range ahead of and after a synchronization point. The 
range ahead of the initial synchronization point is selected to have an 
integral number of sample intervals (e.g., one sample interval). The range 
after the initial synchronization point is selected as having a time value 
commensurate with the duration of the channel impulse response due to the 
expected worst case delay spread and relative to the initial 
synchronization point (Given the delay spread models defined by the 
EIA/TIA TR45.3 committee for the U.S. TDMA digital cellular system, the 
range would be from the initial synchronization point to a point occurring 
T/2 seconds later, where T is the U.S. TDMA symbol interval). 
The delay spread correlation peak is determined by DSD filtering (61) the 
correlated output sequence of the sync word correlator (60) with a DSD 
filter value (1, 1, 1, 1, 1, 1, 1, 1, 1) designed to provide a general 
location of a synchronization point. The location or initial 
synchronization point indicated by the peak output of the DSD filter (61) 
is then averaged with previous initial synchronization points, 
corresponding to previous time slots, using an infinite impulse response 
(IIR) filter (62) for more precise determination of the synchronization 
point. 
Following determination of a constrained window (63) a signal within the 
constrained window is subject to a synchronization point location filter 
(e.g., with tap values of (32, 16, 4, 0, 0, 0, 4, 16, 32)) to define a 
synchronization point and initial channel response through the second 
signal path. A maximum likelihood sequence estimation hypothesis is then 
determined as described above using the calculated synchronization point 
and initial channel response values. Determination of the hypothesis with 
the lowest BER is as above wherein the AMLSE switch control (47) selects 
the hypothesis with the lowest level of mean square error. 
In another embodiment of the invention the constrained search window and 
more precisely determined synchronization point is used under a previous 
embodiment as an input to DSD filters (31 and 32), and SPL filter (33) 
within the second maximum likelihood sequence estimator signal processing 
path. Under such an embodiment the first maximum likelihood sequence 
estimator signal processing path (through use of the max peak correlator 
(48)) provides improved BER performance for flat fading. The second 
maximum likelihood sequence estimator signal processing path provides 
improved BER performance for other than flat fading. 
The many features and advantages of this invention are apparent from the 
detailed specification and thus it is intended by the appended claims to 
cover all such features and advantages of the system which fall within the 
true spirit and scope of the invention. Further, since numerous 
modifications and changes will readily occur to those skilled in the art, 
it is not desired to limit the invention to the exact construction and 
operation illustrated and described, and accordingly all suitable 
modifications and equivalents may be resorted to, falling within the scope 
of the invention. 
It is, of course, to be understood that the present invention is, by no 
means, limited to the specific showing in the drawing, but also comprises 
any modification within the scope of the appended claims.