Fully adaptive modem receiver using whitening matched filtering

• US 5,031,195 A
• Filed: 06/05/1989
• Issued: 07/09/1991
• Est. Priority Date: 06/05/1989
• Status: Expired due to Fees

• Current Assignee: Lenovo Singapore Pte Limited (Lenovo Group Ltd.)
• Original Assignee: International Business Machines Corporation
• Inventors: Eleftheriou, Evangelos, Chevillat, Pierre R.
• Primary Examiner(s): Safourek, Benedict V.
• Assistant Examiner(s): Bocure, Tesfaldet
• Application Number:
  US07/361,174
• Time in Patent Office: 764 Days
• Field of Search: 375/8, 375/34, 375/13, 375/14, 375/99, 375/11, 375/57, 364/724.2
• US Class Current: 375/234
• CPC Class Codes: H03H 21/0018 Matched filters
• CPC Class Codes: H04L 25/03044 using fractionally spaced delay lines or combinations of fractionally integrally spaced taps

Abstract

Fully adaptive modem receiver for data-transmission systems which use trellis-coded modulation (TCM) and comprises an adaptive whitened-matched filter (WMF) and a reduced-state trellis decoder. The WMF consists of an adaptive linear equalizer with fractional-T spaced coefficients, and an adaptive linear predictor. The decoder combines the functions of equalization and TCM decoding. It employs combined intersymbol interference (ISI) and code states which exploit the set-partitioning structure of the underlying TCM code to provide full or reduced-state information about past ISI terms. The decoder branch metric cancels those ISI terms that are not, or are only partially, represented by the trellis states.

Claims

1. A fully adaptive modem receiver having a receiver-filter providing an output signal r(t) in response to an input signal received from a telephone line and, comprising:

a whitened matched filter having a fractional-tap spacing equalizer, said fractional tap-spacing equalizer receiving as inputs sample values obtained by sampling said receiver-filter output signal r(t) at rate k/T, where T denotes the symbol interval and k>

1 is a small integer, each tap having a coefficient C_i, i being an integer in the range from 0 to Nk-1 where N is the length of the equalizer delay line in symbol intervals, said equalizer providing an equalizer output value at each symbol interval n;

an adaptive carrier-phase tracking means connected to the output of said equalizer for providing a phase estimate output signal φ

(n) for compensating for the carrier-phase error in said equalizer output signal, said error resulting from phase jitter and frequency offset, and also providing a carrier-phase error compensated output signal value V_n in each symbol interval n,

a linear predictor receiving the output signal values V_n from said adaptive carrier-phase tracking means and having coefficients P_i where i is an integer in the range from 1 to L, said linear predictor providing, in response to said output signal values V_n for L symbol intervals, a predictor output value according to the relation ##EQU16## and an adder for adding at each symbol interval n said predictor output value to said value V_n and providing a resulting output signal Z_n, whereby the noise present in said output signal Z_n is whitened regardless of whether the additive channel noise is correlated or not, and whereby said output signal contains intersymbol interference;

a sequence decoder having ISI cancellation means for determining the symbol sequence closest in Euclidian distance to the samples of said resulting output signal Z_n provided at the input of said decoder after subtracting ISI estimates provided by said ISI cancellation means, and providing a zero-delay tentative decision a_n in each symbol interval n, and also providing in each symbol interval a final decision a_n-D with a delay of D symbol intervals as the final result; and

an ISI coefficient estimator having also said coefficients P_i providing in response to the zero-delay tentative decision a_n-1 taken at the symbol interval n-i where i is an integer in the range from 1 to L an ISI coefficient estimator output value according to the relation ##EQU17## and adapting means which, in response to said ISI estimator output value, said sequence decoder input signal Z_n, and said zero-delay tentative decision from said sequence decoder, provides an error signal e_n for updating the coefficients of said linear predictor; and

adjusting means which, in response to said carrier-phase error compensated signal V_n and to said zero-delay tentative decision from said sequence decoder, provides an error signal e'"'"'_n for adjusting the coefficients of said franctional-tap spacing equalizer.

2. A modem receiver according to claim 1, wherein:

said adapting means provides a prediction error estimate signal e_n according to the relation ##EQU18## for updating the coefficients P_i.

3. A modem receiver according to claim 1, wherein:

said adjusting means provide an error signal e'"'"'_n according to the relation ##EQU19## for adjusting the coefficients of said fractional-tap spacing equalizer.

4. A modem receiver according to claim 1 or 2 or 3, wherein:

said input signal received from a telephone line originates from a remote transmitting system employing a Trellis-Coded Modulation (TCM) encoder and said sequence decoder is a Viterbi decoder which performs maximum likelihood sequence decoding for determining the coded data bit sequence closest in Euclidian distance to samples at the input of said decoder.

5. A modem receiver according to claim 4, wherein:

at the symbol interval n+1 the coefficient C_i of said franctional-tap spacing equalizer, the coefficient P_i of said linear predictor, and the phase estimate φ

to compensate for said carrier-phase error, are produced as a function of said input sample values, said prediction error e said error signal e'"'"' said estimate φ

, and said zero-delay tentative decision signal a_n, at the symbol interval n are such that ##EQU20## with α

, β

, γ

being adaptation parameters.

6. A modem receiver according to claim 5 wherein:

said sequence decoder is a reduced-state decoder operating on a sub-trellis which is built by employing channel-truncation to K terms with 0≦

K≦

L, and enabling the intersymbol interference (ISI) to be cancelled by said ISI-cancellation means by subtracting at the instant n, ISI terms that are not or are only partially represented by the reduced combined states λ

_n^K of the Viterbi algorithm.

7. A modem receiver according to claim 6, wherein:

said reduced-state decoder utilizes a subset-state trellis which retains m_i bits of information for each ISI terms with 1≦

i≦

K, said decoder comprising;

intersymbol interference estimating means for providing ISI estimates U_n (λ

_n^K) for all trellis states as a function of symbol estimates a_n (λ

_n^K) according to the relation ##EQU21## and subset decoder means for determining, for each of the subsets originating from states λ

_n^K, the signal point a which is closest in Euclidian distance to Z_n -U_n λ

_n^K), and trellis path means for determining the best path in the trellis resulting in an optimum state λ

^opt, by using the survivor-path metric computation; and

path decoder means responsive to said state λ

^opt, for determining a final decision a_n-D, where D denotes the decoding delay.

8. A modem receiver according to claim 7, wherein:

said subset decoder means determines, for every successor state λ

_n+1^K of state λ

_n^K, and for all 2^m.sbsp.1 subsets leading to that state, the branch metric according to the relation

space="preserve" listing-type="equation">Δ

(λ

.sub.n.sup.K, λ

.sub.n+1.sup.K)=min |Z.sub.n -U.sub.n (λ

.sub.n.sup.K)-a.sub.n |.sup.2

where the minimum is taken over the 2^m-m.sbsp.1 signal points (a_n) in the corresponding subset.

9. A modem receiver according to claim 8, wherein:

said trellis path means determines the best path in a trellis having

S.2^m.sbsp.1⁺. . . m K where S is the number of said trellis-coded-modulation states of encoder which has transmitted the signals,

and 2^m.sbsp.1 transitions per state using the survivor-path metric computation.

10. A modem receiver according to claim 9, wherein:

said path decoder means, further to determining a final decision a_n-D, also provides said zero-delay tentative decision a_n which is the symbol estimate corresponding to said optimum state at the instant n.