Adaptive MLSE-VA receiver for digital cellular radio

A method for demodulating a time division multiple access (TDMA) signal having a plurality of symbols, said signal comprising at least a synchronizing portion and a data portion within a time slot, comprising the steps of:

• receiving the signal over a channel;
• filtering the received signal with fixed transversal filter means having a characteristic selected for matching to a fixed signal pulse characteristic of the received signal, the step of filtering the received signal including matching a filter to a square root raised cosine pulse characteristic of the received signal;
• and
• adaptively filtering the filtered signal with adaptive filter means in order to compensate for a time varying impulse response of the channel.

A method for demodulating a time division multiple access (TDMA) RF signal having a plurality of symbols, comprising at least a synchronizing portion and a data portion of a time slot, said method comprising the steps of:

• receiving the RF signal over a channel;
• filtering the received signal with fixed transversal filter means having a characteristic selected for matching a square root raised cosine pulse characteristic of the received RF signal;
• and
• adaptively filtering the filtered signal with adaptive filter means in order to compensate for a time varying impulse response of the channel, the step of adaptively filtering including the steps of,
• initially adapting the adaptive filter means during the synchronizing portion of the filtered signal in accordance with a recursive least squares algorithm;
• and
• subsequently adapting the adaptive filter means during the data portion of the filtered signal in accordance with a normalized least mean square (NLMS) algorithm.

A method for demodulating a time division multiple access (TDMA) RF signal having a plurality of symbols and at least a synchronizing portion and a data portion of a time slot, comprising the steps of:

• (a) receiving the RF signal over a channel;
• (b) determining a channel impulse response of the received signal over the synchronizing portion;
• (c) determining signal elements s(-1), S(0), s(1) through autocorrelation, S being defined by ##EQU11## where k represents the index of summation in said autocorrelation operation, N represents the number of samples of the channel impulse response, h(k) represents the estimate of the channel impulse response h(k), and * denotes conjugate;
• (d) employing a normalized least mean square algorithm for adapting the channel impulse response {h(n)};
• (e) operating a maximum likelihood sequence estimator over 2D symbols, said maximum likelihood sequence estimator being implemented by a modified Viterbi algorithm of decision depth D;
• (f) releasing D symbols;
• (g) employing the released D symbols to update the channel impulse response;
• and
• (h) beginning with a Dth symbol, repeating steps (e), (f) and (g) until a penultimate data block having 2D symbols is received, or, if 2D is not an integral multiple of a number of symbols N.sub.sym, appending {N.sub.sym -(N.sub.sym /2D 2D} symbols to a last block of 2D symbols.

In a digital cellular radio receiver having a maximum likelihood sequence estimator demodulator for receiving signal elements, represented by {s(1)}, and determining a value for said signal elements from an estimated channel impulse response, represented by

• h(n)=[h.sub.n (1)h.sub.n (2) . . . h.sub.n (N)],
• wherein N denotes the number of samples of the channel impulse response, said demodulator including a channel adaption circuit, a matched filter including an adaptive filter matched to the channel impulse response, means coupling the channel adaption circuit to the adaptive filter, a modified Viterbi algorithm generator responsive to said adaptive filter, and a signal reconstruction circuit responsive to said channel adaptation circuit and said modified Viterbi algorithm generator, a method of determining a value for said signal elements comprising the steps of:
• (a) determining in said channel adaptation circuit a value of a first factor .sigma.(n) in accordance with .sigma.(n)=.gamma..sigma.(n-1)+1, with an initial value of .sigma.(0)=0 and .gamma. chosen to be a value between 0.9-0.95;
• (b) determining in said channel adaptation circuit a vector of smoothed channel impulse response coefficients in accordance with the expression ##EQU12## where h.sub.s (n) is the vector of smoothed channel impulse response coefficients denoted by [h.sub.sn (1), h.sub.sn (2) .sub.. . . h.sub.sn (N)], the subscript sn denoting a smoothed estimate at time n;
• nd
• (c) determining in said signal reconstruction circuit a value for the signal elements in accordance with the expression:
• ##EQU13##