Low complexity adaptive equalizer for U.S. digital cellular radio receivers

A method of equalizing and demodulating a transmitted signal having synchronization ("sync") s.sub.pre [n] symbols and message symbols s.sub.sp [n] selected from even and odd .pi./4-shifted differential quadrature phase shift keyed (DQPSK) symbol constellation points {S}, {S}, into decoded symbols at a receive unit comprising the steps of:

• a) receiving the transmitted signal;
• b) synchronizing and sampling the transmitted signal into received samples r[n], where n is an integer, and separating the received data r[n] into sequences of sync samples r.sub.pre [n] and received message samples r.sub.sp [n];
• c) selecting a portion of the received message samples r.sub.sp [n] for decoding;
• d) determining initial channel impulse response (CIR) coefficients from the sync samples, r.sub.pre [n];
• e) determining initial sets of estimated reference constellation points {Z.sup.ref } and {Z.sup.ref } for even and odd constellations respectively, from the initial CIR coefficients and the predetermined set of sync symbols s.sub.pre [n];
• f) rotating {Z.sup.ref } and {Z.sup.ref } to result in a full set of estimated symbol constellation points {Z} and {Z} which account for changes through the channel and intersymbol interference;
• g) determining a most likely path of state transitions over a plurality of time period stages through a trellis, corresponding to the selected portions of the received message samples r.sub.sp [n] where each possible state transition has been preassigned to one of the .pi./4-shifted DQPSK symbol constellation point, {S} or {S}, for even and odd stages, respectively;
• h) tracing back the state transitions, and the corresponding symbol constellation points of the most likely path being the decoded symbols;
• i) repeating steps "c" through "h" for subsequent decoding.