QPSK block-modulation codes for unequal error protection

Unequal error protection (UEP) codes find applications in broadcast cbannels, as well as in otber digital communication systems, where messages bave ditTerent degrees of importance, In this correspon­ dence, binary linear UEP (LUEP) codes combined with a Gray mapped QPSK signal set are used to obtain new efficient QPSK block-modulation codes for unequal error protection. Several examples of QPSK modulation codes that have tbe same minimum squared Euclidean distance as the best QPSK modulation codes, of the same rate and length, are given. In the new constructions of QPSK block-modulation codes, even-length binary LUEP codes are used. Good even-length binary LUEP codes are obtained wben shorter binary linear codes are combined using either the well-known lulu + vi-construction or the so-called construction X, Both constructions have the advantage of resulting in optimal or near-optimal binary LUEP codes of short to moderate lengths, using very simple linear codes, and may be used as constituent codes in the new constructions. LUEP codes lend themselves quite naturally to mUltistage decodings up to tbeir minimum distance, using the decodings of component subcodes. A new suboptimal two-stage soft-decision decoding of LUEP codes is presented and its application to QPSK block-modulation codes for UEP illustrated.