Design of optimal multilevel coded modulation schemes for speech services

This paper considers the requirements of a coded modulation system for the provision of different services within a third generation mobile communications system. In particular it focuses on the BER requirements for speech services, and points out that conventional coded modulation schemes are not optimised for these BER requirements, around 10^-3 . The structure of codes optimised for such BER requirements are discussed, and it is shown that it can be achieved using multilevel coded modulation (MCM). The principles for the design of optimal codes are given, namely that the BER due to the component codes should be equalised. New MCM schemes based on block codes are devised, giving advantages over conventional block coded modulation (BCM) schemes of more than 1 dB, leading to a coding gain of 2.2 dB for a length 31 scheme. Codes for a Rayleigh fading channel are also described, giving improvements over previously-described codes of up to 2.2 dB