A multimode and variable-rate voice communications system with source-matched error protection for mobile communications

This paper presents a performance evaluation of the multimode Fast Variable Rate CELP coder developed by CSELT S.p.A. in the UMTS Code Division Testbe d (CODIT) Project [1] for applications involving channels with time-varying error characteristics. The radio channel is statistically modelled by one Rician or Rayleigh-distributed desired signal and several uncorrelated Rayl eigh plus log-normally shadowed interfering signals, propagating according to two different laws: a dual path loss law with a turning point (microcell ular environment) and a single path loss law (macrocellular environment). F or channels with these error characteristics, each mode of the Fast Variabl e Rate CELP coder has a particular allocation of bits between source and ch annel coding stages, leading to an unequal error protection code tailored t o suit the different error sensitivities of the source bits. It is shown th at, on the basis of the sensitivity evaluation of source bits to errors, ac cording to two different error sensitivity measures, Forward Error Correcti on whose power is matched to the error sensitivity of the source bits can b e used. Increased robustness of source bits to errors can thus be obtained while satisfying bandwidth requirements.