Adaptive-type and -rate modulation and channel coding tradeoffs for speech communication

The objective of this paper is to show the effect on speech quality by examining the tradeoff between the coding gain, provided by channel coding, and modulation adjustments, such as rate- and type-changes. Two basic types of systems are utilized to examine the performance tradeoffs. The first system performs a tradeoff between the voice codec and channel codec bit rates, but with a fixed symbol rate modulation (FRM). The second system is represented by a similar adaptive-rate voice codec as in the first system, but with fixed-rate channel coding and adaptive-rate modulation (ARM). Within both the FRM and ARM systems, we also examine the effect of changing modulation type. For this study, power-efficient and bandwidth-efficient modulation signaling sets are considered. For the power-efficient modulation, we utilized M-ary pulse position modulation (PPM) types. M-ary quadrature amplitude modulation (QAM) types were utilized for the bandwidth efficient modulations. In addition, orthogonal frequency division multiplexing (OFDM) was utilized as the method to format and multiplex the digital data at baseband. Simulation results show, for higher order modulations, that the ARM and FRM approaches, respectively, work best for power-efficient and bandwidth-efficient modulations