Power Optimal Signaling for Fading Multi-Access Channel in Presence of Coding Gap

In a multi-access fading channel, dynamic allocation of bandwidth, transmission power and rates is an important aspect to counter the detrimental effect of time-varying nature of the channel. Most of the existing work on dynamic resource allocation assumes capacity achieving codes for various signaling schemes like TDMA, FDMA, CDMA and successive decoding. For the capacity achieving codes, the rate achievable by the user is log(1+SNR), where SNR denotes the signal to noise ratio of the user at the receiver side. However, codes that are used in practice have a finite gap to capacity, i.e., the achievable rate is log(1+SNR/Gamma) for Gamma > 1. The exact value of Gamma depends on the coding strategy and the desired bit error rate. Many existing resource allocation techniques that are optimal for capacity achieving codes perform sub-optimally in presence of the coding gap. For example, successive decoding does not always minimize the sum power required for providing the desired rate to each of the users for Gamma > 1. The problem of minimizing the sum power while guaranteeing the required rate to each of the users is important for both real-time and non real-time applications, and is addressed here. We obtain the resource allocation that is optimal for the above problem in presence of the coding gap.