On the Capacity of Quantized Gaussian MAC Channels with Finite Input Alphabet

Abstract

In this paper, we investigate the capacity of Gaussian multiple access channels (MAC) with finite input alphabet and `quantized output'. With finite input alphabet and an unquantized receiver, the two-user Gaussian MAC rate region was studied in \cite{harsh08}. In most high throughput communication systems based on digital signal processing, the analog received signal is quantized using a low precision quantizer. In this paper, we first derive the expressions for the achievable rate region of a two-user Gaussian MAC with finite input alphabet and `quantized output'. We show that, with finite input alphabet, the achievable rate region with the commonly used `uniform receiver quantizer' has a significant loss in the rate region compared to that in \cite{harsh08}. It is observed that this degradation is due to the fact that the received analog signal is densely distributed around the origin, and is therefore not efficiently quantized with a uniform quantizer which has equally spaced quantization intervals. It is also observed that the density of the received analog signal around the origin increases with increasing number of users. Hence, the loss in the achievable rate region due to uniform receiver quantization is expected to increase with increasing number of users. We, therefore, propose a novel `non-uniform quantizer' with finely spaced quantization intervals near the origin. For a two-user Gaussian MAC with a given finite input alphabet and low precision receiver quantization, we show that the proposed non-uniform quantizer has a significantly larger rate region compared to what is achieved with a uniform quantizer.