Overloaded multiple access systems: A generalized model and a low-complexity multiuser decoder

Several works have emerged in the past decade that can potentially provide higher spectral efficiency than that offered by existing orthogonal-channelization based air interfaces for a fixed bandwidth expansion factor. Among them, the interleave division multiple access (IDMA) and low-density spreading CDMA (LDS-CDMA) are of special interest because of their overloading capability. In this paper, we present a unified system structure that encompasses both systems. We call this unified structure as codeword interleaved LDS CDMA (cILDS-CDMA) and present an iterative decoding structure. We analyze the decoder performance via an EXIT-chart like approach and demonstrate the advantage of low density (sparse) spreading. Numerical results in AWGN channel show that LDS reduces the decoding latency and the cILDS-CDMA scheme also offer a larger diversity gain in frequency selective fading channels.