On Receiver Design for Uplink Low Density Signature OFDM (LDS-OFDM)

Abstract

Low density signature orthogonal frequency division multiplexing (LDS-OFDM) is an uplink multi-carrier multiple access scheme that uses low density signatures (LDS) for spreading the symbols in the frequency domain. In this paper, we introduce an effective receiver for the LDS-OFDM scheme. We propose a framework to analyze and design this iterative receiver using extrinsic information transfer (EXIT) charts. Furthermore, a turbo multiuser detector/decoder (MUDD) is proposed for the LDS-OFDM receiver. We show how the turbo MUDD is tuned using EXIT charts analysis. By tuning the turbo-style processing, the turbo MUDD can approach the performance of optimum MUDD with a smaller number of inner iterations. Using the suggested design guidelines in this paper, we show that the proposed structure brings about 2.3 dB performance improvement at a bit error rate (BER) equal to 10 −5 over conventional LDS-OFDM while keeping the complexity affordable. Simulations for different scenarios also show that the LDS-OFDM outperforms similar well-known multiple access techniques such as multi-carrier code division multiple access (MC-CDMA) and group-orthogonal MC-CDMA.

A novel evidence theory based row message passing algorithm for LDS systems

“…The LDS structure can be represented by its factor graph in which the transmitted symbols x j , j = 1, 2, · · · , J and the chip observations y m , m = 1, 2, · · · , M are denoted as variable nodes and chip nodes [2], respectively.…”

“…Therefore, the signature matrix for all users is sparse thus facilitating an affordable multiuser detector with the message passing algorithm (MPA) [2]. Along with the rapid development of the internet of things (IoT), the future fifth generation (5G) wireless networks are expected to support massive connectivity with a large number of devices, which is the weakness of current long term evolution (LTE) systems.…”

“…In [4], the authors constructed a joint sparse graph (JSG), combing the single graphs of LDS-OFDM and low density parity check (LDPC) codes. The JSG-OFDM system outperforms similar well-known systems such as group-orthogonal multi-carrier CDMA (GO-MC-CDMA) [5] and turbo structured LDS-OFDM [2]. In [6], the sparse code multiple access (SCMA) system is proposed by replacing the the procedure of bit to QAM (quadrature amplitude modulation) symbol mapping and spreading in LDS systems with codebooks.…”

Fast convergence and reduced complexity receiver design for LDS-OFDM system

“…In message passing algorithm, the schedule is a key that determines convergence rate. The flooding schedule has been applied for LDS-OFDM in [7] and [8], which is summarized as follows.…”

“…Message passing schedule on sparse graphs not only influences the convergence rate, but also affects the system performance. In [7] and [8], flooding schedule is adopted for MUD of LDS-OFDM, where all the nodes update messages simultaneously. Its convergence behavior is not ideal.…”