Factor graph based detection approach for high-mobility OFDM systems with large FFT modes

Abstract

In this article, a novel detector design is proposed for orthogonal frequency division multiplexing (OFDM) systems over frequency selective and time varying channels. Namely, we focus on systems with large OFDM symbol lengths where design and complexity constraints have to be taken into account and many of the existing ICI reduction techniques can not be applied. We propose a factor graph (FG) based approach for maximum a posteriori (MAP) symbol detection which exploits the frequency diversity introduced by the ICI in the OFDM symbol. The proposed algorithm provides high diversity orders allowing to outperform the free-ICI performance in high-mobility scenarios with an inherent parallel structure suitable for large OFDM block sizes. The performance of the mentioned near-optimal detection strategy is analyzed over a general bit-interleaved coded modulation (BICM) system applying low-density parity-check (LDPC) codes. The inclusion of pilot symbols is also considered in order to analyze how they assist the detection process.

Multi-Feedback Interference Cancellation Algorithms for OFDM Systems over Doubly-Selective Channels

“…The list includes the approximate MAP equalizers, namely belief propagation Ungerboeck-MAP (BP-MAP), recursive-SIC-linear-MMSE (RSL) in [3,15], the benchmark AMFB without ICI inside the band [2], the banded MMSE-SIC [4] and MF-SIC [16]. Note that the RSL we used is a modified SIC-MMSE with two taps (Q = 4) in the fourth iteration.…”

“…However, its performance improvement by ordering is very small. On the other hand, the sequential ICI cancellation will introduce the aggregation of andỹ k givenŝk are conditionally independent in Equation (15). This will be true if the residual interference inside the band in Equations (10) and (12) dominates.…”

“…The second term in Equation (19) can also be evaluated according to Equation (15). The LLR of s k (1) for the channel decoder, with the use of Equation (19), can be easily evaluated by:…”

“…However, by introducing the Gibbs sampler, the complexity of generating samples for MAP detection is almost the same as that of tens of sequential ICI cancellations with a relatively large channel matrix. Besides the MCMC-MAP equalizer mentioned above, the authors in [3,15] further investigate the reduced state MAP equalization techniques for uncoded and coded OFDM systems to achieve the benchmarking performance with hardware-realizable complexity. Due to the difficulty of estimating the rapidly varying channel, some joint receiver designs incorporating channel estimation have been proposed for OFDM systems in [16][17][18].…”

Low-Complexity ICI Suppression Methods Utilizing Cyclic Prefix for OFDM Systems in High-Mobility Fading Channels

“…This technique is also called an ICI shortening technique, and it can cooperate with other equalization techniques such as in [2]- [13]. In [18] and [19], low-complexity joint equalization and channel coding algorithms were proposed.…”