Spectrally efficient FDM (SEFDM) systems employ non-orthogonal overlapped carriers to improve spectral efficiency for future communication systems. One of the key research challenges for SEFDM systems is to demonstrate efficient hardware implementations for transmitters and receivers. Focusing on transmitters, this paper explains the SEFDM concept and examines the complexity of published modulation algorithms, with particular consideration to implementation issues. We then present two new variants of a digital baseband transmitter architecture for SEFDM, based on a modulation algorithm which employs the discrete Fourier transform (DFT) implemented efficiently using the fast Fourier transform (FFT). The algorithm requires multiple FFTs, which can be configured either as parallel transforms, which is optimal for throughput or using a multi-stream FFT architecture, for reduced circuit area. We propose a simplified approach to IFFT pruning for pipeline architectures, based on a token-flow control style, specifically optimized for the SEFDM application. Reconfigurable implementations for different bandwidth compression ratios, including conventional OFDM, are easily derived from the proposed implementations. The SEFDM transmitters have been synthesized, placed and routed in a commercial 32 nm CMOS process technology and also verified in FPGA. We report circuit area and simulated power dissipation figures, which confirm the feasibility of SEFDM transmitters.

VLSI Architecture for a Reconfigurable Spectrally Efficient FDM Baseband Transmitter

Spectrally Efficient Frequency Division Multiplexing (SEFDM) systems aim to reduce the utilized spectrum by multiplexing non-orthogonal overlapped carriers. Since the per carrier transmission rate is maintained, SEFDM yields higher spectral efficiency relative to an equivalent Orthogonal Frequency Division Multiplexing (OFDM) system. Yet, due to the loss of the orthogonality, detection of the SEFDM system requires overly complex detectors. In this work, new SEFDM receivers that offer substantial complexity reduction with a competitive Bit Error Rate (BER) performance are presented. The Truncated Singular Value Decomposition (TSVD) is proposed as an efficient tool to overcome the ill conditioning of the system caused by the orthogonality collapse. The performance of the system with respect to the system size and spectrum saving is examined by extensive numerical simulations. It is shown that the TSVD detector outperforms linear detectors such as Zero Forcing (ZF) and Minimum Mean Squared Error (MMSE) detectors in terms of BER. Furthermore, a combination of TSVD with the Fixed Sphere Decoder (FSD) algorithm is proposed and tested for the first time. This novel FSD-TSVD receiver achieves near -optimum performance in terms of BER with a fixed and reduced complexity for systems with bandwidth savings of up to 40%.

A Truncated SVD approach for fixed complexity spectrally efficient FDM receivers

Spectrally Efficient FDM (SEFDM) systems employ non-orthogonal overlapped carriers to improve spectral efficiency for future communication systems. One of the challenges for SEFDM systems is to demonstrate efficient hardware implementations for transmitters and receivers. This paper presents the first VLSI digital baseband transmitter architecture for SEFDM. The transmitter is reconfigurable between three bandwidth compression ratios, including OFDM and Fast OFDM, therefore supporting operation with current OFDM systems. Complexity analysis is presented of the proposed architecture, along with an area and power efficient hardware mapping, implemented using a 65nm CMOS cell library to provide analysis of area and power compared to a baseline OFDM transmitter.

VLSI architecture for a reconfigurable Spectrally Efficient FDM baseband transmitter

Spectrally Efficient Frequency Division Multiplexing (SEFDM) system employs non-orthogonal multiple carriers in order to enhance utilization of bandwidth over the Orthogonal Frequency Division Multiplexing system (OFDM). However, the deliberate loss of orthogonality affects many aspects of the system. In particular, the intercarrier interference results in the system becoming ill-conditioned, thus affecting the performance of the system, and particular to this work, the channel estimation accuracy. Therefore, in this work, a novel channel estimation technique benefiting from the structure of the SEFDM system to tackle the ill-conditioning is tailored. The technique is termed Partial Channel Estimation (PCE) and is based on transmitting pilot symbols invoking the partial orthogonality structure of the system to avoid the ill-conditioning impact and thus, is capable of delivering more accurate estimates. Numerical performance of the proposed estimator confirms favorable estimation accuracy and demonstrates a tangible error performance improvement and complexity reduction.

Robust channel estimation for Spectrally Efficient FDM system

Spectrally Efficient Frequency Division Multiplexing (SEFDM) system proposes enhanced spectrum utilization in contrast to Orthogonal Frequency Division Multiplexing system (OFDM). Spectral efficiency is increased by relaxing the orthogonality condition while maintaining the same transmission rate per individual channel, hence, for the same bandwidth allocation SEFDM offers higher throughput than OFDM. However, the loss of orthogonality necessitates complex algorithms for the recovery of the signal. In this work, we propose a precoding strategy that greatly simplifies the detection of the signal. The strategy facilitates simpler detection for the same bandwidth savings as an equivalent uncoded SEFDM system. The strategy is based on localizing the effects of the lost orthogonality in a portion of the transmitted symbols. Detection of the preserved channels is a simple zero forcing (ZF) estimator and the rest of the symbols can be detected using complex detectors such as the maximum likelihood (ML) detector. Simple architecture of the precoded SEFDM system based on IDFT/DFT blocks for transmission and reception is proposed. Extensive numerical investigations in AWGN channel confirmed favorable bit error rate (BER) performance of the new system with a much reduced complexity.

Precoded Spectrally Efficient FDM system

This paper propose the theoretical analysis of the DFT based overlapped frequency division multiplexing (OvFDM)/non-orthogonal frequency division multiplexing(NOFDM) communication system, and investigates the impact of conventional cyclic-prefix and zero-padding cyclic-prefix respectively for the OvFDM-DFT system in the time-variant frequency-selective fading channels The sub-carriers of the DFT based OvFDM/NOFDM communication system are overlapped together and the bandwidth can be larger than the frequency coherence bandwidth, so a kind of implicit frequency diversity will be exploited by the MLSD receiver, which will improve the system performance Theoretical analysis and simulation results show that zero-padding cyclic-prefix has a much better performance than the conventional cyclic-prefix The implicit frequency diversity ability of the OvFDM-DFT symbols is also verified by the simulations

Zhang Xi-lin

Papers

The Prefix Design and Performance Analysis of DFT-based Overlapped Frequency Division Multiplexing (OvFDM-DFT) System