Safa Isam

VLSI Architecture for a Reconfigurable Spectrally Efficient FDM Baseband Transmitter

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.

A Truncated SVD approach for fixed complexity spectrally efficient FDM receivers

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%.

VLSI architecture for a reconfigurable Spectrally Efficient FDM baseband transmitter

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.

Precoded 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.

FPGA design of a truncated SVD based receiver for the detection of SEFDM signals

This work presents the hardware design of a novel algorithm using Field Programmable Gate Arrays (FPGAs) for the detection of Spectrally Efficient Frequency Division Multiplexing (SEFDM) signals. Previous work has shown that a sub-optimal Truncated Singular Value Decomposition (TSVD) approach is well-suited for use in SEFDM systems. TSVD offers a targeted reduction in complexity while outperforming linear detectors, such as Zero Forcing (ZF) and Minimum Mean Squared Error (MMSE), in terms of Bit Error Rate (BER). This is the first time a hardware design for the TSVD algorithm has been devised for implementation on an FPGA device using Very high speed integrated circuit Hardware Description Language (VHDL). Results show excellent fixed-point performance which are comparable to existing floating-point computer-based simulations. The optimal parameters required to achieve this outcome combined with their effect on system performance are identified. The impact of finite FPGA resources against performance gain is also examined.

Design and Performance Assessment of Fixed Complexity Spectrally Efficient FDM Receivers

Spectrally Efficient FDM (SEFDM) signals employs non-orthogonal and overlapping carriers to provide higher spectrum utilization relative to Orthogonal FDM signals (OFDM). Complex detectors are employed to extract the signal from the intercarrier interference (ICI) created by the loss of orthogonality. Sphere Decoder (SD) is proposed for SEFDM detection as an algorithm that achieves ML bit error rate (BER) performance. However, SD complexity is variable depending on the noise as well as the conditioning of the system. In this paper, the use of Fixed complexity Sphere Decoder (FSD) for the detection of SEFDM signal is proposed. The FSD is more suitable for hardware implementation as it eradicates the variable complexity characteristic of the Sphere Decoder algorithm whilst providing competitive bit error rate (BER) performance. The paper shows how the FSD can be applied to detect SEFDM signals and investigate the performance of the FSD in terms of the bit error rate (BER). Simulations results show that the FSD results in minor error penalties that can be traded-off with complexity.

Characterizing the intercarrier interference of non-orthogonal Spectrally Efficient FDM system

The proposal of Spectrally Efficient Frequency Division Multiplexing (SEFDM) system has added a new dimension for enhancing spectral utilization. SEFDM systems defy the orthogonality principle defined for Orthogonal Frequency Division Multiplexing (OFDM) systems by employing closely packed overlapped subcarriers to save bandwidth. Nevertheless, the loss of orthogonality results in intercarrier interference (ICI) which impede signal extraction. In this paper, the ICI in the SEFDM system is characterized through mathematical derivations and computer modeling and is shown to rely mainly on the main system parameters in terms of the level of bandwidth savings and number of subcarriers. Closed form formulas for the ICI for the continuous time and discrete time SEFDM system are derived which can be used in designing mitigation techniques, in particular, the design of informed detection algorithms.

A practical system for improved efficiency in frequency division multiplexed wireless networks

Spectral efficiency is a key design issue for all wireless communication systems. Orthogonal frequency division multiplexing (OFDM) is a very well-known technique for efficient data transmission over many carriers overlapped in frequency. Recently, several studies have appeared that describe spectrally efficient variations of multi-carrier systems where the condition of orthogonality is dropped. Proposed techniques suffer from two weaknesses: firstly, the complexity of generating the signal is increased. Secondly, the signal detection is computationally demanding. Known methods suffer either unusably high complexity or high error rates because of the inter-carrier interference. This study addresses both problems by proposing new transmitter and receiver architectures whose design is based on using the simplification that a rational spectrally efficient frequency division multiplexing (SEFDM) system can be treated as a set of overlapped and interleaving OFDM systems. The efficacy of the proposed designs is shown through detailed simulation of systems with different signal types and carrier dimensions. The decoder is heuristic but in practice produces very good results that are close to the theoretical best performance in a variety of settings. The system is able to produce efficiency gains of up to 20% with negligible impact on the required signal-to-noise ratio.

Robust channel estimation for Spectrally Efficient FDM system

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.

Peak to average power ratio reduction in spectrally efficient FDM systems

Spectrally efficient FDM (SEFDM) systems are new and attractive multicarrier systems that can significantly enhance spectral utilization. However, as a multicarrier system SEFDM is prone to high peak to average power ratio (PAPR). In this work we present for the first time a study of the PAPR in SEFDM systems. We explore the performance of standard PAPR reduction techniques and propose a novel PAPR reduction algorithm, based on sliding a time window across an extended SEFDM symbol period, therefore termed the SLiding Window (SLW) PAPR reduction technique. Numerical simulations confirm this new technique efficacy in PAPR reduction and show no side effects. Furthermore, a complete transmitter that employs SLW is proposed based on the SEFDM IDFT transmitter. SLW shows remarkable PAPR reduction with no spectral spreading or Bit Error Rate (BER) compromises at a much reduced complexity when compared to standard Partial Transmit Sequence (PTS) and Selective Mapping (SLM) PAPR reduction techniques.