Izzat Darwazeh

Spectrally Efficient FDM Signals: Bandwidth Gain at the Expense of Receiver Complexity

This paper investigates the transmission of Frequency Division Multiplexed (FDM) signals, where carrier orthogonality is intentionally violated in order to increase bandwidth efficiency. In analogy to conventional OFDM, signal generation relies on an Inverse Fractional Fourier Transform (IFRFT) that can be implemented with O(N log2 N) algorithmic complexity. Optimal Maximum Likelihood (ML) detection is overly complex due to the presence of substantial Intercarrier Interference (ICI). Consequently, we investigate an alternative detection mechanism based on the Generalized Sphere Decoding (GSD) algorithm. We examine the bandwidth efficiency and the error performance in Additive White Gaussian Noise (AWGN), for various FDM signal parameters. In particular, we show that it is possible to detect optimally and efficiently FDM signals, with 25% bandwidth gain with respect to analogous OFDM signals. This indicates that the transmission of spectrally efficient non orthogonal FDM signals is tangible.

High-speed generalized distributed-amplifier-based transversal-filter topology for optical communication systems

The use of a distributed-amplifier-based transversal filter as a signal processor for high-rate pulse shaping/filtering is discussed. By showing the analogy between transversal filters and distributed-amplifier topologies, schemes demonstrating practical approaches for the design of such filters are explored. The practicability of one of the different schemes is illustrated via an implemented design. A four-stage 10-Gb/s distributed-amplifier monolithic microwave integrated circuit (MMIC) is constructed using one of the developed schemes and its behavior is discussed.

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.

Bandpass sampling for software radio receivers, and the effect of oversampling on aperture jitter

To maximize reconfigurability in software radio receivers, digitization should occur as close to the antenna as possible. Bandpass sampling allows the digitization of bandpass signals at RF or intermediate frequencies without significantly increasing the sampling rate. This enables a more flexible receiver to be realized allowing for many radio functions to be defined in software. Sampling of signals at high carrier frequencies has associated problems such as the effects of aperture jitter. Jitter can limit the frequency at which digitization occurs and also degrade receiver performance. This paper investigates the effects of jitter on signal quality in bandpass sampling systems.

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%.

Optical SEFDM System; Bandwidth Saving Using Non-Orthogonal Sub-Carriers

We propose and demonstrate a new optical spectrally efficient frequency division multiplexing (O-SEFDM) system, where non-orthogonal and overlapping sub-carriers are employed to provide higher spectral efficiency relative to optical-orthogonal frequency division multiplexing (O-OFDM). The O-SEFDM technique can increase spectral efficiency in both the electrical and optical domains. It is experimentally shown that for bandwidth saving up to 25%, we can achieve the same performance as O-OFDM. This is the first experimental verification of 25% optical faster than the Nyquist rate. Furthermore, for approximately the same spectral efficiency, 4QAM O-SEFDM outperforms standard 8QAM by 1.6 dB. It is experimentally shown that a lower-order modulation format can achieve a better performance by replacing a higher one.

Circuit-Level Timing Error Tolerance for Low-Power DSP Filters and Transforms

In this paper, we present a novel circuit-level timing error mitigation technique, which aims to increase energy-efficiency of digital signal processing datapaths without loss of robustness. Timing errors are detected using razor flip-flops on critical-paths, and the error-rate feedback is used to control a dynamic voltage scaling control loop. In place of conventional razor error correction by replay, we propose a new approach to bound the magnitude of intermittent timing errors at the circuit level. A timing guard-band is created by shaping the path delay distribution such that the critical paths correspond to a group of least-significant bit registers. These end-points are ensured to be critical by modifying the topology of the final stage carry-merge adder, and by using tool-based device sizing. Hence, timing violations lead to weakly correlated logical errors of small magnitude in a mean-squared-error sense. We examine this approach in an finite-impulse response (FIR) filter and a 2-D discrete cosine transform implementation, in 32-nm CMOS. Power saving compared to a conventional design at iso-frequency is 21%-23% at the typical corner, while retaining a voltage guard-band to protect against fast transient changes in switching activity and supply noise. The impact on minimum clock period is small (16%-20%), as it does not necessitate the use of ripple-carry adders and also requires only a bare minimum of additional design effort.

An Improved Fixed Sphere Decoder Employing Soft Decision for the Detection of Non-orthogonal Signals

This letter proposes a hybrid soft iterative method together with Fixed Sphere Decoding (FSD) concurrently optimize performance and complexity. We show that for bandwidth compression factors of up to 25 percent, we can achieve the same performance as Orthogonal Frequency Division Multiplexing (OFDM). For systems with bandwidth compression higher than 25 percent, the complexity/performance trade-offs of the hybrid method are better than those of Truncated Singular Value Decomposition-FSD (TSVD-FSD).

Multi-band carrier-less amplitude and phase modulation for bandlimited visible light communications systems

Visible light communications is a technology with enormous potential for a wide range of applications within next generation transmission and broadcasting technologies. VLC offers simultaneous illumination and data communications by intensity modulating the optical power emitted by LEDs operating in the visible range of the electromagnetic spectrum (~370-780 nm). The major challenge in VLC systems to date has been in improving transmission speeds, considering the low bandwidths available with commercial LED devices. Thus, to improve the spectral usage, the research community has increasingly turned to advanced modulation formats such as orthogonal frequency-division multiplexing. In this article we introduce a new modulation scheme into the VLC domain; multiband carrier-less amplitude and phase modulation (m-CAP) and describe in detail its performance within the context of bandlimited systems.

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.