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Abstract Orthogonal frequency-division multiplexing (OFDM) with multiple transmit and multiple receive antennas (MIMO-OFDM) is considered a candidate for high-data rate communication in various existing and forthcoming system standards. To achieve the usually desired low frame and bit error rates, MIMO-OFDM should be combined with adaptive bit loading (ABL) and forward error correction (FEC) coding, where the former is particularly apt for moderate mobility as considered in, for example, IEEE 802.16e OFDM systems. In this paper, we investigate "simple" coding schemes and their combination with ABL for MIMO-OFDM. In particular, we consider wrapped space-frequency coding (WSFC) and coded V-BLAST with ABL and optimize both schemes to mitigate error propagation inherent in the detection process. Simulation results show that bit-loaded WSFC and V-BLAST optimized for coded MIMO-OFDM achieve excellent error rate performances, close to that of quasioptimal MIMO-OFDM based on singular value decomposition of the channel, while their feedback requirements for loading are low.

Fast burst synchronization is an important requirement in asynchronous communication networks, where devices transmit short data packets in an unscheduled fashion. Such a synchronization is typically achieved by means of a preamble sent in front of the data packet. In this paper, we study fast burst synchronization for power line communication (PLC) systems operating below 500 kHz and transmitting data rates of up to about 500 kbps as it is typical in various PLC network applications. In particular, we are concerned with the receiver processing of the preamble signal and the actual design of preambles suitable for fast burst synchronization in such PLC systems. Our approach is comprehensive in that it takes into account the most distinctive characteristics of the power line channel, which are multipath propagation, highly varying path loss, and disturbance by impulse noise, as well as important practical constraints, especially the need for spectral shaping of the preamble signal and fast adjustment of the automatic gain control (AGC). In fact, we regard the explicit incorporation of these various requirements into the preamble design as the main contribution of this work. We devise an optimization criterion and a stochastic algorithm to search for suitable preamble sequences. A comprehensive performance comparison of a designed and two conventional preambles shows that the designed sequence is superior in terms of (a) fast burst synchronization in various transmission environments, (b) fast AGC adjustment, and (c) compliance of its spectrum with the spectral mask applied to the data transmit signal.

Light-fidelity (LiFi) is a light-based wireless communication technology which can complement radio-frequency (RF) communication technologies for indoor applications. Although LiFi signals are spatially more contained than RF signals, the broadcasting nature of LiFi also makes it susceptible to eavesdropping. Therefore, it is important to secure the transmitted data against potential eavesdroppers. In this paper, an overview of the recent developments pertaining to LiFi physical layer security (PLS) is provided, and the main differences between LiFi PLS and RF PLS are explained. LiFi achievable secrecy rates and upper bounds are then investigated under practical channel models and transmission schemes. Beamforming and jamming, which received significant research attention recently as a means to achieve PLS in LiFi, are also investigated under indoor illumination constraints. Finally, future research directions of interest in LiFi PLS are identified and discussed. This article is part of the theme issue Optical wireless communication. [ABSTRACT FROM AUTHOR]

The tremendous growth of machine-tomachine (M2M) applications has been a great attractor to cellular network operators to provide machine-type communication services. One of the important challenges for cellular systems supporting M2M terminals is coverage, because terminals can be located in spaces in buildings and structures suffering from significant penetration losses. Since these terminals are also often stationary, they are permanently without cellular coverage. To address this critical issue, the third generation partnership project (3GPP), and in particular its radio access network technical specification group, commenced work on coverage enhancement (CE) for long-term evolution (LTE) systems in June 2013. This article reviews the CE objectives defined for LTE machine-type communication and presents CE methods for LTE downlink and uplink channels discussed in this group. The presented methods achieve CE in a spectrally efficient manner and without notably affecting performance for legacy (non- M2M) devices. [ABSTRACT FROM PUBLISHER]

This paper focuses on the problem of optimal bit and power allocation in linear periodically time varying (LPTV) channels in broadband (BB) power line communication (PLC). Previous work has demonstrated that improvements in bit loading can be achieved with LPTV channel adaptation via so-called microslots in time for an orthogonal frequency division multiplexing (OFDM) system. In this paper, we present that the application of a power constraint that is averaged over many microslots can be exploited for further performance improvements through loading. Due to the matroid structure of the optimization problem, greedy-type algorithms are proven to be optimal for the new LPTV-aware bit and power loading. Significant gains are attained especially at reduced transmit-power levels, where the energy per bit-transmission is also low, and for poor (i.e. high attenuation) channel conditions. Furthermore, two mechanisms are utilized to reduce the complexity of the optimal LPTV-aware bit loading and peak microslot power levels: (i) employing representative values from microslot transfer functions, and (ii) power clipping. Our results indicate that the reduced complexity LPTV-aware bit loading with power clipping performs very close to the optimal LPTV-aware bit loading, which makes it an attractive option in a practical setting. [ABSTRACT FROM PUBLISHER]

In this letter, we address two questions concerning the application of multipulse pulse-position modulation (MPPM) for free-space optical (FSO) communications: (1) under what condition would MPPM offer a notable advantage over conventional pulse-position modulation (PPM), and (2) what practical coding scheme should be used to realize that advantage. Focusing on bandwidth-efficient FSO transmission, we find decimated MPPM constellations which are suitable for combining with binary codes and offer significant gains in terms of constellation-constrained capacity over PPM under simultaneous peak-power, average-power, and bandwidth constraints. We then consider labeling design for the popular bit-interleaved coded modulation (BICM) and devise a new multilevel coding (MLC) architecture for MPPM, which we refer to as reduced-layer MLC (RL-MLC). Considering the Poisson FSO channel model as a relevant example, we provide simulative evidence that RL-MLC MPPM with off-the-shelf low-density parity-check codes can outperform any PPM scheme under the same transmission constraints. [ABSTRACT FROM AUTHOR]

In this paper, a unified framework for the analysis of differential detection (DD) schemes in time-variant multiple-input multiple-output Rayleigh fading channels is provided. The present results are very general in that they apply to transmission with differential phase-shift keying, unitary differential space- time modulation (DSTM), and block DSTM and reception with conventional DD (CDD), multiple-symbol DD (MSDD), decision-feedback DD (DFDD), and (differentially) coherent detection (CD). New result for general quadratic forms of Gaussian random variables are derived which allows us to obtain elegant closed-form expressions for the pairwise error probabilities (PEPs) of the dominant error events of the considered detectors. Furthermore, it is shown that a unified treatment of all considered detectors is possible with a properly defined effective signal- to-noise ratio (ESNR) and a useful connection between MSDD and minimum-mean-squared-error (MMSE) interpolation is established. Interesting novel results obtained from this analysis include: (i) DSTM constellations designed for CDD and CD are also optimum for MSDD and DFDD; (ii) the error floor entailed by MSDD and DFDD in time-variant fading decreases exponentially with the observation window size N; and (iii) in time-variant fading with effective normalized fading bandwidth Bh,effT MSDD with N → ∞ suffers only from an SNR loss of (1 - 2Bh,effT) compared to CD, whereas DFDD suffers from a diversity loss of (1 - 2Bh,effT). [ABSTRACT FROM AUTHOR]

Differential space-time modulation (DSTM) using unitary-matrix signal constellations is an attractive solution for transmission over multiple-input multiple-output (MIMO) fading channels without requiring channel state information (CSI) at the receiver. To avoid a high error floor for DSTM in relatively fast MIMO fading channels, multiple-symbol differential detection (MSDD) has to be applied at the receiver. MSDD jointly processes blocks of several received matrix-symbols, and power efficiency improves as the blocksize increases. But since the search space of MSDD grows exponentially with the blocksize and also with the number of transmit antennas and the data rate, the complexity of MSDD quickly becomes prohibitive. In this paper, we investigate the application of tree-search algorithms to overcome the complexity limitation of MSDD. We devise a nested MSDD structure consisting of an outer and a number of inner tree-search decoders, which renders MSDD feasible for wide ranges of system parameters. Decoder designs tailored for diagonal and orthogonal DSTM codes are given, and a more power-efficient variant of MSDD, so- called subset MSDD, is proposed. Furthermore, we derive a tight symbol-error rate approximation for MSDD, which lends itself to efficient numerical evaluation. Numerical and simulation results for different DSTM constellations and fading channel scenarios show that the new tree-search MSDD achieves a significantly better performance-complexity tradeoff than benchmark decoders. [ABSTRACT FROM AUTHOR]

Direct-sequence ultra wideband (DS-UWB) transmission is a strong contender for the physical layer of high data-rate short-range UWB systems. Since long delay spreads in UWB channels cause significant intersymbol interference, DS-UWB systems require equalization. In this second part of two papers, we investigate equalization for DS-UWB with 4-ary biorthogonal keying (4BOK), which is one of the two modulation formats that was proposed for standardization by the IEEE 802.15.3a task group. To this end, we first derive expressions for the bit error rate (BER) accord- ing to the matched-filter bound for 4BOK DS-UWB, which serve as theoretical performance limits for equalization. We then devise structures and methods for filter optimization for low-complexity linear and nonlinear equalization schemes. In this context, we develop a new equivalent multiple-input multiple-output (MIMO) description of 4BOK DS-UWB, which facilitates the design of efficient equalizers using MIMO filter optimization techniques. Furthermore, we propose the application of widely linear processing to these equalizers. Simulation and semianalytical results show that MIMO equalization is greatly advantageous over more obvious non-MIMO schemes and that the proposed MIMO equalizers allow for power-efficient 4BOK DS-UWB transmission close to the theoretical limits with moderate computational complexity. [ABSTRACT FROM AUTHOR]

In this paper, a new class of distributed space-time block codes (DSTBCs) is introduced. These DSTBCs are designed for wireless networks which have a large set of single-antenna relay nodes N, but at any given time only a small, a priori unknown subset of nodes S ⊂ N can be active. In the proposed scheme, the signal transmitted by an active relay node is the product of an information-carrying code matrix and a unique node signature vector of length Nc. It is shown that existing STBCs designed for Nc ≥ 2 co-located antennas are favorable choices for the code matrix, guaranteeing a diversity order of d = min{NS, Nc} if NS nodes are active. For the most interesting case, NS ≥ Nc, the performance loss entailed by the distributed implementation is analytically characterized. Furthermore, efficient methods for the optimization of the set of signature vectors are provided. Depending on the chosen design, the proposed DSTBCs allow for low-complexity coherent, differential, and noncoherent detection, respectively. Possible applications include ad hoc and sensor networks employing decode-and-forward relaying. [ABSTRACT FROM AUTHOR]

In this paper, we consider multiple-bit differential detection (MBDD) of differential offset quadrature phase-shift keying (DOQPSK) with conventional bit-wise differential encoding (DE). We propose two novel MBDD schemes that outperform Simon's MBDD and approach the performance of coherent detection (CD) as the observation window size N increases. The first scheme uses the full available detection window spanning N - 1 bit intervals, and is referred to as F-MBDD. A careful bit-error rate (BER) analysis shows that a significant performance improvement can be realized by reducing the detection window size to N - 3 bit intervals. The resulting improved MBDD (I-MBDD) of DOQPSK with bit-wise DE achieves a similar performance as Phoel's MBDD of DOQPSK with more complex symbol-wise DL For both F-MBDD and I-MBDD, tight upper bounds and accurate BER approximations are derived. In addition, since the complexity per bit decision of brute-force decoding of MBDD of DOQPSK grows exponentially with N, we develop optimum and suboptimum fast decoding algorithms with complexities per bit decision of orders log(N + 1) and N, respectively. [ABSTRACT FROM AUTHOR]

Visible light communications (VLC) has a long history dating back to ancient signal fires, followed by the 1880 Alexander Graham Bell photophone, which transmitted speech wirelessly using modulated reflected sunlight. But it was the emergence of solid state light sources that sparked the imagination of researchers, such as those at Nakagawa Laboratories at Keio University around the turn of the 21st century, to demonstrate that solid state light sources could be used for secondary purposes such as data transmission and positioning. [ABSTRACT FROM AUTHOR]