Raed Mesleh

Indoor optical wireless communication: potential and state-of-the-art

In recent years, interest in optical wireless (OW) as a promising complementary technology for RF technology has gained new momentum fueled by significant deployments in solid state lighting technology. This article aims at reviewing and summarizing recent advancements in OW communication, with the main focus on indoor deployment scenarios. This includes a discussion of challenges, potential applications, state of the art, and prospects. Related issues covered in this article are duplex transmission, multiple access, MAC protocols, and link capacity improvements.

Spatial Modulation - A New Low Complexity Spectral Efficiency Enhancing Technique

The multiplexing gain of multiple antenna transmission strongly depends on transmit and receive antenna spacing, transmit antenna synchronization, and the algorithm used to eliminate interchannel interference (ICI) at the receiver. In this paper, a new transmission approach, called spatial modulation, that entirely avoids ICI and requires no synchronization between the transmitting antennas while maintaining high spectral efficiency is presented. A block of information bits is mapped into a constellation point in the signal and the spatial domain, i.e. into the location of a particular antenna. The receiver estimates the transmitted signal and the transmit antenna number and uses the two information to de-map the block of information bits. For this purpose, a novel transmit antenna number detection algorithm called iterative-maximum ratio combining (i-MRC) is presented. Spatial modulation is used to transmit different number of information bits and i-MRC is used to estimate both the transmitted signal and the transmit antenna number. The results are compared to ideal V-BLAST (vertical-Bell Lab layered space-time) and to MRC. Spatial modulation outperforms MRC. The (bit-error-ratio) BER performance and the achieved spectral efficiency is comparable to V-BLAST. However, spatial modulation results in a vast reduction in receiver complexity.

Indoor broadcasting via white LEDs and OFDM

Recently, visible light communication (VLC) technology has been gaining attention in both academia and industry. This is driven by the progress of white light emitting diode (LED) technology for solid-state lighting (SSL) and the potential of simultaneously using such LEDs for illumination and indoor wireless data transmission. This paper provides an overview about the technology and describes the physical layer implementation of a VLC system based on a modified version of the classical orthogonal frequency division multiplexing (OFDM) modulation technique. Besides, the paper presents a hardware prototype for short-range broadcasting using a white LED lamp. The OFDM system runs on DSP development boards. Off-the-shelf 9 LEDs and a single photodiode (PD) are utilized to build the analog frontends. The prototype allows investigating the influence of the electrical signal-to-noise ratio (SNR), constellation order, and channel coding on the bit-error performance. Theoretical and experimental results on optical path loss show close match. In this context, the influence of the LED beam angle on the horizontal coverage is highlighted.

Generalised spatial modulation

In this paper, a generalised technique for spatial modulation (SM) is presented. Generalised spatial modulation (GSM) overcomes in a novel fashion the constraint in SM that the number of transmit antennas has to be a power of two. In GSM, a block of information bits is mapped to a constellation symbol and a spatial symbol. The spatial symbol is a combination of transmit antennas activated at each instance. The actual combination of active transmit antennas depends on the random incoming data stream. This is unlike SM where only a single transmit antenna is activated at each instance. GSM increases the overall spectral efficiency by base-two logarithm of the number of antenna combinations. This reduces the number of transmit antennas needed for the same spectral efficiency. The performance of GSM is analysed in this paper, and an upper bound on the bit-error-ratio (BER) performance is derived. In addition, an algorithm to optimise the antenna combination selection is proposed. Finally, the performance of GSM is validated through Monte Carlo simulations. The results are compared with traditional SM. It is shown that for the same spectral efficiency GSM performs nearly the same as SM, but with a significant reduction in the number of transmit antennas.

OFDM Visible Light Wireless Communication Based on White LEDs

White LEDs are set to penetrate many areas of everyday life. An interesting property of these devices (in addition to their lightening capabilities) is that they can be utilised for data transmission. In the past, primarily OOK (on-off keying) has been used for digital data modulation of such devices. OOK imposes limitations on the achievable data rates. Therefore, in this paper OFDM is considered in combination with higher order modulation schemes. A hardware demonstrator with an entire link chain (transmitter and receiver) is developed and measured BER (bit error ratio) results are reported. The system uses pilot sub-carriers to correct frequency synchronisation errors, training sequences for channel estimation and time synchronisation routines. Forward error correction (FEC) coding is used. It is shown that for COFDM (coded OFDM) with QPSK (quadrature phase shift keying) modulation and a single LED, a BER of 2 times 10-5 is achieved for a distance of 90 cm between transmitter and receiver.

Trellis Coded Spatial Modulation

Trellis coded modulation (TCM) is a well known scheme that reduces power requirements without any bandwidth expansion. In TCM, only certain sequences of successive constellation points are allowed (mapping by set partitioning). The novel idea in this paper is to apply the TCM concept to the antenna constellation points of spatial modulation (SM). The aim is to enhance SM performance in correlated channel conditions. SM considers the multiple transmit antennas as additional constellation points and maps a first part of a block of information bits to the transmit antenna indices. Therefore, spatial multiplexing gains are retained and spectral efficiency is boosted. The second part of the block of information bits is mapped to a complex symbol using conventional digital modulation schemes. At any particular time instant, only one antenna is active. The receiver estimates the transmitted symbol and the active antenna index and uses the two estimates to retrieve the original block of data bits. In this paper, TCM partitions the entire set of transmit antennas into sub-sets such that the spacing between antennas within a particular sub-set is maximized. The scheme is called trellis coded spatial modulation (TCSM). Tight analytical performance bounds over correlated fading channels are proposed in this paper. In addition, the performance and complexity of TCSM is compared to the performance of SM, coded V-BLAST (vertical Bell Labs layered space-time) applying near optimum sphere decoder algorithm, and Alamouti scheme combined with TCM. Also, the performance of all schemes with turbo coded modulation is presented. It is shown that under the same spectral efficiency, TCSM exhibits significant performance enhancements in the presence of realistic channel conditions such as Rician fading and spatial correlation (SC). In addition, the complexity of the proposed scheme is shown to be 80% less than the V-BLAST complexity.

Optical Spatial Modulation

In this paper, a power and bandwidth efficient pulsed modulation technique for optical wireless (OW) communication is proposed. The scheme is called optical spatial modulation (OSM). In OSM, multiple transmit units exist where only one transmitter is active at any given time instance. The spatially separated transmit units are considered as spatial constellation points. Each unique sequence of incoming data bits is mapped to one of the spatial constellation points, i.e., activating one of the transmit units. This is the fundamental concept of the spatial modulation (SM) technique. In OW communication systems, the active transmitter radiates a certain intensity level at a particular time instance. At the receiver side, the optimal SM detector is used to estimate the active transmitter index. An overall increase in the data rate by the base 2 logarithm of the number of transmit units is achieved. The optical MIMO (multiple-input multiple-output) channel and the channel impulse response are obtained via Monte Carlo simulations by applying ray tracing techniques. It will be shown in this paper that the optical MIMO channel is highly correlated if transmitter and receiver locations are not optimized, which results in a significant power penalty. The power efficiency can be improved by increasing the number of receive units to enhance receive diversity and/or by using soft and hard channel coding techniques. Conversely, it is shown that aligning transmit and receive units creates nearly uncorrelated channel paths and results in substantial enhancements in system performance even as compared to the diversity or coding gain. The resultant aligned scheme is shown to be very efficient in terms of power and bandwidth as compared to on-off keying, pulse position modulation, and pulse amplitude modulation. In this paper also, the upper bound bit error ratios of coded and uncoded OSM are analyzed. The analytical results are validated via Monte Carlo simulations and the results demonstrate a close match.

Practical Implementation of Spatial Modulation

In this paper, we seek to characterize the performance of spatial modulation (SM) and spatial multiplexing (SMX) with an experimental testbed. Two National Instruments (NI) PXIe devices are used for the system testing: one for the transmitter and one for the receiver. The digital signal processing (DSP) that formats the information data in preparation for transmission is described, along with the DSP that recovers the information data. In addition, the hardware limitations of the system are also analyzed. The average bit-error ratio (ABER) of the system is validated through both theoretical analysis and simulation results for SM and SMX under the line-of-sight (LoS) channel conditions.

On the Performance of Different OFDM Based Optical Wireless Communication Systems

This paper analyzes the performance of indoor orthogonal frequency division multiplexing (OFDM) optical wireless communication systems in the presence of light emitting diode (LED) nonlinear distortions. There are several forms of optical OFDM using intensity modulation [7th Int. Symp. on Communication Systems Networks and Digital Signal Processing (CSNDSP), 2010, pp. 566-570]. In this paper, DC-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) are considered. ACO-OFDM produces a half-wave symmetry time signal at the output of the OFDM modulator by special assignment of subcarriers, thus allowing signal clipping at the zero level and avoiding the need for DC bias at the expense of data rate reduction. DCO-OFDM assigns data to all possible subcarriers to increase the data rate. However, half-wave symmetry signals cannot be achieved and a high DC bias is needed to convert the bipolar signal to a unipolar signal before modulating the LED intensity. This paper considers a practical LED model and studies the performance of both systems in terms of average electrical OFDM signal power versus bit error ratio in the presence of an additive white Gaussian noise (AWGN) channel. In addition, DC power consumption and the transmitted optical power for the two systems are compared. The analytical results are validated through Monte Carlo simulations and the obtained results demonstrate close match. It is shown that LED clipping has significant impact on the performance of both systems and an optimum system design should take into account the OFDM signal power, DC-bias point, and LED dynamic range.

Generalised Sphere Decoding for Spatial Modulation

In this paper, Sphere Decoding (SD) algorithms for Spatial Modulation (SM) are developed to reduce the computational complexity of Maximum-Likelihood (ML) detectors. Two SDs specifically designed for SM are proposed and analysed in terms of Bit Error Ratio (BER) and computational complexity. Using Monte Carlo simulations and mathematical analysis, it is shown that by carefully choosing the initial radius the proposed sphere decoder algorithms offer the same BER as ML detection, with a significant reduction in the computational complexity. A tight closed form expression for the BER performance of SM-SD is derived in the paper, along with an algorithm for choosing the initial radius which provides near to optimum performance. Also, it is shown that none of the proposed SDs are always superior to the others, but the best SD to use depends on the target spectral efficiency. The computational complexity trade-off offered by the proposed solutions is studied via analysis and simulation, and is shown to validate our findings. Finally, the performance of SM-SDs are compared to Spatial Multiplexing (SMX) applying ML decoder and applying SD. It is shown that for the same spectral efficiency, SM-SD offers up to 84% reduction in complexity compared to SMX-SD, with up to 1 dB better BER performance than SMX-ML decoder.