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Summary: This paper considers multiuser downlink transmission and detection in a multiple input‐multiple output (MIMO) visible light communication (VLC) system. We combine two promising techniques: space‐shift keying (SSK) and optical orthogonal codes (OOCs). Multiuser downlink transmission is realized by allowing each activated LED to transmit user‐specific OOC simultaneously. An OOC despreader is implemented at the front end of each user terminal (UT) to extract the desired signal and mitigate multiuser interference (MUI). The second part of UT receiver aims to extract the index of the corresponding active LED based on maximum likelihood (ML) or zero‐forcing (ZF) algorithm. The figure of merit we used to access the performance of the proposed downlink multiuser SSK system is the average symbol error rate (SER). Comprehensive Monte Carlo simulations have been undertaken for different system setup to evaluate SER at UT receiver. It is demonstrated that for a specified SER, the required SNR for ML‐based detection algorithm is 3 dB less than the ZF detector in the MUI‐free scenario. On the other hand, the ZF detector outperforms ML detector in the case with residual MUI and limited channel state information (CSI). Both algorithms are computationally attractive, hence can be put into practice in multiuser VLC MIMO system. [ABSTRACT FROM AUTHOR]

Abstract This paper deals with the design of blind mobile station (MS) receiver for time-hopping (TH) ultrawide-band (UWB) impulse radio (IR) system employing antipodal pulse amplitude modulation (PAM). In the presence of multipath fading channel, batch-mode minimum output energy (MOE) receiver, as well as a blind channel estimator, is first developed. To reduce the computational complexity, we propose two blind adaptive algorithms to determine the weight vector of the MS receiver. The rational of the algorithms premises on iteratively maximizing the minimum possible receiver's output energy. The first (indirect) approach is derived by first developing an adaptive blind channel estimator, then the updated channel impulse response (IR) is used to calculate the MS receiver's weight vector. Meanwhile the second (direct) method jointly and iteratively optimizes the weight vector and channel IR to improve system performance. Simulation results demonstrate convergence of both algorithms. Moreover, the algorithms are shown to be robust to multiuser interference (MUI) and near-far problems.

A novel concept of time reversal division multiple access (TRDMA) is proposed recently as a low complexity, energy efficient wireless multi-user access method. In this paper, we first generalize TRDMA as a pre-filtering based space division multiple access communication system, where each user's signature waveform is derived from the information of channel impulse responses (CIRs) between basestation and location specific multiple mobile stations. We develop two pre-filtering schemes: one is referred to as time reversal matched filter. The benefit of the time reversal matched filter is to fully collect energy at the sampling instant of the intended mobile station (spatial and temporal focusing). The second pre-filter, which is referred to as zero-forcing pre-filter (ZFPF), is derived to meet the zero-forcing criterion such that multi-user interference is completely eliminated. However, because the uplink CIR is generally different from the downlink CIR, we present extensive analysis on the performance degradation of the ZFPF caused by this mismatched effect. Furthermore, to improve the robustness to mismatch, we propose a simple yet effective robust ZFPF. We investigate and compare the performances of both schemes under various scenarios in terms of power consumption and averaged signal-to-interference-plus-noise power ratio. It is shown in both analytical and simulation results that satisfying performance can be achieved by the proposed pre-filtering system. Moreover, we demonstrate that the proposed robust-ZFPF has improved the performance in mismatched condition. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

ABSTRACT In this paper, a novel M-ary pulse-width modulation (PWM) coded optical code division multiple access (O-CDMA) system is proposed. At the transmitter end, in order to increase the number of transmitted bits per pulse, each chip of signature sequence is further divided into M time slots to realise the M-ary PWM signal format. Two types of detection schemes are developed, and the performance in terms of symbol error rate is analysed. In the first scheme, the correlation receiver is employed followed by an M-level threshold detection. To enhance the multiuser interference suppression capability, we apply the chip-level-based diversity detection in the second scheme. Adding optical hard-limiters at the front end of the receiver, both schemes are free from the near-far problem that is suffered severely in O-CDMA system employing pulse-amplitude modulation (PAM). Numerical results demonstrate that the proposed M-ary PWM O-CDMA system with the chip-level receiver achieves better symbol error rate and multiuser interference mitigation capability than the correlation receiver. Moreover, the performance of the proposed M-ary PWM O-CDMA system is much better than the M-ary pulse-amplitude modulation scheme. And the complexity is substantially reduced as we compare to the existing M-ary pulse-position modulation O-CDMA system. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]