Zaichen Zhang

Adaptive Modulation Schemes for Visible Light Communications

A major limitation of existing visible light communication (VLC) systems is the limited modulation bandwidth of light-emitting diodes used in such systems. Using adaptive modulation to improve the spectral efficiency for radio communications has been well studied. For VLC with various physical layer schemes, however, how adaptive modulation works is not well understood yet. The goal of this paper is to provide an in-depth analysis of the achievable spectral efficiency of adaptive modulation for three different schemes for high speed VLC: dc-biased optical orthogonal frequency division multiplexing (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), and single-carrier frequency-domain equalization (SC-FDE). We will show that in the low signal-to-noise ratio region, the ACO-OFDM-based adaptive modulation scheme outperforms the other two schemes. SC-FDE-based adaptive modulation achieves a better performance than the DCO-OFDM-based scheme, and it is much simpler than the other two schemes.

MIMO-OFDM visible light communications system with low complexity

In visible light communication (VLC) systems with intensity modulation (IM) and direct detection (DD), the modulation bandwidth of light emitting diode (LED) is limited, and dispersion of the channel might cause inter-symbol interference. Also, the transmitted signal must be nonnegative. Thus most modulation schemes for radio communications cannot be used directly for such a system. This paper develops a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) VLC system that overcomes the modulation bandwidth limitation of LED and satisfies the requirements of the transmitted signal for IM/DD. Also, two transmit data streams require only one inverse discrete Fourier transform (IDFT) block, resulting in a low computational complexity. The transmitter architecture, receive algorithm, bit error rate performance, channel capacity analysis, and simulation results are provided.

An Optimum DC-Biasing for DCO-OFDM System

Selecting an appropriate direct current (dc) bias is crucial to obtain positive and low-peak-to-average-power-ratio electrical signals in the dc optical orthogonal frequency-division multiplexing (DCO-OFDM) system. In this letter, our goal is to propose an optimization strategy of dc bias under given lower and upper clipping bounds. We characterize the mean square error between the original signals and the clipped ones as the metric of clipping distortion. Finding the optimum dc bias can be formulated as a nonconvex optimization problem. However, the optimum dc bias can be obtained from a numerical method. The simulation results show that the bit error ratio performance can be improved when employing the optimum dc bias obtained by the proposed numerical method.

A Novel Receiver for ACO-OFDM in Visible Light Communication

This letter proposes a novel receiver for asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) system in visible light communication to enhance its performance. In ACO-OFDM, the even subcarriers are distorted by the clipping noise and are simply discarded in the conventional receiver. In this letter we show that the signals on the even subcarriers can be viewed as data-dependent precoded OFDM. A novel receiver based on minimum mean square error criteria is then proposed to exploit the even subcarriers to collect additional diversity and coding gains, leading to improved bit error rate performance. Simulations show that the proposed receiver provides significant signal-to-noise ratio gain (up to 10 dB) over the conventional receiver, especially in non-line-of-sight channels. Moreover, the proposed receiver is also superior to some existing advanced ones.

Comparison of optical OFDM-IDMA and optical OFDMA for uplink visible light communications

In this paper we compare two candidate multiple access schemes for visible light communications (VLC), namely, optical orthogonal frequency division multiplexing interleave division multiple access (O-OFDM-IDMA) and optical orthogonal frequency division multiple access (O-OFDMA). Both of them belong to multicarrier transmission schemes and are tailored for VLC channels by asymmetrically clipping at zero after OFDM modulation. We compare their bit error rate (BER) performance, receiver complexities as well as peak-to-average power ratios (PAPRs) in different scenarios. Simulations show that O-OFDM-IDMA is more power efficient than O-OFDMA, especially for higher throughput, at the cost of higher computational complexity. The PAPRs of O-OFDM-IDMA and O-OFDMA do not show much difference. Those results provide a useful guidance for the design of future networks based on VLC.

Transmit Beamforming for MIMO Optical Wireless Communication Systems

This paper focuses on transmit beamforming for multiple-input multiple-output optical wireless communication (OWC) systems with intensity modulation and direct detection (IM/DD). OWC with IM/DD requires the transmitted signals to be nonnegative, for which existing beamforming schemes developed for radio frequency systems cannot be applied directly. We propose effective schemes for OWC over frequency flat and frequency selective channels. For frequency flat fading, the property of the beamforming vector is derived. For frequency selective fading, bit-error rate performances of the proposed scheme with zero-forcing and minimum mean-square error frequency domain equalization receivers are derived, and a suboptimal beamforming vector for frequency selective fading channels is proposed. Compared with asymmetrically clipped optical orthogonal frequency division multiplexing based frequency domain beamforming, the proposed scheme needs much less feedback information and has a better error performance.

OFDM-IDMA communications over underwater acoustic channels

The combination of orthogonal frequency division multiplexing (OFDM) and interleave division multiple access (IDMA), known as OFDM-IDMA, has been proposed as a promising multiple access scheme for wireless communications due to its low decoding complexity and potential of achieving high spectral efficiency. However, most of the work on OFDM-IDMA is in radio frequency (RF) communications scenarios and based on computer simulations. In this paper, we propose OFDM-IDMA over underwater acoustic (UWA) channels with experimental results. We also extend the original OFDM-IDMA to a multiple hydrophones scenario to improve the bit error rate (BER) performance. Experimental results show that with multiple hydrophones, the coded BER performance of OFDM-IDMA approaches zero after only a few iterations, validating the feasibility of our proposed approach over UWA channels.

Improved SNR Evolution for OFDM-IDMA Systems

The bit error rate performance of interleave division multiple access (IDMA) based systems can be predicted by signal-to-noise ratio (SNR) evolution which tracks the average symbol SNR at each iteration and provides a faster solution than brute-force simulations. As the desired SNR in the evolution procedure is hard to obtain, approximate SNR updating formula has been widely adopted in the literature. In this paper a revised SNR updating formula is proposed for orthogonal frequency division multiplexing interleave division multiple access (OFDM-IDMA) systems in Rayleigh fading channels. Theoretical analysis shows that the new updating formula provides a tighter lower bound of the expected SNR in the evolution procedure compared with the existing one. We then verified this by simulations.

OFDM-IDMA with User Grouping

A generalized version of orthogonal frequency division multiplexing interleave division multiple access (OFDM-IDMA) referred to as grouped OFDM-IDMA (G-OFDM-IDMA) is introduced in this paper. By dividing users into groups and transmitting each groups data only on some (as opposed to all) subcarriers, G-OFDM-IDMA can have much lower decoding complexity compared with conventional OFDM-IDMA while preserving the bit error probability (BEP) performance and the bandwidth efficiency. The user grouping problem is formulated into an integer linear programming problem whose suboptimal solution is proposed and compared with the lower bound. The optimization complexity issue is also addressed. Simulations are carried out to test the performance of G-OFDM-IDMA under various system configurations. It is observed that up to 80% complexity could be saved when the conventional OFDM-IDMA is substituted by G-OFDM-IDMA configured according to the suboptimal grouping solution.

On grouped OFDM-IDMA

A generalized version of orthogonal frequency division multiplexing interleave division multiple access (OFDM-IDMA) referred to as grouped OFDM-IDMA (G-OFDM-IDMA) is introduced in this paper. By dividing the users into several groups and transmitting each groups data only on part of the subcarriers, the G-OFDM-IDMA system is capable of having much lower decoding complexity compared with the conventional OFDM-IDMA while preserving the bandwidth efficiency and the bit error rate (BER) performance. We study the performance of the proposed scheme in terms of the decoding complexity and diversity order. We also extend the results to the multiple receive antennae case. Simulations are performed to corroborate the performance of the proposed G-OFDM-IDMA.