Jian Dang

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.

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.

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.

Properties and achievable data rate of a cyclic prefix based imperfect reconstruction filter bank multiple access system

In this study, the authors propose a cyclic prefix (CP) based imperfect reconstruction (IPR) filter bank multicarrier (FBMC) system in the uplink of multiuser networks, known as filter bank multiple access (FBMA). The prototype filter in the system is a common low-pass filter without any perfect reconstruction (PR) or nearly perfect reconstruction (NPR) constraint. CP appended input signal format is applied to avoid inter-symbol interference and facilitate the equalisation task. The received signal is jointly processed by multi-subband equalisation. Besides, the input data symbols can be complex-valued due to IPR design. Some properties and achievable data rate of the proposed system are discussed. Simulations illustrate that the proposed system with uniformly/non-uniformly designed filter bank has a similar achievable data rate to orthogonal frequency division multiple access (OFDMA). Bit error rate performance of the proposed system is better than OFDMA and an error floor always exists when applying conventional PR FBMC to multi-user networks. In addition, OFDMA is more sensitive to carrier frequency offset than FBMA because of full synchronisation requirement. Therefore, it suggests that the proposed CP based IPR FBMA system works well without PR/NPR conditions and more extensive development is feasible for the filter banks in the foreseeable future.

Performance analysis of full-duplex visible light communication networks

Full-duplex transmission can be easily implemented on a visible light communication (VLC) link. In this paper, we investigate the performance of a VLC network with full-duplex optical links. We propose two contention protocols, named U-ALOHA and FD-CSMA, to utilize the full-duplex capability effectively. Their performances in terms of channel utilization and network throughput are analyzed and simulated and compared with a protocol with half-duplex links. The results show that the proposed protocols can effectively exploit the full-duplex capability. U-ALOHA achieves high channel utilization on the downlink channel while FD-CSMA has good performance on both downlink and uplink channels. Both of them outperform the protocol with half-duplex links when the traffic load is sufficiently high.

A Novel Receiver Design for PAM-DMT in Optical Wireless Communication Systems

Pulse-amplitude-modulated discrete multitone (PAM-DMT) modulation is an optical orthogonal frequency division multiplexing scheme, which is compatible with intensity modulation and direct detection channel. In the conventional receiver for PAM-DMT, only the imaginary parts of subcarriers are detected, while the real parts are considered as clipping noise and simply discarded, causing insufficient usage of the received power. This letter proposes a new receiver characterizing temporal and spectral diversity combining for PAM-DMT, which extracts the effective information in clipping noise to recover the data. The proposed receiver achieves a better performance than the conventional receiver with a gain of at least 2.3 dB in Eb/N0 at the system bit error rate of 10-5 in additive white Gaussian noise channel by simulation when using 4-PAM modulation.