Cuiwei He

Angular diversity for indoor MIMO optical wireless communications

In this paper, we present a new type of receiver for MIMO optical wireless communications. The new receiver consists of multiple receiving elements (REs), each made up of a photodetector (PD) placed under an aperture. The directionality of each RE is shown to depend on the relative position of the PD and the aperture. Receiving patterns are calculated for various MIMO receiver designs, demonstrating that significant angular diversity can be achieved within a compact receiver structure. Simulation results are presented for a number of typical indoor scenarios when the transmitted signals are modulated using asymmetrically clipped optical OFDM (ACO-OFDM). For the scenarios considered, the bit error rate is lowest in the middle of the room and highest in the corners. The results indicate that for typical parameter values, reliable communication can be achieved throughout the room.

Performance of Optical Receivers Using Photodetectors With Different Fields of View in a MIMO ACO-OFDM System

In this paper, we analyze the performance of optical receivers using photodetectors (PDs) with two different fields of view (FOVs) in a multiple-input multiple-output optical wireless communication system which uses intensity modulation and direct detection. The novel aspect is that the PDs in the receiver do not all have the same FOV. It is shown that the use of PDs with different FOVs leads to an invertible channel matrix even when the PDs are closely spaced. Simulations for a typical indoor visible light communications scenario where LED lights are used as data transmitters show that the signal-to-noise ratios at the equalizer outputs are much higher than for a receiver of the same dimension where all the PDs have the same FOV. Good performance can be achieved with PDs located in a 3.5 cm by 3.5 cm area. Finally, the overall bit error rate (BER) is calculated for systems using asymmetrically clipped optical OFDM as the modulation scheme. Results are presented for both zero forcing and minimum mean square error equalizers. It is shown that the BER varies with the receiver position, with higher values in the center and the corners of the room.

MIMO Optical Wireless Receiver Using Photodetectors with Different Fields of View

This paper describes a new form of receiver for multiple-input multiple-output (MIMO) optical wireless communications. The new receiver uses a number of photodetectors (PDs), all facing in the same direction but with different fields of view (FOVs). This enables a compact planar receiver structure. The performance of the receiver is analyzed for an indoor visible light communications system where LED lights are used as data transmitters. It is shown that the MIMO channel matrix has full rank for typical receiver positions and that for a given number of PDs, the new receiver provides much greater diversity than a conventional receiver of the same overall dimensions in which all of the PDs have the same FOV. Finally the overall bit error rate (BER) is presented for systems using asymmetrically clipped optical OFDM (ACO-OFDM) as the modulation scheme. Results are presented for both zero forcing (ZF) and minimum mean square error (MMSE) equalization in the receiver. It is shown that the BER varies with receiver position, with relatively high values in the center and the corners of the room.

Performance comparison between spatial multiplexing and spatial modulation in indoor MIMO visible light communication systems

Spatial multiplexing (SMP) and optical spatial modulation (OSM) are two important technologies for indoor multiple-input multiple-output (MIMO) visible light communications (VLC). In this paper, we compare the performance of SMP and OSM systems which are operated with typical configurations in an indoor scenario. Both systems transmit data using asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) as the modulation scheme. It is shown that in order to achieve the same data rate with the SMP, the OSM has to employ large constellations which become impractical when either the number of luminaires or the constellation size of the SMP is greater than four. Simulation results are presented for both MIMO systems using four luminaires as transmitters and a receiver configured with three different front-ends. These receivers include a conventional non-imaging receiver, a prism-based receiver and an aperture-based receiver. The BER results demonstrate that SMP outperforms OSM in terms of both the size of the region in which a receiver can achieve low BER and the BER at typical receiver positions.

Clipping Noise Mitigation in Optical OFDM Systems

This letter describes a new non-linear algorithm for clipping noise mitigation in intensity modulation/direct detection dc biased optical orthogonal frequency division multiplexing (DCO-OFDM) systems. Clipping noise is often the major limitation in DCO-OFDM. In this letter, we show that extra information about the clipped signal can be extracted using a non-linear process and then used to mitigate the clipping noise. The effectiveness of the new algorithm is demonstrated by simulation and in an optical wireless experiment.

Performance Analysis of ACO-OFDM and DCO-OFDM Using Bit and Power Loading in Frequency Selective Optical Wireless Channels

In this paper, we present a detailed comparison of the performance of asymmetrically clipped optical OFDM (ACO-OFDM), ACO-OFDM with diversity combining, and DC-biased optical OFDM (DCO-OFDM) using a range of different bias levels. Comparisons are made for both an additive white Gaussian noise (AWGN) channel and for a typical frequency selective visible light communication channel. Adaptive bit loading is used to maximize the bit rate for the frequency selective channel using a target bit error rate of 10-3. Clipping noise in DCO-OFDM, unlike AWGN, is added at the transmitter, not the receiver. New analytical results are derived which include this effect in the bit-loading calculations. It is shown that diversity-combining alters the spectral distribution of the noise after equalization. The effect of this in bit loading is also analyzed. It is shown that for both the AWGN and the frequency selective channel ACO-OFDM with diversity combining is the most energy efficient at lower bit rates/normalized bandwidths but for higher data rates DCO-OFDM requires the smallest transmit power. For example, for the frequency selective channel for bit rates/normalized bandwidths up to 6, ACO-OFDM with diversity combining is the most power efficient. For higher data rates DCO-OFDM requires the smallest transmit power, but these data rates require very large constellation sizes, for example, the constellation size of 128 for DCO-OFDM for some subcarriers and 2048 for ACO-OFDM with diversity-combining.

Performance of optical receivers using photodetectors with different fields of view in an indoor cellular communication system

This paper describes the use of an optical receiver which has photodiodes (PDs) with two different fields of view (2-FOV) in an indoor visible light communications (VLC) system. Data is transmitted using asymmetrically clipped optical OFDM (ACO-OFDM) from white light emitting diodes (LEDs) which are also used for illumination. A cellular configuration is considered where each transmitter transmits data to receivers within its cell. A minimum mean square error (MMSE) combiner is used in the receiver to separate the wanted signal from the interference. Results are calculated for the signal to interference plus noise ratio (SINR) as a function of the transmitter and receiver characteristics and the relative positions of the transmitters and receiver. It is shown that for most receiver positions the 2-FOV receiver outperforms a conventional receiver in which all of the PDs have the same FOV. It is also shown that increasing the number of PDs in the 2-FOV receiver from two to eight gives negligible improvement in performance. Bit error rate (BER) results are presented for a number of receiver positions showing that at some positions the 2-FOV receiver gives a significant improvement in performance but that at others no improvement is observed.