Recent papers on MIMO optical wireless communications.

C. He, S. Cincotta, M. M. A. Mohammed and J. Armstrong, "Angular Diversity Aperture (ADA) Receivers for Indoor Multiple-Input Multiple-Output (MIMO) Visible Light Communications (VLC)," IEEE Access, Vol. 7, PP. 145282 - 145301, 2019 

Abstract -The angular diversity aperture (ADA) receiver is a new form of optical wireless receiver which is made up of a number of receiving elements (REs) each consisting of a photodiode (PD) located below an aperture in an opaque screen. Previous work on the use of ADA receivers for visible light communication(VLC) analyzes only REs with round apertures and photodiodes with a limited range of parameters. This paper extends the analysis to calculate the channel gains for REs with for both round and square apertures and PDs and for more parameters. Graphs show how the gain varies as a function of the relative orientation of the transmitter and receiver for ten different designs. It is shown that the directionality of an RE depends on the offset of the PD from the aperture and that the field of view depends on the distance between the aperture and the PD. Detailed theoretical analysis and extensive simulation results demonstrate the bit error rate (BER) performance of receivers based on these ten designs in a multiple-input multiple-output (MIMO) VLC application. The round and square designs are found to have very similar BERs. It is shown that, even with high levels of shot noise, good BER results can be achieved throughout a typical scenario. 

T. Q. Wang, C. He and J. Armstrong, "Performance Analysis of Aperture-Based Receivers for MIMO IM/DD Visible Light Communications," Journal of Lightwave Technology, Vol. 35 , Iss. 9, PP. 1513 - 1523, 2017

Abstract-Aperture-based receivers are a new form of receiver for multiple-input multiple-output (MIMO) intensity-modulated direct-detection visible light communication (VLC). These receivers provide a wide field of view and excellent angular diversity using a compact planar structure and so are ideally suited for integration in hand-held devices such as smartphones. It is shown that in typical scenarios to achieve similar performance the photodiodes in a conventional receiver based on spatial diversity would have to be separated by distances greater than 30 cm. An in-depth analysis of the performance of aperture-based receivers is presented. Expressions are derived for the channel gain between an optical transmitter and each receiving element (RE) as a function of the transmission pattern of the transmitter, the design of the RE, and the relative positions of the transmitter and receiver. It is shown that a well-designed receiver consisting of multiple REs can separate signals received from different directions with low-multi stream interference and that the associated MIMO channel matrices are well conditioned. Simulations are performed for a typical indoor VLC scenario in which light-emitting diode luminaires transmit information using asymmetrically clipped optical orthogonal frequency division multiplexing. Results are presented for receivers using both linear and nonlinear equalizers and for both line of sight (LOS) and LOS plus diffuse reception. The diffuse component is shown to improve the bit error rate (BER) performance slightly. It is shown that the BER depends on the receiver position. When a zero-forcing (ZF) linear receiver is used, the BER is dominated by the most attenuated signal, so the performance degrades at the corners of the room. In contrast, the receivers with nonlinear equalizers based on ZF followed by successive interference cancelation achieve low BER throughout the scenario.

C. He, T. Q. Wang and Jean Armstrong, "Performance of Optical Receivers Using Photodetectors With Different Fields of View in a MIMO ACO-OFDM System," Journal of Lightwave Technology, Vol. 33, Iss. 23, PP. 4957 - 4967, 2015

Abstract—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.

T. Q. Wang, Y. A. Sekercioglu, and J. Armstrong, "Analysis of an Optical Wireless Receiver Using a Hemispherical Lens With Application in MIMO Visible Light Communications," Journal of Lightwave Technology, vol. 31, pp. 1744-1754, 2013.

Abstract—White lighting LEDs offer great potential for high speed communications, especially for indoor applications. However, for their widespread adoption, two important issues need to be addressed. They are the lack of diversity in multiple-input multiple output (MIMO) systems and the small field of view of receivers. In this paper, we describe a design using a hemispherical lens in the receiver that solves these problems. By using classical optics, we derive exact expressions for the channel gain and the optical power density of the projected images. Simulation results of a typical indoor scenario show that the new system has a wide field of view, and provides adequate channel gain for angles of incidence as large as 70 degrees. We present the distribution of optical power on the imaging plane for a number of representative indoor scenarios for various receiving positions and tilted receivers. They show that the images of LEDs are clearly distinguishable. This demonstrates the presence of low channel correlations between individual transmitters and receivers. Consequently, this confirms that the new technique is capable of providing significant diversity order for MIMO optical wireless applications.

M.R.H Mondal and J. Armstrong, Impact of linear misalignment on a spatial OFDM based pixelated system, (APCC), 2012 18th Asia-Pacific Conference on Communications, 15 October 2012 to 17 October 2012, pp. 617-622.

Abstract—This paper investigates the effect of fractional linearmisalignment between the transmitter and the receiver for a pixelated MIMO optical wireless system using spatial orthogonal frequency division multiplexing (OFDM). The novelty of this work is in the modeling and analysis of fractional misalignment for spatial OFDM. It is shown that when the receiver is perfectly focused and when fractional misalignment is the only impairment, the received constellation points are phase shifted and attenuated. MATLAB simulations show that the impact of misalignment is the greatest when the fractional offset is equal to half of a pixel. Unlike integral misalignment, the use of equalizers cannot fully compensate the effect of fractional misalignment. It is revealed that when the fractional offset is half of a pixel, the offset causes bit error rate (BER) floor and the degradation is greater for a system with lower number of subcarriers than a system with larger number of subcarriers. We show that the BER degradation caused by fractional offset can be mitigated when a number of higher spatial.