This paper presents a novel massive multiple-input multiple-output (MIMO) transmission in beam domain for optical wireless communications. The optical base station equipped with massive optical transmitters communicates with a number of user terminals (UTs) through a transmit lens. Focusing on LED transmitters, we analyze light refraction of the lens and establish a channel model for optical massive MIMO transmissions. For a large number of LEDs, channel vectors of different UTs become asymptotically orthogonal. We investigate the maximum ratio transmission and regularized zero-forcing precoding in the optical massive MIMO system, and propose a linear precoding design to maximize the sum rate. We further design the precoding when the number of transmitters grows asymptotically large, and show that beam division multiple access (BDMA) transmission achieves the asymptotically optimal performance for sum rate maximization. Unlike optical MIMO without a transmit lens, BDMA can increase the sum rate proportionally to 2K and K under the total and per transmitter power constraints, respectively, where K is the number of UTs. In the non-asymptotic case, we prove the orthogonality conditions of the optimal power allocation in beam domain and propose efficient beam allocation algorithms. Numerical results confirm the significantly improved performance of our proposed beam domain optical massive MIMO communication approaches.
Index TermsOptical massive MIMO communication, transmit lens, Beam division multiple access (BDMA). Optical wireless communication systems rely on optical radiations to transmit information with wavelengths ranging from infrared to ultraviolet [1]. Base station (BS) commonly employs optical transmitters, such as light-emitting diodes (LEDs), to convert the electrical signals to optical signals. Recently, laser diodes (LDs) are considered as potential sources for optical communication due to high modulation bandwidth, efficiency, and beam convergence [2]. User terminals (UTs) employ photodetectors like photodiodes as optical receivers to convert the optical power into electrical current. Optical communications can significantly relieve the crowed radio frequency (RF) spectrum, provide high speed data transmission [3], and achieve simple and lowcost modulation and demodulation through intensity modulation and direct detection (IM/DD) [4]. Thus, optical wireless communication has attracted increasing attention from both academia and industry [5]-[7]. To achieve high data rate in optical communications, multiple separate LED/LD arrays are usually utilized in the BS to provide higher data rate by means of spatial multiplexing. As a result, multiple-input multiple-output (MIMO) technique is a natural progression for optical communication systems [7], [8]. As the nature of optical downlink communication is broadcast network, multiple UTs should be well supported. BSs simultaneously transmit signals to all UTs, resulting in the so-called multi-user interference, consequently degrades the performance. Thus, mul...