Multi-user Optical Wireless Communications Research Articles

Capacity Bounds for the Two-User IM/DD Interference Channel

This paper studies the capacity of the two-user intensity-modulation/direct-detection (IM/DD) interference channel (IC), which is relevant in the context of multi-user optical wireless communications. Despite some known single-letter capacity characterizations for general discrete-memoryless ICs, a computable capacity expression for the IM/DD IC is missing. In this paper, we provide tight and easily computable inner and outer bounds for a general two-user IM/DD IC under peak and average optical intensity constraints. The bounds enable characterizing the asymptotic sum-rate capacity in the strong and weak interference regimes, as well as the generalized degrees of freedom (GDoF) in the symmetric case. Using the obtained bounds, the GDoF of the IM/DD IC is shown to have a ‘W’ shape similar to the Gaussian IC with power constraints. The obtained bounds are also evaluated numerically in different interference regimes to show their tightness, and used to study the performance of on-chip and indoor OWC systems.

Power Allocation for Uplink Multi-User Optical Wireless Communication Systems

Indoor optical wireless communication (IOWC) is a key technology complementing radio communication in 6G. Typically, the downlink of IOWC is implemented using visible light and uplink is implemented using infrared. Though the downlink of IOWC has been studied well, the uplink of multiple-access IOWC requires further research. In this paper, power allocation strategies in the uplink of IOWC with multiple non-cooperative users is studied. Optimizing power allocation in IOWC that maximize sum-rate with power constraints becomes a non-convex problem. First, this paper proposes an alternating maximization algorithm to compute near optimal power allocation policies for small number of users. Next, a phenomenon of invariance is observed when the number of users in the system is large. Under invariance, the optimal power allocation policy of a user becomes independent of other users, thereby facilitating fast and decentralized computation of power allocation strategies. From simulations, it was observed that the invariance can occur even with five to ten users. For the first time, a rigorous analysis of the invariance phenomenon is provided. An algorithm to compute the optimal power allocation at invariance is proposed; this algorithm is proven to achieve the global maxima in sum-rate without the channel knowledge of other users.

High-speed multi-user optical wireless communication between VCSEL-integrated electronic devices

Vertical cavity surface emitting laser (VCSEL) array is increasingly integrated in consumer electronic devices as light sources for biometrics and interaction between terminals. In this study, we proposed a high-speed multi-user optical wireless communication (OWC) method based on a near-infrared VCSEL array, and then experimentally demonstrated a two-user quadrature amplitude modulation (QAM) orthogonal frequency division multiple access (OFDMA) system in a typical indoor environment over a 2-m link. The measured 3-dB bandwidth is around 1.02 GHz for both links, then the maximum sum rate reaches up to 2.7 Gbps for two users with bit-error-rate (BER) of 0 and 3.2×10−3 respectively. The designed VCSEL-based QAM–OFDMA OWC system depicts the feasibility of potential scenarios for the interactive information transfer between terminals.

Capacity Maximization Using Nash Bargaining in Indoor Optical Multiuser Interference-Limited Poisson Channel

In multiuser optical wireless communications (OWC), the multiuser interference becomes significant as the number of users increases. This would lead to a poor quality of service with a low signal-to-interference-plus-noise-ratio (SINR) or even a failure of service. In this paper, we consider coordination among multiple users to maximize the capacity in a multiple-input single-output (MISO) interference-limited discrete-time Poisson channel from a game-theoretic perspective. With power constraints imposed, the resource allocation problem can be interpreted as a downlink beamforming game. To improve the capacity in a power- and interference-limited channel, we model the problem as a cooperative game and propose an altruistic algorithm using the Nash bargaining, where each self-interested user attempts to maximize its capacity through cooperation. Moreover, to compare the performance with a non-game-theoretic approach, a convex optimization framework is developed. It is shown that the proposed Nash bargaining yields significant performance improvement when compared with the Nash equilibrium in the non-cooperative game. It is also found that in a power- and interference-limited channel, self-interested but cooperative users can provide a significant performance improvement over the convex optimization based non-game-theoretic approach.

Interference in multi-user optical wireless communications systems.

Visible light communications (VLC) (including LiFi) represent a subset of the broader field of optical wireless communications. Where narrow beams, typical of free space optical communications are largely free from interference. VLC encompasses use cases involving combined illumination and data access and supporting a wireless access point (AP) model. The use of many units provides scaling of spatial coverage for both lighting and data access. However, AP replication in close proximity creates many interference challenges that motivate the investigation embodied in this paper. In particular, we frame the interference challenge in the context of existing strategies for driving improvements in link performance and consider the impacts of multiple users, multiple sources and multiple cells. Lastly, we review the state of existing research in this area and recommend areas for further study.This article is part of the theme issue ‘Optical wireless communication’.

Spectrum sensing for free space optical communications in strong atmospheric turbulence channel

One of the primary challenges in multi-user optical wireless communications (OWC) is to periodically sense the optical frequency spectrum at the receiver to detect whether the channel is occupied by other transceiver pairs in order to avoid interference. Spectrum sensing enables the user to continuously sense the channel prior to its access to the channel. In this paper, we propose the spectrum sensing of an unknown optical signal over strong atmospheric turbulence channel. The turbulent atmospheric channel causes fluctuations in the received signal intensity, resulting in power loss at the receiver. To mitigate this adverse effect, we present an energy detection based optical spectrum sensing scheme with the spatial diversity reception in the form of a square-law selection diversity. The intensity fluctuations are modeled by a negative exponential distribution. The analytical closed-form expressions for the probability of detection and the probability of false alarm are derived. Based on the derived expressions, we obtain the performance analysis as a function of the system parameters. It is also found that sensing capability is boosted with the square-law diversity scheme.

Cyclic code‐shift extension keying for multi‐user optical wireless communications

An optical code-division multiple access (OCDMA) scheme based on cyclic shift extension of a base code is proposed to enable asynchronous multi-user optical wireless communications for very populated scenarios. This multiple access method is convenient for low–medium data rate communications of a large number of simultaneous transmitters. An expression to obtain a tight upper bound of the bit-error probability of the new scheme is provided which is also confirmed by experimental results. A variant of the proposed communication method which allows the encoding of several bits per symbol is also presented, although its analysis demonstrates that the first simpler strategy reaches similar performance with a significantly lower receiver complexity.