Rate Splitting in Laser-based Optical Wireless Networks

Optical wireless communication (OWC) systems using infrared lasers as transmitters, offer high capacities compared to radio frequency (RF) networks. In this work, the primary focus is on interference management in such systems.
The work introduces for the first time rate splitting (RS) in a laser-based OWC system. Multiple optical access points (APs) in a laser-based system can serve multiple users simultaneously by splitting the message of a user into common and private messages, each message with a certain level of power, while on the other side users decode their messages following a specific methodology. Interestingly, the power must be carefully allocated between these messages to minimize multi-user interference (MUI) and maximize the spectral efficiency of the network. The RS strategy offers higher sum rates compared to traditional interference management schemes. However, in scenarios with high number of users, the performance of RS faces severe limitations due to noise enhancement.
To address this challenge, a two-tier precoding RS scheme called hierarchical rate splitting (HRS) is proposed for a multi-user signal cell OWC network to enhance the sum rate and alleviate channel state information (CSI) requirements. In HRS, users are divided into multiple groups, and outer RS is applied to manage inter-group interference, while inner RS manages intra-group interference. This methodology requires a new message, referred to as the outer common message, to manage inter-group interference. Therefore, the power budget must be used efficiently among the three messages to maximize the sum data rate of the network. In this context, an optimization problem is formulated for power allocation under certain constraints, and the results demonstrate that HRS achieves high performance in dense OWC networks compared to RS and OMA.
In laser-based OWC networks, users might experience severe ICI due to the confined coverage area of the optical AP, therefore the coordination among the optical APs is necessary. A third scheme using blind interference alignment (BIA) with RS is introduced to address these challenges. In BIA-RS, users are spatially divided into different groups, where RS manages intra-group interference and BIA offers coordination among multiple optical APs and allocates non-orthogonal resources to all groups with guaranteed inter-group interference cancellation. To further enhance the performance of this scheme, an optimization problem is formulated for power allocation to maximize the minimum sum rate within each group. The proposed BIA-RS scheme provides higher sum rates compared to benchmarking schemes such as BIA, RS, and NOMA.
Finally, in OWC, power allocation optimization problems are complex. Therefore, deep neural network (DNN) models are introduced to obtain real-time solutions with low computational complexity. The results show the effectiveness of the trained DNN in enhancing the performances of the proposed transmission schemes, HRS and BIA-RS.