Experimental Validation of Near-Far-Resilient Asynchronous Uplink Multiple Access for Underwater Optical Wireless Vehicle Networks
This paper experimentally validates a near-far-resilient asynchronous uplink multiple access (MA) scheme for underwater optical wireless vehicle networks (UOWVNs) based on code division multiple access (CDMA). In UOWVNs, asynchronous packet arrivals and motion-induced delay fluctuations cause chip misalignment, degrade the spreading-code orthogonality, and generate dynamic multiuser interference (MUI) under received-power imbalance (near-far) conditions. Because a 1-Gchip/s chip rate is desirable for 4K video uplinks, continuous sub-nanosecond delay estimation and decorrelator regeneration under vehicle motion can impose real-time implementation complexity. To address this challenge, we propose a novel 1-Gchip/s asynchronous uplink MA scheme for UOWVNs that utilizes a length-16 Hadamard code pair with low aperiodic cross-correlation, packet-embedded pilots for per-user chip alignment, and an adaptive interference cancellation (AIC) receiver. The AIC updates despreading weights once per packet to suppress dynamic MUI without sub-nanosecond relative delay estimation. The proposed scheme was experimentally validated using two underwater optical wireless links, achieving an effective bit rate of 59.9 Mb/s per user. Under near-far conditions, the AIC with three-symbol stacking reached the forward error correction limit at signal-to-interference ratios (SIRs) of approximately -9.5 dB for the desired user 1 case and -8.1 dB for the desired user 2 case, and remained below the limit for relative delays of up to 3 ns at SIRs of -7 dB and -6 dB, respectively. Additionally, user scalability was evaluated by simulation, indicating feasibility for at least five active users, and operation-count analysis revealed linear receiver-complexity scaling.