A Low-latency Collaborative HARQ Scheme for Control/User-plane Decoupled Railway Wireless Networks

ArticleThe recently proposed Control/User (C/U) plane decoupled railway wireless network is a promising new network architecture to meet the communication demands of both train control systems and onboard passengers by completely separating the C-plane and U-plane into different network nodes operating at different frequency bands. Although the system capacity of this network architecture can be highly increased, the forwarding latency of X3 interfaces to link the C-plane and U-plane becomes a serious concern, especially for hybrid automatic repeat request (HARQ) protocols which demand frequent interactions between the C-plane and U-plane. This concern becomes more pronounced for latency sensitive train control. To address this challenging problem, in this paper we propose a low-latency collaborative HARQ scheme. Through a newly designed collaborative transmission framework, the possible spare resources on lower frequency bands of macro cells by excluding those used by C-plane transmissions are utilized to help small cells relay erroneously received data. Compared to the conventional HARQ scheme, to reach the same transmission reliability, the proposed scheme requires fewer retransmissions on average, thereby mitigating the latency problem caused by HARQ retransmissions. Correspondingly, the channel mapping is also redesigned to conform to the proposed collaborative transmission framework. In the theoretical analysis, the expression of the average retransmission times related to the sum of independent Gamma variables is developed. Finally, simulation results show that a great decrease in the retransmission latency is gained by the proposed scheme, but at the sacrifice of few average system transmission rate