A Low-Latency Collaborative HARQ Scheme for Control/User-Plane Decoupled Railway Wireless Networks

The control/user (C/U) plane decoupled railway wireless network is an innovative architecture recently proposed to meet the communication demands of both train control systems and onboard passengers. The core idea is to completely separate the C-plane and the 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 the U-plane becomes a serious problem, particularly for hybrid automatic repeat request (HARQ) protocols that demand frequent interactions between the C-plane and the U-plane. To address this challenging problem, we propose a low-latency collaborative HARQ scheme in this paper. Specifically, we develop a new collaborative transmission framework where the possible spare resources on lower frequency bands of macrocells excluding those used by C-plane transmissions can be utilized to help small cells relay erroneously received data. Compared with the conventional HARQ scheme, the proposed scheme requires fewer retransmissions to reach the same transmission reliability, thereby mitigating the latency problem caused by HARQ retransmissions. Furthermore, channel mapping is also redesigned to conform to the proposed collaborative transmission framework. Through theoretical analysis, we derive the expression of the average number of retransmissions related to the sum of independent Gamma variables. Finally, the results of simulation experiments show that the proposed scheme can largely decrease the retransmission latency for railway wireless networks.