Physical layer forwarding for 5G multi-hop Backhaul networks

In 5G networks, an integrated access and backhaul network is formed to connect IAB-nodes to the core network via an IAB-donor. To support multi-hop communications in this network, an adaptation-layer forwarding mechanism has been specified in the standard. However, the end-to-end delay achieved by such a mechanism cannot satisfy the requirement of delay-sensitive traffic. To this end, a PHY-layer forwarding mechanism is designed to complement the adaptation-layer forwarding mechanism. In the PHY-layer forwarding mechanism, when a transport block (TB) is received, it is first determined whether or not the TB is forwarded at the PHY layer, based on an ingress indicator that is sent prior to the transmission of the TB. If the ingress indicator shows that the TB is to be forwarded at the PHY layer, the indicator is mapped to an egress indicator that will be sent as an ingress indicator for a certain next-hop node. The process of ingress–egress indicator mapping can be conducted in parallel with TB decoding, so it does not cause additional forwarding delay. To improve the reliability of PHY-layer forwarding, the TB is buffered until it is successfully received, which is achieved by exploiting the hybrid automatic repeat request (HARQ) mechanism of 5G new radio (NR). The end-to-end delay of the PHY-layer forwarding mechanism and the adaptation-layer forwarding mechanism are analyzed and also evaluated via simulations. Performance results show that the PHY-layer forwarding mechanism reduces the end-to-end delay by more than 40% compared with the adaptation-layer forwarding mechanism under various system parameters.