###### Abstract  
Round Robin based Intermittent Periodic Transmit (RR-IPT) has been proposed which achieves highly efficient multi-hop relays in multi-hop wireless backhaul networks (MWBN) where relay nodes are 2-dimensionally deployed. This paper newly investigates multi-channel packet scheduling and forwarding scheme for RR-IPT. Downlink traffic is forwarded by RR-IPT via one of the channels, while uplink traffic and part of downlink are accommodated in the other channel. By comparing IPT and carrier sense multiple access with collision avoidance (CSMA/CA) for uplink/downlink packet forwarding channel, IPT is more effective in reducing packet loss rate whereas CSMA/CA is better in terms of system throughput and packet delay improvement. wireless backhaul; multi-hop; intermittent periodic transmit; round robin; multi-channel; CSMA/CA +
Footnote †: journal:  
0000-0002-4891-9900-3890][PERSON] 0000-0002-0002-1899-2244][PERSON] 0000-0002-1898-2284][PERSON] 0000-0002-189-2284  
in hop count [17]. In addition, each node communicates based on a distributed coordination function (DCF), i.e., carrier sense multiple access with collision avoidance (CSMA/CA), so that the system is often dominated by packet collisions. It severely worsens the system capacity. For tree topology based multi-hop networks, we previously proposed intermittent periodic transmit (IPT) protocol which achieves a high efficient packet forwarding in a simplified manner [18]. Its fundamental effectiveness has also been experimentally verified [19; 20; 21] and commercialization is ongoing [22]. Original IPT design was limited for uni-directional forwarding in a string-type topology. Its extensions for bi-directional traffic accommodation and 2-dimensional topologies have been developed. Bi-directional IPT (Bi-IPT) [23] is a simple modification where reverse link (uplink) transmission is also triggered by hearing of forward link (downlink) packet reception. Although Bi-IPT enables collision-free bi-directional duplex, its throughput improvement is limited due to the increase of packet delay especially in uplink. For 2-dimensional extension, path-reservation (PR-IPT) [24] and round robin packet scheduling (RR-IPT) [25] have been proposed. The former scheme forces an extended network allocation vector (NAV) to surrounding nodes on an IPT forwarding path. It prevents active nodes from packet transmission to awaiting nodes and hence it causes frequent retransmissions resulting in packet loss. In addition, RR-IPT with multi-channel packet forwarding [26] was found as an efficient solution keeping IPT's advantages with accommodating up and downlink traffic. In [26], relay nodes have two wireless interfaces, each of which is assigned different channel delivering downlink and uplink traffic respectively. In this scheme, downlink traffic is exclusively assigned to the 1 st channel and forwarded by RR-IPT. The 2 nd channel is assigned uplink traffic with the use of the conventional CSMA/CA. However, its spectral efficiency cannot be maximized since uplink traffic is generally smaller than downlink; overall system capacity enhancement is limited. Meanwhile, there exists a lot of literature investigating multi-channel medium access control (MAC) protocols. Use of multiple channels can enhance the network capacity and it has been a challenge solving their efficient allocation as well as multi-channel hidden terminal problems in distributed/asynchronous manners. Handshakes in multiple channels [27] may cause gratuitous radiation interfering with other wireless nodes. Most of the solutions prepare a control channel [28; 29; 30] dedicated to channel determination. It may cause overhead called control channel bottleneck [29] and it wastes spectrum resources which cannot be used for data transmission. Assuming a single transceiver, channel switching incurs additional delay around several micro seconds [28; 29; 31]. Furthermore, the number of channels available is limited in practical situations.