Transmission Scheduling for PHY-Layer Wireless Network Coding

1. Transmission Scheduling for PHY-Layer Wireless Network Coding APCC 2010, 1 Nov. 2010 Hiroyuki Yomo1,4Masaki Bandai2 Takashi Watanabe3Sadao Obana4 Faculty of Engineering Science, Kansai University, Japan1 Department of Information and Communication Sciences, Sophia University2 Graduate School of Science and Technology, Shizuoka University3 ATR Adaptive Communications Research Laboratories4 This work is supported by a Grant-in-Aid for Scientific Research (A) (no. 20240005) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

2. Agenda Introduction System Model PHY Layer Wireless Network Coding(PL-WNC) Transmission Scheduling for PL-WNC Simulation Results and Discussions Summary

3. Introduction Wireless Network Coding (WNC) Combining/forwarding of packets belonging to different communication flows Throughput improvement in two-way (bi-directional) relay channel Coding in packet level XORWNC (XOR-WNC) Combining in symbol level PHY LayerWNC (PL-WNC) Step 1 Step 2 Step 3 B B B DAC DCA C C C A A A DAC DCA Step 1 Step 2 B B xAC xCA C A C A xB

4. Introduction - Cont.- • Unicast packet exchange among multiple sources through a single relay • Bi-directional traffic flow does not always exist • Creating coding opportunities with overhearing • Transmission scheduling for PL-WNC • Pair-wise scheduling • Challenges • Lack of queue status • Overhearing affected byinterference between nodes 8 7 1 6 R 2 5 3 4

5. System/Traffic Model Star Topology Network One relay node and N neighborsat vertices of a regular N-sided polygon (relay at center, line for N=2) Average SNR for neighbor-relay Average SNR among neighbors is scaled from based on distance (path loss decay factor: 4) Block Rayleigh flat fading channel MAC: polling (slotted channel) BPSK Traffic Model Traffic requiring relay: among neighbors which have average SNR less than S uni-directional unicast sessions per node (destination is randomly selected, fully loaded condition) dNR dNR 1 R 2 N = 2 8 7 1 6 dNR R 2 5 3 4 N = 8

6. PL-WNC 1st Slot: Two nodes transmit packets simultaneously Tx: Node 1 (packet for node 3) and Node 2 (packet for node 4) Rx signal at relay: 2nd Slot: Relay amplifies/forwards the received signal Rx signal at node 3 Node 3 decodes x24from Rx signal in the 1st slot Cancel interferencefrom Rx signal in the2nd slot (noise powerand CSI: known) SNR at node 3 Interference 1st slot 2nd slot 4 3 4 3 R R 1 1 2 2

7. Transmission Scheduling for PL-WNC (1) Scheduling for PL-WNC Select two nodes transmitting simultaneously such that throughput is maximized Best strategy: always select two nodes having bi-directional traffic flow The destination has the a priori information to cancel interference component with probability of 1 Problems Tx queue status at neighbors: unknown at relay Bi-directional traffic does not always exist between neighbors Overhearing failure due to interference mutually caused by end-nodes

8. Transmission Scheduling for PL-WNC (2) Step 1 Relay polls two neighbors (senders) with a single polling message A primary polled (PP) node is selected in turn from neighbors The secondary polled (SP)node is randomly selected out of neighbors with which the PP node needs relaying to deliver packets The polling message includesinformation on the two pollednodes: each polled node knows the other polled node 8 7 PP node SP node 1 6 R 2 5 3 4

9. Transmission Scheduling for PL-WNC (3) Step 2 The polled nodes decide packets to transmit according to Tx queue status (Silent if there is no packet to transmit) If there is a packet destined to the other polled node, transmit that packet Otherwise, calculate the following metric for each possible destination j (case for PP node) Select a destination j* as Transmit only when Avg. SNR for SP-j 8 7 PP node Avg. SNR for PP-j SP node 1 6 R 2 5 3 4

10. Simulation Model Performance criterion: Throughput (average number of successfully received packets per slot) (w/o retransmission) Channel gain: constant over 2 slots Overhead for WNC coding/decoding: Ignored Parameters Packet size:1000 bits Threshold to decide whether there is traffic to be relayed between neighbors: Comparison Target COPE-like XOR WNC [1] W/O WNC [1] H. Yomo et al., “Throughput enhancement using wireless network coding in star topology network,” in Proc. Of WPMC 2009, Sendai, Japan, Sept. 2009

11. Simulation Results (1) N = 16, = 20 [dB] N = 16, = 30 [dB] Tx even with low SIR for overhearing Prevent often simultaneous Txs dB is employed

12. Simulation Results (2) N = 16, S = 1 N = 16, S = 5 PL-WNC-RS: In the 2nd step of PL-WNC scheduling, the polled nodesselect packets to transmit randomly (w/o considering SIR conditionsamong neighbors)

13. Summary We have applied PL-WNC to a scenario where multiple sources exchange packets through a single relay We have proposed a scheduling for PL-WNC Adapting to traffic pattern Decreasing the impact of overhearing failure Throughput is significantly enhanced in PL-WNC with the proposed scheduling Future work Scheduling for the other PL-WNC Evaluation in mesh-network topology Evaluation with rate adaptation

14. XOR-WNC Up-link Phase Relay sends polling messages to neighbors in turn Each neighbor sends a packet to relay if any If there are several packets with different destinations, a packet is randomly selected Each neighbor tries to overhear packets transmitted by the other neighbors Down-link Phase Relay keeps statistics of overhearingamong neighbors Relay decides a combination of packets (to be XORed) and the order of transmissions such that the required number of transmissions is minimized Up-link Phase 4 3 R 1 2 Down-link Phase 4 3 R 1 2

15. XOR-WNC (2) Down-link Phase Relay forwards all the packets gathered in the up-link phase (with and/or without XOR) Relay calculates packet delivery rate (PDR) from overhearing status: If PDR is larger than PDRth, it assumes that overhearing was successful Based on the information on source, destination, and overhearing estimation of each packet, relay decides a combination to apply XOR such that the required number of transmission is minimized: NP-Complete Source: 4 Destination: 1 Overheard by2, 3, 5 (estimation) 4 宛先 1 2 3 5 6 7 8

16. XOR-WNC (3) Heuristic ApproachStep 1: Include all the packets in the head of Tx queues into SCStep 2: Check possibility of XOR for packets in SCStep 3: If coding is possible in Step 2, select a coded set with maximum number of packets included. Insert the coded packet into ST and remove packets included into the coded packet from SCStep 4: If coding is impossible in Step 2, select a packet in the queue having multiple packets to be forwarded. If there is no such a queue, select a packet randomly. Insert the selected packet into STand remove it from SCStep 5: Repeat Step 1-4 until SCbecomes emptyStep 6: Transmit packets in ST 4 1 2 3 5 6 7 8 Destination