Cooperative Spatial Scheduling in Distributed MIMO MAC with Interference Awareness

1. Cooperative Spatial Scheduling in Distributed MIMO MAC with Interference Awareness Pengkai Zhao, You Lu, Babak Daneshrad, Mario Gerla Electrical Engineering/Computer Science, UCLA

2. Outline • Background & Motivation • Contribution • System Model • Net-Eigen MAC: Primary Link • Net-Eigen MAC: Secondary Link • Cooperative Scheduling • Results and Discussion

3. Background & Motivation • MIMO beamforming enabled concurrent spatial access • MIMO is used to enable concurrent links to transmit/receive simultaneously. • Net-Eigen MAC uses MIMO beam-vectors to (a) null the interference of concurrent links and (b) maximize SNR within the desired link • Such concurrent MIMO spatial access provides unique opportunities for other layers in network. Concurrent Link Access

4. Background & Motivation • Prioritized concurrency hierarchy • The earlier the access order, the higher the priority in utilizing channel resource • Because newly accessed link should use beam-vectors to null interference on existing links • This feature can be used by MAC/higher layers for performance improvement. MIMO Node MIMO Node Priority 1 link MIMO Node MIMO Node Priority 2 link MIMO Node MIMO Node Priority 3 link

5. Major Works • Will present initial results for cooperative scheduling of concurrent links with spatial-MIMO access. • Key idea: using prioritized concurrency hierarchy in spatial MIMO MAC to schedule (at each time frame): • Active concurrent links per time frame • Prioritized access order within that time frame Link Scheduling Task: (TDMA MAC) (1) Schedule active links per time frame (2) Prioritize access order of these links

6. Major Works • Two major steps: • Modify Net-Eigen MAC to support only two concurrent links (primary link and secondary link). • Compared to original Net-Eigen MAC, such design has less protocol overhead, higher efficiency, and simplified operation. • Present two typical scheduling policies that utilize spatial priorities between primary/secondary links. Modify Net-Eigen MAC to support two concurrent links per time frame Two typical scheduling policies that use prioritized concurrency hierarchy

7. System Model • One-hop Ad Hoc networks with multiple independent links • MIMO-OFDM is adopted as PHY tech • Tx/Rx beam-vectors are applied at each subcarrier • For simplicity, assume 4 antennas per node • MAC layer uses TDMA based protocol • Transmission timeline is separated into independent time frames with equal duration Subcarriers in OFDM BW TDMA MAC

8. Net-Eigen MAC • Modified Net-Eigen MAC • Support only two concurrent links per time frame: primary link and secondary link. They access the channel in a sequential way. • Primary link has higher priority in utilizing the link: • Secondary link should set its Tx/Rx beam-vectors to null interference on primary link. • Primary/Secondary links’ MIMO beam-vectors are formulated via short control packets/channel learning process. Primary Link Access sequential access Secondary Link Access time delay

9. Net-Eigen MAC • Access procedure: • Primary link first accesses channel and sets its Tx/Rx beam-vectors to maximize SNR • After primary link, secondary link accesses channel but introduces null interference on primary link. • Under this constraint, secondary link further updates its Tx/Rx vectors to optimize SNR primary link access secondary link access

10. Primary Link Access • Primary Link Access: • Primary link uses RTS packet to learn the channel. • It runs an SVD decomposition over the channel response. • Its Tx/Rx vectors are set as left/right eigen-vectors of SVD results • See detailed description in Algorithm 1 in paper primary link access

11. Secondary Link Access • Secondary Link Access: Interference Nulling • Secondary link learns its channel to/from primary link via primary link’s control packets . Its initial Tx/Rx vectors are derived to null interference to/from primary link. (See Algorithm 2 in paper) • Secondary Link Access: SNR Optimization • Under the constraint of nulling interference on primary link, secondary link further update its Tx/Rx vectors to optimize efficient SNR • Such optimization is achieved via an SVD decomposition over the efficient nulling-space that is orthogonal to primary link (See Algorithm 3 in paper). secondary link: Interference nulling secondary link: SNR optimization

12. Cooperative Scheduling with Prioritized Spatial Access • Consider an Ad-Hoc like networks with multiple independent links • Use centralized or distributed scheduler that is similar to WiMAX mesh TDMA MAC • Schedule links in the network to be primary/secondary links at different time frames • Given that primary link has higher priority in utilizing the channel, it has higher priority in scheduling TDMA MAC

13. Cooperative Scheduling with Prioritized Spatial Access • Two examples • Max-min scheduling – optimize max-min throughput • LQF scheduling – minimize buffered packets At each data frame: Step 1: Collect achieved long-term throughput from all links Step 2: Schedule the link with the lowest long-term throughput to be primary link Step 3: Schedule the link with the 2nd lowest long-term throughput to be secondary link At each data frame: Step 1: Collect buffered packet number from all links Step 2: Schedule the link with largest buffered packets to be primary link Step 3: Schedule the link with 2nd largest buffered packets to be secondary link

14. Simulation Results • Single-Hop Ad-Hoc networks with 4 independent links • 4 antennas per node, data frame = 5ms. PHY is built on MIMO-OFDM system similar to 802.11n • Ref design: Single Link MAC & SPACEMAC [1] J.-S. Park, A. Nandan, M. Gerla, and H. Lee, “SPACE-MAC: enabling spatial reuse using MIMO channel-aware MAC” ICC’04 [2] P. Zhao and B. Daneshrad, “Net-eigen mac: A new mimo mac solution for interference-oriented concurrent link communications” MILCOM 2011

15. Simulation Results • Max-min Scheduling • Maximize the minimum throughput in the network • Consider 4 links and an asymmetric topology • Investigate max-min throughput results Max-min Scheduling Schedule primary link as the one with lowest long-term rate. Schedule secondary link as the one with 2nd lowest long-term rate.

16. LQF Scheduling Results (buffered packet number) symmetric topology throughput Poisson packet arrival process delay loss ratio

17. Discussions • This study uses spatial MIMO access as underlying MAC protocol to enable concurrent links in the network • Our design schedules concurrent links according to prioritized concurrency hierarchy in spatial MIMO MAC. • Two typical examples • Max-min scheduling: maximize the minimum long-term rate • LQF scheduling: minimize buffered packets • Future works: • Mathematically scale Net-Eigen MAC’s performance and apply it in more complicated network optimization problem. • Extend to multi-hop situations and QoS based scheduling.