Pengkai Zhao, You Lu, Babak Daneshrad, Mario Gerla

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.