4/30/2015 Pengkai Zhao, You Lu, Babak Daneshrad, Mario Gerla Electrical Engineering/Computer Science, UCLA Cooperative Spatial Scheduling in Distributed MIMO MAC with Interference Awareness

Background & Motivation Contribution System Model Net-Eigen MAC: Primary Link Net-Eigen MAC: Secondary Link Cooperative Scheduling Results and Discussion Outline 4/30/2015

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. Background & Motivation 4/30/2015 Concurrent Link Access

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. Background & Motivation 4/30/2015 MIMO Node Priority 1 link Priority 2 link Priority 3 link

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 Major Works 4/30/2015 Link Scheduling Task: (TDMA MAC) (1) Schedule active links per time frame (2) Prioritize access order of these links

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. Major Works 4/30/2015 Modify Net-Eigen MAC to support two concurrent links per time frame Two typical scheduling policies that use prioritized concurrency hierarchy

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 System Model 4/30/2015 Subcarriers in OFDM BW TDMA 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. Net-Eigen MAC 4/30/2015 Primary Link Access Secondary Link Access sequential access time delay

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 Net-Eigen MAC 4/30/2015 primary link access secondary 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 4/30/2015 primary 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 Access 4/30/2015 secondary link: SNR optimization secondary link: Interference nulling

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 Cooperative Scheduling with Prioritized Spatial Access 4/30/2015 TDMA MAC

Two examples Max-min scheduling – optimize max-min throughput LQF scheduling – minimize buffered packets Cooperative Scheduling with Prioritized Spatial Access 4/30/2015 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 2 nd 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

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 n Ref design: Single Link MAC & SPACEMAC Simulation Results 4/30/2015 [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 Single Link MACSPACEMAC This MAC enables one single link at each data frame. This MAC only nulls interference, but has no Max-SNR beam-vector updating for secondary link. (details in [1] & [2])

Max-min Scheduling –Maximize the minimum throughput in the network –Consider 4 links and an asymmetric topology –Investigate max-min throughput results Simulation Results 4/30/2015 Net-Eigen MACSingle Link MAC SPACEMAC Max-min thpt21.3Mbps17.5Mbps16.9Mbps Max-min Scheduling Schedule primary link as the one with lowest long-term rate. Schedule secondary link as the one with 2 nd lowest long-term rate.

LQF Scheduling Results (buffered packet number) 4/30/2015 symmetric topology throughput delay loss ratio Poisson packet arrival process

Discussions 4/30/2015 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.