04/22/23

Pengkai Zhao, You Lu, Babak Daneshrad, Mario GerlaElectrical 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

04/22/23

• 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

04/22/23

MIMONode

MIMONode

MIMONode

MIMONode

MIMONode

MIMONode

MIMONode

MIMONode

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

04/22/23

MIMO Node

MIMO Node

MIMO Node

MIMO Node

MIMO Node

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

04/22/23

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

04/22/23

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

04/22/23

Subcarriers in OFDM BW

TDMAMAC

• 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

04/22/23

Primary Link AccessSecondary Link Access

sequentialaccess

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

04/22/23

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

04/22/23

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

04/22/23

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

04/22/23

TDMAMAC

• Two examples• Max-min scheduling – optimize max-min throughput

• LQF scheduling – minimize buffered packets

Cooperative Scheduling with Prioritized Spatial Access

04/22/23

At each data frame:Step 1: Collect achieved long-term throughput from all linksStep 2: Schedule the link with the lowest long-term throughput to be primary linkStep 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 linksStep 2: Schedule the link with largest buffered packets to be primary linkStep 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 802.11n

• Ref design: Single Link MAC & SPACEMAC

Simulation Results

04/22/23

[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 MAC SPACEMACThis 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

04/22/23

Net-Eigen MAC Single Link MAC

SPACEMAC

Max-min thpt 21.3Mbps 17.5Mbps 16.9Mbps

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.

LQF Scheduling Results (buffered packet number)

04/22/23

1000 2000 3000 4000 5000 60000

5

10

15

20

25

30

average packets per second

link

thro

ughp

ut (M

bps)

Single Link MACProposed DesignSPACEMAC

1000 2000 3000 4000 5000 60000

100

200

300

400

500

600

700

800

average packets per second

end-

to-e

nd d

elay

(m

s)

Single Link MACProposed DesignSPACEMAC

0 1000 2000 3000 4000 5000 60000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

average packets per second

loss

ratio

Single Link MACProposed DesignSPACEMAC

symmetrictopology throughput

delay

loss ratio

Poisson packet arrival process

Discussions

04/22/23

• 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.