MIMO-CAST: A CROSS-LAYER AD HOC MULTICAST PROTOCOL

USING MIMO RADIOS

Soon Y. Oh*, Mario Gerla*, Pengkai Zhao**, Babak Daneshrad***Computer Science Dept., **Electrical Engineering Dept.

University of California, Los AngelesGuangyu Pei, Jae H. Kim

Boeing Phantom Works

Outline

• Multicast ODMRP Operation– Hidden terminal problem

• Multi Point Relay – partial solution of the Hidden Terminal Problem

• The MIMO Solution

• MIMO-CAST

• Simulation Results

S

R

R

R

RJoin Query

Join Reply

Forwarding node

Link

Multicast Mesh

Forwarding Mesh

ODMRP Mesh Structure

A

C

SB

Hidden Terminals in ODMRP

• Source S broadcasts to A and B• In turn A and B re-broadcast• A and B hidden Node C experiences a collision

Multi Points Relayto alleviate the Hidden Terminal Problem

• MPR is a feature of the OLSR routing protocol - used in Link State update dissemination

• Each node chooses a set of nodes (MPR Selectors) in the neighborhood, which will retransmit its packets.

• MPR alleviates, but does not eliminate hidden terminals

24 retransmissions to diffuse a message up to 3 hops

Retransmission node

11 retransmission to diffuse a message up to 3 hops

Retransmission node

Enter the MIMO Solution

• MIMO benefits

AB

CD

Interference range of A

Interference range of B

Reduced interference range of AA

BC

D

E

F

– Beamforming• Achieving space reuse

– Multiple streams• Increasing point-to-point capacity

We exploit the Beamforming feature of MIMO

MIMO Channel: Linear System

ry

y

y

y2

1

tx

x

x

x2

1

rtrr

t

t

hhh

hhh

hhh

...

.

...

...

H

21

22221

11211

where

n

+

+

+

)( )( )( kkk nx Hy )( )( 1 kk yHx

MIMO Beamforming

r = wRHHwTs

wherewT = [wT1 wT2 wT3]T: tx weights,wR = [wR1 wR2 wR3]T: rx weights,H: 3x3 channel matrix,

wT1

wT2

wT3

wR1

wR2

wR3

H

Transmitter Receiver

s r

wR1

wR2

wR3

hT = HwT

Transmitter Receiver

s

r = wRHhTs

where

hT = HwT: equivalent to 3x1 vector channel

• Estimate hT on reception of Channel

Learning Preamble

Nullifying

• On reception of Channel Learning Preamble from A, Node D learns hA = HwA

r = wDHhAs

• To null A, D finds wD

such that wDHhA = 0

• n-1 interferers are nullable with n antennas

A B

D

C

MIMO MAC Protocol

• MIMO-CAST uses Beamforming Technique

• Channel Learning– Before transmitting a data packet, the upstream node sends

a CL (Channel Learning) Preamble including weight vector– Upon receiving CL Preamble, the intended receivers

adjusts their weight vectors (to optimize reception)

• Blocking Interference– If a node receives a non-intended CL Preamble, it

recalculates own weight vector to null the signal

Simulation Settings

• Simulation Environments– Qualnet– 200 nodes in 1000m x 1000m– 1500 bytes/packet– 802.11b 15dBm transmission power– 2Mbps channel capacity and 370m radio range

• Metrics– Packet Deliver Ratio : the fraction of packets received averaged over all

receivers– Throughput : total received byte of data packet divided by the total simulation

time– Average End-to-End delay : the averaged time taken for a packet to be

transmitted across the network form a source to a receiver

• Compare MIMO-CAST with MPR-Multicast with SISO and ODMRP

Hidden Terminal Scenario

2

34

1

Source

Forwarder

Receiver

• Build a topology that emphasizes the hidden terminal problem

• Node 1,2,3, and 4 receive duplicated packets

Packet Deliver Ratio in Hidden Terminal Scenario

• Packet Deliver Ratio– the fraction of packets received averaged over all receivers

Packet Delivery Ratio

0102030405060708090

100

MIMO-CAST MPR Multicast ODMRP

Delivery Ratio

Throughput in Variable Traffic Scenario

• Data sending rate increases from 10packets/s to 200packets/s• 1 source, 100 members among 200 nodes

Throughput with various Traffics

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

10pkts/s 50pkts/s 100pkts/s 150pkts/s 200pkts/s

Traffic (pkts/s)

Throughput(KB/s).

MIMO-CAST

MPR Multicast

ODMRP

Throughput vs Increasing Membership

• Number of receiver increases from 20 members to 100 members• 1 source and 200 packets/s with 1500 byes/packet

Throughput with various Members

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

20 40 60 80 100

Number of Members

Throughput (KB/s)

MIMO-CAST

MPR Multicast

ODMRP

Average End-to-End Delay vs Increasing Membership

• Number of receiver increases from 20 members to 100 members• 1 source and 200 packets/s with 1500 byes/packet

Average End-to-End Delay with Various Members

0.000

0.050

0.100

0.150

0.200

0.250

0.300

20 40 60 80 100

The Number of Members

Average End-to-EndDelay (Millisecond)

MIMO-CAST

MPR Multicast

ODMRP

Conclusions

• MIMO-CAST is a cross-layer multicast protocol– Selective reception feature of the MIMO system is used to improve multicast

(at the network layer)

• Significantly reduces duplicated retransmission

• Reduces delays

• Simulation results confirm MIMO-CAST performs far better than conventional multicast protocols with IEEE 802.11 and a SISO system

• Future work:– Use MIMO to combine (in phase) instead of nullifying competing broadcasts

(cooperative radio approach)– Testbed Implementation

• This will allow verification of the simplifying assumptions used in simulation