MIMO and TCP: A CASE for CROSS LAYER DESIGN

Soon Y. Oh, Mario GerlaComputer Science Dept. University of California, Los Angeles

{soonoh, gerla}@cs.ucla.eduJoon-Sang Park

Dept. of Computer Engineering, Hongik University, Seoul Koreajsp@cs.hongik.ac.kr

Introduction

• TCP performs poorly in wireless networks– Mobility leads to path breaks and TCP time outs

– Radio channel problems (noise, fading, jamming etc) cause pkt loss and throughput degradation

– TCP and IEEE 802.11 time out interactions cause unfairness and capture

• In this paper we focus on Capture

Example of TCP Capture

• FTP Flows 1, 2, 3 use TCP

•The flows “interfere” with each other

• Interference causes capture

Flow 1

Flow 2

d

d

Reception RangeInterference

rangeNode C

Node A

Flow 3

Node B

CAPTURE in the current system

No Fading vs. Rayleigh Fading

0

50

100

150

200

250

300

350

400

450

No Fading Rayleigh Fading

Throughput (Kbits/s) Flow 1 Flow 2 Flow 3

Solutions to the Capture Problem

• Previous solutions– MAC Layer: modify the IEEE 802.11 retransmission

mechanism– Network layer (NRED): Selective drop of packets in the

aggressive flows– Problems – both schemes require non trivial changes in

protocols

• Proposed solution - Physical and MAC Layer– Use MIMO antenna weights to minimize interference– Use SPACE-MAC, a MIMO aware MAC protocol

SPACE-MAC

• Targets Beamforming MIMO

• Enables multiple communications by nullifying interferers

• Uses RTS/CTS exchange to learn about channel A

B

D

F

SPACE-MAC PHY Model

r(t) = wRTHwTs(t)

Where

wT = [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(t) r(t)

Operation of SPACE-MAC

When A wishes to transmit to B

A

B

D

F

Operation of SPACE-MAC

A

B

D

F

1) A sends RTS to B; D and F learn about A

Operation of SPACE-MAC

A

B

D

F

2) B responds with CTS; D and F learn about B

A

B

D

F

Operation of SPACE-MAC

3) D and F beamform; signals from/to B and A are nulled;

4) A and B start communicating

Operation of SPACE-MAC4) While A and B are communicating, D and F also can start talking

A

B

D

F

D

Simulation Settings

• Simulation environment– Qualnet– Rayleigh fading– 512 bytes/packet– 802.11b; 2Mbps channel data rate;370m radio range– 5 antennas for each node

• TCP experiments with:– SPACE-MAC– conventional IEEE 802.11 MAC

CASE 1: 3 FTP/TCP Flows Topology• 3 parallel FTP/TCP flows• d = 350 ~ 400 m

Flow 1

Flow 2

d

d

Reception RangeNode C

Node A

Flow 3

Node B

CASE 1: d = 400m Throughput

• Distance between intermediate nodes: d = 400m• A and C are out of B’s reception range (no nulling)• However, A and C are within B interference range• No interference between A and C

3 FTP/TCP Flows

0

50

100

150

200

250

300

350

802.11 SPACE-MAC

MAC Protocol

Throughput (Kbits/s)Flow 1 Flow 2 Flow 3

CASE 1: d = 350m Throughput

• Distance between intermediate nodes: d = 350m• A and C are within B’s reception range (SPACE MAC can null )• A and C interfere with each other

3 FTP Flows

0

50

100

150

200

250

300

350

802.11 Space-MAC

MAC Protocol

Throughput (Kbits/s)Flow 1 Flow 2 Flow 3

CASE 1: d = 350m Total Throughput

• Distance between intermediate nodes: d = 350m• A and C are within B’s reception range (SPACE MAC can null )• A and C interfere with each other

Total Throughput

0

100

200

300

400

500

600

700

800

802.11 Space-MAC

Throughput (Kbit/s)

CASE 2: More General Scenario

• 100 nodes uniformly distributed in 750m x 750m area• Random 20 FTP/TCP flows• Throughput of each flow

Random 20 Flows

0

50

100

150

200

250

300

350

Recievers

Throughput (Kbits/s)802.11 Space-MAC

CASE 2: More Realistic Scenario

• 100 nodes uniformly distributed in 750m x 750m area• Random 20 FTP/TCP flows• Aggregated throughput

Total Throughput

0

200

400

600

800

1000

1200

1400

1600

802.11 SPACE-MAC

Throughput (Kbit/s)

Conclusions

• “Capture” seriously impacts TCP performance in wireless Ad Hoc networks

• MIMO and SPACE MAC beamforming prevents capture by “deconflicting” the flows

• Moreover, MIMO increases total TCP throughput by reducing interference

• Future work:– Testbed experiments using MIMO and Space MAC