mimo and tcp: a case for cross layer design soon y. oh, mario gerla computer science dept....
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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
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