MIMO and TCP: A CASE for CROSS LAYER DESIGN Soon Y. Oh, Mario Gerla Computer Science Dept. University of California, Los Angeles {soonoh, Joon-Sang Park Dept. of Computer Engineering, Hongik University, Seoul Korea

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 time out interactions cause unfairness and capture In this paper we focus on Capture

Possible causes of “capture” Spatial Reuse –Multiple flows compete for shared medium in the same collision domain –One of the flows may capture the channel Range Dependency –Reception range R 1 and interference range R 2 R 1 < R 2 –Hidden/Exposed terminal problem and packet collision Topology –Unequal channel access opportunity

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 Range Interference range Node C Node A Flow 3 Node B

CAPTURE in the current system

Solutions to the Capture Problem Previous solutions –MAC Layer: modify the IEEE 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) = w R T Hw T s(t) Where w T = [w T1 w T2 w T3 ] T : tx weights, w R = [w R1 w R2 w R3 ] T : rx weights, H: 3x3 channel matrix w T1 w T2 w T3 w R1 w R2 w R3 H  Transmitter Receiver s(t) r(t)

SPACE-MAC PHY Model (Cont.) r(t) = w R T Hw T s(t) + n' where w T = [w T1 w T2 w T3 ] T : tx weights, w R = [w R1 w R2 w R3 ] T : rx weights, H: 3x3 channel matrix, n ’ : Weighted noise r(t) = w R T h T s(t) + n' where h T = Hw T : 3x1 channel vector Estimate h T on reception of RTS/CTS w T1 w T2 w T3 w R1 w R2 w R3 H  Transmitter Receiver s(t) Virtual channel h T Real channel w R1 w R2 w R3 h T = Hw T  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-MAC 4)While A and B are communicating, D and F also can start talking A B D F D

Nullifying r(t) = w D T h A s(t) + n' On reception of RTS from A, Node D learns h A = Hw A Find w D s.t. w D T h A = 0 DA w R1 w R2 w R3 h T = Hw T  Transmitter Receiver s(t) r(t)

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 MAC

CASE 1: 3 FTP/TCP Flows Topology 3 parallel FTP/TCP flows d = 350 ~ 400 m Flow 1 Flow 2 d d Reception Range Node 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

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

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

CASE 2: More General Scenario 100 nodes uniformly distributed in 750m x 750m area Random 20 FTP/TCP flows Throughput of each flow

CASE 2: More Realistic Scenario 100 nodes uniformly distributed in 750m x 750m area Random 20 FTP/TCP flows Aggregated throughput

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