Wireless ad hoc networks: cross layer opportunities

NSF workshopWashington DC Aug 27-28

Mario Gerla

Computer Science Dept, UCLA

www.cs.ucla.edu

Ad hoc networking Current Status

Leading Applications

• Tactical battlefield: – no infrastructure

• Civilian emergency:– infrastructure, if present, was destroyed

• Critical Requirements: scalability, survivability, 100% reliable, QoS, jam protection, etc

• Non critical: Cost, Standards, Privacy

SURVEILLANCE MISSION

SURVEILLANCE MISSION

AIR-TO-AIR MISSION

STRIKE MISSION

FRIENDLY GROUND CONTROL

(MOBILE)

RESUPPLY MISSION

SATELLITE COMMS

Unmanned Control Platform

COMM/TASKING

COMM/TASKING

MannedControl Platform

COMM/TASKING

UAV-UAV NETWORK

Tactical Ad Hoc Network

UAV-UGV NETWORK

Emerging Landscape : “Opportunistic” Ad Hoc networks

Recreational, commercial, education applications

• Vehicle networks• Workgroups (eg, sharing 3G via Bluetooth)• Massive Network games • Patient monitoring

Access to Internet? • available, but - “bypass it” with “ad hoc” if too costly or

inadequate

Tolerant to delays: DTNs

Critical: Cost, Privacy, security, standards

Car to Car communications for Safe Driving

Vehicle type: Cadillac XLRCurb weight: 3,547 lbsSpeed: 65 mphAcceleration: - 5m/sec^2Coefficient of friction: .65Driver Attention: YesEtc.

Vehicle type: Cadillac XLRCurb weight: 3,547 lbsSpeed: 45 mphAcceleration: - 20m/sec^2Coefficient of friction: .65Driver Attention: NoEtc.

Vehicle type: Cadillac XLRCurb weight: 3,547 lbsSpeed: 75 mphAcceleration: + 20m/sec^2Coefficient of friction: .65Driver Attention: YesEtc.

Vehicle type: Cadillac XLRCurb weight: 3,547 lbsSpeed: 75 mphAcceleration: + 10m/sec^2Coefficient of friction: .65Driver Attention: YesEtc.

Alert Status: None

Alert Status: Passing Vehicle on left

Alert Status: Inattentive Driver on Right

Alert Status: None

Alert Status: Slowing vehicle aheadAlert Status: Passing vehicle on left

Co-operative Download: Car Torrent

Vehicle-Vehicle Communication

Internet

Exchanging Pieces of File Later

Vehicular Sensor Network (VSN)

VSN-enabled vehicle

Inter -vehiclecommunications

Vehicle -to-roadsidecommunications

Roadside base station

Vid e o Ch e m.

Sensors

S to ra g e

Systems

P ro c.

Personal Networking: BlueTorrent

A A

B

C

B

C

D D

A

D

B

C

Patient Monitoring

Nurses upload patient data; share data files

in P2P mode

1. Future expectations on wireless network research

• Network layer more tightly coupled with applications– Content sharing, environement sensing

• Besides data forwarding, additional services:– Location aware service discovery,

– content based routing;

– P2P networking

– Data collection, processing, filtering, storage, dissemination

• Network layer design must interact with:– applications

– PHY Layer

2. Major recent advances/breakthroughs in the physical layer

• Cognitive radios (spectrum scavenging)• MIMOs (for flexible topology designs;

interference mitigation etc)• Cooperative radios• Multi radio devices (BT, 802.11, 3G, etc)

3. Algorithms must adjust to PHY layer

“PHY layer aware” MAC, Network and Transport designs

Examples (based on MIMO):

• Topology control• A MIMO aware MAC protocol: SPACE-MAC• Multi-path Routing & MIMO• TCP & MIMO

MIMO Topology Control/Routing

• Topology control:– Exploit mode flexibility to dynamically shape topology– Meet different customer requirements

Topology with high capacity links: disconnected network

Topology with low capacity links: fully connected network

300Mbps

10Mbps

SPACE2 MAC

When A wishes to transmit to B

A

B

D

F1) A sends RTS to B; F and D learn about A

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

SPACE MAC (cont)

3) F and D beamform such that signals from/to B and A are nulled; then, A and B start talking

A

B

F

D

4) After A and B pair is established, F and D pair also can talk

Two-Path Routing using MIMO

• S sends two independent streams simultaneously to R

• Assume 2 antennas at each node (but extendible to systems with more antennas).

SS RR

sender receiver

relay nodes

MIMO yields 6-fold throughput gain

• In the traditional relay mode, the capacity is C/3.• Simulcast achieves 6-fold throughput increase.

SS RR

sender receiver

TCP and MIMO in Ad Hoc Networks

• Consider three flows in the same wireless domain

• As the flows get closer to each other:– Interference builds up

– Throughput decreases

– Fairness suffers

• Can MIMO Help?

FTP 1

FTP 2

FTP 3

(100, 100) (600, 100)

(350, 350)

(350, 700)(0, 700)

(350, 1050)

(100, 1300) (600, 1300)

(700, 700)

TCP over SPACE MAC (MIMO)Distance = 400m (interference range)

3 F T P / T C P F l o w s

0

50

100

150

200

250

300

350

802.11 S P A C E - M A C

M A C P r otoc ol

Throughput (Kbits/s)

F low 1 F low 2 F low 3

Fig 0. The throughput of 3 FTP/TCP flows with the distance between flows being 400m

TCP over SPACE MAC (MIMO)Distance = 350m (tx range)

3 F T P F l o w s

0

50

100

150

200

250

300

350

802.11 Space-MAC

M A C P rotoc ol

Throughput (Kbits/s)

F low 1 F low 2 F low 3

Fig 0. The throughput of 3 FTP/TCP flows with the distance between flows being 350m

Identify gaps

• Question: How to exploit the wealth of PHY emerging technology?

• Do not limit your scope to LINK capacity gains• Look for cross layer optimization opportunities at

all layers:– MAC

– Network (routing, topology control, multicast, bandwidth scavenging, etc)

– transport,

– applications and PHY layer

The End

Thank You

Simul-Cast

Brian Choi

Mario Gerla

MIMO System Model

s(t)WHVH = r(t)

weight vector w1 = [w11 w21 … wm1]

W V

Assumptions

• Fading is flat (i.e. freq. independent).• Channel is symmetric and quasi-static.• Two subchannels - control channel and data channel• Rich scattering - H is full-rank• Antenna’s capable of transmitting and receiving

signals simultaneously.• We ignore additive channel noise.• Perfect sychronization between nodes

Two Path Routing Problem

• S sends two independent streams under two paths simultaneously to R.

• Assume 2 antennas at each node (but extensible to systems with more antennas).

SS RR

sender receiver

relay nodes

The 6-fold Benefit of MIMO

• If C = (capacity of a point-to-point link) in the traditional relay mode, the capacity is C/3.

• Simulcast achieves 6X throughput increase.

SS RR

sender receiver

Sender

• A wants to send a stream (s1(t)) to B and another stream (s2(t)) to C simultaneously.

AA

BB

CC

s1(t)

s2(t)

s(t) = [s1(t) s2(t)]

Sender: Linear Coding

• B receives rB(t) = s(t)WAHABWBH.

• For B to recover s1(t), B must consume 2 degrees of freedom.

AA

BB

CC

s1(t)

s2(t)

s(t) = [s1(t) s2(t)] HAB

HAC

rB1(t)

rB2(t)

rC1(t)

rC2(t)

WA

WB

Sender: Pre-coding

• If A knows the channel and the steering matrices of B and C, then A can precode its data such that s1(t) is received at rB1(t), s2(t) is received at rC1(t), without interfering each other.

• B and C needs to comsume only one DOF each.

AA

BB

CC

s1(t)

s2(t)

rB1(t)

rB2(t)

rC1(t)

rC2(t)

Dirty Paper Coding

HABwB1H

HACwC1H

Let H = = QR

QR factorization, Q = unitary, R = upper triangular

AA

BB

CC

s1(t)

s2(t)

rB1(t)

rB2(t)

rC1(t)

rC2(t)

wB1

wC1

Dirty Paper Coding

• Let r(t) = [rB1(t) rC1(t)]. Then r(t) = s(t)H.• Multiply s(t) by QH, such that s’(t) = s(t)QH. • Then r(t) = s’(t)H = s(t)QHH = s(t)QHQR = s(t)R

• rB1(t) = s1(t)R1,1 (no interference)

• rC1(t) = s1(t)R1,2 + s2(t)R2,2

• Sender can estimate this interference and subtract it from s(t) before transmitting.

(interference!)

Relay Node

• There is one DOF left for us to use. We use it to simultaneously relay the received data to the next node.

• We set weight vectors such that they are orthogonal to each other.

used to receive data from the previous node

used to send a stream to the next node

Receiver

• This reduces to the problem of spatial multiplexing.• If R knows the channels and the weight vectors used

for both streams, then R can decode the received data.

AA

BB

RR

HAR

HBR

Network-wise Benefit

• If C = (capacity of a point-to-point link) in the traditional relay mode, the capacity is C/3.

• Simulcast achieves 6X throughput increase.

SS RR

sender receiver

Multiple Paths

• We can run OLSR-type of routing protocol for the nodes to pre-determine the paths.

• This suggests a cross-layer approach (between network layer and MAC layer).

Summary

• With MIMO and Pre-coding techniques, one can effectively reduce the DOF consumption at the receiving nodes.

• We can utilize the idle DOF to relay the data simultaneously.

• With two independent simultaneous paths, we can achieve up to 6X throughput increase.