copyright 2003

Exploiting Macrodiversityin Dense Multihop Networks

and Relay Channels

Matthew C. ValentiAssistant ProfessorLane Dept. of Comp. Sci. & Elect. Eng.West Virginia UniversityMorgantown, WVmvalenti@wvu.edu

Neiyer CorrealFlorida Commun. Research LabsMotorolaPlantation, FL 33322

This work was supported in part by the Office of Naval Researchunder grant N00014-00-0655

This presentation does not necessarily represent the views of ONR or Motorola.

© 2

003

Motivation & Goals Embedded networks of sensors and actuators:

Expected to be the enabling technology for several revolutionary new applications.

Low cost, disposable devices.• Single antenna.• Noncoherent detection and hard-decision decoding.• High spatial density, but low duty cycle.• Little or no movement = slow fading.

IEEE 802.15 TG 4 Spatial diversity:

Fading can be mitigated using antenna arrays. However, antenna arrays are too cumbersome for EmNets.

Goal is to achieve spatial diversity in a dense network of low-cost devices, each with a single antenna.

“virtual” antenna array. Emphasis on low cost solutions. A cross-layer approach.

© 2

003

Conventional Antenna Arrays

With a conventional array, then elements are closely spaced (/2) and connected through high bandwidth cabling. Microdiversity.

ReceiverTransmitter

© 2

003

Distributed Antenna Array

With a distributed array, the antennas are widely separated (e.g. different base stations) and connected through a moderate bandwidth backbone. Macrodiversity.

Receiver #2Transmitter

Receiver #1

BackboneNetwork

© 2

003

Virtual Antenna Array

With a virtual array, the antenna elements are widely spaced (attached to different receivers) but are not connected by a backbone. Virtual connection achieved by MAC-layer design. Decentralized macrodiversity.

Receiver #2Transmitter

Receiver #1

Virtual Connection

© 2

003

Assumptions We Do Not Make Most research on ad hoc networks makes the

following simplifying assumptions: Point-to-point communications.

• “Receiver-directed”• Facilitates the adaptation of wired protocols.• Ignores broadcast nature of radio.

Fixed transmission range and circular coverage area.• Ignores effects of fading and interference.

Irregular and time-varying shape to coverage area.• Concept of transmission range ignores the shape of the error

performance curve of practical modulation and coding techniques.

Assumes a “brick-wall” packet error rate, i.e. if inside range, transmission is reliable, but if outside

range it is unreliable.

Packet Error Rate of Bluetooth

5 10 15 20 25 30 3510-4

10-3

10-2

10-1

100

Average Es/No in dB

Pac

ket E

rror

Rat

e

M=1

M=2

M=3M=6

Quasi-static Rayleigh fading channelwith M element antenna array

© 2

003

Related Work Several options for exploiting the broadcast nature of

radio have been proposed. Require maximal-ratio-combining.

Source Destination

Relay

The relay channel (Cover/El Gamal 1979)

Cooperative diversity (Sendonaris/Erkip/Aazhang & Laneman/Wornell 1998)

Source #2

Source #1

Destination #2

Destination #1

Multihop diversity (Boyer/Falconer/Yanikomeroglu & Gupta/Kumar 2001)

Parallel relay channel (Gatspar/Kramer/Gupta 2002)

Source Destination

Source Destination

© 2

003

A Simple Approach toDecentralized Macrodiversity

Source broadcasts to a cluster of relays. Receive diversity effect. Any relay that received the broadcast could forward. Decode-and-forward.

• Error detection code used to determine if correct.• Message re-encoded and forwarded.

A negotiation is needed to determine the forwarding node.

Source Destination

Comments:•All M relays participate in receiving the source transmission.•Only one relay forwards to the destination.•The relay is selected after the source transmits.

© 2

003

Potential Gain with Perfect Negotiation

To illustrate the potential performance gains, we first assess the performance with an idealized MAC protocol. All relays within the source’s range know which

relays have received the message correctly.• This information could be explicitly shared over a

separate control channel.• A better approach is to embed this negotiation process

into the MAC protocol. Which relay forwards?

• Must have received the source transmission.• Should have best SNR from relay-destination.

Instantaneous SNR. Average-SNR: Relay closest to the destination.

© 2

003

Channel Model Quasi-static Rayleigh fading channel.

SNR constant for duration of a packet. Varies from packet to packet. Exponential random variable.

Path loss. Received power at distance dm is:

• Assuming path loss exponent n=3, free-space reference distance do = 1 m, and fc = 2.4 GHz.

Noise spectral density No = 10-18 W/Hz

P cd f

dd

P d Pro c

m

o

n

t m tFHG IKJ FHGIKJ

410

24 3

© 2

003

Simulation Parameters Modulation:

Noncoherent FSK modulation. Short packets (N=80 bits). 1 Mbaud symbol rate. Packet error rate:

Topology: source-destination are 10 m apart. M relays placed in circular cluster of diameter 7 m. Relays are moved after each packet.

pNk ke m

k

mk

N

bg FHGIKJ FHGIKJ FHG IKJ

1

21

2

1

1

SourceCluster of

Relays

Destination

Simulation Results:Source/Relay Power for FER = 10-2

-20 -15 -10 -5 0 5 10-25

-20

-15

-10

-5

0

5

10

Relay Power P2 (in dBm)

Sou

rce

Pow

er P

1 (in

dB

m)

M=1

Receiver-directedBest Avg. SNRBest Instantaneous SNR

M=2

M=3

M=5

M=10

M={2,3,5,10}

© 2

003

Performance Gains

If goal is to minimize the sum of the transmit power of source and relay.

M Receiver-directed

Best Avg.SNR

Best Instantaneous SNR

1 6.69 mW 6.69 mW 6.69 mW

2 5.97 mW 1.61 mW 0.63 mW

3 5.62 mW 0.99 mW 0.28 mW

5 5.05 mW 0.64 mW 0.13 mW

10 4.80 mW 0.38 mW 0.07 mW

© 2

003

Methods for Practical Negotiation Divide time into slots.

One slot for source and each relay. Source transmits during first slot. Relay closest to destination transmits in next slot, if it received

the source transmission properly. Reliable ACK messages needed to prevent

unnecessary transmissions.

SourceDestination 1 2 3 4 5 6

NextSource

Message

© 2

003

Conclusion Energy efficiency can be greatly improved by allowing

multiple relays to receive the transmission. Particularly effective in quasi-static Rayleigh fading channels

and simple modulation. No need to find route in advance. MAC layer needs to resolve which relay is used. Cross-layer approach.

Future work. Include direct connection from source-destination.

• Classic relay channel. Information theoretic capacity. Channel coding for the relay channel. Further development of MAC protocol.

• How to handle unreliable acknowledgements.• Determine which nodes are in the cluster.

D

Source-RelayChannel

(& Decoder)

“Source”Decoder

“Relay”Decoder

Interleaver

Deinnterleaver 2(Xi)2(X’i)

1(Xi)

w (Xi)

V1(Xi)

V2(X’i)

V2(Xi)

iX̂Interleaver Relay-Destination

Channel

D

Source-DestinationChannel

Source

Relay

Destination

iX

Distributed Turbo Codes

40 50 60 70 80 90 10075

80

85

90

95

Average transmitted SNR r of the relay in dB

BPSK relay RSC Relay distributed rate 1/3 PCCC

distributed rate 1/4 PCCCdistributed rate 1/4 SCCCtheoretical bound

Ave

rage

tran

smitt

ed S

NR

s o

f the

sou

rce

in d

B