copyright 2003 Exploiting Macrodiversity in Dense Multihop Networks and Relay Channels Matthew C. Valenti Assistant Professor Lane Dept. of Comp. Sci. & Elect. Eng. West Virginia University Morgantown, WV Neiyer Correal Florida Commun. Research Labs Motorola Plantation, FL This work was supported in part by the Office of Naval Research under grant N This presentation does not necessarily represent the views of ONR or Motorola.

© 2003 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 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.

© 2003 Conventional Antenna Arrays With a conventional array, then elements are closely spaced ( /2) and connected through high bandwidth cabling.  Microdiversity. Receiver Transmitter

© 2003 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 #2 Transmitter Receiver #1 Backbone Network

© 2003 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 #2 Transmitter Receiver #1 Virtual Connection

© 2003 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 Quasi-static Rayleigh fading channel with M element antenna array

© 2003 Related Work Several options for exploiting the broadcast nature of radio have been proposed.  Require maximal-ratio-combining. SourceDestination 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) SourceDestination SourceDestination

© 2003 A Simple Approach to Decentralized 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. SourceDestination 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.

© 2003 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.

© 2003 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 d m is: Assuming path loss exponent n=3, free-space reference distance d o = 1 m, and f c = 2.4 GHz. Noise spectral density  N o = W/Hz

© 2003 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. Source Cluster of Relays Destination

Simulation Results: Source/Relay Power for FER = Relay Power P2 (in dBm) Source Power P1 (in dBm) M=1 Receiver-directed Best Avg. SNR Best Instantaneous SNR M=2 M=3 M=5 M=10 M={2,3,5,10}

© 2003 Performance Gains If goal is to minimize the sum of the transmit power of source and relay. MReceiver- directed Best Avg. SNR Best Instantaneous SNR mW mW1.61 mW0.63 mW mW0.99 mW0.28 mW mW0.64 mW0.13 mW mW0.38 mW0.07 mW

© 2003 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. Source Destination Next Source Message

© 2003 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-Relay Channel (& Decoder) “Source” Decoder “Relay” Decoder Interleaver Deinnterleaver  2 (X i )  2 (X’ i )  1 (X i ) w (X i ) V 1 (X i ) V 2 (X’ i ) V 2 (X i ) Interleaver Relay-Destination Channel D Source-Destination Channel Source Relay Destination Distributed Turbo Codes

Average transmitted SNR  r of the relay in dB BPSK relay RSC Relay distributed rate 1/3 PCCC distributed rate 1/4 PCCC distributed rate 1/4 SCCC theoretical bound Average transmitted SNR  s of the source in dB