A short lesson on wireless connectivity… Aggelos Bletsas March 11, 2005 aggelos@media.mit.edu

A short lesson on wireless connectivity… Multi-hop is not a panacea in wireless communication: “multi-hop is more energy efficient than single hop communication”… That is an oversimplification: a) reception energy costs (a lot). b) route discovery and maintenance costs (a lot). c) mobility costs. d) small duty cycle costs! e) fading costs!

A short lesson on wireless connectivity… Multi-hop is not a panacea in wireless communication: “multi-hop is more energy efficient than single hop communication”… That is an oversimplification: a) such assumption neglects reception energy which is NOT negligible. In fact, in Zigbee radios, where high spectral efficiency, short duration pulses are used, reception power is higher than transmission power. b) route discovery and maintenance costs a lot, and usual studies neglect that cost. c) a longer multi-hop route has higher probability to break, than a shorter due to node mobility. d) energy savings require small duty cycle: you need to make sure that when you transmit, the receiver is awake. The longer the route, the higher the complexity. e) in the presence of fading, multi-hop tx power becomes comparable to single hop (energy gains are smaller…)

A short lesson on wireless connectivity…(2) “Multihop is necessary for scalable communication”… You always need to quantify your criteria of scalability: Recent research on wireless scalability takes into account a parameter neglected in previous studies: the issue of delay. Scalability is measured as function of node throughput and delay and it seems that there is always a trade off. It is still unclear if a shorter duration direct transmission is better than a lower power (but higher delay) multihop transmission. Spatial reuse needs smaller tx power, but smaller tx power means higher duration a “packet” travels in a network…

A simple example…(2) P   P/2 P/2

A simple example… P P/2 P/2 v=2 (free space), impossible…   P/2 P/2 v=2 (free space), impossible… v=3 still impossible… v=4 (cellular telephony/urban conditions) => ρ<=1.585 bps/Hz

Need to rethink wireless… Need to exploit the broadcast character of the wireless medium Need to break away from traditional point-to-point thinking of wireless connectivity… One solution example: Opportunistic Relaying  

Opportunistic Relaying…   Opportunistic relaying = distributed and synchronized selection of the “best” relay path out of M candidates. Performance equivalent to complex space-time coding in distributed antenna arrays!