A short lesson on wireless connectivity…

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

2. 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!

3. 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…)

4. 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…

5. A simple example…(2) P P/2 P/2

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

7. 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

8. 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!