Efficient Collaborative (Viral) Communication in OFDM Based WLANs

1. Efficient Collaborative (Viral) Communication in OFDM Based WLANs Aggelos Bletsas MIT Media Laboratory aggelos@media.mit.edu http://web.media.mit.edu/~aggelos ISART 2003

2. MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs The vision: Collaborative Radio 1) multi-user diversity radio: 2) ad-hoc phased arrays: 3) multiple-input multiple-output systems: (MIMO)

3. MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs OFDM is appropriate for collaborative radio: • 1) Software defined radio • baseband signal through IFFT/FFTs • information recovery through over-sampling • 2) zero ISI regardless the network topology (guard interval) • possibility for combined transmissions/receptions

4. MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs OFDM Properties and Viral Communication (analytically): • Software defined baseband signal through FFT/IFFTs • Information recovery through over-sampling

5. MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs OFDM Properties and Viral Communication (analytically): • Guard interval ensures zero ISI regardless the network topology • (possibility for combined transmission/reception) • and a last property… • Convolution with channel impulse response multiplication of the original symbol with the frequency response. • More analytically…

6. MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs

7. MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs • Baseband representation at each sub-carrier: • based on analogue Amplifying and Forwarding [Laneman 2000] • Intermediate node oversamples at first half and forwards at second half. • Receiver oversamples and receives direct at first half and • relayed and direct at second half.

8. MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs Optimal ML receiver results in Maximum Ratio Combining (MRC). • Noise modeled as complex Gaussian with variance Ni/2 per dimension. • Propagation coefficients according to complex Gaussian distribution… MRC

9. MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs Applying the results in our case: MRC receiver: Probability of error:

10. MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs Comparison with direct (point-to-point) BPSK: ML Receiver

11. Tx Relay Rx MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs Error Probability vs. SNR E1=E2=E/2

12. Tx Relay Rx MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs Error Probability vs. SNR E1=E2=E/2

13. Tx Relay Rx MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs Tx Energy ratio (point-to-point/viral) for given error probability. E/(E1+E2) vs. Pg v=4

14. MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs • Collaboration is possible in OFDM based networks. • Collaboration results in substantial energy savings (one order of magnitude or more). • Energy savings can be realized at the same channel (better spectrum allocation - higher capacity?) • Need for Network channelestimation and • Network Time/Frequency Synchronization. • (complexity increases – challenging problem)

15. MIT Media Laboratory – ISART 2003 Efficient Collaborative (Viral) Communication in OFDM Based WLANs • Visit our web site: • http://www.media.mit.edu/~aggelos