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

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

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

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

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

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

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

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

14. Efficient Collaborative (Viral) Communication in OFDM Based WLANs MIT Media Laboratory – ISART 2003 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 channel estimation and Network Time/Frequency Synchronization. (complexity increases – challenging problem)

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