MultiCarrier CDMA MAC Protocol for WLANs: Enhancing Throughput and Reliability

1. ComNets FFV Multihop MAC Protocol forMC-CDMA based WLANsconfidential Georgios Orfanos RWTH Aachen University, Chair of Communication Networks 31.03.2006

2. Outlook • MC-CDMA • C-DCF • Smart Backoff • Parallel transmission • MAC layer relaying function • Multihop guard interval, extended NAV • Simulated scenario and parameters • Simulation results • Comparison with DCF • Conclusions

3. MultiCarrier – Code Division Multiple Access • Combination SS and OFDM • Frequency spreading • Frequency diversity • IFFT / FFT realization of multicarrier modulation • Multipath robustness • Cyclic prefix • Simultaneous transmissions in the same frequency, through division of the spectrum in parallel channels => codechannels • Spreading factor 4

4. C-DCF • CSMA/CA with 4 parallel codechannels • multichannel system • Selection of codechannel • random • first transfer on cch1 • Data transfer: DCF • NAV per codechannel • each station monitors optionally one or all codechannels • Power control over RTS / CTS • for all data frames

5. Smart Backoff • Backoff procedure spanning over many cchs • Backoff Time = Random · aSlotTime • Another cch seems idle • Another cch is determined idle • No cch is idle

6. Parallel transmission • 1 codechannel =1/4 of frequency channel capacity • Parallel (multichannel) transmission for high load

7. MAC layer relaying function • Multihop connections up to 3 hops • Fig. multihop connection from station 1 to station 4 • Multihop packet handled per station as own packet • MAC header contains the addresses of all forwarding stations • Spatial reuse possible Mac frame Format 2 6 6 6 6 2 6 0-2312 4

8. Multihop guard interval, extended NAV • Multihop guard interval to prioritize forwarder • Duration of one complete transmission window • Smart Backoff at forwarders for bottleneck avoidance • Combined with parallel transmission • Extended NAV • Duration, cch, MSs

9. Simulated scenario and parameters

10. Simulation results (I)Mean values • Achievable throughput allies with theoretical calculation: 2,5 Mbit/sec/cch • Depends on number of hops

11. Simulation results (II)CDF of delay per hop • left: 1,25 Mbit/sec • right: 0,75 Mbit/sec (saturation throughput)

12. Simulation results (III)CDF of end-to-end delay per connection • left: 1,25 Mbit/sec • right: 0,75 Mbit/sec (saturation throughput)

13. Comparison with DCF (I)Throughput

14. Comparison with DCF (II)End-to-end delay

15. Conclusions • Multihop guard interval: • prioritizes the relaying MS on medium access • reduces the end-to-end delay of the multihop connection • Smart Backoff at forwarding MSs: • reduces the medium access time • enables parallel transmission in many cchs, that increases the available capacity and reduces the bottleneck effect. • In multi-channel, multihop environments applying carrier sensing NAV timer per cch (channel) is not enough: • the cch specific NAV per cch and MS proposed is then necessary.

16. Thank you for your attention ! Georgios Orfanos orf@comnets.rwth-aachen.de Any questions?