1. SMACK: Smart ACKnowledgment Scheme for Broadcast Messages in Wireless Networks Aveek Dutta, Dola Saha, Dirk Grunwald, Douglas Sicker, University of Colorado

2. Premise wireless protocols can exploit simultaneous transmission to reduce the cost of reliable multicast/broadcast transmission by orders of magnitude

3. Background OFDM is a modulation mechanism that allows for multi-user communication through orthogonal channels

4. Background OFDM is a modulation mechanism that allows for multi-user communication through orthogonal channels  simultaneous transmission and reception

5. Protocol Summary (1)Each node is assigned a unique “membership id”

6. Protocol Summary (1)Each node is assigned a unique “membership id” (2)An AP sends the broadcast message

7. Protocol Summary (1)Each node is assigned a unique “membership id” (2)An AP sends the broadcast message (3)Clients decode the broadcasted message (if possible)

8. Protocol Summary (1)Each node is assigned a unique “membership id” (2)An AP sends the broadcast message (3)Clients decode the broadcasted message (if possible) (4)Client uses assigned subcarrier specified by “membership id” to send back an ACK

9. Protocol Summary (1)Each node is assigned a unique “membership id” (2)An AP sends the broadcast message (3)Clients decode the broadcasted message (if possible) (4)Client uses assigned subcarrier specified by “membership id” to send back an ACK (5)AP receives composite signal of all subcarriers and demodulates the individual ACK’s

10. How does the AP know when a station is transmitting a tone? Checks if average received signal strength (RSS) is above some constant threshold

11. Varying Signal Power Want: One single threshold to detect clients in the network Problem: signal powers from clients may vary widely Solution: adjusting the transmission power of clients such that received power from all clients are within a tolerable range ▫Existing channel assessment techniques done in CDMA

12. Interference

20. Timing Concerns All subcarriers must be present with sufficient energy within the FFT window. ▫near-far effect ▫different processing power of the client nodes

22. Experimental Setup Using SDR platform with an OFDM transceiver using a Virtex-IV FPGA Used 2.484GHz as the carrier frequency for the experiment

24. Results Can use simple FT to detect multiple tone transmission no matter how dense the subcarrier spacing is This protocol is feasible using reconfigurable radio to meet the timing constraint

25. Exp #1 Evenly Spaced Subcarriers Exp #2 Closely Spaced Subcarriers Exp #3 Contiguous Subcarriers

26. Evenly spaced subcarriers [-26, -16, -6, +6, +11, +16)

27. Closely spaced subcarriers [+6, +8, +10, +12, +14, +16)

28. Contiguous subcarriers [+8, +9, +10, +11, +12, +13]

29. Results Can use simple FT to detect multiple tone transmission no matter how dense the subcarrier spacing is This protocol is feasible using reconfigurable radio to meet the timing constraint

30. Complete System with 1 broadcaster and 2 responders at +12 and -12

31. Conclusion Adaptability of SMACK to any higher level group communication and signaling protocols as long as they require simple “yes/no” answers ▫Reducing Redundant Rebroadcast ▫Parallel Polling