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

Premise

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

Background

•OFDM is a modulation mechanism that allows for multi-user communication through orthogonal channels

Background

•OFDM is a modulation mechanism that allows for multi-user communication through orthogonal channels simultaneous transmission and reception

Protocol Summary

(1) Each node is assigned a unique “membership id”

Protocol Summary

(1)Each node is assigned a unique “membership id”

(2)An AP sends the broadcast message

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)

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

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

How does the AP know when a station is transmitting a tone?•Checks if average received signal

strength (RSS) is above some constant threshold

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

Interference

Timing Concerns

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

nodes

Experimental Setup

•Using SDR platform with an OFDM transceiver using a Virtex-IV FPGA

•Used 2.484GHz as the carrier frequency for the experiment

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

Exp #1 Evenly Spaced

Subcarriers

Exp #2 Closely Spaced

Subcarriers

Exp #3 Contiguous Subcarriers

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

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

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

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

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

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