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