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G. Orfanos, ComNets, RWTH Aachen University
Multihop MAC Protocol forMultihop MAC Protocol forMC-CDMA based WLANsMC-CDMA based WLANs
confidentialconfidential
Georgios Orfanos
RWTH Aachen University, Chair of Communication Networks
31.03.2006
ComNets FFVComNets FFV
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G.Orfanos, ComNets, RWTH Aachen University
OutlookOutlook
• 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
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G.Orfanos, ComNets, RWTH Aachen University
MultiCarrier – Code Division Multiple AccessMultiCarrier – 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
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G.Orfanos, ComNets, RWTH Aachen University
C-DCFC-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
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G.Orfanos, ComNets, RWTH Aachen University
Smart BackoffSmart Backoff
• Backoff procedure spanning over many cchs
• Backoff Time = Random · aSlotTime1. Another cch seems
idle 2. Another cch is
determined idle 3. No cch is idle
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G.Orfanos, ComNets, RWTH Aachen University
Parallel transmissionParallel transmission
• 1 codechannel =1/4 of frequency channel capacity
• Parallel (multichannel) transmission for high load
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G.Orfanos, ComNets, RWTH Aachen University
MAC layer relaying functionMAC 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
Frame Control
Duration/ ID
Address 1
Address 2
Address 3
Sequence Control
Address 4
Frame Body
FCS
2 6 6 6 6 2 6 0-2312 4
Mac frame Format
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G.Orfanos, ComNets, RWTH Aachen University
Multihop guard interval, extended NAVMultihop 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
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G.Orfanos, ComNets, RWTH Aachen University
Simulated scenario and parametersSimulated scenario and parameters
Parameter Value
Max. TxPower 17dBm
Spreading Factor 4
Cwmin 31 slots
Cwmax 255 slots
Number of Subcarriers 48 Data + 4 Pilot
Subcarrier Spacing 0.3125 MHz
Channel Bandwidth 20 MHz
Carrier Frequency 5.25 GHz
Noise Level -93dBm
Path loss Factor 3.5
TxRate Data 12 Mbps
TxRate Control 12 Mbps
RTS/CTS enabled
Symbol Interval 4 µs = 3.2 µ + 0.8 µs
Guard Interval 0.8 µs
Preamble 16 µs
Max. Propagation Delay 0,15 µs
PDU Length 1024 Byte
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G.Orfanos, ComNets, RWTH Aachen University
Simulation results (I)Simulation results (I)Mean valuesMean values
• Achievable throughput allies with theoretical calculation: 2,5 Mbit/sec/cch
• Depends on number of hops
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G.Orfanos, ComNets, RWTH Aachen University
Simulation results (II)Simulation results (II)CDF of delay per hopCDF of delay per hop
• left: 1,25 Mbit/sec• right: 0,75 Mbit/sec (saturation
throughput)
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G.Orfanos, ComNets, RWTH Aachen University
Simulation results (III)Simulation results (III)CDF of end-to-end delay per connectionCDF of end-to-end delay per connection
• left: 1,25 Mbit/sec• right: 0,75 Mbit/sec (saturation
throughput)
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G.Orfanos, ComNets, RWTH Aachen University
Comparison with DCF (I)Comparison with DCF (I)ThroughputThroughput
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G.Orfanos, ComNets, RWTH Aachen University
Comparison with DCF (II)Comparison with DCF (II)End-to-end delayEnd-to-end delay
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G.Orfanos, ComNets, RWTH Aachen University
ConclusionsConclusions
• 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.
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G.Orfanos, ComNets, RWTH Aachen University
Thank you for your attention !
Georgios [email protected]
Any questions?