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

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

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

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

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

G.Orfanos, ComNets, RWTH Aachen University

Parallel transmissionParallel transmission

• 1 codechannel =1/4 of frequency channel capacity

• Parallel (multichannel) transmission for high load

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

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

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

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

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)

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)

G.Orfanos, ComNets, RWTH Aachen University

Comparison with DCF (I)Comparison with DCF (I)ThroughputThroughput

G.Orfanos, ComNets, RWTH Aachen University

Comparison with DCF (II)Comparison with DCF (II)End-to-end delayEnd-to-end delay

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.

G.Orfanos, ComNets, RWTH Aachen University

Thank you for your attention !

Georgios Orfanosorf@comnets.rwth-aachen.de

Any questions?