guard zones and the near-far problem in ds-cdma ad hoc...
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1/6/13
Don Torrieri and Matthew Valenti
Guard Zones and the Near-Far Problem in DS-CDMA Ad Hoc Networks
Extreme mobility (potentially supersonic)
Operational
security (covertness)
Airborne/UAV networks
Intermittent connectivity due to
mobility
Risk of
capture/compromise Special operations
Unattended sensors
Poor channel quality
Platform constraints (size/weight/power)
Low bandwidth
Mobile tank networks
Intermittent
connectivity from terrain/environment
Environmental requirements
(ruggedization) limiting capability
Deployed soldier networks
■ Figure 1. The constraints of the tactical military environment
Features
• Finite network size • Finite number of mobiles • Different fading conditions for
each mobile • Arbitrary spatial distributions • Exclusion zones • Spreading factors
• Near-far problem exists when an interferer is near and a communicator is far from the receiver.
• Exclusion zone: no mobiles are physically present, reflecting the fact that mobiles will always have a minimum physical separation in actual networks.
• Guard zone may be established by the carrier-sense multiple-access (CSMA) protocol; mobiles can be deactivated by the protocol.
Zones
CSMA/CA
Figure 3.2.4 Carrier-sense Multiple Access (CSMA) with Collision Avoidance (CA)
Network Realization
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Outage Prob. vs. SNR, no shadowing
Outage Prob. vs. SNR, shadowing
Outage Prob. vs. Density and Attenuation
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Average Outage Probability
Gi α rex rg Ec Ep 1 3 0 1/12 0.5298 0.3056
1/4 0.2324 0.1683 1/12 1/12 0.5234 0.2592
1/4 0.2256 0.1528 4 0 1/12 0.4129 0.2388
1/4 0.1453 0.1228 1/12 1/12 0.3869 0.1774
1/4 0.1313 0.1026 48 3 0 1/12 0.0644 0.0391
1/4 0.0181 0.0172 1/12 1/12 0.0308 0.0199
1/4 0.0173 0.0165 4 0 1/12 0.0842 0.0494
1/4 0.0177 0.0174 1/12 1/12 0.0335 0.0209
1/4 0.0165 0.0163
Outage Constraint
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Transmission Capacity
• The network efficiency can be quantified by the transmission capacity (TC), which represents the network throughput in bps per unit area.
• It can be found by multiplying the link throughput (excluding outages) by the network density, which is the number of active mobiles per unit area.
• Increasing the guard zone reduces TC due to fewer simultaneous transmissions.
Transmission Cap. vs. Trans. Distance
Transmission Cap. vs. Guard Radius
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Transmission Cap. vs. Interfering Mobiles
Transmission Capacity Constraint
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Conclusions • Trade-offs between exclusion and guard zones
assessed for DS-CDMA and unspread networks • Advantage of an exclusion zone over a CSMA guard
zone is that the number of active mobiles remains constant and higher transmission capacities can be achieved.
• If the processing gain is sufficiently large, a CSMA guard zone is largely ineffective in that it only slightly improves the outage probability at the cost of a considerable decrease in the transmission capacity.
• Guard zone is vital for unspread network.
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