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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.