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On Collision-Tolerant Transmission with Directional

Antennas

Hong-Ning Dai, Kam-Wing Ng, Min-You Wu

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Outline

Introduction Analytical models and the

probability of successful transmission

Lightweight MAC protocol Conclusion

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Motivation Directional antennas can improve network capacit

y Most of MAC protocols with directional antennas ar

e based on RTS/CTS mechanism The effectiveness of this mechanism with direction

al antennas is still questionable Our paper focuses on the questions

How does the success transmission probability vary with the narrower beamwidth of directional antennas?

How effective is the RTS/CTS mechanism in wireless networks using directional antennas?

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Analysis Model Antenna Model

A steerable beam antenna (beamwidth θ) Antenna gain Gd

Interference Model Transmission range Rt Interference range Ri (1)where σis SINR threshold(under two ray ground model) (2)

4 ti RR

ti RR

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Directional Transmission and Directional Reception (DTDR)

There are four transmission and reception modes: Omni-directional Transmission and Omni-directional Rece

ption (OTOR); Directional Transmission and Omni-directional Reception

(DTOR); Omni-directional Transmission and Directional Reception

(OTDR) Directional Transmission and Directional Reception(DTDR)

DTDR has the smallest interference area compared with OTOR, DTOR and OTDR. Only DTDR is considered in this work

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Related Work Many studies focus on MAC protocols with d

irectional antennas Basic-DMAC, Circular-DMAC, DVCS, ToneDMAC, etc.

Capacity analysis on performance evaluation of directional antennas

However, there is no work that studies the connections between the beamwidth of directional antennas and interference, especially for narrow-beam antennas.

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Probability of Successful Transmissions

The probability that a successful transmission is equal to the probability that no nodes can cause collisions with a receiver

(3) where p is the probability that a

node begins to transmit, (4) Under DTDR, the successful tra

nsmission probability is quite high when the beamwidth is lessened enough.

NpeP

2)2

(

2tRN ρthe node density

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Probability of Successful Transmissions (cont.) When θ≤ π/12 (i.e.,

15°), the success probability is always above 98%

the transmission under DTDR is less vulnerable to interference when the beamwidth is narrow.

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Lightweight MAC protocol We propose a lightweight MAC schem

e denoted as Basic Directional Transmission and Directional Reception (BAS-DTDR)

This protocol turns off RTS/CTS The competitor is RTC/CTS based Dire

ctional Transmission and Directional Reception (RTS-DTDR)

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Performance Evaluation Throughput is

calculated by the proportion of time that a node spends transmitting data frames successfully on average

We adopt a discrete Markov chain model

)3()()()(

t)(Throughput data

Isc TIPTSPTCP

SP

)4()()()(

t)(Overhead ctrl

Isc TIPTSPTCP

SP

Numerical Results (short data frame)

N=10 N=20

N=30 N=40

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Results Analysis (short frame) With the increased node density, both RTS-

DTDR and BAS-DTDR degrade. The BAS-DTDR has a much higher throughp

ut than RTS-DTDR protocol. The peak value of BAS-DTDR is almost 20% higher than that of RTS-DTDR.

Reason: When the beamwidth is very narrow, the interfering nodes are so sparse that they cause nearly no collisions at that time.

Numerical Results (long data frame)

N=10 N=20

N=30 N=40

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Results Analysis (long frame) Both RTS-DTDR and BAS-DTDR perform well under

a narrow beam (e.g., beamwidth less thanπ/15) BAS-DTDR still has a higher throughput than RTS-D

TDR because it gets rid of the bulky RTS/CTS mechanism.

When the beamwidth is increased further, the collisions caused by interfering nodes become remarkable, both the throughput of RTS-DTDR and BAS-DTDR degrades.

There exists a trade-off between the arisen interfering nodes and the overhead of control frames

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Discussion When the beamwidth is narrow enough, a transmis

sion can yield a high success probability. Under DTDR, if the beamwidth is less than π/12 (i.

e., 15°), the probability of a successful transmission is greater than 99%

The transmission under this situation can be regarded as a collision-tolerant transmission (the collision probability is quite small)

Under this condition, the collision avoidance mechanisms, such as RTS/CTS, are not necessary to be used.

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Conclusion

This paper studies the performance wireless networks using directional antennas with a narrow beam.

The preliminary results show that RTS/CTS may not be necessary if a narrow beam antenna is used.

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Thank you