1. 1 On Collision-Tolerant Transmission with Directional Antennas Hong-Ning Dai, Kam-Wing Ng, Min-You Wu

2. 2 Outline Introduction Analytical models and the probability of successful transmission Lightweight MAC protocol Conclusion

3. 3 Motivation Directional antennas can improve network capacity Most of MAC protocols with directional antennas are based on RTS/CTS mechanism The effectiveness of this mechanism with directional 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?

4. 4 Analysis Model Antenna Model A steerable beam antenna (beamwidth θ ) Antenna gain G d Interference Model Transmission range R t Interference range R i (1) where σ is SINR threshold (under two ray ground model) (2)

5. 5 Directional Transmission and Directional Reception (DTDR) There are four transmission and reception modes: Omni-directional Transmission and Omni-directional Reception (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

6. 6 Related Work Many studies focus on MAC protocols with directional 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.

7. 7 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 transmission probability is quite high when the beamwidth is lessened enough. ρ the node density

8. 8 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.

9. 9 Lightweight MAC protocol We propose a lightweight MAC scheme denoted as Basic Directional Transmission and Directional Reception (BAS-DTDR) This protocol turns off RTS/CTS The competitor is RTC/CTS based Directional Transmission and Directional Reception (RTS-DTDR)

10. 10 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

11. Numerical Results (short data frame) N=10N=20 N=30N=40

12. 12 Results Analysis (short frame) With the increased node density, both RTS- DTDR and BAS-DTDR degrade. The BAS-DTDR has a much higher throughput 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.

13. Numerical Results (long data frame) N=10 N=20 N=30N=40

14. 14 Results Analysis (long frame) Both RTS-DTDR and BAS-DTDR perform well under a narrow beam (e.g., beamwidth less than π/15 ) BAS-DTDR still has a higher throughput than RTS- DTDR 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

15. 15 Discussion When the beamwidth is narrow enough, a transmission 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.

16. 16 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.

17. 17 Thank you