In the first scenario, the number of stations is six. Each station has no mobility. There is one traffic connection between Node 0 and Node 1 (flow 0-1) and another traffic connection between the Node 4 and Node 5 (flow 4-5). The max number of packets which arrive at the flow 4-5 is 10000. Packets arrive at each connection at a rate of 200 packets/sec and each MSDU packets is 512 bytes in length. This scenario is depicted in Fig. 4-1.
The simulation result is the average of results in 5 runs in which the distance between the Node 0 and Node 1 varies. In these runs, the distance between the Node 0 and Node 1 is far, so the connection of the two nodes needs a relay node to help relay the data. Otherwise, the performance of this connection may get worse due to the high error rate. Besides, the location of Node 2 is more closely to Node 0 and Node 1 than Node 3.
In the following simulation, the opportunistic relaying (OR) will be compared with the proposed relay selection scheme based on NAV (NAV-RS) and different lengths of hello message interval will be used. For example, NAV-RS _0.01 expresses that the proposed relay selection scheme based on NAV which the length of hello message interval is 0.01 second.
Figure 4-1 Topology of Scenario 1
Figure 4-2 Service Delay of 802.11 vs. OR vs. NAV-RS with different hello message intervals
Figure 4-3 Throughput of 802.11 vs. OR vs. NAV-RS in different hello message intervals
We observed the impact of “hello message” frequency on service delay and throughput.
As indicated by Fig. 4-2 and Fig.4-3, when the hello message internal is short as 0.01 seconds, it may cause more collisions and waste more channel usage to transmit the hello message. As a result, the performance is worse. After increasing the length of hello message intervals, we can alleviate the overhead of hello messages, but when the lengths of hello message intervals
exceed 0.05 second, it reduces the accuracy of information obtained by hello messages. Thus, we adopt “0.05 second” as the length of hello message interval in the following simulations.
Figure 4-4 Service Delay of 802.11 vs. OR vs. NAV-RS in Scenario 1
As indicated by Fig. 4-4, when the time is at 50th second, the number of arrived packets of flow 4-5 reached 10000. As a result, the effect caused by the flow 0-1 is gone. The delay decreases in the opportunistic relaying and NAV-RS. But, the influence affected by the flow 4-5 is lower in the proposed NAV-RS. This is because NAV-RS not only selects the relay which can help relay the data but also considers whether the relay candidate is appropriate or not to help relay the data. In this scenario, the opportunistic relaying is more likely to select Node 2 with the relay node because of the better channel quality in the path 0-2-1 due to the location of Node 2 and Node 3. On the other hand, in the proposed NAV-RS, the times to select Node 3 to be the relay node is less because the dense traffic near Node 3 leads the hello message sent by Node 3 will more likely to carry with the NAV information to prevent being selected for relaying.
Figure 4-5 Throughput of 802.11 vs. OR vs. NAV-RS in Scenario 1
From Fig. 4-5, it can be showed that the performance of throughput in our relay selection scheme outperforms that in opportunistic relaying scheme and the performance of throughput with cooperative communication is better than that in 802.11. The decreasing of throughput at about 50th second is because of that the number of arrived packets of flow 4-5 reached 10000 and no packets arrived at the flow 4-5. After about 50th second, the throughput is equal to the throughput of the flow 0-1. The reason for that the proposed NAV-RS outperforms than opportunistic relaying (OR) is the same as the reason for the performance of Service delay which is discussed.
Figure 4-6 Flow 0-1’s throughput of 802.11 vs. OR vs. NAV-RS in Scenario 1
As indicated by Fig.4-6, the performance of throughput in OR is worse than NAV-RS because of the flow 4-5 before the 50th second.
Figure 4-7 Transmission times of 802.11 vs. OR vs. NAV-RS in Scenario 1
The number of the average transmission times in our method is lower than the average transmission times in the opportunistic relaying scheme and 802.11 indicates that the probability to successfully transmit the data is higher in the proposed NAV-RS. In our method, the sender is more likely to select the relay node which is not interfered by other traffics.
Figure 4-8 Loss rate of 802.11 vs. OR vs. NAV-RS
Figure. 4-8 shows that in the proposed NAV-RS, the loss rate is very low while the loss rate is increasing in the opportunistic relaying and 802.11. This is because the impact of the neighbor traffic.
Figure 4-9 Transmission cost of 802.11 vs. OR vs. NAV-RS
In Fig. 4-9, it shows the transmission costs of different protocols. The transmission cost in the worst case of 802.11 is lower because it doesn’t use the cooperative communication.
But in the average, the transmission costs in 802.11 and opportunistic relaying (OR) are both higher than that in the proposed NAV-RS. Both using cooperative communication, the performance of NAV-RS is better than that of opportunistic relaying.
In the second scenario, the number of stations is six. There is one traffic connection between Node 0 and Node 1 (flow 0-1) and another traffic connection between the Node 4 and Node 5 (flow 4-5). Packets arrive at each connection at a rate of 200 packets/sec and each MSDU packets is 512 bytes in length. Different from the first scenario is that Node 1 is near Node 0 at the beginning and moves away from Node 0 at a speed of 4m/sec. So, the communication state of flow 0-1 will get worse with the increasing distance between the two nodes .This scenario is depicted in Fig. 4-10.
Figure 4-10 Topology of Scenario 2
Figure 4-11 Service Delay of 802.11 vs. OR vs. NAV-RS in Scenario 2
In Fig.4-11, at 87th second, the mean service delay increased rapidly. This is because the long distance between Node 0 and Node 1 causes the attenuation of the power and the SNR is too low to transmit packets directly. It’s about the time to start the cooperative mode.
Otherwise, the high error rate may cause the data be dropped frequently. We are showed that the cooperative mode is activated and the performance differs.
The opportunistic relaying scheme may be affected by the other connection causing the delay being higher than that in our proposed relay selection scheme.
Figure 4-12 Throughput of 802.11 vs. OR vs. NAV-RS in Scenario 2
As indicated by Fig. 4-12, the performance of our proposed NAV-RS is the most stable and better than OR and 802.11.
Figure 4-13 Flow 0-1’s throughput of 802.11 vs. OR vs. NAV-RS in Scenario 2
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
4 12 20 28 36 44 52 60 68 76 84 92 100 108 116
Time(s)
Transmission times
802.11 OR NAV-RS
Figure 4-14 Transmission times of 802.11 vs. OR vs. NAV-RS in Scenario 2
Fig. 4-14 shows that the transmission times in our proposed NAV-RS is much lower than that of opportunistic relaying and 802.11. The result is corresponsive to the result of loss rate.
0 0.2 0.4 0.6 0.8 1 1.2
0 16 32 48 64 80 96 11
Time(s)
Loss rate
2 802.11
OR NAV-RS
Figure 4-15 Loss rate of 802.11 vs. OR vs. NAV-RS in Scenario 2 As showed in Fig. 4-15, in the proposed NAV-RS is about to have no loss.
Figure 4-16 Transmission costs of 802.11 vs. OR vs. NAV-RS in Scenario 2
In the third scenario, there are 10 nodes distributed in the simulation topology. There’s at least one connection needs the cooperation relay. The scenario is depicted as Fig. 4-17. We have several runs in this scenario with the packet size of 512 KB and 1024KB for each run.
Figure 4-17 Topology of Scenario 3
0
Figure 4-18 Throughput of 802.11 vs. OR vs. NAV-RS with different packet size In Fig. 4-18, we can see that no matter the packet size is big or not, the performance of throughput in our relay selection scheme is better.
Figure 4-19 Throughput gain of OR vs. NAV-RS
Through Fig. 4-19, it is showed that our relay selection scheme outperforms the opportunistic relaying scheme when there’s other traffic interfering. The opportunistic relaying select the “best” relay based on the instantaneous channel quality between the relay and the source as well as the quality between the relay and destination. But it doesn’t consider about the coexisting connection at the same time. The participation of the relay may affect other connections because the extended sensing area of the relay node. Besides, the transmission by the relay may be interfered by other traffics.
One may notice that with the increasing of the packet size, the improved percentage decreases in our method while increasing in opportunistic relaying. The possible reason is that as the overhead of the hello message in our relay selection scheme may collide with the transmission of data.
In the last scenario, there are 16 nodes randomly distributed in the simulation topology.
There are 8 connections which are randomly set. The traffic type is CBR for each connection.
The packet size is 512 byte and the rate is 512Kb per second. The simulation time is 50 seconds. All traffic are started at 1th second.
Figure 4-20 Throughput of 802.11 vs. OR vs. NAV-RS in Scenario 4
As indicated by Fig. 4-20, the performance of throughput in opportunistic relaying and NAV-RS are better than that in 802.11. The performance of throughput in OR and NAV-RS are similar. The possible reason may be that the location of nodes and connections are randomly set. As a result, sources could not find a better relay for them.
Chapter5 Conclusion and Future Work
At the beginning, the reasons that the wireless networks becomes popular is mentioned but unfortunately there’re still some limitations to the wireless network like the attenuation due to propagation and the fading caused by the multi-path problem. After that, the cooperative communication is introduced which includes why the cooperative communication can help and how the cooperative communication works to improve the throughput and reliability.
Later, we focus on the relay (partner) selection scheme in MAC layer. Several cooperative protocols with relay selection scheme are introduced in related works. By studying the related works, it is observed that most proposed relay selection scheme aim to select the relay by measuring the instantaneous channel state information but rarely consider the interference of other coexisting traffics and the problem of the extended sensing area due to the participation of the relay. Thus, a relay selection scheme based on NAV which considers neighbor traffic of the relay is proposed. In the proposed relay selection scheme, the purpose is to select the relay which is less interfered by other traffic and is qualified to help relay the data.
Through the simulation by using the ns-2 simulator, we can see that the performance of throughput and delays in the proposed scheme are better than that in opportunistic relaying and original 802.11 in some scenarios. After observing the impact of the frequency of hello messages, we have chosen a suitable value as the parameter of lengths of hello message intervals. In randomly distributed networks, sources may not find a better relay for them, but the performances are still better than that in original 802.11.
In the future work, we can do the mathematical analysis for the proposed scheme and the next step is to observer the relevance between the proposed scheme and the issue of hot spot avoidance.
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