Performance Evaluation on Relay schemes

In this simulation, twelve RSs are surrounding the BS uniformly in a circle and RSs are at the cell radius of three fourths. While the desired site location of three-fourth cell radius is not exactly at an intersection, the RS would be placed on the nearest intersection from the desired site. In the following discussions, we compare the proposed QoS_GTE with LMP, LMP with TC, and without relay (w/o Relay), respectively.

Figure shows the modulation order distribution on radio links for TT_based path selection algorithm, LMP path selection scheme, and w/o Relay technique. We can observe that the probabilities of QPSK for TT and LMP are both much lower than w/o

w/o Relay. Moreover, the relay 1_hop links and 2_hop links are almost used in 64_QAM. These is because channels from BS to RSs are usually LOS which causes lower path-loss and RSs re-transmit the information received from BS which can strengthen the transmitted signal power. Therefore, relaying SINR at the receiver would be larger than the no relaying SINR and the modulation order of relay schemes is intuitively higher than the without relay case.

Figure is the average modulation order on system. The overall path modulation order and transmission efficiency are considered to obtain the average modulation order on system. The average overall modulation order of a relay path is the average of its 1_hop link and 2_hop link. However, as using a relay path, it takes two transmission times, which are transmission time from BS to RS and the time from RS to MS, to send the same information. The re-transmission reduces the system efficiency half. Therefore, the average modulation order on system is half of the average overall modulation order when using relay path. Since paths in w/o relay are all direct paths, the average links modulation order for w/o Relay is the same as the average modulation order on system. From Figure we can observe that LMP average system modulation order is smaller than w/o Relay case, which is the contrary of Figure. It has been mentioned that the efficiency of using relay is half of without relay due to the re-transmission in relay path. From this point of view, it can be said that using relay improperly is not good for the system performance. Compare the average system modulation order of two relaying schemes. TT is more than LMP.

It is because that TT decides the path by transmission time, which consists of whole path of path loss, shadowing, and interference, rather than according to the path loss of one hop only. TT selects a path more soundly and accurate in system than LMP does.

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Figure 4.1. Modulation Order Distribution on Radio Links

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Figure 4.2. Average Modulation Order on System

0 QoS_GTE w/o TC decision algorithm, LMP with TC decision algorithm (LMP+TC), LMP only, and without relay (w/o Relay) scheme. QoS_GTE outperforms the other four schemes. The TT chooses the most efficient and good channel condition path. By means of choosing the minimum transmission time path, the system resource using efficiency is taken into account. No matter the direct path or the relay path, it is the most suitable path for system not only due to its higher modulation order. The TC decision algorithm make the several RSs transmit concurrently so that the resource can be fully used. Since the SO_based resource allocation is used, it not only guarantees QoS but also maximizes the throughput. Since the LMP scheme selects the path only based on the bottleneck path-loss, it does not consider the overall path situation and neglects the effects on system while using relay path. This is why the performance of TT scheme is better than LMP scheme.

The throughput of LMP and TT are less than w/o Relay when the traffic load is below 0.75. This is because that twelve RSs share the resource in TDMA mode. In TDMA mode, each RS transmits the information in different symbol time in spite of the usage of sub-channels. If the sub-channels in a symbol time for a certain RS cannot be used entirely, the spare resource is not allowed to be used by other RSs, which results in resource consuming. This is why the relay schemes with good channel quality but poor throughput performance. Therefore, TC is undoubtedly the main factor to increase the system throughput in using relay. By several RSs transmitting concurrently, TC advances the system efficiency and improves the system throughput.

As the traffic load keeps increasing, the throughputs of these five algorithms start to decrease. LMP, LMP+TC, w/o Relay begin to fall down when traffic load is 0.6 and QoS_GTE and TT decline when the traffic load is 0.75. The reason of throughput reducing is that resource is used for QoS guarantee. When the traffic becomes heavy, more urgent packets are waiting to be transmitted. Using SO_based resource system is able to served more users which delay the dropping point of the throughput performance.

Figure(a) and (b) depict the voice and video packet dropping rate, respectively.

The voice packet dropping rates are almost zero despite the varying of traffic load. As the voice packets are urgent, they have the highest Sm and SO_based algorithm will

belongs to voice packets users prior. On the other hand, the video packet dropping rates are almost zero for light traffic load. The video packet dropping rate of QoS_GTE and QoS_GTE w/o TC start to increase when the traffic load is 0.75 and exceed the maximum allowable dropping rate (1%) at traffic load with 0.9. For LMP and LMP+TC, their video packet dropping rates increase for the traffic load higher than 0.6 and exceed the maximum allowable dropping rate at traffic load with 0.9. For the case of w/o Relay, its video packet dropping rate increases for traffic load higher than 0.45 and exceeds the maximum allowable dropping rate at traffic load with 0.6.

Since a voice packet has higher priority than video one, Sm of a voice packet is higher than the one of a video packet when they are in the same urgent situation. By much better than LMP so the video packet dropping rates of LMP and the case of w/o Relay are much higher than the one of TT.

Figure illustrates the non-guaranteed ratio for HTTP traffic. Unlike the real time traffic, packets of HTTP users will not be dropped but still waiting for service when they cannot reach the minimum transmission rate. In Figure, the guaranteed ratios for HTTP traffic are almost the same when the traffic load is light. As the traffic load becomes higher, the non-guaranteed ratio of w/o Relay rises steeply and those of TT, TT+TC keep low. Since we set Sth equal to the average of HTTP maximum and minimum Sm, about half of the HTTP packets with Sm lower than Sth is probably not be served priory. Only when HTTP packet is very urgent and its Sm is higher than Sth,

the HTTP packet may have a chance to be served at the first stage of SO_based

algorithm. Moreover, Sm of voice packets and video packets are usually greater than HTTP packets’. Urgent HTTP packets also have to be waiting for service until real time services are almost done. This is why the non-guaranteed ratio maintains in 2%

at light traffic load. As traffic load grows, the non-guaranteed ratio increases severely because the resource is allocated to the real time service to avoid larger dropping rate.

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(b) Video Packet Dropping Rate

Figure 4.4. Packet Dropping Rate of Real Time Services

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Figure 4.5. Non-guaranteed Ratio of HTTP User

Figure shows the average transmission rate of FTP. Since the Sm of FTP packets are all set to be 0.25 and this value is lower than S_th, FTP packets will be transmitted only in best effort manner. The FTP transmission rate is nearly the same at light traffic load. As the traffic load increases, the average transmission rate increases until a

condition is good. As a result, the system throughput can be enlarged by these high modulation order FTP users. Besides, the FTP traffic arrival rate is almost half of the total traffic arrival rate in our simulation which may cause FTP dominates the throughput as long as the FTP packets are served. This is why the trend of Figure is the same as Figure.

Figure 4.6. Average Transmission Rate of FTP