To investigate the effects of average requested bandwidth on total throughput, the arrival data rate was varied from 100Kbps to 1Mbps. Fig. 7(a) shows the total throughputs achieved by eOCSA, WLFF, and BCO. BCO outperforms eOCSA and WLFF on throughput because of two main reasons. First, eOCSA and WLFF often construct bursts with inferior MCSs because they do not consider subchannel diversity, and therefore, fail to address optimal block exploration. Second, internal fragmentation may occur by using eOCSA and WLFF because they do not consider the requested bandwidth during burst construction.
eOCSA and WLFF cannot achieve the targeted bandwidth (200 Kbps ×20 connections =4Mbps), even when the traffic is light, that is, the requested bandwidth is smaller than 200Kbps. This occurs because the burst B has some unused free i slots if the requested bandwidth is satisfied by fewer slots than A when i B is i constructed on high-quality subchannels and uses an optimal MCS. In this situation, eOCSA and WLFF does not shrink the area of B and, therefore, cannot release the i unused slots to other bursts. In addition, eOCSA and WLFF may construct bursts on the subchannels with unacceptable channel quality, and thus, cannot use any suitable MCS to transmit data because they do not consider subchannel diversity.
Consequently, all slots internal to the burst are invalid. Thus, several unused slots are wasted, as shown in Fig. 7(c), and cannot achieve the targeted bandwidth.
BCO alleviates internal fragmentation by shrinking the number of occupied slots.
Fig. 7(c) demonstrates that BCO experiences a maximum of 1.6 % IUSR. The saved
slots can be used by the following unconstructed bursts with insufficient allocated slots. In addition, BCO constructs a burst in the optimal corner to avoid external fragmentation and to explore an optimal block. Thus, it achieves a superior throughput than eOCSA and WLFF. Figs. 7(a) and 7(d) reveal that, when the requested bandwidth is less than 700 Kbps, BCO achieves the targeted bandwidth with fewer slots, and thus, owns higher EUSRs, because the constructed shrunken bursts already provide sufficient bandwidth, that is, THCal i B( , i)=Wi. However, the EUSR decreases when the required bandwidth increases because more slots are required to fulfill the increasing required bandwidth.
Although the requested bandwidth increases, the throughput should become stable when most slots are used (requested bandwidth exceeds 700Kbps for BCO).
However, in this case, the situation is not actually saturated and their throughputs slightly increase, because, although most slots are used, the burst generally satisfies its requested bandwidth by fewer slots when using an optimal MCS and leaves the unused free slots of the allocated slots to other bursts that use inferior MCSs. That is, the minority of slots in the downlink subframe use optimal MCSs, and the majority use inferior MCSs. The area of the burst using an optimal MCS increases in conjunction with the bandwidth to satisfy the requested bandwidth, and the unused slots, which are left to other bursts with lower MCSs, decrease. Consequently, more slots in the downlink subframe use optimal MCSs and fewer slots use inferior MCSs.
Therefore, the throughput slightly increases. However, a saturated condition is eventually achieved when most slots are efficiently used. Some of the bandwidth area with inferior channel quality remains unused (approximately 10 %) even when the traffic load is heavy, as shown in Fig. 7(d), because BCO shrinks the bursts to prevent throughput anomaly and achieves higher overall throughput, as explained in Section 4.2.
Fig. 7(b) demonstrates that the improvement ratios of BCO increased in conjunction with the requested bandwidth because eOCSA and WLFF reached a saturated condition, whereas BCO did not reach a saturated condition. Under the heavy load of 1Mbpps, BCO achieved 2 and 9 times the throughput achieved by eOCSA and WLFF, respectively.
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Average Requsted Bandwidth of Each Connection (kbps) eOCSA
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Average Requsted Bandwidth of Each Connection (kbps) BCO to eOCSA
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Average Requsted Bandwidth of Each Connection (kbps) eOCSA
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Average Requsted Bandwidth of Each Connection (kbps) eOCSA
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Figure 7. Effects of average requested bandwidth 5.3. Number of Connections
The effects of the number of connections on the total throughput were also investigated. The number of connections was varied from 10 to 50 with the same overall data arrival rates, that is, 16Mbps. Fig. 8(a) reveals that the total throughputs of BCO, eOCSA, and WLFF increased in conjunction with the number of connections.
Under the same overall data rate, the larger the number of connections, the smaller the bandwidth requested by each connection and the smaller the area of each burst. A burst with a smaller area provides all algorithms with more opportunities to construct bursts on high-quality subchannels. It also enables all algorithms to decrease the numbers of unused slots internal and external to the bursts, as shown in Figs. 8(c) and 8(d), respectively, resulting in the increase of the throughput. Fig. 8(b) demonstrates that, the smaller the number of connections, the larger the improvement ratios achieved by BCO, because, when the burst is larger, eOCSA and WLFF are more likely to construct this burst on low-quality subchannels, whereas BCO attempts to avoid this problem.
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(a) Total throughput (b) Improvement ratio
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Figure 8. Effects of number of connections high mean SNR provides bursts with more opportunities to use better MCSs. Fig. 9(b) demonstrates that the improvement ratios of BCO decreased as the channel quality increased. Two main reasons were determined for this occurrence. First, eOCSA and WLFF did not consider subchannel diversity, and thus, failed to address optimal block exploration. Therefore, the increase of throughput was caused by the higher channel quality. However, because BCO considered optimal block exploration, it achieved satisfactory throughput, even when the mean SNR was low. Therefore, as the mean SNR increased, the increasing slope on throughput in BCO was smaller than that in