Simulation Results

Besides numerical results described in Section 3.4, we will show some simulation results to illustrate the benefits of multicarrier system when using the maximum C/I scheduling scheme. Furthermore, we will compare both system throughput and fairness performances of different resource management approaches in the multicarrier systems.

3.6.1 Simulation Methodology

Then, we apply the two practical IEEE 802.16 channel models, SUI-1 and SUI-5, to our simulation. In [28], six SUI channel delay profiles are specified. For the multicar-rier OFDMA system, we first take the appropriate 2048 samples of the channel delay profiles where the sample time

T_s = 1

B , (3.19)

where B is total bandwidth and 2048 is FFT size corresponding to the number of subcarriers. And then we use the fast Fourier transform (FFT) technique [32] to transform from time domain to frequency domain. Hence, we can observe the multi-path fading effect in the frequency domain, (see Fig. 3.4). Finally, observing a long time period of this frequency domain channel models, we pick different time points to represent the channel response of different users. Therefore, we can form a practical channel matrix for simulation to evaluate the system performance. The simulation parameters are listed in Table 3.5.

Fig. 3.4: Time varying with multipath fading model

Tab. 3.5: Simulation Parameters

No. of user 32

FFT size 2048

Total bandwidth 6 MHz Channel model SUI-1 and SUI-5

3.6.2 Effect of Multiuser Scheduling on the Fairness of Multi-carrier System

Figure 3.5 shows the fairness by using the IEEE 802.16a SUI-5 channel models in simulation. For the sake of fitting in with IEEE 802.16a OFDMA physical layer standard, 2048 FFT size used, we divide the total bandwidth into 2,4,8,16 and 32 subchannels. We still observe that when the number of subcarriers increases, the system fairness performance becomes better even in the SUI-5 channel model.

3.6.3 System Performances Comparison of Different Resource Allocation Techniques

Figures 3.6 and 3.7 compares the fairness and throughput performances of different resource management algorithms in SUI-1 and SUI-5 channel models, respectively. In SUI-1 channel model, the fairness performance can be maintained easily. However, SUI-5 channel suffer from more severe fading.

Figure 3.6 shows that dynamic power allocation and maximum C/I scheduling policies do not have obvious difference in fairness performance in SUI-1 channel mod-els. Because frequency and multiuser diversity exist in the multiuser multi-carrier environment, fairness performance is very good. However, the fairness performance

of the maximum C/I scheduling is worse than that of the power allocation scheme about 3.5%. Nevertheless, the value about 0.96 of the fairness index of the maximum C/I scheduling algorithm still means it is a fair resource allocation.

At the same time, we observe Fig. 3.7. We find that the throughput per-formances of the maximum C/I scheduling policy always better than that of the power allocation scheme whether in the SUI-1 channel model or in the SUI-5 channel model. In the SUI-1 channel model, the difference of the throughput performances of the maximum C/I scheduling algorithm and the power allocation scheme is very small. The maximum C/I performs better than the dynamic power allocation about 5%. On the other hand, the throughput performance of the maximum C/I schedul-ing policy is better than that of the power allocation algorithm about 13%. In short, we observe that the system performances of the maximum C/I scheduling algorithm and the power allocation policy are similar in the SUI-1 channel model. However, in the SUI-5 channel model, the maximum C/I scheduling enhance the system through-put about 13% more than the power allocation at the expense sacrificing 3.5% of the fairness performance. Therefore, we concludes that good fairness performance is easily achieved in the multiuser multi-carrier system even when the maximum C/I scheduling adopted.

3.6.4 System Performances Comparison of Different Scheduling Techniques

In addition to comparing the system performance of the power allocation and the maximum scheduling schemes, we compare the system performances of the maximum C/I and proportional scheduling algorithms in this subsection.

Figure 3.8 compares the fairness performance of the maximum C/I scheduling and the proportional fair scheduling in the IEEE 802.16 SUI-1 and SUI-5 channel

models. In the IEEE 802.16 SUI-1 channel model, the fairness performance can be easily maintained. However, because of more severe fading, it is more difficult to maintain the short-term fairness performance in the IEEE 802.16 SUI-5 channel than that in the IEEE 802.16 SUI 1 channel. The proportional fair scheduling takes the great part of frequency diversity and multiuser diversity when the channel variation is not severe, so it also performs well. Furthermore, from the figure, we see that in the IEEE 802.16 SUI-1 channel model, the difference of fairness performance between the maximum C/I and the proportional fair scheduling is insignificant. Even in the IEEE 802.16 SUI-5 channel model, although the fairness of the proportional fair scheduling scheme is still better than the maximum C/I scheduling scheme, the difference of the fairness index between the two scheduling algorithms is less than 3.5%.

Figure 3.9 shows that main advantage of using maximum C/I in a multiuser multi-carrier system. In Fig. 3.9, we compare the throughput performance of both scheduling schemes. In the SUI-1 channel, the throughput performances of the two algorithms are about the same. Interestingly, when consider the SUI-5 channel model with more severe fading, Fig. 3.9 indicates that maximum C/I can take advantage of severer fading and maximize the system throughput. Summarizing from Figs.

3.8 and 3.9, we find that the maximum C/I scheduling can improve the throughput performance by 20% over the proportional fair scheduling at the cost of degrading the fairness index by only 3.5%.

Consequently, the maximum C/I is sufficiently used in the OFDMA system.

We do not need other complicated resource allocation algorithms, such as proportional fair scheduling or power allocation method, to achieve good fairness performance at the expense of throughput. By adopting this simple maximum C/I scheduling schemes, we can obtain good fairness performance and the best throughput perfor-mance simultaneously. In SUI-5 channel model, the maximum C/I improves total system throughput about 13% compared to the power allocation without dynamic

subcarrier allocation. Moreover, the maximum C/I scheduling algorithm even in-crease more than 20% of system throughput than that using the proportional fair scheduling policy.

3.6.5 Discussions

In the scenario described above, we should decide to whom all subcarriers belong every transmission time interval (TTI). It is impractical to do this in such a short time. In fact, because IEEE 802.16a is a fixed wireless application, the channel does not change frequently. Hence, we do not need to schedule users every TTI.

Considering the coherence time of the system, the maximum Doppler frequency is 20Hz (SUI-5 channel), and then we will calculate the coherence time based on (3.20) [33]. Coherence time is the time duration over which two received signals have a strong potential for amplitude correlation. The Doppler spread and coherence time are inversely proportional to one another. The equation (3.20) is defined as the time over which the time correlation function is above 0.5. For example, when the maximum Doppler shift f_d = 2Hz, and the coherence time T_c is about 90 ms. Therefore, the maximum C/I scheduling approach is practical in the system.

T_c = 9

16πf_d (3.20)