Effect of Different Design Factors on Link Reliability and Cell

In Fig. 5.7, link reliability is simulated for literature work (location-TSA), traditional fre-quency reuse scheme and FFR scheme in symmetric environment with Δ_DL : Δ_{U L}= 7 : 7.

We simulated reuse factor equals to two, three and for for traditional frequency reuse scheme and outer region reuse factor equals to two, three and four for FFR scheme. The traditional frequency reuse scheme is the case of inner region radius is determined as zero.

Total spectrum will be divided n parts to avoid inter-cell interference. As the simulation shown, the traditional frequency reuse scheme can maintain higher link reliability which almost achieve 0.95, 0.99 and 1 while n=2, 3 and 4, respectively. The link reliability of traditional frequency reuse scheme with n=2, 3 and 4 always outperform the literature work that implement time slots strategy of considering MS location information because the literature work doesn’t mitigate the effects of inter-cell interference. From views of cell throughput that is illustrated in Fig. 5.8, the literature work have higher cell through-put which equals to 21.8 Mbps than traditional frequency reuse scheme due to use total spectrum in a cell coverage which is better than traditional frequency reuse scheme just can use one n-th ratio of total bandwidth. The cell throughput is almost 15, 13.5 and 11.8 Mbps while using reuse factor equals to two, three and four, respectively. However, the traditional frequency reuse scheme is sacrificed many spectrum to exchange higher link re-liability. Hence, using FFR scheme is better than traditional frequency reuse scheme due to using inner region and outer region to maintain spectrum utilization and mitigate inter-cell interference while MS locates at cell boundary.

In Figs. 5.7 and 5.8 also show the FFR scheme while inner region radius doesn’t equal to zero. Due to increase inner region radius, more spectrum can be used in a cell coverage. More spectrum are assigned to inner region and MS transmit signal in small size of inner region always have higher transmission quality since the link reliability will

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Figure 5.9: Link reliability for traditional frequency reuse scheme and FFR scheme in the Asymmetric traffic environment with Δrand=5-9.

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Figure 5.10: Cell throughput for straditional frequency reuse scheme and FFR scheme in

be promoted in small size of inner region. By observing Fig. 5.8, cell throughput will be improved due to more spectrum can be used and high transmission quality in small size of inner region. Nevertheless, while size of inner region becomes too large, more inter-cell interference come from neighboring cells will be received at receiver and degrade SINR level. Even though more spectrum can be use in a cell, the cell throughput become worse than small size of inner region due to the requirement of link reliability can be satisfied. In symmetric traffic environment, the maximum size of inner region with guaranteeing link reliability can almost achieve inner region radius equals to 800 m and cell throughput can be maximized as 23.4 Mbps while using outer region reuse factor equals to three. As the simulation shown, the FFR scheme is better than literature work by suitable design inner region radius and outer region reuse factor.

Figs. 5.9 and 5.10 shows link reliability and cell throughput for literature work (location-TSA), traditional frequency reuse scheme and FFR scheme in an asymmetric environment with Δ_DL : Δ_{U L} = Δ_rand : (N − Δrand), where Δ_rand = 5− 9 that means each cell may request bandwidth requirement of downlink and uplink within CTS size of -2 to 2. The largest size of generating cross-slot interference is four time slots range.

We can observe the simulation result no matter traditional frequency reuse scheme and of FFR scheme have degrade their link reliability and cell throughput. The cell throughput is 14.4, 13.1 and 11.6 Mbps by using n=2, 3 and 4 for traditional frequency reuse scheme with achieving link reliability 0.91, 0.97 and 0.98, respectively. However, if we suitable design inner region radius equals to 700 m and outer region reuse factor n=3, system can maximize cell throughput to 22.1 Mbps with guaranteeing link reliability is higher than 0.9.

The literature work have little advantages while inner region is small because the literature work is using an approach that avoid MS transmit signal and incur cross-slot interference at cell boundary. Hence, the frame will be divide two parts, CTS region and non-CTS region. The CTS region can be used for MS locates in inner region and non-CTS region can be supply to MS locates in overall cell no matter inner region or outer region. In the

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Figure 5.11: Link reliability for traditional frequency reuse scheme and FFR scheme in the Asymmetric traffic environment with Δrand=3-11.

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Figure 5.12: Cell throughput for traditional frequency reuse scheme and FFR scheme in

asymmetric traffic environment with Δ_rand = 5− 9, there will generate CTS size is four time slots and non-CTS size is ten time slots. While inner region is small, some MS locates in inner region can use the CTS time slots. Although MS transmit signal and incur cross-slot interference effect in inner region, the stronger received signal power can maintain SINR levels and guarantee link reliability. Hence, the link reliability will be improved in small inner region. However, the link reliability still isn’t better than traditional frequency reuse scheme and FFR scheme because the literature work doesn’t mitigate effects of inter-cell interference in each case.

We simulate large size of CTS in the TDD-OFDMA system with Δ_DL : Δ_{U L} = Δ_rand: (N−Δrand), where Δ_rand = 3−11 that means CTS size is eight time slots from -4 to 4. The simulation result is shown in Fig. 5.11 and 5.12. The strong cross-slot interference degrade system performance seriously. The link reliability of traditional frequency reuse scheme by using reuse factor n=2, 3 and 4 are reduced to 0.86, 0.95 and 0.97, respectively.

However, FFR scheme must reduce inner region radius to 600 m and use outer region reuse factor n=3 to guarantee link reliability and achieve cell throughput to 21 Mbps. For literature work, larger CTS region will be generated, lower spectrum utilization is used in overall cell because system must avoid MS located in outer region and incur cross-slot interference effect immediately. Hence, there are less resource unit can be used in outer region while size of CTS region is large. The simulation shows the literature work have poor cell throughput than FFR scheme. Finally, we consider the serious effect of cross-slot interference environment that Δ_DL : Δ_{U L} = Δ_rand : (N − Δrand) and Δ_rand = 1− 13.

The link reliability and cell throughput result are simulated in Figs. 5.13 and 5.14. The FFR scheme can maintain inner region radius equals to 600 m, but system must use outer region reuse factor n=4 to guarantee link reliability and the cell throughput can achieve 20 Mbps.

However, the literature work will waste many resource unit to avoid cross-slot interference effects. In the large size CTS environment, the FFR scheme is better than literature work.

From observing several simulation result, the FFR scheme can effectively reduce

the effects of inter-cell interference by adjusting inner reign radius and outer region reuse factor especially surrounding in random asymmetric environment. We can use revise this two factors to maximize cell throughput with guaranteeing link reliability. However, we will show advantages of FFR scheme in different random asymmetric traffic environments in next section.

5.4.2 Effect of Different Random Asymmetric Traffics on Link