A good interference management (IM) scheme that can mitigate inter-cell interference.
Frequency reuse is a conventional approach to reduce the effects of the inter-cell interfer-ence. However, the generic frequency reuse approach faces the trade-off between spectral efficiency and inter-cell interference mitigation. The former is affected by reuse factor is small and the latter can be achieved by big reuse factor. Hence, the FFR scheme [20] is introduced in this chapter.
FFR is an approach to limit the inter-cell interference. How to use it to get the best performance is an interesting issue. For example, the IEEE 802.16m WiMAX supports frequency reuse factor of one which means that all cells operate at only one frequency band to maximize spectrum efficiency. Nevertheless, the strong inter-cell interference is a main issue. Traditional frequency reuse technique can improve the inter-cell interference, but it will lower spectrum utilization. The concept of FFR is based on the idea of applying a reuse factor of one in the zone near the BS, and a higher reuse factor in the zone near the cell boundary. The FFR technique can reduce the inter-cell interference from neighbor cells for the MS near cell boundary and take care of spectrum utilization.
2.3.1 Traditional Frequency Reuse Scheme
The simplest frequency reuse scheme is a frequency reuse factor-n system. About the tra-ditional frequency reuse scheme is shown as in Fig. 2.3. Total bandwidth can be used in the system will be divide into several partitions (n>1) and assign these partitions to neighboring cells that be shaped as a cluster. In a cluster, the neighboring cell will be assigned different bands to avoid inter-cell interference. For instance, Fig. 2.3 is tradi-tion frequency reuse scheme of n=3, there are not inter-cell interference in each cluster.
And each cell will be surrounded by six immediate neighboring cells which use orthogo-nal bands and each bandwidth equal to one-third of the total bandwidth. The traditioorthogo-nal
Figure 2.3: The architecture of traditional frequency reuse scheme and demonstrates the scenario of spectrum usage ratio.
Figure 2.4: The architecture of fraction frequency reuse scheme by considering interference mitigation at outer region and outer region reuse factor equals to three.
frequency reuse scheme can trade the spectral efficiency for the benefits of completing can-cellation of stronger inter-cell interference from first-tier cells. As we know, the larger n will receive lower inter-cell interference at receiver, but also degrades spectral efficiency in each cells. Hence, use the tradition frequency reuse scheme in the cellular systems is not the best interference management scheme in the view of system performance.
2.3.2 Fractional Frequency Reuse (FFR) Scheme
The fractional frequency reuse scheme is used to increase spectrum efficiency and improve the link quality while MS locate at cell boundary. As Fig. 2.4 shown, each cell divide into two groups, the region of inner and outer. As we realize, if MS arrives to the inner region, they could have good link quality by less propagation lossy than MS locate at outer region. Hence, the MS locates at inner region can tolerate more effects of inter-cell interference from neighboring cells than outer region. On the other hand, the outer region
always has larger propagation lossy since if they also receive inter-cell interference from neighboring cells, signal transmission is almost outage that means the received signal can’t be demodulated. Therefore, FFR scheme is major solution to improve link quality when MS transmits signal at cell boundary and maintain spectrum efficiency. In the inner region always use frequency reuse factor equals to one or smaller than frequency reuse factor of outer region. However, the frequency reuse factor of outer region is always larger than one for improving the effects of inter-cell interference from neighboring cells. For example, there is a FFR scheme with frequency reuse factor is one at inner region and three at outer region in Fig. 2.4. The total bandwidth will classify four parts to assign in different region for a cluster is combined by cell A, B and C. First part is used in inner region and other parts are assigned to outer region of each cells. In a cluster, the inner region shares the same bandwidth to increase spectrum utilization in a cell and the outer region uses different bandwidth to avoid inter-cell interference.
For the FFR scheme, two important design parameters must be noticed are the size of inner region and the outer region reuse factor. The size of inner region is associated with maintaining spectrum efficiency. The outer region reuse factor will affect spectrum utilization and overall inter-cell interference. Two parameters will affect the system per-formance in FFR scheme. Hence, system must design this two factors to improve system performance in difference systems.
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CHAPTER 3
Effect of Cross-Slot Interference in FFR based TDD-OFDMA Multi-Cellular System
Multimedia services are popularly used in the third generation communication systems. In the future, there are more various applications with different uplink/downlink bandwidth requirements. However, the traditional bandwidth requirements of uplink and downlink transmission are always symmetric by voice and messages. No matter using TDD or FDD mode, system always allocates symmetric traffics in each cell. Thus, inter-cell interference always comes from neighboring cells at different entities between transmitter and receiver.
This kinds of inter-cell interference is called non cross-slot interference, such as BS-to-MS and BS-to-MS-to-BS interference. Oppositely, the inter-cell interference is coming from same entities between transmitter and receiver called cross-slot interference, such as BS-to-BS and MS-to-MS cross-slot interference which doesn’t cause in FDD and TDD with fix uplink/downlink ratio among all cells.
Furthermore, more various applications with different uplink/downlink bandwidth requirements are generated. Each cell may support different kinds of uplink/downlink ratio dynamically. Thus, the dynamic TDD mode is popularly researched in the recent years.
The key advantage of the dynamic TDD mode is its capability of flexibly adjusting up-link and downup-link bandwidth by allocating different numbers of time slot. However, the cross-slot interference may be generated in dynamic TDD mode and it becomes the largest resistance of supporting dynamic TDD mode, because will degrade SINR seriously. If
we want to support multimedia service in next generation system, the effects of cross-slot interference must be solved or reduced for guaranteeing link quality.