The mobile WiMAX systems use frequency reuse equal to one due to which they have good frequency efficiency . With the frequency reuse of one, users are affected by interference which degrades the quality of service and also introduces latency in the system. This can be avoided by increasing the frequency reuse to three (or more) and the spectrum requirement will increase three times as required with frequency reuse of one . A technique known as Fractional Frequency Reuse (FFR) is used to solve the interference problem incurred by the users at the edge and the interior of the cells. This can be done by using different transmitting powers for users at the interior and users present at the edge of the cell.
Worldwide Interoperability for Microwave Access (WiMAX) has been deployed in more than 146 countries; hence its popularity cannot be challenged. But with these rising figures the coverage and the number of users are increasing significantly. Enormous number of users also results in the increased interference while using the same spectrum in a particular geographical area. The users comprise of both fixed and mobile users. Fixed WiMAX systems may be less vulnerable to interference in contrast to mobile WiMAX systems, in which a user may be either close to a base station in a cell or away from it due to mobility within a cell or beyond.
The impact of interference is such that the cell capacity and coverage decreases . User applications namely video streaming, video-conferencing, high quality multimedia applications etc. require large bandwidths and enhanced spectrum. Increasing the bandwidth on a continual basis is not possible as the spectrum is scarce and for limited use only. The Fractional Frequency Reuse (FFR) technique is deployed to accommodate wider coverage and high data rates without additional expenses incurred for building base stations and other hardware .
2. TDD Frame Structure for Mobile WiMAX
Figure 2 shows a schematic representation of FFR in mobile WiMAX where F1, F2 and F3 represent the different sub-carrier sets . The six major groups discussed in the previous section are allocated to these sub-carrier sets. Table 1 shows the allotment of different groups to their respective sub-carrier sets.
|Group numbers allotted to sub-carrier set
||Sub-carrier set name
|Group 0, group 1
|Group 2, group 3
|Group 4, group 5
Table 1: Allotment of different groups to their respective sub-carrier sets
The frequency reuse factor at the interior of the cells is one (FR=1) where the cells or sectors operate at the same frequency band, due to which there is a high co-channel interference (CCI) at the interior of the cell . The frequency reuse factor at the edge of the cell is three (FR=3), where the user experiences interference from the three cells in the vicinity. The high CCI at the centre of the cell can be decreased by increasing the frequency reuse factor to three or more by implementing a technique known as segmentation. In segmentation, the sub-channels are grouped into three (or more), each group forming a segment . This can improve the spectral efficiencies by increasing the sub-channels and thereby increasing the number of segments. These segments are used for different sectors by which interferences are avoided, but the capacity of each sector might decrease .
The users located at the edge of a cell (or sector) experience the effects of interference from the neighboring cells or sectors. If the Carrier Interference Noise Ratio (CINR) is high, it means that the interference (and noise) is low, which is required at the edge of a cell (or sector) where the interference is high. Therefore, users having a good CINR must operate in the zone where all the sub-channels are available, i.e., frequency reuse 1 zone as shown in figure 3 and figure 4 . Users located at the edge of the cell (or sector) must operate in the frequency reuse 3 zone, in which a fraction of sub-channels are present . By improving the CINR, high data rates can also be achieved.
In this paper we discussed the mobile WiMAX TDD frame structure in which the sub-channels are divided into six groups (major group) present in the PUSC zone for the downlink frame. Solutions for reducing interference at the cell interior and the cell edge were explained by changing the frequency reuse from one to three.
 Yijie Chen; Wenbo Wang; Tao Li; Xing Zhang; Mugen Peng, “Fractional Frequency Reuse in
Mobile WiMAX”, Communications and Networking in China, 2008. ChinaCom 2008. Third
International Conference on; 25-27 Aug. 2008
 Yuefeng Zhou; Zein, N., “Simulation Study of Fractional Frequency Reuse for Mobile WiMAX”,
Vehicular Technology Conference, 2008; 11-14 May 2008