Praise for Fundamentals of WiMAX

82 Chapter 3 • The Challenge of Broadband Wireless Channelsigated if higher frequency reuse is adopted, as shown in Figure 3.7b. However, as previouslyemphasized, this improvement in the quality of communication is achieved at the sacrifice ofspectral efficiency: In this case, the available bandwidth is cut by a factor of 3. Frequency planningis a delicate balancing act of using the highest reuse factor possible while still having mostof the cell have at least some minimum SIR.3.3.3 SectoringSince the SIR is so low in most of the cell, it is desirable to find techniques to improve it withoutsacrificing so much bandwidth, as frequency reuse does. A popular technique is to sectorize thecells, which is effective if frequencies are reused in each cell. Using directional antennas insteadof an omnidirectional antenna at the base station can significantly reduced the cochannel interference.An illustration of sectoring is shown in Figure 3.8. Although the absolute amount ofbandwidth used is three times before (assuming three sector cells), the capacity increase is infact more than three times. No capacity is lost from sectoring, because each sector can reusetime and code slots, so each sector has the same nominal capacity as an entire cell. Furthermore,the capacity in each sector is higher than that in a nonsectored cellular system, because the interferenceis reduced by sectoring, since users experience only interference from the sectors at theirfrequency. In Figure 3.8a, if each sector 1 points in the same direction in each cell, the interferencecaused by neighboring cells will be dramatically reduced. An alternative way to use sectors,not shown in Figure 3.8, is to reuse frequencies in each sector. In this case, all the time/code/frequency slots can be reused in each sector, but there is no reduction in the experiencedinterference.Figure 3.9 shows the regions of a three-sector cell in various SIR bins of the systems withuniversal frequency reuse and 1/3 frequency reuse. All the configurations are the same as thoseof Figure 3.7 except that sectoring is added. Compared to Figure 3.7, sectoring improves SIR,especially at the cell boundaries, even when universal frequency reuse is adopted. If sectoring isadopted with frequency reuse, the received SIR can be significantly improved, as shown inFigure 3.9b, where both f =1/3 frequency reuse and 120° sectoring are used.Although sectoring is an effective and practical approach to the OCI problem, it is not withoutcost. Sectoring increases the number of antennas at each base station and reduces trunkingefficiency, owing to channel sectoring at the base station. Even though intersector handoff issimpler than intercell handoff, sectoring also increases the overhead, owing to the increasednumber of intersector handoffs. Finally, in channels with heavy scattering, desired power can belost into other sectors, which can cause inter-sector interference as well as power loss.Although the problem of cochannel interference has existed in cellular systems for manyyears, its effect on future cellular systems, such as WiMAX, is likely to be far more severe,owing to the requirements for high data rate, high spectral efficiency, and the likely use of multipleantennas. This is a very tough combination [2, 6]. Recent research approaches to this difficultproblem have focused on advanced signal-processing techniques at the receiver [1, 6] andthe transmitter [15, 29, 35] as a means of reducing or canceling the perceived interference.