Fractional Frequency Reuse in Mobile WiMAX

Frequency spectrum is a limited and increasingly expensive resource. Wireless network operators or WISPs often have to compete in acquiring licenses to operate on frequencies of their choice. Of course, they still have another alternative, that's using free spectrum in license-exempt bands. But then, they have to find the means to control interference from other networks sharing the same band and to limit spillover to other users of the band.

Mobile WiMAX in mobile mode (remember, Mobile WiMAX also supports fixed mode) will be deployed like a cellular network (2G, 3G), requiring a large number of base stations to have a considerable coverage. So in most cases it will operate in licensed bands. Operation in unlicensed bands may be considered only for greenfield deployment where there are no other users of the same spectrum.

Picture. Fractional Frequency Reuse
F1, F2, and F3 are different sets of sub-channels, allocated to users at cell edges.
F = F1+F2+F3. The whole sub-channels (F) are allocated to users at cell centers.

Regardless of licensed or unlicensed spectrum, frequencies have to be used efficiently. Therefore, it's crucial to maintain frequency reuse one. Frequency reuse one is achieved when all sectors within a cell and all cells within a network operate on the same frequency channel. However, frequency reuse one in a cellular network implies that users at a cell edge may get degraded signals due to interference from adjacent cells.

Mobile WiMAX addresses this issue by "tweaking" the frequency reuse one. It works by allowing users at a cell center to operate on all available sub-channels. Cell center is the area closer to a base station (BS) that is particularly immune to co-channel interference. While users at a cell edge are only allowed to operate on a fraction of all available sub-channels. This sub-channels fraction is allocated in such a way that adjacent cells' edges will operate on different sets of sub-channels (see picture above). This is called fractional frequency reuse.

Fractional frequency reuse takes advantage of the fact that a Mobile WiMAX user transmits on sub-channels (because in OFDMA, a channel is divided into sub-channels) and doesn't occupy an entire channel such as in 3G (CDMA2000 or WCDMA). Fractional frequency reuse maximizes spectral efficiency for users at a cell center and improves signal strength and throughput for users at a cell edge.