Fractional frequency reuse is one of methods for increasing the number of channels per unit area in a cellular system. Since strength of radio wave gets weaker in proportion to an increasing distance, inter-radio wave interference becomes insignificant in a place over a predetermined distance, whereby the same frequency channel is usable. Using such a principle, it may be able to raise a subscriber volume considerably in a manner of using the same frequency in several areas simultaneously. This efficient frequency utilization is called a frequency reuse. A unit for dividing an area is called a cell (e.g., a mobile communication cell) and a frequency channel switching between cells to maintain a call is called a handoff. And, a frequency reuse technology is essential to an analog cellular mobile communication system. A frequency reuse rate is one of parameters indicating frequency efficiency in a cellular system. In particular, the frequency reuse rate is a value resulting from dividing a total number of cells (sectors), which use the same frequency simultaneously in a multi-cell structure, by the total number of cells (sectors) of the whole multi-cell structure.
A frequency reuse rate of 1G system (e.g., AMPS) is smaller than 1. For instance, in 7-cell frequency reuse, a frequency reuse rate is 1/7. A frequency reuse rate of 2G system (e.g., CDMA, TDMA, etc.) is better than that of 1G system. For instance, a frequency reuse rate of FDMA-TDMA combined GSM may reach a range of ¼ to ⅓. In case of 2G CDMA system or 3G WCDMA system, since a frequency reuse rate may reach 1, efficiency of spectrum is increased and a network arrangement cost is lowered.
When all sectors in one cell and all cells in one network use the same frequency, a frequency reuse rate can become 1. Yet, if a frequency reuse rate becomes 1 in a cellular network, it means that signal reception performance for users at a cell edge is lowered by the interference with a neighbor cell.
In OFDMA, since a channel is divided by a subchannel unit, a signal is carried on a subchannel and all channels are not used like 3G (e.g., CDMA 2000, WCDMA, etc.). Using this feature, it may be able to simultaneously enhance both throughputs of users at a cell center and users at a cell edge. In particular, since a central area of a cell is located close to a base station, it is safe from co-channel interference from a neighbor cell. Hence, inner users at the cell center are able to use all available subchannels. However, the users at the cell edge may use the available subchannels in part only. On a cell edge between cells adjacent to each other, a frequency is allocated to enable the cells to use different subchannels, respectively. This scheme is called a fractional frequency reuse (FFR).
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