Frequency reuse is one of methods for increasing the number of channels per a unit area in a cellular system. Because a radio wave gets weaker with distance, radio waves interfere less with each other in places apart from each other by a specific distance and thus the same frequency channel can be used in the places. Use of the same frequency in a plurality of areas based on this principle may lead to a great increase in subscriber capacity. The efficient frequency utilization is called frequency reuse. An identifiable geographical unit is called a cell (mobile communication cell) and frequency channel switching between cells for seamless communication is called handoff. In an analog cellular communication scheme, frequency reuse is essential. A frequency reuse ratio is one of parameters representing frequency efficiency in a cellular system. The frequency reuse ratio is calculated by dividing the total number of cells (sectors) using the same frequency by the total number of cells (sectors) in a multi-cell structure.
A 1G system (e.g. AMPS) has a frequency reuse ratio less than 1. For instance, in a 7-cell frequency reuse scheme, the frequency reuse ratio is 1/7. Compared to the 1G system, a 2G system (e.g. CDMA and TDMA) has an improved frequency reuse ratio. For instance, the frequency reuse ratio may reach 1/4 or 1/3 in GSM being a combination of FDMA and TDMA. In 2G CDMA and 3G WCDMA systems, the frequency reuse ratio may reach 1, thus increasing spectral efficiency and reducing network deployment cost.
When all sectors of one cell and all sells of one network use the same frequency, a frequency reuse ratio of 1 can be achieved. However, the frequency reuse ratio of 1 in a cellular network means that users at a cell edge have degraded signal reception performance due to interference from a neighbor cell.
In OFDMA, a channel is divided into subchannels and thus a signal is transmitted on a subchannel, without using the entire channel as is done in 3G (CDMA2000 or WCDMA). This feature of OFDMA may simultaneously improve the throughputs of users at the center of a cell and users at the boundary of the cell (the edge of the cell). Specifically, the cell center is near to a BS and thus safe from co-channel interference of a neighbor cell. Accordingly, inner users at the cell center may use all available subchannels. However, cell-edge users may use only a part of all available subchannels. Frequencies are allocated to users at the boundary between adjacent cells such that each cell may use a different subchannel. This scheme is called FFR (fractional frequency reuse).
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