Dynamic Frequency Selection

In radio resource management for wireless and cellular networks, channel allocation schemes allocate bandwidth and communication channels to base stations, access points and terminal equipment. The objective is to achieve maximum system spectral efficiency in bit/s/Hz/site by means of frequency reuse, but still assure a certain grade of service by avoiding co-channel interference and adjacent channel interference among nearby cells or networks that share the bandwidth.

Channel-allocation schemes follow one of two types of strategy:

In Fixed Channel Allocation or Fixed Channel Assignment (FCA) each cell is given a predetermined set of frequency channels. FCA requires manual frequency planning, which is an arduous task in TDMA and FDMA based systems, since such systems are highly sensitive to co-channel interference from nearby cells that are reusing the same channel. Another drawback with TDMA and FDMA systems with FCA is that the number of channels in the cell remains constant irrespective of the number of customers in that cell. This results in traffic congestion and some calls being lost when traffic gets heavy in some cells, and idle capacity in other cells.

If FCA is combined with conventional FDMA and perhaps or TDMA, a fixed number of voice channels can be transferred over the cell. A new call can only be connected by an unused channel. If all the channel are occupied than the new call is blocked in this system. There are however several dynamic radio-resource management schemes that can be combined with FCA. A simple form is traffic-adaptive handover threshold, implying that calls from cell phones situated in the overlap between two adjacent cells can be forced to make handover to the cell with lowest load for the moment. If FCA is combined with spread spectrum, the maximum number of channels is not fixed in theory, but in practice a maximum limit is applied, since too many calls would cause too high co-channel interference level, causing the quality to be problematic. Spread spectrum allows cell breathing to be applied, by allowing an overloaded cell to borrow capacity (maximum number of simultaneous calls in the cell) from a nearby cell that is sharing the same frequency.

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