Channel Assignment

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

  1. Fixed: FCA, fixed channel allocation: Manually assigned by the network operator
  2. Dynamic:
    1. DCA, dynamic channel allocation,
    2. DFS, dynamic frequency selection
    3. Spread spectrum

FCA:

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.

FCA can be extended into a DCA system by using a borrowing strategy in which a cell can borrow channels from neighbouring cell which is supervised by Mobile Switching Centre (MSC). 

DCA:

A more efficient way of channel allocation would be Dynamic Channel Allocation or Dynamic Channel Assignment (DCA) in which voice channel are not allocated to cell permanently, instead for every call request base station request channel from MSC. The channel is allocated following an algorithm which accounts the following criteria:

  • Future blocking probability in neighbouring cells
  • Reuse distance
  • Usage frequency of the candidate channel
  • Average blocking probability of the overall system
  • Instantaneous channel occupancy distribution

It requires the MSC to collect real time data on channel occupancy, traffic distribution and Received Signal Strength Indications(RSSI). DCA schemes are suggested forTDMA/FDMA based cellular systems such as GSM, but are currently not used in any products.] OFDMA systems, such as the downlink of 4G cellular systems, can be considered as carrying out DCA for each individual sub-carrier as well as each timeslot.

DCA can be further classified into centralized and distributed. Some of the centralized DCA schemes are:

  • First available (FA): the first available channel satisfying reuse distance requirement is assigned to the call
  • Locally optimized dynamic assignment (LODA): cost function is based on the future blocking probability in the neighbouring cells
  • Selection with maximum usage on the reuse ring (RING): a candidate channel is selected which is in use in the most cells in the co-channel set

DCA and DFS eliminate the tedious manual frequency planning work. DCA also handles bursty cell traffic and utilizes the cellular radio resources more efficiently. DCA allows the number of channels in a cell to vary with the traffic load, hence increasing channel capacity with little costs.

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