Channel allocation schemes




In TDMA and FDMA based cellular radio systems and wireless networks, channel allocation schemes are required to allocate channels to base stations and access points and to avoid co-channel interference among nearby cells. A number of approaches have been tried to assign bandwidth to users in an efficient manner while minimizing interference to other users.

FCA
Fixed Channel Allocation or Fixed Channel Assignment (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 irrespectively of the number of customers in that cell. This result in traffic congestion and some calls being lost when traffic gets heavy in some cells, and idle capacity in other cells.

DCA and DFS

A more efficient way of channel allocation would be Dynamic Channel Allocation or Dynamic Channel Assignment (DCA) in cellular systems, and Dynamic Frequency Selection (DFS) in wireless networks with several adjacent non-centrally controlled access-points. DCA schemes are suggested for TDMA/FDMA based cellular systems such as GSM, but are currently not used in any products. DFS is supported by the novel IEEE 802.11h wireless local area network standard.
DFS is also mandated in the 5470-5725 MHz U-NII band for radar avoidance.
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.

DCA vs spread spectrum
Spread spectrum can be considered as an alternative to complex DCA algorithms. Spread spectrum avoids cochannel interference between adjacent cells, since the probability that users in nearby cells use the same spreading code is insignificant. Thus the frequency channel allocation problem is relaxed in cellular networks based on a combination of Spread spectrum and FDMA, for example IS95 and 3G systems. Spread spectrum also facilitate that centrally controlled base stations dynamically borrow resources from each other depending on the traffic load, simply by increasing the maximum allowed number of simultaneous users in one cell (the maximum allowed interference level from the users in the cell), and decreasing it in an adjacent cell. Users in the overlap between the base station coverage area can be transferred between the cells (called cell-breathing), or the traffic can be regulated by admission control and traffic-shaping.
However, spread spectrum gives lower spectral efficiency than non-spread spectrum techniques, if the channel allocation in the latter case is optimized by a good DCA scheme. Especially OFDM modulation is an interesting alternative to spread spectrum in many because of its ability to combat multipath propagation for wideband channels without complex equalization. OFDM can be extended with OFDMA for uplink multiple access among users in the same cell. For avoidance of inter-cell interference, FDMA with DCA or DFS is once again of interest. One example of this concept is the above mentioned IEEE 802.11h standard. OFDM and OFDMA with DCA is often studied as an alternative for 4G wireless systems.

DCA on a packet-by-packet basis
In packet based data communication services, the communication is bursty and the traffic load rapidly changing. For high system spectrum efficiency, DCA should be performed on a packet-by-packet basis. Examples of algorithms for packet-by-packet DCA are Dynamic Packet Assignment (DPA), Dynamic Single Frequency Networks (DSFN) and Packet and resource plan scheduling (PARPS).

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