Channel Allocation Methodology in Mobile Computing.

Channel Allocation:

One of the critical issue in mobile computing is to provide efficient allocation of communication channel for the better performance of the wireless mobile system. The Channel allocation algorithms should be robust and scalable and some of the allocation methods are briefly described in this blog for learning purpose.

What is Channel Allocation??



A given Radio Spectrum or Bandwidth can be divided into a set of disjoint or non interfering radio channels. All such channels can be used simultaneously while maintaining an acceptable radio signal. In order to divide a given spectrum into such channels many techniques such as frequency division(FD), time division(TD), or code division(CD) can be used.

In FD it is done by dividing spectrum into disjoint frequency bands
In TD it is done by Dividing spectrum into disjoint Time period called time slots.
In CD it is done using different modulation code.

An efficient channel allocation strategy should have following features:

1. uses frequency reuse..
2. minimize the connection time.
3. maximize the communication sessions
4.adaptive to load distribution.

Classification of Channel Allocation:

1. Fixed or Static Channel Allocation :

In this a set of nominal channel are permanently fixed or allocated to each cell for its exclusive use. All cells can use all channels assigned to them simultaneously without interference.Such policy is static and can't take advantage of temporary stochastic variations in demands for services.

2. Dynamic channel allocation (DCA):

In dynamic channel allocation all channels are kept in a central pool and are assigned dynamically to radio cells as new call arrive in system. After a call is completed its channel is returned to the central pool for reuse . By this In DCA a channel is eligible for use in any cell provided that the signal interference constraints are satisfied. The main idea of all DCA scheme is to evaluate cost of using each candidate channel and select the one with minimum cost for allocation providing the interference constraints satisfied.

DCA schemes :  based on information used for channel assignment =>
1. Call-by-Call DCA :  the channel allocation is based only on current channel usage condition
2. Adaptive DCA: the channel assignment is adaptively carried out using information on the previous as well as the present channel usage condition.

The best existing dynamic channel allocation policy is BDCL, (Borrowing with Directional channel Locking of Zhang & yuhm 1989). It numbered the channel from 1 to N, partitions and assigns them to cells as in Fixed Allocation. The Channel assign to a channel are its nominal channels. If a nominal channel is available when a call arrives in a cell, the smallest numbered channel is assign to a call. If no nominal channel is available, then the largest numbered free channel is borrowed from the neighbor with most free channels.
When channel in borrowed,accounting the directional effect of which cell not longer use that channel because interference is done. The call gets blocked if there are no channel at all.

this scheme provide channel ordering nominal channel and channel borrowing.