What are Channel Allocation Strategies?
Channel Allocation is a method of assigning the existing channels to the various cells within the cellular system. Channel assignment is mainly done to manage the traffic demands of the cell.
Concept of Channel Assignment
We are aware of the basic idea involved in mobile communication that a radio signal path is maintained between the base station and the mobile station so as to maintain communication. We know that there are various multiple access technique like FDMA, TDMA, and CDMA.
In Frequency Division Multiple Access, the various users of the cell are allotted different frequency bands to establish communication. While in Time Division Multiple Access, the communication is maintained over the same frequency band at different time slots. As against, in Code Division Multiple Access a well-defined pattern code is imposed on the actual signal and then transmission over the channel takes place.
Basically, mobile communication is known to be a wireless network that allows users to connect while roaming.
Need for Channel Assignment
We are aware of the fact that there are number of radio channels in a cell and when a call is made then a specific radio channel is assigned to ensure communication.
However, when call attempts in a cell increases more than the assigned radio channels, then the extra call requests will remain unassigned, and hence the call will get dropped. When a higher number of calls remain unconnected within the cell then this shows poor quality service of the provider.
There is a term called ‘blocking probability’ which is defined as the overall dropped calls as a percentage of overall call requests within the cell.
Generally, service providers ensure that the blocking probability must be less than 3% of each cell and this depends on the number of channels assigned to each cell.
The figure shown above clearly represents that the number of channels assigned is 3 but the number of requested calls is 5. So, here number of calls is more than the overall available channels and so there are 2 dropped calls.
Now, let us have a look at the figure shown below:
Here in this scenario, the number of assigned channels to the cell are more in comparison to overall call requests generated and thus here some of the channels remain unused. This turns out to be a waste of the available channel resources.
Hence channel assignment is necessarily done to ensure good usage efficiency within the channel.
There are few considerations regarding the channel assignment which are as follows:
- The antenna at the base station must be accessible but all devices of the channel hence spacing of the channels must be proper.
- The traffic demand of the cell must be fulfilled by all channels of that cell.
- Cochannel interference must be least as much as possible.
It is to be noted here that in earlier days practically economical approach was that a single antenna must be used for both transmitter and receiver. However, with the advent of technology now it is not necessary to use a common antenna as we can use power-efficient common amplifiers.
Hence, to achieve stable channel assignment point second and third must be more severely considered than the first one.
Methods of Channel Assignment
Channel Assignment is basically done to avoid co-channel and adjacent channel interference in the cellular system as much as possible.
There are mainly two methods of assigning channels to each cell which are fixed channel assignment and dynamic channel assignment. Let us now proceed to understand each method separately.
Fixed Channel Assignment
It is abbreviated as FCA. In this channel assignment technique, each mobile subscriber within the cell is assigned its own frequency channel to ensure communication. Here basically what follows is assigning the voice channels for a long period of time.
The channel numbering is done in increasing order manner according to frequency. No matter what is the overall number of channels in the channel set, the highest and the lowest channel sets are frequency adjacent to each other.
Basically, in the fixed channel assignment technique, the channels to the cell are assigned statically. This means that whatever the traffic demand is the assigned channels will not vary. Hence this method is suitable for uniform traffic load conditions within the cell.
The figure below represents fixed channel assignment allocation:
One can determine the number of channels for a specific cell by the formulation given below:
: N corresponds to the number of overall channels needed to cover a specific coverage area
D is the frequency reuse distance and
R denotes the radius of the cell
This channel assignment offers maximal frequency reuse which is known to be its crucial advantageous factor. However, at the same time, major disadvantageous factor is that channel bandwidth gets wasted and congestion may be a problem when the cell traffic is non-uniform.
Therefore, if we want fixed channel assignment then it is necessary that the expected load in each cell must be estimated along with that the required number of channels must be calculated.
Dynamic Channel Assignment
In this channel assignment technique, the allocated channels to a cell is not static and thus varies depending on the requirement. Here the channel assignment is time-varying in nature and changes according to the traffic demands in the cell. It is abbreviated as DCA and sometimes called the dynamic method.
This method shows suitability in cases when the traffic demand is non-uniform. There is a resource pool that provides the channel to the cells according to the requirement of each cell.
The figure shown above clearly indicates that the channels a, b and c are used by various cells but according to the requirement as we have not preassigned the channels to the cells.
This method is known to be a comparatively more efficient one than FCA because it offers flexibility in changing the demands of the system. Day by day, with the increasing demand of mobile phone users, this approach fulfills today’s needs.
If we talk about the advantage offered by this method then it is known that the channel usage efficiency is quite high. But at the same time the disadvantageous factor is that here randomization exists thus frequency reuse is not maximized. Also, as the process involves real time computation thus, is time-consuming and complex.
It is to be noted here that in this method, once the cell demand gets fulfilled by the channel then the channel gets returned to the pool for further reusability.
The pool from where channels are demanded can be of centralized or distributed.
This is all about channel assignment strategies.