Cellular mobile communication is one of the major branches of wireless communication which gives mobility and service to the user. The main technologies that are currently being used are GSM (Global System for Mobile) and CDMA (Code Division Multiple Access). In Nepal, NTC and Ncell have adopted GSM system, while Sky phone use CDMA technology. Although both GSM and CDMA divide their available frequency band into channels, the main difference between these two technologies is GSM uses certain number of frequency channels in a cell and each user get their own frequency channel, while CDMA uses only one frequency channel in a cell with proper allocation of PN (pseudo-random) code to each user. PN code is a special set of numbers which is mixed with the modulated signal during transmission and transmitted message can be decoded only with the same PN code.
Analysis of GSM system with mathematical model in Matlab is very useful for communication design engineers. My work is to find the optimum cluster size, number of cell required and user support for a GSM system with the help of MATLAB GUI. I put some design equations in this GUI and created simple model as shown below.
A cluster is a group of cell with different set of frequencies. It can only be acceptable numbers like 1,3,4,7,9,12,… so that cluster size 7 means there are only seven different cells in the system that use different frequencies and other cells are only the reused pattern of these cells. By knowing the suitable cluster size one can estimate the channel required and estimate the bandwidth of their system.
Total number of cell refers simply to the number of towers required, i.e. cost of the system (Generally, one cell requires one tower for communication purpose). If we also know the area of a cell, than we can estimate the terrestrial coverage of the system. User support indicates the total number of users using the system. It helps to calculate percentage penetration.
The input parameters required for this GUI are SNR (i.e. accepted level of signal to nose ratio; SNR=18 means system’s SNR should not be less than 18dB), path loss (depends on the terrain or terrestrial condition like it is free space, village area or urban area; for urban n=4), GOS (measure of ability of user to access the trunked system, if GOS=2%, then out of 100 users trying to access a trunked system, 2 users are blocked and only 98 users can access the trunked system), Total area of cell and Channel per Cell(we have to specify at most channel the system can provide for a cell).
Sectoring is the process of splitting a single cell into different sub-cells. I have given the facility of simulating the case of sectoring and type of cell area in this GUI. Sectoring options include (i) no sectoring, (ii) 120o sectoring and (iii) 60o sectoring. Similarly types of cell area are differentiated as small, medium and large. Small cells refer to the average area of 1km2, Medium cells refer to 5km2 and large cells refer to 10km2.
Let us view the GUI simulation for the inputs and outputs as shown in figure.
Input are SNR=15 dB, path loss n=4, GOS=2%, total area of cell=2500 km2, channel per cell=20, no sectoring and medium sized cells. So the cluster size we should use is of 7, i.e. we should use at least seven cell of different sets of frequencies. Using this scheme, we can cover 500 medium cell area and support up to 65500 users
To view source code, click here.
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