Mumbai University > Electronics and Telecommunication > Sem8 > Wireless Networks

Marks: 10 M

Year: May 2016

Factors affecting size of the cellular network and the frequency Planning:

i) The essence of a cellular network is the use of multiple low-power transmitters, on the order of 100 W or less. Because the range of such a transmitter is small, an area can be divided into cells, each one served by its own antenna.

ii) Frequency Reuse In a cellular system, each cell has a base transceiver. The transmission power is carefully controlled to allow communication within the cell using a given frequency band while limiting the power at that frequency that escapes the cell into adjacent cells.

iii) The objective is to use the same frequency band in multiple cells at some distance from one another. This allows the same frequency band to be used for multiple simultaneous conversations in different cells.

iv) A key design issue is to determine the minimum separation between two cells using the same frequency band, so that the two cells do not interfere with each other.

In characterizing frequency reuse, the following parameters are commonly used:

D = minimum distance between centers of cells that use the same frequency band (called cochannels)

R = radius of a cell

d = distance between centers of adjacent cells

N = number of cells in a repetitious pattern (each cell in the pattern uses a unique set of frequency bands) termed the cluster size

In a hexagonal cell pattern, only the following values of N are possible:

$N = I^2 + J^2 + (I X J), I,J = 0, 1,2, 3,...$ Hence, possible values of N are 1, 3,4, 7,9, 12, 13. 16, 19,21, and so on.

The following relationship holds:

${\frac {D}{d}} = \sqrt {3N}$

Now if each cell uses S number of channels with a N cell cluster pattern, the total number of available duplex channels K can be expressed as

K=S•N

i) A geographical area may be covered by replicating the N cell pattern by M times then total available duplex channel capacity (C) in the region is given by C=MSN

ii) This implies that for a constant cell size, lower N value provides a scope for more replication (M) of pattern and hence more capacity. This however, causes more co-channel interference because of reduction of D.

iii) Choice of N is an important issue for designer to decide the interference tolerance for quality of signal strength or to enhance cellular capacity.

iv) Frequency reuse plan utilizes several co-channels cells separated by a distance, D, may cause co-channel interference if D is not sufficiently large.

v) This limits the single-to-noise in the desired cell as the interfering signals from co-channel cells are unwanted. This interference can be reduced by increasing D.

vi) For cells of the same size radiating the same power from each of co-channel base stations, the co-channel interference depends on the ratio D/R.

Interference:

• Interference is one of the major factors affecting the performance of cellular radio systems. Sources of interference consist of another mobile inside the same cell, an ongoing call in a neighboring cell, other base stations transmitting signal in the same frequency band, or any non-cellular system which accidentally transmits energy into the cellular frequency band.

• Interference on voice signals could give rise to cross talk, where the caller hears interference in the background due to the presence of an unwanted transmission. The presence of interference in control channels, gives rise to missed and blocked calls.

• Interference has been accepted as a major obstruction in increasing the capacity of a system and is largely responsible for dropped calls in a network. The two major types of interference that are taken consideration while allocating channels to the calls are co-channel and adjacent channel interference.

i) Co-channel Interference:

• The channel reuse approach is very useful for increasing the efficiency of radio spectrum utilization but it results in co-channel interference because the same radio channel is repeatedly used in different co-channel cells in a network. In this case, the quality of a received signal is very much affected both by the amount of radio coverage area and the co-channel interference.

• Co-channel interference takes place when two or more transmitters located within a wireless system, or even a neighboring wireless system, which are transmitting on the same radio channel. Co-channel interference happens when the same carrier frequency (base station) reaches the same receiver (mobile phone) from two different transmitters.

ii) Adjacent Channel Interference:

• Signals from neighboring radio channels, also called adjacent channel, leak into the particular channel, thus causing adjacent channel interference.

• Adjacent channel interference takes place due to the inability of a mobile phone to separate out the signals of adjacent channels allocated to neighboring cell sites.