Capacity of Cellular CDMA
The capacity of CDMA systems is interference limited. Any reduction in the interference will cause a linear increase in the capacity of CDMA.The link performance for each user increases as the number of users decreases. The directional antennas receive signals from only a fraction of the current users, thus leading to the reduction of interference.Another way of increasing CDMA capacity is to operate in a discontinuous transmission mode (DTX), where advantage is taken of the intermittent nature of speech
Evaluation the capacity of CDMA system:
The voice signals have a duty factor of about 3/8 in landline networks [Bra68], and 1/2 for mobile systems, where background noise and vibration can trigger voice activity detectors.
The average capacity of a CDMA system can be increased by a factor inversely proportional to the duty factor.CDMA can reuse the entire spectrum for all cells, and this result in an increase of capacity by a large percentage over the normal frequency reuse factor.
For evaluating the capacity of CDMA system, first consider a single cell system. The cellular network consists of a large number of mobile users communicating with a base station
For a single cell system under consideration, these weighting factors can be assumed to be equal.For a single-cell system with power control, all the signals on the reverse channel are received at the same power level at the base station.
Let the number of users be N.
Then, each demodulator at the cell site receives a composite waveform containing the desired signal of power S and (N — I) interfering users, each of which has power, S.
Thus, the signal-to-noise ratio is ― (1)
The SNR at the base station receiver can be represented in terms ofEb/N0 given by ― (2)
Where R= Baseband information bit rate, W Bandwidth,
Equation (2) does not take into account the background thermal noise, in the spread bandwidth.
To take this noise into consideration, Eb/N0 can be represented as ― (3)
The number of users that can access the system is thus given as ― (4)
where W/R is called the processing gain.
The background noise determines the cell radius for a given transmitter power. In order to achieve an increase in capacity, the interference due to other users should be reduced.
This can be done by decreasing the denominator of equations (1) or (2).
The first technique for reducing interference is Antenna sectorization