Advantage of CDMA Network

1      Larger Capacity :

let us discuss this issue with the help of Shannon’s Theorem. It states that the channel capacity is related to product of available band width and S/N ratio.

C         = W log 2 (1+S/N)

Where C         = channel capacity

            W        = Band width available

            S/N      = Signal to noise ratio.

It is clear that even if we improve S/N to a great extent the advantage that we are expected to get in terms of channel capacity will not be proportionally increased. But instead if we increase the bandwidth (W), we can achieve more channel capacity even at a lower S/N. That forms the basis of CDMA approach, wherein increased channel capacity is obtained by increasing both W & S/N. The S/N can be increased by devising proper power control methods.

2              Less (Optimum) Power per cell:

Power Control Methods: As we have already seen that in CDMA the entire bandwidth of 1.25Mhz is used by all the subscribers served in that area. Hence they all will be transmitting on the same frequency using the entire bandwidth but separated by different codes. At the receiving end the noise contributed by all the subscribers is added up. To minimize the level of interfering signals in CDMA, very powerful power control methods have been devised and are listed below:

1. Reserve link open loop power control

2. Reserve link closed loop power control

3. Forward link power control

The objective of open loop power control in the reverse link (Mobile to Base) is that the mobile station should adjust its transmit power according to the changes in its received power from the base. Open loop power control attempts to ensure that the received signal strength at the base station from different mobile stations, irrespective of their distances from the base site, should be same.

In Closed loop power control in reverse link, the base satation provides rapid corrections to the mobile stations’ open loop estimates to maintain optimum transmit power by the mobile stations. The base station measures the received signal strength  from the mobile connected to it and compares it with a threshold value and a decision is taken by the base every 1.25 ms to either increase or decrease the power of the mobile.

In forward link power control (Base to Mobile) the cell (base) adjusts its power in the forward link for each subscriber, in response to measurements provided by the mobile station so as to provide more power to the  mobile who is relatively far away from the base or is in a location experiencing more difficult environment.

These power control methods attempt to have an environment which permits high quality communication (good S/N) and at the same time the interference to other mobile stations sharing the same CDMA channel is minimum. Thus more numbers of mobile station are able to use the system without degradation in the performance. Apart from the capacity advantage thus gained power control extends the life of the battery used in portables and minimizes the concern of ill effects of RF radiation on the human body.

3              Seamless Hand-off :

CDMA provides soft hand-off feature for the mobile crossing from one cell to another cell by combining the signals from both the cells in the transition areas. This improves the performance of the network at the boundaries of the cells, virtually eliminating the dropped calls.

4              No Frequency Planning :

A CDMA system requires no frequency planning as the adjacent cells use the same common frequency. A typical cellular system (with a repetition rate of 7) and a CDMA system  is shown in the following figures which clearly indicates that in a CDMA network no frequency planning is required.

5         High Tolerance to Interference :

The primary advantage of spread spectrum is its ability to tolerate a fair amount of interfering signals as compared to other conventional systems. This factor provides a considerable advantage from a system point of view.

6              Multiple Diversity :

Diversity techniques are often employed to counter the effect of fading. The greater the number of diversity techniques employed, the better the performance of the system in a difficult propagation environment.

CDMA has a vastly improved performance as it employs all the three diversity techniques in the form of the following:

A .Frequency Diversity:          A wide band RF signal of 1.25 Mhz being used.

B. Space Diversity:     Employed by way of multipath rake receiver.