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.
