CDMA CONCEPTS-3

CDMA Concept

In CDMA timing plays an important role. All the base stations are synchronised with satellite time. The base stations will start their transmission with reference to the satellite clock. All the base stations will not transmit at the same instant. With reference to the satellite clock, each base station will offset it’s timing by a certain offset. All base stations will have different offsets.

So even if transmission is occurring in a neighbouring cell using the same PN code, then though this come as an interference to the user in the existing cell but it will not effect the receiver in decoding the right information. This is because, information is decoded by correlating the PN codes, since the neighbour base station is offset in time, it will not correlate with the code used in this cell.

CDMA Capacity

The factors deciding capacity are

* Processing Gain
* Signal to Noise Ratio
* Voice Activity Factor
* Frequency Reuse Efficiency

Capacity in CDMA is soft, CDMA has all users on each frequency and users are separated by code. This means CDMA operates in the presence of noise and interference. On top of this the neighbour cells use the same frequencies, which means no re-use. So CDMA capacity calculations should be very simple. No of code channels in a cell, multiplied by no of cells. But it is not that simple. Though the no of code channels available are 64, it may not be possible to use all at one time, since CDMA the frequency is same. Soft capacity means is that all code channels can be sued at one time, but at the cost of quality.

Processing Gain:

P (gain) = 10log (W/R)
W is Spread Rate
R is Data Rate
For CDMA P (gain) = 10log (1228800/9600)
= 21dB

CDMA is spread spectrum technique. Each data bit is spread by a code sequence. This means energy per bit is also increased. This means, that we get some gain out of this. This gain is a factor of spreading rate and actual data rate.

On average, a typical transmission condition requires a signal to noise ratio of 7 dB for adequate voice quality.

Translated into a ratio, signal must be 5 times stronger than noise.

Actual processing gain = P (gain) - SNR

= 21 – 7 = 14dB

CDMA uses variable rate coder

The Voice Activity Factor of 0.4 is considered = -4dB.

CDMA has 100% frequency reuse. Use of same frequency in surrounding cells causes some additional interference.

In CDMA frequency reuse efficiency is 0.67 (70% eff.) = -1.73dB