[[Image:cdma12.jpg|thumb|600 px|centre|Comparison of cellular access schemes]]
==Code Division Multiple Access==
The CDMA technology can be implemented in two ways
* Direct Sequence Spread Sprectrum - DSSS CDMA
* Frequency Hopping - FH CDMA
'''=== Direct Sequence Spread Sprectrum - DSSS CDMA:''' === In this method , the direct sequence(input data) which is spread over a limited bandwidth is multiplied with a code or spreading sequence (a pseudorandom sequence) which will spread the input data over the entire bandwidth of the communication channel. The power density is also reduced and is spread over the frequency spectrum and hence is known as spread spectrum method. The modulation part of DSSS is as shown below<br>.[[Image:cdma13.jpg|thumb|600px|center|CDMA Modulation]]The modulated signal is transmitted over the channel and all users can receive it but only the user which knows the correct code can only decode the message. This is depicted in the figure below.<br>[[Image:cdma14.jpg|thumb|600px|center|CDMA Demodulation]]===Features of CDMA=======Frequency Reuse==== Number of radio channel frequencies are limited. The concept of frequency reuse is based on assigning to each cell a group of radio channels used within a small geographic area. Cells are assigned a group of channels that is completely different from neighboring cells. The coverage area of cells is called the footprint. This footprint is limited by a boundary so that the same group of channels can be used in different cells that are far enough away from each other so that their frequencies do not interfere.
===Frequency Reuse =Soft Handoff==== Handoff means Switching a cellular phone transmission from one The number of radio channel frequencies is limited. The concept of frequency reuse is based on assigning to each cell to another as a mobile user moves into group of radio channels used within a new cellular small geographic area. It is so called because the radio link with the previous sector(s) is not broken before Cells are assigned a link group of channels that is established with a new sector; this type completely different from neighboring cells. The coverage area of handoff cells is described as "make before break"called the footprint. In CDMA due to this soft handoff there This footprint is no interruption of call even at limited by a boundary so that the border of cell site which means more number same group of customers channels can be accommodated automatically increasing the capacity of the cell siteused in different cells that are far enough away from each other so that their frequencies do not interfere.
=== Soft Handoff === Handoff means switching a cellular phone transmission from one cell to another as a mobile user moves into a new cellular area. It is so called because the radio link with the previous sector(s) is not broken before a link is established with a new sector; this type of handoff is described as "make before break". In CDMA, due to this soft handoff, there is no interruption of call even at the border of a cell site which means more number of customers can be accommodated, automatically increasing the capacity of the cell site. ===Multipath Fading==== In a mobile environment , a mobile station will receive one direct signal from the base station and multiple signals which are reflected from obstructions like building buildings and towers. Each signal would have travelled a different length and would be displaced in time. Due to this , when they are combined at the mobile handset, it will cause interference resulting in poor signal quality. This is called known as ''fading''. This problem is handled in a very good way in CDMA. Here , the phase of the multiple signals is modified such the that only positive interference(addition) takes place and the overall signal strength increases. A receiver that implements the above principle is known as a RAKE receiver as shown in the figure below<br>.[[Image:cdma15.jpg|thumb|center|600px|RAKE receiver]]
====Near Far Problem==== The problem is described by taking an example consider a receiver and two transmitters (one close to the receiver; the other far away). If both transmitters transmit simultaneously and at equal powers, then due to the inverse square law the receiver will receive more power from the nearer transmitter. This makes the farther transmitter voice more difficult to understand. Since one transmission's signal is the other's noise the signal-to-noise ratio (SNR) for the farther transmitter is much lower. If the nearer transmitter transmits a signal that is orders of magnitude higher than the farther transmitter then the SNR for the farther transmitter may be below detectability and the farther transmitter may just as well not transmit. This effectively jams the communication channel. In CDMA systems this is commonly solved by dynamic output power adjustment of the transmitters. That is the closer transmitters use less power so that the SNR for all transmitters at the receiver is roughly the same. This sometimes can have a noticeable impact on battery life, which can be dramatically different depending on distance from the base station.<br>
[[Image:cdma16.jpg]]
====Power Control:==== As the propagation losses between BS and MS's are different according to individual communication distances, the received levels at the base station are different from each other when all mobile stations transmit their signals at the same power. Moreover, the received level fluctuates quickly due to fading. In order to maintain the strength of received signal level at BS, power control technique must be employed in CDMA systems.<br>.
[[Image:cdma17.jpg]]<br>
Power control can be implemented in two ways : open loop power control and closed loop power control<br>
'''Effect of Power Control''' Power control is capable of compensating the fading fluctuation. Received power from all MS are controlled to be equal.　Near-Far problem is mitigated by the power control.
[[Image:cdma19.jpg]]
==Quality of Service(QoS)==
CDMA is being accepted as a third generation system and a specific feature of 3G system is that it offers a radio interface adapted for all kinds of services and combination of services(data,voice,image etc). The big issue is of multiplexing of these services which have do not have the same demands in terms of quality of service(QoS) which can be represented as BER(bit error rate),processing delay,frame error rate etc. Different QoS will require different channel encoding and interleaving strategies. The demand of BER can be satisfied when the coding bits have atleast a code dependent ratio Eb/I(ratio of bit energy to interference). There are several influences that might change system performance(BER) and hence Eb/I ratio, of which the most effective is variation of Bit Rate by a step of '''Rate Matching'''. <br><br>