Primary requirements for any wireless mode of communication include high quality of service and data secrecy. Realizing that these two factors are to be fulfilled in the most optimized ways without the costs going too high, **CDMA**, a spread spectrum based technology came into existence.

*Fig. 1: A Representational Image of CDMA Technology*

Initially restricted to the armed forces, this technology was commercially launched in 1995 by Qualcomm Telecommunications and now, as per Q4 of the year 2011, CDMA has over 8 billion voice and data customers in the 122 countries that it operates.

**What CDMA Means?**

**Code:**It refers to the string of binary sequence that the transmitter and the receiver share. This code encodes the information into a low frequency signal before it is transmitted over a channel. This same code is used by the receiver to decode the information. The receiver attains the code with the help of the nearest base station.

**Division:**In CDMA a single channel is divided into numerous slots which can be used by multiple users. This is possible because of the use of unique code.

**Multiple Accesses:**Due to code based communication, multiple users can communicate and access the same channel simultaneously without any undesirable interference and loses.

**Why we need CDMA?**

**History of CDMA**

## Understanding CDMA Terminology

**Understanding CDMA Terminology:**

**1. Cross correlation:**It can be defined as the measure of alikeness that two signals share. When signals are cross correlated with each other, they suffer interference and efficiency losses. It limits multiple signal transmission and hence should be avoided.

**2. Bandwidth:**Spectral width over which the frequencies of a system are defined. In usual signals, the bandwidth needed is less but the power required is quite high. However, in CDMA, power requirement is low but bandwidth required is high. Since we use different coding schemes for all transmissions, multiple users can transmit over a single frequency and bandwidth used in excess is not wasted.

**3. Chip:**It’s a single digital pulse of “spreading sequence” that is superimposed on the information signal to create the low frequency signal.

**4. Spread Spectrum:**Mode of signal transmission where it takes up more bandwidth but reduces itself in terms of power required for transmission. In usual signals, the bandwidth acquired is less but the power is quite high. However, in CDMA, power is low but excess of bandwidth is required.

*Fig. 2:*

*A Figure Representing Bandwidth Before and After Application of Spread Spectrum Technique*

**5. Auto correlation: I**t is that phenomena under which the signal gets interfered with its past and future values which might exist due to unpredictable time delays. It can also be termed as a signal’s cross correlation with itself.

**6. Spreading Sequences:**A spreading sequence is a non-ambiguous identification for a transmitter – receiver pair. Spreading Codes are noise-like and random signals generated at the transmitter. The same signal must be generated at the receiver in synchronization. A spreading sequence consists of C units called chips. The chips are two–valued. The autocorrelation function of a spreading sequence reflects the similarity of these sequences with a replica of itself delayed by a time gap. For a given time gap to be zero the autocorrelation value is one. In any other case than this, the autocorrelation value should be small to minimize the interference among copies of the original signal that are generated and delayed by multi-path propagation. The cross–correlation value of two different spreading sequences represents the interference level for two signals from different wireless terminals with delay ?

_{gap}. This value should be as small as possible for all ?

_{gap}such that a maximum number of subscribers are allowed in the cell.

**. Balance property:**

**. Run property:**

^{n}of all run lengths should be of length n for all finite n.

**. Autocorrelation property:**

**Orthogonal Sequence**

_{gap}= 0. If all transmitters are synchronized and no multi–path is considered, the multiple access interference can be neglected. For any other value of ?

_{gap}they have large cross–correlation values. Therefore orthogonal sequences are only applied if perfect synchronism can be guaranteed within the system. The autocorrelation properties of orthogonal sequences are also poor if ?

_{gap}= 0. This happens if we consider multi–path interference. In such a situation equalization is applied to recover the original signal.

**Pseudorandom Sequence**

^{r}– 1. If P = 2

^{r}– 1, sequence is called Maximal Length sequence or m-sequences. Maximal Length sequences, Gold sequences, and Kasami sequences are few of the popular PN sequences.

**Randomness properties of PN sequence are**

**.**

*Balance property**– Of the 2*

^{r}– 1 terms , 2

^{r-1}are “1” and 2

^{r-1}–1 are “0”. Thus the unbalance is 1/P.

**.**

*Run length property**–*Relative frequency of run length “n” (number of “0”s or “1”s) is 1/ 2

^{n}for n £ r-1 and 1/(2

^{r}– 1) for n = r. One run length each of r-1 zeros and r ones occurs. There are no run lengths for n > r

**.**– The number of disagreements exceeds the number of agreements by unity. Thus again the discrepancy is 1/p

*Autocorrelation property*## Working

**Working**

*Fig. 3: A Diagram Demonstrating Work Process of Code Division Multiple Access*

*Fig. 4: A Figure Detailing About CDMA*

**1. Direct Sequence Spread Spectrum-CDMA**

*Fig. 5: A Diagram Illustrating Direct Sequence Spread Spectrum in CDMA*

*Fig. 6: A Figure Representing Generation of DSSS signals*

**2. Frequency Hopping Spread Spectrum-CDMA**

*Fig. 7: A Diagram Representing Advantages of FHSS System Over DSSS System*

**3. Time Hopping Spread Spectrum-CDMA**

_{frame}. As shown in figure, each frame is divided again into N slots with time duration ?

_{slot}. A single user uses one slot out of k possible slots within one frame and sends data with k times higher data rate in contrast to the situation where the data is transmitted within the whole frame.

*Fig. 8: A Representational Image of Time Hopping Spread Spectrum System*

**4. Hybrid Spread Spectrum-CDMA**

## Advantages

**Advantages of CDMA**

**1. Less multi-path interference**

*Fig. 9: A Figure Illustrating the Basic Principle of RAKE Receiver*

**2. Power Control Mechanism**

**3. Soft Handoff**

*Fig. 10: A Figure Illustrating Process of Combining Information From Multiple Transmitted Packets *

**4. Miscellaneous**

## Applications

**Applications of CDMA**

## Limitations

**Limitations of CDMA**

**3.**Usage Restrictions: Qualcomm Telecommunications holds the major patents of CDMA and hence, its usage is limited.

**4.**Less Popular than GSM: CDMA is still not well spread all over the world as compared to GSM and though more efficient, it will take some time to reach all corners of the world.

**5.**Practical Limitations: In the case of excessive number of CDMA users, there can be problems in giving each user a unique spreading sequence. This may limit the performance as well as the number of users.

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