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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.

## Comments

## excellent

excellent

## outstanding explanation

outstanding explanation

## "This code encodes the

"This code encodes the information into a low frequency signal before it is transmitted over a channel"

Please help me understand why the encoded information is encoded into a low frequency.

If I understand you correctly, the original data is actually a relatively low frequency signal. A much higher frequency is the code which samples the original data. thus, in effect, i now have much more information being transmitted. I could then transmit this increased (relative to original data) data over a signal that is of a lower frequency thus taking much longer to convey the original data.

Have I understood what you were explaining? What does that buy me?

## "Under acquisition, the

"Under acquisition, the receiver acquires the sent signal and generates the decoding sequence which it receives from the base station"

In trying to understand the above, I am thinking of the GPS system.

It is my understanding that the receiver is not given the decoding sequence directly but indirectly through the name of the decoding code (PRN). The receiver must independently somehow know the coding associated with the PRN.

CDMA, inherently would not be "secret" if my understanding is correct.

## "When we go the CDMA way,

"When we go the CDMA way, every user’s voice is converted to a unique code which only the intended recipient instruments can understand."

If I understand you, then the unique code(A) applied to the original data generates a new higher frequency(in terms of how often data is changing) sequence of bits such that if OTHER unique codes(B) are used to decode the encoded original data, the outcome will somehow be understood to NOT be the original data and if the unique code(A) is applied to decode the encoded original data, that the outcome will somehow be understood TO BE the original data.

If I am correct, can you amplify on how I know the decoding for B is understood to not be the original data and similarily, the decoding for A is understood to be the original data.

## "The code here is a

"The code here is a “spreading sequence” of digital bits"

I do not believe I am understanding you and I sincerely do want to understand you.

What I understand so far is that the encoded stream of data is a great deal more data than the original data as one is actually sending every sample taken of the original data (at least twice as many(nyquist) and probably a great deal more). I do not see how that is "spreading" the data, other than one could say that every bit of the original data is now composed of many bits of the encoded data.

Another way of looking at "spreading" might be a result of transmiting each of the coding samples of the original data on a different frequency, then in effect I would be "spreading" a single piece of the original data across many frequencies. Is this what you mean?

If this is so, then the "spreading" method (distributing the original data across many frequencies) has not been explained (as far as I understand) up to this point.

If my understanding is valid, then what I can appreciate is that this CDMA process ends up giving me redundant samples for each original data point. In certain situations this might be a great idea. But we also might appreciate that I could give away the redundant samples. In effect, I am thus giving away sample time slots. If those time slots were assigned a frequency, then effect I am giving away the chance for another original data stream to use that time slot and that frequency.

Thus to ensure there is orthogonality (each data stream is not impacted by another data stream) I am guessing the code applied to the original data is a declaration of when a sample is taken and that this user has exclusive access to that sample time slot and its corresponding frequency at that time slot.

Is my thinking above correct or am I way off base. I have no clue. You are the only guy I have read that has come close to conveying CDMA principles understandable by a non mathematician.