CDMA

Shashwat Shriparvdwivedishashwat@gmail.com

InfinitySoft

Code Division Multiple Access(CDMA) is a spread spectrum technique in which we have to use a wide bandwidth for different applications. But at the same time more than one users can use the same frequency is all done by using a code called pseudo random code (PN) . In addition it can provide an extra data security and it can prevent interference and jamming of signals

Multiple Access is a technique where by many subscribers or local stations can share the use of communication channel at the same time or nearly so ,despite the fact that there individual transmissions may originate from widely different locations . Stated in other way a multiple access technique permit the communication resources of the channel to be shared by a large number of users seeking to communicate with each other. There are subtle difference between multiple access and multiplexing

Multiple Access refers to the remote sharing of a communication channel such a satellite or radio channel by users in highly dispersed locations. On the other hand multiplexing refers to sharing of a channel such as a telephone channel by users confined to a local site

FDMA

TDMA

CDMA

In Frequency Division Multiple Access disjoined sub bands of frequency are allocated to the different users on a continuous time basis. In order to reduce interference between users allocated adjacent channel bands, guard bands are used to act as buffer zones. These guard bands are necessary because of the impossibility of achieving ideal filtering for separating the different users

In Time Division Multiple Access each users is allocated the full spectral occupancy of the channel but only for a short duration of time called time slot. Buffer zones in the form of guard times are inserted between the assigned time slots. This is done to reduce interference between users by allowing for time uncertainty that arises due to system in perfections, especially in synchronizations schemes

In CDMA there is no limitations in the use of frequencies that is we have to use any frequencies allowed at any time that we need. And the same time so many users can also use this same frequency. But it is free from interference and jamming and it can provide an extra data security. CDMA is derived from direct sequence spread spectrum.

When CDMA was first proposed, the industry gave it approximately the same reaction that Columbus first got from Queen Isabella when he proposed reaching India by sailing in wrong direction. However, through the persistence of a single company , Qualcomm, CDMA has matured to the point where it is not only acceptable , it is now viewed as the best technical solution around and the basis for the third-generation mobile system. It is also widely used in the U.S

An airport lounge with many pairs of people conversing. TDM is comparable to all the people being in the middle of the room but talking turns speaking. FDM is comparable to the people being in widely separated clumps, each clump holding its own conversation at the same time as, but still independent of the others. CDMA is comparable to everybody being in the middle of the room talking at once , but with each pair in a different language. The French-speaking couple just hones in on the French, rejecting everything that is not French as noise. Thus, the key to CDMA is to be able to extract the desired signal while rejecting everything else as random noise

In CDMA, each bit time is subdivided into m short intervals called chips. Typically, there are 64 or 128 chips per bit, but in the example given below we will use 8 chips/bit for simplicity.

Each station is assigned a unique m-bit code called a chip sequence. To transmit a 1 bit, a station sends its chip sequence. To transmit a 0 bit, it sends the one’s complement of its chip sequence. No other patterns are permitted.

Consider there are 4 stations A,B,C and D. The chip sequences are given below

A: 0 0 0 1 1 0 1 1 B: 0 0 1 0 1 1 1 0 C: 0 1 0 1 1 1 0 0 D: 0 1 0 0 0 0 1 0

it is more convenient to use a bipolar notation, with binary 0 being -1 and binary 1 being +1 . So the chip sequence will like this

A: (-1 -1 -1 +1 +1 -1 +1 +1) B: (-1 -1 +1 -1 +1 +1 +1 -1) C: (-1 +1 -1 +1 +1 +1 -1 -1) D: (-1 +1 -1 -1 -1 -1 +1 -1)

Each station has its own unique chip sequence. Let us use the symbol S to indicate the m-chip vector for station S, and S for its negation

All chip sequences are pairwise orthogonal , by which we mean that the normalized inner product of any two distinct chip sequences, S and T (written as S T ) is 0 .

And we know that S S = 1 S S = -1

During each bit time , a station can transmit a 1 by sending its chip sequence , it can transmit a 0 by sending the negative of its chip sequence or it can be silent and transmit nothing

For the moment we assume that all stations are synchronized in time, so all chip sequence begin at the same instant.

When two or more stations transmit simultaneously their bipolar signals add linearly. For example if one chip period three stations output +1 and one station output -1 , the result is +2 . One can think of this as adding voltages: three stations outputting +1 volts and one station outputting -1 volts gives 2 volts

To recover the bit stream of an individual station, the receiver must know that station’s chip sequence in advance. It does the recovery by computing the normalized inner product of the received chip sequence (the linear sum of all the stations that transmitted ) and the chip sequence of the station whose bit stream it is trying to recover. If the received chip sequence is S and the receiver is trying to listen to a station whose chip sequence is C , it just computes the normalized inner product , S C

Capacity increases of 8 to 10 times that of an AMPS analog system and 4 to 5 times of an GSM system

Improved call quality, with better and more consistent sound as compared to AMPS systems

Simplified system planning through the use of the same frequency in every sector of every cell

Enhanced privacyImproved coverage characteristics Increased talk time for portablesBandwidth on demand

In an ideal, noiseless CDMA systems the capacity(i.e. , number of stations) can be made arbitrary large, in practice , physical limitations reduce the capacity considerably. First we have assumed that all chips are synchronized in time. In reality such synchronization is not possible

An implicit assumption in our discussion is that the power levels of all stations are the same as perceived by the receiver . CDMA is typically used for wireless systems with a fixed base stations at varying distance from it . The power levels received at the base station depend on how far away the transmitter are

We have also assumed that the receiver knows who the sender is . In principle, given enough computing capacity, the receiver can listen to all the senders at once by running the decoding algorithm for each of them in parallel. In real life suffice it to say that this is easier said than done

The main application of CDMA is in the field of mobile phone communications. Due to the increased needs and limitations of frequencies. Due to the high availability of bandwidth CDMA can use for application like internet accessing in mobile phones. So many protocols have been developing in CDMA the currently used CDMA is called CDMA ONE. We have to expect so many improvements in communication by the use of CDMA

Shashwat Shriparvdwivedishashwat@gmail.com

InfinitySoft