Code Division Multiple Access

Introduction:

Code Division Multiple Access(CDMA) is a communication channel used to send the signal through various communication technologies in CDMA serval users can share the bandwidth and also this CDMA acts as interface several users. Every channel has a specific code in this.CDMA is widely used in cellular-communications for further better use of the technology. In CDMA data can be sent in small pieces over the frequencies at a specific range.

Different CDMA users use different codes to modulate the signal using codes to generate the signal is important in this CDMA this method is used in simple transceiver systems. In this CDMA lower frequencies are multiplied with higher frequencies to get the desired frequency and transmit the carrier through the sine-wave transmitter.

This CDMA sometimes generate mathematical signal between the data strings and vectors. In CDMA signals are received in encoding sequence. In CDMA controlling the signal strength with the transmitter is very important.CDMA maintains the various privacy levels because signals are passed to multipath and spread using pseudo-random code.

Due to these pseudo-random code properties, signals may appear in random form or noise-like properties. The signals which are used to demodulate these signals using encoding are impossible without the knowledge of pseudo-random code.

CDMA also uses jamming. This jamming signal has a certain amount of resistance power to the jamming signal.CDMA also rejects narrow-band frequencies. In CDMA same frequencies can be used for different cells.CDMA is flexible to resource allocation

CDMA is the best example of a mobile network where a large number of data is produced and a small amount of traffic is generated in an irregular interval of time. In this the CDMA allows several users to access simultaneously over a single communication channel. To receive the data synchronization should be done to access the multiple users with the same bandwidth.

The devices sent on the same frequency at the same time can use the whole bandwidth of the transmission channel. Each sender has the unique random number

Advantages:

*All the devices can use the same frequency without planning

*It has huge space compared to frequency space

*Interferences are not coded

*Frequency reuse

*High privacy

*Good voice coding

Perfect power control

Disadvantages:

*High complexity while receiving the signal

*All signals should have the same strength at the receiver.

In CDMA all signals are just not based on a single path because signals are refracted and may be received through different paths. These transmitted signals take different amplitude, phases and may vary in timing. This transmitted data can be disappeared and can be retrieved whenever need it is possible only if the user knows the pseudo-code.

CDMA operates in a multi-path environment. It uses Thermal noise and interferences are also included in CDMA simulation signals.

These simulation models help in increase the performances this also includes error signals in it with the noise level. This same frequency is used for adjacent cells.CDMA allows several users to access simultaneously over the communication channel. To receive the data synchronization should be done in order to receive the data from the same bandwidth.

Each sender has the unique code CDMA delivers the data in a faster way

The data send through different devices sent on the same frequency same time can use the whole bandwidth of the transmission channel to use. In CDMA data can be sent in small-pieces over the frequencies of a specific range.

Different CDMA users use the different code to modulate the signals using of codes to generate the signal is important in this CDMA this method is used in simple transceiver systems.CDMA is widely used in cellular communication systems for further better use of technology.

In this lower frequencies are multiplied with higher frequencies to get the desired frequencies. Signals are received in encoding sequence maintaining the signal strength is very important in this CDMA sometimes

it also generates the signals in mathematical signal between the data strings and vectors, it also acts as an interface to several users.

It is flexible to resource allocation and also it rejects the narrow bandwidth frequencies.

CDMA has perfect power control technology and this power is very important to solve the near and far problems. The best way to control the rower problem is to balance the same power received by all the mobiles to the base stations.

All the power of the received signals from the base station should be at the least level to meet the maximum requirements of the mobile devices at the base stations.

The main advantage of the CDMA is it reuses the resources allocated at the same frequency in every cell. In CDMA due to complicated radio frequencies and irregular cell allocation, it is not easy to allocate the time-slot for the same frequency.

Cellular communication passes from one base station to another base station due to this network also switches from one base station to another base station automatically this kind of behavior is called the “handover technique”.It uses different frequencies to communicate with the base stations and it also changes the frequencies from one base station to another base station during switching.

CDMA consists of two channels

*Access channels and

*Reverse traffic channels

=>Access Channel: Access channels are used by mobile stations to communicate with base stations these accessing channel are used for short signal messages

=>Reverse traffic channel: Reverse traffic channels are individual users to transmit signal from mobile station to one or more base stations.

Overview:

CDMA is very different from time and frequency division multiplexing the user can access the entire bandwidth from entire duration from the group of codes it is possible to select a code from the many other codes.

CDMA allows 61 concurrent users CDMA is a spread spectrum technique

by using code sequence.

In CDMA base station covers a small geographical area called a cell and these cells use the same frequencies in cellular communication devices the frequencies.CDMA uses a wide range of communication to the user for the further betterment of the technology.

The Code division multiple access is different from the time division multiple access and the frequency division multiple access. Let us discuss the frequency division multiple access and the time division multiple access and how it is different from the code division multiple access, in the frequency division multiple access concerns that every user can be differenced with the particularly given range. Every user gets allocated a specific frequency band and provides a gap.

These gaps we call it as the guard bands, we use guard bands because to avoid the interference between the uses of the frequency band, we can be sure that in frequency division multiple access, we can differentiate the user by different frequency band.

When we come to the time division multiple access is mainly concerned with allocating different frequency bands, these different frequency bands are allocated to the different range of spectrums. All these are allocated to different users. This differentiation can be done in a specific time period, we allocate the entire range of frequencies for the particular time period. We differentiate the users.

The users can communicate with each other according to the time slots. We do not have any restrictions for any type of the band of frequencies that we use., in the time division multiple access, the entire frequency band is get allocated for that particular period of time. The Users can have their own type of chances in their allocated communication slots in order to communicate with the messages.

We can pass these TDMA formats to the next user, once the user can be communicated, it gets passed to the next type of user, the user can be able to communicate the messages. But why we use the CDMA when we have the TDMA and the FDMA techniques. We have some of the drawbacks in the TDMA and FDMA, the user is limited to the particular frequency band, they can send the signals that suit the frequency band.

When a particular user wants a higher bandwidth and if he wants to communicate with the very larger bandwidth of the signals, these cannot be done using the FDMA. These get overcome by the TDMA technique because it doesn’t have restrictions on the frequency band, but in the TDMA, the entire band is get allocated for that particular period.

These transmissions can not be used for continuous transmission applications if all the users are to communicate continuously for the larger bandwidth. To overcome these types of problems we use a particular type of technology, it doesn’t have any type of restrictions on the frequency band. The user should be separated only then they can access the spectrum simultaneously.

This type of multiple access is called the code division multiple access, we assign a separate unique code to each that is orthogonal for the codes used by the other users. This unique code is different by the other users, the network is get connected to that particular base station. And these base station is separated by the special type pf the code.