Gsm and Cdma – What Is Cdma In Mobile Computing

Gsm and Cdma – What Is Cdma In Mobile Computing

Gsm and Cdma – What Is Cdma In Mobile Computing – CDMA refers to Code Division Multiple Access. CDMA is cellular technology. In CDMA there are two main systems

• Base Station (BS)
• mobile subscribers or users.

In CDMA Unique codes are assigned to the users for communication with BS or base station.

CDMA allows multiple transmitters to send data over a single channel simultaneously.

This makes it possible to share the same bandwidth by multiple users at the same time.

 

How CDMA works

Codes with specific characteristics can be applied to the transmission to enable the use of code division multiplexing (CDM).

Code division multiple access (CDMA) systems use exactly these codes to separate different users in code space and to enable access to a shared medium without interference.

In CDMA entire bandwidth is being used by multiple users and each user has their unique codes (e.g. PN codes) to recover the data. The system works based on the spread spectrum concept.

The main problem is how to find “good” codes and how to separate the signal from noise generated by other signals and the environment.

CDMA can be used in combination with FDMA/TDMA access schemes to increase the capacity of a cell.

 

 

In contrast to other schemes, CDMA has the advantage of a soft handover and soft capacity. Soft handover means that a mobile station can smoothly switch cells. This is achieved by communicating with two base stations at the same time.

CDMA does this using the same code and the receiver even benefits from both signals. TDMA/FDMA systems perform a hard handover, i.e., they switch base station and hopping sequences (time/frequency) precisely at the moment of handover.

 

Advantages of CDMA

1. In CDMA, specific Codes are applied to the transmission. It is robust against noisy and fading signals.
2. CDMA is a secure system because for hackers it is difficult to decipher the CDMA code used over traffic channels.
3. CDMA allows the whole bandwidth simultaneously and therefore, there is no limit on the number of subscribers per cell, unlike TDMA & FDMA access schemes. All this depends on the number of codes supported by the CDMA base station.
4. In the CDMA system, it is easy to add users.
5. During the handover process between the cells, due to soft handoff in which first connection is established with a new cell then data is transferred, it reduces the chances of disconnection or calls drop.
6. CDMA networks can interoperate with other cellular networks like GSM/LTE, etc., therefore, nationwide roaming is possible without any problem.

 Disadvantages of CDMA

Following are the disadvantages of CDMA

1. In CDMA technology, orthogonal codes are used by mobile subscribers. Orthogonality between the codes must be maintained to recover the data in the future. Mobile subscribers who are very far from the Base Station will acquire more attenuation of data. Therefore, will lose the orthogonality more, hence it will be very difficult to recover the data.
2. CDMA uses a soft handoff scheme. In this type of handoff scheme, mobile needs to establish a connection with the new target cell before disconnecting from the serving cell. This process is more complex as compared to the hard handoff scheme.
3. An increase in the number of mobile users will decrease the Quality of service.
4. Precision code synchronization is needed to recover the original baseband signal.
5. Self Jamming is common in the CDMA system due to loss of orthogonality of PN codes or spreading sequences of different subscribers.
6. Near-far-problem is common in the CDMA system. It needs close control of transmit powers of CDMA handsets. It helps the farthest CDMA mobile Phone to transmit at higher power as compared to CDMA mobile phone which is closer in order to manage a good SNR at the Base station.