G.H. Raisoni College of Engineering, Nagpur (An Autonomous Institute Under UGC Act 1956)

Department of Electronics & Telecommunication Engineering

Session:2014-15

Subject: Mobile Communication

TAE- I

Topic review

CDMA

Submitted By:

1. Piyush Dhakate (29)

Sem/ Section:

8th sem / c

INTRODUCTION

DEFINITION :

A coding scheme, used as a modulation technique, in which multiple channels

are independently coded for transmission over a single wideband channel.

In some communication systems, CDMA is used as an access method that

permits carriers from different stations to use the same transmission equipment by

using a wider bandwidth than the individual carriers. On reception, each carrier can

be distinguished from the others by means of a specific modulation code, thereby

allowing for the reception of signals that were originally overlapping in frequency

and time. Thus, several transmissions can occur simultaneously within the same

bandwidth, with the mutual interference reduced by the degree of orthogonality of

the unique codes used in each transmission .

CDMA permits a more uniform distribution of energy in the emitted

bandwidth Short for Code Division Multiple Access, a digital cellular technology

that uses spread-spectrum techniques. Unlike competing systems, such as GSM that

use TDMA, CDMA does not assign a specific frequency to each user. Instead, every

channel uses the full available spectrum. Individual conversations are encoded with a

pseudo-random digital sequence.

Because Qualcomm Inc. created communications chips for CDMA

technology, it was privy to the classified information. Once the information became

public, Qualcomm claimed patents on the technology and became the first to

commercialize it.

Code Division Multiple Access (CDMA) is the ultimate in inside

communications technology. The fastest growing wireless technology, CDMA

already has tens of millions of users throughout the world. The reasons for this are

simple. CDMA supports current voice and data needs while providing a quick, cost-

efficient evolutionary path to tomorrow’s systems.

CDMA has greater voice quality and call clarity than other technologies

because it filters out background noise, cross-talk and interference. CDMA has fewer

dropped calls because it increases voice and date transmission reliability. CDMA has

greater spectral efficiency because it packs more transmissions into the same space,

resulting in fewer cell sites, which lowers operators' costs. Add to this increased

capacity, enhanced privacy, and improved coverage… why would you use any other

wireless technology?

Working of CDMA

The CDMA uses the spread spectrum technology.The spread spectrum refers

to any system that satisfies the following conditions :

1. The spread spectrum may be viewed as a kind of modulation scheme in

which the modulated(spread spectrum) signal bandwidth is much greater than the

message(baseband) signal bandwidth. Thus, spread spectrum is a wideband scheme.

2.The spectral spreading is performed by a code that is independent of the

message signal. This same ode is also used at the receiver to despread the received

signal in order to recover the message signal (from spread spectrum signal). In secure

communication, this code is known only to the preson(s) for whom the message is

intended.

The spread spectrum increases the bandwidth of the message signal by a

factor N, called the processing gain. If the message signal bandwidth is B Hz and the

corresponding spread spectrum signal bandwidth is Bss Hz, then

Processing gain N = Bss / B

Thus, the key to CDMA is to be able to extract the desired signal while

rejecting everything else as random noise. A somewhat simplified description of

CDMA follows:

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.

Applications of CDMA Technology

Daily applications possible with CDMA

Daily Downloads : ringers

characters

images

horoscopes

Real time stock quotes :

of different stock exchanges

Text Communication : Chat

instant messaging

SMS

e-mail

message board

member search

Sending photos over the air : MMS messages

Position Location Services : navigation assistance

friend finder

Games and Entertainment :

magazine

comic book store

All these services are already being offered in South Korea and Japan.

ADVANTAGES OF CDMA

No SIM card is required.

Improved call quality: CDMA provides better and more consistent sound

quality than systems based on other technologies.

Enhanced privacy when compared to systems using other technologies.

Increased talk time and standby time for mobiles.

They are difficult to intercept for an unauthorized person.

They are easily hidden. For an unauthorized person, it is dufficult to ever

detect their presence in many cases.

They are resistant to jamming.

Capacity increases of 8 to 10 times that of an AMPS Analog system, and 4

to 5 times GSM , because of CDMA’s unique spread spectrum technology.

Many users can share the same carrier frequency, and without time-sharing.

This means that mobile phone service providers can handle more customers on a

CDMA network than on a GSM network.

Improved call quality, with better and more consistent sound , CDMA

systems use precise power control—that is, the base station sends commands to every

mobile phone currently involved in a call, turning down the power on the nearby

ones, and increasing the power of those further away. The result is a nice, even noise

level across the carrier, with lower overall power levels and no spiky interference.

DISADVANTAGE

Collision :

In general, the collisions at the channel is a disadvantage of CDMA system

and can be mitigated by careful selection of the sequence and power control that is

close to perfect.

Roaming :

Since most countries have chosen the GSM standard, “roaming” on CDMA is

limited.

M-commerce :

A CDMA doesn’t have a SIM card, which makes m-commerce difficult.

Conclusion

After these wonderful particulars of CDMA technology I arrive at the conclusion as follows:

Where CDMA scores

Where CDMA needs to scores

CDMA is probably the most interesting multipal access method provided by

spread – spectrum technology.CDMA 2000 and other 3g technology bring

telecommunication into the packet – switched domin, adding a host of new

services and network complexities in the process.

References

1 . Mobile communication- Design fundamentals by William c.y.lee, {john willey}

2. Scribd