Cdma

EE-583 Wireless Communications

By: Robert Skowronski

April 19, 2015 1

Introduction

What is Multiple Access?

General Specifications

Classification of Existing

Systems

Classification of CDMA

Systems

Multiple Access

IS-95

CDMA spread spectrum

◦ CDMA Specs

Comparison between

CDMA and TDMA/FDMA

Advantages of CDMA

Disadvantages of CDMA

CDMA Codes

CDMA Operation

TDMA/FDMA/CDMA

The Near-Far Problem

CDMA Carriers

Summary

References

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CDMA stands for Code Division Multiple Access

A channel access method used by various radio

communication technologies

Where several transmitters can send information

simultaneously over one channel

Allows users to share a band of frequencies

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Used as the access method in many mobile phone

standards (cdmaOne, CDMA2000)

Spread spectrum technology

Special coding scheme where each transmitter is

assigned a code

Digital air interface standard, claiming eight to fifteen

times the capacity of analog

Users have control of a very wide channel bandwidth

1.5 to 5Mhz

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1. When multiple users want to

communicate in a common

geographic area

2. Many people can talk on the

phone at the same time

3.Each phone must communicate

with a base station

Rx: 869-894MHz Tx: 824-849MHz

20 Channels spaced 1250kHz apart

(798 users/channel)

QPSK/(Offset) OQPSK modulation scheme

1.2288Mbps bit rate

IS-95 standard

Operates at both 800 and 1900 MHz frequency bands

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Each user is assigned a unique PN code

Each user transmits its information by spreading with

unique code

Direct Sequence spread spectrum is used

Users are separated by code not by time or frequency

slot

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IS-95 was first operating system to use CDMA

◦ Invented by Qualcomm

◦ Began production in 1995

Being supplanted by CDMA2000 (WCDMA) and

UMTS, fully 3G systems

Both had been using CDMA

Wideband System

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A signal takes up to 6-10 times the bandwidth that it

needs at a minimum

This is used for military communications as well

Much more difficult to tune into a certain frequency

Difficult to block, listen in on, and identify from noise

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All users share the same 1.25MHz bandwidth

They all transmit the same size signal

Unique digital codes are used to separate each of the

mobile phones

.02Mhz of a guard band

Each bit of the conversation is encoded with this

special code

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In TDMA/FDMA, cell design needs more frequency

planning while in CDMA frequency planning is

minimal

TDMA is Band Limited system, CDMA is power

limited system

CDMA systems entire spectrum is used which

enhances voice quality. In TDMA band width available

is small which leads to compromise of transmission

quality

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No limit to the number of concurrent users

Low power requirements

Little cell-to-cell coordination needed by operators

Carrier can serve more subscribers

Precision clocks that do not limit the distance a tower

can cover

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Capacity-can accommodate more users per Mhz of

bandwidth than any other technology

Small phones

Extended reach

Cell size is larger but consumes less power and covers

large areas

Impossible for hackers to decipher the code sent

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Well-defined path to higher data rates

Uses Soft Handoff to reduce drop calls

Variable rate voice orders reduce the rate being

transmitted when speaker is not talking, allows channel

to be packed more efficiently

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Near far problem

Not as mature as the GSM network

Cannot offer international roaming

Demands high pea power on uplink in transient mode

The only limit to the system is the computing prowess

of the base station and its ability to separate noise from

actual data

Self-jamming

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CDMA covers a small portion of the world compared

to GSM

Multiple Path Distortion

GSM has more subscribers

GSM is in more countries overall

Most technologies are patented and must be licensed

from Qualcomm

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The base station and mobile phone use an algorithm for

generating pseudo random numbers

Mathematical function called Walsh Vectors

◦ Generates 128 bit random numbers that are

orthogonal to every other random number generated

◦ Large period

Both the mobile station and base station should

generate the same random numbers (32 bits)

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Time stand and error codes are added to the random

number convoluted with the data

Results in 128 bits that represent only 1 bit of data

The original 9600 bps of conversation has been

multiplied to 1.23 Mbps

Code are designed to never interfere with any other

codes to a very high probability

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Spread Spectrum Multiple Access Technology

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Users near the base station are received with high

power

Users far from the base station are received with low

power

For a path loss exponent of 4 and a cell size of 1km

P2/P1=(1000/50)^4= 52db

Nearby users will completely swamp far away users

Solution: Power Control

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Sprint

Verizon

U.S Cellular

These companies use CDMA to authenticate there

phones on home networks. There is no Sim Card used

like GSM where switching phones is easy.

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Graceful degradation as traffic increases

Strong against interference

CDMA allows efficient use of spectrum by putting all

users on top of each other in time and frequency

CDMA is used as the access method in many mobile

phone standards such as cdmaOne, CDMA2000 (the

3G evolution of cdmaOne), and WCDMA (the 3G

standard used by GSM carriers), which are often

referred to as simply CDMA

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CDMA becomes more important in radio

communication systems because it is superior to the

FDMA and TDMA cellular systems

The military can use this because it is based on the

spread spectrum technique

Channel access method used by various radio

communication technologies

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1.http://en.wikipedia.org/wiki/Code_division_multiple_access

2. K.S. Gilhousen et al, “On the capacity of a cellular CDMA

system,'' IEEE Trans. on Vehicular Tech., May 1991.

3.http://www.math.niu.edu/~beachy/courses/523/cdma_lec.pdf

4. http://www.pcmag.com/article2/0,2817,2407896,00.asp

5. http://www.cdg.org/technology/cdma.asp

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