An overview of CDMACode - Division Multiple Access

Presented By

Saimunur RahmanMatric No: C093003

Dept. of Computer Science & Engineering

International Islamic University Chittagong

Presented to

Abdullahil KafiAssistant Professor

Dept. of Computer Science & Engineering

International Islamic University Chittagong

And all of my classmates …

History of CDMA• In the USSR, the first work devoted to this subject was

published in 1935 by professor D.V. Ageev.

• CDMA was used in 1957, by the young military radio engineer Leonid Kupriyanovich in Moscow , he made a wearable automatic mobile phone called LK-1, with a base station.

• In 1958, Kupriyanovich made the new experimental "pocket" model of mobile phone.

History of CDMA• 1958, the USSR started the development of the "Altay"

national civil mobile phone service for cars

• In 1963 this service started in Moscow and in 1970 Altay service was used in 30 USSR cities.

What is CDMA?

• CDMA stands for Code division multiple access (CDMA)

• Digital technology for delivering mobile telephone services

• Uses spread-spectrum techniques

• One channel carries all transmissions simultaneously

• Communications with different codes

Idea of CDMALet us assume we have four stations 1, 2, 3 & 4.They are connected to the same channel.

The data from station 1 are d1.

The data from station 2 are d2.

The data from station 3 are d3.

The data from station 4 are d4.

Code assigned to the first station is C1

Code assigned to the first station is C2

Code assigned to the first station is C3

Code assigned to the first station is C4

Idea of CDMA

Assume the code for channels

Idea of CDMA

We assume that the assigned codes have two properties

• If we multiply each code by another, we get 0.• If we multiply each code by itself, we get the number of

stations.

Always we must keep these two properties in mind

Idea of CDMA•Station 1 multiplies its data by its code to get d1.c1.

•Station 2 multiplies its data by its code to get d2.c2.

•Station 3 multiplies its data by its code to get d3.c3.

•Station 4 multiplies its data by its code to get d4.c4.

The data that go on the channel are the sum of all these terms, as shown in the box

d1 . c1 + d2 . c2 + d3 . c3 + d4 . c4

Idea of CDMAIf station 1 and 2 are talking to each other. Station 2

wants to hear what station 1 is saying. It multiplies the data on the channel by c1, the code of station 1.

Data = ( d1 . c1 + d2 . c2 + d3 . c3 + d4 . c4 ) . c1

= d1 . c1 . c1 + d2 . c2 . c1 + d3 . c3 . c1 + d4 . c4 . c1

= 4 ₓ d1

= ( 4 ₓ d1 ) / 4

= d1

d1 Was the data's of channel 1.

]0.&0.,0.,4.[ 14131211 cccccccc

Chips•CDMA is based on coding theory.•Each station is assigned a code, which is a

sequence of numbers called chips.

Example of chip sequences

Chips•We can’t select chip sequences randomly, they

were very carefully selected.

•They are called ‘Orthogonal Sequences’ and they have some properties

Lets discuss those properties that must be satisfied by chip sequences …

Chips sequence properties Each sequence is made of N elements, where N is

the number of stations. If we multiply a sequence by a number, every

element in the sequence is multiplied by that element.

2 ∙ [+1 +1 -1 -1] = [+2 +2 -2 -2] If we multiply two equal sequences, element by

element and add the results, we get N. Where, N is the number of elements in each sequence.[+1 +1 -1 -1] ∙ [+1 +1 -1 -1] = 1 + 1 + 1 + 1 = 4

Chips sequence properties If we multiply two different sequences, element

by element and add the results, we get 0.[+1 +1 -1 -1] ∙ [+1 +1 +1 +1] = 1+1-1-1 = 0

Adding two sequences means adding the corresponding elements.[+1 +1 -1 -1] + [+1 +1 +1 +1] = [+2 +2 0 0]

Chips Sequence GenerationFor generating chip sequences we will use Walsh Table.

Walsh Table is a two- dimensional table with an equal number of rows and columns.

Each row is a sequence of chips.

Let’s see a figure for better understanding … …

Chips Sequence Generation

Fig: General rule and examples of creating Walsh tables

Chips Sequence GenerationThe table for N sequences WN

The table for 2N sequences W2N

Where N = 1,2,3 … …

Data representation in CDMAIf the station needs to send bits then

For 0 bit it encodes it as -1.For 1 bit it encodes it as 1.For no signal it interpreted as 0.

Fig : Data representation in CDMA

Encoding (By using example)

•Lets assume,▫Channel 1 sending 0 bit.▫Channel 2 sending 0 bit.▫Channel 3 is silent.▫Channel 4 sending 1 bit

•The data’s at the sender side are translated into -1, -1, 0, +1.

•Each station multiplies the corresponding number by its chip, which is unique for each station.

Encoding (By using example)

• New results are sent to the channel• The sequence of the channel is the sum of all four

sequences as defined before.

[-1 -1 -1 -1] + [-1 +1 -1 +1] + [0 0 0 0] + [+1 -1 -1 +1] =[-1 -1 -3 +1]

Channel NoCn

Datadi

ChipCi

Di * Ci

1 -1 [+1 +1 +1 +1] [-1 -1 -1 -1]

2 -1 [+1 -1 +1 -1] [-1 +1 -1 +1]

3 0 [+1 +1 -1 -1] [0 0 0 0]

4 +1 [+1 -1 -1 +1] [+1 -1 -1 +1]

Decoding (As per previous example)

• Suppose, station 3 (which we said is silent) wants to listen station 2.

• Now, station 3 multiplies the total data on the channel by the code for station 2, which is [+1 -1 +1 -1].

• Then we get,[-1 -1 -3 +1] ∙ [+1 -1 +1 -1] = -1+1-3-1

= -4/4 = -1

-1 was encoded for station 2. -1 means 0. so, we got the value of station 2.

Let’s see the process in a figure

Fig : Sharing channel in CDMA

Encoding (Analysis by using Signal lebel)

It will be very clear if I use Signal level

Lets see …

Encoding (Analysis by using Signal label)

See the figure very carefully

Fig: Encoding the digital signal created by four stations in CDMA

Decoding (Analysis by using Signal label)

See the figure carefully

Fig: Decoding of the composite signal for one in CDMA

Benefits of CDMA

Increased CapacityImproved QualityImproved QualityEnhanced PrivacyImproved CoverageIncreased Portable Talk Time Bandwidth on Demand

CDMA-ApplicationsCDMA for Cellular

CDMA-Short Message Service

Over-the-Air Activation

CDMA Data and Fax

Subscriber Access Control

CDMA for Personal Communications Services

ConclusionTechnology of choice for 3G generation

because of it’sGreater total capacityOutstanding voice quality Fewer dropped calls

It’s use in satellite communication is of great importance

Thank You