CDMA is a digital cellular technology that allows multiple users to access a shared radio frequency channel simultaneously through the use of unique codes assigned to each channel. It works by spreading each phone call's signal over a wide section of the available bandwidth, with different codes used to identify individual users within the band. CDMA has benefits over other technologies including increased capacity, improved quality, enhanced privacy and coverage.

1. An overview of CDMA
Code - Division Multiple Access

2. Presented By
Saimunur Rahman
Matric No: C093003
Dept. of Computer Science & Engineering
International Islamic University Chittagong

3. Presented to
Abdullahil Kafi
Assistant Professor
Dept. of Computer Science & Engineering
International Islamic University Chittagong
And all of my classmates …

4. 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.

5. 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.

6. 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

7. Idea of CDMA
Let 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

8. Idea of CDMA
Assume the code for channels

9. 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

11. 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

12. Idea of CDMA
If 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 ₓd1 [c1.c1 4, c2.c1 0, c3.c1 0 & c4.c1 0]
= ( 4 ₓd1 ) / 4
= d1
d1 Was the data's of channel 1.

13. 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

14. 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 …

15. 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

16. 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]

17. Chips Sequence Generation
For 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 … …

18. Chips Sequence Generation
Fig: General rule and examples of creating Walsh tables

19. Chips Sequence Generation
The table for N sequences WN
The table for 2N sequences W2N
Where N = 1,2,3 … …

20. Data representation in CDMA
If 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

21. 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.

22. Encoding (By using example)
Channel No Data Chip Di * Ci
Cn 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]
• 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]

23. 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.

24. Let’s see the process in a figure
Fig : Sharing channel in CDMA

25. Encoding (Analysis by using Signal lebel)
It will be very clear if I use Signal level
Lets see …

26. Encoding (Analysis by using Signal label)
See the figure very carefully
Fig: Encoding the digital signal created by four stations in CDMA

27. Decoding (Analysis by using Signal label)
See the figure carefully
Fig: Decoding of the composite signal for one in CDMA

28. Benefits of CDMA
Increased Capacity
Improved Quality
Improved Quality
Enhanced Privacy
Improved Coverage
Increased Portable Talk Time
Bandwidth on Demand

29. CDMA-Applications
CDMA for Cellular
CDMA-Short Message Service
Over-the-Air Activation
CDMA Data and Fax
Subscriber Access Control
CDMA for Personal Communications Services

30. Conclusion
Technology of choice for 3G generation
because of it’s
Greater total capacity
Outstanding voice quality
 Fewer dropped calls
It’s use in satellite communication is of great
importance