King Fahd University of Petroleum &Minerals
Electrical Engineering Department
EE-400
presentation
CDMA systems
Done By:
Ibrahim Al-Dosari
221416
Mohammad Al-Doraibi
202647
Outline
1. Multiple Access Techniques
2. Introduction.
3. How CDMA works?
4. Types of CDMA
5. Spreading Codes
6. Multiple Access Interference
7. Detection in the CDMA system
8. Commercial usage of CDMA
9. Advantages and Disadvantages of CDMA
10. Conclusion
Multiple Access Techniques

In many wireless systems, multiple transmitters
attempt to communicate with the same receiver.

There are three widely-used policies:
1. FDMA (Frequency Division Multiple
Access)
2. TDMA (Time Division Multiple Access)
3. CDMA (Code Division Multiple Access
Multiple Access Techniques
FDMA
TDMA
C
C
BOnly possibleBin
A
A
f2
B
f1
CBA
C B for
A CGSM
B ACB A
Used
f0
analog system
A
f0
C
Time
Time
Introduction

Definition:

CDMA is a technology that
allows multiple users to
share the whole spectrum
at all the time unlike TDMA
and FDMA.

CDMA has wider bandwidth
compared to TDMA &
FDMA.
Requires digital
transmission

How CDMA works?
CDMA transmitter:




The voice has to be digitalize (Pulse Code
Modulation (PCM) then compressed)
Each user is given a unique PN code.
The Codes must have low cross-correlation
The transmitter multiplies the code by the data to
get the coded massage (bit)
How CDMA works?
CDMA transmitter:
Ck
R bps
bk (t)
RJ bps
X
Sk (t)
How CDMA works?

Spectrum of the CDMA:
1
0.9
0.8
0.7
noise
massege
After Spreding
Chip Duration is 0.4
ms.
Bit Duration is 6.4 ms.
Processing Gain is 16
0.6
0.5
0.4
0.3
0.2
0.1
0
-5000
-4000
-3000
-2000
-1000
0
1000
2000
3000
4000
5000
How CDMA works?
CDMA Receiver:


The received signal is multiplied again by the
same code that used in the transmitter.
Integrate over bit duration and, then using a
hard decision to get the transmitted massage
(bit)
How CDMA works?
CDMA Receiver:
Hard Decision
C1
Sum over
J chips
∑
RJ bps
R bps
R bps
R bps
R bps
y(t)
Ck
Sum over
J chips
∑
RJ bps
Matched Filter
How CDMA works?
Types of CDMA
1. Frequency Hopping

fixed sequence of frequency values & Time is divided into
slots .

In the first time slot, a given user transmit to the base station
using the first frequency in its frequency hopping sequence.

In the next time interval, it transmits using the second
frequency value in its frequency hopping sequence, and so
on.

This way, the transmit frequency keeps changing in time.
Types of CDMA
2. Direct Sequence

each user transmits its message to the base
station using the same frequency, at the same
time.

Here signals from different users interfere with
each other.

But the user distinguishes its message by using a
special, unique code. This code serves as a
special language that only the transmitter and
receiver understand.
Spreading Codes

There are two types of PN codes orthogonal
codes and shift register codes:

Orthogonal Codes: have zero cross-correlation


Walsh
Shift Register Codes: have low cross-correlation
and they are produced from shift register



M-sequence
Gold (produced from two different m-sequence )
kassami (produced from Gold and m-sequence )
Multiple Access Interference:


Result from the lack of perfect orthogonally
between the spreading codes, that is, the
cross-correlation does not equal to zero
As the number of the users increases, the
more interference will be. Because, each
user consider the other users as a noise.
Detection in the CDMA system:

Multi-Users Detection (MUD)




The capacity and the performance of the CDMA
system increase when MUD is implemented.
The basic principle of MUD is the elimination of
the negative effect of each user on the other.
MUD is also known as joint detection and
interference cancellation .
Multi-user detection considers all users as signals
for each other.
Detection in the CDMA system:

Multi-Users Detection (MUD)


The ultimate technique for the MUD is the optimum
receiver.
Optimum Receiver:

Very complex. So, it is unpractical solution to reduce
the multiple access interference. Sup-optimum detector
will be implemented.
Detection in the CDMA system:

Multi-Users Detection (MUD)

Sup-optimum solution:

Serial Interference cancellation (SIC):



SIC is preferred in the absence of power control.
Serial canceling the interference generated from the other
users.
Cancel the strongest signal first (most negative effect )
Detection in the CDMA system:

Multi-Users Detection (MUD)

Parallel Interference Cancellation (PIC):



PIC would be preferred when the amount of
interference from each user is similar.
In the initial detection stage, all active users are
detected in parallel by a SUD technique.
Then, the obtained interference is subtracted from the
received signal and data detection is performed again
with reduced MAI
Commercial usage of CDMA

CDMA was introduced in the 2nd Generation (early
of the 1990s).


E.g. IS-95 standard, also known as cdmaOne which
support up to 64 users that are orthogonally coded and at
the same time transmitted over 1.25 MHz channel
Used for Cellular Communication System


(824-894 MHz in US Cellular)
3rd Generation: cdma2000

Allow high rate of packet transmitting in
addition of voice transmitting.
Advantages and Disadvantages of
CDMA
Advantages:
CDMA provides :
1. better cost effective.
2. high voice quality.
3. Increased cellular communications security.
4. system capacity is higher than TDMA and
FDMA
5. operate at very low power levels.
Advantages and Disadvantages of
CDMA
Disadvantages:

1. Multiple Access Interference:
Require multi-user detection (MUD)
algorithms to solve.
Advantages and Disadvantages of
CDMA
Disadvantages:

2. Near-far problem.
Where stronger (near to the Base Station)
user masks the weaker user (far from the
Base Station)
3. Requires wideband channel.
Conclusion

The CDMA will allow many signals to be transmitted at
the same channel at the same time. This done by giving
each user a Pseudo-Noise code which is a binary
sequence. This code should have low cross-correlation
between each other.

Multiple access interference has bad effect on the CDMA
system so the multiple user detection is used to reduce
the MAI.
References

http://www.cellular.co.za/technologies/cdma/cdm
a_w_paper.htm

http://www.arcx.com/sites/CDMAvsTDMA.htm
Questions