King Fahd University of Petroleum &MineralsElectrical Engineering Department
EE-400
presentation
CDMA systems
Done By:
Ibrahim Al-Dosari 221416
Mohammad Al-Doraibi 202647
Outline1. Multiple Access Techniques2. Introduction.3. How CDMA works?4. Types of CDMA5. Spreading Codes6. Multiple Access Interference 7. Detection in the CDMA system 8. Commercial usage of CDMA9. Advantages and Disadvantages of CDMA10. 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
Only possible in analog system
TDMA
Used for GSM
A A
B B
C C Freq
uenc
y
Time
f2
f1
f0
C B A C B A C B A C B A
C
A
B
Time
f0
Freq
uenc
y
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:
X
Ck
R bps RJ bps
Sk (t) bk (t)
-5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 50000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1noisemassegeAfter SpredingChip Duration is 0.4
ms.Bit Duration is 6.4 ms.Processing Gain is 16
How CDMA works?
Spectrum of the CDMA:
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:
∑
∑
C1
Ck
RJ bps
Sum over J chips
Sum over J chips
RJ bps R bps
R bps R bps
R bps
y(t)
Matched Filter
Hard Decision
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 CDMA2. 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 CDMADisadvantages:
1. Multiple Access Interference: Require multi-user detection (MUD)
algorithms to solve.
Advantages and Disadvantages of CDMADisadvantages:
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/cdma_w_paper.htm
http://www.arcx.com/sites/CDMAvsTDMA.htm
Questions