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King Fahd University of Petroleum


Title: Slide 1 Author: Mohammad_D Last modified by: Mohammad_D Created Date: 5/16/2006 7:00:48 PM Document presentation format: On-screen Show Company – PowerPoint PPT presentation

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Title: King Fahd University of Petroleum

King Fahd University of Petroleum
Minerals Electrical Engineering Department
EE-400 presentation CDMA systems Done
By Ibrahim Al-Dosari 221416 Mohammad
Al-Doraibi 202647
  • 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
  • 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

  • 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

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

How CDMA works?
  • CDMA Receiver

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
  • 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
  • 4. system capacity is higher than TDMA and
  • 5. operate at very low power levels.

Advantages and Disadvantages of CDMA
  • Disadvantages
  • 1. Multiple Access Interference
  • Require multi-user detection (MUD) algorithms to

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.

  • 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.

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