Code division multiple access

Code division multiple access (CDMA) is a channel access method used by various radio communication technologies.

CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication channel. This allows several users to share a band of frequencies. To permit this without undue interference between the users, CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code).

Code division multiple access

Characteristics of code division multiple access:

  • Spread spectrum techniques use a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. These systems were designed using spread spectrum because of its security and resistance to jamming.
  • CDMA can effectively reject narrow band interference. Since narrow band interference affects only a small portion of the spread spectrum signal, it can easily be removed through notch filtering without much loss of information.
  • CDMA devices use a rake receiver, which exploits multipath delay components to improve the performance of the system.
  • In a CDMA system, the same frequency can be used in every cell, because channelization is done using the pseudo-random codes.
  • Reusing the same frequency in every cell eliminates the need for frequency  planning in a CDMA system;
  • CDMA systems use the soft hand off, which is undetectable and provides a more reliable and higher quality signal.

General Specification of Code division multiple access

  • Rx:  869-894MHz     Tx:  824-849MHz
  • 20 Channels spaced 1250kHz apart (798 users/channel)
  • QPSK/(Offset) OQPSK modulation scheme
  • 2288Mbps bit rate
  • IS-95 standard
  • Operates at both 800 and 1900 MHz frequency bands

Advantages of Code division multiple access techniques:

  • Efficient practical utilization of fixed frequency spectrum.
  • Flexible allocation of resources.
  • Many users of CDMA use the same frequency, TDD or FDD may be used

Multipath fading may be substantially reduced because of large signal bandwidth

  • No absolute limit on the number of users, Easy addition of more users.
  • Impossible for hackers to decipher the code sentBetter signal quality
  • No sense of handoff when changing cells
  • The CDMA channel is nominally 1.23 MHz wide.
  • CDMA networks use a scheme called soft handoff, which minimizes signal breakup as a handset passes from one cell to another.
  • CDMA is compatible with other cellular technologies; this allows for nationwide roaming.
  • The combination of digital and spread-spectrum modes supports several times as many signals per unit bandwidth as Analog modes.

Disadvantages to using Code division multiple access

  • As the number of users increases, the overall quality of service decreases
  • Self-jamming
  • Near- Far- problem arises

Uses of CDMA:

  • One of the early applications for code division multiplexing is in GPS. This predates and is distinct from its use in mobile phones.
  • The Qualcomm standard IS-95, marketed as CDMA One.
  • The Qualcomm standard IS-2000, known as CDMA2000. This standard is used by several mobile phone companies, including the globalstar satellite phone network.
  • The UMTS 3G mobile phone standard, which uses W-CDMA.
  • CDMA has been used in the OmniTRACS satellite system for transportation logistics.

Key elements of CDMA

CDMA is a form of spread spectrum transmission technology. It has a number of distinguishing features that are key to spread spectrum transmission technologies:

  • Use of wide bandwidth:CDMA, like other spread spectrum technologies uses a wider bandwidth than would otherwise be needed for the transmission of the data. This results in a number of advantages including an increased immunity to interference or jamming, and multiple user access.
  • Spreading codes used: In order to achieve the increased bandwidth, the data is spread by use of a code which is independent of the data.
  • Level of security: In order to receive the data, the receiver must have a knowledge of the spreading code, without this it is not possible to decipher the transmitted data, and this gives a measure of security.
  • Multiple access: The use of the spreading codes which are independent for each user along with synchronous reception allow multiple users to access the same channel simultaneously.
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