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What is cdma?

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CDMA

What is CDMA?

CDMA (Code-Division Multiple Access) is a channel access method used by various radio communication technologies. It is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.

CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology. Audio input is first digitized into binary elements. The frequency of the transmitted signal is then made to vary according to a defined pattern (code), so it can be intercepted only by a receiver whose frequency response is programmed with the same code, so it follows exactly along with the transmitter frequency. There are trillions of possible frequency- sequencing codes, which  enhances  privacy and makes cloning difficult.

The original CDMA standard, also known as CDMA One and still common in cellular telephones in the U.S offers a transmission speed of only up to 14.4 Kbps in its single channel form and up to 115 Kbps in an eight-channel form. CDMA2000 and Wideband CDMA deliver data many times faster.



Characteristics of  CDMA:

  • 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 CDMA

  Ø    Rx: 869-894MHz Tx: 824-849MHz

  Ø  20 Channels spaced 1250kHz apart (798 users/channel)

  Ø QPSK/(Offset) OQPSK modulation scheme

  Ø   1.2288Mbps bit rate

  Ø    IS-95 standard

  Ø    Operates at both 800 and 1900 MHz frequency bands

Advantages  of CDMA 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 sent
  • Better 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  CDMA:

  • 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 Global star satellite phone network.
  • The UMTS 3G mobile phone standard, which uses W-CDMA.
  • CDMA has been used in the Omni TRACS satellite system for transportation logistics.


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