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21 What is CDMA? Advantage Disadvantage and its Characteristics




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:

Ø  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.
Ø  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.
Ø  CDMA devices use a rake receiver, which exploits multipath delay components to improve the performance of the system.
Ø  In a CDMA system, the same frequency can be used in every cell, because channelization is done using the pseudo-random codes.
Ø  Reusing the same frequency in every cell eliminates the need for frequency  planning in a CDMA system;
Ø  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:

Ø  Efficient practical utilization of fixed frequency spectrum.
Ø  Flexible allocation of resources.
Ø  Many users of CDMA use the same frequency, TDD or FDD may be used
Ø  Multipath fading may be substantially reduced because of large signal bandwidth
Ø  No absolute limit on the number of users, Easy addition of more users.
Ø  Impossible for hackers to decipher the code sent
Ø  Better signal quality
Ø  No sense of handoff when changing cells
Ø  The CDMA channel is nominally 1.23 MHz wide.
Ø  CDMA networks use a scheme called soft handoff, which minimizes signal breakup as a handset passes from one cell to another.
Ø  CDMA is compatible with other cellular technologies; this allows for nationwide roaming.
Ø  The combination of digital and spread-spectrum modes supports several times as many signals per unit bandwidth as analog modes.

Disadvantages to using CDMA:

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

Uses of CDMA:

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

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