What is CDMA? Advantage Disadvantage and its Characteristics
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.
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.
- 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
- 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.
- Ø As the number of users increases, the overall quality of service decreases
- Ø Self-jamming
- Ø Near- Far- problem arises
- 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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