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.
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.
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.
