Contents
Theory
The frequency spectrum in a practical sense is a finite resource. To effectively
support a large number of users, some technique for sharing the spectrum is
required to minimize mutual interference. Several common techniques have
focused on the use of directional antennas to carefully restrict propagation, the
use of separate frequency slots, or time sharing. Code Division Multiple Access
(CDMA) is a digital technique for sharing the frequency spectrum. CDMA is
based on proven spread spectrum communication technology.
Spread spectrum is a means of transmission in which the signal occupies a
bandwidth in excess of the minimum necessary to send the information; the band
spread being accomplished by means of a code which is independent of the
data, accompanied by synchronized reception with the same code at the receiver
for de-spreading and subsequent data recovery.
The spreading code is usually a pseudo random (pseudo noise, PN) code, and
demodulation is accomplished in part by correlation of the received signal with a
replica of the PN code used in the transmitter.
Different spread-spectrum systems exist, including direct-sequence (DS),
frequency hopping (FH) and time-hopping (TH) systems and various hybrid
forms. These systems can be used as an aid for mitigating the effects of various
types of interference, e.g. caused by jamming or multi path propagation. Spreadspectrum
signals are also used in code-division multiple-access (CDMA)
systems, which have a larger capacity in mobile telecommunications than the
present frequency-division multiple-access (FDMA) and time-division multipleaccess
(TDMA) systems.
The innovative Spread Spectrum communication techniques are based on the
"spreading" of the power spectrum of the transmitted signal on a wide frequency
band, so that the power for frequency unit (W/Hz) sent onto the channel by a
transmitter is very low.
The successful key of these techniques is that the above-mentioned "spreading"
of the transmitted power is obtained by a pseudo random pattern, which
modulates the transmitted signal. This pattern is known only by the receiver that
must detect the signal. It is impossible to recover, and hence demodulate, the
narrow band signal if the spreading pattern is not completely known.
Another suitable characteristic of Spread Spectrum Communications is their
capability of rejecting interference in a natural way. In fact, the receiver recovers
only the signal that must be detected, leaving the other interfering signals on the
transmission wideband, so that the global interfering power is lower than in the
traditional narrow band communication system.
USES OF CDMA
CDMA is a data transmission technology primarily used for data transmission to mobile handsets. CDMA technology is far superior to other technologies in all fields.1. CDMA handsets provide greater coverage. Coding and Interleaving techniques used in CDMA provide the ability to cover a larger area for the same amount of available power used in other systems. Under line of sight conditions, CDMA has 1.7 to 3 times more coverage than TDMA.
2. CDMA allows greater number of simultaneous calls per sector. Where FDMA and TDMA allow 6 and 18 simultaneous calls per sector, CDMA may allow 30 simultaneous calls per sector.
3. CDMA uses soft hand-off techniques between sectors increasing capacity and providing better voice quality. Soft handoff refers to the state where the mobile I is in communication with multiple Base Stations at the same time. Soft handoff is a make-before-break type of handoff, whereby a mobile acquires a target code channel before breaking an existing one.
4. Soft handoff is a special attribute of CDMA that is enabled by universal frequency reuse. Soft Handoff increases Capacity. There are several important reasons to place in soft handoff any additional Base Stations that can be detected by the mobile station, as soon as possible.
5. Improved voice quality — Cell boundaries usually offer poor coverage coupled with increased interference from other cells; therefore, Forward traffic Channel diversity from additional cells will improve voice quality.
6. Controlled mobile station interference —While on a boundary of a cell, the mobile station’s interference to mobile stations in other cells is maximal; therefore, it is important to e able to power-control these cells.
7. Reduced call dropping probabilities—Handoff areas are areas in which the Forward link is most vulnerable. A slow handoff process coupled with a vehicle moving at a high speed may cause the call to be dropped.
8. Increased capacity and coverage— Soft handoff considerably increases both the capacity of a heavily loaded cellular system and the coverage of each individual cell in a lightly loaded system.
4. Better Use of Multipath
One of the main advantages of CDMA systems is their ability to use signals that arrive in the receivers with different time delays. This phenomenon is called multipath.
REFERENCES
1. Applications of CDMA in wireless personal communications Vijay K Garg, Kenneth F Smolik, Joseph E Wilkes2. CDMA: Principles of spread spectrum communication Andrew J Viterbi
3. CDMA for wireless personal communications Ramjee Prasad
4. http://www.qualcomm.com/cdma/university/introductorycourses.html
