CDMA for cellular based communications
Introduction
The cellular communications industry has experienced explosive growth since its inception in 1983. In fact, at the end of 1995 there were more than 85 million subscribers worldwide, with rising growth rates expected to continue through the end of this century. As the wireless communication industry matures, operators are working to increase the number of subscribers on their system, lower the cost per subscriber, generate more revenue and increase market penetration. Mobile communication systems employ multiple access techniques such as FDMA, TDMA and CDMA. These techniques will be briefly discussed in the following section.
a.
Frequency Division Multiple Access (FDMA)
FDMA
is a multiple access technique used by analog systems (such as: AMPS, NAMPS,
TACS,…). It consists of dividing the Radio Frequency (RF) spectrum into channels
of equal bandwidth. Each channel is assigned to one user at a time and cannot
be accessed by any other users until the call is finished or it is handed-off
to another channel.
b.
Time Division Multiple Access (TDMA)
TDMA
is another multiple access technique used by GSM systems in which the RF spectrum
is divided into carriers. Each carrier is then divided into time slots. Only
one user is assigned to each time slot (referred to as channel). Users can transmit
or receive calls only when the carrier serves their time slot. Then channel
is then dropped and the following time slot is served and so on.
c.
Code Division Multiple Access (CDMA)
CDMA
is a wide band technology in which the radio spectrum is divided into carriers
of 1.23 MHz wide. Each user using this band is assigned a unique code (Pseudorandom
Code Sequence). The code is then modulated by the transmitted signal and is
sent over the entire carrier. CDMA is based on the Spread Spectrum (SS) technology
(explained later in detail) which spreads the information to be transmitted
on a larger bandwidth than the original signal. Signals begin at a rate of 9.6
Kbps and are then spread to a rate of 1.23 Mbps. When the signal is received,
the code is removed and the information is returned to a rate of 9.6 Kbps. CDMA's
SS technique was used for military communications for over fifty years. And
in response to the overwhelming demand for cellular communications, CDMA adaptation
to cellular applications began in the late 1980's and was first used commercially
in 1995.