CDMA TECHNOLOGY

CDMA TECHNOLOGY

The world's first cellular networks were introduced in the early 1980s, using analog
radio transmission technologies such as AMPS (Advanced Mobile Phone System).
Within a few years, cellular systems began to hit a capacity ceiling as millions of new
subscribers signed up for service, demanding more and more airtime. Dropped calls and
network busy signals became common in many areas.

To accommodate more traffic within a limited amount of radio spectrum, the
industry developed a new set of digital wireless technologies called TDMA (Time
Division Multiple Access) and GSM (Global System for Mobile). TDMA and GSM used
a time-sharing protocol to provide three to four times more capacity than analog systems.
But just as TDMA was being standardized, an even better solution was found in CDMA.
Commercial Development
The founders of QUALCOMM realized that CDMA technology could be used in
commercial cellular communications to make even better use of the radio spectrum than
other technologies. They developed the key advances that made CDMA suitable for
cellular, then demonstrated a working prototype and began to license the technology to
telecom equipment manufacturers.

The first CDMA networks were commercially launched in 1995, and provided
roughly 10 times more capacity than analog networks - far more than TDMA or GSM.
Since then, CDMA has become the fastest-growing of all wireless technologies, with
over 100 million subscribers worldwide. In addition to supporting more traffic, CDMA
brings many other benefits to carriers and consumers, including better voice quality,
broader coverage and stronger security.
The world is demanding more from wireless communication technologies than ever
before. More people around the world are subscribing to wireless services and consumers
are using their phones more frequently. Add in exciting Third-Generation (3G) wireless
data services and applications - such as wireless email, web, digital picture
taking/sending and assisted-GPS position location applications - and wireless networks
are asked to do much more than just a few years ago. And these networks will be asked to
do more tomorrow.

This is where CDMA technology fits in. CDMA consistently provides better capacity
for voice and data communications than other commercial mobile technologies, allowing
more subscribers to connect at any given time, and it is the common platform on which
3G technologies are built.
CDMA is a "spread spectrum" technology, allowing many users to occupy the same
time and frequency allocations in a given band/space. As its name implies, CDMA
assigns unique codes to each communication to differentiate it from others in the same
spectrum.

Brief Working of CDMA

CDMA takes an entirely different approach from TDMA. CDMA, after digitizing
data, spreads it out over the entire available bandwidth. Multiple calls are overlaid on
each other on the channel, with each assigned a unique sequence code. CDMA is a formof spread spectrum, which simply means that data is sent in small pieces over a number
of the discrete frequencies available for use at any time in the specified range.
In CDMA, each phone's data has a unique code.
All of the users transmit in the same wide-band chunk of spectrum. Each user's signal is
spread over the entire bandwidth by a unique spreading code. At the receiver, that same
unique code is used to recover the signal. Because CDMA systems need to put an
accurate time-stamp on each piece of a signal, it references the GPS system for this
information. Between eight and 10 separate calls can be carried in the same channel
space as one analog AMPS call.

Spread Spectrum Communications

CDMA is a form of Direct Sequence Spread Spectrum communications. In general,
Spread Spectrum communications is distinguished by three key elements:
1. The signal occupies a bandwidth much greater than that which is necessary to send the
information. This results in many benefits, such as immunity to interference and jamming
and multi-user access, which we’ll discuss later on.
2. The bandwidth is spread by means of a code which is independent of the data. The
independence of the code distinguishes this from standard modulation schemes in which
the data modulation will always spread the spectrum somewhat.
3. The receiver synchronizes to the code to recover the data. The use of an independent
code and synchronous reception allows multiple users to access the same frequency band
at the same time.
In order to protect the signal, the code used is pseudo-random. It appears random, but
is actually deterministic, so that the receiver can reconstruct the code for synchronous
detection. This pseudo-random code is also called pseudo-noise (PN).