CDMA technology

Skip to first unread message

May 24, 2005, 8:26:45 AM5/24/05

Short for Code-Division Multiple Access, a digital cellular technology
that uses spread-spectrum techniques. Unlike competing systems, such as
GSM, that use TDMA, CDMA does not assign a specific frequency to each
user. Instead, every channel uses the full available spectrum.
Individual conversations are encoded with a pseudo-random digital
sequence. 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 military technology first used during World War II by English
allies to foil German attempts at jamming transmissions. The allies
decided to transmit over several frequencies, instead of one, making it
difficult for the Germans to pick up the complete signal. Because
Qualcomm created communications chips for CDMA technology, it was privy
to the classified information. Once the information became public,
Qualcomm claimed patents on the technology and became the first to
commercialize it.

May 24, 2005, 9:24:13 AM5/24/05

CDMA is a second-generation digital mobile telephone standard which
takes a different approach to the other, competing standards: GSM
(Global System for Mobile Communications) and TDMA (Time Division
Multiple Access). Where GSM and TDMA divide the available bandwidth
into 'channels' using a combination of frequency bands and time-slices,
CDMA spreads the signal over a wide bandwidth, identifying each channel
using unique digital codes. This means it can provide greater bandwidth
efficiency, and hence a greater potential number of channels.

TDMA and GSM systems commonly start with a slice of spectrum referred
to as one "carrier". Each carrier is then divided into time slots. Only
one subscriber at a time is assigned to each time slot, or channel. No
other conversations can access this channel until the subscriber's call
is finished, or until that original call is handed off to a different
channel by the system. For example, GSM systems create 8 time-division
channels in 200kHz (kilohertz) wide carriers.

With CDMA, all users share the same 1,250kHz wide carrier, but unique
digital codes are used to differentiate subscribers. The codes are
shared by both the mobile station and the base station and are called
"pseudo-random code sequences". Base stations in the system distinguish
themselves from each other by transmitting different portions of the
code at a given time. In other words, the base stations transmit
time-offset versions of the same pseudo-random code.

In order to ensure that the time offsets used remain unique from each
other, CDMA stations must remain synchronised to a common time
reference. The global positioning system (GPS) provides this precise
common time reference. GPS is a satellite-based radio navigation system
capable of providing a practical and affordable means of determining
continuous position, velocity, and time to an unlimited number of

One of the unique aspects of CDMA is that while there are certainly
limits to the number of phone calls that can be handled by a carrier,
this is not a fixed figure. The number of simultaneous connections any
base station is able to handle is the result of a trade-off with the
range of the base station and the quality of each connection. A
standard CDMA connection has a digital transfer rate of 9.6kbps
(kilobits per second), the same as GSM. The voice data part of the
connection is transmitted at a rate of 8kbps. All connections are
shared around the spread spectrum with a maximum transfer rate of
1.23Mbps (Megabits per second), theoretically allowing a maximum of 131
connections, compared to 48 GSM connections in a similar bandwidth.

CDMA supports variable bandwidth connections, at 13kbps and 64kbps for
superior quality speech and faster data connections. This
'bandwidth-on-demand' limits the number of simultaneous connections to
a particular base station.

Traditional uses of spread spectrum are in military operations. Because
of the wide bandwidth of a spread spectrum signal, it is very difficult
to jam, difficult to interfere with, and difficult to identify. This is
in contrast to technologies using a narrower bandwidth of frequencies.
Since a wideband spread spectrum signal is very hard to detect, it
appears as nothing more than a slight rise in the "noise floor" or
interference level. With other technologies, the power of the signal is
concentrated in a narrower band, which makes it easier to detect. The
narrow band is also more prone to interference than the spread-spectrum
of CDMA.

Increased privacy is inherent in CDMA technology. CDMA phone calls will
be secure from the casual eavesdropper since, unlike an analog
conversation, a simple radio receiver will not be able to pick
individual digital conversations out of the overall RF radiation in a
frequency band. TDMA (time division multiple access) and GSM systems
have to add an extra stage of encryption to the signal to ensure
privacy, whereas encryption is inherent to the CDMA system.

Aspects of CDMA technology have been incorporated into the
third-generation IMT-2000 standard, designed to allow interoperability
between the different networks and integration with satellite
technology. This third-generation technology will allow broadband data
access that can be used for voice, video and data communication at
speeds of up to 2Mbps.

Reply all
Reply to author
0 new messages