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3G Data Matrix
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Technology
CDMA (Code Division Multipe Access) allows multiple users to more efficiently share the same radio spectrum. The actual quantitative improvement
over existing analog systems and over competing digital technologies is still a subject of intense debate. However, there seems to be a consensus
that the improvement over analog ("AMPS") is
approximately 6x. Instead of dividing RF spectrum into narrow channels (e.g. 30 kHz each) and assigning one (AMPS) or
more (e.g. IS-136 TDMA) conversations to each channel, CDMA spreads many conversations over a broad spectrum
(1.25 MHz in the case of North American CDMA standard - IS-95). To separate a particular call from the other
calls using the same spectrum at the same time, a unique digital code called a "Pseudo-Random Code Sequence
(PRCS)" is assigned to each user. Many users (up to 64 for IS-95) share the same spectrum, each using their
unique code, and decoders separate the codes at each end in a process exactly like a tuner that separates different
frequencies in more conventional systems.
As a digital multiple access technology that can provide greater capacity and quality on a wireless network, CDMA can be used with cellular networks or personal
communications services (PCS) networks. The following standards have already been defined for each:
- (IS-95) Cellular CDMA Standard - wideband spread spectrum cellular mobile communications first published in July 1993
- (ANSI-J-STD-008) PCS CDMA Standard - similar specification to IS-95 but defined for 1.8 to 2.0 GHz CDMA PCS systems and first published in 1995
Data over CDMA
As of today, work is underway to add both circuit-switched and packet-switched data capabilities to CDMA. In January 1998, the Telecommunications Industry
Association (TIA) published IS-707 as the data specification for the cellular (IS-95) and PCS (J-STD-008) CDMA standards. Its data service options include:
- Circuit-switched data and fax,
- QuickNet Connect, a circuit-switched enhancement that makes data transmission packet-like on the inbound or reverse channel,
- Low-rate packet at 14,400 bps (formerly IS-657),
- Medium-rate packet at up to 64,000 bps.
IS-637 is another CDMA data service specification. However, it provides for two-way short-messaging service (SMS) with only one-way currently available.
Strengths
- Increased Capacity
For reasons mentioned above - a combination of voice coder technology and improved resistance to interference that allows the same frequencies to be re-used more often than in analog systems - CDMA offers increased voice capacity over analog. Its proponents claim capacity advantages over other digital systems.
- Low Power
The properties of spread spectrum permit signals to be transmitted at lower power than analog signals for similar performance - hence devices should have longer battery lives than analog. This is particularly true for voice services.
- Security
CDMA offers two levels of security. To the network operator it includes encryption to prevent fraudulent "cloning" of phones. To the user, the complexity of the pseudo random code sequence makes casual eavesdropping impossible.
- Easy Addition of New Functions
The implementation of CDMA allows many new features, including the different forms of data supported to be added largely as software upgrades are done to the switch. It is one of the claims of CDMA that it requires very little additional equipment to add new services.
Related Technologies
- TDMA - Time Division Multiple Access: another digital cellular technology for providing greater user capacity than is available with analog technology. Developed in the U.S., but with international deployment. Although TDMA is a generic technical term, it is used by convention to refer to the North American "IS-136" standard and derivatives.
- GSM - Global System for Mobile: another digital cellular technology for providing greater user capacity than is available with analog technology. Developed in Europe, but with significant international deployment.
- Personal Communication Services (PCS) - cellular-like system in the 1900 MHz range within North America and in the 1800 MHz range in Europe. ( PCS Information )
Deployment
Most North American carriers have deployed circuit-switched CDMA with QuickNet Connect and fax services at speeds up to 14,400 bps and short messaging service
(SMS). It is not clear which of the possible services will be deployed next. The IS-95B standard permits several CDMA "channels" to be combined to provide a
64 kbps "virtual channel". This can be used for either circuit-switched or packet-switched services. However, the service displaces significant voice capacity,
and may not prove attractive to carriers. In May 1999 the standards body (TIA) voted on a new standard – commonly referred to as 1XRTT and also as
“cdma2000” – that will offer 144 kbps service. Although less mature as a standard than IS95B, it is possible that carriers will leapfrog IS95B and go straight to
CDMA2000 as their next offering - possibly positioning it as a "3G" service.
As for packet data, most carriers see little benefit in low-rate (i.e. 14,400 bps) packet data. CDPD carriers compare it to CDPD and non-CDPD carriers compare it
to 14,400 bps circuit-switched with QuickNet Connect. Medium-rate packet data is another story. Most carriers Sierra has spoken to are very interested in the
concept. However, the issue of which underlying technology will be used - IS95B at up to 64 kbps or cdma2000 at 144 kbps is still uncertain. The author's bet
is on cdma2000.
Markets and Applications
Primarily a voice service, the market for CDMA is anywhere where there is a need for cellular voice services. Its main advantage is to the carrier, by offering
more capacity and lower cost per user. To the user, the advantages are better voice quality and handsets with longer battery life. Secondary services include
the data services discussed above as well as simpler short messaging services. The traditional "vertical" markets that use wireless data today can use the
data services promised by CDMA. In addition, several vendors are promoting the use of the "SmartPhone" concept where the phone itself is the data terminal.
This seems to have some appeal for the casual data user, and integration with short messaging service and simple displays on the phone provide some "horizontal"
market appeal.
Future Outlook
Formerly, the world talked of current digital cellular systems as second generation, or 2G, systems. These were to be followed by third generation (3G) systems
with immensely improved features. However, the industry has become less radical, and a more gradual deployment of interim services has become favoured. A result
of this has been mass confusion of what is 3G and what is not - with the term 2.5G being introduced for interim solutions, and 3G used for publicity. Today,
3G is generally seen as the ultimate goal when all of the world's wireless standards converge to some common level of interoperation, and is generally expected
to use wide bandwidths and combine elements of GSM and CDMA. The two great barriers to this utopia were lack of cooperation between international bodies and
intellectual property rights. Recently, positive developments have emerged in both of these areas.
Returning to more specific prognostications on the North American CDMA market, future (2.5G?) generation CDMA data specifications promise data speeds of more
than one megabit per second using broader frequency allocations. As an example "3XRTT" uses 3 1.25 MHz channels plus guard bands for a total 5MHz occupancy,
and offers speeds of up to 2Mbps. These are probably 2003 to 2005 systems.
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