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CDMA IS-95 (Code Division Multiple Access)




Key Data


CDMA (code division multiple access) 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.

Despite being mostly confined to the US, CDMA systems accounted for roughly 12% of digital subscribers worldwide in June 2002. CDMA shares the 1900MHz frequency bands with GSM in the US but, as mentioned above, spreads each channel across a wider spectrum.

CDMA is currently incompatible with TDMA and GSM, and since most networks are confined to the US, it does not cater for global roaming. The best features of all three standards are being brought together to allow inter-operability for the third-generation networks, such as UMTS (Universal Mobile Telecommunication System) using the IMT-2000 standard.

CDMA TECHNICAL DETAILS

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 users.

SPREAD SPECTRUM

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, with enhanced standards 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.

PRIVACY

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.

FUTURE OF CDMA

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.