IS-95, cdmaOne

- a basic introduction or tutorial about the basics of IS-95 (IS-95A and IS-95B) marketed under the brand name cdmaOne

IS-95 was the first CDMA mobile phone system to gain widespread use and it is found widely in North America. Its brand name is cdmaOne and the initial specification for the system was IS95A, but its performance was later upgraded under IS-95B. It is this later specification that is synonymous with cdmaOne. Apart from voice the mobile phone system is also able to carry data at rates up to 14.4 kbps for IS-95A and 115 kbps for IS-95B.

IS95 / cdmaOne was the fist cellular telecommunications system to use the CDMA - code division multiple access system. Previous systems had used FDMA - frequency division multiple access or TDMA - time division multiple access. With IS-95 being a second generation - 2G system and all the later 3G systems using CDMA as their access system, this meant that IS95 / cdmaOne was a pioneering system.

IS-95 history

The idea for using the form of modulation known as direct sequence spread spectrum (DSSS) for a multiple access system for mobile telecommunications came from a California based company called Qualcomm in the 1980s. Previously DSSS had been mainly used for military or covert communications systems as the transmissions were hard to detect, jam and eavesdrop.

The system involved multiplying the required data with another data stream with a much higher data rate. Known as a spreading code, this widened the bandwidth required for the transmission, spreading it over a wide frequency band. Only when the original spreading code was used in the reconstruction of the data, would the original information be reconstituted. It was reasoned that by having different spreading codes, a multiple access system could be created for use in a mobile phone system.

In order to prove that the new system was viable a consortium was set up and Qualcomm was joined by US network operators Nynex and Ameritech to develop the first experimental code division multiple access (CDMA) system. Later the team was expanded as Motorola and AT&T (now Lucent) joined to bring their resources to speed development. As a result the new standard was published as IS-95A in 1995 under the auspices of the Cellular Telecommunications Industry Association (CTIA) and the Telecommunications Industry Association (TIA). As part of the development of CDMA an organisation called the CDMA Development Group (CDG) was formed from the founding network and manufacturers. Its purpose is to promote CDMA and evolve the technology and standards, although today most of the standards work is carried out by 3GPP2.

It then took a further three years before Hutchison Telecom became the first organisation to launch a system. The IS95 system was widely deployed in North America, and the Asia Pacific region, but there were also networks in South America, Africa, and the Middle East as well as some in Eastern Europe.

With the success of the initial IS95 format, improvements were made and the standard was upgraded to IS95B. The main improvement was that this provided for an increased data rate of 115 kbps as data traffic was starting to be carried.

The basic CDMA system was later further improved and evolved into a 3G system carrying much higher data rates and introducing new improvements. The 3G migration of IS95 was given the brand name cdma2000, and was available in a variety of flavours including cdma2000 1x, cdma2000 1x ev-do (evolution data only or data optimised) and another version was termed cdma2000 1x ev-dv (evolution data and voice), although this version was never seriously deployed.

CDMA within IS-95

The CDMA or code division multiple access system used for IS-95 is very different to other multiple access schemes used in previous cellular systems. However it offers a number of advantages and as a result has been widely used in many cellular technologies.

Note on CDMA:

CDMA, Code Division Multiple Access, is a multiple access scheme used by many 3G cellular technologies, and other forms of wireless technology. It uses a process called Direct Sequence Spread Spectrum where spreading codes are used to spread a signal out over a given bandwidth and then reconstituting the data in the receiver by using the same spreading code. By supplying different spreading codes to different users, several users are able to utilises the same frequency without mutual interference.

Click on the link for a CDMA tutorial

The advantage of using CDMA over FDMA and TDMA is that it enables a greater number of users to be supported. The improvement in efficiency is hard to define as it depends on many factors including the size of the cells and the level of interference between cells and several other factors.

Unlike the more traditional cellular systems where neighbouring cells use different sets of channels, a CDMA system re-uses the same channels. Signals from other cells will be appear as interference, but the system is able to extract the required signal by using the correct code in the demodulation and signal extraction process. Often more than one channel is used in each cell, and this provides additional capacity because there is a limit to the amount of traffic that can be supported on each channel.

IS95 specification summary

Parameter Details
Multiple Access Scheme CDMA
Channel bandwidth 1.25 MHz
Data rate 14.4 kbps - IS-95A
115 kbps - IS-95B

IS-95 summary

IS-95, cdmaOne was a revolutionary system for cellular telecommunications. It paved the way for the other CDMA based 3G systems that were to follow around the globe. It enabled greater levels of spectrum efficiency to be gained while also allowing many other improvements to be introduced. IS-95 itself evolved into the variety of cdma2000 schemes including 1x, 1x ev-do, and the plans were ultimately to migrate onto a 4G system known as UMB - Ultra-Mobile Broadband. However IS-95 proved to be a winner in many areas of the globe, particularly in the Americas and the Far East.

By Ian Poole

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