Code Division Multiple Access has many variants as well. InterDigital (external link), for example, produces a broadband CDMA system called B-CDMA that is different from Qualcomm's (external link)narrowband CDMA system. In the coming years wideband may dominate. But narrowband CDMA right now is dominant in the United States, used with the operating system IS-95. I should repeat here what I wrote at the start of this article. I know some of this is advanced and sounds like gibberish, but bear with me or skip ahead two paragraphs:
Systems built on time division multiplexing will gradually be replaced with other access technologies. CDMA is the future of digital cellular radio. Time division systems are now being regarded as legacy technologies, older methods that must be accommodated in the future, but ones which are not the future itself. (Time division duplexing, as used in cordless telephone schemes: DECT and Personal Handy Phone systems might have a place but this still isn't clear.) Right now all digital cellular radio systems are second generation, prioritizing on voice traffic, circuit switching, and slow data transfer speeds. 3G, while still delivering voice, will emphasize data, packet switching, and high speed access.
Over the years, in stages hard to follow, often with 2G and 3G techniques co-existing, TDMA based GSM and AT&T's IS-136 cellular service will be replaced with a wideband CDMA system, the much hoped for Universal Mobile Telephone System (external link). Strangely, IS-136 will first be replaced by GSM before going to UMTS. Technologies like EDGE and GPRS(Nokia white paper) will extend the life of these present TDMA systems but eventually new infrastructure and new spectrum will allow CDMA/UMTS development. The present CDMA system, IS-95, which Qualcomm supports and the Sprint PCS network uses, is narrowband CDMA. In the Ericsson/Qualcomm view of the future, IS-95 will also go to wideband CDMA.
Excellent writing on this transition period from 2G to 3G and beyond is in this printable .pdf file, a chapter from The Essential Guide to Wireless Communications Applications by Andy Dornan. Many good charts. (454K, 21 pages in .pdf)
Whew! Where we were we? Back to code division multiple access. A CDMA system assigns a specific digital code to each user or mobile on the system. It then encodes each bit of information transmitted from each user. These codes are so specific that dozens of users can transmit simultaneously on the same frequency without interference to each other, indeed, there is no need for adjacent cell sites to use different frequencies as in AMPS and TDMA. Every cell site can transmit on every frequency available to the wireline or non-wireline carrier.
CDMA is less prone to interference than AMPS or TDMA. That's because the specificity of the coded signals helps a CDMA system treat other radio signals and interference as irrelevant noise. Some of the details of CDMA are also interesting. Before we get to them, let's stop here and review, because it is hard to think of the big picture, the overall subject of cellular radio, when we get involved in details.
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