CDMA IS-95-A/IS-95-B Data Networks

The success of the wireless industry led to the search for new technologies to increase the capacity of wireless systems without requiring additional spectrum. Code Division Multiple Access, or CDMA, was one such digital technology developed to address this need for capacity. It provides increased spectrum efficiency or, in other words, allows multiple users to share the same radio spectrum more efficiently. The actual quantitative improvement over existing analog systems and over competing digital technologies is still a subject of intense debate.CDMA is a spread spectrum technology, which means that instead of dividing RF spectrum into narrow channels (e.g., 30 KHz each) and assigning one (AMPS) or more (e.g., TIA/EIA/IS-136 TDMA) conversations to each channel, it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal.

A CDMA call compresses a digital 64-Kbps stream into a standard rate of 9600 bps (or 14400 bps). This is then spread to a transmitted rate of about 1.2288 Mbps. Spreading is done by applying digital codes, unique to each user, to the data bits associated with users in a cell. The spread signal is transmitted along with the signals of all the other users in that cell over a 1.25 MHz channel. When the signal is received, the codes are removed from the desired signal, separating the users and returning the call to a rate of 9600 bps (or 14400 bps). This primary channel is called the fundamental channel (FCH).


In the United States, the spectrum is divided into two bands: one for ordinary public mobile telephony service in the 800-MHz frequency band and the other for PCS services in the 1900-MHz frequency band. New digital wireless technologies operating in the 800-MHz band are required to be compatible with AMPS. The radio functionalities for the 800-MHz band are specified in TIA/EIA-95-A and TIA/EIA-95-B and include AMPS compatibility specification. Those for the 1900-MHz band are specified in ANSI J-STD 008. There is essentially no difference in the CDMA-related functionality between the two standards. The service options for wireless data are specified in TIA/EIA/IS-707.

TIA/EIA/IS-95-A was developed primarily to provide increased voice capacity. Support for data was initially limited to circuit-switched (CS) data. CS data requires that a dedicated connection be maintained between the endpoints in the network at all times in the duration of the CS data call. This implies that a dedicated radio channel is maintained even if no data is being transferred. However, for most applications, the data traffic is bursty, and this results in an inefficient utilization of scarce radio spectrum with CS data.