Code Division Multiple Access

Code Division Multiple Access (CDMA) is a spread-spectrum technology that is used for implementing cellular telephone service. Spread spectrum is a family of digital communication techniques originally used in military communications and control applications.

Spread spectrum uses carrier waves that consume a much wider bandwidth than that required for simple point-to-point communication at the same data rate. This results in the carrier wave looking more like random noise than real communication between a sender and receiver.

Originally, there were two motivations for implementing spread spectrum: to resist enemy efforts to jam vital communications and to hide the fact that communication was even taking place. For cellular telephony, spread-spectrum technology underlies CDMA, which is a digital multiple access technique specified by the Telecommunications Industry Association (TIA) as IS-95.

Commercial applications of CDMAbecame possible because of two key developments. One was the availability of low-cost, high-density digital integrated circuits, which reduce the size, weight, and cost of the mobile phones. The other was the realization that optimal multiple access communication depends on the ability of all mobile phones to regulate their transmitter power to the lowest level that will achieve adequate signal quality.

CDMAchanges the nature of the mobile phone from a predominately analog device to a predominately digital device. CDMAreceivers do not eliminate analog processing entirely, but they separate communication channels by means of a pseudorandom modulation that is applied and removed in the digital domain, not on the basis of frequency. This allows multiple users to occupy the same frequency band; this frequency reuse results in high spectral efficiency.

TDMAsystems commonly start with a slice of spectrum, referred to as a “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, TDMAsystems, designed to coexist with AMPS systems, divide 30 kHz of spectrum into three channels. By comparison, GSM systems create eight timedivision channels in 200-kHz-wide carriers.

Wideband Usage

With CDMAsystems, multiple conversations simultaneously share the available spectrum in both the time and frequency dimensions. The available spectrum is not “channelized” in frequency or time as in Frequency Division Multiple Access (FDMA) or TDMA systems, respectively. Instead, the individual conversations are distinguished through coding; that is, at the transmitter, each conversation is processed with a unique spreading code that is used to distribute the signal over the available bandwidth.

The receiver uses the unique code to accept the energy associated with a particular code. The other signals present are each identified by a different code and simply produce background noise. In this way, many conversations can be carried simultaneously within the same block of spectrum. The following analogy is used commonly to explain how CDMAtechnology works.

Four speakers are simultaneously giving a presentation, and they each speak a different native language: Spanish, Korean, English, and Chinese (Figure C- 6). If English is your native language, you only understand the words of the English speaker and tune out the Spanish, Korean, and Chinese speakers. You hear only what you know and recognize. The rest sounds like background noise. The same is true for CDMA. Each conversation is specially encoded and decoded for a particular user.

Multiple users share the same frequency band at the same time, yet each user hears only the conversation he or she can interpret. CDMAassigns each subscriber a unique code to put multiple users on the same wideband channel at the same time. These codes are used to distinguish between the various conversations. The result of this access method is increased callhandling capacity.

One of the unique aspects of CDMAis that while there are ultimate limits to the number of phone calls that a system can handle, this is not a fixed number. Rather, the capacity of the system depends on how coverage, quality, and capacity are balanced to arrive at the desired level of system performance.

Since these parameters are tightly intertwined, operators cannot have the best of all worlds: 3 times wider coverage, 40 times capacity, and high-quality sound. For example, the 13-kbps vocoder provides better sound quality but reduces system capacity compared with an 8-kbps vocoder. Higher capacity might be achieved through some degree of degradation in coverage and/or quality.

There are still conflicting performance claims for TDMAand CDMA. Since both TDMAand CDMAhave become TIAstandards— IS-54 and IS-95, respectively—vendors are now aiming their full marketing efforts toward the cellular carriers. Proponents of each technology have the research to back up their claims of superior performance.

Of the two, CDMA suffered a credibility problem early on because its advocates made grandiose performance claims for CDMAthat could not be verified in the real-world operating environment. In some circles, this credibility problem lingers today. Of note, however, is that both technologies have been successful in the marketplace, each having been selected by many cellular carriers around the world.

Both are capable of supporting emerging PCS networks and providing such services as wireless Internet access, Short messaging Service, voice mail, facsimile, paging, and video. Although TDMA-based Global System for Mobile (GSM) telecommunications is the dominant standard in the global wireless market, the use of CDMAis growing rapidly.

GSM’s head start in the market gives it a much larger presence and practically guarantees that GSM will continue to lead the digital cellular market for the next 5 years.