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. 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.

System Features

CDMAhas been adapted for use in cellular communications with the addition of several system features that enhance efficiency and lower costs.

Mobile Station Sign-on On power-on, the mobile station already knows the assigned frequency for CDMAservice in the local area and will tune to that frequency and search for pilot signals. Multiple pilot signals typically will be found, each with a different time offset. This time offset distinguishes one base station from another. The mobile station will pick the strongest pilot and establish a frequency reference and a time reference from that signal. Once the mobile station becomes synchronized with the base station’s system time, it can then register. Registration is the process by which the mobile station tells the system that it is available for calls and notifies the system of its location.

Call Processing The user makes a call by entering the digits on the mobile station keypad and hitting the “Send” button. If multiple mobile stations attempt a link on the access channel at precisely the same moment, a collision occurs. If the base station does not acknowledge the access attempt, the mobile station will wait a random time and try again. On making contact, the base station assigns a traffic channel, whereupon basic information is exchanged, including the mobile station’s serial number. At this point, the conversation mode is started. As a mobile station moves from one cell to the next, another cell’s pilot signal will be detected that is strong enough for it to use. The mobile station will then request a “soft handoff,” during which it is actually receiving both signals via different correlative elements in the receiver circuitry. Eventually, the signal from the first cell will diminish, and the mobile station will request from the second cell that the soft handoff be terminated. Abase station does not hand off the call to another base station until it detects acceptable signal strength. This soft handoff technique is a significant improvement over the handoff procedure used in analog FM cellular systems, where the communication link with the old cell site is momentarily disconnected before the link to the new site is established. For a short time, the mobile station is not connected to either cell site, during which the subscriber hears background noise or nothing at all. Sometimes the mobile stations Ping-Pong between two cell sites as the links are handed back and forth between the approaching and retreating cell sites. Other times, the calls are simply dropped. Because a mobile station in the CDMAsystem has more than one modulator, it can communicate with multiple cells simultaneously to implement the soft handoff. At the end of a call placed over the CDMA system, the channel will be freed and may be reused. When the mobile station is turned off, it will generate a power-down registration signal that tells the system that it is no longer available for incoming calls.

Voice Detection and Encoding With voice activity detection, the transmitter is activated only when the user is speaking. This reduces interference levels—and, consequently, the amount of bandwidth consumed—when the user is not speaking. Through interference averaging, the capacity of the system is increased. This allows systems to be designed for the average rather than the worst interference case. However, the IS-95 CDMAstandard requires that no interfering signal be received that is significantly stronger than the desired signal, since it would then jam the weaker signal. This has been called the “near-far problem” and means that high cell capacity does not necessarily translate into high overall system capacity.

The speech coder used in CDMAoperates at a variable rate. When the subscriber is talking, the speech coder operates at the full rate; when the subscriber is not talking, the speech coder operates at only one-eighth the full rate. Two intermediate rates are also defined to capture the transitions and eliminate the effect of sudden rate changes. Since the variable-rate operation of the speech coder reduces the average bit rate of the conversations, system capacity is increased.

Privacy Increased privacy is inherent in CDMAtechnology . CDMAphone calls will be secure from the casual eavesdropper because, unlike a conversation carried over an analog system, a simple radio receiver will not be able to pick out individual digital conversations from the overall RF radiation in a frequency band.

ACDMAcall starts with a standard rate of 9.6 kbps. This is then spread to a transmitted rate of about 1.25 Mbps. “Spreading” means that digital codes are applied to the data bits associated with users in a cell. These data bits are transmitted along with the signals of all the other users in that cell. When the signal is received, the codes are removed from the desired signal, separating the users and returning the call to the original rate of 9.6 kbps.

Because of the wide bandwidth of a spread-spectrum signal, it is very difficult to identify individual conversations for eavesdropping. 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 analog technologies, the power of the signal is concentrated in a narrower band, which makes it easier to detect with a radio receiver tuned to that set of frequencies.

The use of wideband spread-spectrum signals also offers more protection against cloning, an illegal practice whereby a mobile phone’s electronic serial number is taken over the air and programmed into another phone. All calls made from a cloned phone are “free” because they are billed to the original subscriber.

Power Control CDMA systems rely on strict control of power at the mobile station to overcome the so-called near-far problem. If the signal from a near mobile station is received at the cell site receiver with too much power, the cell site receiver will become overloaded and prevent it from picking up the signals from mobile stations located farther away. The goal of CDMA is to have the signals of all mobile stations arrive at the base station with exactly the same power level. The closer the mobile station is to the cell site receiver, the lower is the power necessary for transmission; the farther away the mobile station, the greater is the power necessary for transmission.

Two forms of adaptive power control are employed in CDMAsystems: open loop and closed loop. Open-loop power control is based on the similarity of loss in the forward and reverse paths. The received power at the mobile station is used as a reference. If it is low, the mobile station is assumed to be far from the base station and transmits with high power. If it is high, the mobile station is assumed to be near the base station and transmits with low power. The sum of the two power levels is a constant.

Closed-loop power control is used to force the power from the mobile station to deviate from the open-loop setting. This is achieved by an active feedback system from the base station to the mobile station. Power control bits are sent every 1.25 millisecond (ms) to direct the mobile station to increase or decrease its transmitted power by 1 decibel (dB). Lack of power control to at least this accuracy greatly reduces the capacity of CDMAsystems.

With these adaptive power-control techniques, the mobile station transmits only enough power to maintain a link. This results in an average power requirement that is much lower than that for analog systems, which do not usually employ such techniques. CDMA’s lower power requirement translates into smaller, lightweight, longer-life batteries—approximately 5 hours of talk time and over 2 days of standby time—and makes possible smaller, lower-cost hand-held computers and hybrid computer-communications devices. CDMAphones can easily weigh in at less than 8 ounces.

Spatial Diversity Among the various forms of diversity is that of spatial diversity, which is employed in CDMA, as well as in other multiple access techniques, including FDMAand TDMA. Spatial diversity helps to maintain the signal during the call handoff process when a user moves from one cell to the next. This process entails antennas in two different cell sites maintaining links with one mobile station. The mobile station has multiple correlative receiver elements that are assigned to each incoming signal and can add these.

CDMAuses at least four of these correlators: three that can be assigned to the link and one that searches for alternate paths. The cell sites send the received data, along with a quality index, to the MTSO, where a choice is made regarding the better of the two signals.

Not all these features are unique to CDMA; some can be exploited by TDMA-based systems as well, such as spatial diversity and power control. These already exist in all TDMA standards today, while soft handoff is implemented in the European Digital Enhanced Cordless Telecommunications (DECT) standard, which is based on TDMA.

Summary

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, CDMAsuffered 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.