Introduction to CDMA

Code Division Multiple Access (CDMA) is a radically new concept in wireless communications. It has gained widespread international acceptance by cellular radio system operators as an upgrade that will dramatically increase both their system capacity and the service quality. It has likewise been chosen for deployment by the majority of the winners of the United States Personal Communications System spectrum auctions. It may seem, however, mysterious for those who aren't familiar with it. This site is provided in an effort to dispel some of the mystery and to disseminate at least a basic level of knowledge about the technology.

CDMA is a form of spread-spectrum, a family of digital communication techniques that have been used in military applications for many years. The core principle of spread spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths much wider than that required for simple point-to-point communication at the same data rate. Originally there were two motivations: either to resist enemy efforts to jam the communications (anti-jam, or AJ), or to hide the fact that communication was even taking place, sometimes called low probability of intercept (LPI). It has a history that goes back to the early days of World War II.

The use of CDMA for civilian mobile radio applications is novel. It was proposed theoretically in the late 1940's, but the practical application in the civilian marketplace did not take place until 40 years later. Commercial applications became possible because of two evolutionary developments. One was the availability of very low cost, high density digital integrated circuits, which reduce the size, weight, and cost of the subscriber stations to an acceptably low level. The other was the realization that optimal multiple access communication requires that all user stations regulate their transmitter powers to the lowest that will achieve adequate signal quality.

CDMA changes the nature of the subscriber station from a predominately analog device to a predominately digital device. Old-fashioned radio receivers separate stations or channels by filtering in the frequency domain. CDMA receivers do not eliminate analog processing entirely, but they separate communication channels by means of a pseudo-random modulation that is applied and removed in the digital domain, not on the basis of frequency. Multiple users occupy the same frequency band. This universal frequency reuse is not fortuitous. On the contrary, it is crucial to the very high spectral efficiency that is the hallmark of CDMA. Other discussions in these pages show why this is true.

What is CDMA (Code Division Multiple Access)?

One of the most important concepts to any cellular telephone system is that of "multiple access", meaning that multiple, simultaneous users can be supported. In other words, a large number of users share a common pool of radio channels and any user can gain access to any channel (each user is not always assigned to the same channel). A channel can be thought of as merely a portion of the limited radio resource which is temporarily allocated for a specific purpose, such as someone's phone call. A multiple access method is a definition of how the radio spectrum is divided into channels and how channels are allocated to the many users of the system.

CDMA is altering the face of cellular and PCS communication by:

Current Cellular Standards

Different types of cellular systems employ various methods of multiple access. The traditional analog cellular systems, such as those based on the Advanced Mobile Phone Service (AMPS) and Total Access Communications System (TACS) standards, use Frequency Division Multiple Access (FDMA). FDMA channels are defined by a range of radio frequencies, usually expressed in a number of kilohertz (kHz), out of the radio spectrum.

For example, AMPS systems use 30 kHz "slices" of spectrum for each channel. Narrowband AMPS (NAMPS) requires only 10 kHz per channel. TACS channels are 25 kHz wide. With FDMA, only one subscriber at a time is assigned to a 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.

A common multiple access method employed in new digital cellular systems is the Time Division Multiple Access (TDMA). TDMA digital standards include North American Digital Cellular (know by its standard number IS-54), Global System for Mobile Communications (GSM), and Personal Digital Cellular (PDC).

TDMA systems commonly start with a slice of spectrum, referred to as one "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, IS-54 systems, designed to coexist with AMPS systems, divide 30 kHz of spectrum into three channels. PDC divides 25 kHz slices of spectrum into three channels. GSM systems create 8 time-division channels in 200 kHz wide carriers.

The CDMA Cellular Standard

With CDMA, unique digital codes, rather than separate RF frequencies or channels, are used to differentiate subscribers. The codes are shared by both the mobile station (cellular phone) and the base station, and are called "pseudo-Random Code Sequences." All users share the same range of radio spectrum.

For cellular telephony, CDMA is a digital multiple access technique specified by the Telecommunications Industry Association (TIA) as "IS-95."

In March 1992, the TIA established the TR-45.5 subcommittee with the charter of developing a spread-spectrum digital cellular standard. In July of 1993, the TIA gave its approval of the CDMA IS-95 standard.

IS-95 systems divide the radio spectrum into carriers which are 1,250 kHz (1.25 MHz) wide. One of the unique aspects of CDMA is that while there are certainly limits to the number of phone calls that can be handled by a carrier, this is not a fixed number. Rather, the capacity of the system will be dependent on a number of different factors. This will be discussed in later sections.

CDMA Technology

Though CDMA's application in cellular telephony is relatively new, it is not a new technology. CDMA has been used in many military applications, such as anti-jamming (because of the spread signal, it is difficult to jam or interfere with a CDMA signal), ranging (measuring the distance of the transmission to know when it will be received), and secure communications (the spread spectrum signal is very hard to detect).

Spread Spectrum


CDMA is a "spread spectrum" technology, which means that it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal.

A CDMA call starts with a standard rate of 9600 bits per second (9.6 kilobits per second). This is then spread to a transmitted rate of about 1.23 Megabits per second. 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 a rate of 9600 bps.

Traditional uses of spread spectrum are in military operations. Because of the wide bandwidth of a spread spectrum signal, it is very difficult to jam, difficult to interfere with, and difficult to identify. This is in contrast to technologies using a narrower bandwidth of frequencies. 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 other technologies, the power of the signal is concentrated in a narrower band, which makes it easier to detect.

Increased privacy is inherent in CDMA technology. CDMA phone calls will be secure from the casual eavesdropper since, unlike an analog conversation, a simple radio receiver will not be able to pick individual digital conversations out of the overall RF radiation in a frequency band.


In the final stages of the encoding of the radio link from the base station to the mobile, CDMA adds a special "pseudo-random code" to the signal that repeats itself after a finite amount of time. Base stations in the system distinguish themselves from each other by transmitting different portions of the code at a given time. In other words, the base stations transmit time offset versions of the same pseudo-random code. In order to assure that the time offsets used remain unique from each other, CDMA stations must remain synchronized to a common time reference.

The Global Positioning System (GPS) provides this precise common time reference. GPS is a satellite based, radio navigation system capable of providing a practical and affordable means of determining continuous position, velocity, and time to an unlimited number of users.

"The Balancing Act"

CDMA cell coverage is dependent upon the way the system is designed. In fact, three primary system characteristics-Coverage, Quality, and Capacity-must be balanced off of each other to arrive at the desired level of system performance.

In a CDMA system these three characteristics are tightly inter-related. Even higher capacity might be achieved through some degree of degradation in coverage and/or quality. Since these parameters are all intertwined, operators cannot have the best of all worlds: three times wider coverage, 40 times capacity, and "CD" quality sound. For example, the 13 kbps vocoder provides better sound quality, but reduces system capacity as compared to an 8 kbps vocoder.

CDMA Benefits


When implemented in a cellular telephone system, CDMA technology offers numerous benefits to the cellular operators and their subscribers. The following is an overview of the benefits of CDMA.

  1. Capacity increases of 8 to 10 times that of an AMPS analog system and 4 to 5 times that of a GSM system
  2. Improved call quality, with better and more consistent sound as compared to AMPS systems
  3. Simplified system planning through the use of the same frequency in every sector of every cell
  4. Enhanced privacy
  5. Improved coverage characteristics, allowing for the possibility of fewer cell sites
  6. Increased talk time for portables
  7. Bandwidth on demand


Fiber Optic Data Communications

Fiber optic technology is predominant in the infrastructure of data communications networks.   Since AT&T and GTE installed the first fiber optic communication system in 1977, the obvious advantages of optical fibers over to copper cables led to their almost exclusive use in the physical layers (OSI-Layer 1) of wide area (WAN) and metropolitan area networks (MAN). The fast paced development of local area networks (LAN), triggered by such concepts as fiber to the desk, will clearly move in the same direction.

The Principle of Fiber

Fiber optic signals are light pulses transmitted through a thin strand of glass from one point to another. The total reflection of the light beam caused by the difference in the refractive index (density) of the fiber core and cladding keeps the light wave contained in the fiber. In principle there is no difference between copper based and optical data transmission systems. The encoded data is converted from electrical signals to optical light pulses, sent through the medium (in our case optical fiber cables), converted back and finally decoded for further processing. In short, optical fiber is to light as copper wire is to electricity.


  Light wave the fiber
fiber.gif (2519 bytes)

The Limits of Fiber Optic Data Communications

The one problem with optical fibers is that there are never enough of them. Private networks and service providers are constantly dealing with either a limited physical resource or the high costs of installing more fiber. The limiting factor in fiber optic networks is therefore not the bandwidth of the fiber lines, but the ability needed to transmit the various new high speed applications. Considering that the rational behind all data- and telecommunication networking is economics, this obvious limitation will cause serious concern.  

Optical Fiber

     Optical fibers pack tremendous benefits
    such as:

  • Information carrying capacity reaching into the terabyte range
  • Transmission distance
  • Small size and weight
  • EM profile has no influence
  • Data security 



The Technology of  Wavelength Division Multiplexing

In general there are two forms of multiplexing: wavelength division multiplexing (WDM) and time division multiplexing (TDM). TDM has been the most common method to bring together many lower speed applications over one common higher speed service. TDM creates high speed time slots in the form of frames or cells that allow multiple applications to share the channel. This solution works well when all applications are of the same platform, such as ATM. However, it is not the optimum solution for increasing network bandwidth for all environments.

Wavelength division multiplexing (WDM) is quickly becoming a critical technology for many high speed communication systems. WDM utilizes a similar principle to TDM, except the channel discriminator is a wavelength, instead of time.
  The operation of a WDM system begins with the conversion of each input data stream into separate wavelengths (colors). In the case of optical communication the wavelengths are grouped in transmission windows around 850,1300 and 1500 nm (the available hardware for optical communications is typically centered around these wavelengths). Each application creates a channel that operates at a separate wavelength. The WDM system then combines and simultaneously transmits the channels through the same optical fiber. Since each wavelength is completely isolated from the other, protocols can be mixed within the same link. The combined signals are then separated by the WDM at the other site and converted back to their original wavelength. Essentially, WDM systems create multiple "virtual fiber pairs" from one.

    Since light of different wavelengths do not interfere with each other, multiple wavelength signals can be transmitted through the same optical fiber without error. WDM begins to capture the true bandwidth potential of fiber optics by allowing multiple high speed communication applications to simultaneously share the same fiber. The telecommunications industry has been heavily investing in wavelength division multiplexing technologies. Industry standards are now created around high speed WDM systems. A variety of 4, 8 and even 16 channel systems are now commercially available with 10, 20 and 40 Gbps transmission speeds, respectively. While these represent excellent capacity expansion alternatives for long distance central office telecommunications, they ignore the speed and cost requirements of the data communications applications and metropolitan area networks (MAN).