What is CDMA (Code Division Multiple Access)?

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

Current Cellular Standards

Different types of cellular systems employ various methods of multipleaccess. The traditional analog cellular systems, such asthose 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 bya range of radio frequencies, usuallyexpressed in a number of kilohertz (kHz), out of the radio spectrum. Forexample, AMPS systems use 30 kHz "slices" ofspectrum for each channel. Narrowband AMPS (NAMPS) requires only 10 kHz perchannel. TACS channels are 25 kHz wide.
With FDMA, only one subscriber at a time is assigned to a channel. No otherconversations can access this channel until thesubscriber's call is finished, or until that original call is handed off toa different channel by the system. A common multiple accessmethod 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 sliceof spectrum, referred to as one "carrier". Eachcarrier is then divided into time slots. Only one subscriber at a time isassigned to each time slot, or channel. No otherconversations can access this channel until the subscriber's call isfinished, or until that original call is handed off to a differentchannel by the system. For example, IS-54 systems, designed to coexist with
AMPS systems, divide 30 kHz of spectrum intothree channels. PDC divides 25 kHz slices of spectrum into three channels.
GSM systems create 8 time-division channels in 200kHz wide carriers.

The CDMA Cellular Standard

With CDMA, unique digital codes, rather than separate RF frequencies orchannels, are used to differentiate subscribers. Thecodes are shared by both the mobile station (cellular phone) and the basestation, and are called "pseudo-Random Code
Sequences. "All users share the same range of radio spectrum. For cellulartelephony, CDMA is a digital multiple accesstechnique 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-spectrumdigital cellular standard. In July of 1993, the TIA gaveits approval of the CDMA IS-95 standard. IS-95 systems divide the radiospectrum into carriers which are 1,250 kHz (1.25
MHz) wide. One of the unique aspects of CDMA is that while there arecertainly limits to the number of phone calls that can behandled by a carrier, this is not a fixed number. Rather, the capacity ofthe system will be dependent on a number of differentfactors. This will be discussed in later sections.

CDMA Technology

Though CDMA's application in cellular telephony is relatively new, it isnot a new technology. CDMA has been used in manymilitary 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 bereceived), and secure communications (the spreadspectrum signal is very hard to detect).

Spread Spectrum

CDMA is a "spread spectrum" technology, which means that it spreads theinformation contained in a particular signal of interestover a much greater bandwidth than the original signal. A CDMA call startswith a standard rate of 9600 bits per second (9.6kilobits per second). This is then spread to a transmitted rate of about
1.23 Megabits per second. Spreading means that digitalcodes are applied to the data bits associated with users in a cell. Thesedata bits are transmitted along with the signals of all theother users in that cell. When the signal is received, the codes areremoved from the desired signal, separating the users andreturning the call to a rate of 9600 bps. Traditional uses of spreadspectrum are in military operations. Because of the widebandwidth of a spread spectrum signal, it is very difficult to jam,difficult to interfere with, and difficult to identify. This is incontrast to technologies using a narrower bandwidth of frequencies. Since awideband spread spectrum signal is very hard todetect, it appears as nothing more than a slight rise in the "noise floor"or interference level. With other technologies, the power ofthe signal is concentrated in a narrower band, which makes it easier todetect. Increased privacy is inherent in CDMAtechnology. CDMA phone calls will be secure from the casual eavesdroppersince, unlike an analog conversation, a simple radioreceiver will not be able to pick individual digital conversations out ofthe overall RF radiation in a frequency band.

Synchronization

In the final stages of the encoding of the radio link from the base stationto the mobile, CDMA adds a special "pseudo-randomcode "to the signal that repeats itself after a finite amount of time. Basestations in the system distinguish themselves from eachother by transmitting different portions of the code at a given time. Inother words, the base stations transmit time offset versionsof the same pseudo-random code. In order to assure that the time offsetsused remain unique from each other, CDMA stationsmust remain synchronized to a common time reference. The Global Positioning
System (GPS) provides this precise common timereference. GPS is a satellite based, radio navigation system capable ofproviding a practical and affordable means of determiningcontinuous 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. Infact, three primary system characteristics-Coverage,
Quality, and Capacity-must be balanced off of each other to arrive at thedesired level of system performance. In a CDMAsystem these three characteristics are tightly inter-related. Even highercapacity might be achieved through some degree ofdegradation in coverage and/or quality. Since these parameters are allintertwined, operators cannot have the best of all worlds:three times wider coverage, 40 times capacity, and "CD" quality sound. Forexample, the 13 kbps vocoder provides bettersound quality, but reduces system capacity as compared to an 8 kbpsvocoder.

CDMA Benefits

When implemented in a cellular telephone system, CDMA technology offersnumerous benefits to the cellular operators and theirsubscribers. The following is an overview of the benefits of CDMA.
1.Capacity increases of 8 to 10 times that of an AMPSanalog system and 4 to 5 times that of a GSM system 2.Improved callquality, with better and more consistent sound ascompared to AMPS systems 3.Simplified system planning through the use ofthe same frequency in every sector of every cell
4.Enhanced privacy 5.Improved coverage characteristics, allowing for thepossibility of fewer cell sites 6.Increased talk time forportables 7.Bandwidth on demand

Click below for more information about:

Spread Spectrum Techniques
Spreading Codes
Power Control
Handoff
Forward CDMA Channel
Frequency Plans
Walsh Functions
Short Code
Reverse CDMA Channel
Long Code
Long Code Mask
Reverse Spreading
Linear Feedback Shift Registers
System Time
Forward Spreading
Coverage-Capacity Tradeoff in the Reverse Link
Spreading Bandwidth
Optimum Bandwidth for CDMA