Реферат: Что является CDMA (Разделение Кодекса Многократный Доступ)?

What is CDMA (Code Division Multiple Access)?

One of the most important concepts to any cellular telephonesystem is that of «multiple access», meaning that multiple,

simultaneous users can be supported. In other words, a largenumber of users share a common pool of radio channels and any

user can gain access to any channel (each user is not alwaysassigned to the same channel). A channel can be thought of as

merely a portion of the limited radio resource which istemporarily allocated for a specific purpose, such as someone's phone call.

A multiple access method is a definition of how the radiospectrum is divided into channels and how channels are allocated to the

many users of the system.

                                   CurrentCellular Standards

Different types of cellular systems employ various methodsof multiple access. The traditional analog cellular systems, such as

those based on the Advanced Mobile Phone Service (AMPS) andTotal Access Communications System (TACS) standards,

use Frequency Division Multiple Access (FDMA). FDMA channelsare defined by a range of radio frequencies, usually

expressed in a number of kilohertz (kHz), out of the radiospectrum. For example, AMPS systems use 30 kHz «slices» of

spectrum for each channel. NarrowbandAMPS (NAMPS) requires only 10 kHz per channel. TACS channels are 25 kHz wide.

With FDMA, only one subscriber at a time is assigned to achannel. No other conversations can access this channel until the

subscriber's call is finished, or until that original callis handed off to a different channel by the system. A common multiple access

method employed in new digital cellular systems is the TimeDivision Multiple Access (TDMA). TDMA digital standards include

North American Digital Cellular (know by its standard numberIS-54), Global System for Mobile Communications (GSM), and

Personal Digital Cellular (PDC). TDMA systems commonly startwith a slice of spectrum, referred to as one «carrier». Each

carrier is then divided into time slots. Only one subscriberat a time is assigned to each time slot, or channel. No other

conversations can access this channel until the subscriber'scall is finished, or until that original call is handed off to a different

channel by the system. For example, IS-54 systems, designedto coexist with AMPS systems, divide 30 kHz of spectrum into

three channels. PDC divides 25 kHz slices of spectrum intothree channels. GSM systems create 8 time-division channels in 200

kHz wide carriers.

                                  The CDMACellular Standard

With CDMA, unique digital codes, rather than separate RFfrequencies 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-RandomCode

Sequences.» All users share the same range of radiospectrum. For cellular telephony, CDMA is a digital multiple access

technique specified by the Telecommunications IndustryAssociation (TIA) as «IS-95.» In March 1992, the TIA established the

TR-45.5 subcommittee with the charter of developing aspread-spectrum digital cellular standard. In July of 1993, the TIA gave

its approval of the CDMA IS-95 standard. IS-95 systemsdivide the radio spectrum into carriers which are 1,250 kHz (1.25

MHz) wide. One of the unique aspects of CDMA is that whilethere 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.


Though CDMA's application incellular telephony is relatively new, it is not a new technology. CDMA has beenused in many

military applications, such as anti-jamming (because of thespread signal, it is difficult to jam or interfere with a CDMA signal),

ranging (measuring the distance of the transmission to knowwhen it will be received), and secure communications (the spread

spectrum signal is very hard to detect).


CDMA is a «spread spectrum» technology, whichmeans that it spreads the information contained in a particular signal ofinterest

over a much greater bandwidth than the original signal. ACDMA call starts with a standard rate of 9600 bits per second (9.6

kilobits per second). This is then spread to a transmittedrate of about 1.23 Megabits per second. Spreading means that digital

codes are applied to the data bits associated with users ina cell. These data bits are transmitted along with the signals of all the

other users in that cell. When the signal is received, thecodes 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 thewide

bandwidth of a spread spectrum signal, it is very difficultto jam, difficult to interfere with, and difficult to identify. This is in

contrast to technologies using a narrower bandwidth offrequencies. 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, thepower of

the signal is concentrated in a narrower band, which makesit easier to detect. Increased privacy is inherent in CDMA

technology. CDMA phone calls will be secure from the casualeavesdropper since, unlike an analog conversation, a simple radio

receiver will not be able to pick individual digitalconversations out of the overall RF radiation in a frequency band.


In the final stages of the encoding of the radio link fromthe base station to the mobile, CDMA adds a special «pseudo-random

code» to the signal that repeats itself after a finiteamount of time. Base stations in the system distinguish themselves from each

other by transmitting different portions of the code at agiven time. In other words, the base stations transmit time offset versions

of the same pseudo-random code. In order to assure that thetime offsets used remain unique from each other, CDMA stations

must remain synchronized to a common time reference. TheGlobal Positioning System (GPS) provides this precise common time

reference. GPS is a satellite based, radio navigation systemcapable of providing a practical and affordable means of determining

continuous position, velocity, and time to an unlimitednumber of users.

                                      «TheBalancing Act»

CDMA cell coverage is dependent upon the way the system isdesigned. In fact, three primary system characteristics-Coverage,

Quality, and Capacity-must be balanced off of each other toarrive at the desired level of system performance. In a CDMA

system these three characteristics are tightly inter-related. Even higher capacity might be achievedthrough some degree of

degradation in coverage and/or quality. Since theseparameters 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 kbpsvocoder provides better

sound quality, but reduces system capacity as compared to an8 kbps vocoder.


When implemented in a cellular telephone system, CDMAtechnology offers numerous benefits to the cellular operators and their

subscribers. The following is an overview of the benefits ofCDMA. 1.Capacity increases of 8 to 10 times that of an AMPS

analog system and 4 to 5 times that of a GSM system2.Improved call quality, with better and more consistent sound as

compared to AMPS systems 3.Simplified system planningthrough 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

                                  Click belowfor more information about:

Spread Spectrum Techniques

Spreading Codes

Power Control


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

еще рефераты
Еще работы по компьютерным сетям