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.
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.
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
Forward CDMA Channel
Reverse CDMA Channel
Long Code Mask
Linear Feedback Shift Registers
Coverage-Capacity Tradeoff in the Reverse Link
Optimum Bandwidth for CDMA