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

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

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.

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.

Synchronization

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

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