25-08-2017, 09:32 PM
CDMA TECHNOLOGY
[attachment=42843]
What is CDMA?
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 temporary 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 a "spread spectrum" technique in which each phone in a cell uses a distinct code known to the base station to communicate with the base station. All frequencies in one cell can be used in other cells.
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.
Current Cellular Standard
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 Time Division Multiple Access (TDMA). TDMA digital standards include North American Digital Cellular (known 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.
Multiple Access Comparison
It is easier to understand CDMA if it is compared with other multiple access technologies. The following sections describe the fundamental differences between a Frequency Division Multiple Access Analog technology (FDMA), a Time Division Multiple Access Digital technology (TDMA) and a Code Division Multiple Access Digital technology (CDMA).
FDMA - Frequency Division Multiple Access
FDMA is used for standard analog cellular. Each user is assigned a discrete slice of the RF spectrum. FDMA permits only one user per channel since it allows the user to use the channel 100% of the time. Therefore, only the frequency "dimension" is used to define channels.
TDMA - Time Division Multiple Access
The key point to make about TDMA is that users are still assigned a discrete slice of RF spectrum, but multiple users now share that RF carrier on a time slot basis. Each of the users alternate their use of the RF channel. Frequency division is still employed, but these carriers are now further sub-divided into some number of time slots per carrier.
A user is assigned a particular time slot in a carrier and can only send or receive information at those times. This is true whether or not the other time slots are being used. Information flow is not continuous for any user, but rather is sent and received in "bursts." The bursts are re-assembled at the receiving end, and appear to provide continuous sound because the process is very fast.
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.
The standard data rate of a CDMA call is 9600 bits per second (9.6 kilobits per second). This initial data is "spread," including the application of digital codes to the data bits, up to the transmitted rate of about 1.23 megabits per second. The data bits of each call are then transmitted in combination with the data bits of all of the calls in the cell. At the receiving end, the digital codes are separated out, leaving only the original information which was to be communicated. At that point, each call is once again a unique data stream with a rate of 9600 bits per second.
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.
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 primary source of the very precise synchronization signals required by CDMA systems is the Global Positioning System (GPS). GPS is a radio navigation system based on a constellation of orbiting satellites. Since the GPS system covers the entire surface of the earth, it provides a readily available method for determining position and time to as many receivers as are required.
[attachment=42843]
What is CDMA?
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 temporary 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 a "spread spectrum" technique in which each phone in a cell uses a distinct code known to the base station to communicate with the base station. All frequencies in one cell can be used in other cells.
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.
Current Cellular Standard
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 Time Division Multiple Access (TDMA). TDMA digital standards include North American Digital Cellular (known 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.
Multiple Access Comparison
It is easier to understand CDMA if it is compared with other multiple access technologies. The following sections describe the fundamental differences between a Frequency Division Multiple Access Analog technology (FDMA), a Time Division Multiple Access Digital technology (TDMA) and a Code Division Multiple Access Digital technology (CDMA).
FDMA - Frequency Division Multiple Access
FDMA is used for standard analog cellular. Each user is assigned a discrete slice of the RF spectrum. FDMA permits only one user per channel since it allows the user to use the channel 100% of the time. Therefore, only the frequency "dimension" is used to define channels.
TDMA - Time Division Multiple Access
The key point to make about TDMA is that users are still assigned a discrete slice of RF spectrum, but multiple users now share that RF carrier on a time slot basis. Each of the users alternate their use of the RF channel. Frequency division is still employed, but these carriers are now further sub-divided into some number of time slots per carrier.
A user is assigned a particular time slot in a carrier and can only send or receive information at those times. This is true whether or not the other time slots are being used. Information flow is not continuous for any user, but rather is sent and received in "bursts." The bursts are re-assembled at the receiving end, and appear to provide continuous sound because the process is very fast.
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.
The standard data rate of a CDMA call is 9600 bits per second (9.6 kilobits per second). This initial data is "spread," including the application of digital codes to the data bits, up to the transmitted rate of about 1.23 megabits per second. The data bits of each call are then transmitted in combination with the data bits of all of the calls in the cell. At the receiving end, the digital codes are separated out, leaving only the original information which was to be communicated. At that point, each call is once again a unique data stream with a rate of 9600 bits per second.
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.
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 primary source of the very precise synchronization signals required by CDMA systems is the Global Positioning System (GPS). GPS is a radio navigation system based on a constellation of orbiting satellites. Since the GPS system covers the entire surface of the earth, it provides a readily available method for determining position and time to as many receivers as are required.