6. Sharing the channel
The RF spectrum is a finite resource and is shared between users using multiplexing (sometimes called channelization). Multiplexing is used to separate different users of the spectrum. This section covers multiplexing frequency, time, code, and geography. Most communications systems use
a combination of these multiplexing methods.
6.1 Multiplexing - frequency
Frequency Division Multiple-Access (FDMA) splits the available frequency band into smaller fixed frequency channels. Each transmitter or receiver uses a separate frequency. This technique has been used since around 1900 and is still in use today. Transmitters are narrowband or frequency-limited.
A narrowband transmitter is used along with a receiver that has a narrowband filter so that it can demodulate the desired signal and reject unwanted signals, such as interfering signals from adjacent radios.
6.2 Multiplexing - time
Time-division multiplexing involves separating the transmitters in time so that they can share the same frequency. The simplest type is Time Division Duplex (TDD). This multiplexes the transmitter and receiver on the same frequency. TDD is used, for example, in a simple two-way radio where a button is pressed to talk and released to listen. This kind of time division duplex, however, is very slow. Modern digital radios like CT2 and DECT use Time Division Duplex but they multiplex hundreds of times per second.
TDMA (Time Division Multiple Access) multiplexes several transmitters or receivers on the same frequency. TDMA is used in the GSM digital cellular system and also in the US NADC-TDMA system.
In CDMA systems, users timeshare a higher-rate digital channel by overlaying a higher-rate digital sequence on their transmission. A different sequence is assigned to each terminal so that the signals can be discerned
from one another by correlating them with the overlaid sequence. This is based on codes that are shared between the base and mobile stations.
Because of the choice of coding used, there is a limit of 64 code channels on the forward link. The reverse link has no practical limit to the number of codes available.
6.4 Multiplexing - geography
Another kind of multiplexing is geographical or cellular. If two transmitter/receiver pairs are far enough apart, they can operate on the same frequency and not interfere with each other. There are only a few kinds of systems that do not use some sort of geographic multiplexing.
Clear-channel international broadcast stations, amateur stations, and some military low frequency radios are about the only systems that have no geographic boundaries and they broadcast around the world.
6.5 Combining multiplexing modes
In most of these common communications systems, different forms of multiplexing are generally combined. For example, GSM uses FDMA, TDMA, FDD and geographic. DECT uses FDMA, TDD and geographic
multiplexing. For a full listing see the table in section ten.
6.6 Penetration versus efficiency
Penetration means the ability of a signal to be used in environments where there is a lot of attenuation or noise or interference. One very common example is the use of pagers versus cellular phones. In many cases,
pagers can receive signals even if the user is inside a metal building or a steel-reinforced concrete structure like a modern skyscraper. Most pagers use a two-level FSK signal where the frequency deviation is large and the modulation rate (symbol rate) is quite slow. This makes it easy for the receiver to detect and demodulate the signal since the frequency difference is large (the symbol locations are widely separated) and these different frequencies persist for a long time (a slow symbol rate).
However, the factors causing good pager signal penetration also cause inefficient information transmission. There are typically only two symbol locations. They are widely separated (approximately 8 kHz), and a small
number of symbols (500 to 1200) are sent each second. Compare this with a cellular system such as GSM which sends 270,833 symbols each second.
This is not a big problem for the pager since all it needs to receive is its unique address and perhaps a short ASCII text message.
A cellular phone signal, however, must transmit live duplex voice. This requires a much higher bit rate and a much more efficient modulation technique. Cellular phones use more complex modulation formats (such
as ¹/4 DQPSK and 0.3 GMSK) and faster symbol rates. Unfortunately, this greatly reduces penetration and one way to compensate is to use more power. More power brings in a host of other problems, as described
previously.
Lihat juga
DIGITAL MODULATION; INTRODUCTION
DIGITAL MODULATION ; WHY DIGITAL MODULATION
DIGITAL MODULATION ; USING I/Q MODULATION TO CONVEY INFORMATION
DIGITAL MODULATION ; DIGITAL MODULATION TYPES AND RELATIVE EFFICIENCIES
DIGITAL MODULATION ; FILTERING
DIGITAL MODULATION ; DIFFERENT WAYS OF LOOKING AT A DIGITAL MODULATED SIGNAL TIME AND FREQUENCT DOMAIN VIEW
DIGITAL MODULATION ; SHARING THE CHANNEL
DIGITAL MODULATION ; HOW DIGITAL TRANSMITTER AND RECEIVER WORK
DIGITAL MODULATION ; MEASUREMENT ON DIGITAL RF COMMINICATION SYSTEMS
DIGITAL MODULATION ; SUMMARY
DIGITAL MODULATION ; OVERVIEW OF COMMUNICATION SYSTEM
DIGITAL MODULATION ; GLOSSARY OF TERM
The RF spectrum is a finite resource and is shared between users using multiplexing (sometimes called channelization). Multiplexing is used to separate different users of the spectrum. This section covers multiplexing frequency, time, code, and geography. Most communications systems use
a combination of these multiplexing methods.
6.1 Multiplexing - frequency
Frequency Division Multiple-Access (FDMA) splits the available frequency band into smaller fixed frequency channels. Each transmitter or receiver uses a separate frequency. This technique has been used since around 1900 and is still in use today. Transmitters are narrowband or frequency-limited.
A narrowband transmitter is used along with a receiver that has a narrowband filter so that it can demodulate the desired signal and reject unwanted signals, such as interfering signals from adjacent radios.
Figure 29. Multiplexing - Frequency
6.2 Multiplexing - time
Time-division multiplexing involves separating the transmitters in time so that they can share the same frequency. The simplest type is Time Division Duplex (TDD). This multiplexes the transmitter and receiver on the same frequency. TDD is used, for example, in a simple two-way radio where a button is pressed to talk and released to listen. This kind of time division duplex, however, is very slow. Modern digital radios like CT2 and DECT use Time Division Duplex but they multiplex hundreds of times per second.
TDMA (Time Division Multiple Access) multiplexes several transmitters or receivers on the same frequency. TDMA is used in the GSM digital cellular system and also in the US NADC-TDMA system.
Figure 30. Multiplexing - Time
6.3 Multiplexing - code
CDMA is an access method where multiple users are permitted to transmit simultaneously on the same frequency. Frequency division multiplexing is still performed but the channel is 1.23 MHz wide. In the case of US CDMA telephones, an additional type of channelization is added, in the form of coding.
Figure 31. Multiplexing - Code
In CDMA systems, users timeshare a higher-rate digital channel by overlaying a higher-rate digital sequence on their transmission. A different sequence is assigned to each terminal so that the signals can be discerned
from one another by correlating them with the overlaid sequence. This is based on codes that are shared between the base and mobile stations.
Because of the choice of coding used, there is a limit of 64 code channels on the forward link. The reverse link has no practical limit to the number of codes available.
6.4 Multiplexing - geography
Another kind of multiplexing is geographical or cellular. If two transmitter/receiver pairs are far enough apart, they can operate on the same frequency and not interfere with each other. There are only a few kinds of systems that do not use some sort of geographic multiplexing.
Clear-channel international broadcast stations, amateur stations, and some military low frequency radios are about the only systems that have no geographic boundaries and they broadcast around the world.
Figure 32. Multiplexing - Geography
6.5 Combining multiplexing modes
In most of these common communications systems, different forms of multiplexing are generally combined. For example, GSM uses FDMA, TDMA, FDD and geographic. DECT uses FDMA, TDD and geographic
multiplexing. For a full listing see the table in section ten.
6.6 Penetration versus efficiency
Penetration means the ability of a signal to be used in environments where there is a lot of attenuation or noise or interference. One very common example is the use of pagers versus cellular phones. In many cases,
pagers can receive signals even if the user is inside a metal building or a steel-reinforced concrete structure like a modern skyscraper. Most pagers use a two-level FSK signal where the frequency deviation is large and the modulation rate (symbol rate) is quite slow. This makes it easy for the receiver to detect and demodulate the signal since the frequency difference is large (the symbol locations are widely separated) and these different frequencies persist for a long time (a slow symbol rate).
However, the factors causing good pager signal penetration also cause inefficient information transmission. There are typically only two symbol locations. They are widely separated (approximately 8 kHz), and a small
number of symbols (500 to 1200) are sent each second. Compare this with a cellular system such as GSM which sends 270,833 symbols each second.
This is not a big problem for the pager since all it needs to receive is its unique address and perhaps a short ASCII text message.
A cellular phone signal, however, must transmit live duplex voice. This requires a much higher bit rate and a much more efficient modulation technique. Cellular phones use more complex modulation formats (such
as ¹/4 DQPSK and 0.3 GMSK) and faster symbol rates. Unfortunately, this greatly reduces penetration and one way to compensate is to use more power. More power brings in a host of other problems, as described
previously.
Lihat juga
DIGITAL MODULATION; INTRODUCTION
DIGITAL MODULATION ; WHY DIGITAL MODULATION
DIGITAL MODULATION ; USING I/Q MODULATION TO CONVEY INFORMATION
DIGITAL MODULATION ; DIGITAL MODULATION TYPES AND RELATIVE EFFICIENCIES
DIGITAL MODULATION ; FILTERING
DIGITAL MODULATION ; DIFFERENT WAYS OF LOOKING AT A DIGITAL MODULATED SIGNAL TIME AND FREQUENCT DOMAIN VIEW
DIGITAL MODULATION ; SHARING THE CHANNEL
DIGITAL MODULATION ; HOW DIGITAL TRANSMITTER AND RECEIVER WORK
DIGITAL MODULATION ; MEASUREMENT ON DIGITAL RF COMMINICATION SYSTEMS
DIGITAL MODULATION ; SUMMARY
DIGITAL MODULATION ; OVERVIEW OF COMMUNICATION SYSTEM
DIGITAL MODULATION ; GLOSSARY OF TERM
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