When several communications channels are connected over a common channel, a device called a multiplexer is used. The multiplexer combines multiple incoming (input) signals onto one common communications channel through the process of time, frequency, or code sharing. At the other end of the communication line, a demultiplexer device is used to separate the channels (output) at the receiving end.
When a digital channel is divided into multiple digital sub channels, the separate channels are called logical channels. Each logical channel is assigned a portion of the bits from the digital communications channel.
A device that converts the information signal into a format that is suitable for transmission is called a transmitter. The device that receives and decodes the transmitted signal is called a receiver. When a transmitter and receiver are combined into one device, it is called a transceiver.
Frequency Division Multiplexing (FDM)
Frequency division multiplexing is a process of allowing multiple channels to share a frequency band by dividing up a frequency band into smaller frequency bandwidth channels. Each of these smaller channels provides for a separate communications channel.
Figure below shows how a frequency band can be divided into several communication channels. When a device is communicating on a FDM system using a frequency carrier signal, it’s carrier channel is completely occupied by the transmission of the device. For some FDM systems, after it has stopped transmitting, other transceivers may be assigned to that carrier channel frequency. When this process of assigning channels is organized, it is called frequency division multiple access (FDMA). Transceivers in an FDM system typically have the ability to tune to several different carrier channel frequencies.
Carrier signals can co-exist with each other on an FDM system without interference if they are operating at different frequencies. Because the modulating signal slightly changes the carrier signal, this produces small changes in frequency. This results in a single radio signal that occupies a frequency range, depending on the type and amount of information that is changing the electromagnetic wave. The maximum amount of frequency change is typically called the channel bandwidth. Hence, a carrier signal should not typically operate in areas that other radio carrier signals may occupy.
As a carrier signal is modulated (amplitude, frequency, or phase), several other small energy signals at different frequencies are created. Some of the signals produced by the modulation process fall outside the designated frequency bandwidth. Although the amount of energy that falls outside the designated bandwidth is usually small, they may cause interference with other devices that are communicating on other nearby channels.
To help protect from unwanted interference, when multiple carrier signals are operating in an FDM system, a guard band is usually used to protect adjacent carriers from interference. Guard bands are a portion of a resource (frequency or time) that is dedicated to the protection of a communication channel from interference due to radio signal energy or time overlap of signals. While guard bands protect a desired communication channel from interference, the guard band also uses part of the valuable resource (frequency bandwidth or time period) for this protection.
Time Division Multiplexing (TDM)
Time division multiplexing (TDM) is a process of sharing a single carrier channel by dividing the channel into time slots that are shared between simultaneous users of the carrier channel. When a transceiver communicates on a TDM system, it is assigned a specific time position on the carrier channel. By allowing several users to use different time positions (time slots) on a single carrier channel, TDM systems increase their ability to serve multiple users with a limited number of channels by dividing a frequency band into time slots. Time slots are grouped into repetitive frames. Each communication channel is assigned to one (or several) time slot(s) within a frame.
To allow TDM systems to provide continuous voice communication to a transceiver that can only transmit for brief periods, TDM systems use digital signal processing to characterize and compress digital signals into short time-slices. Figure below shows how a single carrier channel is time-sliced into three communication channels. Transceiver number 1 is communicating on time slot number 1 and mobile radio number 2 is communicating on time slot number 3. Each frame on this communication system has three time slots.
Code Division Multiplexing (CDM)
Code division multiplexing uses a method of spreading an information signal using different codes on a wide bandwidth communication channel (typically digital signals). For CDM channels, the frequency bandwidth of the carrier channel is much larger than the bandwidth of the original information signal. Because the channel bandwidth is very large, information from other channels operating in the same frequency band is relatively small. This allows multiple communications channels to operate in the same frequency bandwidth at the same time. There are various forms of CDM. The most popular forms of spread spectrum include frequency hopping and direct spread spectrum.
Frequency hopping is a multiplexing technology where transceivers may share a frequency band by transmitting for brief periods of time on an individual carrier channels and then hopping to other carrier channels to continue transmission. Each transceiver is assigned to a particular hopping pattern and collisions that occur are random. These errors only cause a loss of small amounts of data that may be fixed through error detection and correction methods.
Direct spread spectrum is relatively new commercialized (verses militarized) modulation technique that is used primarily in cellular and satellite systems. Direct sequence spread spectrum systems mix a relatively long digital code with a small amount of communication data (information signal) to produce a combined signal that is spread over a relatively wide frequency band. To receive the signal, the long code is used to extract the original signal.
Because the energy is spread over a wide bandwidth, multiple spread spectrum channels with different codes can co-exist with minimal interference. Figure 3.8 shows how a single direct sequence spread spectrum communication channel can have several channels. In this example, there are 3 different code patterns that are used for communication channels. When a receiver uses the reference code, a direct sequence spread spectrum system can build a mask as shown in Figure below for each conversation allowing only that information which falls within the mask to be transmitted or received.
Digital Speech Interpolation (DSI)
In addition to multiplexing through channel division, statistical multiplexing can also be used by distributing transmission of a communications channel over idle portions of multiplexed channels. An example of statistical multiplexing is digital speech interpolation (DSI). DSI is a technique that dynamically allocates time slots for voice or data transmission to a user only when the have voice or data activity. This increases the system capacity as transmission for other users can occur when others are silent.
Digital speech interpolation (DSI) is a digital form of a process known as time assigned speech interpolation (TASI). The DSI technique that dynamically allocates channels (usually time slots) for voice or data transmission to a user only when the have voice or data activity. This increases the system capacity as transmission for other users can occur when others are silent.
A system that has DSI capability assigns information transmission based in speech activity. The DSI system senses activities of speech signals and availability of communication channels in a system and dynamically transmits information signals on available communications channels. Because speech conversation is composed of pauses and alternating directions of communications (usually one person speaks at a time), the use of TASI increases the efficiency of a communications system of approximately 2:1. For example, a 96 channel communications circuit that uses TASI can provide service approximately 192 calls.
Figure below shows the process of multiplexing using DSI. This diagram shows a communication circuit that has 96 independent communication channels (one communication link that has 96 time slots). The DSI system monitors the activity of each voice conversation (a voice channel) using a voice activity detector (VAD). The VAD is an electronic circuit that senses the activity (or absence) of voice signals. This is used to inhibit a transmission signal during periods of voice inactivity.
When the VAD detects that speech is active, the DSI system assigns the information to specific time slots on the communications channel. The DSI transmitter system identifies the voice channel at the beginning of the transmission so the DSI receiver can assign it to an output voice channel. When the voice activity detector senses a pause in communication, the DSI transmitter sends an ending message on the channel allowing the channel to be placed back into a pool of available communication channels. The next time the speech activity detector senses the voice channel is again active, the DSI transmitter will select a channel from the pool of available communication channels and the process begins again. Each time, the DSI receiver will assign the information to the correct output voice channel.
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