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1 Principles of Electronic Communication Systems Third Edition Louis E. Frenzel, Jr. © 2008 The McGraw-Hill Companies 2 Chapter 10 Multiplexing and Demultiplexing © 2008 The McGraw-Hill Companies 3 Topics Covered in Chapter 10 10-1: Multiplexing Principles 10-2: Frequency-Division Multiplexing 10-3: Time-Division Multiplexing 10-4: Pulse-Code Modulation 10-5: Duplexing © 2008 The McGraw-Hill Companies 4 10-1: Multiplexing Principles Transmitting two or more signals simultaneously can be accomplished by running multiple cables or setting up one transmitter-receiver pair for each channel, but this is an expensive approach. A single cable or radio link can handle multiple signals simultaneously using a technique known as multiplexing. Multiplexing permits hundreds or even thousands of signals to be combined and transmitted over a single medium. © 2008 The McGraw-Hill Companies 5 10-1: Multiplexing Principles Multiplexing is the process of simultaneously transmitting two or more individual signals over a single communication channel. It increases the number of communication channels so that more information can be transmitted. An application may require multiple signals. Cost savings can be gained by using a single channel to send multiple information signals. © 2008 The McGraw-Hill Companies 6 10-1: Multiplexing Principles Four communication applications that would be prohibitively expensive or impossible without multiplexing are: 1. Telephone systems 2. Telemetry 3. Satellites 4. Broadcasting (radio and TV) © 2008 The McGraw-Hill Companies 7 10-1: Multiplexing Principles Fig. 10-1: Concept of multiplexing. © 2008 The McGraw-Hill Companies 8 10-1: Multiplexing Principles The two most common types of multiplexing are 1. Frequency-division multiplexing (FDM) Generally used for analog information. Individual signals to be transmitted are assigned a different frequency within a common bandwidth. 2. Time-division multiplexing (TDM) Generally used for digital information. Multiple signals are transmitted in different time slots on a single channel. © 2008 The McGraw-Hill Companies 9 10-1: Multiplexing Principles Another form of multiple access is known as code- division multiple access (CDMA). Widely used in cell phone systems to allow many subscribers to use a common bandwidth simultaneously. Uses special codes assigned to each user that can be identified. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 10 In frequency-division multiplexing (FDM), multiple signals share the bandwidth of a common communication channel. All channels have specific bandwidths. A wide bandwidth can be shared for the purpose of transmitting many signals at the same time. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 11 Transmitter-Multiplexers In an FDM system, each signal to be transmitted feeds a modulator circuit. The carrier for each modulator (fc) is on a different frequency. The carriers are equally spaced from one another. These carriers are referred to as subcarriers. Each input signal is given a portion of the bandwidth. The FDM process divides up the bandwidth of the single channel into smaller, equally spaced channels, each capable of carrying information in sidebands. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 12 Fig. 10-2: The transmitting end of an FDM system. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 13 Transmitter-Multiplexers The modulator outputs containing the sideband information are added algebraically in a linear mixer. The resulting output signal is a composite of all the modulated subcarriers. This signal can be used to modulate a radio transmitter, or can itself be transmitted over a single channel. The composite signal can also become one input to another multiplexed system. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 14 Receiver-Demultiplexer In an FDM system, a receiver picks up the signal and demodulates it, recovering the composite signal. The composite signal is sent to a group of bandpass filters, each centered on one of the carrier frequencies. Each filter passes only its channel and rejects all others. A channel demodulator then recovers each original input signal. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 15 Figure 10-4: The receiving end of an FDM system. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 16 FDM Applications: Telemetry Sensors in telemetry systems generate electrical signals that change in some way in response to changes in physical characteristics. An example of a sensor is a thermistor, a device used to measure temperature. A thermistor’s resistance varies inversely with temperature. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 17 FDM Applications: Telemetry The thermistor is usually connected into a resistive network, such as a voltage divider or bridge. The thermistor is also connected to a DC voltage source. The result is a DC output voltage which varies in accordance with temperature. This voltage is transmitted to a remote receiver for measurement, readout, and recording. The thermistor becomes one channel of an FDM system. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 18 FDM Applications: Telemetry The varying direct or alternating current changes the frequency of an oscillator operating at the carrier frequency. Such a circuit is called a voltage-controlled oscillator (VCO) or subcarrier oscillator (SCO). Most VCOs are astable multivibrators whose frequency is controlled by the input from the signal conditioning circuits. A system that uses FM of the VCO subcarriers as well as FM of the final carrier is called FM/FM. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 19 Figure 10-5: An FDM telemetry transmitting system. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 20 Figure 10-7: An FM/FM telemetry receiver © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 21 FDM Applications: Telemetry On the receiving end of a telemetry system, an FM demodulator reproduces the original composite multiplexed signal, which is then fed to a demultiplexer. The demultiplexer divides the signals and reproduces the original inputs. The output of the first FM demodulator is fed simultaneously to multiple bandpass filters, each of which is tuned to the center frequency of one of the specified subchannels. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 22 FDM Applications: Telemetry Each filter passes only its subcarrier and related sidebands and rejects all the others. The demultiplexing process essentially uses filters to sort the composite multiplex signal back into its original components. The output of each filter is the subcarrier oscillator frequency with its modulation. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 23 FDM Applications: Telemetry These signals are then applied to FM demodulators. Also known as discriminators, these circuits take the FM signal and recreate the original dc or ac signal produced by the transducer. The original signals are measured or processed to provide the desired information from the remote transmitting source. In most systems, the multiplexed signal is sent to a data recorder where it is stored for possible future use. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 24 FDM Applications: Telephone Systems For decades, telephone companies used FDM to send multiple telephone conversations over a minimum number of cables. The original voice signal, in the 300- to 3000-Hz range is used to modulate a subcarrier. Lower sideband (LSB) SSB AM was used. Each subcarrier is on a different frequency. Those subcarriers are added together to form one channel. The FDM system has been replaced by an all-digital time multiplexing (TDM) system. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 25 FDM Applications: Cable TV In a cable TV system, TV signals, each in its own 6- MHz channel, are multiplexed on a common coaxial or fiber-optic cable and sent to homes. Each 6-MHz channel carries the video and voice of the TV signal. Coaxial and fiber-optic cables have an enormous bandwidth and can carry more than one hundred TV channels. Many cable TV companies also use their cable system for Internet access. © 2008 The McGraw-Hill Companies 10-2: Frequency-Division Multiplexing 26 FDM Applications: FM Stereo Broadcasting In recording original stereo, two microphones are used to generate two separate audio signals. Two microphones pick up sound from a common source, such as voice, but from different directions. The separation of the two microphones provides sufficient differences in the two audio signals to provide a realistic reproduction of the original sound. FDM techniques are used to transmit these independent signals by a single transmitter. © 2008 The McGraw-Hill Companies 27 10-3: Time-Division Multiplexing In FDM, multiple signals are transmitted over a single channel, each signal being allocated a portion of the spectrum within that bandwidth. In time-division multiplexing (TDM), each signal occupies the entire bandwidth of the channel. Each signal is transmitted for only a brief period of time. TDM can be used with both digital and analog signals. © 2008 The McGraw-Hill Companies 28 10-3: Time-Division Multiplexing Figure 10-12: The basic TDM concept. © 2008 The McGraw-Hill Companies 29 10-3: Time-Division Multiplexing Sampling an analog signal creates pulse-amplitude modulation (PAM). The analog signal is converted to a series of constantwidth pulses whose amplitude follows the shape of the analog signal. The original analog signal is recovered by passing it through a low-pass filter. In TDM using PAM, a circuit called a multiplexer (MUX or MPX) samples multiple analog signal sources; the pulses are interleaved and transmitted over one channel. © 2008 The McGraw-Hill Companies 30 10-3: Time-Division Multiplexing Figure 10-13: Sampling an analog signal to produce pulse-amplitude modulation. © 2008 The McGraw-Hill Companies 31 10-3: Time-Division Multiplexing PAM Multiplexer The simplest time multiplexer operates like a single-pole multiple-position mechanical or electronic switch. It rapidly, sequentially samples multiple analog inputs. The switch arm dwells momentarily on each contact. This allows the input signal to be passed to the output. It then switches quickly to the next channel, allowing that channel to pass for a fixed duration. The remaining channels are sampled in the same way. © 2008 The McGraw-Hill Companies 32 10-3: Time-Division Multiplexing Figure 10-14: Simple rotary-switch multiplexer. © 2008 The McGraw-Hill Companies 33 10-3: Time-Division Multiplexing PAM Multiplexer Four different analog signals can be sampled by a PAM multiplexer. In the following slide of Figure 10-15: Signals A and C are continuously varying analog signals. Signal B is a positive-going linear ramp. Signal D is a constant DC voltage. © 2008 The McGraw-Hill Companies 34 10-3: Time-Division Multiplexing Figure 10-15: Four-channel PAM time-division multiplexer. © 2008 The McGraw-Hill Companies 35 10-3: Time-Division Multiplexing PAM Multiplexer: Commutator Switches Multiplexers in early TDM/PAM telemetry systems used a form of rotary switch known as a commutator. One complete revolution of the commutator switch is referred to as a frame. During one frame, each input channel is sampled one time. The number of frames completed in 1 second is called the frame rate. Multiplying the frame rate by the number of samples per frame yields the commutation rate or multiplex rate, which is the basic frequency of the composite signal transmitted over the communication channel. © 2008 The McGraw-Hill Companies 36 10-3: Time-Division Multiplexing PAM Multiplexer: Electronic Multiplexers In practical TDM/PAM systems, electronic circuits are used instead of mechanical switches or commutators. The multiplexer itself is usually implemented with FETs. FETs are nearly ideal on/off switches and can turn off and on at very high speeds. © 2008 The McGraw-Hill Companies 37 10-3: Time-Division Multiplexing Figure 10-16: A time-division multiplexer used to produce pulse-amplitude modulation. © 2008 The McGraw-Hill Companies 38 10-3: Time-Division Multiplexing Demultiplexer Circuits Once the composite signal is received, it must be demodulated and demultiplexed. The signal is picked up by the receiver. The signal is sent to an FM demodulator that recovers the original PAM data. The PAM signal is then demultiplexed into the original analog signals. © 2008 The McGraw-Hill Companies 39 10-3: Time-Division Multiplexing Figure 10-18: A PAM demultiplexer. © 2008 The McGraw-Hill Companies 40 10-3: Time-Division Multiplexing Demultiplexer Circuits The main problem encountered in demultiplexing is synchronization. For the PAM signal to be accurately demultiplexed, the clock frequencies at the receiver demultiplexer and the transmitting multiplexer must be identical. The sequence of the demultiplexer must also be identical to that of the multiplexer. Such synchronization is usually carried out by a special synchronizing pulse included as a part of each frame. © 2008 The McGraw-Hill Companies 41 10-3: Time-Division Multiplexing Demultiplexer Circuits: Clock Recovery and Frame Synchronization The clock for a demultiplexer is typically derived from the received PAM signal through a clock recovery circuit. After clock pulses of the proper frequency have been obtained, the multiplexed channels must be synchronized. This synchronization is achieved by using a special synchronizing (sync) pulse applied to one of the input channels at the transmitter. © 2008 The McGraw-Hill Companies 42 10-3: Time-Division Multiplexing Figure 10-19: Two PAM clock recover circuits: (a) Closed loop. (b) Open loop. © 2008 The McGraw-Hill Companies 43 10-3: Time-Division Multiplexing Figure 10-20: Frame sync pulse and comparator detector. © 2008 The McGraw-Hill Companies 44 10-3: Time-Division Multiplexing Figure 10-21: Complete PAM demultiplexer. © 2008 The McGraw-Hill Companies 45 10-4: Pulse-Code Modulation The most popular form of TDM uses pulse-code modulation (PCM). With pulse-code modulation, multiple channels of digital data are transmitted in serial form. Each channel is assigned a time slot in which to transmit one binary word of data. The data streams from the various channels are interleaved and transmitted sequentially. © 2008 The McGraw-Hill Companies 46 10-4: Pulse-Code Modulation PCM Multiplexers When PCM is used to transmit analog signals, the signals are sampled with a multiplexer. The signals are then converted by an A/D converter into a series of binary numbers. Each number is proportional to the amplitude of the analog signal at various sampling points. These binary words are converted from parallel to serial format and then transmitted. © 2008 The McGraw-Hill Companies 47 10-4: Pulse-Code Modulation PCM Multiplexers At the receiving end, the various channels are demultiplexed. The original sequential binary numbers are recovered and stored in a digital memory. They are then transferred to a D/A converter that reconstructs the analog signal. © 2008 The McGraw-Hill Companies 48 10-4: Pulse-Code Modulation Figure 10-22: A PCM system. © 2008 The McGraw-Hill Companies 49 10-4: Pulse-Code Modulation PCM Demultiplexers At the receiving end, the PCM signal is demultiplexed and converted back into the original data. If the PCM signal has modulated a carrier and is being transmitted by radio, the RF signal will be picked up by a receiver and demodulated. The original serial PCM binary waveform is recovered and fed to a shaping circuit to clean up and rejuvenate the binary pulses. The original signal is then demultiplexed by a digital demultiplexer using AND or NAND gates. © 2008 The McGraw-Hill Companies 50 10-4: Pulse-Code Modulation Figure 10-24:A PCM receiver-demultiplexer. © 2008 The McGraw-Hill Companies 51 10-4: Pulse-Code Modulation Benefits of PCM PCM is reliable, inexpensive, and highly resistant to noise. The transmitted binary pulses all have the same amplitude and can be clipped to reduce noise. Even when signals have been degraded because of noise, attenuation, or distortion, the receiver only has to determine whether a pulse was transmitted. PCM signals are easily recovered and rejuvenated. © 2008 The McGraw-Hill Companies 52 10-4: Pulse-Code Modulation Digital Carrier Systems The most widespread use of TDM is in the telephone system. Years ago, the telephone companies developed a complete digital transmission system called the Tcarrier system. The T-carrier system defines a range of PCM TDM systems with progressively faster data rates. The physical implementations of these systems are referred to as T-1, T-2, T-3, and T-4. © 2008 The McGraw-Hill Companies 53 10-4: Pulse-Code Modulation T-1 Systems The most commonly used PCM system is the T-1 system for transmitting telephone conversations by high-speed digital links. The T-1 system multiplexes 24 voice channels onto a single line using TDM techniques. Each serial digital word (8-bit words, 7 bits of magnitude, and 1 bit representing polarity) from the 24 channels is transmitted sequentially. Each frame is sampled at an 8-kHz rate, producing a 125-μs sampling interval. © 2008 The McGraw-Hill Companies 54 10-4: Pulse-Code Modulation T-Carrier Systems, and T-2, T-3, and T-4 Systems T-1 systems transmit each voice signal at a 64-kbp/s rate. They are also used to transmit fewer than 24 inputs at a faster rate. T-2 systems are not widely used except as a stepping stone to form DS3 signals. T-1 and T-3 lines are widely used by business and industry for telephone service as well as for digital data transmission. T-2 and T-4 lines are rarely used by subscribers, but they are used within the telephone system itself. © 2008 The McGraw-Hill Companies 55 10-5: Duplexing Duplexing is the method by which two-way communications are handled. Half duplexing means that the two stations communicating take turns transmitting and receiving. Full duplexing means that the two stations can send and receive simultaneously. There are two ways to provide duplexing: Frequency-division duplexing (FDD) Time-division duplexing (TDD). © 2008 The McGraw-Hill Companies 56 10-5: Duplexing FDD is the simplest way to provide full duplex. FDD uses two separate channels, one for sending and another for receiving. The big disadvantage of this method is the extra spectrum space required. However, most cell phone systems use this method because it is the easiest to implement and the most reliable. © 2008 The McGraw-Hill Companies 57 10-5: Duplexing Figure 10-27: Frequency-division duplexing (FDD). © 2008 The McGraw-Hill Companies 58 10-5: Duplexing Time-division duplexing (TDD) means that signals are transmitted simultaneously on a single channel by interleaving them in different time slots. Each time slot may contain one data word, such as 1 byte from an A/D converter or a D/A converter. The primary benefit of TDD is that only one channel is needed, saving spectrum space and cost. © 2008 The McGraw-Hill Companies 59 10-5: Duplexing The TDD method is harder to implement. The key to making it work is precise timing and synchronization between transmitter and receiver. Special synchronizing pulses or frame sequences are needed to constantly ensure that timing will not result in collisions between transmit and receive. Several of the newer third-generation cell phone systems may use TDD. © 2008 The McGraw-Hill Companies 60 10-5: Duplexing Figure 10-28: Time-division duplexing (TDD). © 2008 The McGraw-Hill Companies