Chapter Thirteen: Multiplexing and Multiple-Access Techniques

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# Chapter Thirteen: Multiplexing and Multiple-Access Techniques - PowerPoint PPT Presentation

Chapter Thirteen: Multiplexing and Multiple-Access Techniques. Introduction. Most communication systems require the sharing of channels Shared media is common in cable television, telephone systems, and data communications Two types of combining signals are:

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Presentation Transcript
Introduction
• Most communication systems require the sharing of channels
• Shared media is common in cable television, telephone systems, and data communications
• Two types of combining signals are:
• Multiplexing - combining signals from the same sources
• Multiple-access - combining signals from multiple sources
Review of Hartley’s Law
• Hartley’s law demonstrates the theoretical limit to how much information can be delivered over a medium
• Hartley’s law shows that time and bandwidth are equivalent
• A communications medium can be shared equally by dividing either quantity among users
• The frequency spectrum can be divided by using:
• FDM (frequency-division multiplexing)
• TDM (time-division multiplexing
• CDMA (code-division multiple access)
• FDM/FDMA is the most basic form of multiplexing and has been used since the first days of radio
• Each transmission is assigned a band of frequencies on a full-time basis
• FDM/FDMA is versatile, being used in radio, all types of cable, and optical fiber
Time-Division Multiplexing and Multiple Access (TDM/TDMA)
• TDM is used mainly for digital communication
• Each information signal is allowed all the available bandwidth, but only for part of the time
• In theory, it is possible to divide the bandwidth among all users of a channel
• Continuously varying signals are not well suited to TDM
• Many signals can be sent on one channel by sending a sample from each signal in rotation
TDM in Telephony
• TDM is used extensively in telephony
• Many different standards for TDM exist
• On arrangement is the DS-1 signal
• Consists of 24 PCM voice channels
• Each channel is sampled at 8 kHz with 8 bits per sample
• Each channel therefore has 64 kb/s
• Consists of frames which contain the bits representing one sample from each of the 24 channels
• The multiplexed signal is sent at 8000 frames/sec, giving a total of 1.544 Mb/s transmission rate
TDM Framing
• The framing bits are used to enable the receiver to determine which sample and which bit in the sample are being received at a given time
• The receiver must be able to distinguish between frames in order to decode the signaling information that is sent with the signal
Digital Switching
• One characteristic important to digital communication is the ease and variety of methods available for switching
• Switching signals from one line to another is known as space switching to distinguish it from time switching
Time Switching
• A time switch moves PCM samples from one time slot to another in a TDM signal
Space Switching
• A real switch has to handle very large numbers of subscribers
• One way of accomplishing this is to use a combination of time and space switches
• A digital space switch is a crosspoint type of switch, but very fast
• A digital switch is completely electronic and not mechanical
Time-Space-Time Switching
• In a time-space-time switch, each of the time switches has a separate bus, called a highway, at its output
• Each of the space switches connects two or more time switches at its input to two or more others at its output as shown below:
• One of the problems facing communication systems is the proliferation of devices using limited available bandwidth, such as CB and cordless telephone systems
• One approach to solve this problem is use a complex, computer-controlled system of frequency reuse
• The problem with this approach is the delegation of strong central control to government or service providers
• One technique to solve these problems is the use of spread-spectrum communication
• This technique, as the name implies, spreads the signal over a broader spectrum of frequencies than is usual
• By using a smaller portion of a greater bandwidth, less interference is produced between competing signals
• Spread-spectrum signals use very low power and may have a signal-to-noise ratio of less than one
Types of Spread-Spectrum Systems
• There are two important types of spread-spectrum systems:
• Frequency-hopping
• Direct-sequence
Frequency-Hopping Systems
• Frequency-hopping systems are the simpler of the two systems available
• A frequency generator is used that generates a carrier that changes frequency many times a second according to a programmed sequence of channels known as pseudo-random (PN) noise sequence
• It is called this because if the sequence is not known, the frequencies appear to hop about unpredictably
Direct-Sequence Systems
• Direct-sequence systems inject pseudo-random noise (PN) into the bit stream that has a much higher rate than the actual data to be communicated
• The data to be transmitted is combined with the PN
• The PN bits are inverted when real data is represented by a one and leave the bit stream unchanged when a data zero is transmitted
• The extra bits transmitted this way are called chips
• Most direct-sequence systems use a chipping rate of at least ten times the bit rate
Direct-Sequence Spectrum
• The use of high-speed PN sequence results in an increase in the bandwidth of the signal, regardless of the modulation scheme used to encode the signal
Reception of Spread-Spectrum Signals
• The type of receiver used for spread-spectrum signals depends upon how the signal is generated
• For a frequency-hopped signal, a conventional narrowband receiver is needed that hops in the same way and is synchronized to the transmitter
• One way to synchronize the signals is to transmit a tone on a prearranged channel at the start of each transmission before it begins hopping
• A more reliable method is to for the transmitter to visit several channels in a prearranged order before beginning a normal transmission
Reception of Direct-Sequence Spread-Spectrum
• Direct-sequence spread-spectrum transmissions require a wideband receiver with autocorrelation incorporated into it
• Autocorrelation involves multiplying the the received signal by a signal generated at the receiver from the PN code
• When the input signal corresponds to the PN code, the output will be large; at other times, the output will be small
Code-Division Multiple Access (CDMA)
• For code-division multiple access, all that is required is for each transmitter to be assigned a different pseudo-noise (PN) sequence
• If possible, orthogonal sequences should be used
• The PN sequence for the transmitter is only given to the receiver that is to operate with the transmitter
• The receiver will then only receive the correct signals and ignore all others