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Chapter Thirteen: Multiplexing and Multiple-Access Techniques

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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Chapter Thirteen: Multiplexing and Multiple-Access Techniques

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  1. Chapter Thirteen:Multiplexing and Multiple-Access Techniques

  2. 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

  3. 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)

  4. Frequency-Division Multiplexing and Multiple Access (FDM/FDMA) • 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. Time Switching • A time switch moves PCM samples from one time slot to another in a TDM signal

  10. 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

  11. 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:

  12. Spread-Spectrum Systems • 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

  13. Spread-Spectrum Communication • 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

  14. Types of Spread-Spectrum Systems • There are two important types of spread-spectrum systems: • Frequency-hopping • Direct-sequence

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

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