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: –Multiplexing - combining signals from the same sources –Multiple-access - combining signals from multiple sources

Review of Hartleys Law Hartleys law demonstrates the theoretical limit to how much information can be delivered over a medium Hartleys 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)

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

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 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:

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

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

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