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DIGITAL BASEBAND TRANSMISSION and digital baseband transmission and recording pdf system in digital communication system
Inside this Chapter

  • Introduction
  • A Baseband Digital Communication System
  • Introduction to Discrete PAM Signals (Digital Data Formats)
  • Line Coding and its Properties
  • Various PAM Formats or Line Codes
  • Unipolar RZ and NRZ
  • Polar RZ and NRZ
  • Bipolar NRZ (Alternate Mark Inversion (AMI)]
  • Split Phase Manchester Format
  • Polar Quaternary NRZ Format
  • High Density Bipolar (HDB) Signalling
  • B8ZS Line Code
  • Power Spectra of Discrete PAM Signals (Various Line Codes)
  • Power Spectral Density (psd) of NRZ Unipolar Format
  • Power Spectral Density (psd) of NRZ Polar Format
  • Power Spectral Density (psd) of NRZ Bipolar Format
  • Power Spectral Density (psd) of the Manchester Format
  • Comparison of Various Discrete PAM Formats on the Basis of Power Spectra
  • Difference between Source Coding and Line Coding
  • Introduction to Matched Filter and Intersymbol Interference (ISI)
  • Integrate and Dump Filter (i.e., Receiver)
  • The Optimum Filter (i.e., Optimum Receiver)
  • Matched Filter
  • The Correlator
  • Intersymbol Interference (ISI)
  • Cause of Intersymbol Interference (ISI)
  • Nyquist’s Criterion for Distortionless Baseband Binary Transmission
  • Ideal Solution
  • Concept of Eye Pattern

Although a significant portion of communication, these days, is in analog form, it is now being replaced rapidly by digital communication. This chapter deals with the problems of transmitting digital data over a channel. Hence, the starting messages are assumed to be digital. Infact, to begin with, we shall consider the binary case where the data consists of only two symbols i.e. 1 and 0. We assign a distinct waveform (i.e. pulse) to each of these two symbols. The resulting sequence of these pulses is transmitted over a channel. At the receiver end, these pulses are detected and are converted back to binary data (i.e. 1 s and 0 s).
As a matter of fact, the pulse code Modulation (PCM), Delta modulation (DM), Adaptive delta modulation (ADM) etc. are used to convert an analog signal to digital data. This digital data is the sequence of binary symbols. This digital data needs to be transmitted over the channel. In digital data stream, the occurrence of l’s and 0’s is not exactly equal. Therefore, the signal contains frequencies right from very low frequencies to high frequencies. This means that the baseband transmission of digital data requires a low-pass channel. In other words, this channel should transmit very low frequencies as well highest frequency that can be represented by the signal. Thus, the low-pass channel used to transmit digital data can be approximated by the low-pass filter (LPF). When the digital data is transmitted over a channel, each received pulse is affected somewhat by adjacent pulses. This means that a signal received at a particular time is due to the transmitted puke and some interference due to the adjacent pulses. This interference due to adjacent pulses is, known as Intersymbol Interference (ISI). Because of this intefference (ISI), the received signal is distorted from the expected value. Therefore, errors are introduced in the detection of the symbols. To control the effect of ISI, care is taken so that the interference between the transmitted symbols is minimum. This needs pulse shaping.
Further, we know that any transmission channel adds noise to the signal. This noise is known as the channel noise. The channel noise is normally additive white noise. Thus, a noisy signal is received at the receiver. Ir. the presence of such noise, the receiver should decide which of the possible waveform was transmitted. When this decision is made, then the symbol (1 or 0) is recovered exactly without any noise. Therefore, the channel noise can introduce errors in the signal and decision for a particular symbol can be wrong. This means that the ISI and channel noise simultaneously affect the received signal and create errors in the signal. Special cares must be taken to reduce the noise and increase the signal to noise ratio.
In this chapter, first, we shall discuss the digital representation of signals. Then, we shall discuss ISI and method to minimize it.
Also, it may be noted that the optimum detection of such a pulse signal involves the use of a linear time invariant (LTI) filter known as a matched filter.
            A baseband digital communication system is made up of several elements as described under:

  1. Source

The input to a digitial system is in the form of sequence of digits. The input can be the output from such sources as a data set, a computer, a digitized voice signal (PCM or DM), a digital facsimile or television or telemetry equipment. Most of the discussion in this chapter is restricted to the binary case (i.e. communication schemes using only two symbols).

  1. Multiplexer

            Generally speaking, the capacity practical channel of transmitting data is much largo, than the data rate of individual sources. To utilize this capacity effectively, we combine several sources through a digital multiplexer using the process of interleaving. Thus, a channel is time-shared by several messages simultaneously.

  1. Line Coder

            The output of a multiplexer is coded into electrical pulses or waveforms for the purpose of transmission over the channel. This process is known as line coding or transmission coding. There are several possible ways of assigning waveforms (i.e. pulses) to the digital data. In the binary case (two symbols), for example, conceptually the simplest line code is on-off, where a 1 is transmitted by a pulse p(t) and a 0 is transmitted by no pulse (i.e. zero signal), as shown in figure 6.1(a). Another commonly used code is polar, where 1 is transmitted by a pulse p(t) and 0 is transmitted by a pulse—p(t) as shown in figure 6.1(b). The polar scheme is the most power efficient code, since for a given noise immunity (i.e. error probability) this code needs the least power. Another   popular code in PCM is bipolar, also known as pseudoternary or alternate mark inversion (AMI), where 0 is encoded by no pulse and 1 is encoded by a pulse p(t) or —p(t), depending on whether the previous 1 is encoded by —p(t) or p(t). In short, pulses representing consecutive 1’s alternate in sign, as shown in figure 6.1(c). This code has the advantage that if an error is made in the detection of pulses, the received pulse sequence will violate the bipolar rule and the error is immediately detected (although not corrected).
FIGURE 6.1 Some line codes (a) On-off (RZ). (b) Polar (RZ), (c) Bipolar (RZ), (d) On-off NRZ), (e) Polar (NRZ)

Binary l’s and 0’s, such as in PCM signaling, may be represented in various serial-bit signaling formats called line codes.

Another line code that in the past appeared promising is the duobinary (and modified duobinary) proposed by Lender. Although this code is better than the bipolar in terms of bandwidth efficiency, it has lost its appeal due to some practical problems and will not be discussed here.
In our discussion so far, we have used half-width pulses just for the sake of illustration. We can select other widths. However, full-width pulses are often used in some applications. Whenever full-width pulses are used, the pulse amplitude is held to a constant value throughout the pulse interval (it does not have a chance to go to zero before the next pulse begins). For this reason these schemes are known as nonreturn-to-zero (NRZ) schemes in contrast to return-to-zero (RZ) schemes as shown in figure 6.1(a), (b), (c). Figure 6.1(d) shows an on-off NRZ signal, whereas figure 6.1(e) illustrates a polar NRZ signal.

  1. Regenerative Repeater

            Regenerative repeaters are used at regularly spaced intervals along a digital transmission line to detect the incoming digital signal and regenerate new clean pulses for further transmission along the line. This process periodically eliminates, and thereby combats, the accumulation of noise and signal distortion along the transmission path. If the pulses are transmitted at a rate of Rb pulses per second, we require the periodic timing information—the clock signal at Rb Hz-to sample the incoming pulses at a repeater. This timing information can be extracted from the received signal itself if the line code is chosen properly. The polar signal in figure 6.1(b). for example, when rectified, results in a periodic signal of clock frequency Rb Hz which contains the desired periodic timing signal of frequency Rb Hz. When this signal is applied to a resonant circuit tuned to frequency Rb, the output, which is a sinusoid of frequency Rb Hz, can be used for timing. The on-off signal can be expressed as the sum of a periodic signal (of clock frequency) and a polar signal, as shown in figure 6.2. Because of the presence of the periodic component, we can extract the timing information from this signal using a resonant circuit tuned to the clock frequency. Thus, a bipolar signal, when rectified, becomes an on-off signal. Hence, its timing information can be extracted the same way as that for an on-off signal.
FIGURE 6.2 An on-off signal is the sum of a polar signal and a clock frequency periodic signal.
The timing signal (i.e., the resonant circuit output) is sensitive to the incoming bit pattern. In the on-off or bipolar case, a ‘0’ is transmitted by “no pulse”. Hence, if there are too many 0’s in a sequence (no pulses), there is no signal at the input of the resonant circuit and the sinusoidal output of the resonant circuit. starts decaying, thus, causing error in the timing information. We shall discuss later the ways of overcoming this problem. A line code in which the bit pattern does not affect the accuracy of the timing information is said to be a transparent line code. The polar scheme (where each bit is transmitted by some pulse) is transparent, whereas, the on-off and bipolar schemes are non-transparent.