1. Chapter Seven: Digital Communication

2. Introduction Many signals in modern communication systems are digital
Additionally, analog signals are transmitted digitally
Digitizing a signal results in reduced distortion and improvement in signal-to-noise ratios

3. Types of Signal Transmission

4. Channels and Information Capacity
All practical communication channels are band-limited
There are theoretical limits to the rate at which data may be transmitted
The relationship between time, information capacity, and channel bandwidth is given by Hartley’s Law:

5. Shannon-Hartley Theorem
There is a limit to the amount of data that can be sent in a given bandwidth:

6. Pulse Modulation
Nyquist showed that it is possible to reconstruct a band-limited signal from periodic samples, as long as the sampling rate is at least twice the frequency of the of highest frequency component of the signal
Several types of sampling are available for pulse modulation

7. Sampling Rate Errors
Sampling rates that are too low result in aliasing or foldover
The figures illustrate correct and incorrect sampling rates:

8. Sampling Sampling alone is not a digital technique
The immediate result of sampling is a pulse-amplitude modulation (PAM) signal
PAM is an analog scheme in which the amplitude of the pulse is proportional to the amplitude of the signal at the instant of sampling
Another analog pulse-forming technique is known as pulse-duration modulation (PDM). This is also known as pulse-width modulation (PWM)
Pulse-position modulation is closely related to PDM

9. Analog Pulse-Modulation Techniques

10. Pulse-Code Modulation
Pulse-Code Modulation (PCM) is the most commonly used digital modulation scheme
In PCM, the available range of signal voltages is divided into levels and each is assigned a binary number
Each sample is represented by a binary number and transmitted serially
The number of levels available depends upon the number of bits used to express the sample value
The number of levels is given by: N = 2m

11. Quantizing
The process of converting analog signals to PCM is called quantizing
Since the original signal can have an infinite number of signal levels, the quantizing process will produce errors called quantizing errors or quantizing noise
The dynamic range of a system is the ratio of the strongest possible signal that can be transmitted and the weakest discernible signal
In a linear PCM system, the maximum dynamic range is found by:
DR = ( m) dB

12. Companding Companding is used to improve dynamic range
Compression is used on the transmitting end and expanding is used on the receiving end, hence companding

13. Coding and Decoding
The process of converting an analog signal into PCM is called coding, the inverse operation is called decoding
Both procedures are accomplished in a CODEC

14. PCM Coding

15. Delta Modulation
In Delta Modulation, only one bit is transmitted per sample
That bit is a one if the current sample is more positive than the previous sample, and a zero if it is more negative
Since so little information is transmitted, delta modulation requires higher sampling rates than PCM for equal quality of reproduction

16. Line Codes
Line codes are methods of converting binary numbers back into analog voltages or currents
The simplest line code is to use the presence or absence of a voltage/current to indicate the logic state
Unipolar NRZ (non-return-to-zero) means that there is no requirement for a signal to return to zero at the end of each element
RZ (return-to-zero) methods are used to eliminate low-frequency ac components and dc components

17. Bipolar NRZ Code

18. Time-Division Multiplexing
There are two basic types of multiplexing:
Frequency-division multiplexing (FDM
Time-division multiplexing
In TDM, each information signal is allowed to use all available bandwidth
In theory, it is possible to to divide the bandwidth or the time among the users of a channel
Continuously variable signals, such as analog, are not well adapted to TDM because the signal is present all the time

19. TDM in Telephony TDM is used extensively in telephony
The most common standard is the DS-1 signal, which consists of 24 PCM voice channels, multiplexed using TDM
Each channel is sampled at 8 kHz with 8 bits per sample, which gives a bit rate of 64 kb/s for each voice channel
The samples must be transmitted at the rate they were obtained to be reconstructed
The overall bit rate is Mb/s
The whole system is known as a T1 Carrier

20. Digital Signal Hierarchy
Coax, fiber-optic
4032
DS-4 T4
Fiber optics
560.16
8064
DS-5 T5
Coax, microwave
44.736
672
DS-3 T3
Low-capacitance twisted-pair
microwave
6.312 96
DS-2 T2
Twisted-pair
3.152 48
DS-1C
T1C
1.544 24
DS-1 T1
Typical Medium
Bit Rate
(Mb/s)
Voice
Channels
Signal
Carrier

21. Data Compression
Data compression is a technique used to reduce the bandwidth to transmit an analog signal in a digital form
The exact bandwidth necessary is dependent upon the modulation scheme

22. Lossy and Lossless Compression
There are two main categories of data compression:
Lossless compression involves transmitting all of the data in the original signal but using fewer bits. Lossless compression generally looks for redundancies in the data
Lossy compression allows for some reduction in the quality of the transmitted signal. Lossy compression involves reducing the number of bits per sample or reducing the sampling rate

23. Vocoders
A vocoder (voice coder) is an example of lossy compression applied to human speech
A typical vocoder reduces the amount of data that needs to be transmitted by constructing a model of the human vocal system

24. Vocoder Types
There are two main ways of generating the excitation signal in a linear predictive vocoder:
Pulse Excited Linear Predictive (PELP)
Residual Excited Linear Predictive (RELP)