Chapter Seven: Digital Communication

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

Types of Signal Transmission

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:

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

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

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

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

Analog Pulse-Modulation Techniques

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

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 = (1.76 + 6.02m) dB

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

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

PCM Coding

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

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

Bipolar NRZ Code

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

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 1.544 Mb/s The whole system is known as a T1 Carrier

Digital Signal Hierarchy Coax, fiber-optic 274.176 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

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

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

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

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)