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Chapter 1: Introduction
Introduction to Course
The study of communication systems has two main areas: o Studying how communication systems work. o Studying how communication systems perform in the presence of noise. To study the first area, student must be familiar with signal analysis (i.e. Fourier techniques). To study the second area, student must be familiar with the probability theory and random processes. This is because the noise signal is inherently random. The study of communication systems means covering both areas in depth (it is though impossible to do this in a single course). Fourier techniques have been covered in Signal Analysis course (the pre-requisite of this course). In this course, we will mainly study the analog communication systems; in particular, the analog modulation and demodulation techniques (linear and angle modulations) and the noise effect on the behavior of analog communication systems.
Very Basic Background
In the past, messages were carried using basic methods, for example, by runners or carrier pigeons. This was adequate for distances and data rates of the age. Today, people communicate with each other using modern communication systems which are based on electrical communication. Such a technology (electrical communication) allows the transmission of signals over very long distances at very high speeds. Examples of electrical communication systems are: telephones, TVs, mobiles, faxes, computer networks, etc.
Communication system
A typical communication system can be modeled as follows:
Figure 1: Schematic of communication system.
The components of a communication system are:
Source: originates a message, such as a human voice, a television picture, a teletype message (used for telegraph) or data. Input transducer: converts nonelectrical messages (e.g. human voice, television picture, teletype message) into electrical waveforms (signals) referred to as baseband signal or message signal. Transmitter: modifies (adjusts) the message signal to make it possible (efficient) for transmission. Channel: is the medium on which the modified (ready) signal is transmitted. A communication channel can be a wire, a coaxial cable, an optical fiber or a radio link (i.e. wireless). Receiver: reprocesses the received signal by undoing the modification made at the transmitter and channel. Output transducer: converts electrical messages into its original nonelectrical (baseband) form (e.g. human voice, television picture, teletype message). Destination: is the unit to which the message is communicated (transmitted).
Distortion and Noise
Distortion: is any deformation (alteration) of the signal due to traveling in the channel. For example, attenuation or expansion in the signal width (Figure 2).
Figure 2: Example of a signal distorted with increasing the distance.
Attenuation: o It is a reduction in the strength of a signal due to loss of energy while passing through different materials. o Cables usually have attenuation factor measured per unit distance. The less the attenuation per unit distance, the more efficient the cable. o Repeaters – with amplifiers – can be used to compensate for loss of signal strength (attenuation). However, this is not helpful with the presence of noise (as we will see later). Noise: is undesirable (unwanted) signal. Noise signal is random (probabilistic) and unpredictable.
Figure 4: Feature of some vowels in American English. (a) vocal tract profiles, (b) acoustic waveforms, and (c) frequency spectra for all vowels.
Digital signals: have a finite (limited) number of values.
Examples:
Printed English language consists of 26 letters, 10 numbers, a space and a set of punctuation marks. Thus, a text is a digital message made of about 50 symbols. Another example is the Morse-coded telegraph message. It is made up of only two symbols (mark and space or dot and dash). It is therefore described as a binary message (Figure 5).
Note that a digital message constructed with M symbols is described as an M-ary message (e.g. English text). So the binary message is a special case of the M -ary message with M = 2.
Figure 5: Morse-code for English alphabets.
Noise immunity of digital signals
Generally, the task of the receiver in any communication system is to extract a message from a distorted and noisy signal at the channel output. Message extraction is often easier from digital signals than from analog signals. Consider the example shown in Figure 6.
Figure 6: Example of a transmitted signal.
This example shows one possible way to represent Morse code. In particular:
The mark (or dot ) can be represented by a rectangular pulse of amplitude A/. The space (or dash ) can be represented by a rectangular pulse of – A/.
The only decision at the receiver is to select between two possible pulses received, not the details of the pulse shape. The decision can easily be made even if the pulses are distorted or noisy.
Assume that the digital message in Figure 6 is distorted (without noise) as shown in Figure 7, or distorted (with the presence of noise) as shown in Figure 8. In both cases, it is still possible to recover the original message as we need only to make a simple binary decision, whether the received pulse is positive or negative, as follows:
If it is positive, the receiver will interpret it as mark. If it is negative, the receiver will interpret it as space.
Note that this works only if the distortion and the noise are within limits. If the distortion and noise levels are very high, the positive parts may become negative (and vice versa), so the receiver will make a wrong interpretation of the signal.
Figure 7: Received distorted signal (without noise).
Figure 8: Received noisy signal (with both distortion and noise).
In contrast, the waveform (signal shape) in analog messages is so important, as even a small distortion or interference can cause error in the received signal (Figure 9).
Repeaters used in analog systems basically consist of filters and then amplifiers (they are not “regenerative” repeaters).
Example 1:
Choose the correct answer:
At each regenerative repeater station in digital communication system, the received signal is:
a) Filtered and then amplified. b) Amplified and then filtered. c) Detected, corrected and retransmitted. d) Detected and a clean signal is transmitted.
Example 2:
A digital signal with 1 mV amplitude is to travel along 3000 km. If the amplitude is attenuated by 0.2 mV every 10 km, find the minimum number of regenerative repeaters required to withstand (resist) this distortion (given that the minimum amplitude which can be interpreted by the receiver as high voltage is 0.2 mV).
Solution:
Min. amplitude allowed is 0.2 mV Every 10 km, amplitude is attenuated by 0.2 mV Every 40 km, amplitude is attenuated by 0.8 mV and amplitude becomes 0.2 mV. No. of repeaters = 3000 / 40 = 75
Due to the noise immunity advantage of digital systems over analog ones, almost all communication systems being installed today are digital.
Why do we study analog communication?
The answer is because of the following reasons:
Old analog communication facilities such as AM and FM radio broadcasting are still in use (the quality of received signal is still acceptable). The only way to understand digital communication is to study the modulation and demodulation techniques used in analog communication systems.
Frequency spectrum:
Frequency spectrum allocation is the division of the electromagnetic spectrum into radio frequency bands (Figure 11 and Figure 12). This spectrum management is regulated by governments in most countries. Radio propagation does not stop at national boundaries. Due to technical and economic reasons, governments have sought to harmonize the allocation of RF bands and their standardization. A number standards bodies work on standards for frequency allocation. International Telecommunication Union (ITU) is a specialized agency of the United Nations (UN) that is responsible for issues that concern information and communication technologies. The ITU mainly coordinates the shared global use of the radio spectrum, promotes international cooperation in assigning satellite orbits, works to improve telecommunication infrastructure in the developing world, and assists in the development and coordination of worldwide technical standards.
In Saudi Arabia, the Communications and Information Technology Commission (CITC) is responsible of all communication and information technology issues including the allocation of radio frequencies.
Figure 11: The complete frequency spectrum (zero frequency DC to light).
Figure 12: Electromagnetic spectrum of radio frequency (30 kHz – 300 GHz).
Channel and Signal Bandwidth:
Generally, the bandwidth is a measure of the width of a range of frequencies from the minimum to the maximum frequency values, measured in hertz (Hz ≡ 1/s). In continuous-time signals, Hz means number of cycles (oscillations) per second. In discrete-time signals, Hz means number of samples per second. Channel bandwidth: is the range of signal bandwidths allowed by a communication channel without significant loss of energy (attenuation). Figure 13 shows various channel types with their bandwidths.
Signal Power:
The signal power Ps is related to the quality of transmission. Increasing Ps strengthens the signal pulse, and reduces the effect of channel noise and interference. The channel bandwidth B and the signal power Ps are “exchangeable” (i.e. one may use less bandwidth if willing to increase the power, or one may reduce the power if given a bigger bandwidth). The two primary communication resources are: bandwidth and the transmitted power. In a given communication channel, one resource may be more available than the other. For example: o A typical telephone channel has a limited bandwidth (~3 KHz), but the power is less restrictive (power is taken from the main power supply). o On the other hand, in space vehicles, huge bandwidth is available but the power is severely limited (no power source is available on space, except the batteries installed in the vehicle).
Sinusoidal waveform
Figure 15: Sinusoidal signal formula.
The sinusoidal signal (waveform) shown is reoresented mathematically as:
𝑣(𝑡) = 𝐴 cos(𝜔 0 𝑡 + 𝜃)
Where:
𝐴: is the maximum amplitude. If 𝑣(𝑡) is a voltage signal, then 𝐴 unit is Volt.
𝜔 0 : is the radian frequency in (radian/sec). This is also called “radial frequency”, “angular frequency” or “angular speed” (which is the magnitude of angular velocity).
𝜃: is the phase angle (usually written in degrees).
𝑇 0 : is the period of the signal (since it is periodic). It is measured in second.
𝑓 0 : is the cyclic (or cyclical) frequency in hertz. It is the reciprocal of the period 𝑇 0. Recall that Hz ≡ 1/s.
Modulation
Analog signal generated by the message source is called “baseband signal” (such a signal has low frequency components, see Figure 14 above). Baseband signals may be directly transmitted without modulation (e.g. audio telephone signal with approx. 3 KHz bandwidth is transmitted on twisted pair wire with almost the same bandwidth). However, baseband signals are not always suitable for direct transmission. When signal and channel frequencies do not match, channel cannot be moved (see the figure below). In this case, the signal must be moved to the right channel bandwidth. Thus, the message signal must be modified for possible (efficient) transmission. This modification is called “modulation”.
Figure 16: The need for modulation.
In the modulation process, the baseband (message) signal modulates a carrier signal. Carrier is a sinusoid of high frequency. Through modulation, one parameter of the carrier signal – e.g. amplitude, frequency or phase – is varied in proportion to the baseband signal m ( t ). We have three types of modulation: o Amplitude Modulation (AM). o Frequency Modulation (FM). o Phase Modulation (PM). In AM, the carrier amplitude varies in proportion to the amplitude of m ( t ), as shown in Figure 17. In FM, the carrier frequency varies in proportion to the amplitude of m ( t ), as shown in Figure 18. In PM, the carrier phase varies in proportion to the amplitude of m ( t ). Demodulation is a reverse process carried out at the receiver to recover (reconstruct) the original baseband (message) signal m ( t ).
Figure 19: Electromagnetic spectrum in terms of wavelength.
If d is the antenna dimension, d α λ or d α 1/ f. This means that:
As frequency f decreases ⇒ the wavelength λ increases ⇒ antenna dimension d increases.
For speech signal: o f : 100 Hz → 3 KHz o λ : 100 Km → 3000 Km Example: to transmit a speech signal, the antenna size would be a fraction or more of its wavelength (i.e. 100 Km → 3000 Km which is really too large!). For 100 MHz carrier signal, the corresponding wavelength is 3 m. So the antenna size will be on the order of 3 m. If the baseband message signal is transmitted on a high frequency carrier signal, then the antenna size will be reduced much.
2. Simultaneous transmission of multiple signals (multiplexing):
Modulation allows multiple signals to be transmitted at the same time in the same geographical area. Consider TV signals, without modulation, multiple video signals will be interfering with each other (because all video signals inherently have the same bandwidth of approx. 4.5 MHz). If carriers are chosen sufficiently far apart in frequency, the TV channels (signals) will not overlap (no interference will occur). This process is called “frequency division multiplexing” (FDM); see the figure below. FDM is the method of transmitting several signals simultaneously over nonoverlapping frequency bands (similar approach is used in AM and FM radio broadcasting).
Figure 20: Example of FDM.
Besides FDM, there is another multiplexing method called “time division multiplexing” (TDM). This is the method of dividing time between signals. It is suitable for pulse trains; see the figure below.
Figure 21: Example of TDM.
Modulation is the process to move the message signal into a specific frequency band dictated by the physical channel. AM / FM radios and old TV signals are based on analog modulation techniques. 2G, 3G, HDTV and DSL are based on digital modulation techniques.
The main focus of this course:
Analog modulation is the focus of this course. This type of modulation has two categories: o Linear: amplitude modulation (AM) – covered in Chapter 4. o Exponential: angle modulation including both frequency and phase modulation (FM & PM) – covered in Chapter 5. The performance (behavior) of analog communication system with the presence of noise is also studied in this course. Note that throughout the course, we will use f and ω interchangeably. Recall that: ω = 2π f
