TM355 Communications Technology Block 1 PDF

TM355: COMMUNICATIONS TECHNOLOGY BLOCK 1 PART 1: CHANNELS FOR COMMUNICATIONS 1 Prepared by: Dr. Naser Zaeri Arab Open University  OUTLINE Introduction Analogue and digital signals Optical f...

TM355: COMMUNICATIONS TECHNOLOGY BLOCK 1 PART 1: CHANNELS FOR COMMUNICATIONS 1 Prepared by: Dr. Naser Zaeri Arab Open University  OUTLINE Introduction Analogue and digital signals Optical fibre Prepared by: Dr. Naser Zaeri 2  1. INTRODUCTION (1/6) The simplest type of communications system consists of a transmitter that sends a signal along a channel to a receiver. This first part of Block 1 is mainly concerned with the physical channel: the properties of optical fibre and copper cable, and the behaviour of radio waves. Each of these transmission media has its own merits and limitations, and this text discusses the suitability of the various types of media for different applications. There may be many transmitters and/or receivers, and it is necessary to ensure that the correct sender communicates with the correct recipient. Prepared by: Dr. Naser Zaeri 3  1. INTRODUCTION (2/6) One example is a local area network (LAN) – a computer network covering a relatively small area, such as a company site. Another example is the ordinary telephone system, referred to as the public switched telephone network (PSTN). Prepared by: Dr. Naser Zaeri 4  1. INTRODUCTION (3/6) Figure 1.1 distinguishes between the access network (the local exchange and its connections to the subscribers it serves) and the core network, which is everything beyond the local exchange. The technical demands of the access and core networks are rather different. The access network may serve millions of subscribers, each on a different site, but with only short distances involved. In contrast, trunk lines in the core network carry multiple calls between two places that may be hundreds of miles apart. Prepared by: Dr. Naser Zaeri 5  1. INTRODUCTION (4/6) Different techniques have evolved to meet these demands. At one time, the entire system was built with copper cables, which carry information as electrical signals. Later the technology of optical fibres was developed, in which messages are sent using light rays that are constrained to follow a path within the fibre. Optical fibre has a number of advantages over copper, particularly over long distances, so it now predominates in the core network, although copper still retains a key place in the access network. Between them, optical fibre and copper cable serve most of the needs of the PSTN and other fixed networks. Prepared by: Dr. Naser Zaeri 6  1. INTRODUCTION (5/6) Mobile communications systems require radio. This is a form of electromagnetic radiation like light, and can propagate through air or space in various ways. While the term ‘radio transmitter’ brings to mind the tall masts for radio and television broadcasting, smaller transmitters are everywhere: in mobile phones, computers (to provide a WiFi connection). Cables confine signals to a defined route, but radio waves spread out over a wide area. Prepared by: Dr. Naser Zaeri 7  1. INTRODUCTION (6/6) Radio is thus a shared medium, so many different transmissions are competing for attention everywhere, and a receiver has to be able to select the right one. The shared nature of radio leads to problems of resource allocation. Radio also has security implications, as signals can easily be intercepted. Prepared by: Dr. Naser Zaeri 8  2. ANALOGUE AND DIGITAL SIGNALS (1/2) The term ‘signal’ describes the form in which a message is sent along a communications channel. In the case of copper cable, the signal is a varying electrical voltage. For both optical fibre and radio, the signal is a varying electromagnetic wave. Prepared by: Dr. Naser Zaeri 9  2. ANALOGUE AND DIGITAL SIGNALS (2/2) Figure 1.2(a) illustrates part of a speech signal as produced by the microphone in a telephone. The voltage signal follows the air vibrations and is called an analogue signal because the voltage is analogous to the fluctuating air pressure. Figure 1.2(b) represents a data signal as might be seen on a cable to a computer on a LAN (such a connection between a home PC and a router.) Prepared by: Dr. Naser Zaeri 10  2.1 BENEFITS OF TRANSMITTING DIGITAL SIGNALS (1/2) Whenever a signal is sent along a communications channel, two things happen to it: it gets smaller (it attenuates) and it gets distorted (the shape changes). This is illustrated in Figure 1.3 for both an analogue signal and a digital signal. It is possible to compensate for attenuation by amplifying the received signal. With digital signals, on the other hand, we can in principle get rid of the distortion entirely by the process of regeneration, provided the distortion is not too great (threshold detection). Another reason for using digital technologies in communications is that voice, music and video can all be handled by the same techniques as computer data if they are first converted to a digital form. Prepared by: Dr. Naser Zaeri 11 2.1 BENEFITS OF TRANSMITTING DIGITAL SIGNALS (2/2) Prepared by: Dr. Naser Zaeri 12  2.2 CONVERTING ANALOGUE TO DIGITAL (1/3) Analogue-to-digital converters (ADCs) and digital- to-analogue converters (DACs) are electronic devices that convert between analogue and digital in each direction. To convert an analogue signal to digital form, it is first sampled by measuring its value at regular intervals in time. Prepared by: Dr. Naser Zaeri 13  2.2 CONVERTING ANALOGUE TO DIGITAL (2/3) The next step is to encode each of the possible quantisation levels with a binary number. To restrict the measured values to a discrete set, the values are quantised. Since a binary number n bits long can take any of 2𝑛 different values, the number of quantisation levels allowed is normally a power of two. Thus a 4-bit number can represent 24 = 16 different levels, and an 8-bit number can represent 28 = 256 different levels. Prepared by: Dr. Naser Zaeri 14  2.2 CONVERTING ANALOGUE TO DIGITAL (3/3) For a given range of variation, a large value of n improves the accuracy of the conversion because the quantisation levels are closer together; conversely, a small n results in a smaller amount of binary data at the expense of conversion accuracy. In converting an analogue signal to digital, it appears that information has been lost in two different ways: a) The signal is not measured at every instant of time but only at the sampling points. b) An approximation has been made by rounding the samples to the nearest quantisation level. Prepared by: Dr. Naser Zaeri 15  2.3 ANALYSING SIGNALS 2.3.1 SINUSOIDS (1/2) Sinusoids are important not only because they turn up naturally in a wide variety of situations, but also for their mathematical simplicity. A sinusoid is a periodic signal  repeats at regular time intervals. A section of a periodic signal between two consecutive corresponding points, such as the maxima, is called a cycle, and the duration of a cycle is the period. The number of cycles in one second is the frequency. The unit of frequency is the hertz (Hz), where 1 Hz = 1 cycle per second. f=1/T Amplitude is the maximum value of the sinusoid. Prepared by: Dr. Naser Zaeri 16  2.3.1 SINUSOIDS (2/2) Another characteristic of a sinusoidal signal is its phase. This relates to the point that the sinusoid has reached at a particular time. For example, at zero time the signal is zero and rising. Shifting the signal to the right or left changes its phase. One of the reasons that sinusoids are so significant in communications is that they form the components of other periodic signals. Prepared by: Dr. Naser Zaeri 17 2.3.2 OTHER PERIODIC SIGNALS (1/4) An important result in communications theory, due to Joseph Fourier (1768–1830), shows that any periodic signal may be expressed as a sum of multiple sinusoids. The graph on the left of Figure 1.7(a) shows a sinusoid as it progresses in time, while the graph on the right shows another representation of the signal: a function of frequency.  These are known respectively as time-domain and frequency-domain representations. The frequency domain is also known as the spectrum. Prepared by: Dr. Naser Zaeri 18 2.3.2 OTHER PERIODIC SIGNALS (2/4) Prepared by: Dr. Naser Zaeri 19 2.3.2 OTHER PERIODIC SIGNALS (3/4) Any signal can be represented in either the time or the frequency domain. A sinusoid, is shown as a single line in the frequency domain because it represents a single frequency of a particular strength. Figure 1.7(b) shows a periodic signal with a different shape (or waveform as it is sometimes called), known as a saw-tooth wave from its appearance. The corresponding frequency-domain representation reveals that it is made up of the sum of sinusoids of decreasing amplitude.  Notice that the frequencies of these sinusoids are exact whole-number multiples of the lowest frequency. These higher-frequency sinusoids are called harmonics. Prepared by: Dr. Naser Zaeri 20 2.3.2 OTHER PERIODIC SIGNALS (4/4) The waveform in Figure 1.7(c), known as a square wave, may be regarded as a binary signal with alternate 1s and 0s. In the frequency domain there are again sinusoids at multiples of the lowest frequency, though in this case the even multiples (or harmonics) are missing. A consequence of this theory is that a communications channel that can transmit, say, a 1 MHz sine wave correctly may not be able to transmit a 1 MHz periodic signal with a different waveform, unless it can also transmit sinusoids at 2 MHz, 3 MHz and so on. A complete frequency-domain representation would also include phase. For many purposes, though, phase is less important than frequency; it is not a consideration in allocating radio spectrum, for example. Prepared by: Dr. Naser Zaeri 21 2.3.3 NON-PERIODIC SIGNALS (1/2) Non-periodic signals (also known as aperiodic signals) also have both a time-domain and a frequency-domain representation, but the details are different.  There are no longer lines at particular frequencies; instead, the spectrum is spread out over a continuous range of frequencies. Prepared by: Dr. Naser Zaeri 22 2.3.3 NON-PERIODIC SIGNALS (2/2) For example, a speech signal ranges from around 100 Hz to a few thousand Hz (for telephone-quality speech, a range of 300 Hz to 3400 Hz is often assumed). In practical communications, exactly periodic signals are the exception. Signals that carry real information, such as speech, music or video, do not repeat endlessly. Prepared by: Dr. Naser Zaeri 23  3. OPTICAL FIBRE (1/2) Optical fibre can transmit large amounts of information rapidly over long distances using light signals, and so has become the preferred technology for trunk cable communications and increasingly for LANs. A basic optical-fibre link has three main components (Figure 1.10): a suitable source of light (not necessarily within the visible range), controlled by input data in the form of an electrical signal the optical fibre itself, which carries the resulting pulses of light to … a detector, which converts the pattern of light and dark back to an electrical signal. Prepared by: Dr. Naser Zaeri 24 3. OPTICAL FIBRE (2/2) Prepared by: Dr. Naser Zaeri 25  3.1 ELECTROMAGNETIC RADIATION (1/6) Electromagnetic radiation includes radio waves, light, the radiation felt as heat, and ultraviolet radiation from the Sun. Light, as an electromagnetic wave, is a wave pattern carried by interdependent electric and magnetic fields. In electromagnetic wave, electricity and magnetism are in fact intimately related. Electric fields, like magnetic fields, are invisible but can exert forces on objects. Strong electric fields form in thunderclouds and break down when lightning strikes. Prepared by: Dr. Naser Zaeri 26  3.1 ELECTROMAGNETIC RADIATION (2/6) Electric fields and magnetic fields can both store and release energy. They are also linked together by a set of relationships known as Maxwell’s equations. Changing electric fields generate changing magnetic fields, and vice versa. The effect of all of this is that disturbances in the fields can be self-sustaining, and spread outwards like waves in a pond. Prepared by: Dr. Naser Zaeri 27  3.1 ELECTROMAGNETIC RADIATION (3/6) Figure 1.13 represents a an electromagnetic wave in terms of its electric and magnetic components. The axis along the direction of travel measures distance along the wave. The other axes represent the strength of the electric and magnetic fields at any point along the wave. Prepared by: Dr. Naser Zaeri 28  3.1 ELECTROMAGNETIC RADIATION (4/6) The electric field and magnetic field are both sinusoidal and are at right angles to each other. This is just a snapshot of the wave at one particular instant in time. The whole wave pattern moves forward at the speed of light, which in free space (a vacuum) is about 300 000 000 m/s = 3×108 m/s and is conventionally denoted c. All electromagnetic waves travel at this same speed in free space. Prepared by: Dr. Naser Zaeri 29  3.1 ELECTROMAGNETIC RADIATION (5/6) In free space, the electric field contains half the power in an electromagnetic wave and the magnetic field contains the other half. As the wave moves forward, it conveys energy. So, for example, light from the Sun can be converted to electrical energy by solar panels. The power generated by such a panel is simply the energy it produces per unit time. Prepared by: Dr. Naser Zaeri 30  3.1 ELECTROMAGNETIC RADIATION (6/6) Note on Energy and Power: Power is the rate of flow of energy, and is measured in watts (W). Energy and power are related as follows: energy = power × time. Example: Suppose a 2 kW kettle takes three minutes to boil. Three minutes is 0.05 hours, so the energy used is: 2000 W × 0.05 h = 100 Wh or 0.1 kWh (kilowatt-hours). However, although energy is quoted as kWh on electricity bills, the SI unit of energy is the joule (J), where energy in joules is the power in watts multiplied by the time in seconds. Repeating the kettle example using SI units gives: 2000 W × 180 s = 360 000 J or 360 kJ. Prepared by: Dr. Naser Zaeri 31 3.1.1 WAVELENGTH AND FREQUENCY Electromagnetic waves are characterised by their wavelength, the distance between two consecutive peaks (or other corresponding points.) Light waves have short wavelengths, measured in nanometres (nm, 10−9 m). Light of different wavelengths is perceived as different colours, for example 650 nm is red and 520 nm is green. A wave can also be described by its frequency, where: 𝒄=𝝀×𝒇 Prepared by: Dr. Naser Zaeri 32  3.1.2 THE ELECTROMAGNETIC SPECTRUM Figure 1.14 shows the complete electromagnetic spectrum. Prepared by: Dr. Naser Zaeri 33  3.2 LIGHT WAVES IN OPTICAL FIBRE (1/3) In optical fibres, light travels through highly transparent glass along guided paths, so there are some differences from propagation in free space. For one thing, light travels significantly more slowly in glass than in a vacuum (or in air). The speed of light, 𝒗, in a medium such as glass is found by: 𝒗 = 𝒄/𝒏 where “𝒏” is the refractive index and depends on the material. It is around 1.5 for most optical glasses. Prepared by: Dr. Naser Zaeri 34  3.2 LIGHT WAVES IN OPTICAL FIBRE (2/3) So what keeps light guided along its path in an optical fibre? Why does the light not just stop when it comes to the first bend? Optical fibres work because the refractive index is not the same all the way across the fibre, but is higher in the central core than it is in the cladding around the core. Light can change direction by two processes, refraction and reflection, which take place in lenses and mirrors respectively. Refraction can occur when a ray of light travels from one medium to another medium with a different refractive index. The light continues within the second medium, but on a diverted path (Figure 1.15a). Prepared by: Dr. Naser Zaeri 35  3.2 LIGHT WAVES IN OPTICAL FIBRE (3/3) However, if light is directed from one medium towards another with a lower refractive index, and it hits the boundary at a sufficiently small angle, it is not refracted but reflected back into the first medium. This process is known as total internal reflection (Figure 1.15b). Thus in an optical fibre, provided the light enters the fibre from the right direction (not too large an angle from the axis of the fibre), it will continue all the way along the fibre, relying on total internal reflection to keep it on course. Prepared by: Dr. Naser Zaeri 36  3.3 TYPES OF FIBRE (1/4) The diameter of the core is a key feature of an optical fibre. the larger the diameter of the fibre, the more light it will let through. A large-diameter fibre also has some practical advantages: aligning fibres to join them together becomes easier. However, when it comes to transmitting data over distances at high data rates, it turns out that a large diameter is not necessarily the best. A fibre with a core diameter that is large in comparison with the wavelength is known as a multimode fibre because light can travel along it in a variety of ways. Prepared by: Dr. Naser Zaeri 37  3.3 TYPES OF FIBRE (2/4) Commonly, the diameter of the core for a multimode fibre is 50 μm, much larger than the wavelengths typically used (which are in the order of 1.5 μm). The cladding diameter is 125 μm. From Fig. 1.16, some paths that the light can take are more direct than others, meaning two rays of light that set off at the same time may not reach the other end of the fibre exactly simultaneously. A fibre (as the one shown in Fig. 1.16) where the refractive index changes abruptly between core and cladding is called a step-index fibre. Prepared by: Dr. Naser Zaeri 38  3.3 TYPES OF FIBRE (3/4) Another variant of the multimode fibre is called a graded-index fibre (Fig. 1.17)  the refractive index varies smoothly from a maximum in the centre of the core to a minimum within the cladding. This means waves that take slightly longer paths travel slightly faster, and so different waves setting off at the same time arrive nearly simultaneously at the other end of the fibre. Prepared by: Dr. Naser Zaeri 39  3.3 TYPES OF FIBRE (4/4) If the core diameter of a step-index fibre is reduced, there are fewer ways or modes in which the wave can propagate. There comes a point where only one mode can propagate, and signals all travel along the same path. This happens when the core is a few wavelengths in diameter – typically 10 μm. This type of fibre is called a single-mode fibre, and it provides the best performance over long distances in a number of respects. For this reason, single-mode fibre is preferred over multimode types for long-haul communications. The cladding diameter is typically 125 μm, the same as for multimode fibre. Prepared by: Dr. Naser Zaeri 40  TM355: COMMUNICATIONS TECHNOLOGY BLOCK 1 PART 1 (CONT.): CHANNELS FOR COMMUNICATIONS 1 Prepared By: Dr. Naser Zaeri Arab Open University  OUTLINE 3.4: limitations of optical fibre 3.5: Components of an optical-fibre link 4. Copper cable 5. radio 6. Analogue modulation Prepared By: Dr. Naser Zaeri 2  3.4 LIMITATIONS OF OPTICAL FIBRE 3.4.1 ATTENUATION AND DECIBELS (1/4) Modern optical fibres are extremely transparent and so can carry optical signals long distances. However, some of the energy supplied by the source is eventually absorbed by the fibre as the signal passes along it. The process by which the signal gradually loses power as it travels along a transmission medium is called attenuation. Manufacturers of optical fibres specify how much a fibre attenuates in terms of so much attenuation in decibels (dB) per kilometre length. The decibel is a logarithmic measure of the ratio between two powers. Prepared By: Dr. Naser Zaeri 3  3.4.1 ATTENUATION AND DECIBELS (2/4) More specifically, Power loss (or gain) in decibels (dB) is given by: 𝑃2 𝑃 = 10𝑙𝑜𝑔10 𝑃1 Where 𝑃1 is the transmitted power and 𝑃2 is the received power. Note: The two powers to be compared may represent a loss, as in fibre attenuation, or a gain, as when a signal is amplified. 0 dB, meaning ‘no loss’. increasing a power by 3 dB doubles the power attenuating a power by 3 dB halves the power. Prepared By: Dr. Naser Zaeri 4  3.4.1 ATTENUATION AND DECIBELS (3/4) Example: Suppose a fibre-optic link consists of three sections. Half the power is lost in the first section, another half of the remaining power is lost in the second section, and 95% of the remaining power is lost in the third section. Find the total power loss in dB at the end of the third section. Sol.: The total fraction remaining is: 0.5 × 0.5 × 0.05 = 0.0125 𝑃2 0.0125𝑃1  In dB: 𝑃 = 10𝑙𝑜𝑔10 = 10𝑙𝑜𝑔10 = -19 dB. 𝑃1 𝑃1 Prepared By: Dr. Naser Zaeri 5  3.4.1 ATTENUATION AND DECIBELS (4/4) Although multimode fibre has a higher figure for attenuation, it is generally preferred for short-distance applications because of lower component costs and greater ease of use than single- mode fibre. 6  3.4.2 PULSE SPREADING (1/3) As well as attenuation, various effects distort signals in optical fibres by smearing out sharp transitions between light and dark sections of the signal. This limits the data rate that can be obtained over a length of fibre, because parts of the signal start to merge into each other. As with attenuation, the effects are cumulative  the longer the fibre, the worse it gets. Figure 1.21 illustrates the problem: The signal transmitted is called a pulse, so the effect is known as pulse spreading. Prepared By: Dr. Naser Zaeri 7  3.4.2 PULSE SPREADING (2/3) One reason for pulse spreading in multimode fibres: different path lengths result in different timings for the trip through the fibre  This is called multimode distortion (Fig. 1.22) and is the main cause of pulse spreading in multimode fibres. Prepared By: Dr. Naser Zaeri 8  3.4.2 PULSE SPREADING (3/3) Although this effect is eliminated in single-mode fibres, other mechanisms can still cause pulse spreading: Dispersion, or ‘chromatic dispersion’, is caused by light of different wavelengths travelling at different speeds. Polarisation mode distortion affects single-mode fibres and is caused by another variation in the speed of light: the speed varies with the orientation of the light wave in the fibre. Prepared By: Dr. Naser Zaeri 9  3.5 COMPONENTS OF AN OPTICAL-FIBRE LINK 3.5.1 OPTICAL TRANSMITTERS AND DETECTORS (1/3) Optical transmitter: converts input data in the form of an electrical signal into a light signal that is sent along the fibre. There are two main types, both semiconductor devices: the light-emitting diode (LED) and the laser diode LEDs for optical transmitters are similar in principle to the LEDs seen in displays and as indication lights in consumer goods. However, rather than emitting visible light, they emit in the infrared region of the spectrum, where optical fibres are most transparent. Laser diodes are also found in CD, DVD and Blu-ray drives, where they read and write data from the disc. Prepared By: Dr. Naser Zaeri 10  3.5.1 OPTICAL TRANSMITTERS AND DETECTORS (2/3) LEDs are inexpensive compared to laser diodes and so are used in some multimode fibre systems. However, they have a number of disadvantages: they are lower in power and emit over a range of wavelengths, leading to dispersion. LEDs emit a relatively broad cone of radiation, whereas a laser diode emits a strongly aligned beam. Laser diode is much more efficient at transferring its energy to the fibre. The data rate that can be obtained from the transmitter depends on how fast the beam can be modulated. Again, the laser diode has an advantage over the LED in the speed at which it can switch. Prepared By: Dr. Naser Zaeri 11  3.5.1 OPTICAL TRANSMITTERS AND DETECTORS (3/3) At the other end of the fibre, a detector converts the light signal back into an electrical signal. The type of detector commonly used is called a photodiode. It provides a current output that varies with the intensity of the light it receives. Prepared By: Dr. Naser Zaeri 12  3.5.2 OPTICAL AMPLIFIERS (1/3) Optical fibres attenuate markedly over long distances. For example, the best-case attenuation in Table 1.1 is 0.35 dB km−1, which over a 300 km link would amount to 105 dB, a huge drop in power. One way of increasing the range of an optical-fibre link is to use a repeater or regenerator. These are devices that counteract the effects of attenuation by restoring an optical signal to its original form. The optical signal is converted back to an electrical signal, which is then processed electronically and retransmitted optically. Prepared By: Dr. Naser Zaeri 13  3.5.2 OPTICAL AMPLIFIERS (2/3) Repeaters and regenerators are distinguished by the extent of the processing that is carried out, the term ‘repeater’ tending to include simpler devices than ‘regenerator’. A repeater amplifies the signal to bring it back to its original amplitude, but at the same time it may also amplify any noise that is mixed with the signal. A regenerator does further processing, so that the degraded received pulse would be reshaped and retimed as well as being restored to its original amplitude. Thus the regenerated pulse is a copy of the original transmitted pulse with any noise removed, provided the signal has not deteriorated too far. 14  3.5.2 OPTICAL AMPLIFIERS (3/3) In principle, any distance can be covered using a chain of regenerators, but there are some practical disadvantages: these devices have to be powered and maintained, and many of them may be needed to cover long distances. This is a particular problem for international cables, which often run under the ocean, making power provision and maintenance very difficult. Optical amplifiers have been developed as a better solution for long-haul links. They amplify the optical signal directly, without converting it back to an electrical signal. The idea: allows energy to be ‘pumped’ into the atoms of a material and then released later when ‘stimulated’ by radiation, thus creating more radiation of the same wavelength. Prepared By: Dr. Naser Zaeri 15  4. COPPER CABLE The sender and receiver in most communications systems operate with electrical signals Copper cable is a simple solution because it does not involve any conversions to other types of energy (light or radio waves). Prepared By: Dr. Naser Zaeri 16  4.1 TRANSMISSION IN CABLES (1/2) Figure 1.26 shows a general type of circuit in which a power source sends power to a load. The source and load might be a battery and bulb in a torch, or the house mains supply and a toaster, or (in the case of communications) a transmitter and receiver. Voltage is the force that sends the current around the circuit. It is measured in volts (V). Batteries may be 1.5 V, 9 V and so on, and in the UK electricity is supplied to houses at a nominal 230 V. Current represents the movement of electrons and is measured in amperes or amps (A). 17 4.1 TRANSMISSION IN CABLES (2/2) Communications signals with high frequencies or data rates involve effects in cables that are not apparent in other circuits, such as house wiring. At the high frequencies and data rates used in communications, the speed of a signal in a copper cable depends on the construction of the cable. A typical figure is 2 ×108 m/s, which is comparable to the speed of signals in optical fibres. Prepared By: Dr. Naser Zaeri 18 4.1.1 MAGNETIC AND ELECTRIC FIELDS (1/2) A conductor carrying current has both magnetic and electric fields associated with it. A magnetic field encircles a conductor carrying an electric current. The field may be strong enough to exert appreciable forces on ferrous objects, particularly when the field is concentrated as it is in an electric motor. Conductors also have associated electric fields. In high-voltage lines, the electric field ionises the intervening atmosphere and creates a spark. Prepared By: Dr. Naser Zaeri 19 4.1.1 MAGNETIC AND ELECTRIC FIELDS (2/2) The low voltages and currents used in computer networks and phones generate tiny fields in comparison to these examples. Signal propagation in cables can in fact be regarded as another type of electromagnetic wave. Because of the wave nature of signals in cables, effects such as reflection are possible, whereby some of the signal moves back along the cable. Prepared By: Dr. Naser Zaeri 20 4.1.2 CROSSTALK AND RADIATION Commonly, copper cables carrying different signals run alongside each other for convenience – they may, for example, share a conduit along a street. Many cables have multiple pairs of conductors rather than just one, so that several independent signals can be carried. There is a potential problem, though, in that the electric and/or magnetic fields associated with one pair of conductors may couple with the conductors next to them to some extent, so that a weak version of the signal is transferred to the other conductor pair. This is called crosstalk and can be minimised by appropriate design of the cable. Prepared By: Dr. Naser Zaeri 21 4.2 ATTENUATION AND DISTORTION IN CABLES (1/2) All ordinary conductors have a property called resistance: electrons do not flow along conductors entirely freely, but are subject to frequent collisions. This results in loss of electrical energy, which is converted to heat. Resistance is particularly important in power distribution, where it must be kept as low as possible to avoid wasting energy, but it also affects communications systems that use copper cables. The two conductors in a pair are kept apart by a non-conducting material, usually plastic, known as an insulator or dielectric. Prepared By: Dr. Naser Zaeri 22 4.2 ATTENUATION AND DISTORTION IN CABLES (2/2) Resistance and leakage both increase in proportion to the length of the cable, so they need to be taken into account particularly over long-distance links. At high frequencies or high data rates, the material separating the conductors can have an effect in increasing the attenuation over and above that observed at low frequencies. This is because a small part of the energy in the electromagnetic fields is wasted as heat in a process known as dielectric loss. Distortion can also occur in cables when signals of different frequencies travel at different speeds (just as in optical fibre.) Prepared By: Dr. Naser Zaeri 23  4.3 TYPES OF CABLE (1/2) Figure 1.28 illustrates two of the many types of cable that are used for transmitting digital data and other high-frequency signals: unshielded twisted pair (UTP) and coaxial cable. In a UTP cable, a pair of conductors is twisted together along its length. The effect of the twisting is that any interference entering the cable will affect both conductors equally, and can be cancelled out by using a receiver that is sensitive only to the difference in voltage between the two conductors. Prepared By: Dr. Naser Zaeri 24  4.3 TYPES OF CABLE (2/2) So the twisting gives some protection against crosstalk. In coaxial cable, the two conductors take the form of a centre conductor with a conducting shield around it. An advantage of this construction is that the electric and magnetic fields are confined within the shield. This gives the cable good immunity to interference and minimises losses due to radiation. A common use of coaxial cable is for connection to TV antennas. Prepared By: Dr. Naser Zaeri 25  5. RADIO Radio waves are another form of electromagnetic radiation but at a much lower frequency than light or infrared, 300 GHz often being regarded as the upper limit. Their electric and magnetic fields are generated directly from electrical signals in structures known as antennas, or sometimes aerials. A receiving antenna converts a radio signal back to an electrical signal. An antenna simply consists of one or more conductors, and is effective at launching (or receiving) a radio wave at a particular frequency or band of frequencies by virtue of its physical shape and dimensions. An antenna that is effective at transmitting at a given frequency is also effective at receiving at the same frequency. Prepared By: Dr. Naser Zaeri 26 5.1 BANDWIDTH AND RECEPTION (1/3) The amount of spectrum occupied by a signal is called the bandwidth. The bandwidth is equal to the difference between the highest and lowest frequencies, 𝒇𝟐 − 𝒇𝟏. The larger the bandwidth, the more information the signal can convey. The centre frequency of this transmission, halfway between 𝒇𝟏 and 𝒇𝟐. In many modulation schemes, the centre frequency is the same as the frequency of the unmodulated signal. Prepared By: Dr. Naser Zaeri 27 5.1 BANDWIDTH AND RECEPTION (2/3) Prepared By: Dr. Naser Zaeri 28 5.1 BANDWIDTH AND RECEPTION (3/3) Figure 1.30 shows what the response of an ideal receiver would look like, with the response of a typical practical receiver by way of comparison. The range of frequencies that the receiver responds best to is called the passband: Extends from a lower cut-off frequency to a higher cut-off frequency. The cut-off frequencies are the points where the signal strength in volts has fallen to 0.707 of the maximum (a voltage drop of this magnitude corresponds to a drop in power of 3 dB). Prepared By: Dr. Naser Zaeri 29 5.2 SOME PROPERTIES OF RADIO WAVES 5.2.1 THE INVERSE SQUARE LAW The inverse square law describes the reduction in power with distance from the transmitter, due to spreading. Figure 1.31 represents a wave moving outwards from a transmitter, which is assumed here to radiate equally well in all directions – isotropically. A receiver that is n times as far from the transmitter will receive 𝟏 of the power. 𝒏𝟐 30  5.2.2 REFLECTION (1/2) Radio waves can be both reflected by specular reflection (from smooth, flat surfaces) and scattered. Radio waves are reflected by many surfaces, including the ground, buildings and vehicles. They are particularly well reflected by metal surfaces, for example in satellite dishes. Prepared By: Dr. Naser Zaeri 31  5.2.2 REFLECTION (2/2) Scattering occurs when reflecting objects or features are small compared to the wavelength. Radio waves can be scattered by a variety of objects, ranging from small dust particles to meteors high in the atmosphere. Scattering by intervening objects or particles can result in a loss of useful energy between the transmitter and receiver, reducing the received signal. Prepared By: Dr. Naser Zaeri 32  5.2.4 ABSORPTION Radio waves can be absorbed by the medium they travel through. The attenuation due to absorption is measured in decibels per metre or kilometre. The loss due to absorption is separate from, and additional to, any power loss due to spreading as accounted for by the inverse square law. Absorption is dependent on frequency, and this is one of the reasons why certain frequency bands are preferred for different radio applications. Prepared By: Dr. Naser Zaeri 33  5.2.5 DIFFRACTION Diffraction: is the spreading or bending of an electromagnetic wave when it passes through a gap or encounters a sharp corner. Diffraction is very dependent on the dimensions of a gap or the sharpness of an edge. Prepared By: Dr. Naser Zaeri 34  5.4 PROPAGATION 5.4.4 PROPAGATION MODELS Propagation: at low frequencies, long-distance transmission depends on the state of the ionosphere, while multipath propagation causes fading at high frequencies. Propagation in terrestrial environments is complex, and so computer models have been developed to help with prediction. The inverse fourth-power law is often invoked as a first approximation for propagation at VHF and above in typical terrestrial environments. With the inverse fourth-power law the received power decreases in proportion to 1 𝑑4. Prepared By: Dr. Naser Zaeri 35  6. ANALOGUE MODULATION In communications, the types of signal that can be transmitted over a channel are often very different from the original message signal. Radio waves at the frequencies found in sound are not easy to transmit and receive, and do not propagate well. A transmitter that converts a sound signal directly to a radio wave, while possible, is of limited use anyway; if there was more than one of them, they would effectively talk over each other. The idea of using different frequencies from the original message is not confined to radio, but also applies to other media such as cable, in the process known as modulation, the message signal is converted to a suitable form for transmission Modulation is usually a matter of varying one or more properties of the sine wave in a way that represents the information to be conveyed. Prepared By: Dr. Naser Zaeri 36 6.1 AMPLITUDE MODULATION (1/4) In amplitude modulation (AM) the amplitude of the carrier waveform is altered in proportion to the information signal, referred to as the modulating signal. The term envelope is used to describe the varying strength, or shape, of the modulating signal After modulation, the carrier’s amplitude takes on the envelope of the modulating signal. The modulated signal is created by multiplying the modulating signal and the carrier signal together. Done using a device known as a mixer. A mixer is used to shift power at one frequency to power at another frequency. Prepared By: Dr. Naser Zaeri 37 6.1 AMPLITUDE MODULATION (2/4) The carrier waveform is usually generated using a local oscillator, which is an electronic circuit that produces a periodic electronic signal – in this case a sine wave with frequency 𝑓𝑐. Advantage of AM is its simplicity. Disadvantage of AM signal is highly susceptible to noise. 38 6.1 AMPLITUDE MODULATION (3/4) Mathematically: if the carrier waveform is represented by 𝑣𝑐 𝑡 = 𝑣𝑐 cos(𝜔𝑐 𝑡), and the information or modulating signal is also represented by a sinusoid, 𝑣𝑚 𝑡 = 𝑣𝑚 cos(𝜔𝑚 𝑡), then 𝑣𝐴𝑀 𝑡 = 𝑣𝑚 cos(𝜔𝑚 𝑡) x 𝑣𝑐 cos(𝜔𝑐 𝑡) 𝑣𝑚 𝑣𝑐 = cos 𝜔𝑐 + 𝜔𝑚 𝑡 + cos 𝜔𝑐 − 𝜔𝑚 𝑡 2  The modulated signal comprises two components, known as the upper sideband, at (𝜔𝑐 + 𝜔𝑚 ), and the lower sideband, at (𝜔𝑐 - 𝜔𝑚 ). Prepared By: Dr. Naser Zaeri 39 6.1 AMPLITUDE MODULATION (4/4) Bandwidth of the modulated signal, 𝐵𝐴𝑀 , is twice that of the original information signal, 𝐵𝑚  𝐵𝐴𝑀 = 2𝐵𝑚. Prepared By: Dr. Naser Zaeri 40 6.2 FREQUENCY MODULATION (1/3) In frequency modulation (FM) the frequency of the carrier waveform is altered in proportion to the envelope of the modulating signal, so the amplitude and the phase remain the same. The modulated signal is usually created using a voltage-controlled oscillator (VCO). This is an electronic circuit that takes a voltage signal as an input and produces a periodic electronic signal – in this case a sine wave – as an output. Prepared By: Dr. Naser Zaeri 41 6.2 FREQUENCY MODULATION (2/3) Prepared By: Dr. Naser Zaeri 42 6.2 FREQUENCY MODULATION (3/3) The frequency deviation, Δf: defined as the maximum deviation of the FM-modulated frequency from the carrier frequency. The modulation index, β: ratio of the frequency deviation to the highest frequency component in the modulating signal (modulating frequency), 𝑓𝑚 : ∆𝑓 𝛽= 𝑓𝑚 Bandwidth of frequency modulated signal is: 𝐵𝐹𝑀 = 2 ∆𝑓 + 𝑓𝑚 = 2(1 + 𝛽)𝑓𝑚 Prepared By: Dr. Naser Zaeri 43  6.3 PHASE MODULATION In the third analogue modulation technique, phase modulation (PM), the phase of the carrier waveform is altered in proportion to the amplitude of the modulating signal. Prepared By: Dr. Naser Zaeri 44  TM355: COMMUNICATIONS TECHNOLOGY BLOCK 1 PART 2: AN INTRODUCTION TO THE THEORY OF SIGNALS 1 Prepared By: Dr. Naser Zaeri Arab Open University  OUTLINE Introduction Time and frequency domains Digital modulation Quadrature modulation schemes Prepared By: Dr. Naser Zaeri 2  1. INTRODUCTION In this part of Block 1: We focus is on the underlying theory behind signals and modulation. We understand the time and frequency domains and why these are important. Prepared By: Dr. Naser Zaeri 3 2. TIME AND FREQUENCY DOMAINS 2.1 TIME-DOMAIN REPRESENTATION OF SINE WAVES [1/3] Basic radio waves, called carrier waves, are sinusoidal. Time-domain representation of a signal: how the strength of the signal varies over time. Mathematically represented by: 𝑣 𝑡 = 𝐴 sin 2𝜋𝑓𝑡 + 𝜑 = 𝐴 sin(𝜔𝑡 + 𝜑) A is the amplitude (or maximum value) measured in volts (V) f is frequency measured in hertz (Hz). T is time period (measures in second), where f=1/T. ω is the angular frequency = 2𝜋f, measured in radians per second (rad/s). 𝝋 is the relative phase, measured in radians or degrees. Prepared By: Dr. Naser Zaeri 4 2.1 TIME-DOMAIN REPRESENTATION OF SINE WAVES [2/3] Note: the relative phase 𝝋, is the phase of the signal where it: describes the position of the waveform relative to time at 0 seconds. describes where in the cycle of the waveform it begins its first cycle. It can be negative or positive in value, with a negative value representing a delay and a positive value an advance in time. Measured in either degrees (°) or radians (rad), and 360° or 2𝜋 rad is equivalent to a shift in a sinusoidal waveform through a complete period. If 𝜑 = +𝜋/2, this would result in a shift of the waveform to the left by a quarter of a cycle  cosine wave: 𝑣 𝑡 = 𝐴 co𝑠(𝜔𝑡) Prepared By: Dr. Naser Zaeri 5 2.1 TIME-DOMAIN REPRESENTATION OF SINE WAVES [3/3] Sine waves and cosine waves are the same shape but, they start at different points in the cycle  phase shift between them. Both are referred to as sinusoids and both are used to represent electromagnetic waveforms. Prepared By: Dr. Naser Zaeri 6 2.2 FREQUENCY-DOMAIN REPRESENTATION OF SINE WAVES [1/2] A sinusoid waveform can be plotted in terms of its frequency. The frequency-domain representation is often much more useful than the time-domain representation when it comes to designing communications systems and understanding how they work. Prepared By: Dr. Naser Zaeri 7 2.2 FREQUENCY-DOMAIN REPRESENTATION OF SINE WAVES [2/2] The phase component of the waveform can be illustrated similarly, in a separate diagram with frequency as the x-axis. These two diagrams together give the sinusoid’s complete frequency-domain representation – also called the spectrum. Often the plot of amplitude against frequency that provides the interesting information phase diagram is frequently omitted. Prepared By: Dr. Naser Zaeri 8  2.3 PERIODIC SIGNALS AND THE FOURIER SERIES [1/6] The spectrum, with a distinct frequency component represented by a single line or spike at frequency 𝒇, is the simplest example of a discrete spectrum. All periodic signals have a discrete spectrum  spectrum can be represented by a line or series of lines (or spikes) at specific frequencies. Periodic signals that are not simple sinusoids have spectra that contain multiple spikes. This is because, all periodic signals can be represented as a sum of sinusoidal components – that is, a series of different sine or cosine waves – and each spike represents one of these components. Each frequency component of a periodic signal is a multiple, known as a harmonic, of the fundamental frequency 𝒇. This series of components is the Fourier series of a signal. 9  2.3 PERIODIC SIGNALS AND THE FOURIER SERIES [2/6] In general terms, a Fourier series looks like the following equation: 𝒗(𝒕) = 𝑨𝟎 + 𝑨𝟏 cos(𝝎𝒕 + 𝝋𝟏 ) + 𝑨𝟐 cos(𝟐𝝎𝒕 + 𝝋𝟐 ) + 𝑨𝟑 cos(𝟑𝝎𝒕 + 𝝋𝟑 ) + … + 𝑨𝒏 cos(𝒏𝝎𝒕 + 𝝋𝒏 ) The frequency of each component is a multiple of the fundamental frequency: 𝜔, 2𝜔, 3𝜔…. The first term, 𝑨𝟎 , represents a component at zero frequency. Prepared By: Dr. Naser Zaeri 10  2.3 PERIODIC SIGNALS AND THE FOURIER SERIES [3/6] For example, a periodic square waveform can be represented by the following Fourier series: Prepared By: Dr. Naser Zaeri 11  2.3 PERIODIC SIGNALS AND THE FOURIER SERIES [4/6] Figure 2.9(a) shows the original square waveform (with V=1), Figures 2.9(b) to (e) show the four sinusoidal components that were represented as spikes in Figure 2.8. 12  2.3 PERIODIC SIGNALS AND THE FOURIER SERIES [5/6] Figure 2.10(a) shows the result of adding the first two sinusoidal components (Figures 2.9b + c). This is a much closer approximation to the original square wave than part (b) on its own. Adding the third and fourth sinusoids brings the result closer still to the original, as seen in Figure 2.10(b) and (c) respectively. An infinite number of components is needed to reconstruct the square wave exactly. 13  2.3 PERIODIC SIGNALS AND THE FOURIER SERIES [6/6] Although a square wave theoretically has an infinite bandwidth (because in theory the series of high-frequency components continues indefinitely), the energy in the components decreases with increasing frequency. This means that if only the first few components are used, not much information in the reconstruction of the original signal is lost. Prepared By: Dr. Naser Zaeri 14  3. DIGITAL MODULATION [1/4] Digital modulation has many advantages over analogue modulation: Has a greater immunity to noise and other types of interference. Enables techniques to be used such as error control coding, which enables the performance to be improved, Encryption for securing transmissions. Prepared By: Dr. Naser Zaeri 15  3. DIGITAL MODULATION [2/4] The three basic digital modulation schemes are: amplitude-shift keying (ASK) frequency-shift keying (FSK) phase-shift keying (PSK). The term ‘keying’ originates from the keys used to generate early telegraphy messages. 16  3. DIGITAL MODULATION [3/4] There are a number of variants of each of these schemes that have been developed to overcome the challenges each one faces and in response to the need for higher data rates. One that is of particular interest, and is widely used in today’s systems, combines elements of PSK and ASK and is called quadrature amplitude modulation (QAM). Popular because they are more spectrally efficient than other schemes. Can achieve higher data rates without requiring more spectrum. Prepared By: Dr. Naser Zaeri 17  3. DIGITAL MODULATION [4/4] There are many factors to take into account when comparing digital modulation schemes and deciding which one is most suitable for a specific application: bandwidth efficient power efficient performs well in multipath fading channels cost effective and simple to implement. Unfortunately there isn’t one scheme that satisfies them all. There are trade-offs to be made: Before a modulation scheme is chosen for a particular application, it is important to consider the most important requirement. Prepared By: Dr. Naser Zaeri 18 3.1 THE FREQUENCY DOMAIN AND NON-PERIODIC SIGNALS [1/4] We saw how the Fourier series can be used to represent a periodic signal and consequently illustrate the spectra of such signals. The Fourier transform is used to give the frequency- domain representation of non-periodic signals. Whereas periodic signals have spectra comprising discrete frequency components, non-periodic signals have spectra comprising a continuous spectrum, which means a continuous distribution of frequency components. Prepared By: Dr. Naser Zaeri 19 3.1 THE FREQUENCY DOMAIN AND NON-PERIODIC SIGNALS [2/4] Continuous spectra are often described using the terms spectral nulls, main lobe and side lobes.  The spectral nulls are where the spectral curve crosses the x-axis – that is, where the spectral energy is zero. The first pair of spectral nulls in this example occur at -1/𝜏 and 1/ 𝜏. Prepared By: Dr. Naser Zaeri 20 3.1 THE FREQUENCY DOMAIN AND NON-PERIODIC SIGNALS [3/4] The main lobe is the large area in the middle, bounded by the first pair of spectral nulls, and is the range of frequencies that contain most of the signal’s energy. The side lobes are what look like ripples that occur on either side of the main lobe with much smaller amplitudes. As the pulse waveforms get narrower, the spacing between the first pair of spectral nulls gets larger. That is, the narrower a pulse, the broader its spectrum. For example, if a pulse has duration 10 ms, most of the spectral energy lies within a 200 Hz range. If a pulse has a much shorter duration, say 10 ns, most of the spectral energy lies within a 200 MHz range. Prepared By: Dr. Naser Zaeri 21  3.1 THE FREQUENCY DOMAIN AND NON-PERIODIC SIGNALS [4/4] This relationship shows why, if everything else remains the same, as data rates increase (i.e. more data is transmitted per second, which means a shorter pulse duration) a larger bandwidth is needed. 22  3.2 AMPLITUDE-SHIFT KEYING (ASK) [1/4] ASK is a special case of AM, so it is the amplitude of the carrier signal that is modified. The simplest version of this is also called on–off keying (OOK). Suffers from problems: synchronisation the receiver have difficulty establishing exactly how many bits have been received might potentially think that the transmission has ended (while it is not!). Prepared By: Dr. Naser Zaeri 23  3.2 AMPLITUDE-SHIFT KEYING (ASK) [2/4] Other versions of ASK use different (non- zero) amplitudes to represent 1 and 0 also exist. Bandwidth of an ASK signal is generally approximated by BASK = 2B, where B is the bandwidth of the modulating signal. 24  3.2 AMPLITUDE-SHIFT KEYING (ASK) [3/4] Representation in time domain: the modulated signal would look like a segment of a sinusoid – that is, a short burst of a sinusoid. Prepared By: Dr. Naser Zaeri 25  3.2 AMPLITUDE-SHIFT KEYING (ASK) [4/4] In frequency domain, the resulting spectrum is a function of the spectra of the original two signals. In this case, it is the original spectrum of the pulse but shifted to a centre frequency, 𝑓𝑐. Prepared By: Dr. Naser Zaeri 26 3.3 FREQUENCY-SHIFT KEYING (FSK) FSK is a type of frequency modulation, so it is the frequency of the carrier signal that is modified. The bandwidth of an FSK-modulated signal: BFSK = 2(Δf + B), where Δf is the separation of the two frequencies used: Prepared By: Dr. Naser Zaeri 27  3.4 PHASE-SHIFT KEYING (PSK) PSK is the most widely used in one form or another. There are many variations of PSK. In BPSK (Binary Phase-Shift Keying), 0 and 1 are represented by segments of sinusoids that differ in their phase. At the demodulator, distinguishing between the two segments is easier if their phases differ by as much as possible, so they are invariably separated by half a cycle (equivalent to 𝜋 radians or 180°). A BPSK-modulated signal is almost as simple to produce as an ASK signal, but with the added advantage that it is less susceptible to additive noise. Bandwidth: BPSK = 2B. 28 3.5 POWER EFFICIENCY AND BIT ERROR RATES [1/2] With respect to noise and interference, power efficiency is concerned with what signal power levels a transmitter needs to operate at in order to ensure that the number of bits received in error is at an acceptable level. Bit error rate (BER) is defined as the number of bits received in error divided by the number of bits transmitted; therefore a low BER is desirable. Different schemes result in different BERs, and plotting these against the ratio of signal power to noise power is a useful indicator of their power efficiency. Prepared By: Dr. Naser Zaeri 29  3.5 POWER EFFICIENCY AND BIT ERROR RATES [2/2] For example, take a bit error rate of 1 error in 10,000,000 bits transmitted –which is equivalent to a BER of 10−7. If this can be achieved at a lower transmitting power by one scheme than a second, then the first would be deemed to be more power efficient. PSK requires less signal power to achieve the same BER performance as FSK and ASK, meaning that PSK is the most power efficient. Prepared By: Dr. Naser Zaeri 30  4. QUADRATURE MODULATION SCHEMES Driven by the need for higher data rates, multiple variations of the three basic digital modulation schemes have been developed. Here, we are concerned with higher-order modulation methods that use an increased number of carrier states to achieve higher data rates. We look at how elements of PSK and ASK can be combined to enable non-binary schemes to be used, which can achieve higher data rates without requiring more spectrum. Prepared By: Dr. Naser Zaeri 31  4.1 SYMBOLS AND BAUD [1/3] The different carrier states are what are known as symbols. If there are more than two possible carrier states – that is, more than two symbols available – then it is possible for each symbol to represent more than one bit. In Figure 2.23 there are four possible amplitude levels.  it is possible to associate each symbol uniquely with a 2-bit binary number  four possible 2-bit binary numbers: 11, 10, 01 and 00. Prepared By: Dr. Naser Zaeri 32  4.1 SYMBOLS AND BAUD [2/3] If the number of bits that can be represented by a symbol, n, and the number of available symbols, M, then: 𝑀 = 2𝑛. Data rates: the number of bits transmitted per second. Sometimes the symbol rate is also used in communication systems The term baud refers to the number of symbols per second, where one baud is one symbol per second. Prepared By: Dr. Naser Zaeri 33  4.1 SYMBOLS AND BAUD [3/3] Increasing the number of bits a symbol can represent means that higher data rates can be achieved. The two most widely used digital modulation schemes in wireless communications systems are: Quadrature Phase-Shift Keying (QPSK) Quadrature Amplitude Modulation (QAM). Higher-order PSK and ASK are used in quadrature modulation methods. Quadrature techniques are used in WiFi, digital television, 3G and 4G mobile telecommunications. Prepared By: Dr. Naser Zaeri 34 4.2 QUADRATURE PHASE-SHIFT KEYING (QPSK) [1/8] BPSK uses segments of two sinusoidal waves that differ in phase by a half-cycle (𝜋 radians or 180°) to represent its two symbols. Instead of thinking of Figure 2.24(b) as a phase-shifted version of Figure 2.24(a), we can think of it now as having an amplitude of −1 relative to Figure 2.24(a). Prepared By: Dr. Naser Zaeri 35 4.2 QUADRATURE PHASE-SHIFT KEYING (QPSK) [2/8] The essence of quadrature modulation methods is the application of complementary pairs of amplitudes to two simultaneous sinusoidal waves differing in phase by one-quarter of a cycle. It is customary to refer to one of these waves as the I wave, or in-phase wave or component, and the other as the Q wave, or quadrature wave or component. Remember: a sine wave and a cosine wave happen to be a quarter of a cycle apart  I wave can also be thought of as a sine wave and the Q wave as a cosine wave. Prepared By: Dr. Naser Zaeri 36 4.2 QUADRATURE PHASE-SHIFT KEYING (QPSK) [3/8] The preservation of the quadrature relationship between the I and Q waves, despite the inversion of either or both of the waves, means that both waves can be modulated using BPSK and the quadrature relationship between them is preserved.  This is the basis of QPSK. 37 4.2 QUADRATURE PHASE-SHIFT KEYING (QPSK) [4/8] Another way of describing the I and Q waves is that they are orthogonal to each other. Orthogonality is an important concept in communications systems. ‘Orthogonal’ in the context of two orthogonal lines, in which it means they are ‘at right angles’ or perpendicular to each other. It can also mean ‘statistically independent’ or uncorrelated’. If two signals are orthogonal: when they are transmitted simultaneously, one can be completely recovered at the receiver without any interference from the second, and vice versa. Prepared By: Dr. Naser Zaeri 38 4.2 QUADRATURE PHASE-SHIFT KEYING (QPSK) [5/8] Signal constellation diagram: the points on the diagram represent the different symbols that the carrier can assume. For BPSK: there is no quadrature wave and the two symbols are simply represented as two points on the I-axis (x-axis) at +1 and −1. 39 4.2 QUADRATURE PHASE-SHIFT KEYING (QPSK) [6/8] Signal constellation diagram for QPSK: can be thought of as two BPSK waveforms in quadrature.  two symbols on the I-axis and two on the Q-axis. QPSK can achieve twice the data rate of a comparable BPSK scheme for a given bandwidth  Makes QPSK such an attractive option. Prepared By: Dr. Naser Zaeri 40 4.2 QUADRATURE PHASE-SHIFT KEYING (QPSK) [7/8] Figure 2.30 shows a different, but equally valid, representation. This constellation diagram looks like the original one in Figure 2.29, but rotated through 𝜋/4 radians or 45°. Prepared By: Dr. Naser Zaeri 41 4.2 QUADRATURE PHASE-SHIFT KEYING (QPSK) [8/8] Constellation diagrams are useful for representing the symbols of a modulation scheme. As the number of symbols increases, the scheme becomes more bandwidth efficient  the more symbols there are on the constellation diagram, the more data bits are transmitted per symbol  higher data rates for a given bandwidth. However, as the constellation becomes more densely packed with symbols, the distance separating the points becomes smaller.  can have negative effects on performance. Prepared By: Dr. Naser Zaeri 42  4.3 QUADRATURE AMPLITUDE MODULATION (QAM) [1/2] In considering how to increase the number of available symbols further than QPSK can provide, there are two options: The first possibility: the amplitude of the modulated signal remains fixed but the number of different phases the carrier wave can take continues to increase. This is the principle applied in M-PSK schemes or PSK of the order M, where M is the number of possible symbols. Prepared By: Dr. Naser Zaeri 43  4.3 QUADRATURE AMPLITUDE MODULATION (QAM) [2/2] The second possibility: is to combine QPSK with ASK techniques and introduce more amplitude levels as well as phases. This is the principle of quadrature amplitude modulation (QAM). Schemes of this kind are referred to as M-QAM or QAM of the order M. 16-QAM is much more widely used in practice than 16-PSK, along with higher-order schemes such as 64- QAM, 128-QAM and 256-QAM. This is because it performs better in noisy channels. Prepared By: Dr. Naser Zaeri 44