Transcript of "Emi protection for_communication_systems"
EMI Protection forCommunication Systems
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EMI Protection forCommunication Systems Kresimir Malaric artechhouse.com
Contents Preface xiii CHAPTER 1 Communications Systems 1 1.1 Components of Communications Systems 1 1.2 Transmitter Systems 2 1.2.1 Transmitter 3 1.2.2 Randomization 4 1.2.3 Encryption 5 1.2.4 Encoder 5 1.2.5 Interleaving 9 1.2.6 Modulation 10 1.2.7 Mixer (Upconverter) 10 1.2.8 Filter 11 1.3 Receiver Systems 11 1.3.1 Filter 11 1.3.2 Mixer (Downconverter) 12 1.3.3 Demodulator 12 1.3.4 Deinterleaver 12 1.3.5 Decoder 13 1.3.6 Decryptor 15 1.3.7 Derandomizer 15 1.3.8 Demultiplexer 16 1.3.9 Received Power 16 1.4 User Interface 18 1.4.1 Graphical User Interface (GUI) 18 1.4.2 Voice User Interface (VOI) 19 1.5 Antenna Systems 19 1.5.1 Duplexer 19 1.5.2 Antenna 20 1.6 Power Supplies 22 1.6.1 Power Supply Types 23 1.6.2 Power Amplifier 23 v
vi Contents 1.7 Considerations for Voice Versus Data 23 1.7.1 Text 23 1.7.2 Images 24 1.7.3 Voice 24 1.7.4 Video 24 Selected Bibliography 24 CHAPTER 2 Electromagnetic Spectrum Used for Communications 27 2.1 Electromagnetic Spectrum 27 2.1.1 Extra Low Frequency (ELF) 28 2.1.2 Super Low Frequency (SLF) 28 2.1.3 Ultra Low Frequencies (ULF) 29 2.1.4 Very Low Frequency (VLF) 29 2.1.5 Low Frequency (LF) 29 2.1.6 Medium Frequency (MF) 29 2.1.7 High Frequency (HF) 29 2.1.8 Very High Frequency (VHF) 29 2.1.9 Ultra High Frequency (UHF) 29 2.1.10 Super High Frequency (SHF) 30 2.1.11 Extra High Frequency (EHF) 30 2.1.12 Infrared (IR) 30 2.1.13 Visible 30 2.2 Spectrum Division 30 Selected Bibliography 33 CHAPTER 3 Electromagnetic Properties of Communications Systems 35 3.1 Fundamental Communications System Electromagnetics 35 3.1.1 Smith Chart 39 3.1.2 Snell’s Law of Reflection and Refraction 42 3.2 Wave Generation and Propagation in Free Space 44 3.2.1 Maxwell’s Equations 44 3.2.2 Wave Propagation 46 3.2.3 Wave Polarization 47 3.2.4 Fresnel Knife-Edge Diffraction 48 3.2.5 Path Loss Prediction 51 3.3 Wave Generation and Propagation in the Terrestrial Atmosphere 53 3.3.1 Absorption and Scattering 53 3.3.2 Wave Propagation in the Atmosphere 54 Selected Bibliography 55 CHAPTER 4 Electromagnetic Interference 57 4.1 Electromagnetic Interference with Wave Propagation and Reception 57 4.1.1 Additive White Gaussian Noise (AWGN) 57
Contents vii 4.1.2 Thermal Noise 58 4.1.3 Shot Noise 58 4.1.4 Flicker (1/f ) Noise 58 4.1.5 Burst Noise 59 4.1.6 Noise Spectral Density 59 4.1.7 Effective Input Noise Temperature 59 4.2 Natural Sources of Electromagnetic Interference 59 4.2.1 Lightning and Electrostatic Discharge 59 4.2.2 Multipath Effects Caused by Surface Feature Diffraction and Attenuation 64 4.2.3 Attenuation by Atmospheric Water 65 4.2.4 Attenuation by Atmospheric Pollutants 67 4.2.5 Sunspot Activity 68 4.3 Manmade Sources of Electromagnetic Interference 69 4.3.1 Commercial Radio and Telephone Communications 69 4.3.2 Military Radio and Telephone Communications 74 4.3.3 Commercial Radar Systems 74 4.3.4 Industrial Sources 75 4.3.5 Intentional Interference 76 Selected Bibliography 77 CHAPTER 5 Filter Interference Control 79 5.1 Filters 79 5.1.1 Lowpass Filter 80 5.1.2 Highpass Filter 80 5.1.3 Bandpass Filter 81 5.1.4 Bandstop Filter 83 5.1.5 Resonator 83 5.2 Analog Filters 85 5.2.1 Butterworth Filter 85 5.2.2 Chebyshev Filters 86 5.2.3 Bessel Filters 87 5.2.4 Elliptic Filters 88 5.2.5 Passive Filters 88 5.2.6 Active Filters 91 5.3 Digital Filters 91 5.3.1 FIR Filters 93 5.3.2 IIR Filters 94 5.4 Microwave Filters 97 5.4.1 Lumped-Element Filters 97 5.4.2 Waveguide Cavity Filters 98 5.4.3 Dielectric Resonator 100 Selected Bibliography 101
viii Contents CHAPTER 6 Modulation Techniques 103 6.1 Signal Processing and Detection 103 6.2 Modulation and Demodulation 105 6.2.1 Analog Modulations 105 6.2.2 Digital Modulation 112 6.3 Control of System Drift 120 Selected Bibliography 120 CHAPTER 7 Electromagnetic Field Coupling to Wire 123 7.1 Field-to-Wire Coupling 123 7.1.1 Skin Effect 123 7.1.2 Unshielded Twisted Pair (UTP) 125 7.1.3 Ferrite Filter 126 7.2 Electric Field Coupling to Wires 128 7.3 Magnetic Field Coupling to Wires 131 7.4 Cable Shielding 132 7.4.1 Tri-Axial Cable 133 7.4.2 Cable Termination 133 7.4.3 Shielded Twisted Pair Cables 134 Selected Bibliography 136 CHAPTER 8 Electromagnetic Field-to-Aperture Coupling 137 8.1 Field-to-Aperture Coupling 137 8.1.1 Shielding Effectiveness (SE) 138 8.1.2 Multiple Apertures 138 8.1.3 Waveguides Below Cutoff 140 8.2 Reflection and Transmission 141 8.2.1 Electric Field 145 8.2.2 Magnetic Field 146 8.3 Equipment Shielding 147 8.3.1 Gasketing 147 8.3.2 PCB Protection 148 8.3.3 Magnetic Shield 149 Selected Bibliography 151 CHAPTER 9 Electrical Grounding and Bonding 153 9.1 Grounding for Safety 154 9.1.1 Shock Control 154 9.1.2 Fault Protection 155 9.2 Grounding for Voltage Reference Control 156 9.2.1 Floating Ground 156 9.2.2 Single Point Ground 157
Contents ix 9.2.3 Multipoint Ground 158 9.2.4 Equipotential Plane 158 9.3 Bonding for Current Control 159 9.3.1 Bonding Classes 160 9.3.2 Strap Bond for Class R 160 9.3.3 Resistance Requirements 162 9.4 Types of Electrical Bonds 162 9.4.1 Welding and Brazing 163 9.4.2 Bolting 163 9.4.3 Conductive Adhesive 164 9.5 Galvanic (Dissimilar Metal) Corrosion Control 164 Selected Bibliography 166 CHAPTER 10 Emissions and Susceptibility—Radiated and Conducted 167 10.1 Control of Emissions and Susceptibility—Radiated and Conducted 167 10.1.1 Sources of Electromagnetic Interference 167 10.1.2 Test Requirements for Emission and Susceptibility 171 10.1.3 Standard Organizations 173 10.2 Commercial Requirements 177 10.3 Military Requirements 178 10.3.1 Specific Conducted Emissions Requirements Mil-Std 461E 178 10.3.2 Specific Conducted Susceptibility Requirements Mil-Std 461E 179 10.3.3 Radiated Emissions Requirements Mil-Std 461E 181 10.3.4 Radiated Susceptibility Requirements Mil-Std 461E 182 Selected Bibliography 182 CHAPTER 11 Measurement Facilities 185 11.1 Full Anechoic and Semianechoic Chambers 185 11.1.1 Absorbers 187 11.1.2 Ferrite Tiles 189 11.2 Open Area Test Site (OATS) 191 11.3 Reverberation Chamber 193 11.4 TEM Cell 195 11.4.1 Characteristic Impedance 196 11.4.2 Higher-Order Modes 197 11.4.3 TEM Cell Construction 198 11.4.4 Parameter Measurements 200 11.5 GTEM Cell 201 11.5.1 GTEM Cell Characteristics 203 11.5.2 GTEM Cell Construction 203 11.5.3 GTEM Cell Parameter Measurement 204 11.5.4 Current Distribution at Septum 211 Selected Bibliography 212
x Contents CHAPTER 12 Typical Test Equipment 215 12.1 LISN—Line Impedance Stabilization Network 215 12.2 Coupling Capacitor 216 12.3 Coupling Transformer 217 12.4 Parallel Plate for Susceptibility Test 217 12.5 Coupling Clamps and Probes 218 12.5.1 Capacitive Coupling Clamp 219 12.5.2 Current Probe 220 12.6 Injection Clamps and Probes 221 12.6.1 Current Injection Probe 221 12.6.2 EM Clamp 221 12.6.3 Electrostatic Discharge (ESD) Generator 223 12.7 EMI Receiver 224 12.8 Spectrum Analyzer 225 12.9 Oscilloscopes 225 Selected Bibliography 225 CHAPTER 13 Control of Measurement Uncertainty 227 13.1 Evaluation of Standard Uncertainty 227 13.1.1 Type A Evaluation of Standard Uncertainty 227 13.1.2 Type B Evaluation of Standard Uncertainty 228 13.2 Distributions 228 13.2.1 Normal (Gaussian) Distribution 229 13.2.2 Rectangular Distribution 229 13.2.3 U-Shaped Distribution 230 13.2.4 Combined Standard Uncertainty 230 13.2.5 Expanded Uncertainty 231 13.3 Sources of Error 231 13.3.1 Stability 231 13.3.2 Environment 231 13.3.3 Calibration Data 231 13.3.4 Resolution 232 13.3.5 Device Positioning 232 13.3.6 RF Mismatch Error 232 13.4 Definitions 232 Selected Bibliography 232 Appendix A Communication Frequency Allocations 235 A.1 Frequency Allocation in the United States 235 A.2 International Frequency Allocation 245
Contents xi Appendix B List of EMC Standards Regarding Emission and Susceptibility 255 B.1 Cenelec 255 B.2 Australian Standards 256 B.3 Canadian Standards 256 B.4 European Standards 258 B.5 Other Standards 259 Acronyms and Abbreviations 261 Glossary 265 About the Author 267 Index 269
Preface Communication today is not as easy as it was in the past. Protecting numerous com- munication services, which are operating in the same or adjacent communication channels, has become increasingly challenging. Communication systems have to be protected from both natural and manmade interference. Electromagnetic interfer- ence can be radiated or conducted, intentional or unintentional. Understanding physical characteristics of wave propagation is necessary to comprehend the mecha- nisms of electric and magnetic coupling in the communication signal paths. Differ- ent modulating techniques, as well as encoding and encrypting, can improve bit error rates (BER) and signal quality. Communication systems must be designed properly, so that the performance of their system capabilities is not subject to degra- dation or complete loss due to electromagnetic interference. Although there are numerous books available on electromagnetic compatibil- ity, signal processing, and electromagnetic theory, there is no book offering a com- prehensive description of technologies for the protection of communication systems, which includes discussions on the improvement of existing communication systems and the creation of new systems. The book provides laymen with basic information and definitions of problems regarding electromagnetic interference in communication systems. In addition, it gives an experienced practitioner knowl- edge of how to solve possible problems in both digital and analog communication systems. The examples given in the book are intended for an easier comprehension of otherwise demanding electromagnetic problems. The book’s primary audience includes designers, researchers, and graduate students in the area of communications. The book is organized into 13 chapters dealing with fundamental concerns of developers and users of communication systems. • Chapter 1 gives an overview of communication system components. • Chapter 2 deals with the use of the electromagnetic spectrum for communica- tions. • Chapter 3 describes wave propagation in free space and the terrestrial atmo- sphere. • Chapter 4 discusses natural sources of electromagnetic interference, such as attenuation of atmospheric water or lightning, as well as numerous manmade sources of electromagnetic interference. • Chapter 5 covers analog, digital, and microwave filters. • Chapter 6 deals with signal processing and modulation/demodulation issues in communication systems. xiii
xiv Preface • Chapters 7 through 9 are devoted to electromagnetic field-to-wire and aper- ture coupling, as well as to electrical grounding and bonding. • Chapter 10 gives the commercial and military requirements for radiated and conducted emission and susceptibility. Facilities for EMI measurement such as TEM and GTEM cells, open area test sites, and reverberation chambers are covered in Chapter 11. • Chapter 12 includes the description of coupling capacitors and transformers, coupling and injection clamps and probes, and other test equipment. • Chapter 13 deals with the control of measurement uncertainties. • Appendix A gives a list of communication frequencies, and Appendix B gives a list of EMC standards. The program TEM-GTEM on the CD-ROM accompanying this book works in the LabVIEW environment on a PC Windows operating system. The TEM-GTEM program requires prior installation of LabVIEW Run-Time Engine 8.6 – Windows 2000/Vista x64/Vista x86/XP on any computer which does not already have LabVIEW 8.6 installed. The Run-Time engine can be found on the CD-ROM in the folder: runtimeengine. The second folder, tem-gtem, contains the tem-gtem.exe as well as Installation.doc and Instructions.doc file. The application TEM-GTEM has two programs: TEM and GTEM. The first program, TEM, calculates the characteristic impedance Z0 (in ohms), and cutoff fre- quencies (fc) for modes TE01, TE10, TE11, TM11, TE02, TE12, TM12, and TE20 depend- ing on the TEM-cell dimensions. The second program, GTEM, calculates the cutoff (fc) and the associated stimulated resonant frequencies (fr1, fr2, and fr3) in mega- hertz for higher-order modes H10, H01, H11, H20, E11, and E21. The resonances are also shown graphically. A detailed explanation on the theory for the TEM cell and the GTEM cell can be found in Sections 11.4 and 11.5, respectively. I wish to thank Artech House editors Lindsey Gendall, Barbara Lovenvirth, and Mark Walsh for their encouragement in writing this book. Finally, I thank my wife Blazenka and my parents Marija and Vladimir for their love and support through- out this project.
CHAPTER 1Communications Systems1.1 Components of Communications Systems A communication system usually consists of the information source, transmitter, channel, receiver, antenna systems, amplifier, and end user (Figure 1.1). It converts information into a format appropriate for the transmission medium. A transmitting antenna’s purpose is to effectively transform the electrical signal into radiation energy, whereas the receiving antenna’s purpose is to effectively receive the radiated energy and its electric signal transformation for further process- ing at the receiver. Communication systems can be either analog or digital. Historically, analog systems (Figure 1.2) are simpler but less resilient to interference. Analog communi- cation systems convert (modulate) analog signals into modulated signals. Signals that are analog are converted into digital bits by sampling and quantization, also called digitization and coding. Digital systems can reconstruct original information, are better protected from interference, and have the potential to code signals, thus enabling larger amount of data transportation. It is important that the information sent from the source and the information sent to the end user are similar as possible (i.e., identical in the case of digital infor- mation). Even though digital systems are used more for communications, analog systems will not be neglected in this and other chapters. With digital systems, there is a source and channel coder instead of signal pro- cessing in the transmitter, as is the case when dealing with analog systems. In the receiver, there are also channel and source decoders instead of the signal processing of analog systems. The modulator and demodulator should be designed to lessen the distortion and noise from the channel. The channel transports the signals using electromagnetic waves. There is always noise in the channel along with the useful signal. The source coder (Figure 1.3) converts the analog information to digital bits using analog to digital conversion (A/D). The transmitter converts the signal (ana- log) or bits (digital) into a format that is appropriate for channel transmission. Dis- tortion, noise, and interference are brought into the channel. The receiver decodes the received signal back into the information signal and then the source decoder decodes the signal back to the original information (analog or digital). 1
2 Communications Systems Source of Transmitter Receiver User information Channel Figure 1.1 Communication system. Transmitter in baseband Source Processing Modulator RF stage Channel User Processing Demodulator RF stage Receiver in baseband Figure 1.2 Analog communication system. Transmitter in baseband Source Channel Digital RF stage Source coder coder modulator Channel Digital RF stage User Decoder Decoder demodulator Receiver in baseband Figure 1.3 Digital communication system.1.2 Transmitter Systems A transmitter system is used to send information/data from one user to another. The source information can be analog or digital. Digital systems are usually more com- plex than the analog ones and require more modules. Both the transmitter and receiver systems should have the same complexity. For example, if we have a modu- lator on the transmitter side, there should be a demodulator on the receiver side. The same applies for the coder, multiplexer, and so forth. Usually, transmitter sys- tems have a multiplexer, randomization, encryption, an encoder, interleaving, a modulator, a mixer (upconverter), a power amplifier, a filter, a duplexer, and a
1.2 Transmitter Systems 3 transmitting antenna. Not all systems require all of the mentioned modules; this depends on the type of communication used and/or on the required security level. 1.2.1 Transmitter The multiplexer provides multiple dedicated channels to users and combines the data (bits) from every channel into one combined stream of data bits. These bits are organized into frames; a frame has a fixed length of bits. Every user is allocated a specific position in the frame. There must be a synchronization code or sequence of bits inside the frame in order to provide the ability to identify each frame and the rel- ative position of the bit inside the frame. There should also be a clock for the correct transmission of the data. In this way several signals can share one communication line, instead of having one line for every signal (Figure 1.4). Multiplexers (MUX) can range from two input signals up to sixteen or more. For more input signals, a cascade (consisting of simpler multiplexers) is used. Typi- cally they are 2/1, 4/1, 8/1, or 16/1, indicating the number of input signals and only one output signal. Figure 1.5 shows a 4/1 multiplexer. This means that four input signals share one communication line. The input signals are I0, I1, I2, and I3. The output is Z. Which signal will pass from input to output is decided on the basis of control signals a1 and a0 as shown in Table 1.1. Input E enables (E = 1) or disables (E = 0) the multiplexer. MUX DEMUX Conversation 1 Conversation 1 Conversation 2 Conversation 2 Conversation 3 Conversation 3 Conversation 4 Conversation 4 Conversation 5 Conversation 5 Figure 1.4 Use of one line for several communications. MUX 4/1 I0 I1 I2 Z I3 E a1 a0 Figure 1.5 Multiplexer 4/1.
4 Communications Systems Table 1.1 Combination Table for Multiplexer 4/1 E a1 a0 Z 0 x x 0 1 0 0 I0 1 0 1 I1 1 1 0 I2 1 1 1 I3 1.2.2 Randomization Randomization is used to ensure the even number of 0s and 1s in the data informa- tion, which should be randomly distributed. The process of randomization is car- ried out by the exclusive or adding (XOR), which adds a bit from a selected bit sequence to each bit within the multiplexer frame, except for the synchronization bits. The bit sequence that is used to randomize is called pseudorandom or pseudonoise (PN) sequence. The random distribution of the bit sequence matches the Gaussian distribution. This function happens simultaneously with each multiplexing frame. PN codes can be generated using a series of shift registers and logic gates in feedback as shown in Figure 1.6. There is also a modulo-2 adder (adding without carry). The shift registers receive a clock signal every Tc seconds. The feedback lines can be used to obtain dif- r ferent output codes. For r shift registers, a maximum of 2 − 1 bit sequence can be produced. This means that with four shift registers, a maximum of fifteen bit sequences can be achieved. After that, the combinations will start repeating them- selves. It is possible to connect the feedback gates to produce a shorter sequence, but shorter sequences are less random and will repeat more often. A circuit configured to produce the maximum sequence of nonrepeating bits for a given number of shift registers is called a maximal length PN code generator. The main characteristic of this maximal length code is that the Modulo 2 sum of any sequence with a shifted version of itself will produce another shifted version of the same sequence. All com- binations will appear only once except all the 0 combinations, as this state will cause no changes to occur in the shift register values or in the output. The number of 1s will always be 1 larger than the number of 0s, independent of the length of the code. PN signal out Modulo 2 adder Shift register 1 2 3 4 r Clock signal Figure 1.6 Pseudonoise generator.
1.2 Transmitter Systems 5 This type of PN generator is used in the transmitter to modulate a continuous wave signal, as well as in the receiver, where an identical PN generator is used to demodulate the received signal. 1.2.3 Encryption Encryption is used to protect the data should it be intercepted. Usually the bitstream is changed (encrypted) in such a way that it would be difficult to reconstruct the original bitstream without a decryption device. A problem develops if there is an error in the received bitstream, which results in an additional error in the decryption process. This is called error extension. Encryption has been used in wars and for information protection for a long time now. It can be used in computer systems and communication systems for authorization, copyright protection, and other applica- tions. For the encryption process, an encryption key is required. It is usually 40 to 256 bits long. The longer the key (cipher strength), the harder it is to break the code. There are two methods available: the secret and the public key. With the secret key, both sender and receiver use the same key to encrypt and decrypt the bitstream. This is the fastest method, but there is the problem of getting the secret key to the receiving side. With the public key, each recipient has a private key that is kept secret and a public key known to everyone. The sender uses the public key to encrypt the data, whereas the recipient uses the private key to decrypt the data. In this manner the private key is never transmitted, and thus is not vulnerable to inter- ception. The most spread encryption standards are the Data Encryption Standard (DES) and the Advanced Encryption Standard (AES). DES (Figure 1.7) is the most widely used encryption standard, dating from the 1970s. It has blocks of 64 bits at a time, and the key length is 56 bits. The 64 bits of the input block to be enciphered are first subjected to initial per- mutation. The permutated input block becomes the input to a complex key-depend- ent computation. The output of that computation, called the preoutput, is then subjected to permutation, which is the inverse of the initial permutation. The com- putation which uses the permuted input block as its input to produce the preoutput block consists of 16 iterations of a calculation depending on the cipher function. Today DES is considered insecure because a key of 56 bits is not long enough. That is why in 2002, AES was adopted, which is capable of processing data blocks of 128 bits using cipher keys with lengths of 128, 192, and 256 bits. More on AES can be found in “Announcing the Advanced Encryption Standard (AES),” which is free to download from the Internet [National Institute of Standards and Technol- ogy (NIST), http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf]. 1.2.4 Encoder Encoders are used in transmitter systems for detection and correction of errors that may occur during transmission due to noise or interference. Coding can also be used for compressing the information. Most encoders add the redundant (known) bits expanding the data (information bits). This slows the traffic. How many redundant bits will be added depends on the surrounding of our communication service (inter- ference) and on the importance of the information being transmitted in real time.
6 Communications Systems Input (64 bits) Key (64 bits) Initialization Initial permutation Key permutation Left half (32 bits) Right half (32 bits) Left half (28 bits) Right half (28 bits) Cipher Binary rotation Binary rotation Round 1 function Subkey #1 (48 bits) Permutation Cipher Round 2 function Binary rotation Binary rotation Subkey #2 (48 bits) Permutation Round 16 Cipher function Binary rotation Binary rotation Subkey #16 (48 bits) Final permutation Finalization Permutation Output (64 bits) Figure 1.7 Data Encryption Standard algorithm. The differential encoder, convolutional encoder, Reed Solomon coding, and Golay encoder are most often used in communication systems. 220.127.116.11 Differential Encoder Differential encoding of data is required for modulations such as duobinary and dif- ferential phase shift keying. These modulation types are used for optical links and high data rates of 10 to 40 Gbps. The principle of differential encoding is shown in Figure 1.8. dk c k = ck−1 d XOR 1 bit period c k−1 delay Figure 1.8 Differential encoder.
1.2 Transmitter Systems 7 Let dk be a sequence of binary bits that are the input to a differential encoder and let ck be the output of the differential encoder. Then we have c k = c k −1 ⊕ d k (1.1) where ⊕ is the modulo 2 addition. The direct implementation of the above equation is the use of an exclusive –OR (XOR) gate with a delay in the feedback path of 1 bit period delay. At 40 Gbps, 1 bit period is equal to 25 ps. 18.104.22.168 Convolutional Encoder Information data is susceptible to errors. For useable data, there are methods of encoding information. This means organizing the 0s and 1s so that errors can be corrected. Convolutional encoding is applied to the data link signal in order to cor- rect bit errors that might occur during transmission, which results in coding gain for the system. Through the convolutional encoding/decoding process, the majority of transmission errors will be corrected before they are passed onto the decryption process. Codes have three primary characteristics: length, dimension, and minimum dis- tance of a code. The code’s length is the amount of bits per code word. The code dimension is the amount of actual information bits contained within each code word and the minimum distance is the minimum number of information differences between each code word. Convolutional codes are commonly specified by three parameters: (n, k, m) where n is the number of output bits, k is the number of input bits and m is the number of memory registers. The quantity k/n is called the code rate R and is a measurement of coding efficiency: k R= (1.2) n Commonly k and n parameters range from 1 to 8 and the code rate accordingly from 1/8 to 7/8. Memory registers, m, can range from 2 to 10. Another parameter, the constraint length K, is defined by: K = k ⋅ ( m − 1) (1.3) which represents the number of bits in the encoder memory that affects the genera- tion of n output bits. A convolutional encoder can be made with a K-stage shift register and n modulo-2 adders, where K is called the constraint length of the code. An example of such an encoder with K = 3 and n = 2 is shown in Figure 1.9. For each bit entering into the register, the output switch samples n = 2 code bits out (u1 and u2); hence the rate of the code k/n is 1/2. Each output code bit will be a function of the input bit (located in the leftmost stage of the register) plus two of the earlier bits (in the rightmost stages). The larger the constraint length K, the greater the number of past bits that have an effect on each output code word.
8 Communications Systems u1 First code bit Input bit Output m branch word Second u2 code bit Figure 1.9 Convolutional encoder: K = 3, rate = 1/2. 22.214.171.124 Reed-Solomon Coding As with convolutional encoding, RS coding adds redundant bits and creates code words that enable the decoding process to correct errors. RS differs from convolutional encoding by performing block encoding (using bytes) rather than bitwise encoding. Because of the block encoding, RS is eight times faster than convolutional encoding. The incoming data stream is first packaged into small blocks, which are treated as a new set of k symbols to be packaged into a super-coded block of n symbols, by appending the calculated redundancy. Such symbols can either be comprised of one bit or of several bits (symbol code). There- fore, the information transfer rate is reduced by a factor called code rate (R), and the modulator is expanded by the ratio: 1 n = (1.4) R k A Reed-Solomon decoder can correct up to t symbols that contain errors in a code word, where 2t = n − k (1.5) A Reed-Solomon code word is generated using a special polynomial. All valid code words are exactly divisible by the generator polynomial. The general form of the generator polynomial is ( g( x ) = x − a1 )( x − a )K( x − a ) i+1 i+ 2t (1.6) The code word is constructed using: c( x ) = g( x )i( x ) (1.7) where g(x) is the generator polynomial, i(x) is the information block, c(x) is a valid code word, and a is referred to as the primitive element of the field.
1.2 Transmitter Systems 9 126.96.36.199 Golay Code Marcel J. E. Golay discovered the possible existence of a perfect binary (23, 12, 7) code, with error-correcting capability t = 3, that is, capable of correcting all possible patterns of three errors in 23 bit positions, at the most. So the Golay (23, 12, 7) code 12 is a perfect linear error-correcting code consisting of 2 = 4,096 code words of length 23 and a minimum distance of 7. Golay also defined the parity check matrix for this code as: H = ( MI11 ) (1.8) where I11 is the 11 × 11 identity matrix and M is a 11 × 12 defined matrix. Since the code’s length is relatively small (length = 23), the number of redundant bits is 11, and the dimension is 12, the Golay (23, 23, 7) code can be encoded by simply using look up tables (LUTs). A look up table is an array that holds a set of precomputed results for a given operation. This array provides access to results faster than com- puting the result of the given operation each time. Beside the perfect binary Golay code, there is the extended binary Golay code that encodes 12 bits of data in a word with a length of 24 bits, so that a triple-bit error can be corrected and a quadru- ple-bit error detected. 1.2.5 Interleaving Interleaving is used to intermix the bits of the code words generated through convolutional encoding. The motivation for interleaving is to compensate for burst or sequential errors, which can otherwise exceed the capability of the decoder to correct errors. Each code word generated through convolutional encoding can only correct a limited number of errors that occur in that code word. Sequential errors can cause multiple errors in a single code word, which can exceed the error-correct- ing capability of the decoding process. Interleaving distributes bits in such a way that, if sequential errors do occur, they will be distributed over multiple code words. For example, seven errors in a single code word will be distributed during interleav- ing into seven code words each having a single error. While the decoder may not be able to recover data in a code word with seven errors, it can easily recover a single error in seven code words. The disadvantage of interleaving is the delay created by writing a block of bits into memory, intermixing the bits, and then pulling the bits from memory. This delay is dependent on the number of bits that are interleaved at a time and the data rate of the aggregate bitstream. Interleaving is performed only on a finite block of bits at a time. Similar to multiplexing, interleaving requires framing the aggregate bitstream and adding synchronization bits. Interleavers are divided into periodic and pseudorandom. In periodic interleavers, symbols of the transmitted sequence are scrambled as a periodic func- tion of time. Periodic interleavers can be either block or convolutional.
10 Communications Systems 1.2.6 Modulation Modulation is the process of changing one or more parameters of an auxiliary sig- nal, depending on the signal that carries the information. This auxiliary signal is called the transmission signal. The signal that carries the information (and controls the parameter changes of the transmission signal) is called the modulation signal. The result of the modulation is the modulated signal. The process is performed in a device called the modulator, which converts the total digital bitstream into the radio frequency (RF) analog signal. The digital bitstream is usually modulated into the intermediate frequency (IF), which after amplification is upconverted to the transmit frequency. There are many analog and digital modulations that are used in communication systems. Analog modulations include: amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), and several others. Digital modulations include frequency shift keying (FSK), phase shift keying (PSK), amplitude shift key- ing (ASK), quadrature amplitude modulation (QAM), pulse code modulation (PCM), and others. Chapter 6 will discuss more on modulation and demodulation. 1.2.7 Mixer (Upconverter) Mixers are used in transmitter systems for easier processing of the signal. It is much cheaper and easier to amplify the signal at a lower intermediate frequency (IF) than at a higher radio frequency (RF). The mixer inputs two different frequencies (one of them is a local oscillator fre- quency) and mixes them. The result is the sum and difference of the input signals. The frequency that is not needed must be filtered out. Figure 1.10 shows the mixer, which has a local oscillator frequency added to or subtracted from the input frequency. For upconversion of the frequency, the local oscillator frequency fLO is added to the input signal frequency fin: f out = f in + f LO (1.9) True systems mixers will produce more than just the sum and difference of the input signals. There will be intermodulation products from the input signals. If a second signal fin2 arrives at the input with the fin, the mixer will generate intermodulation products at its output due to inherent nonlinearity, in the form Mixer Input Output signal signal Local oscillator Figure 1.10 The mixer.
1.3 Receiver Systems 11 ± m ⋅ f in ± n ⋅ fi in2 (1.10) where m and n are positive integers, which can assume any value from 1 to infinity. The order of the intermodulation is defined as m + n. Accordingly, 2 fin − fin2, 2 fin2 − fin, 3 · fin and 3 · fin2 are third order products by definition. The first two products are called two-tone third-order products as they are generated when two tones are applied simultaneously at the input. Two-tone third-order products are very close to the desired signals and are very difficult to filter out. 1.2.8 Filter Filtering of the frequency range is an important part of every communication sub- system—hence the transmitter. Filtering is the ability to select the frequency range we wish to process and to block all other frequencies. Filters can be analog or digi- tal. Figure 1.11 shows the symbols used for lowpass, bandpass, and highpass filters, depending on which frequency range needs to be processed further. On lower frequencies, LC filters are used, and on higher frequencies (such as microwave) the microstrip is used. Filters will be discussed in more detail in Chapter 5.1.3 Receiver Systems The receiver system largely depends on the transmitter system. If in a communica- tion system a multiplexer is used on the transmitter’s side, there must be a demultiplexer on the receiving side. The same applies for other blocks mentioned in the previous section, which on the receiving side are placed in reverse order. Some receivers must deal with very small signals. Better and more expensive receivers will introduce very little noise themselves. Again, depending on the complexity of the communication system, the following blocks are optional: filter, downconverter, demodulator, deinterleaver, decoder, derandomizer, and demultiplexer. 1.3.1 Filter The filter is part of every receiving system. It selects the frequency band of use to be processed further and it stops signals on all other frequencies. The selectivity of the filter is shown by Q factor which can be calculated as fc Q= (1.11) f 2 − f1 Figure 1.11 Lowpass, bandpass, and highpass filters.
12 Communications Systems where f2 and f1 are frequencies where the power drops by 50% (3 dB), and fc is the central or resonant frequency as shown in Figure 1.12. 1.3.2 Mixer (Downconverter) On the receiving end of the communication system there is also a mixer, which in this case serves as a downconverter. It is necessary to downconvert the received RF frequency because it is much easier to amplify the signal at intermediate frequencies (IF) than at RF frequencies. Here, again, the local oscillator is necessary, and the output frequency is obtained as f out = f in − f LO (1.12) where the input frequency fin must be greater than fLO; otherwise an error will occur. The other result, that is, the adding of the two frequencies, will be filtered out. Again, intermodulation products may occur here. That is why it is necessary to take into consideration all possible transmitters in the vicinity (depending on the applica- tion, this can be up to 50 km) and calculate the intermodulation in order to deter- mine whether additional filtering is required. 1.3.3 Demodulator The demodulator converts an analog RF signal into a digital bitstream. It extracts the original information from the modulated carrier wave. There are different types of demodulation, such as envelope detection, differential, coherent, and synchro- nous demodulation. Demodulation and demodulators will be discussed in greater detail in Chapter 6. 1.3.4 Deinterleaver If a bitstream was interleaved in the transmission process, deinterleaving is required in the receiving process to reassemble the code words created by the encoder. Should any errors have occurred before deinterleaving, they will be distributed depending on the selected algorithm. Figure 1.13 shows the deinterleaving of an array of three element structures. Synchronization bits must be present to recognize when one frame is finished and the other is starting. Bandwidth −3dB f1 fc f2 Figure 1.12 Selectivity of the filter.
1.3 Receiver Systems 13 X A X B X C X A X B X C X A X B X C X A X B A 3 A 2 A 1 A 0 Y0 X C B 3 B 2 B 1 B 0 Y1 C 3 C 2 C 1 C 0 Y2 Figure 1.13 Deinterleaving an array of three element structures. 1.3.5 Decoder The decoder in the receiver system must match the encoder that was used in the transmitter system. 188.8.131.52 Differential Decoder If differential encoding was used in the transmitter system, differential decoding must be performed in the receiving system. The differential encoding process does not introduce redundant bits, but transforms the waveform by converting the space signal (zeros) into transitions. Accordingly, the decoding process converts transi- tions back to spaces. Since a single bit error affects two transitions, the differential decoding process doubles any bit error, corresponding to a 3-dB loss to the system. The decoder decodes the binary input signal. The output is the logical difference between present and previous input. The input and the output are related with m(t 0 ) = d (t 0 ) XOR initial condition parameter value (1.13) m(t k ) = d (t k ) XORd (t k −1 ) where d is the differentially encoded input, m is the output message, tk is the kth time step, and XOR is the logical exclusive-or operator.
14 Communications Systems 184.108.40.206 Viterbi Decoder The Viterbi decoder is used together with convolutional coding, and is applied to the data link signal to correct errors that may have occurred during the RF transmis- sion. The encoding/decoding process adds what is referred to as coding gain, which may be necessary for the successful data link transmission. The decoding corrects errors before the decryption process of total bitstream. Correction of the errors occurs because the convolutional encoder (or transmit side of the data link) creates code words, which contain data bits with added redundant bits. The redundant bits allow the decoder to detect and correct errors that may exist in each code word. The process of decoding is much more complicated than the encoding process, and limits the speed of the bitstream that needs to be decoded. If the convolutional code uses 2n possible symbols, the input vector length is K · n for positive integer K. If decoded data uses 2k possible output symbols, the output length will be K · n. The integer number K is the number of frames processed in each step. The entry into the decoder input can be a real number (positive real is logical zero, while negative real is logical one), 0 and 1 (0 is logical zero, 1 is logical 1). The latter is called a hard decision. The third possible input is a soft decision. It can be any integer between 0 and 2b − 1, where b is the number of the soft decision bit b parameter. Here 0 is the most confident decision for logical zero, 2 − 1 is the most confident decision for logical one, and other values are less confident decisions. Table 1.2 shows the decisions for three bits. 220.127.116.11 Reed-Solomon Decoding The Reed-Solomon encoder and decoder are commonly used in data transmission and storage applications, such as: broadcast equipment, wireless LANs, cable modems, xDSL, satellite communications, microwave networks, and digital TV. The block diagram of the RS decoder is shown in Figure 1.14. The received code word r(x) is the original (transmitted) code word c(x) plus additional errors: r( x ) = c( x ) + e( x ) (1.14) Table 1.2 3-Bit Soft Decision Input Value Decision 0 Most confident zero 1 Second most confident zero 2 Third most confident zero 3 Least confident zero 4 Least confident one 5 Third most confident one 6 Second most confident one 7 Most confident one
1.3 Receiver Systems 15 r(x) Si Xi Syndrome Error L(x) Error Error Error c(x) Input calculator polynomial locations magnitudes corrector Yi Output v Figure 1.14 Reed-Solomon decoder. The decoder will try to identify the position and the magnitude of maximum t errors (or 2t erasures) and correct the errors and erasures. A Reed-Solomon code word has 2t syndromes (Si) that depend only on errors and not on the transmitted code word. The syndrome is calculated by substituting the 2t roots of the generator polynomial g(x) into r(x). To find the symbol error location, solving simultaneous equations with t unknowns is necessary. First, the error locator polynomial (L(x)) is found using the Berklekamp-Massey or Euclid’s algorithms, with v being the num- ber of errors. The roots of the polynomial [i.e., the error locations (Xi)], are found with the Chien search algorithm. Next, the symbol error values (Yi) are found using the Fornay algorithm. 18.104.22.168 Golay Decoder The Golay coding can detect up to four bit errors in 24 bits (12 information bits) and correct up to three bit errors in 24 bits. If in 24 received bits there are three or less errors, the Golay decoding algorithm will detect the errors and correct them. If four errors appear, they will be detected but the exact pattern will not be deter- mined. An error message will be displayed. If there are more than four errors, the Golay decoding will not provide the actual error pattern, and the information in the 12 bits will be lost. 1.3.6 Decryptor Decryption is the process that reconstructs the original signal, which was altered through the encrypter in the transmitter. Decryption is required in the receiver sys- tem only if the encrypter was used in the transmitting system. Encryption is used as a protection means from signal interception. Error extension is possible during decryption (where multiple errors will be added for every error bit received). For decryption of the Data Encryption Standard (DES), the same encryp- tion algorithm (Figure 1.7) is used, with the same key, but reversed key schedule (16, ..., 1). 1.3.7 Derandomizer When randomization is used in the transmitting system, a derandomization of the data bitstream must be done in the receiving system. If synchronization bits are not randomized, they do not need to be derandomized. Synchronization bits identify the multiplexing frame, which is derandomized by Modulo 2 adding the same PN sequence that was used to randomize the frame to the bits within the frame.
16 Communications Systems 1.3.8 Demultiplexer The demultiplexer (Figure 1.15) is a device that receives data from one input and distributes it on 2n possible outputs, where n is the number of control bits. Table 1.3 shows the combination table for the demultiplexer 4/1. The data c coming to input “Z” will be distributed to four outputs I0, I1, I2, and I3 according to the controlling combinations of a1 and a0. All other outputs will have 0 as the output value. The demultiplexer recreates the user channels from the total bitstream. The bitstream is organized into multiplexing frames with a fixed bit length. Every user channel is allocated a specific bit position inside the frame. Inside the frame there are synchronization bits, which are used in the demultiplexing process, in which the bits are distributed to the appropriate user channel. This is not all that has to be thought of when considering the receiver system. Received power, sensitivity, required ratio of signal to noise, and noise factor are just some of the important parameters that have to be taken into consideration when planning a communication link. 1.3.9 Received Power Received power (Pr) at the receiving point is calculated using the effective area of an antenna (λ2/4π) and power density (Pt /4 · π · d2) as 2 λ2 Pt ⎛ λ ⎞ Pr = ⋅ = Pt ⎜ ⎟ (1.15) 4⋅ π 4⋅ π ⋅ d 2 ⎝4⋅ π ⋅ d⎠ where Pr is the received power, d is the distance from the transmitter to the receiver, Pt is the transmitted power, and λ is the wavelength of the signal. There are transmit- ting and receiving antenna gains Gt and Gr, for the antenna so the previous expres- sion can be written as 2 ⎛ λ ⎞ Pr = Pt ⋅ Gr ⋅ Gt ⎜ ⎟ (1.16) ⎝4⋅ π ⋅ d⎠ DEMUX 1/4 I0 I1 Z I2 I3 a1 a0 Figure 1.15 Demultiplexer 1/4.
1.3 Receiver Systems 17 Table 1.3 Combination Table for Demultiplexer 1/4 a1 a0 Z I0 I1 I2 I3 0 0 c 0 0 0 c 0 1 c 0 0 c 0 1 0 c 0 c 0 0 1 1 c c 0 0 0 22.214.171.124 Receiver Sensitivity Receiver sensitivity is the minimum level of signal at the input of the receiver, which is required to achieve a sufficient level of signal-to-noise ratio for the demodulation. The sensitivity is determined with thermal noise Pterm, required ratio of signal to noise (S/N)req for demodulation, and noise factor (NF) as ⎛S⎞ Pr min = Pterm ⋅ ⎜ ⎟ ⋅ NF (1.17) ⎝ N ⎠ req Receivers have the lowest level of signal strength required to process the infor- mation without loss of data. With digital systems, a lower received signal strength will result in a lower rate of received information. Typically receivers have a sensi- −9 −13 tivity ranging from −60 to −94 dBm (10 to 4 × 10 W). 126.96.36.199 Thermal Noise The thermal noise of the receiver is defined as Pterm = k ⋅ T ⋅ B (1.18) −23 where k is the Boltzmann constant 1,38⋅10 J/K, T is the temperature in Kelvin (290–300K), and B is the frequency range width in hertz. The density of the thermal noise at room temperature (290K) is 204 dBw/Hz. The width of the frequency chan- nel B is determined by the receiving filter width. Required ratio signal to noise in the receiver is the ratio of signal to noise required for a certain quality of the link (i.e., relative number of bits or frames with errors). The ratio of signal to noise is the difference between received signal and noise: S N[dB] = 10 log( S ) − 10 log( N ) (1.19) For analog systems, the S/N ratio must always be above zero. In digital systems (spread spectrum), the signal can be buried in the noise. The higher the bit rate, the larger the signal to noise ratio must be. 188.8.131.52 Noise Factor The noise factor of the receiver (NF) is the ratio of signal to noise at the input and output of the receiver:
18 Communications Systems S N in NF = (1.20) S N out This ratio can be from a fraction of a decibel for low noise microwave convert- ers [0.3 dB for low noise block downconverters (LNB) for satellite applications] to 30 or 40 dB for spectral analyzers; typically the ratio ranges from 2 to 10 dB. This is actually the noise the receiver itself introduces into the system. The noise threshold is the sum of the thermal noise and the noise factor.1.4 User Interface The user interface is the means for people to interact with the communication sys- tem. It consists of input of some sort and output. The input can be a command using a keyboard, voice, or text. We have heard of the phrase “user-friendly,” which means that it is simple to operate a certain device. When designing an application, a lot of care is taken to make a suitable user interface. Nowadays, there are hearing aids and other tools available for individuals with a handicap. Normally, the user interface is graphical (GUI), but it can also be operating via voice or touch. 1.4.1 Graphical User Interface (GUI) The graphical user interface interacts with electronic devices through icons or visual indicators. Touch screens are one of the GUI types. There are also the command line and text user interfaces, which use a keyboard to type the commands. Main touchscreen technologies are resistive and capacitive. Resistive LCD touchscreen monitors rely on a touch overlay, which is composed of a flexible top layer and a rigid bottom layer separated by insulating dots attached to a touchscreen controller. The inside surface of each of the two layers is coated with a transparent metal oxide coating that facilitates a gradient across each layer when voltage is applied. Pressing the flexible top sheet creates electrical contact between the resistive layers, and closes a switch in the circuit. The control electronics alternate voltage between the layers and pass the resulting X and Y touch coordinates to the touchscreen control- ler. The touchscreen controller data is then passed on to the computer operating sys- tem for processing. Capacitive touch screens work by placing a very small charge at each of the four corners of the screen. When a finger touches the screen, the touch controller determines the change of capacitance of the screen from each of the four points and provides a touch value at the correct location. Surface acoustic wave touch screen technology is based on sending acoustic waves across a clear glass panel with a series of transducers and reflectors. When a finger touches the screen, the waves are absorbed, causing a touch event to be detected at that point. Because the panel is all glass, there are no layers that can be worn, which results in durability. Infrared technology is based on the interruption of an infrared light grid in front of the display screen. The touch frame contains a row of infrared LEDs and photo transistors, each mounted on two opposite sides to create a grid of invisi- ble infrared light.
1.5 Antenna Systems 19 1.4.2 Voice User Interface (VOI) The voice user interface can be activated through speech. Today hands-free com- mands are possible. The possibility of error when inputing a command or data by voice is higher than when entering it through a keyboard. Figure 1.16 shows a possi- ble user interface for a communication device.1.5 Antenna Systems Antenna systems consist of a duplexer and an antenna used to transmit and receive information from one user to another. There are many types of antennas that can be used for a communication system, depending on the frequency of use, power, appli- cation, and even international standard regulations. Most communication systems use the same antenna for transmitting and receiving a signal. This normally requires two antennas, which have to be physically separated. This is impractical, except in some cases of high interference when antenna diversity could be an option. That is why in most cases a single antenna is used for both transmitting and receiving the signal. This is possible with the use of a duplexer. 1.5.1 Duplexer The duplexer makes it possible for receiver and transmitter systems to use the same antenna; otherwise it would be necessary to use two antennas. The duplexer has fil- ters, which isolate the transmitting frequency from the receiving frequency. Since the transmitting and receiving frequency are usually not the same (because of inter- ference), there must be a separation between them. The duplexer must be designed to operate in the frequency band used by both the receiver and the transmitter. It also must be able to operate on the power from the power amplifier. When working at the transmitting frequency, it must reject the noise from the receiver and vice versa. The duplexer can be made with a hybrid ring, cavity notch, and a band-pass/ band-reject design. Figure 1.17 shows the design of a reject duplexer using notch cavities. Audio Signal Transmitter processor processor Screen User Communication interface interface Keyboard Figure 1.16 User interface.
20 Communications Systems To antenna Optimal cable length Cavity tuned to Cavity tuned to transmitter frequency receiver frequency To transmitter To receiver Figure 1.17 Reject duplexer with notch cavities. Using only two notch cavities would probably not provide sufficient isolation for most situations. The cavity in the transmitter is tuned to the receiver frequency, and the cavity in the receiver is tuned to the transmitter frequency. That means that the cavity in the transmitter area will pass the transmitting frequency and notch (reject) the receiving frequency. The same applies for the cavity in the receiver area, which will pass the receiver frequency and notch the transmitter frequency. If there are other strong signals present, this design will not be enough. A more advanced design must include four to six cavities. Two or three cavities in each leg are more effective than just one. Usually the cavities require tuning with a spectrum analyzer or wattmeter. 1.5.2 Antenna The antenna is a device that transforms a guided electromagnetic wave from the transmission line (waveguide or cable) into a space wave in free space. The antenna actually makes a transition between the guided wave in the transmission line and the space wave in free space. The most important characteristics of an antenna are: radiation pattern, directivity, impedance, gain, and affective area. 184.108.40.206 Radiation Pattern Electric field intensity falls with 1/d, where d is the distance from the antenna. To measure the electric or magnetic field from the antenna, we have to be far enough from the antenna (only the radiating field exists). This happens at the distance d, 2D 2 d = (1.21) λ where D is the largest dimension of the antenna and λ is the wavelength of the sig- nal. Then, knowing the electric field E, the magnetic field H can be calculated from E H= (1.22) η
1.5 Antenna Systems 21 where η is the impedance of free space, that is 120π or 377Ω. Usually the radiation pattern is given in two perpendicular planes: horizontal and vertical; it usually has one main lobe and several sidelobes. 220.127.116.11 Directivity Often the goal of an antenna is to have most of the radiation in just one direction with much less radiation in other directions. The directivity angle is calculated as the angle where the power density is one half of the maximum and the field density drops for a factor of 1/ 2. Directivity is defined as the ratio of the power density radiated by the antenna in the direction of maximum intensity and the power den- sity radiated by the isotropic radiator. The isotropic radiator radiates equally in all directions. Antennas with higher directivity are used to radiate as much energy to the receiver as possible. At the same time, dispersion of the signal in unwanted directions is diminished, thus making the interference to other systems smaller. 18.104.22.168 Antenna Impedance Antenna impedance is the ratio of the voltage and current at the antenna. The most power from the generator will be given to the antenna if the antenna and generator impedances are complex conjugates (i.e., Z a = ZG ). That means that their * resistances must be equal, whereas their reluctances must be equal in magnitude but of opposite signs. Usually generators have an output impedance of 50Ω or 75Ω, so the antenna will have to be of the same impedance if possible. 22.214.171.124 Gain Gain is related to the power received from the generator and represents the number showing how much larger the power from the isotropic radiator must be compared to the received power of the antenna, in order for the radiation from the isotropic radiator to be the same as the radiation from the observed antenna in the direction of maximum radiation. For an ideal antenna without losses, the gain would be equal to directivity. Gain of the antenna is usually given in decibels. 126.96.36.199 Effective Area The effective area of a receiving antenna, Aeff, is defined as the ratio of received power, Pr, absorbed on a matched load connected to the antenna, and power den- sity of the incident electromagnetic wave, Sr: Pr A eff = (1.23) Sr The power density of the transmitting antenna in the maximum direction is equal to
22 Communications Systems Gt Pt Sr = (1.24) 4⋅ π ⋅ d 2 where Gt and Pt are gain of the transmitting antenna and power of the transmitting antenna. The relation that connects the effective area and gain for all antennas is λ2 A eff = ⋅G (1.25) 4⋅ π 188.8.131.52 Antenna Types There are many types of antennas, depending on the type of the application needed. They can be divided into four groups: electrically small antennas, wideband anten- nas, resonant antennas, and aperture antennas. Electrically small antennas are much smaller in dimension than the wavelength associated with the frequency on which they operate. They have small directivity and radiation effectiveness. They include Hertz’s dipole and monopole. To increase directivity, antenna arrays can be built. By changing the phase of the supplying cur- rents, different radiation patterns can be obtained. Wideband antennas have a stable radiation pattern, gain, and impedance in the wide frequency range. The gain is small to medium. The biconical antenna and log-periodic antenna are examples of this type of antenna. Resonant antennas oper- ate in one or more selective frequency ranges. They have a small to medium gain. The microwave microstrip antenna is a resonant antenna. Aperture antennas receive and radiate electromagnetic waves through an aperture. They have large gain, which increases with the frequency. The horn antenna and parabolic dish are examples of this type of antenna. 184.108.40.206 Smart Antenna Systems A smart antenna system uses multiple antenna elements including signal processing to optimize its radiation pattern depending on the signal environment. The smart antenna interference is smaller, which enables reuse of the frequency more often. This can also improve the capacity of the link. Greater signal gain will result in lower power requirements at the receiving system with a smaller size and battery. The power amplifier used can be cheaper, with less total power consumption.1.6 Power Supplies Power in communication systems is necessary for the operation of electronic com- ponents. For simple systems, a DC supply is sufficient. For the high power of a transmitter, a power amplifier is necessary. The transmitter system usually requires more power than the receiving system. The majority of power is used to amplify the signal before reaching the antenna. In calculating the communication link, a free space loss must also be taken into consideration. In addition, cable loss and match-
1.7 Considerations for Voice Versus Data 23 ing losses require the transmitted power to be raised. For small transmitting power, a simple 12-V DC supply is sufficient. Larger power requires power amplifiers. 1.6.1 Power Supply Types Linear power uses a transformer to convert the voltage from the mains to a lower voltage. Converters, which transform 120-V or 220-V AC into a lower DC voltage (typically 12V or 24V), are often used for electronic circuits. There are many types available. An uninterruptible power supply (UPS) must be used in applications for which a constant power supply is necessary. UPS usually takes the power from the AC mains and charges its own battery at the same time. If there is a loss of power, the battery will provide the necessary power for some time. There are solutions where the UPS charges a battery with energy generated from internal combustion engines or turbines. Batteries are also often used for mobile communications. In some situations solar power might be used, especially in areas with a lot of sun. 1.6.2 Power Amplifier Power amplifiers are used to increase the level of the signal, both in transmitter and receiver systems. They are used to amplify the low-level signal to a higher value. Power gain is described as ⎛P ⎞ G(dB) = 10 log10 ⎜ out ⎟ (1.26) ⎝ Pin ⎠ where Pin is input power and Pout output power. Power amplifiers can be divided in classes A, B, AB, C, D, and E. Class A uses 100% of the input signal. This amplifier is inefficient and is used for small signals or low power amplification. Class B uses 50% of the input signal. It is more efficient than class A, but subject to signal distortions. Class AB is a combination of class A and class B. It uses more than 50% of the signal. Class C uses less than 50% of the signal. Distortions are high, but so is the efficiency. Class D uses switching (on/off) for high efficiency. It can be used in digital circuits. There are also some other special classes.1.7 Considerations for Voice Versus Data Input information into the communication system can be voice or data (text, pic- tures, video, and so forth). In this section just some of the codecs are mentioned. There are many more; some of them are obsolete, while others are being developed. 1.7.1 Text ASCII uses 7 bits per character. Extended ASCII uses 8 bits per character.
24 Communications Systems Table 1.4 Data Rates for Audio Codecs ADPCM G.711 G.729a Sample Rate 8 KHz 8 KHz 8 KHz Effective Sample Size 8 bits 4 bits 1 bit Data Rate 64 Kbps 32 Kbps 8 Kbps 1.7.2 Images The Graphics Interchange Format (GIF) (lossless compression) uses 8 bits per pixel and a 256 color palette. The Joint Photographic Exchange Group (JPEG) (lossy compression) format most often uses a 10:1 compression. 1.7.3 Voice Pulse code modulation (PCM) has 8,000 samples per second—with 8 bits per sec- ond it results in 64 Kbits per second. Compression techniques are adaptive differen- tial pulse code modulation (ADPCM) (32 Kbps) and residual excited linear predictive coding (8–16 Kbps). Audio music requires 32–384 Kb/s. The audio signal is sensitive to delay and jitter. Latency is the end-to-end delay from mouth to ear. It must not exceed 100 ms for excellent quality. For acceptable quality it should not exceed 150 ms. For higher delays an echo canceller is required. There is a propagation delay in free space, which depends on the frequency used and the distance between the transmitter and receiver. Packetization delay is the time required to create an audio packet and send it on a network—it depends on the codec. Table 1.4 gives the data rates for some audio codecs. The G.711 codec used in telephony works at 64 Kbps. 1.7.4 Video H.261 coding uses 176 by 144 or 352 by 258 frames at 10–30 frame/sec. MPEG-2 and HDTV use 1,920 by 1,080 frames at 30 frames/sec.Selected Bibliography Balanis, C. A., Antenna Theory—Analysis and Design, New York: John Wiley & Sons, 2005. Brown, S., and Z. Vranesic, Fundamentals of Digital Logic with VHDL Design, New York: McGraw-Hill, 2001. Couch, L. W., Digital and Analog Communication Systems, 6th ed., Upper Saddle River, NJ: Prentice-Hall, 2001. Dunlop, J., and D. G. Smith, Telecommunication Engineering, London, U.K.: Chapman and Hill, 1994. Diffie, W., and M. E. Hellman, “Privacy and Authentification: An Introduction to Cryptogra- phy,” Proceedings of the IEEE, Vol. 67, No. 3, March 1979, pp. 397–428. Hanna, S. A., “Convolutional Interleaving for Digital Radio Communications,” Proc. 2nd Inter- national Conference on Personal Communications: Gateway to the 21st Century, 1993, Vol. 1, pp. 443–447.
1.7 Considerations for Voice Versus Data 25 Federal Information Processing Standards Publications 197: “Announcing the Advanced Encryp- tion Standard (AES),” http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf. Gardiol, F. E., Introduction to Microwaves, Dedham, MA: Artech House, 1984. Morelos-Zaragoza, R. H., The Art of Error Correcting Coding, New York: John Wiley & Sons, 2006. Sklar, B., Digital Communication: Fundamentals and Applications, Upper Saddle River, NJ: Prentice-Hall, 2001. Xiong, F., Digital Modulation Techniques, Norwood, MA: Artech House, 2000.
CHAPTER 2Electromagnetic Spectrum Used forCommunications2.1 Electromagnetic Spectrum The electromagnetic (EM) spectrum of an object is the distribution of electromag- netic radiation from that object. The EM spectrum (Figure 2.1) covers frequencies from 3 Hz (ELF) to gamma rays (30 ZHz) and beyond (cosmic rays). The corresponding wavelengths λ can range from thousands of kilometers to a fraction of an atom size (Table 2.1). The frequency and the wavelength are related by the following expression: c λ= (2.1) f where c is the speed of light—approximately 30,000,000 m/s. The energy of the particular range is defined as E = h⋅ f (2.2) where f is the frequency in hertz and h is the Planck’s constant, 6.62606896e−34 Js. Energy can be expressed in eV, where 1 eV is approximately 1.60217653e−19 J. One eV is equal to the amount of energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of 1 volt. It is also the energy needed to break the chemical bond in the cell. The higher the frequency, the higher the energy in each photon (Table 2.1). Table 2.2 gives the prefix converters used in Table 2.1. The spectrum is divided in decades. The radio spectrum (including microwaves) is considered to cover frequencies from 9 kHz to 300 GHz, that is, from VLF to SHF. Most communications take place in the radio spectrum, but the infrared and the visible spectrum can be used as well. The use of frequency bands for communi- cation is discussed latter in Sections 2.1.1 to 2.1.13. 27