Wireless Connectivity
Description
Wireless connectivity has become an indispensable part, a commodity associated with the way we work and play. The latest developments, the 5G, next-generation Wi-Fi and Internet of Things connectivity, are the key enablers for widespread digitalization of practically all industries and public sector segments. This immense development within the last three decades have been accompanied by a large number of ideas, articles, patents, and even myths. This book introduces the most important ideas and concepts in wireless connectivity and discusses how these are interconnected, whilst the mathematical content is kept minimal. The book does not follow the established, linear structure in which one starts from the propagation and channels and then climbs up the protocol layers. The structure is, rather, nonlinear, in an attempt to follow the intuition used when one creates a new technology to solve a certain problem.
The target audience is:
* Students in electronics, communication, and networking
* Wireless engineers that are specialized in one area, but want to know how the whole system works, without going through all the details and math
* Computer scientists that want to understand the fundamentals of wireless connectivity, the requirements and, most importantly, the limitations
* Engineers in energy systems, logistics, transport and other vertical sectors that are increasingly reliant on wireless technology
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Petar Popovski is a Professor of Wireless Communications at Aalborg University and Fellow of the IEEE. He received his Dipl.-Ing and M. Sc. degrees in Communication Engineering from the University of Sts. Cyril and Methodius in Skopje and the Ph.D. degree from Aalborg University in 2005. He has over 300 publications in journals, conference proceedings, and edited books. He holds over 30 patents and patent applications. He received an ERC Consolidator Grant (2015), the Danish Elite Researcher award (2016), IEEE Fred W. Ellersick prize (2016), and IEEE Stephen O. Rice prize (2018). He is featured in the list of Highly Cited Researchers 2018, compiled by Web of Science. His research interests are in the area of wireless communication and communication theory.
Content
Foreword xv
Acknowledgments xix
Acronyms xxi
1 An Easy Introduction to the Shared Wireless Medium 3
1.1 How to Build a Simple Model for Wireless Communication 4
1.1.1 Which Features We Want from the Model 4
1.1.2 Communication Channel with Collisions 4
1.1.3 Trade-offs in the Collision Model 7
1.2 The First Contact 9
1.2.1 Hierarchy Helps to Establish Contact 9
1.2.2 Wireless Rendezvous without Help 11
1.2.3 Rendezvous with Full-Duplex Devices 12
1.3 Multiple Access with Centralized Control 12
1.3.1 A Frame for Time Division 13
1.3.2 Frame Header for Flexible Time Division 14
1.3.3 A Simple Two-Way System that Works Under the Collision Model 15
1.3.4 Still Not a Practical TDMA System 18
1.4 Making TDMA Dynamic 19
1.4.1 Circuit-Switched versus Packet-Switched Operation 19
1.4.2 Dynamic Allocation of Resources to Users 20
1.4.3 Short Control Packets and the Idea of Reservation 22
1.4.4 Half-Duplex versus Full-Duplex in TDMA 24
1.5 Chapter Summary 25
1.6 Further Reading 25
1.7 Problems and Reflections 26
2 Random Access: How to Talk in Crowded Dark Room 29
2.1 Framed ALOHA 30
2.1.1 Randomization that Maximizes the ALOHA Throughput 32
2.2 Probing 35
2.2.1 Combining ALOHA and Probing 39
2.3 Carrier Sensing 39
2.3.1 Randomization and Spectrum Sharing 39
2.3.2 An Idle Slot is Cheap 41
2.3.3 Feedback to the Transmitter 43
2.4 Random Access and Multiple Hops 45
2.4.1 Use of Reservation Packets in Multi-Hop 47
2.4.2 Multiple Hops and Full-Duplex 47
2.5 Chapter Summary 48
2.6 Further Reading 48
2.7 Problems and Reflections 48
3 Access Beyond the Collision Model 53
3.1 Distance Gets into the Model 53
3.1.1 Communication Degrades as the Distance Increases 53
3.1.2 How to Make the Result of a Collision Dependent on the Distance 55
3.2 Simplified Distance Dependence: A Double Disk Model 57
3.3 Downlink Communication with the Double Disk Model 58
3.3.1 A Cautious Example of a Design that Reaches the Limits of the Model 61
3.4 Uplink Communication with the Double Disk Model 62
3.4.1 Uplink that Uses Multi-Packet Reception 64
3.4.2 Buffered Collisions for Future Use 64
3.4.3 Protocols that Use Packet Fractions 66
3.5 Unwrapping the Packets 68
3.6 Chapter Summary 69
3.7 Further Reading 70
3.8 Problems and Reflections 70
4 The Networking Cake: Layering and Slicing 75
4.1 Layering for a One-Way Link 75
4.1.1 Modules and their Interconnection 75
4.1.2 Three Important Concepts in Layering 77
4.1.3 An Example of a Two-Layer System 78
4.2 Layers and Cross-Layer 79
4.3 Reliable and Unreliable Service from a Layer 81
4.4 Black Box Functionality for Different Communication Models 84
4.5 Standard Layering Models 86
4.5.1 Connection versus Connectionless 87
4.5.2 Functionality of the Standard Layers 88
4.5.3 A Very Brief Look at the Network Layer 89
4.6 An Alternative Wireless Layering 91
4.7 Cross-Layer Design for Multiple Hops 92
4.8 Slicing of the Wireless Communication Resources 94
4.8.1 Analog, Digital, Sliced 94
4.8.2 A Primer on Wireless Slicing 96
4.8.2.1 Orthogonal Wireless Slicing 96
4.8.2.2 Non-Orthogonal Wireless Slicing 98
4.9 Chapter Summary 100
4.10 Further Reading 100
4.11 Problems and Reflections 100
5 Packets Under the Looking Glass: Symbols and Noise 105
5.1 Compression, Entropy, and Bit 105
5.1.1 Obtaining Digital Messages by Compression 106
5.1.2 A Bit of Information 106
5.2 Baseband Modules of the Communication System 107
5.2.1 Mapping Bits to Baseband Symbols under Simplifying Assumptions 108
5.2.2 Challenging the Simplifying Assumptions about the Baseband 109
5.3 Signal Constellations and Noise 110
5.3.1 Constellation Points and Noise Clouds 110
5.3.2 Constellations with Limited Average Power 113
5.3.3 Beyond the Simple Setup for Symbol Detection 114
5.3.4 Signal-to-Noise Ratio (SNR) 116
5.4 From Bits to Symbols 117
5.4.1 Binary Phase Shift Keying (BPSK) 117
5.4.2 Quaternary Phase Shift Keying (QPSK) 118
5.4.3 Constellations of Higher Order 119
5.4.4 Generalized Mapping to Many Symbols 122
5.5 Symbol-Level Interference Models 123
5.5.1 Advanced Treatment of Collisions based on a Baseband Model 124
5.6 Weak and Strong Signals: New Protocol Possibilities 126
5.6.1 Randomization of Power 127
5.6.2 Other Goodies from the Baseband Model 129
5.7 How to Select the Data Rate 130
5.7.1 A Simple Relation between Packet Errors and Distance 130
5.7.2 Adaptive Modulation 132
5.8 Superposition of Baseband Symbols 134
5.8.1 Broadcast and Non-Orthogonal Access 135
5.8.2 Unequal Error Protection (UEP) 137
5.9 Communication with Unknown Channel Coefficients 138
5.10 Chapter Summary 141
5.11 Further Reading 142
5.12 Problems and Reflections 142
6 A Mathematical View on a Communication Channel 147
6.1 A Toy Example: The Pigeon Communication Channel 147
6.1.1 Specification of a Communication Channel 149
6.1.2 Comparison of the Information Carrying Capability of Mathematical Channels 150
6.1.3 Assumptions and Notations 151
6.2 Analog Channels with Gaussian Noise 151
6.2.1 Gaussian Channel 152
6.2.2 Other Analog Channels Based on the Gaussian Channel 152
6.3 The Channel Definition Depends on Who Knows What 154
6.4 Using Analog to Create Digital Communication Channels 158
6.4.1 Creating Digital Channels through Gray Mapping 158
6.4.2 Creating Digital Channels through Superposition 161
6.5 Transmission of Packets over Communication Channels 163
6.5.1 Layering Perspective of the Communication Channels 163
6.5.2 How to Obtain Throughput that is not Zero 164
6.5.3 Asynchronous Packets and Transmission of "Nothing" 167
6.5.4 Packet Transmission over a Ternary Channel 169
6.6 Chapter Summary 171
6.7 Further Reading 171
6.8 Problems and Reflections 172
7 Coding for Reliable Communication 177
7.1 Some Coding Ideas for the Binary Symmetric Channel 177
7.1.1 A Channel Based on Repetition Coding 177
7.1.2 Channel Based on Repetition Coding with Erasures 179
7.1.3 Coding Beyond Repetition 181
7.1.4 An Illustrative Comparison of the BSC Based Channels 182
7.2 Generalization of the Coding Idea 183
7.2.1 Maximum Likelihood (ML) Decoding 187
7.3 Linear Block Codes for the Binary Symmetric Channel 188
7.4 Coded Modulation as a Layered Subsystem 192
7.5 Retransmission as a Supplement to Coding 194
7.5.1 Full Packet Retransmission 195
7.5.2 Partial Retransmission and Incremental Redundancy 197
7.6 Chapter Summary 199
7.7 Further Reading 199
7.8 Problems and Reflections 199
8 Information-Theoretic View on Wireless Channel Capacity 203
8.1 It Starts with the Law of Large Numbers 203
8.2 A Useful Digression into Source Coding 204
8.3 Perfectly Reliable Communication and Channel Capacity 207
8.4 Mutual Information and Its Interpretations 209
8.4.1 From a Local to a Global Property 209
8.4.2 Mutual Information in Some Actual Communication Setups 211
8.5 The Gaussian Channel and the Popular Capacity Formula 214
8.5.1 The Concept of Entropy in Analog Channels 214
8.5.2 The Meaning of "Shannon's Capacity Formula" 215
8.5.3 Simultaneous Usage of Multiple Gaussian Channels 217
8.6 Capacity of Fading Channels 219
8.6.1 Channel State Information Available at the Transmitter 219
8.6.2 Example: Water Filling for Binary Fading 221
8.6.3 Water Filling for Continuously Distributed Fading 222
8.6.4 Fast Fading and Further Remarks on Channel Knowledge 223
8.6.5 Capacity When the Transmitter Does Not Know the Channel 225
8.6.5.1 Channel with Binary Inputs and Binary Fading 225
8.6.5.2 Channels with Gaussian Noise and Fading 229
8.6.6 Channel Estimation and Knowledge 230
8.7 Chapter Summary 232
8.8 Further Reading 233
8.9 Problems and Reflections 233
9 Time and Frequency in Wireless Communications 237
9.1 Reliable Communication Requires Transmission of Discrete Values 237
9.2 Communication Through a Waveform: An Example 239
9.3 Enter the Frequency 242
9.3.1 Infinitely Long Signals and True Frequency 242
9.3.2 Bandwidth and Time-Limited Signals 245
9.3.3 Parallel Communication Channels 247
9.3.4 How Frequency Affects the Notion of Multiple Access 248
9.4 Noise and Interference 250
9.4.1 Signal Power and Gaussian White Noise 250
9.4.2 Interference between Non-Orthogonal Frequencies 252
9.5 Power Spectrum and Fourier Transform 255
9.6 Frequency Channels, Finally 258
9.6.1 Capacity of a Bandlimited Channel 259
9.6.2 Capacity and OFDM Transmission 261
9.6.3 Frequency for Multiple Access and Duplexing 261
9.7 Code Division and Spread Spectrum 263
9.7.1 Sharing Synchronized Resources with Orthogonal Codes 263
9.7.2 Why Go Through the Trouble of Spreading? 265
9.7.3 Mimicking the Noise and Covert Communication 268
9.7.4 Relation to Random Access 269
9.8 Chapter Summary 270
9.9 Further Reading 270
9.10 Problems and Reflections 270
10 Space in Wireless Communications 275
10.1 Communication Range and Coverage Area 276
10.2 The Myth about Frequencies that Propagate Badly in Free Space 278
10.3 The World View of an Antenna 280
10.3.1 Antenna Directivity 280
10.3.2 Directivity Changes the Communication Models 282
10.4 Multipath and Shadowing: Space is Rarely Free 283
10.5 The Final Missing Link in the Layering Model 286
10.6 The Time-Frequency Dynamics of the Radio Channel 288
10.6.1 How a Time-Invariant Channel Distorts the Received Signal 288
10.6.2 Frequency Selectivity, Multiplexing, and Diversity 291
10.6.3 Time-Variant Channel Introduces New Frequencies 292
10.6.4 Combined Time-Frequency Dynamics 295
10.7 Two Ideas to Deal with Multipath Propagation and Delay Spread 296
10.7.1 The Wideband Idea: Spread Spectrum and a RAKE Receiver 297
10.7.2 The Narrowband Idea: OFDM and a Guard Interval 299
10.8 Statistical Modeling of Wireless Channels 300
10.8.1 Fading Models: Rayleigh and Some Others 301
10.8.2 Randomness in the Path Loss 303
10.9 Reciprocity and How to Use It 303
10.10 Chapter Summary 305
10.11 Further Reading 305
10.12 Problems and Reflections 305
11 Using Two, More, or a Massive Number of Antennas 309
11.1 Assumptions about the Channel Model and the Antennas 310
11.2 Receiving or Transmitting with a Two-Antenna Device 311
11.2.1 Receiver with Two Antennas 311
11.2.2 Using Two Antennas at a Knowledgeable Transmitter 313
11.2.3 Transmit Diversity 314
11.3 Introducing MIMO 315
11.3.1 Spatial Multiplexing 317
11.4 Multiple Antennas for Spatial Division of Multiple Users 319
11.4.1 Digital Interference-Free Beams: Zero Forcing 320
11.4.2 Other Schemes for Precoding and Digital Beamforming 322
11.5 Beamforming and Spectrum Sharing 325
11.6 What If the Number of Antennas is Scaled Massively? 327
11.6.1 The Base Station Knows the Channels Perfectly 328
11.6.2 The Base Station has to Learn the Channels 329
11.7 Chapter Summary 331
11.8 Further Reading 331
11.9 Problems and Reflections 331
12 Wireless Beyond a Link: Connections and Networks 335
12.1 Wireless Connections with Different Flavors 335
12.1.1 Coarse Classification of the Wireless Connections 335
12.1.2 The Complex, Multidimensional World of Wireless Connectivity 337
12.2 Fundamental Ideas for Providing Wireless Coverage 339
12.2.1 Static or Moving Infrastructure 340
12.2.2 Cells and a Cellular Network 341
12.2.3 Spatial Reuse 343
12.2.4 Cells Come in Different Sizes 345
12.2.5 Two-Way Coverage and Decoupled Access 347
12.3 No Cell is an Island 348
12.3.1 Wired and Wireless Backhaul 348
12.3.2 Wireless One-Way Relaying and the Half-Duplex Loss 349
12.3.3 Wireless Two-Way Relaying: Reclaiming the Half-Duplex Loss 351
12.4 Cooperation and Coordination 355
12.4.1 Artificial Multipath: Treating the BS as Yet Another Antenna 355
12.4.2 Distributing and Networking the MIMO Concept 357
12.4.3 Cooperation Through a Wireless Backhaul 359
12.5 Dissolving the Cells into Clouds and Fog 360
12.5.1 The Unattainable Ideal Coverage 360
12.5.2 The Backhaul Links Must Have a Finite Capacity 362
12.5.3 Noisy Cooperation with a Finite Backhaul 363
12.5.4 Access Through Clouds and Fog 364
12.6 Coping with External Interference and Other Questions about the Radio Spectrum 366
12.6.1 Oblivious Rather Than Selfish 366
12.6.2 License to Control Interference 367
12.6.3 Spectrum Sharing and Caring 369
12.6.4 Duty Cycling, Sensing, and Hopping 371
12.6.5 Beyond the Licensed and Unlicensed and Some Final Words 372
12.7 Chapter Summary 374
12.8 Further Reading 374
12.9 Problems and Reflections 375
Bibliography 377
Index 381
1
An Easy Introduction to the Shared Wireless Medium
We start by describing wireless communication through an analogy with a conversation within a group of people, named Zoya, Yoshi, and Xia. We will refer to these and some other characters throughout the book; the characters will stand for wireless devices, base stations, or similar. The data that they want to communicate to each other is the content of their speech, which is part of the conversation. Regardless of the speech content, the conversation can only take place if the participants follow some conversation protocol, such that at a given time only one person speaks while the others listen. How do they agree who gets to speak and who gets to listen? One way would be, before starting the actual conversation, to have them agree upon which conversation protocol should be followed. In that case the information exchanged in that preliminary conversation cannot be regarded as useful data, but rather as metadata, also called protocol information or control information. The metadata is necessary in order to enable the conversation to take place. But then, how do they agree on the protocol for exchanging the metadata?
These questions can go on to infinity, but in a normal situation the communication protocol is agreed upon by either sticking to certain rules of politeness or following visual cues and gestures that facilitate the conversation. In other words, the metadata is exchanged by using a visual communication channel that is different from the speech communication channel. However, in a commonly encountered wireless communication system there is only one communication channel through which both the data and the metadata should be sent. This is not to say that it is not possible to have one wireless communication channel for data and a separate one for metadata; even if such separation exists, then what is the protocol for exchanging the meta-metadata that is used to agree how to send the metadata?
This gets obviously complicated, but the bottom line is that we will always hit the problem of communicating over a single shared wireless channel. Now, taking the fact that there is a single channel for communicating both the data and the metadata, the key point of the analogy with the conversation is to put Zoya, Yoshi, and Xia in a dark room, such that they have only speech as a means of communicating (we exclude tactile communication) and no visual cues can be of help. In that setting, the audio channel should be used both to coordinate the conversation and to carry the actual content of the conversation.
This is the common situation in which wireless communication systems operate and will be the subject of this chapter. Here are some examples of the questions that will be discussed. If Zoya and Yoshi want to talk to each other, how do they agree who talks first and who listens first? If both Xia and Yoshi want to talk to Zoya, how should they agree who takes a turn to speak at a given time, so that they don't all talk simultaneously? Solutions to these problems are provided by various protocols for controlling the access to the medium; hence the name MAC (medium access control) protocols, and they are of central importance in wireless communication systems.
1.1 How to Build a Simple Model for Wireless Communication
1.1.1 Which Features We Want from the Model
The main feature of the wireless communication medium is the fact that the medium is shared, in the same way in which the air through which the sound propagates is shared among the people having a conversation. MAC protocols enable multiple wireless communication users and devices to share the medium and send/receive data.
First, we must agree on how the system operates and what it takes to have a signal from one communication node received correctly at another node. In other words, we need to settle on a suitable system model: a set of assumptions that will allow us to talk about communication protocols and principles in a setting that is simple, but sufficient to contain the necessary properties of a shared wireless medium. We build the initial model by relying on a common sense analogy with the spoken conversation, as it captures three fundamental properties of wireless communication: broadcast, interference, and half-duplex operation. We illustrate these features by observing a conversation between Zoya, Yoshi, and Xia:
- Half-duplex: A given person, e.g. Zoya, cannot speak and listen at the same time.
- Broadcast: If Zoya has information to convey to Yoshi and Xia, then, provided that both Yoshi and Xia are listening, Zoya needs to say her message only once, and not repeat it individually to Yoshi and to Xia.
- Interference: If Yoshi and Xia speak simultaneously, Zoya will not understand either of them.
The descriptions above are arguably not always correct, but they do represent what is common sense for a conversation. Furthermore, the analogy of the communication problems with the conversation between Zoya, Yoshi, and Xia is useful, but it has its limitations, which will be pointed out when necessary.
1.1.2 Communication Channel with Collisions
For the purpose of this chapter, we define a communication channel to be the physical resource that is used for a wireless transmission. In that sense, in spoken communication, the channel is created by the audible vibrations that take place in air or even another sound-propagating medium. It is useful to note that the communication channel is not the whole physical medium with all the vibrations, since there are vibrations that cannot be registered by ear and thus do not carry useful audio information. Furthermore, spoken communication uses a single communication channel: one cannot switch to another channel, such as in a TV receiver, in order to listen to the desired speaker and avoid the undesired one.
As already stated above, our discussion will be limited to the case in which all nodes use a single communication channel. In reality that can be, for example, a certain frequency to which all the nodes are "tuned". Here we use the term "frequency" as it is used in a common language for, say, a TV frequency. One may argue that Zoya and Yoshi can agree to one frequency, while Xia and Walt can agree to tune to another frequency and in this way they do not need to share the channel with the link Zoya-Yoshi. This is indeed possible and we will discuss it in later chapters, when we introduce the notion of separation in frequency. On the other hand, it is also true that Zoya and Yoshi should first use some communication channel to agree upon which frequency they will use for communication. This agreement is, again, metadata or control information, such that the corresponding channel is often denoted as a control channel and can be shared by multiple nodes to come to an agreement about the frequency. For example, if Zoya decides to communicate with Xia, then she knows that she should try to find Xia at the control channel and, upon contacting her, use the control channel to decide which channel/frequency they should both be tuned to in order to communicate the useful data. However, the control channel is a common, shared communication channel and therefore the question of how to share that channel to send metadata remains valid.
The communication model used in this chapter is called a collision model. This is because the central assumption of the model is that if two or more nodes transmit simultaneously, then the interference that they cause to each other is manifested as a collision at the receiver. Upon collision, the receiver does not manage to retrieve any data successfully. Another assumption in the model, not really related to the issue of collision, is that a node operates in a half-duplex manner and cannot receive while transmitting. Most of the wireless systems that we encounter today are not full-duplex, that is, do not transmit and receive simultaneously at the same frequency channel. However, although technologically more complex, it is also possible to have full-duplex operation. Therefore, throughout the chapter we will occasionally revise the half-duplex assumption and discuss the changes that the full-duplex can bring into the design of a specific protocol or algorithm.
The communication between the wireless nodes is based on data packets. A transmitting node is capable of sending bits per second (bps) such that a packet of duration contains bits. All packets have the same duration, unless stated otherwise. In the collision model, a packet is treated as the smallest, atomic unit of information, such that either the whole packet is received correctly or it is lost. In other words, it is not possible to receive only some bits of the packet correctly. A packet sent by Zoya to Yoshi is received correctly if:
- Yoshi is in the communication range of Zoya such that the distance between them is less than m;
- No other communication node that is within m of Yoshi transmits while Yoshi is receiving the packet from Zoya.
The first condition above indicates that each transmission is omnidirectional. Due to the basic property of reciprocity in electromagnetic/radio propagation (see Section 10.9), each reception in our model is also omnidirectional. From this it follows that Yoshi receives a signal as long it is sent from a distance less than m, regardless of the actual position. The ingredients of the collision model are illustrated in...
