Preface

Wireless communication is an important area of research these days. However, the promise of wireless communication has not matured as expected. This is because some of the important principles of electromagnetics were not adhered to during system design over the years. Therefore, one of the objectives of this book is to describe and document some of the subtle electromagnetic principles that are often overlooked in designing a cellular wireless system. These involve both physics and mathematics of the concepts used in deploying antennas for transmission and reception of electromagnetic signals and selecting the proper methodology out of a plethora of scenarios. The various scenarios are but not limited to: is it better to use an electrically small antenna, a resonant antenna or multiple antennas in a wireless system? However, the fact of the matter as demonstrated in the book is that a single antenna is sufficient if it is properly designed and integrated into the system as was done in the old days of the transistor radios where one could hear broadcasts from the other side of the world using a single small antenna operating at 1?MHz, where an array gain is difficult to achieve!

The second objective of this book is to illustrate that the main function of an antenna is to capture the electromagnetic waves that are propagating through space and prepare them as a signal fed to the input of the first stage of the radio frequency (RF) amplifier. The reality is that if the signal of interest is not captured and available for processing at the input of the first stage of the RF amplifier, then application of various signal processing techniques cannot recreate that signal. Hence the modern introduction of various statistical concepts into this deterministic problem of electromagnetic wave transmission/reception is examined from a real system deployment point of view. In this respect the responses of various sensors in the frequency and the time domain are observed. It is important to note that the impulse response of an antenna is different in the transmit mode than in the receive mode. Understanding of this fundamental principle can lead one to transmit ultrawideband signals through space using a pair of antennas without any distortion. Experimental results are provided to demonstrate how a distortion free tens of gigahertz bandwidth signal can be transmitted and received to justify this claim. This technique can be achieved by recasting the Friis's transmission formula (after Danish-American radio engineer Harald Trap Friis) to an alternate form which clearly illustrates that if the physics of the transmit and receive antennas are factored in the channel modelling then the path loss can be made independent of frequency. The other important point to note is that in deploying an antenna in a real system one should focus on the radiation efficiency of the antenna and not on the maximum power transfer theorem which has resulted in the misuse of the S-parameters. Also two antennas which possess a century bandwidth (i.e., a 100:1 bandwidth) are also discussed.

The next topic that is addressed in the book is the illustration of the shortcomings of a MIMO system from both theoretical and practical aspects in the sense that it is difficult if not impossible to achieve simultaneously several orthogonal modes of transmission with good radiation efficiency. In this context, a new deterministic methodology based on the principle of reciprocity is presented to illustrate how a signal can be directed to a desired user and simultaneously be made to have nulls along the directions of the undesired ones without an explicit characterization of the operational environment. This is accomplished using an embarrassingly simple matrix inversion technique. Since this principle also holds over a band of frequencies, then the characterization of the system at the uplink frequency can be used to implement this methodology in the downlink or vice versa.

Another objective of the book is to point out that all measurements related to propagation path loss in electromagnetic wave transmission over ground illustrate that the path loss from the base station in a cellular environment is approximately 30?dB per decade of distance within the cell of a few Km in radius and the loss is 40?dB per decade outside this cell. This is true independent of the nature of the ground whether it be urban, suburban, rural or over water. Also the path loss in the cellular band appears to be independent of frequency. Therefore in order to propagate a signal from 1?m to 1?kilometer the total path loss, based on the 30?dB per decade of distance, is 90?dB. And compared to this free space path loss over Earth, the attenuation introduced by buildings, trees and so on has a second order effect as it is shown to be of the order of 30-40?dB. Even though this loss due to buildings, trees and the like is quite large, when compared to the free space path loss of approximately 90?dB over a 1?km, it is negligible! Also, the concept of slow fading appears to be due to interference of the direct wave from the transmitting antenna along with the ground wave propagation over earth and also emanating from it and generally occurs when majority of the cell area is located in a near field environment of the base station antenna. These concepts have been illustrated from a physics based view point developed over a hundred years ago by German theoretical physicist Arnold Johannes Wilhelm Sommerfeld and have been validated using experimental data where possible. Finally, it is shown how to reduce the propagation loss by deploying the transmitting antenna closer to the ground with a slight vertical tilt - a rotation about the horizontal axis - a very non-intuitive solution. Deployment of base station antennas high above the ground indeed provides a height-gain in the far field, but in the near field there is actually a height loss. Also, the higher the antenna is over the ground the far field starts further away from the transmitter.

Finally we introduce the concept of simultaneous transfer of information and power. The requirements for these two issues are contradictory in the sense that transmission of information is a function of the bandwidth of the system whereas the power transfer is related to the resonance of the system which is invariably of extremely narrow bandwidth. To this end, the various concepts of channel capacities are presented including those of an American mathematician and electrical engineer Claude Elwood Shannon, a Hungarian-British electrical engineer and physicist Dennis Gabor, and an American electrical engineer William G. Tuller. It is rather important to note that each one of these methodologies is suitable for a different operational environment. For example, the Shannon capacity is useful when one is dealing with transmission in the presence of thermal noise and Shannon's discovery made satellite communication possible. The Gabor channel capacity on the other hand is useful when a system is operating in the presence of interfering signals which is not white background noise. And finally the Tuller capacity is useful in a realistic near field noisy environment where the concept of power flow through the Poynting vector is a complex quantity. Since the Tuller capacity is defined in terms of the smallest discernable voltage levels that the first stage of the RF amplifier can handle and is not related to power, the Tuller formula can be and has been used in the design of a practical system. Tuller himself designed and constructed the first private ground to air communication system and it worked in the first trial and provided a transmission rate which was close to the theoretical design. It is also important to point out that in the development of the various properties of channel capacity it makes sense to talk about the rate of transmission only when one is using coding at the RF stage. To Shannon a transmitter was an encoder and not an RF amplifier and similarly the receiver was a decoder! Currently only two systems use coding at RF. One is satellite communication where the satellite is quite far away from the Earth and the other is in Global Positioning System (GPS) where the code is often gigabits long. In some radar systems, often a Barker code (R. H. Barker, "Group Synchronizing of Binary Digital Systems". Communication Theory. London: Butterworth, pp. 273-287, 1953) is used during transmission. It is also illustrated how the effect of matching using both conventional and non Foster type devices have an impact on the channel capacity of a system.

The book contains four chapters. In Chapter 1, the principle of electromagnetics is developed through the Maxwellian principles where it is illustrated that the superposition of power does not apply in electrical engineering. It is either superposition of the voltages or the currents (or electric and magnetic fields). The other concept is that the energy flow in a wire, when we turn on a switch to complete the electrical circuit, does not take place through the flow of electrons. For an alternating current (AC) system the electrons never actually leave the switch but simply move back and forth when an alternating voltage is applied to excite the circuit and cause an AC current flow. The energy flow is external to the wire where the electric and the magnetic fields reside and they travel at the speed of light in the given dielectric medium carrying the energy from the source to the load. Also, the transmitting and receiving responses of simple antennas both in time and frequency domains are presented to illustrate the...