Chapter 2
Background of backscatter communication
2.1 Discovery and evolution
Backscattering communications have a long and interesting history that dates back to the early days of radar in the 19th century. By then, James C. Maxwell had already published his work "A Dynamical Theory of the Electromagnetic Field" [2.1], establishing the theoretical foundations of electromagnetic waves:
Maxwell’s equations. Maxwell’s work was the culmination of a set of evidences presented by scientists such as Hans Christian Orsted, André-Marie Ampère, and Michael Faraday during the 18th century that suggested a link between electricity and magnetism. Faraday showed that electricity could influence the behavior of a magnet and vice versa, planting the seed for Maxwell’s work. The scientific community had been skeptical for more than a decade about the idea of Maxwell’s fields until his theory was proven in 1888 by Heinrich R. Hertz [2.2]. By the end of the 18th and early 19th centuries, wireless communica- tions were becoming a reality thanks to the work of Hertz himself, Aleksander Popov, Nikola Tesla, and Guglielmo Marconi with the creation of the first Morse code transceivers. In the first decade of the century, the work of Ernst F.W.
Alexanderson and Reginals Fessenden made the first radio broadcast[2.3] and amplitude modulation possible. At the dawn of World War I, wireless commu- nications were still in the beginning stage and the main method of communi- cation was wired. However, after the incessant destruction of communication
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lines, they soon realized the great advantage of wireless communications, and the 4 years of conflict were vital for the improvement of radio communications.
The invention of the vacuum tube in 1904 by physicist John Ambrose Fleming [2.4] and its subsequent industrial production in 1916 allowed radio transceivers to be miniaturized and integrated into planes, cars, and boats. By 1918, with the end of the war, two-way radio communications and radio broadcasting were already a reality.
After the war, the study of electromagnetic fields was booming, and an- other major breakthrough was on the horizon: radio detection and ranging (radar). Years earlier, while attempting to prove Maxwell’s theoretical work, Hertz discovered that radio waves were affected by metal objects and experi- mented with reflectors to observe this effect. The reflection of radio waves is known as backscattering, and it was the foundation for subsequently developing radar. Later, in 1904, Christian Hülsmeyer delved into Hertz’s observations, determining that reflected radio waves could be used to detect distant metal objects, and he patented a naval detection system. The system, a predecessor to radar, did not attract much attention at the time. In 1917, Nikola Tesla es- tablished the theoretical principles of modern radar. In the 1930s, the pre-war environment in the face of the threat of World War II caused several countries to begin developing radar in parallel. The current radar model was created in 1935 in England by Robert Watson-Watt. However, Germany, the United States, Japan, and the Soviet Union developed their own radars in parallel, pri- marily for military purposes, such as Freya, Wurzburg, and SCR-268/270. With the outbreak of World War II, radar evolution progressed rapidly, driven by the war. The main problem with radar at that time was that it could not differenti- ate between enemy and allied targets. To solve this problem, the backscattered radar signals began to be modulated, giving rise to identification friend or foe (IFF) transponders. The race against time to win the war allowed for an in- crease in operating frequency, surpassing the pre-established barrier in the VHF band and giving rise to the first microwave radars. The main advantages of the frequency increase were the reduction in antenna size and the increase in preci- sion due to narrow beam widths. One of the most important microwave radars was the SCR-584 [2.5]. After the war, although the development of radar slowed down, new advances such as the monopulse tracking radar, phased-array radar, synthetic-aperture radar (SAR), and pulse Doppler radar continued [2.6].
In 1945, the Second World War came to an end, marking the start of a new era: radio-frequency identification (RFID). American inventor Leon Theremin created a completely passive listening device to spy on the Soviet Union. Al- though this device was not an identification tag but an audio modulation de- vice, it is considered the predecessor of RFID because of its complete passivity, much like RFID tags. Three years later, in 1948, Harry Stockman published his work"Communication by Means of Reflected Power" [2.7], which examined the theoretical basis of communications through backscatter. In his conclusions, Stockman stated the following:
-Evidently considerable research and development work has to be done before the remaining basic problems in reflected-power communication are solved, and before the field of useful applications is explored.-
Harry Stockman As expected, without any apparent military applications, it took more than two decades for several companies to show interest in RFID. However, the devel- opment of RFID was already underway. In 1964, Roger F. Harrington presented a general formulation for electromagnetic backscatter in his work "Theory of Loaded Scatters" [2.8]. From 1970, the number of academic contributions in the field of RFID skyrocketed, and private entities began to show interest in the potential applications of RFID tags in fields such as transportation, in- ventory control, animal tagging, and personnel control. Mario W. Cardullo officially patented the first RFID tag with rewritable memory in 1973. Shortly thereafter, in 1975, scientists Alfred Koelle, Steven Depp, and Robert Freyman presented a notable contribution titled "Short-Range Radio-Telemetry for Elec- tronic Identification Using Modulated Backscatter" [2.9]. It described a simple and cost-effective electronic identification system capable of operating at dis- tances of tens of meters, encouraging large-scale commercialization of RFID.
By the end of the 20th century, millions of RFID tags were being used around the world [2.10]. It is worth mentioning that the evolution of RFID, like much of electronics, can be attributed to the invention of the transistor by William B. Shockley, John Bardeen, and Walter Brattain in 1948 and its subsequent evolution into field-effect transistors [2.11].
Currently, the field of backscattering continues to generate a lot of interest, not only for further improving existing technologies such as radar and RFID but
also for developing modern backscattering communications. In the 21st century, scientific efforts have focused on addressing two of the main limitations of RFID:
the imperative use of dedicated readers and the limited communication range, which had long been restricted to few meters. In 2013, Joshua R. Smith et al.
from the University of Washington presented their work "Ambient Backscatter:
Wireless Communication Out of Thin Air" which proposed a communication system powered solely by RF signals from the environment. This breakthrough removed the first limitation of RFID, the need for dedicated readers. Over the last decade, researchers have successfully studied the use of various ambient signals as a carrier source and decoding backscatter communications using non- dedicated receivers. In 2017, the second limitation of RFID and backscatter communications was finally overcome with the introduction of the first long- range backscatter communication system based on LoRa signal reflection, with a maximum range of nearly half a kilometer [2.12]. Today, the system has been further improved to enable communication at distances of over 1 kilometer [2.13].
Figure 2.1 depicts the key discoveries and significant events in the evolution of backscatter communications.
Figure 2.1: Cronologic evolution of RF backscattering