Radio-Frequency Identification (RFID)

Introduction

Ambient backscatter uses existing radio frequency signals, such as radio, television and mobile telephony, to transmit data without a battery or power grid connection. Each such device uses an antenna to pick up an existing signal and convert it into tens to hundreds of microwatts of electricity. It uses that power to modify and reflect the signal with encoded data. Antennas on other devices, in turn, detect that signal and can respond accordingly. Initial implementations can communicate over several feet of distance, even with transmission towers up to 10.5 kilometres (6.5 mi) away. Transmission rates were 1k bits per second between devices situated 0.45 metres (1 ft 6 in) apart inside and 0.75 metres (2 ft 6 in) apart outside, sufficient to handle text messages or other small data sets. Circuit sizes can be as small as 1 sq. mm. This approach would let mobile and other devices communicate without being turned on. It would also allow unpowered sensors to communicate, allowing them to function in places where external power cannot be conveniently supplied. With the advancements in Wireless Power Transfer (WPT), researchers are looking for backscatter communications for low-cost tracking. One of the first instances of the use of backscatter occurred in World War II. With the advent of RADAR, both sides could tell when an aircraft was approaching. However, there was no method to identify whether that aircraft was friendly or an enemy. The German Air Force is credited with discovering that they could identify their aircraft by having their pilots manoeuvre their planes in a certain manner which changed their Radar Cross-Section (RCS), and therefore the response on the radar screen (Dobkin, 2008). This method is a precursor to how backscatter communications work and will be discussed in this Chapter. Few years later in 1937, the British debuted an identify friend or foe (IFF) transponder system called the XAE, and later Mark I, as a more secure method of identifying their aircraft. This system, when hit with a RADAR pulse, would send out a signal which identified the aircraft. It is accredited with being the first active RFID (Radio Frequency Identification) system.

As early as 1945, Soviet Scientist Leon Theremin had designed a passive method of listening to conversations. This device, now known as the `Great Seal Bug,' allowed the Soviets to spy on the American embassy in Moscow for seven years until its discovery (Finkenzeller, 2003). It used a resonator which, when sound wave vibrated a membrane, modulated any sound in the room onto an external RF (Radio Frequency) signal that Soviet Spies pointed at the American Embassy. The Americans were unable to discover this device because it had no active components which to detect. It is accredited with being one of the first passive RFID systems despite not being officially disclosed until much later.

During the time that the Great Seal Bug was in use, Harry Stockman officially referenced the first method of communication by reflected radio waves in a paper published in 1948 (Stockman, 1948). While much of the necessary technology for performing true passive, backscattered communication did not exist, Stockman nevertheless foresaw the benefits of reflected radio waves for passive communication. It wasn't until a few years later in 1952 that D.B. Harris filed a U.S. Patent referencing the first relative of the modern RFID tag (Harris, 1952). While Harris's power transfer mechanisms were not as advanced as Brown's, the path for integration was set.

The 1960s and 1970s represented a huge leap in the progress of RFID tags thanks to advancements in semiconductor technology and interest by the community. Practical applications such as Sensormatic and Checkpoint electronic article surveillance (EAS) devices for retailers (Landt, 2005), the Ratheon Raytag (Freedman, 1973), and Los Alamos National Laboratory nuclear material and cattle tracking (Koelle et al., 1975; Landt, 2001) increased the interest in RFID technology and led to further investment and development by the government and industry alike. The work out of Los Alamos led to the development of passive UHF RFID which is directly linked to the logistical tracking and road tolling systems currently in use.