Photonics and Microwaves

RFID Systems

Radio Frequency Identification is a short-range wireless system with the aim to identify an object in the environment and trace it. Through the identification, we can distinguish one object to the other in the environment, the goal is the acquisition of data. The key components are three:

1. TAG: which is a Radio Frequency transceiver with a very simple integrated circuit, connected to an antenna.
2. READER: which is a Transceiver controlled by microprocessor, it is used for receiving data from the TAG.
3. MANAGEMENT SYSTEM: is an information system connected with reader and database through a network.

TAG

We have three kind of TAG:

• PASSIVE: They do not have a battery for energy supply, so, they extract the required energy from the Radio signal frequency coming from the Reader.
• HYBRID: They use battery for integrated circuit supply but not for Transceiver.
• ACTIVE: They have a battery for energy supply.

READER

Is a element which is required to acquire information from the TAGs, it's a real Transceiver. In particular, Readers for the passive TAGs have to transmit particular modulators able to transfer energy from the reader to tags to supply the electronics.

TAG-READER Communication

For Passive tags, there is no power source for the transmission. Then, the TAG sends back an EM wave without power source. The communication is based on techniques depending on if the TAG is in:

• NEAR-FIELD of the Reader
• FAR-FIELD of the Reader

R_near = 2

π

1) NEAR-FIELD: The field distribution is very close to quasistatic one and usually inductive coupling is used.

Using induction, for power coupling from READER to TAG can be seen as a transformer where two antennas are loops, and the communication happens through the linked flux between the loops. Then, the varying current flowing in the primary winding produces a varying magnetic field. The varying magnetic flux links with the secondary winding causing induced electromotive force. We have 2 cases:

• NO TAG: The magnetic field produced by reader antenna isn't linked and the secondary winding is an open-circuit. Then, the input impedance of the primary winding only depends on primary parameters.
• YES TAG: The magnetic field produced by Reader Antenna is partially linked with the antenna of the TAG. The transformer with secondary winding is connected to a load, so the input impedance of the primary winding also depends on the secondary load.

2) FAR-FIELD: We use the EM coupling which is based on radar principle, so the back-scattering. The mechanism is simple: the wave impinges on the TAG Antenna and it induces a current on it. The induced current on the antenna produces a back-scattered wave, which intensity depends on the induced current amplitude. It's possible to modulate the amplitude of the back-scattered wave according to the impedance modulation.

If we could describe the TAG-READER Communication, two conditions must be satisfied:

1. ACTIVATION: The harvested energy must be enough to feed the chip.
2. READING: The back-scattered wave must be strong enough to be detectable by the Reader Antenna and the Receiver.

The maximum distance TAG-READER is computed by the FRIIS formula:

LASER (Light Amplification by Stimulated Emission of Radiation)

COHERENCE: Describes all the properties of the correlation between physical quantity of a single wave at different moments → Temporal Coherence

at different points → Spatial Coherence

1. Correlation between the wave and itself delayed by τ (perfect!*) → Bandwidth: ε 0
2. If we increase the distance, we maintain the same coherence of radiation.

The advantages are: Ultrashort pulses and High focusing and low diffraction.

A LASER can be considered an electronic oscillator: (see at top of page)

The amplification is based on the interaction between radiation and matter.

Radiation-Matter Interaction: Radiation is analyzed in terms of → classical physics

E=hν - Frequencies

Matter is analyzed in terms of quantum physics with discrete set of allowed energy values.

We have a presence of different energy levels and the photons can move from one level to another by acquisition of or by losing energy. The latter is due by two sources which are,

1. Kinetic energy or Radiation-Matter interaction, so the EM radiation in the material where EM as multiple photons. If we have a mass of atom from lower level to upper level we talk of Absorption, if we have the inverses we talk of Emission which is divided in two classes: Spontaneous (fixed frequency but random phase/abstraction) and Stimulated (fixed frequency and since the fact we have 2 photon called twin, we have same phase and polarization too)

In terms of probability we have:

W₂₁ = σ_a ⋅ F_ph σ₂₁ = τ_spτ

A₂₁ = Z_sp

σ = Emission cross section

The number of transitions depends on the populations, which is calculated by:

ΔN₂ = W₂₁ N₂ = d_s ⋅ F_ph = N_sp ⋅ Az₁ ⋅ N₂ = z_sp ⋅ N₁

ΔN_e = Ne ⋅ Δt = σ_e ⋅ F_ph ⋅ dx = σ_e ⋅ F_ph ⋅ dz

If we now consider: We neglect spontaneous emission.

I ⋅ dS = [ΔNe - ΔN_A] ⋅ dv ⋅ E_ph dI ⋅ dS =

we know that dv = ds ⋅ dz, ΔN_e = σ_e ⋅ N₂ E_ph, ΔN_A = σ_a ⋅ N₂

dI ⋅ dS = I

G_AIN = g ⋅ σ_e N₂ - σ_a N₁

AMPLIFICATION:

g - σ_a ⋅ N₁ ⋅ N₂ + ΔE

ATTENUATION: g

* In the spontaneous emission we have 1 photon but the particle spontaneously decay.

SINGLE OBJECT

Processors: An on board Synthesizer outputs the chirped signal. Once the chirped signal is amplified and transmitted, the receiver antenna detects the reflected delayed signal version of the same chirped signal. This signal, after the low-noise amplifier, is mixed with the original one to generate the intermediate frequency signal at the beat frequency. Intermediate frequency signal is low-pass filtered to eliminate any high-frequency contributions.

MULTIPLE OBJECT

Two targets equivalent to the radar with different relative velocities. Similar to single object, single intermediate frequency tone measured by the radar, but now contains phase information of all the targets. To resolve and detect every multiple object, a discrete "Doppler-FFT" technique is applied over a sequence of range-FFT peaks, which corresponds to a sequence of N equispaced transmitted signals, called frame. Each peak of Doppler-FFT corresponds to the phase difference of one detected target. Relative velocities of the targets can be expressed through velocity resolution (1) and maximum velocity (2).

1. V_req is the minimum velocity difference between objects for the radar to be able to detect them separately.
2. V_max is the unambiguous velocity measurement as possible only if the phase difference ΔΦ is less than 2π radians. This is inversely proportional to chirp period.

*FMCW radar is focused on measuring the distance and speed of the objectives through the frequency variation. *Homopulse radar is used to obtain an accurate information on the direction of the objectives by analyzing the signal received by different antenna.