Coding and Modulation - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 182

Pulse/Pause-
length coding:

START SYNC 1 0 11 0 0 1 0

Figure 6.4 Possible signal path in pulse-pause coding

Various boundary conditions should be taken into consideration when selecting a
suitable signal coding system for an RFID system. The most important consideration
is the signal spectrum after modulation (Couch, 1997; Mausl, 1985) and suscepti-
bility to transmission errors. Furthermore, in the case of passive transponders (the
transponder's power supply is drawn from the HF field of the reader) the power sup-
ply must not be interrupted by an inappropriate combination of signal coding and
modulation procedures.

6.2 Digital Modulation Procedures

Energy is radiated from an antenna into the surrounding area in the form of electro-
magnetic waves. By carefully influencing one of three signal parameters — power,
frequency, phase position — of an electromagnetic wave, messages can be coded and
transmitted to any point within the area. The procedure of influencing an electromag-
netic wave by messages (data) is called modulation , and an unmodulated electromag-
netic wave is called a carrier .
By analysing the characteristics of an electromagnetic wave at any point in the area,
we can reconstruct the message by measuring the change in reception power, frequency
or phase position of the wave. This procedure is known as demodulation .
Classical radio technology is largely concerned with analogue modulation proce-
dures. We can differentiate between amplitude modulation , frequency modulation and
phase modulation , these being the three main variables of an electromagnetic wave.
All other modulation procedures are derived from one of these three types. The pro-
cedures used in RFID systems are the digital modulation procedures ASK (amplitude
shift keying), FSK (frequency shift keying) and PSK (phase shift keying) (Figure 6.5).
In every modulation procedure symmetric modulation products — so-called side-
bands — are generated around the carrier. The spectrum and amplitude of the
sidebands are influenced by the spectrum of the code signal in the baseband and
by the modulation procedure. We differentiate between the upper and lower sideband.

6.2.1 Amplitude shift keying (ASK)

In amplitude shift keying the amplitude of a carrier oscillation is switched between
two states u 0 and u 1 (keying) by a binary code signal. U 1 can take on values between
u 0 and 0. The ratio of u 0 to u 1 is known as the duty factor m .