The block diagram in Figure 6.1 describes a digitalcommunication system. Similarly,data transfer between reader and transponder in an RFID system requires three main functional blocks. From the reader to the transponder – the direction of data transfer – these are: signal coding (signal processing) and themodulator(carrier circuit) in thereader(transmitter), thetransmission medium (channel), and the demodulator (carrier circuit) and signal decoding (signal processing) in the transponder (receiver).
A signal coding system takes the message to be transmitted and its signal representation, and matches it optimally to the characteristics of thetransmission channel. This process involves pro-viding the message with some degree of protection against interference or collision and against intentional modification of certain signal characteristics (Herter and L¨orcher, 1987). Signal coding should not be confused with modulation, and therefore it is referred to ascoding in the baseband.
Modulation is the process of altering the signal parameters of a high frequency carrier, i.e. its amplitude, frequency or phase, in relation to a modulated signal, the baseband signal.
The transmission medium transmits the message over a predetermined distance. The only trans-mission media used in RFID systems are magnetic fields (inductive coupling) and electromagnetic waves (microwaves).
Demodulation is an additional modulation procedure to reclaim the signal in the baseband. As there is often aninformation source(input) in both the transponder and the reader, and information is thus transmitted alternately in both directions, these components contain both amodulatorand a demodulator. This is therefore known as amodem(modulator – demodulator), a term that describes the normal configuration (Herter and L¨orcher, 1987).
The task of signal decoding is to reconstruct the original message from the baseband-coded received signal and to recognise anytransmission errorsand flag them as such.
6.1 Coding in the Baseband
Binary ones and zeros can be represented in various line codes. RFID systems normally use one of the following coding procedures: NRZ, Manchester, Unipolar RZ, DBP (differential bi-phase), Miller, differential coding on PP (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; M¨ausl, 1985) and susceptibility to transmission errors. Furthermore, in the case of passive transponders (in which the transponder’s power supply is drawn from the RF RFID Handbook: Fundamentals and Applications in Contactless Smart Cards, Radio Frequency Identification and Near-Field Communication, Third Edition. Klaus Finkenzeller
2010 John Wiley & Sons, Ltd
Receiver
Figure 6.1 Signal and data flow in a digital communications system (Couch, 1997)
NRZ coding:
Figure 6.2 Signal coding by frequently used serial formats or line codes in RFID systems
field of the reader) the power supply 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 electromagnetic waves.
By carefully influencing one of three signal parameters – power, frequency, phase position – of an
Table 6.1 Signal coding in the baseband
NRZ code A binary 1 is represented by a ‘high’ signal and a binary 0 is represented by a ‘low’
signal. The NRZ code is used almost exclusively with FSK or PSK modulation.
Manchester code
A binary 1 is represented by a negative transition in the half-bit period and a binary 0 is represented by a positive transition. The Manchester code is therefore also known as split-phase coding (Couch, 1997). This code is often used for data transmission from the transponder to the reader, based upon load modulation using a subcarrier.
Unipolar RZ code
A binary 1 is represented by a ‘high’ signal during the first half-bit period, a binary 0 is represented by a ‘low’ signal lasting for the entire duration of the bit.
DBP code A binary 0 is coded by a transition of either type in the half-bit period, a binary 1 is coded by the lack of a transition. Furthermore, the level is inverted at the start of every bit period, so that the bit pulse can be more easily reconstructed in the receiver (if necessary).
Miller code A binary 1 is represented by a transition of either type in the half-bit period, a binary 0 is represented by the continuance of the 1 level over the next bit period. A sequence of zeros creates a transition at the start of a bit period, so that the bit pulse can be more easily reconstructed in the receiver (if necessary).
Modified Miller code
In this variant of the Miller code each transition is replaced by a ‘negative’ pulse. The modified Miller code is highly suitable for use in inductively coupled RFID systems for data transfer from the reader to the transponder. Due to the very short pulse durations(tpulseTbit)it is possible to ensure a continuous power supply to the transponder from the RF field of the reader even during data transfer.
Differential coding
Every binary 1 to be transmitted causes a change (toggle) in the signal level, whereas the signal level remains unchanged for a binary zero. Differential coding can be generated very simply from an NRZ signal by using an XOR gate and a D flip-flop.
Figure 6.4 shows a circuit to achieve this.
Pulse-pause coding
In pulse-pause coding (PPC) a binary 1 is represented by a pause of durationt before the next pulse; a binary 0 is represented by a pause of duration 2t before the next pulse (Figure 6.4). This coding procedure is popular in inductively coupled RFID systems for data transfer from the reader to the transponder. Due to the very short pulse durations(tpulseTbit)it is possible to ensure a continuous power supply to the transponder from the RF field of the reader even during data transfer.
1 0 1 1 0 0 1 0
Pulse/Pause-length coding:
START SYNC
Figure 6.3 Possible signal path in pulse-pause coding
electromagnetic wave, messages can be coded and transmitted to any point within the area. The procedure of influencing an electromagnetic wave by messages (data) is calledmodulation, and an unmodulated electromagnetic wave is called acarrier.
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 asdemodulation.
Classical radio technology is largely concerned with analogue modulation procedures. 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
Clock Data in (NRZ)
Data out (differential) XOR
D Q
Figure 6.4 Generating differential coding from NRZ coding
Carrier
Sideband P
f
Figure 6.5 Each modulation of a sinusoidal signal – the carrier – generates so-called (modulation) sidebands
derived from one of these three types. The procedures used in RFID systems are the digital mod-ulation procedures ASK (amplitude shift keying),FSK (frequency shift keying) andPSK (phase shift keying).
In every modulation procedure symmetricmodulation products – so-calledsidebands – are gen-erated 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)
Inamplitude shift keying the amplitude of acarrier oscillation is switched between two statesu0
andu1(keying) by a binary code signal.U1can take on values betweenu0 and 0. The ratio ofu0
tou1 is known as theduty factor m.