Sổ tay RFID (P6)

RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification, 6 Coding and Second Edition Klaus Finkenzeller Copyright  2003 John Wiley & Sons, Ltd. ISBN: 0-470-84402-7 Modulation The block diagram in Figure 6.1 describes a digital communication 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 trans-fer — these are: signal coding (signal processing) and the modulator (carrier circuit) in the reader (transmitter), the transmission 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 represen-tation and matches it optimally to the characteristics of the transmission channel. This process involves providing the message with some degree of protection against inter-ference or collision and against intentional modification of certain signal characteristics (Herter and Lorcher, 1987). Signal coding should not be confused with modulation, and therefore it is referred to as coding 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 transmission 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 an information source (input) in both the transponder and the reader, and information is thus transmitted alternately in both directions, these components contain both a modulator and a demodulator. This is therefore known as a modem (Modulator — Demodulator), a term that describes the normal configura-tion (Herter and Lorcher, 1987). Noise Transmitter n(t) Receiver Information source m(t) Signal processing Carrier circuit s(t) r(t) Channel Carrier circuit Signal processing To information sink (user) m(t) Figure 6.1 Signal and data flow in a digital communications system (Couch, 1997) 184 6 CODING AND MODULATION The task of signal decoding is to reconstruct the original message from the baseband coded received signal and to recognise any transmission errors and 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 coding (Figure 6.2). NRZ code A binary 1 is represented by a ‘high’ signal and a binary 0 is rep-resented 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). The Manchester code is often used for data transmission from the transponder to the reader based upon load modulation using a subcarrier. NRZ coding: 1 0 1 1 0 0 1 0 Manchester coding: (bi-phase) 1 0 1 1 0 0 1 0 Unipolar RZ coding: 1 0 1 1 0 0 1 0 DBP 1 0 1 1 0 0 1 0 Miller coding: 1 0 1 1 0 0 1 0 Modified Miller coding: 1 0 1 1 0 0 1 0 Differential coding: 1 1 0 1 1 0 0 1 0 Figure 6.2 Signal coding by frequently changing line codes in RFID systems 6.1 CODING IN THE BASEBAND 185 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 (tpulse ¿ Tbit) it is possible to ensure a con-tinuous power supply to the transponder from the HF field of the reader even during data transfer. Differential coding In ‘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 6.3 shows a circuit to achieve this. Pulse-pause coding In pulse-pause coding (PPC) a binary 1 is represented by a pause of duration t 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 (tpulse ¿ Tbit) it is possible to ensure a contin-uous power supply to the transponder from the HF field of the reader even during data transfer. Data in Data out (NRZ) (differential) XOR Clock D Q Figure 6.3 Generating differential coding from NRZ coding 186 6 CODING AND MODULATION Pulse/Pause-length coding: START SYNC 1 0 1 1 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 u0 and u1 (keying) by a binary code signal. U1 can take on values between u0 and 0. The ratio of u0 to u1 is known as the duty factor m. 6.2 DIGITAL MODULATION PROCEDURES 187 P Carrier Sideband f Figure 6.5 Each modulation of a sinusoidal signal — the carrier — generates so-called (mod-ulation) sidebands To find the duty factor m we calculate the arithmetic mean of the keyed and unkeyed amplitude of the carrier signal: uˆm = uˆ0 + uˆ1 (6.1) The duty factor is now calculated from the ratio of amplitude change uˆ0 − uˆm to the mean value uˆ : m = uˆˆm = uˆ0 uˆmuˆm = uˆ0 + uˆ1 (6.2) In 100% ASK the amplitude of the carrier oscillation is switched between the carrier amplitude values 2uˆm and 0 (On-Off keying; Figure 6.6). In amplitude modulation using an analogue signal (sinusoidal oscillation) this would also correspond with a modulation factor of m = 1 (or 100%) (Mausl, 1985). The procedure described for calculating the duty factor is thus the same as that for the calculation of the modulation factor for amplitude modulation using analogue ûm û1 ûm û0 t m = 0.5; (ASK 50%) Figure 6.6 In ASK modulation the amplitude of the carrier is switched between two states by a binary code signal ... - --nqh--