Digital Signal Processing Reference
In-Depth Information
Coding and 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
)andthe
modulator
(
carrier circuit
)
in the
reader
(
transmitter
), the
transmission medium
(
channel
), and the
demodulator
(
carrier circuit
)and
signal decoding
(
signal processing
)inthe
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
(
Mo
dulator —
Dem
odulator), a term that describes the normal configura-
tion (Herter and Lorcher, 1987).
Noise
n(t)
Transmitter
Receiver
Information
source
m(t)
To information
sink (user)
m(t)
s(t)
r(t)
Signal
processing
Carrier
circuit
Carrier
circuit
Signal
processing
Channel
Figure 6.1
Signal and data flow in a digital communications system (Couch, 1997)
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