Digital Signal Processing Reference
In-Depth Information
the wireless channel (i.e. isi), but has opened at the same time a whole new
world of challenges for the analog designer. During the design of a transceiver,
none of the elements in the transmitter/receiver chain may be ignored or over-
valued, whether it concerns the analog front-end, the digital back-end or the
wireless channel itself.
1.3
Onwards to software defined radio?
Before continuing with the discussion of higher performing data networks,
some reflection on the role of the analog front-end, the digital back-end and
their mutual interaction is needed. First of all, it should be noted that our main
goal is to transfer digital information from one location to another using a quite
hostile and sometimes crowded interface: the wireless channel. The non-ideal
properties of this information carrier include heavy signal attenuation, interfer-
ence between co-cell users, self-interference due to multipath reflections and
the frequency-dependent passband characteristic of the channel. It is our goal
to make optimal use of the possibilities of both analog and digital techniques to
convert the input signal from a representation of bits into a stream of electro-
magnetic symbols, dump this signal on the wireless interface and try to find out
at the receiver side what the transmitter was trying to say in the first place. In
the previous section it was already suggested that it is not always easy to make
a justifiable choice for the modulation method or to find out where to draw
the dividing line between the analog front-end and the digital back-end. The
reconfigurability of digital hardware and the flexibility of software make digi-
tal signal processing especially suited for operation in a dynamically changing
environment.
For example, remember the case of ofdm, where transmission over subbands
that suffer from destructive fading could be avoided by rerouting the data
stream over the remaining subchannels. Furthermore, using digital error cor-
rection techniques, the integrity of the received information can be guaran-
teed. Even analog information can benefit from a conversion to a discrete-time
discrete-value representation: thanks to the regenerative character of digital in-
formation, noise does not accumulate over a digital transmission system. This
raises the question whether the analog front-end still plays any significant role
in the transmission chain. Of course, it is clear that sooner or later, the dig-
ital signal must leave the mathematical world in order to be applied to the
antenna
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as a real-world signal in analog form (i.e. using voltage, current,
frequency, charge or
foobar
units).
There is something that deserves some extra attention, though. One part of
the modulation process is almost never performed in the digital domain: the
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Antenna: impedance converter between our circuit and the wireless channel.
