Digital Signal Processing Reference
In-Depth Information
the large amount of available bandwidth that a decent data throughput can be
achieved. However, the low modulation depth used by a low-psd radio system
does not necessarily lead to an equivalent reduction of the dynamic range in
the front-end of the receiver. The problem here is caused by the fact that the
linearity requirements are not dictated by the dynamic range of the signal-of-
interest itself, but by the power level of in-band interferers. It is the combina-
tion of a high bandwidth and dynamic range that forms the biggest challenge
for wideband radio receivers. On the other hand, even when every technical
requirement has been met, the capabilities and power consumption of the re-
ceiver may not be reflected in a higher throughput of the system: after all, the
signal-of-interest is still below the noise floor of the channel.
In the case of ofdm-modulated wideband radio, for example, a lot of effort
is put in obtaining an accurate digital representation of the entire frequency
band. Because the actual signal-of-interest is only a small portion of the total
information that can be represented by the digitized ofdm symbol, this will
inevitably result in a large amount of overhead data. This is because in ofdm,
both modulation and demodulation are performed in the digital domain. By de-
sign, the transmitter of such a system is forced to represent the entire frequency
band in a mathematical form before it can be converted to the analog domain.
The same reasoning holds for the receiver. This time, there are some additional
difficulties which are caused by interference and the linearity of the input stage.
The only remaining task for the analog processing circuits (which includes the
da/ad converter) is to convert between a baseband digital representation and
the passband-rf analog form of the time-domain ofdm signal.
The particular problems that are faced in a wideband low-psd radio system
(linearity, sparse spectral information density and in-band interference) sug-
gests that trying to maintain control over the complete (de)modulation process
in the digital domain at all costs may not result in the most power efficient sys-
tem. It is the purpose of this section to introduce an architecture for wideband
radio systems in which part of the modulation process is outsourced to the ana-
log domain. The presented approach tackles the problems caused by the nature
of wideband radio at their roots. Some part of the signal processing will still
reside in the digital domain, because of its flexibility and the ability of digital
processing to exercise active control over the analog subsystem. However, for
efficiency reasons, a considerable amount of signal processing will thus oc-
cur in the analog domain. As will become clear in the following sections, the
analog subsystem is not only used as a dumb interface between the digital pro-
cessor and the analog outside world (i.e. the antenna terminal), but it also plays
an important role in the
bandwidth compression
4
of the rf antenna-signal.
4
The clearest example of bandwidth compression is the conversion from passband to baseband.
