Digital Signal Processing Reference
In-Depth Information
manipulations can be performed on numerical data, but also because software
algorithms are not necessarily hard-coded in the firmware of a system. They
can be updated and modified if needed.
2
All those developments have recently led to the concept of
software defined
radio
(sdr). In an sdr-based system, the role of analog circuits is pushed into
background, ideally up to the point where the analog-to-digital (ad) converter
is directly connected to the antenna.
Given the fact that - at least at the time of writing - there are no widespread im-
plementations of this ideal concept, it is certainly questionable whether it is a
good idea to disown, ignore, and utterly forsake analog circuits and move over
to an all-digital implementation. The reason must be looked for in the distinc-
tion between the characteristics of the baseband signal and the rf waveform
being transmitted over the wireless channel. The search for insight leads us
to the baseband-rf interface and, more specific, its position in the transceiver
chain. The baseband-to-rf transformation can be explained in two different
ways, depending on the actual conversion process. In either case however, the
goal is to reshape baseband analog or digital information in such a way that it
can be transmitted in a specific frequency band over the air interface.
3
In the first approach, information is directly attached to the rf-carrier. It is
achieved by modifying one or more properties of the rf-carrier over time,
hence the name (de)modulation. Some of the most familiar examples are on-
off keying (e.g. morse code), amplitude modulation (am broadcasting, video
channel of analog tv) and frequency modulation (fm radio, audio channel of
analog tv). One way or another, all these modulation techniques 'modulate'
the carrier by changing the amplitude, phase (frequency) or a combination of
both. It should be kept in mind that the above-mentioned modulation methods
stem from the era of radio receivers where a crack in the glass of the second
if vacuum tube was more of a concern to electrical engineers than, say, band-
width efficiency. Every modulation type was developed with a feasible analog
implementation of the (de)modulator in mind. A noteworthy observation is
that there is only little freedom to control the bandwidth usage of such directly
modulated systems.
For example, in case of am radio, variations in the amplitude of the analog
audio signal are directly translated to amplitude variations of the rf-carrier.
The bandwidth usage of am radio - twice the bandwidth of the baseband audio
signal - is merely a secondary consequence of the modulation type. It cannot be
2
In the assumption of a general-purpose processor. Due to speed or power reasons, sometimes dedicated
(non-programmable) functional blocks are required. They are generally implemented as high performance
extensions of the capabilities of the general-purpose processor.
3
In fact, this is frequency division multiple access (fdma) in its most basic form. It allows multiple users
to operate at the same time on the same physical wireless channel.
