Software Defined Radio, SDR
- an overview, tutorial and information about the basics of the software
defined radio, SDR, that is being looked at increasingly in a number of areas
including the Joint Tactical Radio System, JTRS.
The software defined radio (SDR) has been the aim of many
radio developments for a number of years. One initiative named the Joint
Tactical Radio System (JTRS) has looked at this type of radio for military
applications. However the software defined radio (SDR) is equally applicable for
many other applications in the commercial arena as well. As the majority of the
radio is contained within the software, the hope is that the physical upgrades
for users for applications such as the change from the 2G to 3G cellular systems
would simply consist of a software upgrade. Other similar changes in use could
be implemented purely by changing the software, and leaving the hardware
unchanged.
Definition
There are many ways to describe a software radio, but one
definition that seems to encompass the essence of the SDR is that has a generic
hardware platform on which software runs to provide functions including
modulation and demodulation, filtering (including bandwidth changes), and other
functions such as frequency selection and if required frequency hopping.
In essence software defined radio technology is the
technology where software modules run on a generic hardware platform consisting
of digital signal processing (DSP) processors as well as general purpose
processors to implement the radio functions to transmit and receive signals.
In an ideal world the signal at the final frequency and at
the correct level would emanate, but currently this is not possible because of
some of the hardware limitations. Currently the digital to analogue, and
analogue to digital conversion is not sufficiently fast to generate signals at
all frequencies, and obviously the required power levels could not be generated
for high power transmitters. Accordingly analogue radio frequency elements are
still required.
Key features
There are many advantages of the software defined radio. The
first is the flexibility and reconfigurability. This is of great importance
because it enables radios to be changed, upgraded and enhanced, simply by
changing the software. In this way, one hardware platform can be used in
multiple radios, each which has considerably different characteristics. It is
even possible to perform over the air uploads of software to provide the latest
uploads, of a complete reconfiguration of the radio. With many areas of
electronics experiencing rapid changes in standards - the cellular
telecommunications sector is one such area - changes to the standards, or even
complete changes like those from 2G to 3G could in theory be covered by a
software upload. In this way a much greater level of future-proofing could be
accommodated.
Connectivity is another major issue for the software defined
radio. All radios need to have interfaces both over the air interface at the
radio signal frequency, and also at the base-band interface. With most
transmissions that occur these days carrying digital data, rather than analogue
signals, it is necessary to use the right data formats and exchange protocols.
This problem can be overcome using a software defined radio, because it is
possible to enable a radio to communicate with a new set of users simply by
uploading another software module that enables exchanges using another protocol.
This is particular important for global roaming using mobile equipment. As
different standards are used in different areas it is necessary to be able to
cater for different ones dependent upon the area where the equipment is to be
located. Again this can be illustrated using the cellular telecommunications
example. It is found that although GSM is currently the most widely used format,
there are still areas where other standards are required. By using an SDR, it
would be possible to add the additional functionality for the new standards
using different software modules.
Interoperability is another issue. As so much of the radio is
contained within software, it will be possible to integrate other associated
software functions into the system more easily. Thus the aim is to be able to
integrate other third party software applications into the basic radio, thereby
increasing the functionality.
SDR Architecture
The software defined radio (SDR) contains a number of basic
functional blocks. The radio can be split into three basic blocks, namely the
front end, the IF section and the base-band section as shown below. Each of the
sections undertakes different types of functions and therefore is likely to use
different circuit technologies.
Block Diagram of a Generic Software Defined Radio
The front end section uses analogue RF circuitry and it is
responsible for receiving and transmitting the signal at the operational
frequency, coupling the radio to the antenna or its feeder. It also changes the
signal to or from the intermediate frequency.
Thus on the receive path the front end serves is connected to
the antenna input using matching circuitry to ensure the optimum signal
transfer. It then amplifies the signal and applies it to a mixer with a signal
from a local oscillator to down-convert it to the intermediate frequency.
On the transmit path the front end takes the signal from the
IF, first up-converting it to the final frequency where it is amplified to the
required level, passed through suitable matching circuitry to ensure the maximum
power transfer and then presented at the antenna connection to be routed to the
antenna either directly or via a feeder.
The IF section performs the digital to and from analogue
conversions. It also contains the processing that undertakes what may be thought
of as the traditional radio processing elements, including filtering, modulation
and demodulation and any other signal processing that may be required.
On the receive path the signal enters the DAC where it is
digitized and enters the DDC, the Digital Down Converter, where the signal is
processed and demodulated to provide the baseband signal for the baseband
processor.
Similarly on the transmit side the signal arrives from the
baseband processor and is modulated onto the carrier and conditioned as
required. It is then converted from its digital format to analogue using a
digital to analogue converter.
The DDC and DUC require significant levels of processing.
This is required to perform all the processing on the actual signals in digital
format. This processing must be achieved in real time for te system to be able
to operate satisfactorily. As a result the processors are implemented in either
stock DSPs or ASICs. In fact to achieve the full programmability and
reconfigurability needed for a software defined radio the signal processors may
be implemented as FPGAs. In this way the circuit can be totally reconfigured if
needed.
The final stage of the radio is the baseband processor. It is
at this point that the digital data is processed, with protocols being
accommodated and the data payload assembled or disassembled from the datastream.
Although not as demanding as the DDC and DUC areas, with protocols becoming ever
more complicated and demanding, the level of processing is increasing in these
areas as well.
Summary
A full software defined radio is one that can be fully
programmed. In most instances today this means that the baseband as well as the
DDC and DUC are programmable. For full reconfigurability, the RF section of the
SDR should also be programmable, but the current state of the art DAC and ADC
technology is not able to support the bandwidth, dynamic range, and smapling
rate required to implement this commercially. However as technology advances,
this too will no doubt become possible in time.
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