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Software (analog and digital) chip design, which

Software defined radio (SDR)(also known as “software radio”) is a radio communication
system where components that have been traditionally implemented in hardware(e.g.
mixers, filters, amplifiers, modulator/demodulators, detectors etc.) are instead implemented
by means of software on a personal computer or embedded system.
3.1 SDR Concept
Prior to the proliferation of digital signal processing technology in radio systems most
transceiver functions were implemented in analog circuitry. This confined the capabilities
of the transceiver to the limitations of the analog technology. Some complex communications
algorithms were simply impossible to implement with analog components for a given
project budget. Since analog circuitry is specified for a certain function it is difficult to
multitask, thus analog systems tend to be physically large and power hungry. Although it
is true that in some applications such as filtering, it can outperform its digital counterpart,
an analog systems performance is jeopardized by environmental variations.
Digital signal processors (DSPs), field programmable gate arrays (FPGAs), and microprocessors
allow analog circuits such as filters, equalizers, and phase-locked loops (PLL)
to be packed into one chip, consuming a fraction of the power, area, and cost. This has led
to the implementation of sophisticated signal processing algorithms such as convolutional
encoding, interleaving, and dynamic power control in small hand-held devices such as cel6
Harmonic Rejection Filter Design for Software Defined Radio Application
lular phones.
Todays transceivers consist of a radio-frequency (RF) front end, and a baseband processing
section. The RF front-end is a loose term referring to the analog circuitry between
the antenna and the data converters. The main functions of the RF front end are to modulate
and demodulate the carrier with and from the data, respectively. Base band signal processing,
voice processing, user interface, power management, and networking functions are
done by a combination of analog and digital chips. Mixed signal (analog and digital) chip
design, which would allow the integration of many of the current analog and digital functionalities
into one chip, is a popular concept in todays wireless industry. Texas Instruments
for example, has announced that by 2004 it will introduce a one-chip GSM phone.
Software radio strives to pack as much of the transceivers functionality into a programmable
signal processor as possible. A block diagram of an ideal software radio is
shown in Figure 3.1 where the data converters are placed very close to the antenna.
Fig. 3.1: In an ideal software radio the RF front end is eliminated.
In this system, the RF front end is eliminated and the DSP is tasked with the modulation
and demodulation, in addition to the baseband signal processing. Thus, if the DSP is
programmable, the characteristics of the radio can be significantly defined by the software
that it runs on. A designer can alter the performance of the radio simply by reprogramming
the DSP.
Dept.of ECE, SJCET, Palai Page 7
Harmonic Rejection Filter Design for Software Defined Radio Application
This concept has far-reaching implications in the wireless communications industry.
Base station transceiver equipment at cell sites will no longer become obsolete with changes
in wireless standards. Thus, migration to newer and more powerful systems will be inexpensive.
Wireless switches, access points, and routers will no longer have to be replaced
with system upgrades, but reprogrammed. Satellites and other spacecraft can be reprogrammed
from Earth to alter their transmission and reception characteristics, thus making
them more powerful and flexible.
Interoperability is another potential benefit of software radio systems. Different wireless
communication systems operating on different standards can communicate with each
other. This has been a major focus of the US military because different battlefield units use
different communication systems.
Software radios can drastically reduce time to market because software modifications
can be done at a fraction of the time of hardware modifications. Complex 3G handsets
take many months to design and implement. Any errors in this process can lead to a delay
of many months for the product to reach market. Software radio systems cut down this
correction time significantly.
New software features and upgrades can be downloaded to the handset automatically
or on demand, thus greatly enhancing the degree and quality of services available to cellular
customers. Handsets will not become obsolete as often as they do now with changing
standards, thus saving customers money.
Signal processing algorithms such as filtering, encoding/decoding, equalization, and
modulation/demodulation can be adaptively altered remotely. For example, in current
CDMA systems the base station controls the power emissions of the handsets to minimize
the near-far effects, as well as multiuser interference. This can be applied to all parameters
of the handset, and as a result, transmission quality and capacity can increase. The concept
of cognitive radio, which seeks to make radio systems intelligent and adaptive to their
environments, is a future goal for software radios.
Dept.of ECE, SJCET, Palai Page 8
Harmonic Rejection Filter Design for Software Defined Radio Application
3.2 SDR Achitecture
A traditional or typical receiver, besides the classic demodulation, performs three
other operations: (1) carrier frequency tuning to select the desired signal, (2) filter to separate
it from others received, and (3) amplification to compensate transmission losses. Most
traditional receivers have used conventional heterodyne schemes for almost a century. The
superheterodyne internals blocks are shown in Figure 3.2.
Fig. 3.2: Superheterodyne Receiver’s Interbnal Block.
In the previous scheme, after the signal enters through the antenna, it is typically
amplified by an RF stage that operates only in the frequencies of interest region. Then, the
signal is passed to the mixer which receives the local oscillator contribution by its other
input. The local oscillators frequency is set by the radios tuning control 11. The mixer is
in charge of translating the signal to the Intermediate Frequency (IF).
Typically, the oscillators frequency is set to a value that ensures that its difference
from the desired signals frequency is equal to the IF. The next stage is a bandpass filter that
attenuates every signal except a specific portion of the spectrum. The bandwidth of this
stage limits the band width of the signal thats being received.
At the end, the demodulator recovers the original modulating signal from the IF amplifiers
output employing one of several alternatives. Further processing of the signal depends
on the purpose for which the receiver is intended.


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