Electronic Design= Cognitive Radios & SDRs (long)
Andre Kesteloot
andre.kesteloot at verizon.net
Wed Oct 26 16:32:46 CDT 2011
SDR And CR Boost Wireless Communications
October 12, 2011 09:40 AM 11/10/2011
<http://electronicdesign.com/issues/issue.aspx?issueid=11/10/2011>
Electronic Design
Louis E. Frenzel
<http://electronicdesign.com/author/1843/LouisEFrenzel.aspx>
Software-defined radio (SDR) used to be rare and exotic. But today, most
modern radios use SDR's architecture and techniques. Each year with
continuing advances in ICs and other technologies, SDR becomes more
capable and widespread. In fact, new techniques like cognitive radio
(CR) are making SDR more useful and beneficial to wireless communications.
*SDR Defined*
SDR uses software to perform some of the signal processing in a receiver
and transmitter. For example, a traditional receiver using the
ubiquitous superheterodyne architecture performs all signal processing
with basic electronic circuits (Fig. 1a
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig1.jpg>).
The superheterodyne downconverts the input signal to an intermediate
frequency (IF) for demodulation and other processing.
Early SDR receivers (Fig. 1b
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig1.jpg>)
replaced the demodulator with an analog-to-digital converter (ADC) after
the IF stage and performed the demodulation and some filtering in a
digital signal processor (DSP). Today, because ADCs sample faster, DSPs
can handle more functions.
To make DSPs work, the amplitude and phase of the signals both must be
known. This has led to an architecture that divides the received signal
into two paths, one producing an in-phase (I) signal and a 90° shifted
quadrature (Q) signal. A basic carrier signal has the form:
V = A_c cos(2?f_c t + ?)
Where fc is the carrier frequency, ? is the phase, and A_c is the
carrier amplitude. Any of these may be varied for modulation. For
demodulation in the digital domain, a single signal is insufficient for
existing algorithms. Therefore, the modulated signal is converted into
the I and Q signals:
V = I(t) cos(2?f_c t) + Q(t) sin(2?f_c t)
With the quadrature signals, any variations in amplitude, frequency, or
phase can be detected and used in a demodulation or other process.
Figure 2
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig2.jpg>
shows a modern I/Q SDR receiver. A low-noise amplifier (LNA) usually
boosts the input signal from the antenna before it is applied to the two
mixers. The mixers develop the I and Q signals. Both receive a local
oscillator (LO) signal from the phase-locked loop (PLL) frequency
synthesizer. Note the 90° shift between the LO signals to the two mixers.
The LO frequency is set to the signal frequency so the difference signal
from the mixers is zero without modulation. With modulation, the
difference is the baseband or original modulating signal. This
architecture is called direct conversion or zero IF.
After the baseband signals have been filtered in low pass filters to
eliminate the sum components at the output of the mixers, the signals
are digitized in a pair of ADCs. The digital baseband signals are then
processed with digital downconverters (DDCs) to lower the sampling rate,
making them more compatible with the digital signal processing circuits.
The digital signal processing circuits then use both the I and Q signals
to perform the demodulation, equalization, and additional filtering as
the application demands.
In a modern SDR transmitter, the DSP modulator divides the data to be
transmitted into I and Q signals and feeds them to digital upconverters
(DUCs) that boost their sample rate (Fig. 3
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig3.jpg>).
The I and Q signals are next sent to digital-to-analog converters (DACs)
that produce the final baseband signal. The signals are then low pass
filtered and sent to the mixers that upconvert the signal to the final
transmitted frequency. The signal is finally sent to a power amplifier
before being applied to the antenna.
All modern SDR transceivers use some basic variation of the receiver and
transmitter circuits shown here. Of course as ADCs and DACs get faster,
the digital processing moves closer to the antenna. The ultimate
receiver then becomes simply a filter at the antenna to limit the
bandwidth and a LNA before a fast ADC (Fig. 4
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig4.jpg>).
Then, the DSP performs all other processing like demodulation and
filtering. Commercially available amateur radio and shortwave receivers
covering up to 30 MHz already use this advanced architecture.
Many functions are now performed digitally:
* Filtering (low pass, high pass, band pass, and band reject)
* Modulation (AM, FM, PM, FSK, BPSK, QPSK, QAM, OFDM, etc.)
* Demodulation
* Equalization
* Compression
* Decompression
* Spectrum analysis
* Predistortion
New modulation methods and related procedures are generally known as
waveforms. By changing waveform software, a radio for one application
like FM voice could be reprogrammed for high-speed data on a different
frequency with a different protocol.
The advantages of SDRs lie in the greater simplicity of the hardware.
Standard RF circuits are reduced to a minimum, keeping the cost of ICs
low. DSP software improves operation with functions (like filters) that
provide better performance than equivalent analog circuits. Digital
signal processing also can compensate for some failing of RF components.
Furthermore, the flexibility of reprogramming allows defects to be
fixed, new features to be added, upgraded operations to be included, and
performance to be improved. An SDR of flexible design can be quickly
changed with software to accommodate new modulation methods, new
protocols, and other major adjustments that would ordinarily require new
hardware.
The downsides of SDR include software complexity, development costs and
time, limited frequency range for some applications, and generally
higher power consumption.
*SDR's Hardware*
SDR requires fast ADCs, DACs, and DSPs. Sampling rates of ADCs have been
rising for years and now extend well into the gigahertz region. Many
SDRs use a low IF architecture and an ADC boasting a range of 100
Msamples/s to several hundred Msamples/s. Even more is possible.
National Semiconductor's (now Texas Instruments) recent ADC12Dxx00RF can
sample at rates up to 3.6 Gsamples/s (see "ADCs Sample RF Directly
<http://electronicdesign.com/article/analog-and-mixed-signal/ADCs-Sample-RF-Directly-.aspx>"
at www.electronicdesign.com). This dual-channel, 12-bit ADC can be used
with a clock phase shift that allows interleaved or alternate channel
sampling for even faster conversion. DAC sampling speed is closely
following this trend.
While fast conversion is essential, the DSP must be fast enough to keep
up. That has not been a problem as most processors have easily kept
pace. SDR is software, of course, but you still need hardware that can
be implemented physically in several forms.
For instance, you can write code to run on a general-purpose processor
(GPP). This may not be an optimal approach as some of the algorithms
call for math procedures that are awkward to handle on most GPPs.
However, an Intel or AMD dual-core processor used in most PCs today does
a great job in some applications. Some GPPs also have special
instructions like the multiply and accumulate (function) that are so
commonly used in DSP algorithms.
Then you can also use a DSP designed specifically to handle signal
processing code. It has a special architecture (usually Harvard),
memory, and arithmetic logic unit (ALU) instruction sets that make the
DSP fast.
Texas Instruments' popular line of DSPs has been used for years in SDR,
such as the C5000 and C6000 series. Analog Devices and Freescale also
have general-purpose DSPs. Like any processor, DSPs are fully
programmable so they're very flexible in applications where changes,
additions, and updates may be required in the future. Clock speeds to 1
GHz are common in DSPs today.
More and more SDR designs are using FPGAs. The signal processing
algorithms such as the fast Fourier transform (FFT) can be reduced to
digital logic and quickly implemented in an FPGA. Since the cost of
FPGAs has steadily declined, they have become a major alternative to
DSPs. FPGAs are faster than some other processors with some functions
but still have the flexibility of reprogramming. Altera and Xilinx
support SDR on their FPGAs.
Finally, hard logic is also common today. When implementing fixed
standards like cellular radio specifications, the flexibility or
reprogramming is not necessary. Therefore, algorithms can be implemented
in fixed on-chip logic. It is fast, uses less chip area, and can bring
about a major decrease in power consumption. Such blocks of logic are
generally called accelerators.
Many cell-phone basestation ICs like TI's TMS320TC6614 system-on-a-chip
(SoC) are examples that use accelerators. Figure 5
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig5.jpg>
shows the 6614 with its ARM GPP and four 66x general-purpose
floating-point DSP cores. Note the accelerator logic on the right. Most
of the Layer 1 accelerators use DSP algorithms for the many SDR functions.
*Real SDR Transceivers*
Many SDRs have been developed for the military under the Joint Tactical
Radio System (JTRS). This U.S. Department of Defense program aims to
develop a complete line of SDR radios for voice, data, and video that
can be used to form ad hoc networks on the battlefield. The program has
been around since the late 1990s, with good progress over the years.
The whole basis of JTRS is the Software Communications Architecture
(SCA). This open-architecture platform standard defines how the hardware
and software work together. One of the primary objectives is to develop
software that is fully transferrable between different hardware
platforms, making all military radios multifunctional and interoperable.
The latest version, designated SCA 2.2.2, was recently made available to
further improve the programmer's ability to make the software more
flexible and scalable. Called SCA Next, the software helps make programs
smaller and require less testing.
SCA does not have specific provisions for cognitive features. But over
the past few years, the U.S. Defense Advanced Research Projects Agency
(DARPA) has been testing cognitive enhancements to SCA like Dynamic
Spectrum Access that will hopefully be available in the coming next
generation of JTRS radios.
The Thales Communications AN/PRC-148 JTRS Enhanced Multiband (JEM)
Inter/Intra Team Radio, which is a JTRS radio, covers all the HF, VHF,
and UHF military frequencies from 30 to 512 MHz (Fig. 6
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig6.jpg>).
Power output can be selected from 0.1 to 5 W. A wide range of modes and
waveforms is available.
*Cognitive Radio*
Cognitive radio (CR) expands the definition of SDR to include features
that make a radio intelligent. The Wireless Innovation Forum defines CR
as "Radio in which communication systems are aware of their environment
and internal state and can make decisions about their radio operating
behavior based on that information and predefined objectives. The
environmental information may or may not include location information
related to communicant systems."
CRs are sometimes called adaptive radios that automatically adjust their
behavior or operations to achieve specific objectives. They can sense,
learn, and adapt. They have an internal memory that stores instructions
for various situations. Stored knowledge about their own capabilities
makes it possible for these radios to make their own decisions.
A CR can also access external databases for additional decision-making
intelligence. It senses by listening to a channel assessing the presence
of other signals, their characteristics, and the noise background. The
CR learns from its experience as well. With all of the knowledge it has
or can access, the CR becomes a super-intelligent radio.
The transmitter (TX) and receiver (RX) are full frequency-agile SDRs
with a mix of applicable waveforms and all the related SDR hardware and
software (Fig. 7
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig7.jpg>).
A separate cognitive processor engine runs the cognitive aspects of the
radio. It gets inputs (M) from the RX and TX to monitor their condition
and parameters. It uses these inputs along with others to make decisions.
Other inputs can come from policy instructions stored in memory that
define ways to operate under different conditions. External databases
may also be accessed. Some CR units get location information by GPS.
Decisions are then made, and controls (C) are issued to the radios to
achieve the desired result.
CRs are also a good example of artificial intelligence (AI) in action.
AI is a collection of software that is used to store and use knowledge
to solve problems. It can use standard algorithms but it can also draw
on several AI techniques such as expert systems, natural language
processing, neural networks, fuzzy logic, and search techniques. AI/CR
emulates the human user by assessing the situation and making decisions
based on existing knowledge and taking actions to achieve the desired
best result.
One important aspect of CR is dynamic spectrum access (DSA), which
allows the CR to tune to a channel in the frequency spectrum for its
operation after it decides that the channel is unused. A DSA radio uses
unused spectrum, producing greater efficiency of limited spectrum space.
A cognitive transceiver essentially tells the SDR what to do in the way
of frequency of operation, modulation, power level, protocol, and other
factors and makes corresponding adjustments automatically. A CR is
software that monitors the SDR and delivers commands and control
instructions as needed.
CRs primarily seek to solve two major wireless problems: limited
spectrum and interoperability between different radios or wireless
systems. A CR can find open spectrum and use it. It also can change its
waveforms or protocols to adapt to radios of a different nature, making
communications possible or more reliable.
There are also several different classifications of CRs. For example, a
policy-based radio is programmed with a predefined set of capabilities
like waveforms and procedures. The radio is used by selecting one of
several different preprogrammed fixed functions. The fixed functions are
loaded during manufacturing, selected by the user, or downloaded over
the air.
Another CR form is a fully reconfigurable radio. This fully generic
transceiver can operate over a wide frequency and power range. This type
of radio can be fully reconfigured on the fly for new applications or
communications conditions.
*CR Examples*
The xG Technology xMax carrier-class CR system for mobile communications
uses the unlicensed industrial, scientific, and medical (ISM) band
spectrum in the 902- to 928-MHz range. In its first iteration, it was
deployed as a trial garrison and battlefield cellular radio system for
the U.S. Army.
The system was tested earlier this year at Fort Bliss and the White
Sands Missile Range as part of a military Network Integration Evaluation
(NIE) process used to validate and test systems for the field. The
system did well based on written media accounts and quotes from Army
personnel.
The prototype xMax system used a compact mobile basestation and its own
TX70 handset. It also used a 900-MHz time division duplex (TDD) digital
radio with cognitive technology in the handsets themselves. The system
supported Voice over Internet Protocol (VoIP) calls and texting (SMS) as
well. The handset included a full Wi-Fi radio.
The xMax system adheres to the Federal Communications Commission's part
15 rules for the 902- to 928-MHz spectrum. Radios can radiate up to 4 W
(EIRP), and that goes for the basestations too. The system divides the
spectrum into 18 1.44-MHz channels and uses robust binary phase-shift
keying (BPSK) modulation. Access is time division multiple access
(TDMA), and each channel can handle up to 12 voice calls.
The cognitive feature of the radio listens to the band in use to
determine where any interference is and then switches to a frequency
with the lowest noise levels. The xG handsets scan the band 33 times per
second looking for the interference and identifying the clear spots
where a good link can be formed. It then notifies the basestation,
causing the frequency to change as needed to keep a clean connection.
Now xG Technology is moving into its second-generation xMax design. This
new xMax system drops the special handsets and uses standard smart
phones. This is something that the military wants as soldiers can use
standard off-the-shelf smart phones, laptops, or tablets to save money.
In this new system, the smart phones talk to a bridging device called
the xMod (Fig. 8
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig8.jpg>).
This device resembles the Novatel MiFi devices that let multiple laptops
use Wi-Fi connections to talk to it and then backhauls these connections
back to the Internet via a connection to the cellular network. The xMod
works in a similar fashion supporting a Wi-Fi (or a direct USB wire)
connection to commercial smart phones or computers and using the
cognitive xMax network to connect back into the Internet or military
network.
This new arrangement adds high-speed data connectivity to the system.
Furthermore, it adds a managed VoIP capability via a smart-phone app.
Normal 3G and 4G cellular smart phones use the cellular systems'
standard voice service via 2G or 3G cellular links, which currently
isn't of the VoIP kind. The xMax system puts a special app on the smart
phone that permits it to identify and prioritize the voice packets. The
xMax system then can provide landline quality voice, even though it is a
100% complete IP-based system.
The new system modifies the xMax waveform by leveraging orthogonal
frequency division multiplexing (OFDM). Each of the previously defined
18 1.44-MHz channels is further subdivided into 128 subcarriers. Radio
access is TDD. Another key addition, multiple input multiple output
(MIMO), greatly improves the range, reliability, and data rate. The xMod
devices use a 2x4 MIMO system with four receiver chains and two transmit
chains.
The cognitive capabilities as well as the radio itself are primarily
implemented in software, commonly called SDR. The system's mix of CR
techniques, MIMO, and advanced signal processing maximizes range,
reliability, and throughput. The substantial processing complexity
required leverages a new generation processor capable of supporting 50
GOPS in both the xMod and basestation. This kind of processing power is
brand new and now available in a size and at a power consumption level
suitable for battery powered devices like the xMod.
Rick Rotondo of xG explained that the cognitive capabilities and
interference mitigation algorithms allow the xMax to operate reliably in
"white spaces" as well as "gray spaces." White space is defined as the
unused TV channels that have been blessed by the FCC for unlicensed use.
It's used to mean a 6-MHz channel free of any interference from TV
broadcasters.
However, white spaces can rapidly become gray spaces due to wireless
microphones that operate in these channels as well as other white space
devices operating nearby. In fact, the 900-MHz band that xG cut its
teeth on is heavily used and "dark gray" with interference from cordless
phones, wireless security systems, telemetry radios, and other devices.
So, the system had to operate reliability in the presence of high
interference from day one. The result is that the xG cognitive system
lets you maximize the capacity of gray spaces by reliably letting you
operate where other radio systems may not be able to operate.
As for applications, xG Technology will soon offer an advanced system
suitable for the military, rural broadband, and the enterprise. The new
system will include the 902- to 928-MHz band and the 5.8-GHz ISM band.
Future systems may also use the 700-MHz spectrum.
In addition, xG is evaluating its options in the TV white space arena
both here in the U.S. as well as in the U.K., where the system would be
a good fit. Imagine a powerful mobile voice and data cellular system
that could leverage the free white space 6-MHz TV channels and offer the
same services and better economics than commercial 3G and 4G cellular
systems.
Speaking of white spaces, this is another excellent application for CR.
White spaces comprise the unused 6-MHz TV channels that were abandoned
in 2009 with the switch from analog to digital TV. TV stations still use
channels 2 through 51 (54 to 698 MHz), though many of them are unused.
The open channels vary widely by region but represent a huge waste of
valuable spectrum.
The FCC has approved the use of these channels in a license-free
service. The guidelines call for low power and knowledge of the
available local channels. The FCC and a number of other organizations
like Spectrum Bridge and Telcordia have developed comprehensive
databases logging the TV stations and other wireless devices and
services using these channels in most U.S. locations. To use the
channels, a white space radio must access the database to see if it is
being used. If it is, another channel is selected, preventing interference.
White space radios fall into two categories: basestations and customer
premise equipment (CPE) terminals. The CPE terminals may be mobile. If
they want to transmit, they send their location based on GPS coordinates
to the basestation that accesses the database to see if the desired
channel is open. If it is, the CPE is notified that it can transmit.
In some systems, the CPE actually listens to the desired channel to
assess the presence of other signals. In any case, both the basestation
and CPE radios use forms of cognitive radio to make intelligent
decisions on what channel to use and when.
White space radios promise to make more efficient use of the unused TV
spectrum, but CR methods make it practical to do so without
interference. One key application for white space is wireless broadband
in rural areas. There are still many places where good, high-speed
Internet connections are not available. White space is well suited to
this use.
The lower-frequency and non-line-of-sight (NLOS) characteristics of the
white space channels make long-range connections of several miles not
only possible but also reliable. In one potential business model,
wireless Internet service providers (WISPs) would help fulfill the
federal government's National Broadband Initiative.
Remote monitoring and control is another potential use for white spaces.
Machine-to-machine (M2M) applications like Smart Grid connections, video
surveillance cameras, medical patient monitoring, and sensor networks
all would benefit from access to white spaces.
Designed specifically for white space use, the RuralConnect IP Version
II (RCIP VII) SDR/CR creates point-to-point and point-to-multipoint
networks with priority routing for voice, data, and video (Fig. 9
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig9.jpg>).
The target application is rural broadband access but it can be used in
IP video surveillance, well and pipeline monitoring, smart metering, and
traffic signal communications.
The RCIP VII operates in the 470- to 786-MHz range. It uses TDD and can
achieve data rates of 4, 6, 8, 12, or 16 Mbits/s in a 6-MHz channel
using quadrature phase-shift keying (QPSK) or 16-phase quadrature
amplitude modulation (16QAM). Its transmit power is +30 dBm, and its
receive sensitivity is in the --86- to --89-dBm range. Security is by
AES-128 with a shared secret key. The units are designed to use the
Telcordia database and operate under the FCC's Part 15 rules as well as
the Ofcom regulations in the U.K.
The Neul NeulNET white space radio system is designed for M2M and rural
broadband in the white spaces as well (see "First Commercial White Space
Radios Target M2M And Broadband Applications
<http://electronicdesign.com/article/communications/First-Commercial-White-Space-Radios-Target-M2M-And-Broadband-Applications.aspx>"
at www.electronicdesign.com).
*Developing SDR/CR*
An SDR transceiver is mainly a software project. Once you get a firm RF
platform, the DSP processing choice is next. It could be a standard DSP,
an FPGA, or some GPP. For first projects, a reference design with vendor
development tools really speeds and simplifies things. Other than that
there are few choices beyond going it alone.
One approach is to use GNU Radio, which is an open-source development
platform for SDR. It includes a set of signal processing routines
including modulation for Gaussian minimum shift keying (GMSK),
phase-shift keying (PSK), QAM, OFDM, and a few others. The software also
includes error correcting codes like Reed-Solomon, Viterbi, and turbo
codes. There are routines for optimized filters, FFT, equalizers and
timers. And, it allows coding in C++ or Python. The software runs under
Windows, Linux, or MacOS.
The GNU Radio software relies on a basic RF platform called the
Universal Software Radio Peripheral (USRP). It consists of several
choices of RF boards covering frequency ranges up to 5.9 GHz. Also
included is a complete data acquisition system of fast ADCs and DACs and
related support circuitry. A USB port provides basic I/O.
Ettus Research, a good source for USRP, makes a line of RF boards
covering different frequency ranges that use a basic I/Q architecture
and are full duplex capable. Transmit/receive switching is provided.
Transmitter power and receiver gain are controllable. Typical bandwidth
capability is 30 MHz. You can use either a standard DSP like the TI
OMAP3 that combines an ARM GPP and a TI C64xx DSP or a Xilinx Spartan 3A
DSP1800 or Altera Cyclone FPGA for the software.
As for the ADC and DAC, some models of the Ettus USRP use 100-Msample/s,
14-bit ADCs and 400-Msample/s, 16-bit DACs. Other models use
64-Msample/s, 12-bit ADCs and 128-Msample/s, 14-bit DACs.
If you're just beginning with SDR/CR or if you want to teach it, a great
new choice is National Instruments' USRP product (Fig. 10
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig10.jpg>). National
Instruments owns Ettus and uses its basic hardware in two products, the
NI USRP 2120 with a frequency range of 50 MHz to 2.2 GHz and the NI USRP
2921 with a frequency range of 2.4 to 5.5 GHz.
Both transceivers use the standard direct conversion architecture and
output their I/Q signals via a 1-Gigabit Ethernet port to a PC. Signals
up to 25 Msamples/s baseband may be steamed this way. The bandwidth is
50 MHz. The units cost about $4000 each.
The digital signal processing and other procedures run on the PC.
National Instruments' LabVIEW software with the Modulation Tool Kit is
used for development. Advanced software can be downloaded to NI's Flex
RIO PXI module with its Xilinx FPGA.
The NI USRP was developed primarily for university instruction and
research, though it makes a good learning platform for anyone just
beginning with SDR/CR. When you purchase a two-unit bundle, you get the
full set of instructional materials developed at the University of Texas
and Stanford University. The bundle costs about $6000. For initial
exploration and prototyping, including projects with OFDM and MIMO, the
NI USRP appears to be a good place to start
Fig 10. The National Instruments' Universal Software Radio Peripheral
(USRP) uses standard I/Q architecture radios with 100-Msample/s
sampling. Designed for SDR and CR development, the USRP uses NI's
LabVIEW software with Modulation Tool Kit. The DSP runs on an external
PC.
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig10.jpg>Fig
9. The Carlson RuralConnect IP basestation mounts near the antenna. The
contour map shows coverage of three basestations in rural areas.
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig9.jpg>Fig 8.
The xG Technology xMod wireless bridge links multiple standard smart
phones (and laptops) via Wi-Fi to the xMax wireless network.
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig8.jpg>Fig 7.
The transmitter and receiver in a typical CR are frequency-agile SDR
units. The cognitive processor manages the cognitive processes with
inputs from a policy memory, external data bases, and in some cases GPS
location information.
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig7.jpg>Fig 6.
The Thales AN/PRC-148 JEM military radio uses the JTRS SCA software
platform to ensure military radio interoperability.
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig6.jpg>Fig 5.
The Texas Instruments TMS320TC6614 is a basestation-on-a-chip IC used
for implementing pico, micro, metro, and other small basestations. All
baseband functions are performed here. The RF circuitry is external.
Note the hardware accelerators on the right that use hardwired logic to
speed up the many DSP SDR functions.
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig5.jpg>Fig 4.
The ultimate SDR receiver uses only an input bandpass filter, an ADC,
and a DSP. All demodulation, filtering, and other functions are
performed in the DSP.
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig4.jpg>Fig 3.
In an SDR transmitter, modulation is performed in the DSP. Then, the I/Q
architecture creates two orthogonal signals that are combined and
upconverted to the final frequency for transmission.
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig3.jpg>Fig 2.
A modern software-defined receiver uses the I/Q architecture to divide
the signal into two orthogonal paths. The I and Q channels are needed to
recover any type of modulation by digital signal processing algorithms.
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig2.jpg>Fig 1.
A conventional legacy radio receiver (a) uses the standard analog
superheterodyne architecture with analog circuitry performing all
functions. A more advanced superheterodyne receiver (b) uses digital
demodulation with DSP.
<http://electronicdesign.com/Content/UserStorage/17928/63883-fig1.jpg>
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