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Solutions for
Addressing SDR Design and
Measurement Challenges
Using COTS Technology and an Integrated
Design-to-Test Flow to Quickly Develop
Optimal SDRs
Application Note
Overview Problem
As the communications industry continues to Designing and testing an SDR presents a
grow and evolve, it is becoming more critical to number of difficult challenges. Engineers could
quickly, easily and cost effectively modify radio start the design process from scratch, but a
devices to support new and emerging technolo- substantial amount of time and resources are
gies (e.g., Mobile WiMAXTM and 3GPP Long required to implement the baseband algorithms
Term Evolution (LTE)), without requiring new from the ground up, negatively impacting time-
hardware. Software Defined Radio (SDR)--a to-market and introducing risk. There are other
radio in which some or all of the physical factors affecting time-to-market as well, namely
layer functions are software defined--is one that the design's baseband and RF sections
technology promising to answer this need. SDRs may be designed by several different teams,
are typically comprised of an RF/analog section each potentially using different tools pieced
and a baseband section that utilizes program- together in a disconnected flow. These teams
mable processing technologies like FPGAs, DSPs may not be collaborating and their different
and general-purpose processors (GPPs). tools may not be well integrated, opening the
door for significant system integration risks
In contrast to traditional radio devices, SDRs when the baseband and RF hardware are
provide an efficient and comparatively inexpen- combined. Rapidly evolving standards such as
sive way to enable multi-mode, multi-band and/ LTE further add to the design challenge because
or multi-functional wireless devices that can the standards are subject to interpretation,
be enhanced using software upgrades. Such which introduces risk. Developing an SDR, while
capabilities make the SDR well suited for the maximizing resources and minimizing risk and
communications market which serves a wide time-to-market, now demands an alternate
variety of radio signal formats. For example, solution--one that incorporates commercial
many engineers who are currently developing off-the-shelf (COTS) physical layer algorithm
SDRs are working on OFDMA physical layer modeling with an integrated design-to-test flow
waveforms like Mobile WiMAX and LTE. capable of supporting both baseband and RF
methodologies.
Solution
Utilizing a COTS system design solution
with built-in OFDMA physical layer
algorithm models offers an ideal way for
engineers developing SDRs to maximize
their resources and minimize time-to-market
cycles. Using commercial algorithm models
as a baseline starting point, engineers can
then customize algorithms to create their
own proprietary SDR implementation.
Design risk is mitigated through the use of
an integrated simulation design environment
that incorporates baseband and RF designs
into a single design-to-test flow. During
the design phase, baseband and RF team
hand-offs are supported at each step in the
flow--as the design transitions from algo-
rithm to system to baseband (e.g., FPGA)
FIGURE 1: Agilent's SystemVue, VSA Software and MXA signal analyzer work together to
and RF design to integration. The FPGA and
provide an integrated, SDR flow for FPGAs and RF.
RF designs are verified together in the same
simulation environment, at each stage of
the development process, prior to hardware
implementation. During the test phase, the
hardware implementation is demodulated
and requires a general-purpose signal
analyzer with the speed, accuracy and
scalability to perform the range of measure-
ments necessary to characterize signal
quality.
Agilent's SystemVue system design solution
offers an integrated design environment for
SDR system design (Figure 1). Algorithm
models can be modified using the existing FIGURE 2: Agilent's MXA signal analyzer delivers the fast speed, accuracy and scalability
SystemVue algorithms as a starting point, necessary to demodulate today's SDR designs.
with the Baseband Exploration Library (BEL)
option. This is especially useful for modifying During the design process, SystemVue's When the design progresses to the test
the algorithms for a particular version of algorithm references are used to generate phase, during hardware implementa-
a standard, or modifying a commercial reference vectors to facilitate baseband tion, it is demodulated using Agilent's
standard for a custom/proprietary SDR development when hand writing HDL code. fast, scalable, midrange MXA signal
implementation. Additionally, SystemVue's Alternatively, fixed-point simulation models are analyzer (Figure 2). The MXA features
algorithm references provide an indepen- used to model algorithms, with the baseband power suite--a standard, complete set
dent check of standards interpretation HDL being generated from the fixed-point of powerful one-button measurements
relative to the baseband design and models. RF behavioral models are used to for characterizing signal quality that
implementation--a feature which is construct RF transmitter/receiver designs. include ACPR, channel power, occupied
especially useful given the complex and bandwidth, spectrum emissions mask,
evolving nature of standards like Mobile The baseband HDL can be co-simulated in CCDF, burst power, and spurious emission.
WiMAX and LTE. SystemVue, together with the RF design. It also supports more than 50 modulation
Integrated instrumentation links with types. Running the VSA software in the
Agilent test equipment and with Agilent's MXA enables advanced signal modula-
Vector Signal Analysis (VSA) software tion analysis and troubleshooting of 30
enable SDR RF/baseband testing (demodu- additional modulation types.
lation) at every step of the design phase.
2
Using the VSA software with SystemVue
during the design phase, and with the MXA
signal analyzer for measurements during
the test phase, provides the engineer with
a true design-to-test capability. Agilent's
connected solutions makes the integration
of these instruments possible, enabling
signals to be downloaded and captured
to and from signal sources and analyzers
(Figure 3).
WiMAX IQ Modulator Design
Example
If not designed properly, impairments from
an IQ modulator can significantly degrade
the performance of OFDMA systems. A FIGURE 3: With Agilent's connected solutions, signals can be picked off along different stages
Mobile WiMAX OFDMA IQ modulator of a receiver chain to measure BER with an MXA using RF/IF inputs or baseband IQ inputs.
can be designed using the integrated SDR
flow in Figure 1. Here, SystemVue is used
for both the fixed-point baseband and
RF design (Figure 4). First the WIMAX IQ
modulator design, consisting of IQ pairs
generated with Agilent's WiMAX design
library stored in lookup tables, is imple-
mented in SystemVue and a fixed-point
design is created. Next, the functionality
of the fixed-point design is verified with
the VSA software and HDL code for the
FPGA is generated. The HDL and RF
design are then co-simulated together to
verify functionality before moving into the
FPGA synthesis process and VSA WiMAX
demodulation analysis of the mixed-signal
design (HDL and RF) is performed. Note
that instead of using machine-generated
HDL for co-simulation, custom/proprietary
FIGURE 4: The upper left graphic is a block diagram of the WiMAX OFDMA IQ modulator design imple-
HDL could have alternatively been brought mented in SystemVue. The bottom graphic is the SystemVue fixed-point design with various sections
into SystemVue and co-simulated. of the design annotated to show which part of the block diagram they correspond to.
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Finally, FPGA synthesis is performed to
generate a .bit file that will be used to con-
figure/program the FPGA target. Because
the FPGA board has on-board D/A convert-
ers, the FPGA output is converted to an
analog IF and measured/demodulated with
the Agilent MXA signal analyzer. The result-
ing FPGA implementation of the WiMAX IQ
modulator is shown in Figure 5.
Improving FPGA and RF Team
Communication/Collaboration
The WiMAX OFDMA IQ modulator design
offers a prime example of how SystemVue
can be used as a catalyst for improving
communication and collaboration between
FPGA and RF teams and as a result, mini- FIGURE 5: The analog IF output on the SDR FPGA board (after the DAC) is demodulated by an MXA
analyzer and VSA software.
mizing design risk. In this case, the HDL co-
simulation was combined with an RF trans-
mitter that was designed in SystemVue.
The transmitter design consisted of an Summary of Results
IF-to-RF upconverter, IF/RF filtering, pre-
amplifier, and a power amplifier (PA). A While SDRs present a host of benefits
VSA simulation measurement element was for engineers developing multi-mode,
then connected at the pre-amplifier output multi-band, multi-functional wireless The Power of X
and the PA output. The FPGA HDL code devices, these benefits come at the cost
The MXA signal analyzer, working with
was used as the simulation signal source. of challenges which need to be addressed
SystemVue and the VSA software, is a key
to meet time-to-market demands. The product in Agilent's comprehensive Power of
The waveform distortions analyzed include time and resources required to design X suite of test products. These products grant
both baseband impairments (fixed point OFDMA SDR physical layer waveforms engineers the power to gain greater design
impairments and FIR) from the HDL being from scratch, and the risk associated with insight, speed manufacturing processes, solve
co-simulated, as well as the RF non-linear employing different baseband and RF tough measurement problems, and get to
impairments introduced to the waveform by flows and disconnected tools are some of market ahead of the competition.
the pre-amplifier and PA. Significant distor- the key challenges. Agilent SystemVue's
tion was observed at the PA output (e.g., algorithm development capability and inte- Offering the best combination of speed and
constellation dispersion and spectral re- grated mixed-signal design environment scalability, and created and supported by
growth), but only moderate distortion was can help accelerate SDR design activities renowned worldwide measurement experts,
observed at the pre-amplifier output. The while minimizing baseband/RF system inte- Agilent's X products are helping engineers
output of the PA was heavily compressed gration risks. Agilent's MXA signal analyzer bring innovative, higher-performing products to
and therefore, it introduced significant dis- and VSA software provide the test capabil- emerging markets around the globe. To learn
tortion to the SDR waveform. In addition, ity necessary to test the hardware imple- more about Agilent's suite of X products please
significant spectral re-growth was observed mentation, minimizing risk and completing visit: www.agilent.com/find/powerofx.
on the RF spectrum which, if measured, the integrated SDR design-to-test flow.
would impact a range of metrics like ACLR
or ACPR.
4
Related Applications Remove all doubt www.agilent.com
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