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Using SystemVue to Integrate
a Flexible R&D Testbed for LTE
Application Note
Introduction
As wireless communications evolve, the structure of wireless systems is becom-
ing ever more complex. New test signals and measurements are needed to test
the performance of these systems according to the quickly evolving wireless
standards. Sometimes, however, the required waveforms and measurements
are not available on existing hardware instruments. At such times it is valuable
to have software in the test system that can generate the new waveforms and
perform advanced measurements.
To construct test systems that meet the requirements of new wireless standards,
a number of different instruments are used. If we simply connect these instru-
ments one by one and manually configure the test system, the task of getting
this system to operate correctly will be difficult and time-consuming. A much
more efficient approach is to integrate all the instruments and automate the
system-level performance tests using a software tool.
System-level performance tests usually require a golden transmitter and a golden
receiver to provide test references. However, during the initial development of
a new product, the product’s transmitter and receiver are not yet complete. It is
useful, therefore, to have a software receiver to test the new product design and
early hardware.
This application note describes an integrated solution for testing wireless
communication systems based on the quickly evolving LTE standard. Agilent
SystemVue is recommended as the core software in this test solution to
integrate all test instruments, create new test waveforms, enable advanced mea-
surements, and provide a software reference receiver. An integrated test system
configured for LTE base station receiver measurements is used as an example. It
is well known that receiver sensitivity is an important measurement for describ-
ing LTE receiver performance. To test receiver sensitivity according to the latest
3GPP LTE specifications, Agilent proposes a test system using SystemVue with a
built-in reference receiver and certain auto-configuration capabilities. In this con-
figuration, this system can be used to test both frequency division duplex (FDD)
and time division duplex (TDD) uplink receivers that are specified in LTE. Receiver
performance curves for throughput vs. signal-to-noise ratio and for block-error-
rate vs. signal-to-noise ratio are generated quickly, with reasonable test accuracy.
Test System Structure and Functions
The basic configuration of the integrated test system, shown in Figure 1, includes
the SystemVue software, a device under test (DUT), and instruments such as
vector signal generators with deep ARB memory, vector signal analyzers with
deep capture memory, and a channel emulator. In this configuration, baseband
data is generated by SystemVue and sent to the PXB channel emulator. Faders
can be used in the channel emulator to set up single input-single output (SISO)
or multiple input-multiple output (MIMO) channels with multipath fading. The
signal generators, driven by the channel emulator, generate receiver RF and
IF signals that are used for testing the DUT. The signal analyzers capture the
received signals from the DUT outputs and send them back to SystemVue. In the
SystemVue platform, the software receiver can demodulate, deframe, and decode
received signals and provide a measure of receiver performance. In Figure 1, the
test data flow is indicated by the red arrows.
SystemVue plays the key role in integrating all the instruments in the test
system. The flow of test system control is shown in Figure 1 by the blue arrows.
The main functions of SystemVue include the following:
•
Test sequence configuration,
which invokes the operation of all involved
instruments and the DUT in the desired order
•
Instrument configuration
,
which sets up all hardware instruments properly
prior to testing
•
Advanced waveform generation,
which generates baseband waveforms as
well as baseband and RF signals through the signal generators by means of
specific instrument control protocols such as LAN, GPIB, and USB
•
DUT configuration
,
which conditions the device for testing and can also
provide FPGA programming capability in applications such as software
defined radio (SDR) or cognitive radio
•
DUT output capture,
which is typically performed using a vector signal
analyzer by digitizing the DUT output signals and streaming the captured
data back to SystemVue for further analysis
•
Advanced measurements,
which may include BER, BLER, or throughput
•
Golden receiver,
which facilitates the advanced measurements.
Next we take a closer look at these SystemVue functions as they are used in
the LTE test system.
NxN9020A
Signal Analyzers
NxE4438C
Signal Generators
SystemVue
N5106A PXB
.
.
Figure 1. Test system structure
2
Managing instrument
configuration
SystemVue integrates instrument hardware and software to provide test signals
to the DUT and to capture DUT outputs in a synchronized test system. Without
integration, each instrument would function on its own; hence it would be
impossible to perform complex and challenging tests such as BER, BLER, sensi-
tivity, and throughput.
SystemVue also automatically controls instruments through SCPI commands
so that instruments can be programmed to perform operations in the desired
order. Stimulus-measurement test sequences can be automated as well as the
subsequent post-processing of data.
Generating standard and
custom waveforms
SystemVue can generate test waveforms based on international standards such
as 3GPP LTE and WiMAX™. SystemVue also can generate complex waveforms
including mixed-mode waveforms, multi-modulation waveforms, waveforms that use
specific framed data, or waveforms with special modulation data. Generated wave-
forms can be automatically downloaded to vector signal generators. Waveform
generation also can be sequenced to support flexible and more complex DUT
testing.
SystemVue automatically configures a DUT to its proper test conditions. Since
field programmable gated array (FPGA) technology is broadly used in today’s
hardware design, SystemVue can provide HDL co-simulation. Additionally,
SystemVue can convert design control elements into HDL, which can be synthe-
sized to program FPGA inside the DUT, tremendously simplifying the design of
SDR and cognitive radio products.
Configuring the DUT
Data captured by vector signal analyzers in the system can be automatically
streamed back to SystemVue through either a SystemVue-VSA link or a math
language instrument link. Acquired data can be further processed in SystemVue
for analysis or for use in advanced measurements.
Acquiring and processing DUT
output data
Extending the measurement
capabilities of instruments
SystemVue performs generic measurements such as BER, BLER, and throughput
to extend the measurement capability of the signal analyzers in the system.
More specific measurements defined in the standards—for example, reference
sensitivity power level, adjacent channel selectivity, and blocking—can also be
performed in SystemVue.
Using SystemVue’s “golden”
reference receiver
Receiver testing using instruments always requires a “golden” receiver and
source. The software golden receiver in SystemVue can be embedded in the test
system for troubleshooting and evaluating the performance of receiver designs.
This golden receiver can also be used to evaluate and fine tune transmitter
design to ensure the transmitter meets critical specifications. Unlike hardware
receivers, the software golden receiver in SystemVue can be modified easily to
test new standards that overlap existing standards.
Integrated MIMO test system
using SystemVue
The basic structure of the LTE integrated test system described earlier can be
used directly for testing SISO designs and can be extended for testing MIMO
designs. SystemVue supports MIMO applications by providing MIMO signals via
multiple signal generators, including wideband signal generators, and multiple
signal analyzers, including logic signal analyzers and multi-channel scopes.
3
3GPP LTE Sensitivity Test
To test 3GPP LTE systems [1-5], SystemVue provides an LTE physical layer
(PHY) model library. These PHY models are intended to be a baseline system for
designers to understand what nominal or ideal performance can be expected
from a system. Use of the PHY models can be extended to evaluate degraded
system performance caused by factors such as poorly performing components.
Aspects of the LTE physical layer supported in SystemVue’s LTE PHY models
include the following:
FDD and TDD modes.
In the 3GPP LTE system, downlink and uplink transmis-
sions are organized into radio frames for the uplink and downlink to support
both FDD and TDD modes.
Downlink physical channels and signals.
For the downlink, the orthogonal
frequency domain multiple access (OFDM) waveform is supported. This sup-
port includes the LTE downlink physical channels, which correspond to a set
of resource elements carrying information originating from higher layers. The
downlink physical channels are as follows:
• Physical downlink shared channel (PDSCH)
• Physical broadcast channel (PBCH)
• Physical multicast channel (PMCH)
• Physical control format indicator channel (PCFICH)
• Physical downlink control channel (PDCCH)
• Physical hybrid ARQ indicator channel (PHICH)
The following downlink physical signals are also supported:
• Reference signal
• Synchronization signal
Uplink physical channels and signals.
Requirements for the LTE uplink differ
from downlink requirements in several ways. Power consumption is a key con-
sideration for user equipment (UE) terminals. The high peak-to-average power
ratio (PAPR) and related loss of efficiency associated with OFDM signaling are
major concerns. As a result, an alternative to OFDM was sought for use in the
LTE uplink. Single carrier-frequency domain multiple access (SC-FDMA) is well
suited to the LTE uplink requirements. The basic transmitter and receiver archi-
tecture is very similar (or nearly identical) to OFDMA architecture, and it offers
the same degree of multipath protection. Importantly, because the underlying
waveform is essentially single-carrier, the PAPR is lower.
The SystemVue LTE PHY library includes support for uplink physical channels,
which are listed below:
• Physical uplink shared channel (PUSCH)
• Physical uplink control channel (PUCCH)
• Physical random access channel (PRACH)
The uplink physical signals are also supported:
• Reference signal
• Sounding reference signal
4
Channel coding/decoding.
A channel coding/decoding model set is provided for
both the downlink and uplink channel codecs. Models for CRC, convolutional
encoding and viterbi decoding, turbo encoding and turbo decoding, a scrambler/
de-scrambler, an interleaver/de-interleaver, and HARQ are included.
Modulation.
A modulation model set includes mappers and de-mappers for
QPSK, 16QAM, 64 QAM, OFDM, and SC-FDMA.
Multiplexing.
The multiplex models provide OFDM and SC-FDMA symbol multiplex-
ing and de-multiplexing, along with downlink and uplink framing and de-framing
for the downlink/uplink transceiver.
Additionally, receiver models in the LTE PHY library can be used for constructing
both downlink and uplink receivers that implement timing synchronization,
frequency synchronization, and channel estimation. Measurement models in the
library provide basic measurements including waveform, spectrum, constella-
tion, and EVM measurements. Receiver measurements include BER, BLER, FER,
throughput, and reference sensitivity power level.
Reference sensitivity test based
on 3GPP Technical Specification
36.141
In the 3GPP LTE test specification [6], the reference sensitivity power level is
defined as the minimum mean power received at the antenna connector at
which a throughput of 95% shall be met for a specified reference measurement
channel. To set up the test for LTE receiver sensitivity, all system parameters
must be set to align with the LTE test specification [6] as defined in Table 1.
Table 1. Test setup from 3GPP Std FRC Test Parameters (TS 36.141 v8.50, “Base Station
Conformance Testing” [6])
Reference channel
A1-1
A1-2
A1-3
A1-4
A1-5
Allocated resource blocks
6
15
25
3
9
DFT-OFDM Symbols per subframe
12
12
12
12
12
Modulation
QPSK
QPSK
QPSK
QPSK
QPSK
Code rate
1/3
1/3
1/3
1/3
1/3
Payload size (bits)
600
1544
2216
256
936
Transport block CRC (bits)
24
24
24
24
24
Code block CRC size (bits)
0
0
0
0
0
Number of code blocks - C
1
1
1
1
1
Coded block size including 12bits
trellis termination (bits)
1884
4716
6732
852
2892
Total number of bits per sub-frame
1728
4320
7200
864
2592
Total symbols per sub-frame
864
2160
3600
432
1296
5
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