A
GREAT ER
M EA SU R E
O F
C O N F I D E N C E
Meeting the Test
Challenges of 4G LTE
Mark Elo
RF Marketing Director, Keithley Instruments, Inc.
Introduction
As RF designers work rapidly to create
products that support the new Long Term
Evolution (LTE) cellular wireless standard,
understanding the new test requirements for
LTE assumes ever greater importance as this
technology becomes more widespread. For the
wireless industry, these changes are nearly as
significant as the move from analog to digital,
requiring new measurements and new test
equipment. Here we look at what LTE is and
the main challenges to testing for engineers.
What is LTE?
LTE is a next-generation wireless network
technology for 3GPP cellular networks
promising very high data rates providing
significant improvements to end-user experience
by allowing operators to provide common
applications and services across other fixed and
wireless access technologies. Sometimes called
4G, LTE-based networks are OFDM–based
(orthogonal frequency division multiplexing)
with MIMO (multiple-input multiple-output)
transmission schemes.
Testing Challenges
Two primary challenges affect LTE test
requirements:
• Moving from single-carrier to multi-carrier
OFDM modulated signals
• Moving from SISO (single-input singleoutput) to MIMO signal stream transmissions
• OFDM
Meeting the Test Challenges of 4G LTE
OFDM signals have multiple subcarriers
that are precisely aligned and occupy a wide
channel bandwidth (up to 20 MHz), which
are more complex than the traditional singlecarrier signals to whic h most RF engineers
are accustomed. Measuring these signals from
several dimensions is important to ensure that
the radios are working properly and to quickly
diagnose problem areas when they are not.
For instance, measuring EVM (error vector
magnitude), a modulation quality measurement,
on each subcarrier across the entire frequency
channel can detect problems due to amplifier
or filter roll-off, frequency response ripple, or
narrow-band interference. Similarly, measuring
the composite EVM of whole time slots across
the entire transmission frame can detect
problems due to amplifier heating effects,
droop, and switching or frequency error.
OFDM signals have higher PAR (Peakto-Average Ratio) values than single-carrier
signals, which increase the likelihood of bit
error rate due to transmitter power amplifier
gain compression. Unlike WiMAX however,
LTE compensates for this by using a different
modulation scheme (SC-FDMA) for the
mobile device. Although improving the power
consumption of the amplifier, the baseband
processing becomes more complex and
ultimately more power hungry.
MIMO
The move from SISO to MIMO signal stream
transmissions requires new measurements
and test equipment capable of measuring
multiple signal streams. MIMO is one of the
most important shifts in commercial radio
technology since the move from analog to
digital transmission techniques. Next-generation
standards such as WiMAX, HSPA+ and LTE
are all MIMO-based systems, presenting many
new challenges for commercial communication
equipment designers. As users demand more and
more services and more reliable connections,
MIMO systems today provide higher throughput
or greater coverage, however they will evolve to
encompass techniques such as beam forming, by
increasing the amount of transmitters, receivers,
and antennas in a device.
MIMO measurements are made of the
composite multi-stream data channel and of
the individual signal streams. A number of new
measurements helps determine the quality of
the signal and signal channel in MIMO systems
including channel response, the power of the
individual spatial streams of an N-by-M MIMO
transmission, matrix condition, the ability of
the receiver to separate the multiple signal
stream transmissions, as well as constellation
diagrams.
The performance of a MIMO system is
dependant on the behavior of the channel. So,
the transmitter and receiver must be tested using
a multitude of channel models, both from predefined standards and by user-defined models,
to ensure the design maintains performance
across a multitude of environments. The
channel distorts the signal in many ways. For
example, reflections off of surrounding objects
can cause multiple instances of the signal to
arrive at the transmitter at different times. This
multi-path phenomenon introduces amplitude
degradation and time and phase delays. The
more channel distortion added to the signal,
the more likely the receiver algorithms can
solve for the originally transmitted signals.
If the transmitter or the receiver further adds
amplitude, time, and phase errors, the channel
will not be accurately modeled, and the symbols
will not be effectively resolved.
Test Equipment
Beyond ensuring high measurement integrity,
test instrumentation for LTE will ideally be fast
and flexible. SISO measurements will continue
to be made on MIMO infrastructure and user
equipment. MIMO test equipment that is able to
make SISO measurements and is also scalable to
eight precisely-synchronized MIMO channels
for signal generation and signal analysis
will save time and money. Since LTE will be
incorporated into equipment employing other
wireless standards such as GSM, W-CDMA,
and WLAN, test instruments capable of testing
multiple standards, including non-cellular with
signal bandwidths up to 40 MHz, also provide
time and cost advantages.
January 2009
1
Next-generation test instrumentation for LTE
will have high-speed DSP technology for signal
processing instead of using the traditional
microprocessor, giving the capability to test
highly complex multiple wireless standards
quickly.
For beam forming, test equipment needs
to have the ability to control the phase and
amplitude of each source to be able to build
the required RF radiated patterns based on the
calculated knowledge of the channel.
For effective, accurate MIMO measurements, test equipment, such as signal sources
and analyzers, needs to phase align their local
oscillators and time align frequency references,
D/A, and A/D sample rates in order to minimize
their contribution to the channel. Ideally, phase
errors of less than a degree and a nanosecond of
timing alignment will yield an accurate result.
Conclusion
The growing prevalence of the LTE standard
poses unprecedented new test challenges for RF
designers. The signals are vastly more complex
and require new and more extensive testing.
For this reason, test equipment manufacturers
must design instruments that offer superior
measurement integrity and are more flexible
in order to accommodate these new testing
demands and to help designers save time and
reduce the cost of test.
WLAN System
Tx
About the Author
Mark Elo (melo@keithley.com) is
marketing director, RF/Wireless for
Keithley Instruments. He joined the
company in 2006 after working for
Agilent Technologies in marketing and
R&D management positions. Elo holds
a bachelor’s degree in engineering with
honors from the University of Salford,
Lancashire, England, and an MBA from
Herriot Watt University in Edinburgh,
Scotland.
Real MIMO Wireless System
Tx1
Tx2
Rx
Rx1
Rx2
Figure 1. Typical MIMO system setup used in LTE systems.
Figure 2. Keithley’s 4X4 MIMO RF Test System, a flexible test system to meet current and
future testing needs.
Specifications are subject to change without notice.
All Keithley trademarks and trade names are the property of Keithley Instruments, Inc.
All other trademarks and trade names are the property of their respective companies.
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© Copyright 2009 Keithley Instruments, Inc.
2
January 2009
Printed in the U.S.A.
No. 3006
012109 WO
Meeting the Test Challenges of 4G LTE