MIMO in LTE Operation and Measurement - Excerpts On LTE Test: Application Note
MIMO in LTE Operation and Measurement - Excerpts On LTE Test: Application Note
MIMO in LTE Operation and Measurement - Excerpts On LTE Test: Application Note
and Measurement
Excerpts on LTE Test
Application Note
Introduction
In the specifications, the terms input and output apply to the medium
between the transmitters and receivers, including the RF components of both
known as the channel. Thus, a base station with two transmitters provides
two inputs to the channel, the MI part, and a handset with two receive chains
takes two outputs from the channel, the MO part. This is true only if the data
transmitted and received is independent, and is not just a copy of the same
data, as explained below.
SISO
Tx
Rx
SIMO
Tx
Rx0
Rx1
MISO
Tx0
Rx
Tx1
Figure 3. Multiple Input Single Output
(MISO) radio channel access mode
MIMO
Tx0
Rx0
Tx1
Rx1
Five multi-antenna techniques have been defined for LTE to improve the
downlink performance:
1. Receive diversity at the mobile
2. Transmit diversity using SFBC at the eNB (evolved Node B)
3. MIMO spatial multiplexing at the eNB, for one or two users
4. Cyclic Delay Diversity (CDD) at the eNB, used in conjunction with
spatial multiplexing
5. Beamsteering (user specific)
The first two are relatively conventional diversity methods. The third and fourth
methods make use of space frequency coding mechanisms to spread data
across multiple antennas. Cyclic delay diversity introduces deliberate
delays between the antennas to create artificial multipath. It is applied more
dynamically in LTE than in other radio systems. The techniques are applied
differently, depending on the type of physical signal or physical channel.
Both SIMO and MISO are employed in 3rd generation (3G) cellular systems,
and will be rolled out in LTE networks. Their purpose is to improve the integrity
of connections and to improve error rates, particularly where the connection
suffers poor SNR (for example, at the edge of a cell). Conventional phased-array
beamsteering introduces phase and amplitude offsets to the whole of the signal
feeding each transmitting antenna. The intention is to focus the signal power
in a particular direction. The same technique of applying phase and amplitude
offsets can be used on the receiving antennas to make the receiver more
sensitive to signals coming from a particular direction. In LTE, the amplitude
and phase of individual resource blocks can be adjusted, making beamsteering
far more flexible and user-specific. Beamsteering does not increase data rates
but has an effect similar to diversity in terms of increasing signal robustness.
The effectiveness of beamsteering increases with the number of transmitting
antennas, which allows for the creation of a narrower beam. The gains
possible with only two antennas are generally not considered worthwhile; thus,
beamsteering is generally only considered for the four-antenna option.
User Equipment (UE) diversity reception (SIMO) is mandatory for the UE.
It is typically implemented using maximum ratio combining. In a cellular
environment, the signal from a single receive antenna will suffer level
fluctuations due to various types of fading. With the wider nature of the LTE
channel bandwidths there may also be a noticeable frequency dependency on
the signal level. By combining the signal received from two antennas, the UE
can recover a more robust signal. Receive diversity provides up to 3 dB of gain
in low SNR conditions.
Figure 6. Overall system performance is improved since MIMO can potentially double
the data capacity
With the channel constantly changing due to fading and multipath effects, and
Doppler frequency shift due to handset movement, amongst others, condition
number versus frequency changes constantly across the RF-channel spectrum
as illustrated in Figure 7.
Reference signals (or pilots) at regular frequency locations in the output of each
transmitter provide a way for the receivers to estimate the channel coefficients.
In general, each data pipe will not have the same performance. LTE uses
feedback mechanisms known as pre-coding and eigenbeamforming both
forms of closed-loop MIMO, where the handset requests changes to the
cross-coupling of the transmitter outputs to give the best match to the channel
characteristics.
Figure 7. Condition number as a
function of sub-carrier frequency
The terms codeword, layer, and precoding have been adopted specifically for LTE
to refer to signals and their processing. Figure 8 shows the processing steps to
which they refer. The terms are used in the following ways:
Codeword: A codeword represents user data before it is formatted for
transmission. One or two codewords, CW0 and CW1, can be used depending
on the prevailing channel conditions and use case. In the most common
case of Single User MIMO (SU-MIMO), two codewords are sent to a single
handset UE, but in the case of the less common downlink Multi-User MIMO
(MU-MIMO), each codeword is sent to only one UE.
Layer: The term layer is synonymous with stream. For MIMO, at least two
layers must be used. Up to four are allowed. The number of layers is always
less than or equal to the number of antennas.
Precoding: Precoding modifies the layer signals before transmission. This
may be done for diversity, beamsteering or spatial multiplexing. The MIMO
channel conditions may favor one layer (data stream) over another. If the
base station (eNB) is given information about the channel (e.g. information
sent back from the UE), it can add complex cross-coupling to counteract the
imbalance in the channel. In a 2*2 arrangement, LTE uses a simple 1-of-3
precoding choice, which improves performance if the channel is not changing
too fast.
Eigenbeamforming (some times known simply as beamforming) modifies
the transmit signals to give the best carrier to interference and noise ratio
(CINR) at the output of the channel.
RV index
Payload
Code block
segmentation
Channel
coding
QPSK
/16 QAM
/64 QAM
CW0
Scrambling
Scrambling
Code block
concatenation
Circulat
buffer
Spatial multiplexing
Tx Div (DCC/SFBC)
Antenna
number
Resource
element
mapper
Modulation
mapper
[ d]
CW1
Rate
matching
Layer
mapper
[ X]
OFDM signal
mapper
Precoding
[y]
Resource
element
mapper
Modulation
mapper
OFDM signal
mapper
Figure 8. Signal processing for transmit diversity and spatial multiplexing (MIMO) The symbols d, x and y are used in the specifications
to denote signals before and after layer mapping and after precoding, respectively
Figure 9 shows how both codewords are used for a single user in the downlink.
It is also possible for the codewords to be allocated to different users to create
multiple user MIMO (MU-MIMO). Depending on the channel information available
at the eNB, the modulation and the precoding of the layers may be different to
equalize the performance.
1 UE
1 eNB
User
data
Precoding
Multiplex
into
codewords
User
data
Cross channel
de-mapping
Demultiplex
Map into
layers
(streams)
The channel
Received
codeword
The precoding choices are defined in a lookup table known as the codebook. A
codebook is used to quantize the available options and thus limit the amount of
information fed back from the receiver to the transmitter. Some of the precoding
choices are straightforward; for example, Codebook Index (CI) 0 is a direct
mapping of codewords to layers and CI 1 applies spatial expansion.
Number of layers
Codebook index
[]
[]
[]
[]
[ ]
[ ]
[ ]
1 1
2 1
1 1 0
2 0 1
1 1
2 1
1 1 1
2 1 1
1 1
2 j
1 1 1
2 j j
1 1
2 j
Table 1 shows the codebook choices for one and two layers. Note only the twolayer case employs spatial multiplexing. Precoding with one layer is limited to a
0 , 90 or 180 phase shift.
In operation, the UE sends a message to the eNB scheduler with the codebook
index most closely matching the channel, although the system can be configured
for multiple codebook values, one for each resource block group. To use this
information while it is still valid, the scheduler has to respond rapidly, within
milliseconds, depending on the rate of change of the channel. If the UE is
instructed to provide channel information more regularly, the information will be
more accurate but the proportion of resources used for signalling will increase
and place higher demands on the eNB.
SU-MIMO is within the scope of LTE but at the time of this writing has not
yet been fully defined. To implement SU-MIMO the UE would require two
transmitters. This is a significant challenge in terms of cost, size and battery
consumption, and for these reasons SU-MIMO is not currently a priority for
development. Also, the increased data rates in the uplink that might be possible from SU-MIMO are not as important as they are in the downlink due to
asymmetrical traffic distribution. Lastly, if the system is deployed to be uplinkperformance-limited, it may be impractical to increase the transmit power from
the UE sufficiently to achieve the SNR needed at the eNB receivers.
1 eNB
2 UE
Figure 10. SU-MIMO in the downlink with two antenneas; Codebook 0 shown
LTE already requires fundamental changes in base station and handset design
and test due to the higher data rates, wider allowable signal bandwidths, and
increasing integration and miniaturization in the handset, for example:
The requirement to handle 6 different channel bandwidths from 1.4 to 20 MHz
and both Frequency Division (FDD) and Time Division Duplex (TDD) modes.
Flexible transmission schemes and virtually infinite operating permutations
in which the physical channel configuration has a large impact on RF
performance.
Handset components complying with the multi-gigabit DIgRF v4 standard,
which removes the potential communication bottleneck between the
baseband and radio frequency integrated circuits (RFICs), require crossdomain (digital in, RF out) measurement capability. A digital test source must
emulate both data traffic and the encapsulated protocol stack within the
digital interface that controls RFIC functionality.
The DigRF high-speed digital serial interface in the handset must be treated
as a transmission medium where analog impairments can degrade quality
and degrade bit error rate (BER), and care must be taken connecting test
equipment to avoid disturbing signal flow.
Information transfers between the handset RF and baseband ICs must
comply with strict timing constraints. Therefore it is important for the test
environment to measure precisely when each frame is sent from one IC to
the other and provide real-time detection of timing violations.
Added to these are the specific challenges resulting from the need to support
multi-antenna techniques including diversity, beamsteering and MIMO.
10
Receiver Design
and Test
Modern receivers utilize the same building blocks as classic designs; however,
today there is a higher degree of integration with single components performing
multiple functions, particularly in handsets where space is at a premium
(meaning there will likely be fewer places where signals can be injected or
observed for testing).
11
Here we will focus on a subset of receiver design and test considerations, specifically: open and closed loop operation, wide bandwidths, cross-domain signal
analysis, affects of the channel, and finally precoding and the LTE codebook.
Wide bandwidths
Next, we consider that one of the unique aspects of LTE is that it supports six
channel bandwidths ranging from 1.4 MHz to 20 MHz. To simplify system operation and roaming, handsets must support all of these bandwidths, even though
actual deployment in any one area may be restricted to fewer bandwidths. The
LTE 20 MHz bandwidth is significantly wider than the maximum bandwidths of
todays other cellular systems; therefore, special attention to phase and amplitude flatness is required during receiver design. Filters, amplifiers and mixers
in particular now have to operate correctly over multiple channel bandwidths.
The LTE signal structure contains Reference Signals (RS) that are spread in
both frequency and time over the entire LTE signal. The UE and eNB receivers
can use these signals along with Digital Signal Processing (DSP) techniques to
compensate for amplitude and phase-linearity errors in the receiver. Flatness
needs be tested across each supported bandwidth and band, particularly at the
band edges where the duplex filter attenuates the edge of the signal.
12
The VSA software can be run in a number of Agilent spectrum analyzers, logic
analyzers and oscilloscopes for demodulating various modulation formats, and
offers a unique way to analyze the ADC performance by being able to make
traditional RF measurements directly on digital data. This approach gives a
designer the ability to quantify the ADC contribution to the overall system
performance and compare it to RF measurements made earlier in the block
diagram (shown in Figure 11) using the same measurement algorithms.
If the RFIC has analog IQ outputs, these can be analyzed using an oscilloscope
or Agilent MXA signal analyzer. If the RFIC interface uses DigRFv3 or v4, the
signal can be captured with the Radio Digital Cross Domain (RDX) tester.
These digital or analog IQ signals can then be analyzed with the same 89601A
VSA software. For the baseband developer, hardware probes are available for
analog IQ and DigRF interfaces. The VSA software provides numerical EVM
performance measurements for verification, as well as more detailed graphical
information useful during product development to isolate the source of signal
impairments. If Gaussian noise is added to the signal as the impairment, a
relationship can be drawn between EVM and the raw BER. Alternatively, the
captured IQ signals can be fed into a simulated receiver such as Agilent design
software for LTE. The precise design of a receiver determines the performance,
so the results may differ depending on a vendors specific implementation.
13
In normal operation the receiver will have to deal with a complex and continuously changing channel, but using such a fading channel means testing is not
repeatable when looking to ensure that the basic baseband operation is correct.
A fading channel, built from simple phase and timing differences between paths,
provides a deterministic signal that can be designed to verify the receivers performance limits. Adding noise to such a channel can readily create a test signal
in which some subcarriers are more difficult to demodulate than others. For
dual-source testing the standard RF phase and baseband timing alignment that
can be achieved using a common frequency reference and frame synchronization signal is sufficient for most purposes.
Figure 12. Continuous faded path receiver test configuration for RF,
analog or digital interfaces using the N5106A (PXB) MIMO receiver tester
14
Figure 13. Spatial multiplexed signals without precoding (top) and with precoding
(bottom) to match the channel
15
Figure 14. Impact of phase errors on precoding effectiveness, with example performance values
In LTE, the codebook index method is used to facilitate channel precoding, with
a small number of codes used to minimize the system overhead in signalling.
This means that the codebook index provides an approximation to the channel,
implying some level of residual error. Figure 14 shows that once a codebook is
chosen to equalize the EVM, the actual EVM still depends on the phase match
between the transmitters.
The rectangular block at the center of Figure 14 represents the region in which
codebook 1 would be chosen as the best fit. The diagonal lines show how the
EVM of each stream varies with phase error. EVM is used as a performance metric, but BER could be also used. In the best circumstance, the codebook exactly
fits the channel state and the performance of both layers is made the same.
As the phase between the transmitters varies indicating that a mismatch in
the codebook choice or variation in the channel occurred after the channel station information was provided the layer performance separates. At extremes,
the performance of the layers can be swapped. For receiver measurements, the
significance of precoding errors shows the need for a fixed RF phase relationship at the output of the signal generators being used for a test. The term phase
coherence is used to signify that the RF phase at the outputs of two or more
generators is being maintained at a specified frequency. When it is necessary
to guarantee that phase will not change versus frequency, a test configuration
such as shown in Figure 15 can be used.
16
Figure 15. Configuring multiple signal generators for timing and phase synchronization
Transmitter Design
and Test
From the perspective of the RF engineer, LTE promises a dauntingly wide range
of design and measurement challenges, arising from a number of factors:
The requirement to handle six channel bandwidths from 1.4 to 20 MHz
The use of different transmission schemes for the downlink orthogonal
frequency division multiplexing (OFDMA) and uplink single carrier frequency
division multiplexing (SC-FDMA)
Flexible transmission schemes in which the physical channel configuration
has a large impact on RF performance
Specifications that include both FDD and TDD transmission modes
Challenging measurement configurations resulting from the spectral, power
and time variations due to traffic type and loading
Further challenges resulting from the need to support multi-antenna
techniques such as TX diversity, spatial multiplexing (MIMO) and beamsteering
The need for making complex tradeoffs between in-channel, out-of-channel
and out-of-band performance
As with the development of other modern communication standards, the design
task involves troubleshooting, optimization and design verification with an eye
toward conformance and interoperability testing.
The next section discusses general challenges of transmitter design and some
basic verification techniques, starting with basic characteristics and then moving on to LTE-specific aspects, specifically: output power and power control,
out-of-channel and out-of-band emissions, power efficiency, high-peak power
including crest factor and predistortion, and phase noise.
17
Power efficiency
Next, power efficiency is a critical design factor for both eNB and UE transmitters and the design must meet power consumption targets while ensuring
that the transmitter meets the outputpower, modulation quality, and emission
requirements. There are no formal requirements for power efficiency, although
this may change in the future with increased environmental awareness.
Instead, power efficiency remains an ever-present design challenge to be met
through design choices and optimization.
18
19
A Systematic
Approach to Verifying
Transmitter Quality
Spectrum
meaurements
Vector
measurements
Digital demodulation
measurements
Constellation and
modulation quality analysis,
advanced analysis
Each of the verification sequence stages are represented below with example
measurements, beginning with RF spectrum measurements (Figure 17), then
proceeding to vector measurements (Figure 18), and finally to digital demodulation (Figure 19). The latter shows:
Trace A the IQ constellation (which shows the analyzer has locked to and
demodulated the signal)
Trace B power and frequency from a single FFT
Trace C the error vector spectrum
Trace D the error summary
Trace E the EVM in the time domain as a function of symbol
Trace F the frame summary
Figure 17. Spectrum of 5 MHz downlink showing power, OBW, and center frequency
20
Figure 18. CCDF measurements of uplink signals: from left to right, QPSK, 16QAM,
64QAM and the AWGN reference curve
Figure 19. Example analysis of digitally demodulated 5 MHz LTE downlink signal using
Agilent 89601A VSA software
21
While many transmitter measurements are a straightforward matter of connecting the transmitter RF output directly to an RF signal analyzer input and
measuring signal characteristics and content, some measurements will require
connecting, probing and measuring at early or intermediate points in the transmitter signal chain. Figure 20 shows a typical transmitter block diagram and the
possible ways in which signals can be injected or probed at different points.
RF
RF
\
Transmitter
Receiver
Signal
generator
Signal analyzer
Vector
signal analysis
software
Analog
IQ
Oscilloscope
Digitized
IQ
Upconversion
and
amplification
Amplification
and
downconversion
D to A
conversion
A to D
conversion
Baseband processing
Analog
IQ
Digitized
IQ
Signal studio
software
Baseband
digital interface
Logic analyzer
DUT
Figure 20. Stimulus and analysis of different point in the UE block diagram
22
Analyzer Configuration
Measurement steps
Turn Demodulator ON
Sync to P-SS or RS
Codebook index = 0
TX Diversity or spatial
multiplexing ON
Codebook index = 0
Measure signals using a
two-input analyzer
All codebook values
23
Combining Simulation
and Measurement to
Address Hardware
Testing
Figure 21. Complete transmitter and receiver with faded MIMO channel using Agilent
SystemVue software.
As shown in Figure 21, combining simulation with test offers a number of benefits.
Simulation is a powerful and flexible way to model both baseband and RF design
elements as well as RF path impairments and the creation of pre-coded MIMO
channels. As the design is turned into functioning physical blocks, combining
simulation with test instrumentation allows stimulation and analysis of the blocks
in a real-world environment.
One powerful example is to create a MIMO dual-transmitter source and perform
coded BER measurements on a complete MIMO dual-receiver and baseband
combination. The transmitter payload can be either digital or analog IQ data,
combined with control and pre-coding, with real-time error analysis provided by
comparing the receiver data output with the sent data. Stress-testing the receivers by applying known fading and channel coupling scenarios, while measuring
real-time BER, builds confidence that the design will work correctly in the realworld. The block diagram of such a system is shown in Figure 22.
24
Conclusion
This overview of MIMO and LTE has shown some of the engineering challenges
associated with implementing spatial multiplexing and other system features of LTE.
This knowledge can help receiver and transmitter designers improve their designs
through insight gained from key measurements.
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Revised: October 1, 2009