Hello, I am Craig Grimley working with Agilent Technologies on LTE development.
Common themes in modern wireless communications are the use of multi-antenna techniques, such
as “Beamforming” and “Spatial Multiplexing”.
In order to improve the cell capacity and throughput.
A common problem we hear from customers today is the inability to verify, validate and visualize
the signal at the RF antenna.
That is why I am here today.
I would like to introduce to you the Agilent N7100 multi-channel signal analysis solution
along with the 89600 VSA software installed with the TD-LTE measurement application.
Together, this solution will solve that problem.
Let us focus on how beamforming and spatial multiplexing are used within 3GPP LTE.
LTE defines many downlink transmission modes, which we will only briefly summarize here.
Release #8 defines transmission modes #1-7 covering: SISO, transmit diversity, open loop
single-user MIMO, closed loop single-user MIMO, closed loop multi-user MIMO, rank #1
spatial multiplexing, and single layer beamforming on Port #5.
Release #9 introduced transmission mode #8 supporting dual layer beamforming on ports
#7 and 8, and finally release #10 introduced transmission Mode #9 supporting up to 8 layer
transmissions.
For today’s discussion, we will focus on transmission modes #7 and 8, which are currently
the main development focus for initial TD-LTE market deployments.
They will help summarize the LTE-defined downlink signal processing flow, specifically from
a transmission mode #7 and 8 perspective.
As with other transmission modes, the PDSCH (Data) transport block information is channel
encoded and rate matching applied, producing either 1 or 2 code words, which are then mapped
onto layers.
Note that for transmission modes #7 and 8, the precoding block is non-codeword based,
and so it is left up to the base station to determine the optimum beamforming precoding
to apply.
This is based on UE feedback and can also be derived from direct measurement of the
uplink sounding reference signal.
Also note that beamforming precoding can be dynamic and vary on a per subframe and resource
block basis to adapt to changing channel conditions.
For demodulation purposes, transmission modes #7 and 8 include the mapping of UE-specific
reference signal with an HPDSCH resource block.
It is important to note that the UE reference signal must undergo the same beamforming precoding
as per the associated PDSCH (Data).
This concept is illustrated on the processing diagram.
The beamforming precoding is calculated by the base station to ideally maximize the signal-to-noise
ratio observed by the target UE device, but equally, the chosen precoding will attempt
to minimize transmission interference to other UE devices within the same or adjacent sounds.
As well as producing user-specific beam patterns, the base station also has the capability of
shaping a desired sector-wide broadcast beam pattern for common control channel content,
which is received by all user devices within the cell.
This is possible when a number of beamforming antenna elements is greater than the number
of configured cell RS Ports.
LTE defines resource element mappings for each of the following reference signal types:
transmission mode #7 single layer UE-specific reference signals for port #5, transmission
mode #8 dual-layer UE-specific reference signals for ports #7 and 8, as well as common cell-specific
signals for ports #0-3.
All these reference signals need to be verified both in terms of baseband correctness and
relative magnitude and phase observed at the RF antenna.
Let us consider a typical TD-LTE MIMO beamforming transmission modes #7 and 8 test configuration.
A typical 8 element physical antenna configuration used within TD-LTE today is shown.
It consists of 2 groups of antenna elements; each group is cross-polarized at 90 degrees
to each other.
Antenna Group #0 consists of antenna elements #1-4 polarized at plus 45 degrees.
Antenna Group #1 consists of antenna elements #5-8 polarized at minus 45 degrees.
Each antenna element within a group is spatially separated by approximately 1/2 the RF carrier
wavelength.
This provides a high degree of antenna element correlation within the antenna group, which
is good from a beamforming point of view.
Since each of the 2 groups are cross-polarized relative to each other, there is a low correlation
between each of the 2 antenna groups, which is good from a spatial multiplexing point
of view.
A typical TD-LTE base station is illustrated, comprising of eNB base band and remote radio
head.
The remote radio head provides 8 antenna feeds, which are connected to an RF antenna calibration
coupler unit, for test purposes.
Note that the base station calibration of the RF antenna is achieved using a dedicated
calibration port connection between the remote radio head and the RF antenna calibration
coupler.
Verification of the base station calibration performance is a very important aspect of
beamforming tests.
The calibration coupler output is typically fed into an RF downlink channel emulator,
shown here in an 8 x 2 configuration to emulate the downlink channel characteristics.
The 2 channel emulator RF outputs are received by the user equipment test device.
In our example, the user equipment test device transmits the uplink signal on 2 output ports,
which can be connected to an RF uplink channel emulator in a 2 x 8 configuration to emulate
the uplink channel feedback characteristics.
Finally, to complete the feedback loop, the 8 RF outputs from the uplink channel emulator
are coupled back into the base station’s 8 receive antenna ports.
A common problem we hear from customers today is the inability to verify and visualize the
beamforming signal at the RF antenna.
This is important in order to validate base station RF antenna calibration performance,
baseband encoded beamforming weighting algorithm correctness, and demodulation of MIMO single
and dual-layer EVM at the RF antenna.
The solution to this problem is the Agilent N7100 multi-channel signal analyzer along
with the 89600 VSA software installed with the TD-LTE measurement application.
The N7100 multi-channel signal analyzer can support 8 phase-coherent RF measurement channels
and using appropriate RF splitters and attenuators can easily be integrated into a typical TD-LTE
base station test setup as illustrated here.
The Agilent 89600 VSA software provides an easy to use N7100 correction wizard.
This can be used along with an Agilent MXG or ESG-C calibration signal source in order
to correct for all the RF cabling and connectors used within the measurement setup.
This allows direct corrective measurements of your 8 RF antenna beamforming performance.
With the 89600 VSA software plus N7100 multi-channel signal analyzer, we can start by viewing the
time-synchronized RF signal capture from all 8 antenna elements.
Enabling the 89600 VSA spectrogram feature, we can immediately gain insight into the frequency
resource activity.
This allows us to quickly build up a picture of per subframe RF activity levels for user-specific
resource block scheduling as well as common control channels and signals.
Note this feature does not require any demodulation at this point and is a very useful debugging
tool when investigating unexpected RF or scheduling issues, especially when those issues impair
normal demodulation of the signal.
Prior to demodulating the TD-LTE capture signal, we need to configure the 89600 VSA software
antenna group parameter with the appropriate number of elements and spacing used to match
our physical RF Antenna configuration.
As mentioned earlier, the beamforming weightings can be changing on an individual resource
block basis; therefore, we may choose to view the UE-specific weighting results on a per
resource block basis or alternatively, per user allocation.
The TD-LTE measurement application provides a rich set of demodulation results to help
you verify and visualize your MIMO beamforming signals.
Available results include: IQ constellations, EVM result metrics, detected resource allocations,
UE-specific reference signal weights, cell-specific reference signal weights and impairments and
UE-specific and common broadcast antenna beam patterns.
Let us take a closer look at some of these result traces.
The demodulated IQ constellations are displayed per spatial multiplexing layer and provide
a quick visual indication of the signal’s modulation quality correctness.
The frame summary trace provides access to individual EVM and power metrics associated
with each channel and signal type.
It also provides a color key for all channel type results, which is reused throughout the
VSA traces.
The detected allocations trace lets you visualize the resource block allocations for each user-specific
transmission plus resource allocations used by common control channels.
Measured UE-specific reference signal weights are presented in table format for each of
the 8 antenna elements.
Weightings can be evaluated in both magnitude and phase down to the individual resource
block allocations associated with each user transmission.
Separate UE-specific reference signal weight traces are available for each spatial multiplexing
layer.
To help you visualize the beamforming performance, the VSA software also presents a resulted
combined beam pattern trace associated with each antenna group.
Measured UE-specific reference signal weights from the first 4 input channels are used to
compute the antenna group #0 beam pattern trace.
This process is repeated for the second 4 input channels to compute the result in antenna
group #1 beam pattern trace.
Note that a separate beam pattern trace can be visualized for each resource block associated
with each user device.
Similar to how the IQ constellation provides a quick visual health check of modulation
quality, the antenna group beam pattern trace provides a quick visual health check of beamforming,
baseband encoding and RF calibration quality.
Any identified anomalies can be investigated in detail using the UE-specific weight trace
metrics.
Channel frequency, magnitude and phase response traces can be viewed simultaneously for all
8 antenna elements along with the VSA-supported common tracking error trace.
The VSA MIMO info trace report cell-specific reference signal metrics and impairments measured
for all 8 antenna elements.
Reporting metrics include: cell RS power, EVM, timing, phase, symbol clock and frequency
error, allowing you to verify the common broadcast beam pattern weightings associated with each
antenna element.
The VSA Software also extracts these relative antenna weightings in order to produce the
cell RS derived sector-wide broadcast beam pattern, shown here in blue.
The user-specific and common broadcast beam pattern trace results can be viewed in either
IQ polar format or alternatively in log magnitude (dB) format.
Both formats supports VSA markers for easy tracking of main lobe peak levels and azimuth
locations during live measurement updates.
VSA markers can also be used to read out various beam pattern characteristics like null depth
azimuth location and main lobe to side lobe levels.
A key metric to be verified within TD-LTE beamforming transmissions is beamforming gain.
The VSA software has added a new beamforming gain results trace specifically for this purpose.
It reports dB difference between each UE-specific beam pattern and the common cell-specific
broadcast beam pattern to produce a beamforming gain trace result for each user allocation.
The beamforming gain results can be viewed for each individual resource block associated
with each user’s allocation.
In summary, the Agilent N7100 multi-channel signal analysis solution along with 89600
VSA software installed with TD-LTE measurement application provides an 8 channel phase coherent
measurement solution to enable verification of your beamforming signals.
Today’s demonstration has shown the power of Agilent’s measurement solution to help
you visualize your RF antenna beamforming signals for TD-LTE.
For more information, please visit the URL below, and thanks for watching.