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Written by Manuel Mielke | August 18, 2020

Code-selective electromagnetic field (EMF) measurements (part 2)

In part 1 of the article series about electromagnetic field (EMF) measurements, we explained the different measurement concepts for measuring the electromagnetic field strength. We weighed the pros and cons of these methods, namely the frequency-selective and code-selective methods, and given the rollout of 5G and its beamforming capabilities, concluded that the code-selective EMF measurement method will prevail. Therefore, let’s discuss this method in more detail and examine measurement results obtained with our EMF solution comprising the R&S®TSMA6 scanner and the QualiPoc Android software.

Code-selective electromagnetic field (EMF) measurements

A bit of history – code-selective measurements in LTE?

For code-selective EMF measurements in LTE, the measurement receiver measures on cell-specific synchronization and reference signals, decodes the PCI, and, optionally, the operator and other layer 3 messages. In such a case, the received power can be allocated to a particular cell, carrier, and band.

The location of the synchronization and reference signals in the time and frequency domain is standardized. This means that such signals are an excellent approximation of the maximum received/radiated power since they are spread over the entire spectrum and are not power boosted (at least in lower transmission modes).

When it comes to higher transmission modes in LTE, beamforming is applied to UE-specific signals. Their power offset to standard reference signals and location in the time and frequency domain, however, is unknown to a passive, calibrated receiver. This means that extrapolation factors are applied to project the total radiated power, usually assuming the worst case of power-boosting of the UE specific signals.

The need for extrapolation factors in 5G NR

As explained in part 1 of this series, 5G NR broadcasts a minimum set of cell-specific signals and layer 3 messages. Therefore, similar factors have to be applied for reliable 5G NR electromagnetic field (EMF) measurements. As a reminder, the definition of extrapolation factors is typically country-specific. Fundamental extrapolation factors are:

  • Beam/gain offset between SSB and data beams
    It is expected that data/UE-specific beams have a much lower beamwidth and/or more power than SSB beams to increase the SINR further. The corresponding data has to be requested from the network operators or infrastructure suppliers.
UE-specific signal component transmitted with higher power compared to SBB beams
UE-specific signal component (CSI-RS) is transmitted with higher power compared to SSB beams.
UE-specific signal component transmitted using extremely narrow beams
UE-specific signal component (CSI-RS) is transmitted using extremely narrow beams compared to SSB beams.
  • Uplink and downlink relation factor
    In the case of TDD, the relation between uplink and downlink significantly affects the power radiated by the gNodeB, which is downlink (DL) only. If more slots are reserved for the uplink, the radiated power decreases.

    The relation factor depends on the network configuration that has to be requested from the network operators. Non-standalone (NSA) networks could be an exception if the uplink (UL) is operated in LTE, and the 5G NR carrier is used for DL only.
Flexible scheduling of user data across frequency and time domain
Flexible scheduling of user data (uplink and downlink) across the frequency and time domain.
Zero span-mode
Zero span-mode: TDD slots are visible and a gate can be configured on an uplink slot to trigger the spectrum measurements.
  • Projection of synchronization signal block power on the total 5G NR carrier spectrum
    Synchronization signal blocks only have a bandwidth of 3.6 MHz to 57.6 MHz depending on the subcarrier spacing. The total bandwidth of 5G NR carrier can be up to 400 MHz. This requires another extrapolation factor, which can be requested from the operators or determined using a mobile phone with an active subscription for the particular 5G NR network.

Code-selective measurements with extrapolation factors are applied during post-processing, and the results typically reflect the worst-case value (maximum emission measurements, for example, with a full load, max. output power).

It is also very important to mention that extrapolation factors consider the network configuration, and it is, therefore, helpful to know the network configuration details (signal power offsets, TDD configuration, etc.).

Code-selective measurement solution

When performing a code-selective measurement, the antenna is typically pivoted to determine the maximum of received power within a certain area or room. Code-selective measurement receivers can typically detect the center frequency of the carrier automatically. They are measuring on defined signals and can distinguish between the signal and noise.

The R&S®TSMA6 autonomous network scanner is an example of a measurement system that is capable of performing code-selective maximum emission measurements in 5G NR. It detects 5G NR carriers automatically and decodes and measures on SSBs and PCIs. By applying the antenna factor of the directive measurement antenna, which has to be provided via a .csv file, and summing up all SSBs per PCI, the R&S®TSMA6 delivers a reliable and unique result in mV/m per PCI.

The software supporting code-selective electromagnetic field (EMF) measurements is QualiPoc Android. Running on a smartphone or tablet, QualiPoc Android connects to the R&S®TSMA6 via Bluetooth®.

Bluetooth-connected code-selective measurement solution

QualiPoc Android directly outputs the power [dBm] of every detected SSB or electric field strength values [V/m]. According to the code-selective measurement procedure, as defined by the Swiss Federal Institute of Metrology (METAS), the QualiPoc algorithm sums up the maximum EMF values of all SSBs belonging to a PCI.

The Android-based software, installed on a tablet or smartphone, captures the code-specific RSRP value from the receiver, considers the antenna factor/gain, and performs all mathematical operations to convert dBm into V/m.

QualiPoc searching for max. electric field strength in mV/m
QualiPoc screenshots when searching for the max. electric field strength in mV/m (left) and marking the cell after the maximum is found (right).
QualiPoc displaying PCI and SSB-based real-time EMF measurement values
QualiPoc displaying PCI and SSB-based real-time EMF measurement values in mV/m.

As mentioned earlier, maximum emission measurements consider the worst-case scenario. This requires searching for the maximum EMF value in a certain area by pivoting the measurement antenna before saving the final EMF measurement value per PCI.

The following screenshot illustrates the code-selective EMF measurement procedure:

Code-selective electromagnetic field (EMF) measurement procedure

In summary, let’s recall that while frequency-selective measurements are simpler and give a quick pass/fail result they do not provide the details of the different signals contributing to the final result.

The code-selective method, on the other hand, provides all the details and enables operators and infrastructure suppliers to find the best compromise between adhering to country-specific EMF exposure limits and providing optimized network coverage and capacity – a particularly complex task given 5G NR’s air interface with flexible beamforming concepts for broadcast and data signals.

Read our white paper "EMF measurements in 5G networks" and get more in-depth information about measuring electromagnetic pollution, the 5G NR signal structure, and the discussed EMF measurement procedures.

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