Introduction
The introduction worldwide of the fifth generation (5G) of mobile telecommunications [1] is well underway. In contrast to second to fourth generation (2G–4G) mobile technologies (such as Global System for Mobile communications (GSM), Universal Mobile Telecommunications System (UMTS), and Long Term Evolution (LTE)), the 5G New Radio (NR) technology will make use of a huge span of radiofrequencies (RF), split in two broad ranges: one spanning from 410 MHz to 7.125 GHz (‘sub-6 GHz’), and the other from 24.25 GHz to 52.6 GHz (‘mmWaves’). Furthermore, one of the main technological advances introduced or enhanced in 5G NR will be the widespread use of Massive Multiple-Input Multiple-Output (MaMIMO), in which many antenna elements (up to hundreds) can be used to narrow and steer the transmit beam in order to optimize the signal at the receiver device.
Guidelines on limiting the human exposure to electromagnetic fields (EMF) have been issued by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and the Institute of Electrical and Electronics Engineers (IEEE) based on decades of scientific research [2], [3]. These guidelines have formed the basis for recommendations by internationally recognized institutions such as the World Health Organization (WHO), the US Federal Communications Commission (FCC) [4], and the International Telecommunications Union (ITU), as well as the Recommendation of the European Council [5]. However, some countries or regions (such as Brussels, Belgium) have adopted their own, more strict legal regulations, which may delay or even impede the deployment of 5G networks due to EMF saturation where current limit levels have already been reached with pre-5G telecommunications infrastructure [6]–[9].
In the last few years, there have been a few publications discussing how to properly assess the exposure levels from 5G base stations [10]–[15], some of which include numerical studies and preliminary measurements. However, as of yet, there is no standardized method available.
EMF exposure assessment methods for time-variant mobile telecommunications signals have relied on the measurement and subsequent extrapolation of user-independent signals that are transmitted continuously (or periodically) at constant power, independent of the traffic load [16]–[18]. These signals differ from one telecommunications technology to the other (Table 1): i.e., the Broadcast Control Channel (BCCH) for GSM, the Common Pilot Channel (CPICH) for UMTS, and the cell-specific reference signal (CRS), synchronization signals (SS) and physical broadcast channel (PBCH) for LTE. In the case of NR, there is no CRS, but the ‘always-on’ signal components are, as in LTE, the primary and secondary synchronization signals (PSS and SSS) and the PBCH. The PSS and SSS are used by user devices to find, identify, and synchronize to a network, while the PBCH contains a minimum amount of system information. Together, these signals form the SS/PBCH block (also denoted as SS block or SSB).
Although previous studies (e.g., [14], [15]) have discussed extrapolation methods based on measuring the power of the SSB, none have been molded into a feasible assessment methodology, nor have they been tested in the field.
This paper presents a comprehensive description of a measurement methodology to assess the RF-EMF exposure of a 5G NR base station on site. First, we describe the main principles of the 5G NR physical layer that are important for an accurate assessment. Second, we introduce and discuss the proposed measurement equipment and methods to measure or calculate the time-averaged instantaneous exposure and the theoretical (and actual) maximum exposure. And lastly, the proposed methodology was validated in-situ in the vicinity of a 5G NR base station operating at 3.5 GHz in Düsseldorf, Germany.
5G New Radio and RF-EMF Exposure
The accurate assessment of an RF signal requires that the settings of the measurement device be optimized to the characteristics of the considered signal. Here, we discuss the main principles of the physical layer of 5G NR that are important for RF exposure assessment. More detailed information is out of the scope of this paper but can be found in the 3GPP technical specifications [1].
A. 5G NR Grid Structure
Like 4G LTE, 5G NR supports both frequency division duplexing (FDD) and time division duplexing (TDD) and signals are modulated by using Orthogonal Frequency Division Multiplexing (OFDM) with a cyclic prefix. Moreover, 5G NR also uses a grid structure consisting of subcarriers in the frequency domain and OFDM symbols in the time domain. The basic granularity of the 5G NR resource grid (i.e., in frequency and time) is the resource element (RE), which spans one OFDM symbol in time and one subcarrier in frequency.
In the frequency domain, the grid structure is further organized in resource blocks (RBs), with each RB consisting of twelve contiguous subcarriers. The total number of RBs available for data transmission (
In the time domain, the structure is organized in frames. A 5G NR radio frame is 10 ms long and consists of ten subframes of each 1 ms. A subframe is further divided into slots, which each comprise 14 (in the case of a normal cyclic prefix) or 12 OFDM symbols (in the case of an extended cyclic prefix). The number of slots and the duration of a symbol depend on the SCS. For example, in the case of an SCS of 30 kHz, a subframe consists of two slots and the symbol duration is 35.68
The SS/PBCH block, which comprises the constant-power signal components of 5G NR, spans four OFDM symbols in the time domain and 240 contiguous subcarriers, or 12 RBs, in the frequency domain (Fig. 1). As opposed to the constituting signal equivalents in LTE, in 5G NR the SSB is not fixed to the center frequency of the radio channel, but instead its position (denoted by
B. Time-Averaged Instantaneous Exposure
Assessment of the time-averaged instantaneous exposure level
C. Theoretical Maximum Exposure
The exposure level at an evaluation point in line of sight (LOS) of a 5G NR base station will reach the maximal value when the traffic load is at its maximum (i.e., when the 5G NR frame is completely filled with downlink data) and all traffic is transmitted at the maximum possible gain
D. Actual Maximum Exposure
Since (1) assumes that the radio frame is fully occupied (i.e., at 100% slot occupation) with downlink traffic and broadcast/control data which is all continually transmitted at the highest possible gain in the direction of the evaluation point,
Proposed Measurement Procedure
A. Overview of 5G NR Assessement Procedure
The proposed measurement methodology consists of five steps:
Step 1 “Spectrum overview” — We perform an overview measurement of the telecommunications frequency range to identify the RF signals that are present at the measurement location and in particular the 5G NR signal from the base station under test.
Step 2 “Locating the SS burst” — An important step in the assessment of a 5G NR signal based on extrapolation of the SSB power, is the determination of the actual location of the SS burst. In this step, we identify
as well as the numerology of the SSB(s).SS_{REF} Step 3 “Obtaining the field level per RE of the SSB” — We measure the electric-field strength per resource element of the dominant SSB,
.E_{RE,SSB} Step 4 “Measuring the instantaneous field level” — We determine the time-averaged instantaneous electric-field strength over the channel bandwidth,
, measured during a certain time,E_{avg} (e.g., 6 or 30 min).T_{avg} Step 5 “Post-processing” — We extrapolate
to the theoretical maximum exposure levelE_{RE,SSB} by using (1). We then compare the obtained exposure levels with the relevant exposure limits such as those proposed by ICNIRP [2] of IEEE [3] (in this case, the reference levels for the electric-field strength). Furthermore, we can calculate the actual maximum exposure in case additional deduction factors are known (Section II-D).E_{max}
While 5G NR demodulation software can assist in locating the SSB, identifying its numerology, and measuring its power per RE [15], we focus in this paper on a procedure usable with a common spectrum analyzer (SA). The general method outlined here remains the same in either case.
Finally, the specific steps to be taken depend on the objective of the measurement. If the only objective is to determine the maximum theoretical exposure, step 4 is unnecessary. Likewise, only steps 1 and 4 are needed if the time-averaged power is the sole quantity of interest.
B. Spectrum Analyzer Measurement Setup
The measurement setup used for this study consisted of a Rohde & Schwarz FSV spectrum & signal analyzer FSV30 connected to a Clampco Sistemi AT6000 tri-axial antenna. An SA setup measures the received power
In order to capture the total electric-field level, all three orthogonal components (
Furthermore, the R&S FSV30 was equipped with option R&S FSV-K14 to use it in ‘spectrogram mode’. Besides offering a graphical overview of successive measurement sweeps or traces as a function of time (i.e., the ‘spectrogram’), this option also allowed us to store a high number of measurement traces (up to 20,000 for the R&S FSV-30) and exporting them with a minimum of lag or ‘blind time’.
The SA settings proposed for each step of the measurement procedure can be found in Table 2 and will be discussed in the following section. It is important to note that the mentioned settings may be specific to our measurement equipment, and equivalent settings for other equipment can be used.
C. Discussion of Measurement Settings
1) SA Settings for Step 1
In the first step, a spectrum overview measurement is used to identify the RF signals present in the frequency range used by telecommunication signals (e.g., 700 MHz – 6 GHz). The proposed settings can be found in Table 2.
In order to distinguish between different telecommunication signals (2G–5G), the resolution bandwidth (RBW) is set to a value approximating the minimal bandwidth of the existing telecommunications signals, which is 200 kHz (used by 2G). By using a peak detector in combination with a long sweep time (SWT) and maximum-hold mode, and measuring until the display of the SA is relatively stable, all non-continuous but repetitive signals present at the measurement location are detected. The measurement time per sample is set equal to the duration of one 5G NR radio frame (i.e. 10 ms), by configuring the SWT accordingly.
It is important to note that, with these settings, the measured power levels provide only an indication of the peak values (typically a large overestimate due to the effect of modulation) and no further conclusions can be made.
2) SA Settings for Step 2
After the present 5G NR channel(s) is/are identified, the frequency positions of their broadcast signals are located. If, at the location of assessment,
The center frequency (CF) of the considered 5G NR channel is obtained from the previous step (or from operator information). The frequency span is set to 100 MHz, which is the largest BW for sub-6 GHz 5G NR signals, and the RBW to 1 MHz, as this is the widest possible setting for our measurement setup narrower than the minimum bandwidth of the SSB (i.e.,
In the absence of traffic, which may be transmitted at a higher gain than the broadcast signals, these settings result in the highest power levels when the SA sweeps the (exact) frequency and time range of an SSB, i.e., when within the measurement time per sample, exactly two (SCS 15 kHz) or four symbols (30 kHz) were transmitted at the same power. Hence, by plotting the maximum power per frequency over all measurement traces, we are able to identify the SSB frequency range.
3) SA Settings for Step 3
Thirdly, the power distribution of the REs that are part of the SSB is determined. As we are looking for a recurrent signal of a certain duration (which depends on the structure of the SS burst [1]), aligning measurement samples in time should show us when the SS burst was transmitted. Then, we retain only those samples that were measured during the dominant SSB of the SS burst. The proposed settings for this measurement can be found in Table 2.
In order to continuously measure the power received in the SSB frequency range, we opt for a zero-span measurement, i.e., a measurement of the received power within a certain frequency band as a function of time, with
4) SA Settings for Step 4
The time-averaged instantaneous electric-field strength can be measured with an SA in both frequency and zero-span mode. The proposed settings for both measurements can be found in Table 2.
Depending on the SA specifications, it is possible to measure a 100-MHz bandwidth signal at once in frequency mode. In this case,
In zero-span mode, on the other hand, the RBW of most commercially available SAs is too narrow to completely contain the signal spectrum within the passband of the instrument (e.g., for the FSV30, the maximum RBW is 28 MHz in zero-span mode). In this case, the measurement is split in a number of contiguous parts to cover the whole channel bandwidth of the 5G NR signal. For each part
In-Situ Validation
A. Description of the Location and Tests
The proposed exposure assessment methodology was validated in LOS of a 5G NR base station, operating at 3.5 GHz, situated on the upper level of a parking building in Düsseldorf, Germany, on 28 May 2019 (Fig. 2). This site was chosen as it was available for testing purposes and the location was suitable to conveniently position the measurement equipment. The base station antenna was situated at a height of about 12 m above the floor level. The amount of car traffic during the measurements was minimal and assumed to have no influence on the measurements.
Although the base station was not part of a commercial network, one user equipment (UE) was available for testing purposes. We investigated six test cases, described in Table 3. First, Steps 1 and 2 procedure were followed in the case without UE and thus without traffic (T1). Then Steps 3 to 5 of the procedure were validated with three representative use cases, namely a voice call (using WhatsApp, T2), a video call (WhatsApp, T3), and video streaming (on YouTube, T4), and with downlink and uplink traffic forced at 100% capacity (by using the iPerf tool, https://iperf.fr/) of the base station (T5) and the UE (T6a and T6b in Table 3), respectively.
Most of the tests (T1 to T6a) were conducted at Pos. 1, at a distance of 62 m to the base station antenna and approximately 7 m to the UE. To explore the influence of the UE, another test with 100% uplink (T6b) was conducted at Pos. 2, at a distance of approximately 3 m to the UE and 66 m from the antenna, and at a different azimuth angle to the base station. The height of the measurement probe was 1.5 m above floor level.
The base station was set to operate constantly with a fixed beam in order to validate the methodology in a well-controlled environment.
B. Step 1 – Overview Measurement
First, we performed a spectrum overview measurement (with settings of Table 2) at Pos. 1 during T1. As can be seen in Fig. 3, the RF signals observed at this location included earlier-generation mobile telecommunications signals in the frequency range 700–2700 MHz (i.e., frequency bands around 800 MHz, 925 MHz, 1800 MHz, 2100 MHz, and 2600 MHz), a few other, non-identified signals at frequencies of up to about 3 GHz (at 460 MHz and 2880 MHz), and finally, a 5G NR signal at approximately 3.52 GHz.
It should be noted that the specific frequency allocations are dependent on the country and mobile operator. Parts of the frequency spectrum are auctioned by the government, and the operators themselves choose which bands they employ for which technologies.
C. Step 2 – Locating the SS Burst
The second step consisted in locating the CF of the SS burst,
With the proposed approach, a 7-MHz wide bump in the spectrum was observed on the left side of a 40-MHz wide 5G NR channel (Fig. 4). With no data traffic assumed (since the UE was not connected) the characteristics of this bump (i.e., CF of 3516 MHz and width of about 7 MHz) reveal not only the approximate position of the SS burst but also its SCS: as the 20 RBs of the SSB cover 7 MHz, the SCS was 30 kHz. Furthermore, we obtained the bandwidth of the 5G NR signal, which was 40 MHz. In Fig. 4, we also show two 7-MHz bandwidth parts corresponding to GSCN values of 7857 (black dashed lines) and 7858 (red dotted lines). It is immediately clear that we can distinguish the former as the only candidate, and thus
D. Step 3 – Electric-Field Strength Per Resource Element of the SSB
To determine the electric-field strength per RE of the SSB, zero-span measurements were performed for each UE test (T2–T6b, Table 3) with an RBW of 1 MHz, a CF of 3515.52 MHz, and a measurement time per sample of 35.63
Three examples are shown in Fig. 5, depicting waterfall reconstruction plots of measurements during 1 s (50 times two radio frames) of the
Located in subframe 0 of the first frame in Figs. 5 and 6, a four-symbol-long SS burst was identified—so in this case, there was indeed only one, cell-wide SSB beam—and its default 20-ms period confirmed. The received rms powers per symbol of the (one and thus dominant) SSB were gathered from all captured traces (roughly 56 per electric-field component when measuring during 1 min, Table 2), and after applying a deduction factor
Whereas we measured an
To validate the method described above, we also calculated
E. Step 4 – Instantaneous Electric-Field Strength
The time-averaged instantaneous electric-field strength
The instantaneous exposure levels (measured in zero-span) ranged from 0.288 V/m (video streaming, T4) to 3.716 V/m (at 100% downlink load, T5), with the latter reflecting a worst-case downlink exposure scenario. In comparison to the scenarios with a single UE, the exposure level was about 130 to 170 times higher when the 5G NR channel was fully occupied with downlink resources (T5) (Table 3). In any case, all values were well below the ICNIRP/IEEE reference level of 61 V/m at 3.5 GHz [2], [3].
F. Step 5 – Post-Processing
1) Time-Division Duplexing
In Figs. 5 and 6, the difference in the occurrence of traffic data between the tests can be observed. The allocation of downlink traffic is variable in Fig. 5(b), and whereas Fig. 5(b) shows that roughly three out of every four slots (in fixed subframes) were allocated to downlink traffic, from Fig. 6 we observe that slots allocated to uplink traffic were essentially complementary, barring a few slots that were allocated to neither (e.g., last slot of subframe 9). From these results (29 downlink slots out of 40), we obtain a factor
2) Difference in Gain Between SSB and Traffic
Since we can distinguish SSB signals from traffic signals in the measurements of Step 3 (Fig. 5), it is possible to derive an approximation of the maximum gain difference between broadcast and traffic beams.
In Fig. 7, we compared the distributions of the SSB (orange) and the traffic samples (green) for the
3) Maximum Electric-Field Strength
Based on an
Discussion and Conclusion
In this paper we introduced and tested on a 5G site for the first time a comprehensive methodology to assess in-situ the exposure to radiofrequency (RF) electromagnetic fields (EMFs) emitted by fifth generation New Radio (5G NR) base stations.
The proposed five-step measurement methodology consists of (1) a spectrum overview to identify the 5G NR channels; (2) the identification of the frequency position
The procedure was validated in LOS of a 5G NR base station operating at 3.5 GHz in Düsseldorf, Germany. At the validation site, one user equipment (UE) was available with which various tests (100% downlink or uplink, voice call, video call, and video streaming) were performed. At a distance of 62–66 m from the base station radio, electric-field levels per RE of the SSB of 0.032–0.067 V/m were measured and extrapolated to a (conservative) theoretical maximum field strength of 5.537 V/m (4.715 V/m when accounting for TDD), while the time-averaged electric-field levels ranged between 0.288 V/m for a single UE scenario (video streaming) and 3.716 V/m for a 100% downlink scenario. All these values are well below the ICNIRP reference level of 61 V/m at 3.5 GHz [2].
Frequency-selective extrapolation was previously discussed by Keller [15], who stated two preconditions for it to work: (1) REs outside the SSB are not transmitted at higher power or antenna gain than the SSB REs, and (2) SSB REs are transmitted at a constant power and constant gain. While we agree with the second, the former is not a precondition for our proposed extrapolation method. To account for the difference in antenna gain between broadcast and traffic signals, we introduced the factor
The assessed base station was not part of a commercial network and it was set to transmit with a fixed beam. Moreover, just one UE was available for tests. While this allowed to validate the proposed methodology in a well-controlled environment but for very different traffic scenarios, additional tests should nevertheless be carried out in a live network to generalize the methodology. For example, it is possible that the current method and SA settings for Step 2 (Table 2) are not adequate to identify the SS burst frequency position in the presence of traffic signals. It is also important to note that the extrapolation of
In addition, although we were unable to perform tests with 5G NR signals at higher frequencies (‘mmWaves’), the procedure should remain valid, providing that the measurement settings of Table 2 are adjusted to account for wider channel bandwidths as well as SCS of 120 kHz and 240 kHz.
Finally, since the focus of this paper was on the measurement of base station downlink exposure, uplink traffic contributions were unwanted. In the case of TDD, uplink traffic can contribute to the measured field levels using the SA method if a UE was in the vicinity of the measurement probe (such as at Pos. 2, Fig. 6). The influence of UE on the measurements and the distance beyond which uplink signals from UE do not impact the measurement results should be further evaluated in future work.