In situ LTE exposure of the general public: Characterization and extrapolation
Abstract
In situ radiofrequency (RF) exposure of the different RF sources is characterized in Reading, United Kingdom, and an extrapolation method to estimate worst-case long-term evolution (LTE) exposure is proposed. All electric field levels satisfy the International Commission on Non-Ionizing Radiation Protection (ICNIRP) reference levels with a maximal total electric field value of 4.5 V/m. The total values are dominated by frequency modulation (FM). Exposure levels for LTE of 0.2 V/m on average and 0.5 V/m maximally are obtained. Contributions of LTE to the total exposure are limited to 0.4% on average. Exposure ratios from 0.8% (LTE) to 12.5% (FM) are obtained. An extrapolation method is proposed and validated to assess the worst-case LTE exposure. For this method, the reference signal (RS) and secondary synchronization signal (S-SYNC) are measured and extrapolated to the worst-case value using an extrapolation factor. The influence of the traffic load and output power of the base station on in situ RS and S-SYNC signals are lower than 1 dB for all power and traffic load settings, showing that these signals can be used for the extrapolation method. The maximal extrapolated field value for LTE exposure equals 1.9 V/m, which is 32 times below the ICNIRP reference levels for electric fields. Bioelectromagnetics 33:466–475, 2012. © 2012 Wiley Periodicals, Inc.
INTRODUCTION
It is important to assess the typical range of human exposure from emerging wireless network technologies and to characterize the exposure of the general public [World Health Organization (WHO), 2010]. Long-term evolution (LTE) is a new mobile network technology marketed as the fourth generation (4G) of radio technologies [3GPP, 2009, 2011]. High data rates of LTE systems (targets from 10 to 300 Mbit/s) will enable mobile broadband access to telecommunication services. The world's first publicly available LTE service was started in Stockholm (Sweden) and Oslo (Norway) in 2009. In other countries, network operators are planning rollouts.
The objective of this study is twofold. Firstly, we aim to characterize in situ exposure of the different radiofrequency (RF) sources in the frequency range between 80 MHz and 3 GHz at different locations (denoted as short-term or spot measurements) in the city of Reading, United Kingdom (UK). Here, an LTE test network is active and consists of different base stations (BS) working in a single frequency network (SFN) configuration. The first purpose of this study is thus to provide a range of typical RF exposure values in an urban environment, extend the current knowledge of LTE exposure, and compare the LTE contribution with other sources, including the recent studies of Joseph et al. [2010] and Bornkessel [2011]. Secondly, we propose an extrapolation method for LTE exposure to determine the worst-case value from instantaneous exposure (maximal emission level when the BS is operating at full capacity), similar to the method based on the Broadcast Control Channel (BCCH) signal for Global System for Mobile Communications (GSM), and the Common Pilot Channel (CPICH) for Universal Mobile Telecommunications System (UMTS) [CENELEC, 2008; IEC, 2010]. To validate the proposed LTE extrapolation method we will examine the influence of the traffic load on the constant signals [e.g., reference signal (RS) and secondary synchronization signal (S-SYNC)], which will be used for the extrapolation, and examine the influence of the output power of the BS on the measured LTE signals. Finally, the extrapolation method will be applied to all measurement data at different locations in Reading and will be compared with the typical LTE exposure values obtained from the short-term measurements.
Procedures for measurements in the vicinity of GSM and UMTS BSs have been developed by Olivier and Martens [2007]; for Worldwide Interoperability for Microwave Access (WiMAX) by Joseph and Martens [2006] and Kim et al. [2008]; for BSs by Joseph et al. [2008]; and for Wi-Fi access points by Foster [2007] and Verloock et al. [2010]. Assessment of exposure to electromagnetic fields of emerging wireless systems, such as LTE is only scarce. Up to now, only Joseph et al. [2010] and Bornkessel [2011] investigated LTE exposure assessment. Joseph et al. [2010] determined LTE exposure at 2.6 GHz in the world's first commercial LTE network in Stockholm, Sweden. Using spectrum analyzer (SA) measurements, RF exposure of various sources was determined at different locations, and contributions and LTE exposure levels were characterized. Bornkessel [2011] executed measurements at 800 MHz and 2.6 GHz around 7 LTE BSs in Stuttgart, Düsseldorf, Mönchengladbach, Kyritz, and Munich, Germany. A sweeping method with extrapolation to maximum operational state of the station was used to characterize LTE exposure.
The Electronic Communications Committee (ECC) [2004], the European Committee for Electrotechnical Standardization (CENELEC) [2008], and the International Electrotechnical Commission (IEC) [2010] proposed an extrapolation of the value of the BCCH channel for GSM and the CPICH for UMTS in order to determine the field strength for maximal traffic. These methods were discussed and compared with temporal measurement data by Joseph et al. [2009], Joseph and Verloock [2010], and Mahfouz et al. [2011]. It is assumed that these values remain constant [ECC 2004; CENELEC, 2008]. In this study, we will investigate whether a similar extrapolation method can be applied to LTE. Moreover, a validated in situ extrapolation method for LTE is proposed and compliance with the guidelines of the International Commission on Non-Ionizing Radiation Protection (ICNIRP) for general public exposure [ICNIRP, 1998] is checked. The novelties of this article are thus the following: the characterization of in situ exposure to LTE signals in an urban environment, and a proposal of an in situ validated extrapolation method for LTE worst-case exposure [including multiple-input and multiple-output systems (MIMO)]; and the in situ investigation of the influence of traffic and power settings of LTE BSs on the extrapolation method and the determination of actual and worst-case (extrapolated) LTE exposures. The procedures, methodologies, and results of this study can be used by authorities and epidemiologists to estimate the exposure from LTE and other RF emitting sources and determine which sources cause the highest exposures.
MATERIALS AND METHODS
Measurement Locations
The measurements are executed in February 2011 at 40 locations in an urban environment of Reading, UK, where a trial LTE network is operational and consists of 7 LTE BS in a SFN at 2.680 GHz with a channel bandwidth of 20 MHz. The measurement locations, outdoor and indoor, are randomly selected across Reading in order to compare BS exposure of various sources. In total, 38 positions are outdoor locations and 2 positions are indoor locations. Nine positions are line-of-sight (LOS) with both a GSM/UMTS and LTE BS, and 2 positions are LOS with only a GSM/UMTS BS. The maximal power input to the LTE BS antennas is typically 40 W.
Short-Term Measurements
The “short-term” or spot measurements are executed to characterize in situ exposure of distinct RF sources at different locations.
Measurement equipment
The measurement setup for the short-term measurements consisted of tri-axial Rohde and Schwarz TS-EMF isotropic antennas (dynamic range of 1 mV/m–100 V/m for a frequency range of 80 MHz–3 GHz) in combination with a SA (frequency range of 9 kHz–6 GHz; FSL6, Rohde and Schwarz, Zaventem Belgium). The measurement uncertainty for the electric field is ±3 dB (−29% to 41%) for the considered setup [CENELEC, 2008]. This uncertainty represents the expanded uncertainty evaluated using a confidence interval of 95% (thus estimated at the level of twice the standard deviation, corresponding to a confidence level of 95% in the case of a normal distribution).
Measurement procedure
We assessed the electric field levels for the present signals in the frequency range between 80 MHz and 3 GHz at different locations. The electric field levels generated by LTE transmitters are compared with other RF sources.
During the measurements and processing of the different signals, a worst-case approach is used: the SA is used in “maximum-hold mode” (max-hold) with the root-mean-square (RMS) detector, that is, maximum values are retained for each component for about 5 s to 1 min until the signal stabilizes. Max-hold measurements of all present signals are executed for about 30 min at each location, depending on the number of frequency bands to be measured. The measurement probe is positioned at 1.5 m above the ground [CENELEC, 2008]. The settings of the SA for the different RF signals are specified by Joseph et al. [2012].
LTE Analyzer Measurements
The LTE analyzer measurements are executed to develop an extrapolation method for LTE exposure to determine the worst-case exposure from instantaneously measured exposure values.
Measurement equipment
Two measurement setups were used. The first setup consisted of an LTE analyzer with Romes software (TSMW, Rohde and Schwarz; sensitivity of −123 dBm). This analyzer is used to scan the different cell IDs present and the corresponding power of the reference signal (PRS). This equipment is denoted here as “LTE analyzer I.”
In the second setup, a SA with EUTRA/LTE downlink software is used (FSV, Rohde and Schwarz; sensitivity of −80 dBm). This analyzer is used to measure common channel powers of reference and synchronization signals. This equipment is denoted here as “LTE analyzer II.”
Both analyzers are used in combination with tri-axial Rohde and Schwarz TS-EMF isotropic antennas (dynamic range of 2.5 mV/m–200 V/m for the frequency range of 2–6 GHz) to analyze the LTE signals present. The two types of analyzers are used to ensure correct measurements of the LTE channels and enable a comparison.
LTE signals and measurement procedure
LTE uses Orthogonal Frequency-Division Multiple Access (OFDMA) for downlink communication [3GPP, 2011]. The physical layer of LTE consists of physical channels and physical signals. The primary synchronization signals (P-SYNC), S-SYNC, RS, and Physical Broadcast Channel (PBCH) signals are transmitted continuously at a constant output power, independent of the traffic load [3GPP, 2011]. Regardless of the LTE bandwidth, the P-SYNC, S-SYNC, and PBCH use a fixed number of subcarriers around the center frequency of the LTE channel. P-SYNC and S-SYNC use 62 subcarriers within 72 reserved subcarriers around the central frequency. PBCH uses 72 subcarriers around the central frequency. The RS is carried on specific pre-defined resource elements (the smallest modulation structure in LTE is one subcarrier in the frequency domain, and one symbol in the time domain [3GPP, 2009, 2011]). The RS power measurement is defined as the linear average over the power contributions of the resource elements that carry cell-specific RSs. The RS power is also denoted as the energy per resource element (EPRE).
LTE analyzer I, in combination with the Romes software, scans the present LTE cells and measures their specific characteristics (e.g., RS power, S-SYNC power). The analyzer continuously measures the different LTE signals present with a measurement rate of 10 cycles per second. For each orthogonal field component, the measurements are logged during 5 min. To process the data, the averaged values of each measurement cycle are used.
The LTE analyzer II is used to measure the averaged time-domain frame power (channel power), the RS power, and the S-SYNC power of the LTE cell with the highest power. Furthermore, the analyzer can give an allocation summary of the physical signals and channels of each subframe. For each orthogonal field component, the results are logged during 1 min with a measurement rate of about 1 Hz. To process the data, the averaged values of all the measurement cycles are used.
Extrapolation to worst-case value
(1)
(2)Data Analysis
We consider in this study as exposure metrics the electric field strength E (V/m) of an RF signal, the total electric field Etot (V/m) of all RF signals present, and the power density S (W/m2). Furthermore, exposure ratios (ER) and ERS and average (AC) and maximal (MC) contributions are defined.
(3)
(4)
(5)RESULTS
Results of Short-Term Measurements
Spectral overview
Figure 1 shows the electromagnetic spectrum from 80 MHz to 3 GHz measured at an outdoor position, LOS with GSM, UMTS, and LTE antennas. The signals that are significantly present in the frequency range from 80 MHz to 3 GHz are FM, Terrestrial Digital Audio Broadcasting (T-DAB), Terrestrial Trunked Radio (TETRA), GSM at 900 MHz (GSM900), GSM1800, UMTS–High Speed Packet Access (UMTS-HSPA), and LTE. Based on the signals present in the spectral overview, narrowband measurements are executed at 40 locations.
Spectral overview in the frequency range 80 MHz–3 GHz.
Table 1 lists the wireless signals that are detected at the measurement locations. In this table, the corresponding frequency ranges and reference values according to the ICNIRP guidelines for general public exposure are also listed [ICNIRP, 1998]. Only the downlink frequency bands are measured. Table 1 also provides the sensitivities of the measurement equipment.
| RF signal | Frequency band (MHz) | Reference value, Eref (V/m) | Sensitivity (V/m) |
|---|---|---|---|
| FM | 100 | 28.0 | 0.014 |
| T-DAB | 174–230 | 28.0 | 0.009 |
| TETRA | 390 | 28.0 | 0.002 |
| GSM900 | 900 | 41.3 | 0.002 |
| GSM1800 | 1800 | 58.3 | 0.004 |
| UMTS-HSPA | 2100 | 61.0 | 0.006 |
| LTE | 2600 | 61.0 | 0.005 |
- FM, frequency modulation; T-DAB, Terrestrial-Digital Audio Broadcasting; TETRA, terrestrial trunked radio; GSM, Global System for Mobile Communications; UMTS, Universal Mobile Telecommunications System; HSPA, High Speed Packet Access; LTE, long-term evolution.
Exposure values
During the short-term measurements the LTE SFN network radiated at maximal power (100%) and no, or only limited, traffic (test network) was present. Table 2 lists the minimal (Emin), maximal (Emax), and average (Eavg) measured electric field strengths, and the maximal (Smax) and average (Savg) power density values for each RF signal. Based on the maximal electric field and power density values, the ER and ERS, respectively, are determined. “Total all” in Table 2 represents the total exposure of all measured RF signals, while “total GSM/UMTS-HSPA/LTE” corresponds with the total exposure due to mobile telecommunication signals. These total values are provided to compare the influence of mobile telecommunications on the exposures. Furthermore, for each signal the percentage of the number of locations (n) where a certain signal is above the sensitivity (which is considered as a present signal) of the measurement system is mentioned.
| RF signal | n (%) | Variation 40 meas. locations | Eavg (V/m) | Smax (mW/m2) | Savg (mW/m2) | ER (%) | ERS (%) | |
|---|---|---|---|---|---|---|---|---|
| Emin (V/m) | Emax (V/m) | |||||||
| FM | 100.0 | 0.055 | 3.49 | 0.59 | 32.26 | 2.35 | 12.45 | 1.55 |
| T-DAB | 100.0 | 0.028 | 0.42 | 0.08 | 0.47 | 0.03 | 1.50 | 0.02 |
| TETRA | 100.0 | 0.004 | 0.59 | 0.07 | 0.93 | 0.05 | 2.11 | 0.04 |
| GSM900 | 100.0 | 0.033 | 2.80 | 0.32 | 20.81 | 1.12 | 6.79 | 0.46 |
| GSM1800 | 100.0 | 0.045 | 3.12 | 0.36 | 25.83 | 1.15 | 5.35 | 0.29 |
| UMTS-HSPA | 75.0 | 0.026 | 1.03 | 0.24 | 2.82 | 0.34 | 1.69 | 0.03 |
| LTE | 32.5 | 0.035 | 0.47 | 0.15 | 0.59 | 0.10 | 0.77 | 0.01 |
| Total all | — | 0.094 | 4.46 | 0.93 | 52.86 | 4.98 | — | — |
| Total GSM/UMTS-HSPA/LTE | — | 0.058 | 4.24 | 0.56 | 47.70 | 2.55 | — | — |
- Exposure ratio (ER) = maximal field value/ICNIRP reference level; Total all: total exposure of all measured RF signals; Total GSM/UMTS-HSPA/LTE: total exposure due to mobile telecommunication signals.
All measured electric field values (Table 2) in Reading satisfy the ICNIRP guidelines [ICNIRP, 1998]. The maximal total value Etot equals 4.5 V/m (52.9 mW/m2) and is mainly due to the GSM signals (2.8 V/m for GSM900 and 3.1 V/m for GSM1800). Total exposure varies between 0.09 and 4.5 V/m. When considering the individual RF signals, the highest maximal and averaged electric field value is obtained for the FM signal, and equals 3.5 and 0.6 V/m, respectively. The lowest average electric field values are measured for T-DAB and TETRA (<0.1 V/m).
Concerning the cumulative exposure due to wireless telecommunication systems (GSM900, GSM1800, UMTS-HSPA, and LTE), electric field strengths ranged from 0.06 to 4.2 V/m. The highest maximal and averaged value is obtained for GSM1800 and equals 3.1 and 0.4 V/m, respectively. For GSM900 an average value of 0.3 V/m is obtained.
In contrast to GSM (n = 100%), UMTS-HSPA (n = 75% above a sensitivity of 0.006 V/m; Table 1), and LTE (n = 32.5% above a sensitivity of 0.005 V/m; Table 1) are not measurable at each measurement location. Not measurable means that the values are below the sensitivity of the SA setup with the tri-axial probe. Lower electric field values are measured for UMTS-HSPA (0.2 V/m, on average) than for GSM. This difference might be explained by less coverage and use of UMTS-HSPA technology by the general public. Highest exposure values due to LTE are obtained at locations LOS with LTE BSs, with a maximum of 0.5 V/m and an average of 0.2 V/m for LTE.
In Joseph et al. [2010], narrowband measurements were performed in Stockholm, Sweden. Unlike Reading, a commercial LTE network is deployed in Stockholm. Similar electric field strengths of 0.8 V/m maximally and 0.2 V/m on average, were obtained for LTE in Stockholm. Bornkessel [2011] found median extrapolated exposure values of 0.2 V/m (corresponding to 0.4% of the ICNIRP reference levels). The highest extrapolated exposure was found to be 7.5 V/m. On average, our measured exposure values agree well with the ones obtained in the other studies. Additional supporting information about the exposure values can be found in the online version of this article.
Exposure ratios
Exposure ratios (%) are used to compare the maximal measured electric field strengths and power density values with the ICNIRP reference levels [ICNIRP, 1998]. The ER are listed in Table 2.
The ER vary from 0.8% (LTE) to 12.5% (FM) for the 40 measurement locations, or from 0.01% (LTE) to 1.6% (FM) when considering ERS values. The highest ER occurs for FM, followed by GSM (900 and 1800), TETRA, UMTS-HSPA, T-DAB, and LTE. In Stockholm, a higher ER (1.2%; commercial network) is obtained for LTE than in Reading (0.8%) [Joseph et al., 2010].
Exposure contributions
Table 3 lists the AC and MC of the different RF signals measured in Reading. The highest contributions are obtained for FM and equal 56.3%, on average (maximum 99.3%). The AC is 29.3% for GSM (900 and 1800) and 6.8% for UMTS-HSPA. At positions that are LOS with a telecommunication BS (GSM, UMTS and/or LTE), higher contributions for GSM, UMTS-HSPA, and LTE are obtained. The AC of the GSM signals (900 and 1800) is higher than the contributions of UMTS-HSPA and LTE. The AC of the LTE signal equals 0.4%; the maximal LTE contribution equals 4.9%. In Stockholm, higher contributions of LTE to the total electric field values were measured (commercial network) and equal 4.1% on average, and a maximum of 23.2% [Joseph et al., 2010]. At each measurement location where the T-DAB and TETRA signals are measured, these signals contribute an average of 7.3% to the total value.
| RF signal | AC (%) | MC (%) |
|---|---|---|
| FM | 56.3 | 99.3 |
| T-DAB | 6.3 | 58.5 |
| TETRA | 0.9 | 5.4 |
| GSM900 | 11.9 | 71.8 |
| GSM1800 | 17.4 | 87.1 |
| UMTS-HSPA | 6.8 | 56.4 |
| LTE | 0.4 | 4.9 |
Results of LTE Analyzer Measurements
Influence of power
The influence of the power of the LTE BS on the in situ RS and S-SYNC signals is investigated with the two setups described previously (LTE analyzers I and II). During these tests no data load is present on the investigated LTE cells. The output power is set to 50% and 100% of the maximal total output power of the measured cell.
Table 4 lists the deviations Δ (dB and %) of the electric field values of RS (ERS) and S-SYNC (ES-SYNC) signals for the different power settings (50% and 100%) at two locations. The deviations are lower than 1 dB, showing that the in situ RS and S-SYNC signals are transmitted continuously at a constant level that is not related to the output power of the BS, and thus can be used for the extrapolation method of Equation (1).
| Position | Output power | ERS (dB µV/m) | ES-SYNC (dB µV/m) | ERS (mV/m) | ES-SYNC (mV/m) |
|---|---|---|---|---|---|
| 3 | 50% | 78.1 | 95.7 | 8.0 | 61.0 |
| 100% | 78.3 | 96.1 | 8.3 | 63.6 | |
| Δ (dB) | 0.2 | 0.4 | — | — | |
| Δ (%) | — | — | 2.8 | 4.4 | |
| 36 | 50% | 58.9 | 76.8 | 0.9 | 6.9 |
| 100% | 58.3 | 76 | 0.8 | 6.3 | |
| Δ (dB) | 0.6 | 0.8 | — | — | |
| Δ (%) | — | — | 7.1 | 9.6 |
Influence of traffic
The influence of traffic of the LTE BS on the RS and S-SYNC signals is investigated. The measurements are performed LOS with an LTE site. During the measurements the power is set to the maximal total output value of the investigated LTE cell. By downloading a data file on user equipment, traffic is created on the LTE signal. A defined number of resource blocks are allocated for this user to set a specific data load level. The download test (20 MHz channel) is performed for 0% (0 resource blocks in use), 25% (24 resource blocks in use), 50% (48 resource blocks in use), 75% (72 resource blocks in use), and 100% (100 resource blocks in use) of the maximal data load for the cell.
Figure 2 shows the electric field of the entire LTE channel for different traffic loads as a function of the frequency, measured with the SA (max-hold with a resolution bandwidth (RBW) of 1 MHz and a sweep time (SWT) of 20 s [Joseph et al., 2010]). The increasing amount of traffic load can clearly be seen in this figure. The increase in the signal level is proportional to the traffic load percentage. The used bandwidth (also indicated in Fig. 2) agrees excellently with the theoretically calculated transmission bandwidth used by the data load (Table 5); the maximal deviation is 0.66 MHz.
Electric field of the LTE signal as a function of the frequency measured with the spectrum analyzer (RBW 1 MHz, SWT 20 s, RMS detector) for different traffic percentages.
| Number of resource blocks in use for data load | Theoretically calculated bandwidth (MHz) | Measured bandwidth (MHz) |
|---|---|---|
| 0 | 0 | 0 |
| 24 | 4.3 | 4.6 |
| 48 | 8.6 | 9.3 |
| 72 | 13.0 | 13.5 |
| 100 | 18.0 | 18.6 |
Table 6 lists the electric field values measured for the RS, S-SYNC signals, and the entire LTE channel (“channel”) during the different traffic loads. The deviation Δ between the maximal and minimum value for the different traffic loads is also calculated for each signal, and is shown in the table.
| % Traffic (100% power) | ERS (dB µV/m) | ES-SYNC (dB µV/m) | Channel: E (dB µV/m) |
|---|---|---|---|
| 0% | 78.3 | 96.1 | 102.7 |
| 25% (24 RB) | 78.3 | 96.0 | 106.5 |
| 50% (48 RB) | 78.4 | 96.0 | 108.3 |
| 75% (72 RB) | 78.1 | 95.9 | 109.3 |
| 100% (100 RB) | 77.9 | 95.6 | 110.4 |
| Δ (Emax − Emin) (dB) | 0.5 | 0.5 | 7.7 |
| % Traffic (100% power) | ERS (mV/m) | ES-SYNC (mV/m) | Channel: E (mV/m) |
|---|---|---|---|
| 0% | 8.2 | 63.8 | 136.7 |
| 25% (24 RB) | 8.2 | 63.1 | 211.3 |
| 50% (48 RB) | 8.3 | 63.1 | 260.7 |
| 75% (72 RB) | 8.0 | 62.4 | 292.1 |
| 100% (100 RB) | 7.9 | 60.3 | 330.0 |
| Δ (%) | 5.9 | 5.9 | 141.5 |
Table 6 shows that, on average, ERS = 78.2 dB µV/m (0.008 V/m) and ES-SYNC = 95.9 dB µV/m (0.062 V/m) at the considered LOS position (see also the theoretical difference of 17.9 dB (a factor of 61.7) between RS and S-SYNC signal levels). For all traffic percentages, the deviations between maximal and minimal measured values are lower than 0.5 dB. This shows that the RS and S-SYNC signals are emitted at a constant power that is not related to the traffic load of the BS. Thus, both the in situ RS and S-SYNC signals can be used to apply the method of Equation (1).
The electric fields for the whole LTE channel increase with an increasing traffic load from 102.7 dB µV/m or 0.14 V/m (0% traffic) to 110.4 dB µV/m or 0.33 V/m (100% traffic), resulting in a maximal deviation Δ of 7.7 dB between 0% and 100% traffic (LTE analyzer II). We can conclude from Table 6 that the RS and S-SYNC signals measured in situ are constant, irrespective of the traffic load, and that the total LTE channel signal level increases up to 7.7 dB for increasing traffic percentages.
Validation of the extrapolation method
In this section, the extrapolation method for LTE signals to assess the worst-case value based on the measurement of the RS signal (Eq. 1) is validated in situ at one selected location. The measurements are executed at a position that is LOS and in the main lobe direction of the BS sector. At this position, the LTE signal originating from a specific sector is dominant and contributions from other LTE cells are negligible. The LTE field levels are well above the limited sensitivity of LTE analyzer II. The validation is performed as follows. Firstly, the RS signal is measured with LTE analyzer I and extrapolated to the maximal exposure level Emax [Eq. 1; extrapolation factor of 30.79 dB (a factor of 1999.5)]. Secondly, the actual electric field 
of the total LTE channel is measured for 100% traffic load and 100% output power with the LTE analyzer II. Finally, the extrapolated field Emax and the actual maximal field 
are compared.
The actual measured maximal field 
at this location equals 110.4 dB µV/m (0.3 V/m) while the extrapolated value is 109.0 dB µV/m (0.3 V/m) resulting in a deviation of 1.4 dB. Thus, good agreement is obtained between these worst-case values. This deviation is considered to be low because the values are not measured at the same time, resulting in environmental variations due, for example, to car traffic, and not at exactly the same location because different equipment is used, etc.
LTE measurement results for all 40 locations
With LTE analyzer I the LTE signal is measured at 40 locations in Reading. The power of the LTE BSs is set to its maximum (100%). At each location, the worst-case electric field value is calculated for all detected cell IDs by applying the extrapolation method of Equation (1), using the measured RS signal and the extrapolation factor of 30.79 dB.
Table 7 summarizes the extrapolated electric field values (Emax in V/m and dB µV/m) of the LTE signal for all locations. The resulting maximal power density Smax and exposure quotient (EQ) in %, that is, 100(Smax)/LS, LS = ICNIRP reference level for power density [IEC, 2010] are also listed. The highest electric fields are obtained at outdoor locations in LOS with LTE BSs. The maximal value is measured in LOS and in the direction of the main lobe of an LTE BS and equals 1.9 V/m (32 times below the ICNIRP level or ER = 3.2%) or a power density of 9.9 mW/m2 (994 times below the ICNIRP level or ERS = 0.1%). On average, the extrapolated electric field value equals 0.15 V/m and the power density equals 0.06 mW/m2.
| LTE extrapolated values, Emax | Short-term measurements | |||||
|---|---|---|---|---|---|---|
| Emax (dB µV/m) | Emax (V/m) | Smax (mW/m2) | EQ (%) | ESA (dB µV/m) | ESA (V/m) | |
| Variation | 53.5–125.7 | 0.001–1.9 | 5.9 × 10−7–9.9 | 0.00–0.11 | <90.8–113.49 | <0.035–0.47 |
| Average | 103.72 | 0.15 | 0.06 | 6.3 × 10−4 | 104.0a | 0.15a |
- EQ = 100(Smax)/LS; LS = ICNIRP reference level for power density.
- a Average LTE values of SA cannot be compared with LTE extrapolated values because of the worse sensitivity of the SA (13 locations above detection limit) in comparison with the LTE analyzer (measurements at all locations above detection limit).
Table 7 also summarizes the results of short-term measurements of the LTE signal measured with the SA (ESA). The maximal value is obtained at the same location as for the extrapolation and equals 0.5 V/m. For all locations, the extrapolated values are higher than the values obtained by the short-term measurements due to a low actual traffic load in comparison to the worst-case estimate of the extrapolation method with 100% traffic. Bornkessel [2011] found median extrapolated exposure values of 0.2 V/m, which agrees very well with our results (0.15 V/m; Table 7). The highest extrapolated exposure was found to be 7.5 V/m (12.3% of the ICNIRP field strength reference values), while 1.9 V/m was obtained as the maximal value. This value is lower because we did not search specifically for the highest LTE exposures but performed the measurements at randomly selected locations. Moreover, Bornkessel [2011] studied both 800 MHz and 2.6 GHz transmitters while the SFN considered here operates at 2.6 GHz. Additional supporting information about the extrapolated values can be found in the online version of this article.
DISCUSSION AND CONCLUSIONS
In situ (LTE) exposure of the general public in Reading, UK, is characterized and an extrapolation method to estimate worst-case LTE exposure is proposed.
All electric field levels satisfy the ICNIRP reference levels with a maximal total electric field value of 4.5 V/m. The total values are dominated by the FM signal (at 55.0% of the measurement locations). The maximal electric field value for FM equals 3.5 V/m. Exposure levels for LTE of 0.2 V/m on average, and 0.5 V/m maximally are obtained. Contributions of LTE to the exposure are limited to 0.4%, on average. ER from 0.8% (LTE) to 12.5% (FM) are obtained.
An extrapolation method is proposed and validated to assess the worst-case LTE exposure value including MIMO. For this method, the RS is measured and extrapolated to the worst-case value with an extrapolation factor, similar to the method for BCCH for GSM and CPICH for UMTS. The influence of the traffic load and output power of the BS on in situ RS and S-SYNC signals are lower than 1 dB for all power and traffic load settings, showing that these signals can be used for the extrapolation method. The maximal extrapolated field value for LTE equals 1.9 V/m, which is 32 times below the ICNIRP reference levels for electric fields (3.2%).
Narrowband measurements at more locations and other environments will be part of future research using the proposed methodology once LTE networks are deployed elsewhere. Alternative extrapolation methods based on the PBCH signals can be considered in the future. Future research will also consist of the investigation of the temporal behavior of LTE signals and the influence of usage traffic.
Acknowledgements
W. Joseph is a Post-Doctoral Fellow of the FWO-V (Research Foundation–Flanders). The authors wish to thank Vodaphone and Ericsson for their cooperation.
