Report on: Measurement of Cellular Base-station Emissions Using a Newly Developed RF Field Mapping System*

Measurement System Design

A conceptual block diagram of the instrument is shown in Figure 1. It makes use of omni-directional antennae to convert the incident power density to a readily measured signal. The signals in the two cellular frequency bands are fed to two tuned receivers, which ensure that only the desired frequencies are processed, and which perform the measurement of signal power. The measured signal power levels in the two bands, which are proportional to power density, are then converted to a digital format which is sent to a computer for storage.

The other part of the system is a commercial Global Positioning Satellite (GPS) receiver which is used to detect the location of the measurement point and send it to the computer for storage. Thus the computer stores three essential pieces of information; the geographic position of the measurement point and the power density in both cellular frequency bands. Because of the design of the receivers, the sum of all individual signal carriers in each band are measured simultaneously rather than one at a time. This gives essentially a snapshot of the exposure conditions in an instant and allows the envelope of the composite signal, consisting of multiple carriers, to be viewed in time.

Figure 1: Block diagram of the GLOBE system.

For the purpose of brevity, the instrument system has been given the acronym GLOBE for "Geographically Located Observations of Base-station Emissions". The frequency bands monitored by the system are the Analog cellular band and the Personal Communications Services (PCS) cellular band. It should be mentioned that the term "Analog" is used in this report for traditional reasons since originally, only analog mobile services were offered in this band. Currently, digital mobile services are offered in the same band while the higher PCS band is used only for digital mobile services. The frequency assignments for the two bands are given in Table 1.

Table 1:

Transmit frequencies used by cellular systems in Canada. (Note: the transmit frequency used by one terminal of the system is the receive frequency of the other terminal.)
	Analog Band	PCS Band
Mobile Handset Transmit	824 - 849 MHz	1850 - 1895 MHz
Base-station Transmit	869 - 894 MHz	1930 - 1975 MHz

In the version of the GLOBE system used for this report, the two frequency bands each have their own antenna and receiver. The output DC voltages of the receivers, which are proportional to the logarithm of the incident power density at the antennae, are connected to the inputs of a multiplexed analog-to-digital converter (ADC). The multiplex operation is carried out by a relay, which is controlled by the computer to select the desired measurement channel. The 8-bit ADC then converts the signal and makes the output available on a DB25 connector. A standard parallel cable is used to connect the ADC output to the laptop computer.

Receiver Design

The block diagram of the Analog-band receiver is shown in Figure 2(a). It consists of a chain of amplifiers, filters and a logarithmic amplifier/detector. The bandpass filter is utilized to define the reception bandwidth of the receiver. To further attenuate possible PCS-band (1900 MHz) signals that may be present with the Analog-band ones, a band-stop filter is added at the end of the chain. Detection of the signal is performed by the logarithmic amplifier/detector, which provides a DC voltage, which is proportional to the logarithm of the incident power density. A hard-wired switch is used to route the RF signal either to the log-amp/detector or a test port (coaxial connector) if linear gain measurements are required.

Figure 2(a): Block diagram of Analog-band receiver.

The signal gain of the Analog-band receiver is approximately 27 dB across the Analog cellular frequency band. The signal attenuation over the PCS base-station transmit band of frequencies (1930 - 1975 MHz) is 38 dB, giving the receiver a 65 dB selectivity in terms of Analog to PCS frequencies. This selectivity allows an input PCS power level of up to -9 dBm before the detection threshold is reached. Further selectivity is obtained from the monopole antenna that is tuned to give a good impedance match to the receiver at Analog frequencies but a poor match at PCS frequencies.

The Analog-band receiver has a power density measurement range of 1.4 x 10^-4 W/m² to 2.8 x 10^-9 W/m². When normalized to the Health Canada Safety Code 6 [Health Canada, 1999] limit of 5.9 W/m ² for the Analog band, this measurement rage extends from 40 thousand times below to 2 billion times below the SC 6 limit. Since this range is too sensitive for measurements near cellular base stations, the receiver is operated with a 20 decibel (dB) fixed attenuator between the antenna and the first amplifier which shifts the measurement range from 400 times below to 20 million times below the SC6 limit level.

The block diagram of the PCS-band receiver is given in Figure 2(b). The receiver has a different arrangement of the filters than the Analog-band one in order to pr event harmonics of analog-band signals from producing false outputs. This is especially important in situations where a weak PCS-band signal is being measured in the presence of strong Analog-band signals. Tests showed that signal levels, in the Analog band, of up to -9.0 dBm produced no detectable output with this arrangement. The selectivity of the PCS-band receiver to Analog-band signals was measured to be 76 dB.

Figure 2(b): Block diagram of PCS-band receiver.

The PCS-band receiver has a power density measurement range of 5 x 10-3 W/m² to 1 x 10^-7 W/m². When normalized to the Safety Code 6 limit of 10 W/m² for the PCS band, this measurement rage extends from 2 thousand times below to 100 million times below the SC 6 limit. From experience with measurements of PCS-band power densities, it was found that this range is not too sensitive so that additional attenuation of the signal was unnecessary.

Software

The controlling software for the GLOBE system was written in QuickBasic using a number of subroutines to read the latitude and longitude coordinates from the GPS receiver, retrieve the data from the multiplexer/converter and select the band of measurement by sending the appropriate signal to the multiplexer/converter. The subroutines are flexible and allow a number of functions to be performed with minimal programming expense.

The system is economical in terms of computer resources since it makes use of a single RS232 or serial port for communications with the GPS receiver and a single LPT or parallel port for communications with the mulitiplexer/converter. QuickBasic is a DOS application requiring very little memory; thus the system can be configured to operate with old or obsolete laptop computers. Measurement data is stored as a text file, which can be used by spreadsheets for later analysis and display.

Antennae

Both bands made use of a cylindrical monopole antenna mounted on a ground plane. The length of the monopole is approximately one -quarter wavelength long, at midband, while the square ground plane is at least one wavelength on each side (14" for the Analog band and 10" for the PCS band). This arrangement allowed the receiver electronic modules to be mounted close to the antenna connector on the underside of the ground plane as illustrated in Figure 3.

Figure 3: Photographs of PCS-band antenna and receiver electronics module.

The reception patterns of both antennae were measured to be roughly donut shaped, omni-directional patterns in the horizontal plane. The half-power beamwidths in the vertical plane extended from 4 degrees above the horizon to 70 degrees above the horizon. This type of pattern has a null directly over the axis of the cylindrical monopole (90 degrees). Thus, this type of receiving antenna is not well suited for measurements directly under a base station antenna where the angle of incidence is greater than 70 degrees.

For the monopoles used in this version of the GLOBE, additional measurement errors are incurred for reception angles outside the 4 to 70 degree range. For best accuracy, the ideal distances from the base station are from 15 m to 600 m for a 40 m high base station and 36 m to 1.4 km for a 100 m high base station. Later versions of the GLOBE have alleviated this problem by making use of two different types of antennae, one similar to a monopole in its reception pattern and one which is able to receive signals from directly overhead. The combination of the two gives almost hemispherical coverage for the GLOBE instrument.