Source power and radiation pattern variability
In telecommunication systems, transmitted power may vary in accordance with the usage of channel (channel allocation) and power requirements in certain atmospheric conditions. Some channels are turned on or off as per need basis in dynamic channel allocation. Variation in channel occupancy (where amount of data transmitted over a channel varies; even if no data is transmitted, channel carrier remains) also alters the amount of transmitted power.
To compensate for adverse propagation conditions, the base station uses automatic power control that results in variation of transmitted power. Sometimes, advanced telecom systems use active antennae that can dynamically change their radiation patterns. These variabilities impose a greater challenge for measurements, since the exact state of the transmitter at the time of measurement may not be known.
Assessment of radiation levels
Assessment of exposure levels can be done either by measurement, by numerical calculations or by EM software simulations. All these methods have almost similar levels of uncertainty and accuracy depending on the method, equipment or software used.
Assessment by measurement
An isotropic field probe may be used for broadband measurement of electric field intensity (V/m) or power density (watt/sqm). For a normally compliant radio-communication site, overall measured value of E field or power density with broadband measurement should be within 10 per cent of the reference levels prescribed by ICNIRP for occupational/general public. With broadband measurement, the overall picture of radiation is obtained, but it is hard to get information about contributions to overall radiation level made by individual sources such as GSM, UMTS, CDMA, WiMAX or any other radio-communication system (AM/FM/TV).
A typical test setup for E field measurement is shown in Fig. 5. Some broadband antennae like bi-conical, log-periodic or ridge-guide may be used for measurement in the desired radio-communication frequency band (GSM 900/1800, HSDPA, WiMAX, etc). In such type of antennae, it is required to align the antenna in the plane of incident E field and, thus, the knowledge of polarisation of the radiation source is required.
Another type of antenna/probe that is increasingly being favoured for use with the spectrum analyser is the tri-axial isotropic antenna/probe. This is made by three independent broadband sensing elements placed orthogonally to each other. It simplifies the measurement procedure that ideally determines RMS field strength independent of direction and polarisation of the radiation source, since it affords measurement of E field components along all three axes (X, Y and Z).
Measured value of Ex, Ey and Ez, the electrical field strengths corresponding to each axis of the tri-axial isotropic probe, are first corrected for their antennae factors (for each axis) as:
where E is the electric field strength in volts/metre, V is voltage received by the spectrum analyser and AF is the antenna factor.
Unit of AF is 1/metre. Antenna factor is antenna specific and normally supplied by the manufacturer. Now, the resultant E field strength is added geometrically (root sum of squares).
Frequency selective measurement
As described above, broadband measurement facilitates measurement of the combined effect of all radiating sources. To measure individual response of each and every radiating source, very high sensitive frequency selective intelligent instruments are required. To measure the effect of any particular radiation source, all channels of the source should be transmitting at full power. For example, in the case of GSM, measurement is to be done when all voice channels are running at full load (all control channels are continuously radiating at full power).
Here, the measuring instrument should be operated in the matching bandwidth range and, thus, E field strength is measured for all broadcast control channels (BCCH).
In similar fashion, field calculations can be done for other services like CDMA and WiMAX. Once all peak E field levels are available for all operators running different services, their ratios are calculated with respect to individual field quantity (ELimit) limits (depending on frequency of operation). For compliance, RMS of these ratios should be less than one.
Assessment of exposure levels by measurement is advantageous in many ways. In this case, effect of all radiating sources is accessed with real parameters and real environment factors including obstacles, reflection from metallic objects and high-rise buildings. Further, little knowledge of radiating source is required.
On the other side, it is difficult to confirm and check that all sources are radiating at full load. Moreover, there is no antenna/probe covering the whole frequency band. Measurement is not possible for radiating sources that do not exist yet. Effect of the presence of staff and equipment on the field distribution has to be avoided. Some sort of post-processing is required because of different limits for different operating frequencies.