Frequently asked questions
Why are two measurement types reported?
In the design of this survey an attempt has been made to account for the fact that the measurements are a snapshot of the RF EME present at an instant in time. It is generally expected that base station antennas will emit differing levels of RF EME at different times (of the day or week). Two measurement types are reported: the maximum level of RF EME from this base station and the highest level of RF EME actually measured.
What is the reported maximum level of RF EME from this base station?
This value can be thought of as the maximum level of RF EME present at the measurement location due to this particular base station if it were handling as many calls as it possibly can. In order to determine this figure measurements are made of the control channels and pilot signals from this base station. These measurements are then scaled up (extrapolated) to the maximum operating capacity of the base station using knowledge of the equipment installed at the base station.
What is the reported highest level of RF EME actually measured?
The highest actually measured values include the stable control channel or pilot signals as well as the variable traffic signals present at the time of the measurements. They can be thought of as typical levels of RF EME present at a location coming from all nearby base stations (some could be a kilometre or more away). These are 'real' measurements in the sense that they record what was actually detected at the time of the measurement. However, they may not represent the greatest RF EME level that could be emitted by this base station.
Does the RF EME from a base station vary?
Mobile phone networks are designed to work at less than full capacity most of the time. For the network operators it is a matter of finding a balance between deploying expensive infrastructure that may lie mostly idle and satisfying customer demand at peak usage times which may occur infrequently. Even when there are no mobile phone users accessing the services of a particular base station, that base station is transmitting a certain level of RF EME. In simple terms it may be thought of as a beacon calling out to find any mobile phones in the vicinity wishing to use its services. This signalling information provides a relatively constant background (detected variations in these signals are largely due to scattering and interference effects once the signal has left the antenna).
What are control channel and pilot signals?
For the mobile phone networks to function, every base station must advertise its presence to nearby mobile phones whilst at the same time not interfering with the signals coming from other base stations (of either the same or a competing network operator). This is achieved in part through the use of control channels (in GSM) and pilot signals (for spread-spectrum technologies like UMTS and WCDMA). These values are independent of the number of calls being made at the time of the measurements. They can be considered to be the minimum level of RF EME present at the location if no calls were being made via this base station.
What are extrapolated measurements?
The full power (or extrapolated) value is calculated by multiplying the measured control channel and pilot signal values by a factor to give the maximum possible output of that base station. This factor is dependent upon the equipment installed at the base station and is generally somewhere between four and ten. It also often varies for each of the different services installed on a single base station, so it may be ten for one operator’s UMTS service and four for another operator’s GSM service. The resulting full power value can be thought of as the maximum level of RF EME present at the location when this base station is handling as many calls as it possibly can.
Why do measurements sometimes exceed predictions?
In some cases, the measurement of demodulated UMTS (or WCDMA) pilot signals was not performed. Extrapolation was done assuming that only the pilot signal was present when in fact a mixture of pilot and traffic signals contributed to the measured value. This assumption leads to over-estimates of the actual RF EME levels, typically by a factor of 2 to 5.
There are currently five surveyed base stations where the extrapolated maximum RF EME level exceeds the prediction from the environmental EME report. For four of these sites (Bulleen East, Maidstone, Palmerston and Lower Sandy Bay) the UMTS (or WCDMA) pilot signals were not demodulated, rather the extrapolation was done assuming that only the pilot signal was present. The fifth site is Drummoyne, here the pilot signals were demodulated and the discrepancy is more difficult to explain. At Bulleen East, Maidstone, Drummoyne and Lower Sandy Bay the differences between the extrapolated and calculated values are well within the error bounds on the measurement.
What is the significance of the UMTS (W-CDMA) pilot signal?
Demodulation of the UMTS (or WCDMA) pilot signals is the process whereby the device measuring the RF EME is able to distinguish between the components used for transmitting traffic (phone calls) from the signalling information used by the network to establish and maintain connections between the base station and a subscriber’s phone (pilot signal). This requires specialist equipment that is not commonly employed by all NATA accredited RF measurement consultants. In the design of the ARPANSA base station RF survey it was decided that although desirable, it was not necessary for the RF measurement consultants to demodulate the UMTS (or WCDMA) signals, since the non-demodulated signals will always be greater and thus give an over-estimate of the maximum output capability of the base station. Another consequence is that sometimes the reported maximum level of RF EME from a base station may exceed the predicted maximum values.
Are the predicted levels accurate?
The physics and mathematics used to describe the propagation of RF EME is well established. It is possible to perform calculations that match measurements very accurately under ideal circumstances. For instance, in an open environment on level ground, with good line of sight to the antenna and when all required input parameters are known precisely. However, it is almost never the case in real world situations encountered in this survey. Measured levels of RF EME can be influenced by objects in the environment (buildings, vehicles, vegetation) as well as the topography of the surroundings (undulating ground between antenna and detector).
In order to make the large number of calculations feasible the predictions used in environmental EME reports make a number of simplifying assumptions:
- base station antennas operate without losses (i.e. all power out of the RF amplifier reaches the antenna input port),
- there are no obstructions (such as buildings, vehicles, vegetation) in the vicinity of the base station
- base stations operate at maximum capacity all the time
- the ground surrounding the base station is flat
These assumptions are generally expected to result in predicted RF EME levels that are higher than reality. Although in some situations this is not the case.