A.4 Reverberation chamber constraints

37.5443GPPConformance testingRelease 16TSUniversal Terrestrial Radio Access (UTRA) and Evolved UTRA (E-UTRA)User Equipment (UE) Over The Air (OTA) performance

A.4.1 General

The main objective of this section is to define basic parameters of the reverberation chambers suited for the Tx and Rx measurement of UMTS, and E-UTRA UEs.

The reverberation chambers have for a couple of decades been used for some types of EMC measurements. A reverberation chamber is a metal cavity that is sufficiently large to support many resonant modes, and it is provided with means to stir the modes. The measurements in the reverberation chamber are fast and repeatable, provided the chambers utilize efficient stirring methods. The following sections describe how the reverberation chamber can be used for measurements of TRP and TRS.

A.4.2 Positioning and mode stirring facilities

  1. The reverberation chamber shall be equipped with mode-stirring facilities in such a way that enough number of independent power samples can be achieved for the accuracy requirement stated in this standard to be fulfilled. Possible mode-stirring methods include platform stirring, polarization stirring and mechanical stirring with fan-type stirrers, irregular shaped rotational stirrers, or plate-type stirrers. Also, frequency stirring is possible if the type of measurement allows for a frequency-averaged value, but this is not necessary if the chamber is sufficiently large and well stirred.
  2. For the DUT positioning on head phantom or for the DUT positioning on laptop ground plane phantom, the DUT should be placed in such a way that a sufficient number of independent samples are obtained to fulfil the uncertainty requirements. For example, for chambers utilizing platform stirring the DUT should be placed on the edge of the turntable in order to maximize rotation of the DUT. In addition, the DUT must not be closer than 0.5 wavelengths to other electromagnetic reflective objects inside the chamber and 0.7 wavelengths to absorbing objects.
  3. For the DUT positioning on laptop ground plane phantom the real functional laptop is laid on the floor of the chamber, supplies power to the DUT and controls the state of the DUT. Both the USB cable and the real functional laptop are properly setup in order to have a negligible impact on the measurements: the real functional laptop is fully wrapped up with anechoic absorbers. The real functional laptop can be placed outside the chamber if the connection to the DUT can be maintained and if the communication interface between DUT and real functional laptop has negligible impact on the measurements.
  4. A schematic picture of the measurement setup is provided in Figure A.4.2-1.
  6. Figure A.4.2-1: A schematic picture of the reverberation chamber measurement setup

A.4.3 Polarization imbalance and receiving antennas

It is important that the statistical distribution of waves in the chamber corresponds to the chosen test environment. Present knowledge about reverberation chambers limits this to isotropic environments, i.e. the TRP and TRS parameters can be measured. Since the probability of each polarization is equal in the isotropic environment, a check of the polarization imbalance in the reverberation chamber must be done.

The polarization imbalance can be obtained during the calibration measurement by measuring both when the calibration antenna is oriented for vertical polarization and when it is oriented for horizontal polarization. These two values shall differ by less than the specification in Table A.4.3-1. In order to obtain values for comparison with the results in the table, the reference levels shall be measured for both orientations of the calibration dipole at 8 different positions of the dipole inside the chamber. The average, standard deviation and maximum deviation shall be evaluated by comparing results for both polarizations over the whole set of 8 measurements. Alternatively, the levels for the two polarizations of the calibration antenna at 1 MHz intervals between 1900 MHz and 2200 MHz can be measured, and thereafter the average, standard deviation and maximum of the difference between the two sets of values over these frequency ranges are calculated.

An effective way of avoiding polarization imbalance is to use polarization stirring, i.e. using three orthogonal linearly polarized receiving antennas. These three receiving antennas may be monopoles connected orthogonally to three different and orthogonal walls (including ceiling/floor) of the chamber. The three antennas are below referred to as the three fixed wall mounted antennas.

Table A.4.3-1: Specifications of differences of measured reference levels in each frequency band between using vertically and horizontally polarized calibration dipoles

Maximum tolerable value

Average

0.2 dB

Standard deviation

0.5 dB

Maximum

1.0 dB

A.4.4 Measurement antennas

It is important that the measurement antennas are configured in such a way that the statistical distribution of waves in the chamber in average corresponds to an isotropic environment.

A.4.5 Chamber size and characteristics

The reverberation chamber shall have a volume large enough to support the number of modes needed for the stated accuracy at the lowest operating frequency. If the UE is moved around in the chamber during the measurement, the volume of the reverberation chamber can be reduced. Also, frequency stirring can be used to improve the accuracy. However, this will reduce the resolution of the results correspondingly.

The reverberation chamber can be loaded with lossy objects in order to control the power delay profile in the chamber to some extent. However the reverberation chamber should not be loaded to such an extent that the mode statistics in the chamber are destroyed. It is important to keep the same amount of lossy objects in the chamber during calibration measurement and test measurement, in order not to change the average power transfer function between these two cases. Examples of lossy object are head and hand phantoms.

Furthermore, the DUT must not be closer than 0.5 wavelengths to other electromagnetic reflective objects inside the chamber and 0.7 wavelengths to absorbing objects.

A.4.6 Shielding effectiveness of the chamber

  1. The shielding effectiveness of the chamber shall be as large as needed for the interference from other sources not to influence the measured parameters. This means that the requirements of the shielding is specific to each test site and may vary accordingly.

The recommended level of the shielding effectiveness is -100 dB from 800 MHz to 4 GHz. See Annex H for more details on shielding effectiveness validation.