In June 2006 3GPP published their first SISO OTA test specifications in TR 25.914 . Most of the procedural aspects were similar to CTIA but 3GPP further defined performance requirements as part of the conformance test regime. This step took considerable time because, although the TRP and TIS metrics were conceptually simple; many implementation factors complicated the performance definition. These factors included the frequency band in question, the presence of other frequency bands, the primary operating band of the device, and the device “mechanical” mode—e.g., whether the device was open or shut, whether the antenna was extended, or how the device was being held. Within the 3GPP OTA specifications, the CTIA TIS metric was renamed TRS and the two terms can now be considered synonymous. The actual conformance tests based on the requirements in 25.914 are specified in 34.114 .
Figure 6.10-1 shows two different ways that the probe antenna in the anechoic chamber can be positioned in elevation relative to the DUT.
Figure 6.10-1. Example of a spherical positioner system
with a moving probe antenna (left), and with multiple
probe antennas (right).
In the left hand figure the probe antenna can move in an arc to vary the elevation angle θ by arbitrary increments, whereas in the right hand figure multiple antennas are mounted at discrete angles and the elevation is altered by switching between antenna elements. Other methods can be used including hybrid systems that mix antenna switching with physical movement. The device itself is shown on a rotating table where the azimuth angle φ can be varied. A sampling grid of 15 degrees in both azimuth and elevation is considered sufficient to maintain the required accuracy. Since TRP measurements are quick to make, this sampling frequency is not a problem. However, TRS measurements require a search to be made to find the reference sensitivity for each angle of arrival (AoA) and therefore a coarser sampling grid of 30 degrees can be used with some loss off accuracy but considerably reduced test time. Given the number of test permutations and taking into account frequency bands, mechanical modes, and phantom loading, SISO OTA testing can take many days of expensive anechoic chamber time per device. Since the time and cost of SISO OTA test is significant, 3GPP further specified in 25.914 an alternative test method based on a stirred-mode reverberation chamber as shown in Figure 10.6-2.
Figure 6.10-2. Schematic picture of the reverberation chamber measurement setup
The reverberation chamber is a metal box in which the test signal is launched from a fixed measurement antenna. The signal then reflects from the internal surfaces to create a standing wave pattern. A mode stirrer consisting of a rotating angular reflector is then rotated slowly over time to create a long term uniform field. Due to the nature of the reverberation chamber it is not possible to predict the exact field at any one time. But it has been shown empirically, after careful chamber calibration, that TRP and TRS results obtained using the reverberation chamber method are of a similar accuracy to those performed by averaging many single AoA measurements performed in an anechoic chamber. The primary advantage of the reverberation chamber is its much smaller size and lower cost compared to an anechoic chamber.