ble platform that moves the device under test to different posi- tions in steps. This stirring method is very effective, in particular in small chambers. © 2009 by Taylor & Francis Group, LLC Measuring Performance of 3GPP LTE Terminals In polarization stirring [17], the transmissions between the device under test and each of three orthogonal wall-mounted antennas are measured successively. Thus, a good polarization balance is achieved. An additional, very effective stirring method is referred to as frequency stirring. This corresponds to averaging the measured quantity over a frequency band and is done during the processing of measured results. 12.2.1 Basic Measurements-Reflection and Transmission Coefficients Figure 12.4 shows how the reflection S11 and the transmission S12 between two antennas inside the chamber are measured. From physics it can be argued that S11 of the antenna under test must consist of two contributions: one the Sa from the antenna itself, as if it were located in free space, and the other S° due to the chamber; thus: (12.1) The S11 contribution is deterministic, whereas S°C is random as a result of the mode stirring in the chamber. If the number of independent values of SL is large enough, it must get a complex Gaussian distribution with a zero mean. Therefore, it is possible to determine Sa by complex averaging the measured S11 over all stirrer positions [18]: (12.2) stirrer Excitation antenna Deterministic part Figure 12.4 Example of measured S-parameters of two antennas in a reverbera- tion chamber as a result of mode stirring and illustration of the deterministic and random contributions to them. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Using the same argument, the transmission coefficient S21 must also consist of two parts: one deterministic Sd the same as in free space, and another statistic part S due to the chamber; that is, S21 = S21 +S21 (12.3) The deterministic part Sale in Equation (12.3) represents the direct coupling bet