## 2 Materials and Methods
### Measurement Setup and Procedure  
Measurements followed the frequency swept method [5]. A Rhode & Schwarz ZVA67 vector network analyzer (VNA) (Rohde & Schwarz, Munich, Germany) executed the measurements of the frequency response of the radio channel. This equipment transmits a frequency-swept signal in the band of interest and gathers the corresponding received signal at the other ends. It also computes the S-parameters of the measured channel as a function of the frequency. To measure at 40 GHz, the transmitter was connected to an amplifier through a 4 m coaxial cable and then to the antenna through a 1 m coaxial cable, making this way a total length of 5 m. At 60 GHz only the 4 m cable was necessary, as the amplifier was a wave guide one and was connected directly to the transmitting antenna. Furthermore, the receiver end consisted of three omnidirectional antennas connected to three VNA receiving ports through three 2 m coaxial cables. Despite future mobile terminals working at millimeter wave frequencies will probably use directive antennas that will filter some multipathcomponents; by using omnidirectional ones in our measurement system we will be able to get a better idea about all the multipath components present in the environments.  
The measured frequency bands extend 3 GHz around 41.5 GHz and 60.5 GHz. Each complex frequency response comprises 1001 equally separated frequency samples resulting in a frequency resolution of 3 MHz. A thru calibration of the measurement setup was performed before the field measurements were taken.  
The transmitting antenna is attached to the ceiling, whereas the three receiving antennas are mounted close to each other on a stand placed over a straight-line automatized rail, as depicted in Figure 1. Those antennas are mounted in an equilateral triangle with sides of 4 cm for the 40 GHz case and 2.5 cm for the 60 GHz one, following the scheme of Figure 2. These distances are equivalent to 5.3 wavelengths at 40 GHz and 5 wavelengths at 60.5 GHz. The measured mutual coupling shows values below \(-\)35 dB and \(-\)25 dB at 40 GHz and 60.5 GHz, respectively. The antennas were placed 77 cm above the floor. The antenna labeled as \"3\" was the closest to the transmitter; the other two are labeled as \"2\" and \"4\". Note that the number of the antenna corresponds with the S parameter measured on the VNA, considering label \"1\" of the transmitter antenna. For example, S\({}_{31}\) corresponds to the channel response of receiver antenna number \"3\".  
Figure 1: Measurement setup of the receiver.  
Figure 2: Triangular placement of three receiving antennas at 60 GHz band (**a**) scheme; (**b**) final realization.  
The receiver antennas move over a certain distance on the rail, controlled by an indexer and a laptop. The movement of the stand occurs along 220 cm, in steps of 1.875 mm for the 40 GHz channel and 1.25 mm for the 60 GHz channel. These step lengths are a quarter of the wavelength of their corresponding center frequency. Measurements are repeated with the rail placed at different positions along the corridor, enlarging the entire measurements longer than the length of the rail itself.  
A tailor-made Matlab program was developed to capture and process the data. This program runs on a laptop connected to the indexer controlling via a serial RS232 connection the step-by-step motor of the rail and to the VNA using TCP/IP over a UTP cable.