## 3 TVWS-DTV Coexistence: Field Measurements
### Field Tests Considering Short Distances
#### 3.1.1 Field Test Setup and Methodology Used
Figure 4: Spectrum of the GFDM signal transmitted by one of the antennas of the base station for a power of (**a**) 1 dBm, (**b**) 12 dBm, and (**c**) 22 dBm. The red vertical lines represent the bandwidth limits considered for the main channel, while the green and blue lines indicate the boundaries of the adjacent channels used for ACPR measurement. The DTV modulator used is a professional equipment manufactured by Hitachi Kokusai Linear. The output signal of this modulator is delivered to a channel filter to eliminate traces of image frequency or other spurious signals that may compromise the measurements. After filtering, the signal passes through RF attenuators adjusted to provide power around \(-77\) dBm at the input of the DTV STB (\(P_{\text{RX}}\) DTV) (after the impedance adapter from 50 \(\Omega\) to 75 \(\Omega\)), since this value corresponds to the minimum sensitivity recommended by the standard for DTV STBs. Thus, the proposed coexistence tests evaluate the coexistence of DTV with TVWS in the condition of receiving the DTV signal at the standardized sensitivity limit. The TVWS signal generated by the BS is received via an antenna and is filtered to minimize the influence of other signals present in the channel. An RF isolator, model RFSL6105-01 from RFCI based in San Jose, CA, USA, was used to prevent the signal generated in the setup from being transmitted via the antenna. The TVWS signal is then combined with the DTV signal from an RF combiner, model 11667A, from Agilent based in Santa Clara, CA, USA. It is important to note that the combiner used attenuates the signals at its input by 6 dB. The resulting combined signal, called the setup output signal, is used in two ways: for test signal evaluation, using measurements collected with a spectrum analyzer, MT8222B manufactured by Anritsu based in Atsugi, Japan, in ACPR mode, with the RBW settings of 10 kHz, a VBW of 300 Hz, and a span of 33 MHz, while the center frequency depended on the channel being analyzed; and as the signal applied to the input of the DTV STB, after the correct impedance adaptation (from 50 \(\Omega\) to 75 \(\Omega\)). All measurements taken on the spectrum analyzer take into account the insertion loss of the impedance adapter, which is 5.7 dB, to adequately display the signal actual levels applied at the input of the DTV STB. Three commercial DTV STBs from different manufacturers are used, which are the same equipment implemented in laboratory tests. The video output of the STBs was connected to a TV monitor, through which it is possible to visualize the video signal and, thus, conduct subjective analyses of the DTV signal quality. The subjective analysis of video quality, referred to as DTV STB Status, runs for 60 s on each of the three STBs and is classified as follows:  
* OK--video viewed without freezing or artifacts;
* Intermittent--video viewed, but with occasional freezing or artifacts;
* Not OK--no video playback or permanently frozen video. In order to better illustrate the test conditions, Figure 6 presents a photograph of the setup described above, whose block diagram is shown in Figure 5.  
Figure 5: Block diagram of the MS setup in short-distance tests. In terms of channel allocation, tests were planned for DTV reception in the presence of interference applied to channels \(n+1\), \(n-1\), \(n+2\), and \(n-2\), where position \(n\) refers to the channel occupied by the desired signal, i.e., the DTV signal. The transmission frequency of the 5G-RANGE was set to UHF channel 28, while the DTV signal was adjusted to UHF channels 26, 27, 29, or 30, to allow tests with interference on channels \(n+2\), \(n+1\), \(n-1\), and \(n-2\), respectively. A filter was configured for each of these channels to properly tune the output signal of the DTV modulator. Table 4 summarizes the frequencies planned for the field test, the channel of application of the interfering signal for each condition, and the expected power to be applied at the input of the DTV STBs.