In the context of identifying spectrum resources, much of the research, mainly by academia, has been directed toward the operation of autonomous WSDs. In previous section, main spectrum sensing techniques and algorithms have been presented. It is inevitable that the specification of the regulatory emission limits for the operations of such autonomous WSDs in DTT bands has to be based on worst-case geometry relating to the interfering and victim and worst-case sensing (i.e. hidden node) environments. Consequently, adequate protection of the DTT service can results in very stringent WSD sensing levels, applied uniformly at all locations.
2.2 White Space Database 31
The above issue can be resolved if the WSDs operate with assistance from geo- location database.
The most proper methodologies (e.g. algorithms and parameters and criteria) to be adopted in order to identify available resources for the WSD, whilst granting the most adequate level of protection to incumbent services, should be established by the Administrations, which intend to adopt the geo-location database approach to authorise WSDs. In the following, the protection of incumbent DTT service is taken as an example, even though a similar approach can be adopted also for other services [11]. A WSD, which intends to operate in a specific location, sends its information to the geo-location database and receives information on the available frequencies with associated values of maximum permitted EIRP. As explained in section 1.4 (see figure 1.7), the steps for population of the geo-location database are refereed as ”translation process”. In this section the focus is on the first step of the translation process, that is the identification of spectral resources not used by incumbent services and thus potentially available as white spaces [11].
In the following, different methodologies to determine the amount of spectrum potentially available as white space are described. The proposed methodologies have been applied in Italy in order to fill the geo-location database with the information on the available frequencies.
2.2.1 Threshold-based approach for the population of the geo-location database
This is a simple methodology to identify spectrum available as white space: in a specific location the received signal power on each channel is evaluated by means of a proper propagation model [44], and if the estimated power on a channel is below a certain threshold it can be deduced that there are no licensed users for that channel in the proximity of the investigated location. Therefore a WSD is allowed to transmit its signal, provided that the specific emission requirements are met [17, 45, 10]. The threshold can be determined as a function of different parameters such as the incumbent service to be protected (e.g. DTT, PMSE) or the level of protection to be granted.
In particular, the WS estimation based on the threshold approach is performed as follows:
1. compute the power received on a given channel and in a given pixel (e.g. 600 m x 600 m) by a receiving antenna (e.g. omnidirectional with 0 dBi gain)
assumed at a specific above ground level;
2. compare the received power against a specific threshold;
3. if the received power is below the threshold, the channel is considered as vacant; 4. iterate steps 1- 3 for all the channels from 21 to 60 and for all the pixels of the
considered area.
Figure 2.3 (on the right) shows the flowchart of the threshold-based approach in order to fill the geo-location with information on the potentially available pixels.
2.2.2 Location Probability approach for the population of the geo- location database
This approach is particularly focused on the protection of the DTT service which is the one of paramount interest for Italian scenario. According to this approach, the usage of a specific channel is prevented within the coverage area of each DTT trans- mitter employing that channel, in order to guarantee a wanted Location Probability related to proper field strength threshold [46]. DTT field strength levels are eval- uated using accurate propagation models, which take into account also diffraction phenomena. Predicted values are then employed to identify the DTT protection area and the paired zone outside the coverage area where the channel is potentially available for WSDs.
According to this approach, protection of DTT service is guaranteed evaluating a-priori the service area contour of each DTT transmitter. Potentially available white spaces can be located only outside the computed contour, which is estimated by means of propagation analysis, considering the Reference Planning Configurations (RPCs) specified in [46].
In table 2.1 the minimum median received field strength levels of the GEO-06 RPC at 650 MHz frequency are shown for fixed reception with respect to different location probabilities values.
The potentially available white spaces are calculated based on the following approach:
1. for each pixel and for each channel, the field strength level Erx(dBµV/m) con-
sidering all possible DTT transmitters is evaluated with a suitable propagation model;
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Table 2.1: DTT reference planning configurations
Location Probability Fk,min dBµV/m at 10 m
99 % 60
95 % 56
50 % 48
1 % 34
2. the calculated field strength Erx is compared with the selected planning con-
figuration threshold Fk,min (table 2.1);
3. if Erx is ≥ Fk,min the pixel in within the portected service contour, hence
the channel in occupied (DG = 1); if Erx is < Fk,min the pixel is outside the
portected service contour, hence the channel is vacant (DG = 0).
The logical architecture of the Location Probability- approach is show in figure 2.3 (on the left).