In this section we present the results of the spectrum measurements. We analyze the raw samples collected over all sweeps in Fig. 2.29 as a spectrogram segregated into three sub- plots corresponding to the urban environment class.
Raw power-level samples fed from the spectrum analyser are compared to the detection threshold θ ( f ), accordingly the channel occupancy at a certain frequency f in a certain
Fig. 2.28 Noise analysis of the experiment’s spectrum analyser, represented in terms of a normalized histogram. Indicating the chosen detection threshold of 5% CCDF.
sweep number (spectrum sample) j is determined according to the following rule:
B( f , j) = (
1 (Busy) : Ps( f , j) > θ ( f )
0 (Free) : Ps( f , j) ≤ θ ( f ),
(2.31)
where Ps( f , j) is the sensed signal power at frequency f and a sweep number j. The rep-
resentation of (2.31) is a two dimensional Boolean matrix as depicted in Fig. 2.30, as an occupancy graph, where the figure only shows the results up to 3GHz since no significant spectrum utilization was detected above this frequency (the setup is 400 to 6000 MHz).
The null utilization appearing above 3 GHz is referred to the fact that this band is al- located to multiple special applications such as aeronautical, satellite and radio astronomy [173]. We suspect that the transmission over this band might be occurring below the detec- tion threshold of our setup, especially the satellite downlink, requiring a high gain antenna to receive an interpretable signal.
In order to quantify the level of spectrum occupancy, we calculate the duty cycle of the measured signal, that is the number of the occupied samples divided by the total number of collected samples. In other words, the duty cycle represents the percentage of time when a channel is busy. For a binary occupancy analysis, the calculation is performed as the
Fig. 2.29 The spectrogram obtained from the raw samples collected in the three different urban classes. From top to bottom (high, medium and low) population density.
Fig. 2.30 The occupancy graph obtained by comparing the raw samples against the detection threshold. Dark spots indicate a busy spectral resource (B = 1) while the white spaces indict free spectral resources (B = 0). From top to bottom (high, medium and low) population density. following: D( f ) = Occupied Samples Total Samples = ∑Mj=1B( f , j) M , (2.32)
where M is the total number of sweeps. The duty cycle can take values within the range D( f ) ∈ [0%, 100%], indicating the level of occupancy of a certain channel. We plot the duty cycle in Fig. 2.31for the three classes of urban environment. It can be easily noticed that the duty cycle is different between the three classes, concluding that if a channel has a high utilization in the dense urban environment does not imply that it can’t be opportunistically accessed in another urban environment such as rural or suburban.
The frequency range 402 - 460 MHz is allocated to multiple types of users, mainly Mete- orological satellite uplink, radio astronomy, and mobile UHF narrow band radios (licensed and amateurs) [173]. This band could be considered as a candidate for DSA by oppor-
Fig. 2.31 Duty cycle obtained by comparing the number of occupied samples to the total number of collected samples for each frequency line. From top to bottom (high, medium and low) population density.
tunistically accessing multiple narrow band spectrum holes left by licensed and/or amateur users. A special care is required for radio astronomy and space research bands, since radio telescopes demands extremely low terrestrial interference.
The band 520 - 820 MHz is originally allocated to analog and digital TV broadcasting, however, new allocation to LTE (Long Term Evolution) has taken place in 700 MHz band (694 - 820 MHz). The apparent occupancy variation in the 520 - 694 MHz band can be re- ferred to the low signal-to-noise ratio when receiving a TV signal with a street-level antenna inside a dense urban environment. Identifying spectrum holes in this band requires further investigation with smaller bandwidth measurements.
Cellular networks in Australia originally operates in three main bands 900 MHz, 1800 MHz and 2100 MHz [174], later the bands 700 MHz and 2500 GHz were allocated in 2013 (under the Digital Dividend Act [175]). We can notice the large difference in spectrum uti- lization of LTE bands between the three classes of environments, where network operators are naturally deploying the eNBs (LTE base stations) in locations expected to provide higher
return on investment. These bands might be considered as potential for location-dependent dynamic spectrum access. In addition, uplink sub-bands might also be considered as a can- didate for DSA techniques, a trend that is progressively aiming to utilize these sub-bands for Device-to-Device communication (D2D), as recently advised by 3GPP [25]. D2D can greatly enhance the network coverage by relaying transmission between eNBs and User Equipment (UEs) (see Sec. 4.1).
The band 918 - 926 MHz (centered at 922 MHz) is dedicated for Industrial, Scientific and Medical application (ISM) in addition to RFID. Systems operating over this band should be designed to be interference resilient. We can notice a very low utilization of this band in both rural and suburban environment, thus it might be considered as a potential candidate for DSA applications.
We can notice a null duty cycle in the frequency band 960 - 1700 MHz in suburban and rural environments against a very low duty cycle in dense urban environment. A large portion of this band (about 316 MHz) is allocated to aeronautical radionavigation 960 - 1215 MHz, 1300 - 1350 MHz and 1559 - 1610 MHz. These bands are carrying a Safety-of-Life Service, and they are protected from interference by the International Telecommunication Union [176], thus these bands are not feasible for DSA. The frequency range of 1400 - 1427 MHz is also protected for radio astronomy since the hydrogen spectral line (1420 MHz) has very low atmospheric absorption. Another protected region is the band 1,260 - 1,300 MHz which is dedicated to radio navigation and defence applications, thus it is very difficult to operate secondary users in these bands, not only because of the technical difficulties, but also due to the expected legislative issues.
Some of the allocated spectrum for mobile satellite services and satellite broadcasting lays within the range of this experiment, however it is not possible to capture such signals without directional antennas, due to the low SNR. Dynamic spectrum access for satellite bands is being investigated in the literature [177,178], where initial studies indicate possible mutual use of such bands between terrestrial services (such as cellular communication), and satellite services.
Our setup was not able to detect significant transmission above 3 GHz, where the ma- jority of this band is designed for high gain directive antennas, such as point-to-point links and satellite-earth transmission. Accordingly we cannot comment on the utilization and the opportunity to use DSA, while some literature papers suggest the possibility to use underlay DSA techniques in this band [177].