## 4 Experimental Set Up, Test and Results
### Layer 2-3 Tests  
For the hub, we simulated all the possible errors managed by the software. The breakage of the ARDUINO card, which does not allow the ZigBee communication, quick battery discharge that signals a problem with the PV panel, no connection with the Internet and faults of the Raspberry-Pi. In all cases, the server correctly signals the problem.  
Considering the special features of a WiMAX system, even for the previously described system, some reliability test have been performed. Due to the decay of the working distance with introduction of obstacles leading to multi-path fading, the IEEE 802.16.1 standard [58, 59] does not provide any range data achievable with WiMAX devices.  
Our system works in a rural area some kilometers away from the Base Station essentially in a Line of Sight (LOS) scenario obtained by mounting the receiving antenna on top of an extendable tripod and placing the system on the roof of the church, whilst the BS was on the roof of a building itself [60].  
Carrier to Interference plus Noise Ratio (CINR) and Received Signal Strength Indicator (RSSI) are the considered radio parameters, which allow verifying the reliability of a WiMAX connection. The first is a parameter that judges the quality of the WiMAX signal. The higher the value of the CINR, the biggest throughput a link can maintain. The second is a measurement of the total received power of the frame preamble and is used by the subscriber station to determine the received signal level from the BS [61]. The 802.16 standard supports link adaptive modulation and channel coding (AMC) and it adapts to signal degradation by dropping to a lower modulation [62] in order to maintain a constant bit error rate (BER) of 1 \(\times\)10\({}^{-6}\).  
The AMC is controlled on the BS side and it applies the Modulation and Coding Schemes (MCS), which defines the most suitable modulation rate for current radio channel, and leads to the highest data rate possible. WiMAX uses mainly quadrature amplitude modulation (QAM) with its more powerful 64-QAM5/6 in downstream mode; it is optional in upstream [59]. Maximum modulation upstream is usually 16-QAM3/4.  
The nearest BTS antenna offering the WiMAX service is located near Orte, about 13.2 km away from the site of interest. Even if the distance is quite large and beyond the \"cell-size\" of the BTS (the area covered by the BTS, estimated at about 10 km through simulations), the situation is still lucky since it is almost in LOS (Line Of Sight) as shown in the elevation profile of Figure 6.  
Even if the church is placed out of the WiMAX estimated cell edge, the WiMAX radio link has been successfully tested using the outdoors model HES-319M Customer Premise Equipment (CPE) [54]: the links have been correctly established with poor but stable radio parameters that can still guarantee a QPSK1/2 modulation in UpLink and QPSK3/4 in DownLink.  
As shown in Figure 7, DownLink RSSI at the CPE is about \(-\)80 dBm (Received Signal Strength Indication) while UpLink RSSI at the BTS is about \(-\)110 dBm. We could not expect higher performances in UpLink, since it is strictly dependent on the maximum output power of the CPE, the carrier operator profiles and the average traffic load on the BTS.  
Figure 6: Distance between the site and the BTS with elevation profile.  
In order to verify the real available bandwidth on the WiMAX connection, the software tool Iperf version 2.0.5 [63,64] has been used to check the TCP throughput; the results are shown in Table 5 where for a single CPE, the optimal number of parallel data streams to obtain the maximum throughput is equal to 5. Default settings of the program fix the duration of the single test reported in the second column, while the third column shows the average bandwidth in that time interval and in this scenario the UpLink throughput is enough to upload a standard 8 MP JPG picture in about 1 min.