# Opportunistic Sensor Data Collection with Bluetooth Low Energy
## 2 Related Work
### Bluetooth Low Energy Performance  
The trade-off between energy consumption, latency, network size, and throughput for one of the BLE modes (namely, connection-based mode) is studied in [8], providing experimental, theoretical and simulation results. These results show that using a coin cell battery, lifetimes of up to 14.1 years can be achieved. The average latency of one round-trip for very low BER values (e.g., \(10^{-6}\)) is smaller than 2 ms, being below 2 s in the worst conditions considered (BER = \(10^{-3}\)). The number of sensor nodes (slaves in BLE terminology) simultaneously connected to a mobile entity (a master in BLE terminology) could be up to 5917. The maximum experimental throughput at the application layer is 58.48 kbps. These performance results provide a solid foundation to propose an OSDC connection-based BLE solution.  
The BLE device discovery mechanism has been deeply studied [22; 23; 24]. An analytical model validated by simulation is presented [25]. Results are useful to understand the behaviour of an OSDC scenario with multiple devices performing discovery at the same time. However, the work does not include any empirical study, node lifetime and amount of information exchanged are out of scope, and the connection phase is not considered.  
With regard to sensor node energy consumption, authors in [23] develop a BLE energy consumption analytical model validated with empirical measurements. Since the study does not focus on the OSDC use case, the impact of some relevant aspects on performance, such as the contact time or the maximum amount of information that can be exchanged, are not considered.  
BLE throughput has been evaluated in several works. The maximum application layer throughput reported in experimental studies is between 60 and 80 kbit/s [8; 20; 26]. These low values are mainly due to limited hardware and setup capabilities. Throughput at the link layer is studied analytically and empirically in [27]. The maximum throughput obtained empirically is 122.6 kbit/s, achieved with the connection-based approach. In this way, two interesting contributions of our work are the achievement of a greater empirical maximum of 156.5 kbit/s, and the fact that this value remains almost constant until distances of approximately 200 m. As explained in Section 5.5.1, we obtained this maximum with an aggressive setup of the BLE modules employed [28], and it far exceeds the one announced by their manufacturer (i.e., 60 kbit/s), but matches perfectly the analytically obtained one. To the best of our knowledge it is the highest empirical BLE throughput published in the literature.  
Because OSDC suffers from the non-static nature of its nodes, it is interesting to know the behaviour of BLE in dynamic scenarios. This is empirically shown in [29], using off-the-shelf smartphones in an Inter-Vehicular Communications (IVC) scenario. Maximum communication ranges beyond 100 m and robust connection up to 50 m are obtained. The authors show the connection time between two vehicles as a function of their relative speed. In an urban scenario, contact times between 50 s and 10 s, and packet transmissions between 464 and 85 packets, are achieved with speeds from 20 km/h to 60 km/h, respectively. Finally, an interference test is conducted where BLE communications coexist with Wi-Fi ones, concluding that BLE is interference-resilient. These results empirically demonstrate the capability of BLE for data collection even in challenging scenarios. However, they cannot be extended because a very specific testbed setup is employed. In this way, this paper completes and expands that work.