# Performance Evaluation of Bluetooth Low Energy: A Systematic Review
## 3 BLE Performance
### Power Consumption
\(M\) is the number of pairs of _scanners_ and _advertisers_ existing in the communications, while \(p_{0}\) is the failure probability due to the interference with other devices. Adapted from [78]. Figure 17: Average discovery latency and energy consumption of the _advertiser_ according to \(T_{s}\) (\(T_{a}\) = 0.64 s and \(T\) = 2.56 s) [78]. The green line shows the model proposed by [76], while the black and blue lines positioned in the lower part show the models analyzed in [78]. The crosses represent the simulation based on the theoretical model. As can be noticed, when \(T_{s}<T_{a}\), the two results do not agree anymore. On the other side, the red line [79] and the two blue and black lines positioned in the upper part of the figure represent the data relative to the latency, analyzed in Section 3.4. \(M\) is the number of pairs of _scanners_ and _advertisers_ that can communicate with each other, while \(p_{0}\) is the failure probability due to the interference with other devices. Adapted from [78]. For a large number of devices in a discovery process, there is the possibility of collision and hence of an unsuccessful discovery. For this reason, [PERSON] et al. [80] proposed an enhanced discovery mechanism by employing a backoff strategy for BLE devices, in order to reduce collisions during advertising processes. The results they have shown in this paper highlighted that the proposed mechanism improves the performance in terms of energy consumption, as well as latency (Section 3.4). A power analysis and a battery life estimation in _advertising_ mode for a concrete application has been done by [PERSON] et al. in [3], using the nRF51822 BLE IC. They extrapolate the power profile and the energy consumption when transmitting a triple BLE advertisement, which goes from around 30 uJ to less than 40 uJ, depending on the transmission power (range from \(-\)21 dBm to 5 dBm). During the connection mode, it is more difficult to examine the power consumption because it is strongly variable, depending on several parameters, such as the packet payload, the _conInterval_, the number of _slaves_ per _master_, the type of communication (_one-way_ or _round-trip_), and so on. In [68; 70; 81; 82], the wave form of the current in the BLE IC is modeled in detail and described in order to calculate the value of the average current in the _connEvent_. The average current consumption, estimated for the minimum _connInterval_ of 7.5 ms, is \(\sim\)3200 uA, while the consumption with a _connInterval_ of 2 s is about \(\sim\)13 uA. The operating voltage for the BLE IC is 3 V.  
In addition to this, the relation between the energy consumption and the _ConnInterval_ has been evaluated in [50; 70]. It has been seen that the average consumptive current decreases exponentially at the raising of the _connInterval_, supposing a BER = 0, as shown in Figure 18. However, bit errors may significantly affect the current consumption, depending on each _connInterval_ and _connSlaveLatency_. [PERSON] et al. [84] have also analyzed the variability in power consumption depending on the effective number of byte per packet, comparing two different IC: one developed by Texas Instruments, the CC2541 [77], and one by Nordic Semiconductor, the nRF51822 [67]. These two BLE IC use different types of architecture: the first is based on Intel 8051, while the second is an ARM MCU; it would be also interesting to investigate how performance varies in these different micro  
Figure 18: Average current consumption, per variation of _connInterval_, measured in a CC2540 _slave_. The operating voltage of this IC is 3 V. It is a _one-way_ communication with _connSlaveLatency_ = 0. The Texas Instruments BLE module CC2540 [83] has a USB interface that implies a more relevant drawn current compared to I2C or SPI interfaces (e.g., the CC2541 [77] module has an I2C interface). A BLE module with a USB interface could be a good reference for a _central_ node rather than a _peripheral_ one. Adapted from [50]. According to the results exposed above, [PERSON] et al.