## 4 Noise Mitigation Techniques  
The state of the art is plenty of proposals for mitigating the effects of noise in PLC networks. There are proposals that involve using novel modulation schemes. In [62], for instance, a modulation scheme called OPP-MC-CDMA (Orthogonal Poly-Phase-based Multicarrier Code Division Multiple Access) is proposed to overcome the effects of bursty noise and multipath frequency-selective fading in BPL networks. Compressive sensing is also a very common technique to mitigate the effects of impulsive noise in OFDM systems [63], in general, and in PLC networks [64; 65], in particular. In [66] a modification of compressive sensing based on Partial Fourier Matrices is proposed, the main advantage of such a modification being that no redundancy is added, so there is no decrease in the transmission data rate. In order to mitigate the effects of impulsive noise, there are also proposals which involve simple memoryless nonlinearity techniques, such as clipping, blanking or a combination of both [67; 68]. Another proposal to overcome noise conditions harsher than regular ones in the field is the use of data concentrators with three phase injection capabilities [69]. But sometimes a very powerful noise source generates a very high-level wideband noise which results several dB above the NBPLC signal, making the communications impossible in part or even in the whole network, as it has been shown in Section 3. In current practice, this kind of situation are corrected by the installation of filters at the customer premises, in addition of investigating if the interference emission levels are above regulatory limits [70]. Filters are designed to mitigate conducted noise generated by domestic or industrial appliances in the PRIME frequency band. They are installed between the interfering element (typically located at the customer's premises) and the smart meter, as shown in Figure 18.  
Figure 18: Detail of filter installation. The filters used in this kind of scenarios must meet very constrained and sometimes conflicting requirements, e.g.,: for safety reasons, they have to bear currents up to tens of amperes without any risk of burning; they have to be reasonably compact (as a rule-of-thumb, their dimensions round \(5\times 5\times 10\) cm); they have to present high input impedance so that the automatic recovery mechanism of the smart meter works; and their cost have to be low, since potentially a high number of filters may be deployed. Therefore, in these scenarios the best filter is not the one that better attenuate the noise (otherwise a low pass filter with a cut-off frequency before the beginning of the PRIME frequency band would be fair enough), but the one that presents the best trade-off between all the aforementioned requirements. As a result, one of the main objectives of the OSIRIS project [41] was to develop a filter that combats common noises in \"last mile\" of AMI reasonably well and that could be massively deployed. In order to achieve this goal, the configurable filter shown in Figure 19 was designed with the aim of easily comparing the effectiveness of different configurations. This configurable filter allows selecting different filter configurations by opening or closing the switches (S). The most relevant configurations for the target study are: (1) the use of one or two inductances; (2) the insertion of a capacitor to neutral; and (3) the different capacitors and resistors and their influence on the tip of the attenuation curve. Hence, in the next sections the effectiveness of the four configurations shown in Figures 20-23, ranging from simple and cheap configurations to more sophisticated ones, are assessed in different noise scenarios. Figures 20-23 also show the frequency response of the considered filter configurations in the PRIME frequency band. Figure 19: Electric schematic of the configurable filter. Figure 20: Schema and frequency response of Filter F2.