## 6 Deployment Guidelines for NB PLC System Deployment
### PLC Signal Injection in SSs
#### 6.1.2 Single _versus_ Three-Phase Injection  
Many electricity systems are three-phase systems. PLC LV signal injection at SSs takes place in a set of bus-bars in panels from where electricity cables flow towards the PoS. These panels are connected at the secondary winding of the MV to LV transformers. Inside the panel, three-phase and one neutral bars are found, mounted either on a metallic framework or inside a metallic enclosure. The minimum requirement to inject a PLC signal is via two conductors (typically connected to one phase and one neutral cable, as most common LV connectivity from triangle--star MV to LV transformers configuration suggests. However, a three-phase system might offer the possibility of using all phases to inject PLC signal. PLC signal, when injected in phase and neutral, progresses through the conductors, and may be coupled to the rest of phases either capacitively or inductively (the explanation is to be found on the parasitic elements of the cabling or its layout, with cables running in parallel for long distances). Reference [120] explores single _versus_ three-phase injection in a NB PLC system [117; 146; 147]. The work mentions some early experiments using passive couplers to divide the signal coming from a two-conductor injection-prepared device to produce three-phase coupling. The conclusions of this simple configuration were discouraging, as transmit power within EN 50065-1 [53; 148] regulations reduce transmitted voltage level by 6 dB, and even worst, at the reception path, being the coupling mechanism purely additive, the worst case phase noise is present. As a result, SNR shows a bigger degradation than with a single-phase injection, especially if the phase with the best noise situation can be used for the primary injection. This same reference configured a second experiment to overcome the limitations of the former passive signal injection, together with EN 50065 regulation. A PRIME Base Node is used to configure a virtual three-phase active injection device, with the help of the so-called two Auxiliary Nodes. This \"virtual device\" shows certain limitations over a pure and intelligent three-phase injection system, as the results show. These limitations [99] are related to both the physical and the MAC layer. Physical layer is a limiting factor, as the injection at each phase tends to configure longer distance PLC links than those based on PLC signal repetition. Thus, typical signal fades, or more commonly high noise occurrences, in systems working near the SNR performance limit, get PLC hops compromised by these sudden noise burst, causing unavailability. From a MAC layer perspective, as three-phase transmissions are sharing the same PLC media, their transmissions must be coordinated, because without the coordination of the Base Node and the two Auxiliary Nodes, transmissions would not be optimized. Moreover, this coordination needs to take into account the rest of the network. Notwithstanding these considerations, the results obtained showed that the availability of the results obtained with the single-phase injection were slightly better than those of the virtual three-phase injection (99.5% to 99.1%, respectively). With such close results, it seems that there is room for the existence of a three-phase injection device, overcoming the limitations of the virtual device configured, that could be used in the context of more intelligent solutions to better control the behavior of its networks. The implementation of these smarter three-phase solutions could possibly use some sort of MIMO techniques [149], and specifically should overcome the handicaps identified in this early experiment to bridge the gap of the performance seen, and to offer even greater availability in the context of moreaggressive noise situations than the benign ones that can be presumed from the high figures of the availability results.