Resolution of Interference Issues Will Make Power Line Communications an Attractive Option
- Written by Stefano Galli
The smart grid will employ many kinds of communications technologies, ranging from fiberoptics to wireless and to wireline, including Power Line Communications and broadband access modes like xDSL. Among the wireline options, several PLC technologies are already available or being standardized. But this wide availability of PLC technologies also has some drawbacks.
For non-interoperable Power Line Communications (PLC) technologies that are well separated in frequency, good analog front-end design can eliminate interference between them. There are also many non-interoperable PLC technologies that operate in the same band, however, which can lead to mutual harmful interference in the absence of isolating or attenuating mechanisms between them.
In fact, signals generated within the premises interfere with each other, and with signals generated outside the premises as well, because power line cables connect low-voltage transformers to a set of individual homes or a set of multiple dwelling units without isolation. As the interference increases, both from indoor and outdoor sources, PLC stations will experience a decrease in data rate as packet collisions increase, or even complete service interruption.
Thus, power line cables are a shared medium (just like coax and wireless) and do not provide links dedicated exclusively to a particular subscriber (unlike xDSL). Unfortunately, approaches based on Frequency Division Multiplexing (FDM) for separating the various communications channels as in WiFi or coax are not suitable because only a relatively small band is available for PLC.
Mechanisms that limit the harmful interference caused by non-interoperable neighboring devices are available today. These mechanisms are typically resource sharing protocols based on CSMA (Carrier Sense with Multiple Access) or TDMA that allow PLC devices to share the medium in an orderly way; they are commonly referred to as "coexistence mechanisms."
The issue of PLC coexistence was first addressed in 1992 with the European standard CENELEC EN 50065, which enables low-data-rate-narrowband PLC technologies to operate in the CENELEC C band (125–140 kHz) without interference. IEEE 1901.2 and ITU-T G.hnem (G.9955/G.9956) address coexistence among next-generation OFDM-based high-data-rate narrowband PLC devices operating up to 500 kHz. The case of broadband PLC—where devices operate above 1.8 MHz—was handled in depth during the standardization of the IEEE 1901 Broadband over Power Lines standard. There is a broadband PLC coexistence scheme called Inter-System Protocol (ISP) that has been incorporated in the IEEE 1901 standard and also ratified as ITU-T Recommendation G.9972.
For the specific case of coexistence between the two solutions provided in the IEEE 1901 standard (one based on Windowed-OFDM and one based on Wavelet-OFDM), Panasonic proposed to IEEE's 1901 working group a mechanism called the Inter-PHY Protocol (IPP). The IPP is a TDMA-based distributed coexistence protocol and is a radical conceptual departure from previous designs based on CSMA (with or without hybrid delimiters).
Although the IPP was originally designed to enable coexistence between devices equipped with either the IEEE 1901 Wavelet-OFDM or the IEEE 1901 FFT-OFDM physical layers (PHYs), it was soon recognized that the IPP could also be an excellent tool for regulating simultaneous access to the channel of both IEEE 1901 and non-IEEE 1901 devices, for example the ones based on the ITU-T G.hn standard (ITU-T G.996x Recommendations). Panasonic slightly modified the original IPP scheme to extend coexistence to G.hn devices and proposed this enhanced mechanism, the Inter-System Protocol (ISP), to both ITU and IEEE.
The ISP is now a mandatory part of the IEEE 1901 standard and is also specified in ITU-T Recommendation G.9972. The ISP-related part of IEEE 1901 is not identical to ITU-T G.9972 as there are a few features that are not supported by both specifications, but as a result of the efforts in SGIP PAP 15, IEEE 1901 compliant devices are now also ITU-T G.9972 compliant. Thus, IEEE 1901 compliant devices implementing either one of the two IEEE 1901 PHYs can coexist with each other and with other broadband PLC devices that implement ITU-T G.9972.
As there are already deployed broadband-PLC technologies that do not implement ISP, it is important to understand to what extent the existing installed base of legacy technologies can be an issue for the newer ISP-enabled devices that will be deployed. The first consideration is that this is a minor issue as the installed base of broadband PLC devices is still very small when compared to other LAN technologies such as WiFi. Secondly, a lesser known but important benefit of ISP is its capability of in many cases eliminating the interference created by an installed base of devices that does not use ISP, but can be still controlled in an ISP-compliant manner by newer ISP-enabled broadband PLC technologies.
The ISP coexistence scheme specified in IEEE 1901 and ITU-T G.9972 can be used to ensure that:
- in-home, access, and smart grid standards-based broadband PLC technologies will coexist
- the operation of smart grid and home networking devices can be decoupled and allowed to mature at their traditional obsolescence rate
- utilities and service providers can avoid resolving service issues caused by interference between non-interoperable PLC devices supporting different applications
Not everybody agrees on the usefulness of PLC coexistence, regardless of whether it applies to broadband or narrowband PLC. In fact, there are those who believe that coexistence will foster the proliferation of non-standardized solutions that, in turn, will cause a delay in aligning the market behind a single PLC standard. Although the concern that coexistence could stand in the way of interoperability may be valid to some extent, it is important to understand that the usage of the PLC spectrum is not regulated so that any PLC technology can use channel resources without having any legal obligation to protect other PLC technology from interference. Thus, any deployed PLC technology can be a source or victim of interference to or from the installed base of PLC devices if a common coexistence mechanism is not supported, as there is not enough bandwidth to implement FDM as in the WiFi and coax cases.
It is difficult to foresee widespread adoption of PLC without the protection of coexistence. Basically, coexistence can be seen as a form of "insurance" that PLC devices deployed by some will not stop working due to interference created by neighboring non-interoperable devices deployed by others. Hopefully, this insurance will allow utilities and other consumers to adopt PLC with a higher degree of confidence and ramp-up its deployment. This is the necessary first step for allowing the market to down-select from the various PLC technologies on the basis of field-proven performances.