Radio Amateurs Fear Interference from Next Gen G.fast Broadband Tech

A group of radio amateurs have warned that the next generation G.fast (ITU G.9700 / aka – FTTC2) broadband technology, which BT and Huawei plan to “field trial” in the UK alongside Fibre-to-the-Distribution-Point (FTTdp) lines for potential speeds of up to around 1000Mbps (here), could cause significant interference for them.

The G.Fast specification is due to be finalised during early 2014 (here) but radio amateurs on the Interference.org.uk forum have been quick to point out that its maximum “sampling frequency” might eventually reach 423MHz, which they fear could interfere with them as well as other areas like civil aviation, business radio, FM, DAB and a host of other services in the same region of radio spectrum.

The Register has been similarly quick to note that the sampling frequency only happens inside the DSLAM itself, which are usually found at your local telephone exchange and provide a vital link in the connectivity chain for ADSL, VDSL (FTTC) and similar DSL technologies (though interference can “leak” out). Earlier this year a similar concern was put to the ITU and their response is as follows.

ITU Position on G.Fast Interference in the 400MHz Band

[We] noted your concerns about spectral use up to 400 MHz. We want to point out that the highest frequency ever considered for this technology is 212 MHz, with the determined version of the ITU-T G.fast PSD specification being limited to frequencies up to 106 MHz. ITU-T Q4/15 fully recognizes the competence of ITU-R on spectral usage. We hope this clarification somewhat eases your concerns.

It’s of course entirely possible that future innovations could seek to go higher than the currently outlined specification and indeed some hardware manufacturers are already playing around with the idea of 400Mhz kit. Similarly Ofcom’s drive to improve radio spectrum efficiency, which often focuses upon the delivery of new broadband services, came sometimes come at the behest of other perhaps less significant uses.

Huawei’s Viewpoint on G.fast Spectrum

VDSL2 [FTTC] currently works on 17MHz [ISPr ED: BT uses this] or 30MHz, while G.fast will work on 106MHz or even 212MHz. Of course, the frequency spectrum cannot be extended infinitely. Like spectrum resources in the wireless communication sector, spectrum resources in the fixed communication sector must be properly planned, to prevent conflicts with spectra already in use and to reserve space for future technologies.

For example, the Office of Communications (Ofcom) in the UK has defined a strict ANFP (access network frequency plan) for spectrum application. The ADSL2+ spectrum is allowed only in exchanges, and the VDSL2 spectrum can be deployed only at FTTC street cabinet sites. The G.fast spectrum may need to avoid the frequency bands that are already in use.

G.fast technology will use the 106MHz frequency band in the initial stage and 212MHz in the future. The wider the frequency band, the higher bandwidth G.fast can achieve. However, higher frequencies also mean shorter transmission distances, higher costs, and greater power consumption. The frequency band that is ultimately used is a compromise between performance, costs, and implementation.

At present it’s still early days for G.Fast and we’re unlikely to see a fully developed commercial product for a few more years. Both G.Fast and FTTdp face many hurdles and even if all those can be overcome then it would still represent a fairly expensive upgrade for BT. Never the less there’s every indication that this is the direction BT has chosen to take.