Interference from Very High Speed Digital Subscriber Line (VDSL/VDSL2/G.Fast)

It is official: Openreach do not offer protection to the radio spectrum

UKQRM wrote to Openreach to confirm if they provided protection to the radio spectrum from their VDSL2 deployment. This is their answer:

Further to our previous email regarding your interference query.

I had previously asked our Chief Engineering Team to review your email and they have confirmed that Openreach VDSL DSLAMs do not apply notches at the amateur bands to the DSL signals.

If we can help with anything else then please let us know.

Kind regards

Joe McGill
CEO Complaints Business Manager
Openreach
Web: www.openreach.co.uk

There is a mandatory requirement in the VDSL2 specification to support notching of up to 16 arbitary bands. However, it is up to the operator (Openreach in this case) to specify which notches will be applied, if at all. The internationally assigned Radio Amateur bands are given in the standard as examples of frequencies to be protected.

The following extracts from the VDSL2 specification External link Rec. ITU-T G.993.2 (01/15) mention notches (also known as stop-bands). The wording in previous DSL standards is very similar concerning notches.

1. Scope

A requirement for downstream and upstream transmitters to notch, simultaneously, 16 arbitrary operator-defined RFI bands;

7.2.1.2 Egress control

VDSL2 transmitters shall be able to reduce the PSD of the transmitted signal to a level below −80 dBm/Hz in 16 arbitrary frequency bands simultaneously. An example list of frequency bands (the amateur radio bands) is shown in Table 7-1. The value of −80 dBm/Hz shall be accounted for in the determination of MREFMASK (see Tables 7-3 through 7-5).

Table 7-1 – Amateur radio bands

Band Start (kHz)	Band Stop (kHz)
1800	2000
3500	4000
7000	7300
10100	10150
14000	14350
18068	18168
21000	21450
24890	24990
28000	29700

The specific RFI bands to be notched are configured in the CO-MIB by the operator and set during the ITU-T G.994.1 handshake phase of initialization (see clause 12.3.2). The egress control parameters specified in [ITU-T G.997.1] are the start and stop frequencies of each frequency band in which the transmit PSD shall be reduced to a level below −80 dBm/Hz. The PSD slopes forming the notch are vendor discretionary.

10.4.1 Data subcarriers

The subcarriers shall be indexed from i = 0 to i = MSI, where MSI is the index of the highest loaded subcarrier (i.e., the maximum index in the MEDLEY set). The values of MSI may be different for upstream and downstream transmission and are denoted as MSI_us and MSI_ds respectively. The index of the highest loaded subcarrier (MSI_us or MSI_ds ) will be restricted by the selected profile and band plan as shown in Table 6-1. Specifically, MSI us shall be equal to or lower than the "index of the highest supported upstream data-bearing subcarrier" (6.2.10) and MSI_ds shall be equal to or lower than the "index of the highest supported downstream data-bearing subcarrier" (6.2.9). Transmission will take place on NSC subcarriers, with NSC_us ≤ MSI_us and NSC_ds ≤ MSI_ds ; the subcarrier with index i=0 shall not be used. NSC_us + NSC_ds shall always be less than 4096.

The subcarriers to be used for data transport in the upstream and downstream directions (MEDLEY_us and MEDLEY_ds sets, respectively) shall be determined during initialization, as specified in clause 12.3.3.

NOTE – The subcarriers used for data transmission depend on channel characteristics, such as loop attenuation and noise, and on the specific requirements on the PSD of the transmit signal, such as notching of amateur radio bands, PSD reduction at low frequencies to share the loop with POTS or ISDN, and others.

The following extract is taken from the European Telecommunications Standards Institute's External link TS 101 270-2 concerning VDSL2 protection to the internationally assigned Radio Amateur bands.

8.1.5.4 Mask M1 (notched)

Notching is implemented in the internationally standardized amateur radio bands (see table 17) to limit the transmitted PSD within these designated bands. The notching is provided to reduce the effect of unwanted radiated emissions from VDSL causing undue interference to existing licensed users of that part of the spectrum.

A VDSL transceiver conforming to the requirements of Mask M1 shall be able to reduce the PSD simultaneously to below -80 dBm/Hz in one or more of the internationally standardized Amateur Radio bands listed in table 17.

It is desirable to implement programmability of the notch frequencies to cater for national and regional variations.

UKQRM note: This specification was drawn up before the 5 MHz band became widely allocated. This band is therefore offered no protection at this time!

Radio interference from VDSL

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Page updated: 15th August 2021