Addendum to Consultation on Amendments to SRSP-520, Technical Requirements for Fixed and/or Mobile Systems, Including Flexible Use Broadband Systems, in the Band 3450-3650 MHz

September 2021

Note: Extension to the Comment Period for Consultation on Amendments to SRSP-520, Technical Requirements for Fixed and/or Mobile Systems, Including Flexible Use Broadband Systems, in the Band 3450-3650 MHz

Expand all content / collapse all content

1. Extension of time for comments

On August 6, 2021, ISED initiated a Consultation on Amendments to SRSP-520, Technical Requirements for Fixed and/or Mobile Systems, Including Flexible Use Broadband Systems, in the Band 3450-3650 MHz (the Consultation). The initial deadline for submitting comments was August 23, 2021. Several comments requested further time to provide submissions in the Consultation. As a result, Innovation, Science and Economic Development Canada (ISED) is inviting further comments and extending the deadline to October 15, 2021.

The Consultation referred to studies that were reviewed while developing the proposal. Some comments requested clarification as to which studies ISED had reviewed so that stakeholders could have reviewed them prior to submitting comments. Additional information on these studies, including some of the factors that ISED considered in the development of the proposed Standard Radio Systems Plan (SRSP) are set out below.

2. Considerations

As indicated in the Consultation, there is preliminary evidence that radio altimeters, which are used on aircraft, operating in 4200-4400 MHz band to aid in aeronautical navigation, may be susceptible to harmful interference from flexible use broadband operations in the 3450-3650 MHz band (3500 MHz band) under certain conditions. Radio altimeters are critical sensors for automatic flight guidance systems, which include automated landing systems. In addition, several other aeronautical safety-related systems rely on inputs from radio altimeters (e.g. terrain awareness and warning systems).

Given that radio altimeters are critical in aeronautical services, the amendments proposed in the SRSP are based on protecting the lives and safety of Canadians while still allowing the deployment of 5G operations in the 3500 MHz band. 

2.1 Determination of potential for harmful interference

ISED reviewed a number of publicly available studies conducted by other entities (see annex A), including the October 2020 Radio Technical Commission for Aeronautics (RTCA) report, the 2019 and 2020 studies from Aerospace Vehicle Systems Institute (AVSI), a study conducted by the aircraft manufacturer ATR, and studies from regulators in the United Kingdom (UK), France (available only in French), Australia and Japan (available only in Japanese, however summarized in English). Some studies highlighted the potential for harmful interference to radio altimeters operating in the 4200-4400 MHz band from 5G operations in adjacent and nearby bands, while other studies did not.

For instance, the RTCA report studied the potential impact on radio altimeters’ performance from 5G base stations and user equipment expected to be operating in the 3700-3980 MHz band in the United States (US). It concluded that 5G base stations presented a risk of harmful interference to radio altimeters, which could negatively impact their performance. Moreover, based on the assessment of radio altimeters subjected to emissions in the 3700-4200 MHz band, the AVSI reports stated that the protection from interference signals could not be guaranteed. ATR reported systematic erroneous radio altimeter indications linked to the existing mobile base stations around airports. Coexistence studies conducted by Japan for the 3600-4200 MHz and 4400-4900 MHz bands, by Australia for the 3575-3700 MHz and 3700-4200 MHz bands, and by France for the 3400-3800 MHz band employed different regulatory contexts, interference analysis methodologies and assumptions, but reached similar conclusions that 5G equipment in adjacent bands may cause harmful interference to radio altimeters. A study performed by Ofcom in the UK for the 3800-4200 MHz band concluded that their proposed low and medium power technical conditions for base stations would not pose any interference risk to these aeronautical sensors. However, a study submitted to the International Civil Aviation Organization (ICAO) by the UK Civil Aviation Authority indicated that high-power base station operation in the frequency range 3600-3800 MHz were more likely to cause interference to 4200-4400 MHz radio altimeters than low power base station operation in the frequency band 3800-4200 MHz.  In summary, the review of these studies in the context of Canadian technical and spectrum policies for the 3500 MHz band supported ISED’s initial view that there may be a potential of harmful interference to radio altimeters from 5G operation and, therefore, a risk to the lives and safety of Canadians.

The vast majority of the information relied on by ISED was in public studies available to all parties. Some comments questioned what evidence ISED reviewed in developing the proposed measures and raised concerns regarding reliance on confidential information. Although ISED has had confidential discussions related to radio altimeters with equipment manufactures and other national regulators, the proposed changes to the SRSP set out in the Consultation were developed using well known and publicly available information as discussed above.

As per its usual process, ISED conducted preliminary calculations to assess the potential for interference based on publicly available studies in the context of Canada’s technical rules on the 3500 MHz band. This calculation (see annex B) evaluated coexistence between radio altimeters and 5G equipment in Canada based on the International Telecommunication Union (ITU) Recommendation ITU-R M.2059-0, Operational and technical characteristics and protection criteria of radio altimeters utilizing the band 4200-4400 MHz and the maximum permitted power of 5G equipment in Canada, as defined in RSS-192, Flexible Use Broadband Equipment Operating in the Band 3450-3650 MHz. ISED considered an aircraft approaching and/or landing in the vicinity of an outdoor transmitting base station using the technical characteristics of all radio altimeters defined in ITU-R M.2059-0. Harmful interference to radio altimeters from signals outside their band of operation (4200-4400 MHz) was calculated to be possible, which could include signals from future flexible use deployment of 5G systems in the 3500 MHz band around airports and heliports, warranting further investigation into the issue. These calculations performed by ISED are based on the noted publicly available information and could be performed by an experienced engineer in the field.

ISED is committed to ensuring the safety of Canadians and thus determined that measures were warranted to protect the operation of radio altimeters while international and domestic studies continue. Since ISED cannot rule out the use of worst-case altimeters described by the aviation industry in the RTCA report on Canadian territory at this time, ISED proposed to adopt a cautious approach to define the mitigation measures until additional studies are completed.   

2.2 Establishment of exclusion/protection zones and power flux density limit

Following the preliminary calculation mentioned above, ISED examined regulatory measures already put in place by other countries to protect radio altimeters, such as those of Japan and France. These measures are all public and were available for review by all parties. ISED noted that regulators in other countries such as the United States and in the European Union are still studying the matter. Similar to the framework established by France (available only in French), the SRSP proposed the use of exclusion and protection zones to mitigate interference to aircrafts around certain runways. The runways identified are those where automated landing is authorized, based on advice from Transport Canada. In addition to the identification of exclusion zones and protection zones, ISED also defined a power flux density (pfd) limit for protection zones proposed in the SRSP based on the study conducted by the RTCA (report SC-239). Annex C of this document provides a description of the calculation used for the establishment of the pfd limit. The intent of the pfd limit was to provide a clear requirement to operators in enabling their deployment of 5G systems within the protection zones. These mitigation measures take into account the characteristics of Canadian flexible use systems such as maximum permitted power levels.  

2.3 National antenna down-tilt requirement

In November 2020, France’s Agence Nationale des Fréquences (ANFR) imposed a national antenna down-tilt requirement as part of its mitigation measures to protect radio altimeters in the 3400-3800 MHz band. This requirement was scaled back in March 2021 once more information was obtained on the types of radio altimeters used on military and emergency services  helicopters. In Canada, helicopters are used in low altitude military operations, search and rescue operations and medical evacuations all over the country. After initial consultation with Transport Canada and the Department of National Defence, ISED proposed a national antenna down-tilt requirement for 5G base stations similar to the original requirement in France. Given that ISED cannot confirm all types of radio altimeters in use on helicopters in Canada at this time, the national down-tilt requirement is proposed to protect the lives and safety of Canadians pending further investigation.

3. Next steps

ISED will review the comments received and, as required, publish an updated SRSP after the closing of this Consultation.

As indicated previously, ISED anticipates that additional evidence from domestic and international studies will be available throughout the balance of this year and into 2022. Further changes are expected to be proposed to the SRSP at that time, through a future consultation. In the meantime, ISED plans to continue gathering further information from all stakeholders.

4. Submitting comments

Stakeholders are requested to provide their comments in electronic format (Microsoft Word or Adobe PDF) by email. Stakeholders may amend their comments or provide further comments on any issues relating to the Consultation prior to October 15, 2021.

Paper submissions should be mailed to the following address:

Innovation, Science and Economic Development Canada
Senior Director, Terrestrial Engineering and Standards
235 Queen Street (6th Floor, East Tower)
Ottawa ON K1A 0H5

5. Obtaining copies

All spectrum-related documents referred to in this paper are available on ISED's Spectrum Management and Telecommunications website.

Annex A: Publicly available studies and reports (in the language of the source)

Annex B: ISED’s preliminary calculation

ISED conducted preliminary calculations to assess the potential for interference based on  publicly available studies in the context of Canada’s technical rules on the 3500 MHz band. The calculation evaluated coexistence between radio altimeters and 5G equipment in Canada based on the International Telecommunication Union (ITU) Recommendation ITU-R M.2059-0, Operational and technical characteristics and protection criteria of radio altimeters utilizing the band 4200-4400 MHz and the maximum permitted power of 5G equipment in Canada, as defined in RSS-192, Flexible Use Broadband Equipment Operating in the Band 3450-3650 MHz.

Assumptions

The following assumptions were made: 

  • the base station’s out-of-band antenna pattern was assumed to be the same as the in-band antenna pattern
  • the antenna main beam (excluding antenna side-lobe contributions) of the base station was assumed to be pointing in the direction of the aircraft
  • the radio altimeter (RA) antenna patterns are typically horizontally polarized; cross-polarization isolation was not taken into account due to varying flight vectors of aircraft
  • the radio frequency environment was assumed to have a single point source (i.e. no consideration for aggregate power from multiple sources, diffraction and/or multipath effects)

Radio altimeter protection criteria

This calculation used the following radio altimeter protection criteria found in (ITU-R M.2059-0, ICAO ACP-WGF29/WP11):

Desensitization

\[ I_{TX,IF} ≤ N - 6\:dB \]

where ITX,IF is the interference power threshold, and N is the effective receiver thermal noise power.

Front end overload

\[ I_{RF} ≤ P_{T,RF} \]

where IRF is the total peak interference signal power at the receiver input, and PT,RF is the input power threshold defined in Table 1 and Table 2 of ITU-R M.2059-0.

False altitudes

\[ I_{D} < I_{T,FA} \]

where ID is the interference power at the detector, and IT,FA = –143 dBm/100 Hz following the instantaneous altimeter local oscillator. This criterion is limited to the Frequency Modulated Continuous Wave (FMCW) radio altimeter design as opposed to the pulsed radio altimeters.

As recommended in  ITU-R M.2059-0, p. 18, the three (3) protection criteria are considered in all cases since no clear demarcations between these interference effects.

Although ICAO recommends adding the aeronautical safety margin of at least 6 dB to the radio altimeter protection criteria in any coexistence study, no safety margin was applied in the calculation.

Radio altimeter frequency dependent rejection

Radio altimeters employ bandpass filters to provide a frequency-dependent rejection (FDR), also known as RF selectivity, of unwanted signals outside the 4200-4400 MHz band. The RF filter attenuation of 24 dB per octave (up to a maximum of 40 dB) was applied to the radio altimeters in the calculation, described in  ITU-R M.2059-0 (see table B1).

Table B1: FDR of radio altimeters described in ITU-R M.2059-0
Interference frequency, MHz RF filter attenuation, dB
≤ 4200 Attenuated at 24 dB per octave to a maximum of 40 dB
4200 0
4300 0
4400 0
≥ 4400 Attenuated at 24 dB per octave to a maximum of 40 dB

To calculate the number of octaves as defined in the FDR, the edge (i.e. 4200 MHz) of the radio altimeter operating frequency band was used (see figure B1). Therefore, one (1) octave corresponds to the half or double of the 4200 MHz frequency.

Figure B1: Plot of the RF selectivity curve

Plot of the RF selectivity curve
Description of figure 1

This figure shows the plot of the radio frequency selectivity curve based on a 24 dB per octave (up to a maximum of 40 dB) from the 4200 MHz band edge. The x-axis represents the frequency range from 1000 MHz to 4200 MHz and the y-axis represents the attenuation from 0 dB to 40 dB.

Methodology

Due to the potential of unobstructed path between the radio altimeter’s antenna and the base station’s antenna, the Friis transmission equation with the free-space path loss model according to the ITU-R P.525-4 (Equation 4) was utilized in the calculation. Six (6) analog and four (4) digital radio altimeters (FMCW and pulsed) were considered based on the technical characteristics defined the table 1 and table 2 of ITU-R M.2059-0.

The calculation considered an aircraft approaching and/or landing in the vicinity of an outdoor transmitting base station. Both non-active antenna system (non-AAS) base station equipment and active antenna system (AAS) base station equipment were analyzed using the technical requirements contained in RSS-192, issue 4.

Coexistence of radio altimeters and non-AAS base stations

A non-AAS outdoor base station with a 10 MHz channel bandwidth at the 3515 MHz center frequency of the block was analyzed. The base station’s transmitter output power and unwanted emission levels were according to sections 8.6 and 8.7 of the RSS-192, issue 4 (see table B2).

Table B2: Transmitter output power and unwanted emission levels for the non-AAS outdoor (type 1) base station
Power spectral density, dBm/5 MHz Power spectral density, dBm/MHz Power requirement Frequency range of emission, MHz
68 61 e.i.r.p. 3510-3520
21 14 e.i.r.p. 3520-3525
15 8 e.i.r.p. 3525-3530
13 6 e.i.r.p. 3530-3650
21 14 e.i.r.p. 3650-3655
15 8 e.i.r.p. 3655-3660
13 6 e.i.r.p. 3660-3690
N/A –13 Conducted power >3690

Additional cases were analyzed to account for the susceptibility of interference within the radio altimeter’s 4200-4400 MHz operational band (i.e. in-band interference) and from outside the 4200-440 MHz band (i.e. out-of-band interference) from non-AAS outdoor base station.

  1. Coexistence of radio altimeters and a non-AAS outdoor base station from emissions in the 3510 MHz-3690 MHz range.

    The radio altimeters were assumed to receive an interference signal based on the values in table B2:

    • 61 dBm/MHz power spectral density from 3510 MHz-3520 MHz
    • 14 dBm/MHz power spectral density from 3520 MHz-3525 MHz
    • 8 dBm/MHz power spectral density from 3525 MHz-3530 MHz
    • 6 dBm/MHz power spectral density from 3530 MHz-3650 MHz
    • 14 dBm/MHz power spectral density from 3650 MHz-3655 MHz
    • 8 dBm/MHz power spectral density from 3655 MHz-3660 MHz
    • 6 dBm/MHz power spectral density from 3660 MHz-3690 MHz

    The RF filter attenuation was determined to vary from 6.21 dB at 3510 MHz to 4.48 dB at 3690 MHz.

  2. Coexistence of radio altimeters and a non-AAS outdoor base station from emissions in the 3690 MHz-4200 MHz range.

    The radio altimeters were assumed to receive an interference signal of -13 dBm/MHz power spectral density (conducted) from 3690 MHz-4200 MHz (see table B2). A maximum base station antenna gain of 17 dBi was used based on table B1of SRSP-520. The RF filter attenuation was determined to vary from 4.48 dB at 3690 MHz-0 dB at 4200 MHz.

  3. Coexistence of radio altimeters and a non-AAS outdoor base station from emission in the 4200 MHz-4400 MHz range.

    The radio altimeters were assumed to receive an interference signal of -13 dBm/MHz power spectral density (conducted) from 4200 MHz-4400 MHz. A 17 dBi base station antenna gain and a 0 dB RF filter attenuation was applied in this case.

Coexistence of radio altimeters and AAS base stations

An AAS outdoor base station with a 10 MHz channel bandwidth at the 3515 MHz center frequency of the block was further analyzed. The base station’s transmitter output power and unwanted emission levels were according to sections 8.6 and 8.7 of the RSS-192, issue 4 (see table B3).

Table B3: Transmitter output power and unwanted emission levels assumed for AAS outdoor (type 1) base station
Total radiated power (TRP) density, dBm/5 MHz TRP density, dBm/MHz Frequency range, MHz
47 40 3510-3520
16 9 3520-3525
12 5 3525-3530
1 –6 3530-3650
16 9 3650-3655
12 5 3655-3660
1 –6 3660-3690
N/A –13 >3690

Analogously to the non-AAS base station case, in-band interference and out-of-band interference from the AAS outdoor base station to radio altimeters were assessed.

Results

Examples of calculated minimum separation distances (using MATLAB) between radio altimeters and outdoor non-AAS base station to potentially prevent in-band interference and out-of-band interference are provided in table B4 and table B5, respectively. Shorter distances were found in the AAS base station case since no antenna gain was applied to the total radiated power (TRP) values in the calculation.

Table B4: Summary of calculated minimum separation distances between radio altimeters and the outdoor non-AAS base station to prevent potential in-band interference*
Radio altimeter number referenced in ITU-R M.2059-0 Radio Altimeter Type Minimum separation distance for desensitization to not occur, km Minimum separation distance for false altitude reports to not occur, km
1 Analog, FMCW 0.9 0.9
2 Analog, FMCW 0.4 0.8
3 Analog, FMCW 1.0 0.9
4 Analog, pulsed 6.2 N/A
5 Analog, pulsed 4.0 N/A
6 Analog, pulsed 5.1 N/A
7 Digital, FMCW 0.4 0.8
8 Digital, FMCW 1.5 1.4
9 Digital, FMCW 0.7 0.8
10 Digital, pulsed 11.6 N/A

* Case with -13 dBm/MHz conducted PSD emission limit at 4200 MHz, 17 dBi base station antenna gain and 0 dB RF filter attenuation

Table B5: Summary of calculated minimum separation distances between the radio altimeters and the outdoor non-AAS base station to prevent out-of-band interference*
Radio altimeter number referenced in ITU-R M.2059-0 Radio altimeter type Minimum separation distance for front end overload to not occur, km
1 Analog, FMCW 0.6
2 Analog, FMCW 8.5
3 Analog, FMCW 14.4
4 Analog, pulsed 2.7
5 Analog, pulsed 2.1
6 Analog, pulsed 2.1
7 Digital, FMCW 0.7
8 Digital, FMCW 5.4
9 Digital, FMCW 9.0
10 Digital, pulsed 4.8

* Case with 61 dBm/MHz e.i.r.p. density emission limit at 3515 MHz and 6.16 dB RF filter attenuation

Based on the separation distances calculated above in table B4 and table B5,  radio altimeter protection criteria may not be satisfied in certain base station deployment scenarios

Conclusion

Harmful interference to radio altimeters from signals operating outside their band of operation (4200-4400 MHz) was calculated to be possible, which could include signals from future flexible use deployment of 5G systems in the 3500 MHz band around airports and heliports, warranting further investigation into the issue.

Annex C: Establishment of power flux density limit in protection zones

Given similarity to Canada’s rules (e.g. maximum permitted power level and frequency of operation), France’s framework (available only in French) was used as the basis to define the exclusion and protection zones in the proposed revision to SRSP-520, which were developed to protect radio altimeters on aircraft from harmful interference during an airport approach in the landing phase of a flight.

Annex D: Definition of exclusion zones of the Consultation on Amendments to SRSP-520, Technical Requirements for Fixed and/or Mobile Systems, Including Flexible Use Broadband Systems, in the Band 3450-3650 MHz document describes exclusion zones as rectangular areas around airport runways where automated landing is authorized in Canada. Exclusion zones were developed to protect radio altimeters from harmful interference in scenarios where an aircraft is operating at 200 feet above ground–where an error in a radio altimeter reading could be catastrophic for automated landing systems.

Annex E: Provisions applicable to protection zones, E.1 Definition of protection zones of the Consultation on Amendments to SRSP-520, Technical Requirements for Fixed and/or Mobile Systems, Including Flexible Use Broadband Systems, in the Band 3450-3650 MHz document describes protection zones as rectangular areas that extend from the edge of the exclusion zones. For each runway there are two protection zones, each extending from either end of an exclusion zone. Protection zones were developed to protect radio altimeter from harmful interference in scenarios where an aircraft is operating between 200 feet and 1000 feet above ground—a situation where air crew workload is high.

The power flux density (pfd) limit in the protection zones was calculated by taking into account the height of the aircraft at the boundary of the exclusion and protection zones defined in the revision to the SRSP, which is 91.44 metres (300 feet). A power spectral density (PSD) value of -41.5 dBm/MHz was assumed for the received power interference threshold not to be exceeded to avoid harmful interference. This value was estimated by interpolating the interference tolerance mask values specified in figure 9-1 of the Radio Technical Commission for Aeronautics (RTCA) report for an aircraft at 200 feet and 1000 feet. This value corresponds to the interference tolerance mask value for a fundamental emission from a base station operating with a centre frequency of 3750 MHz interfering with an aircraft operating using a radio altimeter in RTCA usage category 1. Usage category 1 covers commercial air transport airplanes, both single-aisle and wide-body.

The pfd was calculated as follows:

\[ pfd = PSD {-} 30 {-} 10log_{10}(A_{r}) \]

where:

pfd: is the pfd limit in dBW/MHz at 91.44 metres (300 feet) above ground;

PSD: is the received power interference threshold not to be exceeded to avoid harmful interference to an aircraft operating within the protection zones defined in the revision SRSP-520, assumed to be -41.5 dBm/MHz; and

Ar: is the effective aperture of the receiver antenna, calculated as follows:

\[ A_{r} = \left( \frac {c^{2}} {4 π f^2} \right) \]

where:

c: is the speed of light, assumed to be 3.0 x 108 m/s; and

f: is the centre frequency of the base station, assumed to be 3650 MHz.