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PreviousSP 5150 MHz — Spectrum Utilization Policy for Licence-exempt Wireless Local Area Networks in the 5 GHz Range (Issue 2)
Appendix 3 — Excerpts from Annex 1 to Recommendation ITU-R M.1652
Dynamic Frequency Selection in Wireless Access Systems Including Radio Local Area Networks for the Purposes of Protecting the Radiodetermination Services in the 5 GHz Band
Annex 1 to Recommendation ITU-R M.1652
The use of DFS in WAS including RLANs for the purpose of protecting the radiodetermination service in the 5 GHz band
1. Introduction
1.1 DFS
Resolution 736 (WRC-2000) calls, inter alia, for studies on the feasibility of sharing between the mobile service for WAS2 and the radiodetermination service in the frequency bands 5 250–5 350 and 5 470–5 725 MHz. Link budget calculations have shown that interference mitigation techniques are required to enable sharing of WAS with other services such as radar systems. This Recommendation describes the interference mitigation technique(s) DFS3 as specified in the 5 GHz RLAN standards, with performance calculations based on typical implementations.
WAS and radars operating in the 5 GHz band will interfere when operating at the same frequencies and within range of each other.
DFS has then been envisaged to:
- ensure a spread of the loading across the available spectrum of the WAS under the field of view of a satellite to reduce the aggregate emission levels at the satellites of the FSS (feeder links) and EESS (active) from WAS; and
- avoid co-channel operation with other systems, notably radar systems.
Extension of the use of DFS as described herein allows WAS to avoid interfering with the radiodetermination service. The general principle applied is that WAS should detect interference and identify radar interferers and shall not use those frequencies used by the radar.
1.2 Objective of the use of DFS with respect to radars
The objective of using DFS in WAS is to provide adequate protection to radars in the 5 GHz band. This is achieved by avoiding the use of, or vacating, a channel identified as being occupied by radar equipment based on detection of radar signals.
For the purpose of this Annex, a discussion of radiodetermination systems in the 5 GHz range utilized in determining DFS characteristics can be found in Annex 3.
The implementation of radar detection mechanisms and procedures used by WAS are outside the scope of this Annex. The main reasons for this are that:
- WAS design affects implementation;
- practical experience may lead to innovative and more efficient means than can be formulated today;
- different manufacturers may make different implementation choices to achieve the lowest cost for a given level of performance; therefore only performance criteria rather than specifications for a particular mechanism should be given in regulatory documents.
2. DFS performance requirements
The DFS performance requirement is stated in terms of response to detection of an interference signal.
5 GHz WAS should meet the following detection and response requirements.
Procedures for compliance verification should be incorporated in relevant industry standards for RLANs.
2.1 Detection requirements
The DFS mechanism should be able to detect interference signals above a minimum DFS detection threshold of -62 dBm for devices with a maximum e.i.r.p. of < 200 mW and -64 dBm for devices with a maximum e.i.r.p. of 200 mW to 1 W4 averaged over 1 µs.
This is defined as the received signal strength (RSS) (dBm), normalized to the output of a 0 dBi receive antenna, that is required to be detected within the WAS channel bandwidth.
2.2 Operational requirements
The WAS should be able to perform channel availability check: A check during which the WAS listens on a particular radio channel for 60 s to identify whether there is a radar operating on that radio channel.
The WAS should be able to perform in-service monitoring: Monitoring of the operating channel to check that a co-channel radar has not moved or started operation within range of the WAS. During in-service monitoring the radar detection function continuously searches for radar signals in-between normal WAS transmissions. This requires the use of quiet spaces between successive WAS transmissions (See Annex 4) [Not attached in this document].
If the WAS has not previously been in operation or has not continuously monitored the channel with in-service monitoring, it should not start transmission in any channel before completion of a channel availability check.
2.3 Response requirements
A channel that has been flagged as containing a radar signal, either by a channel availability check or in-service monitoring, is subject to a 30 min period (non-occupancy period) where it cannot be used by the WAS device in order to protect scanning radars. The non-occupancy period should start at the time when the radar signal is detected.
Additionally, in the band 5 600–5 650 MHz, if a channel has been flagged as containing a radar, a 10 min continuous monitoring of the flagged channel is required prior to use of that channel. Otherwise, other appropriate methods such as channel exclusion would be required.
Channel move time is defined as the period of 10 s needed by a WAS to cease all transmissions on the operating channel upon detection of an interfering signal above the DFS detection threshold. Transmissions during this period will consist of normal traffic for typically less than 100 ms and a maximum of 200 ms after detection of the radar signal. In addition, intermittent management and control signals can be sent during the remaining time to facilitate vacating the operating channel. The aggregate time of the intermittent management and control signals are typically less than 20 ms.
2.4 Summary of the requirements
Table 1 provides a summary of the requirements described above. An example of the operating procedures is given in Annex 2 [Not attached in this document].
| Parameter | Value |
|---|---|
| DFS detection threshold | -62 dBm for devices with a maximum e.i.r.p. of < 200 mW and -64 dBm for devices with a maximum e.i.r.p. of 200 mW to 1 W averaged over 1 µs |
| Channel availability check time | 60 s |
| Non-occupancy period | 30 min |
| Channel move time | ≤ 10 s |
Footnotes
2 Throughout this Recommendation the term "WAS" denotes "wireless access systems including RLANs".
3 The DFS feature was specified in the 5 GHz RLAN standards initially in order to mitigate interference among uncoordinated RLAN clusters, and to provide optimized spectral efficiency for high-capacity, high bit-rate data transmission.
4 In practice, it may not be necessary for each device to implement full DFS functionality, provided that such devices are only able to transmit under the control of a device that ensures that all DFS requirements are fulfilled.