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Foreword

It is required by the Wireless Telegraphy Act, 1949 and 1998 that no radio apparatus shall be installed or used in the United Kingdom, except under the authority of a licence granted by the Secretary of State. It is a condition of such a licence that the performance of the radio equipment meets certain minimum standards laid down in the appropriate Interface Requirement document. Compliance will have been declared in accordance with the RTTE Directive.

This document details the frequency assignment criteria and principles that will be employed by the RA in the selection of frequencies for use by compliant fixed point to point digital radio equipment operating in the band or frequency range specified.

These assignment criteria are subject to updating and amendment, and intending operators / manufacturers should consult the latest version of this document complete with any amendments. Single copies of this document are available free from the RA library at the address below:

ADDRESS:

Radiocommunications Agency
Library Services
Wyndham House
189 Marsh Wall
Docklands
London E14 9SX

Switchboard and 24 hour enquiry service:
Telephone: 020 7211 0502/0505
Fax: 020 7211 0507

1. General

1.1 Introduction

This document outlines the frequency assignment criteria and principles that will be employed by the RA in the selection of frequencies for use by fixed point-to-point digital radio services operating in the frequency range 5925 MHz to 6425 MHz.

The channel plan shall be in accordance with that given in Clause 5.

This document has been generated taking into account the following equipment capacities:

1 x STM-1 in 30 MHz ---------------------------(EN 300 234) --------------- Class 5

2 x STM-1 in 30 MHz ---------------------------(EN 301 127) --------------- Class 5

The equipment performance parameters relating to these capacities can be found in the relevant UK Interface Requirement document.

1.2 Licensee's responsibility

The Foreword of this document states that the establishment, use or installation of transmitting or receiving apparatus is subject to the issue of a licence by the Secretary of State. The licensee must ensure that equipment conforms with and is maintained to the standard set out in the relevant UK Interface Requirement document.

1.3 Link Length Policy

The RA operates a link length policy to promote the use of the highest possible frequency band for the distance over which the link is to operate. Links transferring STM-1 and 2 x STM-1 data rates will be licensed where the path length is ³ 16 km. At distances below 16 km there may be some expectation that a higher frequency band could be used, depending on the availability of frequencies and equipment.

1.4 Other Services

1.4.1 Satellite Services

WRC 2000 considered the use of the 5925 MHz to 6425 MHz band by FSS earth stations on maritime vessels. Further discussion is ongoing within the ITU. For the latest information on this issue please contact the Radiocommunications Agency. The Lower 6 GHz band is also shared with FSS terrestrial uplinks. Co-ordination between FSS earth stations and FS systems will be undertaken by FTSLU during the assignment period.

2. Transmitting and receiving installations

2.1 General

The transmitting and receiving installations shall conform to Clauses 0 and 0 below. The installations shall be implemented in accordance with good engineering practice.

2.2 Antenna directivity

The antenna installation at the licensed sites shall be such that the antenna pattern will not exceed the co-polar and cross-polar RPE given in the relevant equipment specification.

2.3 Antenna polarisation

The linear plane of polarisation of emissions for a particular radio link will be confirmed by the Radiocommunications Agency.

The antenna alignment surface shall be aligned as precisely as possible to the true vertical or true horizontal and the misalignment shall be no greater than 3° . Links using ATPC and engineered to meet very high levels of performance may require alignment accuracy in the order of 1⁰.

2.4 Adaptive Transmitter Power Control (ATPC)

In order to help maximise use of the frequency band ATPC must be employed on all bi-directional radio links (including passive repeater end stations) operating with a normal assigned fade margin of 20dB or more. ATPC must be activated by detection of a drop in received signal level at the distant end of the radio link caused by fading along the path. A minimum ATPC range of 10dB must be used and will be assumed by the assignment tool. In the reduced power configuration the EIRP must be at least 10dB below the maximum assigned value detailed in section 2.5 below and will be known as the normal EIRP (See also section 3.11).

ATPC operation will not be assumed for radio links operating with assigned fade margins of 15dB or for uni-directional links. ATPC operation will be optional for radio links operating with an assigned fade margin between 15dB and 20dB but will need to be declared by the operator / applicant.

2.5 Equivalent isotropically radiated power (EIRP)

Where appropriate two values of assigned EIRP will be stated for each transmitting station in the licence. The maximum assigned value of EIRP must not be exceeded under any condition. The normal assigned EIRP is the level for operation under normal steady propagation conditions during which ATPC has not been activated. This will also be the level used by the assignment system for interference assessment purposes. A tolerance of ± 3 dB on these assigned values will be allowed in practice. The upper limit for the maximum EIRP normally assigned will be 50dBW.

For uni-directional links and bi-directional links not employing ATPC the normal assigned EIRP and the maximum assigned EIRP value will be the same. (i.e. the ATPC range will be 0dB).

3. Principles of assignment and EIRP derivation.

3.1 Normal assignment

A normal frequency assignment for a single or multi-section bi-directional link shall comprise of a pair of radio frequencies of corresponding channel number, one from each of the low and high frequency groups, and antenna polarisation.

3.2 Multi-section links and repeater stations

3.2.1 Multi-section links

In the case of a multi-section link the direction of transmission of the two frequencies shall alternate for successive repeater sections so that the transmitter frequency assigned to any station shall be taken from only one of the frequency groups in the lower or upper half of the band.

3.2.2 Passive Repeaters

Passive repeater operation using either back to back antennas or planar reflectors will be allowed only under certain specific conditions. Use of these systems will be judged on a case by case basis and are expected to only be used for availabilities of 99.99% or less under the following conditions:

1. At remote sites power provision is extremely difficult.
2. There is a clear spectrum saving by the use of one repeater station to overcome the obstructed path rather than going around the obstruction with two or more conventional links.
3. Use of the higher power needed for this operation does not unnecessarily sterilise surrounding area and block further normal assignments.
4. Higher Performance antennas (if available) should be used at the end stations along with as large as reasonably possible repeater station antennas to maximise the repeater gain.

Detail of the assignment process modifications to deal with passive repeater operation are given in Annex E.

3.3 Go - Return separation

The frequency difference between a pair of corresponding go and return frequencies shall be 252.04 MHz.

3.4 STM-1 Parallel links operating over the same path

Due to the fading characteristics in this frequency band the interference assessment for parallel links operating over the same path will be calculated by assuming that fading of wanted and interfering signals is uncorrelated. However, it is expected that cross-polar operation in adjacent channels will be possible for the majority of links assigned to an availability of 99.99%. Co-polar operation may need to be separated by at least two channels.

3.5 Antenna discrimination

In assigning frequencies for links which are in the same geographical area due consideration shall be taken of the antenna discrimination. When available, data derived from a manufacturer guaranteed Radiation Pattern Envelope (RPE) will be used. If this is not available the appropriate RPE specified in the relevant UK Interface Requirement will be assumed.

3.6 Path Clearance

It will usually be assumed that each hop has a clearance from obstructions of not less than 0.577F between the transmitting and receiving antennas at the two stations under conditions corresponding to values of the ratio K greater than 0.7.

3.6.1 Obstructed Paths

Obstructed paths will be allowed where their need is clearly demonstrated, with an EIRP increased up to a maximum of 6dB above the unobstructed path assignment level providing that the maximum EIRP limit of 50dBW is not exceeded. For obstruction losses in excess of 6dB operators will be expected to accept the possible lower link availability. The following conditions will apply:

1. Where use of the higher power needed for this operation does not unnecessarily sterilise surrounding area and block further normal assignments;
2. The increase in EIRP will only be allowed if the obstruction cannot be overcome by increasing the antenna height;
3. Higher Performance antennas should be used to minimise the sterilisation of surrounding areas, considering the higher than normal EIRP associated with this type of operation;
4. The increased EIRP will only be allowed on link availabilities below or equal to 99.99%.

Detail of the assignment process modifications required to deal with operation over an obstructed path is given in Annex D.

3.7 Path Loss

3.7.1 Median Path Loss

The median path loss between two stations will be equal to the free space basic transmission loss. Atmospheric gaseous absorption is considered negligible in this frequency band.

3.8 Availability

Availabilities greater than 99.99% are to be agreed with the RA on a case-by-case basis.

3.9 Receiver Input Levels

Table 1: Receiver reference signal levels.

The RA will in general examine applications for the use of radio links on the assumption that the median signal level of the receiver input is as detailed in Table 1 and a maximum transmitter power shall be assigned accordingly. The levels in Table 1 are derived from the indicative link budgets given in Annex A.

3.10 Fade margin

3.10.1 General

The two main factors considered, that cause the wanted signal to fade are clear-air fading and rain fade. Clear-air fading is considered dominant below 10GHz and a combination of clear-air effects and rain fade become important above 10GHz with rain fade eventually dominant at higher frequencies. Consequently for the Lower 6 GHz band only clear-air effects are considered.

3.10.2 Clear-Air Fade Margin

The total clear-air fade margin, M₂ depends on path length and terrain, service availability required and a geoclimatic factor. The value for the fade margin (M₂) is derived from ITU Recommendation ITU-R P. 530-7 clause 2.3. The methods described in the recommendation calculate the percentage of time that a specific fade depth (A) is exceeded. In order to calculate the required fade margin for a particular percentage of time (unavailability) an iteration process is required. This iteration process starts using the minimum fade margin value given below and calculates the percentage of time that this fade depth will be exceeded. If this percentage time is too large then the fade depth is increased and the process repeats until a figure for fade depth relating to the percentage of time required is obtained to within 0.1dB.

The process is described in more detail in Annex C.

3.10.3 Minimum Fade Margin

This frequency band is currently shared with the Fixed Satellite Service and it is possible that, in the future, satellite transmit terminals will also be licensed on maritime vessels. In this shared environment a minimum fixed link fade margin of 35 dB is desirable. Therefore, the RA will aim to incorporate a fade margin ³ 35 dB in the assigned EIRP. The RA will inform the link applicant when this is not possible.

3.11 Equivalent Isotropically Radiated Power calculation

The licence schedule will detail the maximum assigned EIRP for each transmitting station and the normal EIRP where ATPC is in operation based on the following calculations:

Maximum EIRP = Rx Median signal level + Rx station feeder losses - Rx antenna gain + Path Loss

Normal EIRP = Maximum EIRP - ATPC Range

The Rx Median signal level is the receiver minimum input level plus the fade margin. The maximum co-polar gain figure for the receiving antenna is used.

4. Interference assessment

4.1 General

The link to be assigned is co-ordinated with all other links in the same frequency band within a co-ordination zone radius. The co-ordination zone radius is typically 250km. The size of the co-ordination zone may be reviewed from time to time as operational experience is gathered. Interference to and from the proposed link is assessed taking into account the path profile between the two stations. Use is made of antenna radiation patterns to obtain the gain of antennas in the direction of unwanted signals.

The interference evaluation considers two conditions:

1. Median wanted signal against the unwanted signal enhanced by clear-air effects.
2. Faded wanted signal against median unwanted signal.

The combination producing the highest interference signal is taken.

The procedures followed are in line with ITU-R PN. Rec. 452 and outlined in the flow diagram shown in Annex F.

4.2 Interference Limits

The interference limits are derived as follows:

Interference limit = Reference sensitivity for BER = 10^-6 (See Annex A)) - W/U ratio.

4.3 Co and Adjacent channel limits

The maximum co-channel and adjacent channel interference, at the receiver input, from a single unwanted source is shown in Tables 2 and 3. These figures are based on a minimum allowable N/I of +3dB that translates to an interference margin of 2dB shown in Annex A. W/U ratios for single-entry interferers, relating to mixed capacity digital systems, are given by the matrices in Annex B. The above limits are set on the basis that the wanted signal is at the reference sensitivity input level of Table 1.

Table 2: Single entry co-channel interference limits

Table 3: Single entry adjacent channel interference limits

4.4 Multiple interferers

In Tables 2 and 3 and Annex B, the single-entry W/U ratios include allowances for multiple digital interferers. The allowances are 4dB for co-channel interferers and 6 dB for adjacent channel interferers independent of bit-rate.

5. Channel Plan

5.1 Derivation of radio frequency channels for the Lower 6 GHz frequency band in the U.K.

An arrangement, based on the CEPT Recommendation 14-01, with 29.65 MHz channel spacing provides a total of 8 'go' and 8 'return' channels.

Let f_o be the frequency of the centre of the band of frequencies occupied - 6175 MHz.

f_n be the centre frequency of one radio channel in the lower half of the band

f'_n be the centre frequency of one radio frequency channel in the upper half of the band

The frequencies in MHz of the individual channels are expressed by the following relationship:

Lower half of the band: f_n = (f_o - 259.45 + 29.65 n) MHz

Upper half of the band: f'_n = (f_o - 7.41 + 29.65 n)

See Table 4 for the actual carrier frequency.

Fig. 1: Channel Arrangement for the 5925 MHz to 6425 MHz Frequency Band.

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1. Annex A: Receiver input levels and interference levels.

Receiver input levels and maximum permitted interference levels for digital systems operating in the frequency band 5925 MHz to 6425 MHz.

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Annex B: Wanted to unwanted levels.

Tables B1 to B6 give the Wanted to Unwanted (W/U) ratio for all frequency offsets up to three times the mean value of the sum of the wanted and unwanted channel spacing.

The figures in Annex B tables have been derived on the following basis:

Co-channel and Adjacent channel W/U ratios for like with like RA allocated systems have been linked to tables 2 and 3 and therefore to the appropriate European equipment standard. W/U ratios for all other combinations have been derived from the methodology detailed in document RSWG (97) 13 Rev.1. This RSWG paper is available from the Radiocommunications Agency. W/U ratios for systems originally managed by British Telecom on an exclusive basis are based on frequency offset rejection levels calculated by British Telecom.

Offsets of 44.49 MHz and 74.14 MHz have been included into the tables where equipment occupies a bandwidth of 29.65 MHz. This measure is planned to overcome the interference potential created by the narrow centre-gap used in CEPT Recommendation 14-01.

Note: Wanted to Unwanted ratios for the protection of B.T. systems are referenced to the following receiver reference sensitivities for a BER of 10^-6:

Tables B1 and B1a - RA Wanted System STM-1 in 29.65 MHz (EN 300 234)

Tables B2 and B2a - RA Wanted System 2xSTM-1 in 29.65 MHz (EN 301 127 Co-channel Cross Polar)

Tables B3 - Wanted System BT 34 Mbit/s in 29.65 MHz (QPSK)

Tables B4 - Wanted System BT STM-1 in 29.65 MHz (64QAM)

Tables B5 - Wanted System BT STM-1 in 29.65 MHz (128 QAM)

Table B6 - Wanted System BT STM-1 in 90 MHz (RBQPSK)

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Annex C: Detail on Fade Margin calculation.

1. Procedure

The calculation of the clear air fade margin is carried out along the guidelines laid out in ITU Recommendation ITU-R P.530.7 clause 2.3. Since the calculations detailed in this recommendation revolve around calculating percentage of time that a fade depth is exceeded an iteration process is required. This process that follows is that given ITU-R P 530-7 and all references refer to that Recommendation:

Starting with a minimum fade depth (A) given in clause 3.10.3 perform the calculation given by equation (23) of clause 2.3.2 of the recommendation. This calculation requires parameter q_a which in turn requires parameters q_t and q'_a. Parameter q_{t is} derived from equation (22) and q_a is derived from equation (21). Equation (21) in turn requires parameter p_w derived from the procedure of clause 2.3.1.3 for a fade depth, A of 25dB. (See NOTE below).

The result from equation (23) will be the percentage of time (also labelled p_w) that a fade depth A, equal to the minimum is exceeded. If the percentage of time is too high (i.e. the fade depth is too low) then the figure for fade depth, A is doubled (i.e. 15dB becomes 30dB) and the calculation process is repeated.

If the fade depth to be evaluated is greater than 25dB then only the method of clause 2.3.1 need be applied (see 2.3.2 paragraph e)) in order to calculate the percentage of time. If the percentage of time calculated is now too low (i.e. the fade depth is too high) then the calculation process is repeated for a fade depth equal to the mean between the last two figures. This iteration process is repeated until the fade depth figure to within 0.1dB is found that returns the required time percentage.

NOTE: The assignment procedure implemented covering clause 2.3.2 paragraphs a) to d) of the recommendation calculate the fade depth for A = 25dB only. The calculation for A = 35 dB is not implemented. Investigation of this procedure uncovered some deficiencies with this method and so the use of 25dB only was implemented on the advice of the ITU Study Group involved. This issue will be investigated further in future updates to Recommendation 530.

2. Method for small percentages of time

This section, clause 2.3.1 is referred to by the process described above and describes a method to calculate K the geo-climatic factor over the path concerned, the path inclination and finally the percentage of time that a given fade depth, A is exceeded for small percentages of time.

If the link is inland then K is calculated from equation (4) where:

p_L = 5 for the UK

C_Lon = 3 for Europe

C_Lat is given by equations (5), (6) or (7)

C₀ = 1.7 for lower-altitude antenna in the range 0 - 400 m above mean sea level; or

C₀ = 4.2 for lower-altitude antenna in the range 400 - 700 m above mean sea level; or

C₀ = 8.0 for lower-altitude antenna more than 700 m above mean sea level.

If a fraction of the path is over the sea or coast then equations (12) and (13) are implemented to obtain K.

Path inclination is given by equation (18).

Finally the percentage of time that a fade depth is exceeded is calculated using equation (19).

3. Conversion from average worst month to average annual distributions

Clause 2.3.4 equations (30) and (31) are used for conversion from average worst month to average annual distributions. This conversion is only required for frequencies that require iteration between the clear air fade calculations and rain fade calculations.

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Annex D: Assignments over an obstructed path.

Assignments over an obstructed path will be allowed with an increased EIRP up to a maximum of 6dB over the value that would be assigned for a normal assignment having assumed a path clearance of 0.577F, where F is the first fresnel zone radius.

Any required increase in EIRP will be assessed using the following procedure:

By calculating the first Fresnel zone radius, F at the obstructed point in the path:

Where d is the overall path length and d₁ and d₂ are the distances from each end of the link to the obstruction point.

By calculating the extra attenuation due to the obstruction to be added to the path loss from the expression:

(Radians assumed)

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Annex E: Assigning Passive Repeater Stations

Consider a link from A to D comprising two hops A to B and C to D where B and C are a co-located passive repeater station.

Back to Back Antenna Type

It is necessary to calculate the EIRP from the central repeater station and determine the required input level to the repeater station. This involves both standard and non standard assignments in the following manner:

1) Enter the data for all sites and make the feeder loss at both B and C equal to the loss between the two antennas.

2) Perform a standard assignment for the link A to B. From the EIRP and antenna gain at B calculate the required receive level at C. Ignore the direction A to B.

3) Carry out a non-standard assignment from C to D using the calculated receive level from 2) at station C.

4) From the EIRP and antenna gain at C calculate the required receive level at B.

5) Repeat the assignment from A to B using the receive level calculated in 4) at station B.

Step 3) provides the final EIRP at station D and step 5) provides the final EIRP at station A.

It may be useful in some frequency bands to change polarisation at the repeater station.

Plane Reflector

The procedure for these types of passive repeater is the same as points 2) to 5) above except that B is the same point as C and the gain of the reflector needs to be calculated from the following expression:

The antenna gain for point B should be half the value Grep for each hop A to B(C) and C (B) to D. Interference at the reflector only needs to be considered if it arrives from the same direction as the wanted signal.

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Annex F: High-level flow diagram for Recommendation ITU-R P.452

Note: the following diagram is based on ITU-R P.452-7 (1995 P Series Fascicle), but there are no differences between recent versions in the high-level structure of the method.

High-level flow diagram for Recommendation ITU-R P.452

Document History

Document Information:

Title: Frequency Assignment Criteria RA 374.

Technical Editor: Brian Harrison.

Filename: 374p 0001 00

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