Guard Bands Between BFWA Operators In The 28GHz Band

28GHz band BFWA inter-operator guard bands

Introduction

At the last Consultative Group meeting the RA presented the proposals to issue licences in the 28GHz band for BFWA in three equal packages in a number of regions. The spectrum packages are paired blocks of 2x112MHz arranged on the 27.5-29.5GHz band CEPT channel plan in a manner consistent with developments regarding the anticipated ERC Decision for FS/FSS use of the band.

The original proposals were for three equal spectrum packages which were assumed to include guard band requirements between the operator assignments in order to facilitate co-existence between operators in the same area. This leads to the situation that the entire 112MHz sub-bands will not be available for deployment without some constraints brought about by the need to ensure successful co-existence.

Since this time, developments with the draft ERC Decision have resulted in the possibility of an "extra" 56MHz paired channel being available to the Fixed Service in the FS exclusive part of the 28GHz band. This could provide an opportunity to include a measure of "fixed protection" between operators and help to alleviate some of the co-ordination issues. This "extra" channel is subject to approval of the ERC Decision relating to geographical band segmentation in the 28GHz band.

BFWAtg(00)23 Appendix 1 illustrates this band layout.

The case for "inclusive" guard bands

In taking the decision to include guard band requirements within the assigned blocks the following considerations have been taken into account:

	Studies within CEPT and ETSI have shown that the variety of technologies, duplexing and multiple access methods lead to differing requirements for guard band size.
	These studies have also shown that deployment scenarios impact the guard band requirements.
	The draft ERC Recommendation for FWA in the 26GHz band suggests that "that the estimated size of the guard band needed is equal to the greater channel used by the two neighbouring systems;"
	To provide absolute protection between operators in all cases, some studies suggest that guard bands between the assignments may need to be up to at least two equivalent channel widths.
	Different studies conclude differently on the required guard band size.
	Under certain conditions these large guard bands might be unnecessary and lead to wasted spectrum which could otherwise be used under certain conditions.
	Guard bands of two equivalent channel widths represent a significant proportion of the spectrum available for BFWA.
	Through limiting the amount of spectrum available, "hard" guard bands would also colour the technology choice for an operator.
	The RA wishes to maximise the spectrum available and used for BFWA services.

However there remain some drawbacks to this approach. These are summarised below:

	It may be difficult for competing operators to co-operate in order to minimise guard band size, interference and deployment constraints on each other.
	An agreement needs to be reached over the acceptable level of potential interference between operators given that complete protection cannot be guaranteed without excessive guard bands.
	The band plan leads to the situation where the centre 2x112MHz assignment has a greater co-operation burden with operators on either side of the assignment. This may lead to limitations on the technology choice for this operator.

Operator Co-operation

Operator co-ordination is cited as an acceptable method of determining the guard band requirements in draft ERC Recommendation covering FWA in the 26GHz band by consideration of the guard band as an "edge band".

In the first instance, care should be exercised when planning use of the outermost channels of an assignment that co-existence issues with the neighbouring frequency assignment are examined. To enable operators to co-operate and co-ordinate then it will be necessary for the exchange of an agreed set of parameters and characteristics to allow operators to mutually agree co-ordination. Such a list can be generated following a review of the appropriate methodologies.

Issues might arise since the choices made by one operator within his assignment could impact on the neighbouring assignments to some extent. This can occur where differing channelisation schemes or duplexing method are used for example. It is difficult to foresee all the possibilities and there will be a need to address these issues almost on a case by case basis. However it must be stressed that the need to protect a neighbouring assignment will lead to some constraint on use in the outermost channels and that it is in the interest all concerned that every effort is made to minimise impact and deploy in a responsible manner.

The "extra" Channel

As detailed in the introduction above there is the possibility of a further 56MHz paired channel being available within the exclusive allocation to Fixed Services in this band. Having decided upon three equal 112MHz paired licences this leaves this extra channel unused which is both wasteful and undesirable.

One use of this channel could be to insert a "hard" 28MHz wide guard band between the 112MHz licences to help reduce some of the co-ordination issues between operators and provide at least some level of protection (as illustrated in Appendix 1). In order not to appear wasteful of spectrum these 28MHz channels could be considered as "edge channels" as defined within the CEPT SE19 co-existence work. This means that they could be used for deployment subject to close inter-operator co-operation.

This approach could have number of advantages;

	Some degree of protection is provided between operators which may provide a level of comfort.
	Amount of inter-operator co-operation needed may be reduced.
	Edge bands could provide greater flexibility for "moving around" co-ordination difficulties.

The operator in the centre 112MHz block is less constrained by the need to co-operate with competing operators at each end of the assignment.

¹Draft Recommendation "USE OF THE BAND 24.5-26.5 GHz FOR FIXED WIRELESS ACCESS" See BFWAtg(00)04.

Operator Guidelines

Useful sources of methodologies for assessing the guard band requirements can be found in the draft ERC Report covering FWA deployment in the 26GHz and 28GHz bands. This employs a so-called Interference Scenario Occurrence Probability (ISOP) to identify an acceptable likelihood of interference and hence an appropriate guard band. Alternatively a draft ETSI report (TM4 WI DTR/TM04090) covers co-existence between both P-MP systems and P-P systems and uses C/I coupled with a percentage of cell suffering/generating unacceptable interference to judge successful co-existence. Additionally it is likely that the IEEE 802.16 BFWA project will produce another variation in the next few months.

 Further Action

If appropriate the RA should develop a "reference methodology" based upon one of the above reports once they have reached a sufficiently stable state which can be used in cases where disagreement exists.

²Draft ERC document "Report on the analysis of the coexistence of two FWA cells in the 24.5-29.5 GHz bands" See BFWAtg(00)05.

³Rules for the co-existence of point-to-point and point-to-multipoint systems using different access methods in the same frequency band.

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