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A previous study funded by Ofcom [Richardson et al, 2004] has indicated that there are spectrum gains to made as a result of introduction of ATPC in P-P fixed service bands, especially those operating at frequencies where rain is a significant attenuator. However, that study was limited in scope, dealing only with a simplified scenario of two parallel links, separated by distances of ~ 1-10km. The previous study left us with several questions with respect to the implementation of ATPC in P-P fixed services:

• During rain events, does the use of ATPC increase harmful interference to neighbouring P-P systems, exceeding the frequency assignment criteria? And if so, how often and how badly will the criteria be exceeded?
• Over what geographical area can rain fading be considered to be correlated? This will have an impact on the worst case scenarios for the use of all-ATPC and mixed deployments.
• What is the most efficient way of maximising the increase in packing density and spectrum utilisation through the use of ATPC, without exceeding the assignment criteria?
• What frequency bands will provide the best results in terms of spectrum utilisation as a result of ATPC implementation, taking into account existing technologies, and inter-service sharing?

Spectrum Efficiency Benefits of Introducing ATPC

The potential advantages of ATPC reported in the literature include:

• Reduced average power consumption.
• Extended equipment mean time between fades (MTBF).
• Elimination of the ‘upfade’ problem in receivers.
• Improved outage performance due to the reduced influence of adjacent channel interference (ACI).
• Easier frequency co-ordination in areas of high radio-relay station density.

It is this last point that is fundamental to the objectives of this study, since an increase in spectrum utilisation is dependent on the ability to reduce the co-ordination distance for systems employing ATPC without compromising the quality of service of neighbouring links through excessive interference.

However, it is vital to emphasise that ATPC should only be used to combat temporary fading of the wanted link rather than interference from the unwanted link(s). Otherwise, a situation could arise where two ATPC systems repeatedly increase their Tx power in response to each other’s interference until both are transmitting at their maximum Tx power. This situation would reduce to the non-ATPC case, completely negating any spectrum efficiency benefits gained as a result of employing ATPC in the first place. To avoid this situation, ATPC links must be designed and deployed correctly to take into account the interference generated by neighbouring links so that ATPC is used to combat rain fading, rather than interference.

The US administrations (National Spectrum Managers Association, NSMA) already permit a co-ordination advantage to be claimed for links operating at frequencies below 12.2 GHz [National Spectrum Managers Association Working Group 18 Recommendation on Adaptive Power Control (WG18-91-032)]. The general approach is to follow the standard frequency assignment and co-ordination process defined in the Handbook on Digital Radio-Relay Systems [ITU-R, 1996], but to claim a co-ordination advantage on the basis of a reduced Txco-ord Power. This advantage can be claimed if the following conditions are met:

• the wanted and unwanted paths are de-correlated,
• the link with ATPC is at Txmax Power for less than 0.01 % of the time.

Project status

The study is well underway. A comparison of the total span of spectrum required to assign sets of links has already be performed. The first set of results has indicated that:

• It is less spectrally efficient to assign a mixture of low and high efficiency-class links than when the links are re-specified to use the lowest, or highest, available efficiency classes, keeping the capacity of every link the same.
• It is more spectrally efficient to specify links with the highest available efficiency class than with the lowest available class. Also, the high efficiency class assignments required 75% of the spectrum needed for the low class assignments. However, a high minimum isolation is required between high-efficiency class links due to the higher W/U ratio.

The final findings have now been published - see below.

• Impact of Introducing Automatic Transmit Power Control in P-P Fixed Service systems operating in bands above 13GHz [pdf]
  Final Report

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