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C.1 Ofcom's intention is that a fully flexible, tradable licence would define the right to transmit in terms of transmitted power and a 'spectrum mask'. The spectrum mask would be specified in terms of conditions experienced at a defined geographic boundary. Together, these conditions would be broad enough to be technology neutral, but sufficiently tightly defined that geographic licence boundaries could be managed effectively.

C.2Ofcom's intention is to define these boundary conditions in terms of common physical engineering dimensions (e.g. Watts per Hertz). However, spectrum masks in different licence classes will need to be defined using different parameters. For example, definition using power spectral density will be suitable for many licence classes, but not all frequency bands or technologies.

C.3 For example, the Radio Regulations provide definitions for effective or equivalent radiated power in relation to three 'reference' antennas, which in turn are used for different broad frequency ranges. These are:

• equivalent isotropically radiated power (e.i.r.p.);
• effective radiated power (e.r.p.); and
• effective monopole radiated power (e.m.r.p.).

C.4 The first two of these are in common use for licensing and notifying the characteristics of stations already. Throughout the Radio Regulations power flux density (p.f.d), either at the Earth's surface or at some geographic boundary or test point, is used extensively as either an interference limit or a co-ordination threshold (or trigger) level. Some specific examples are the e.i.r.p. limits for terrestrial and space services sharing frequency bands above 1GHz, which may also include directionality by specifying e.i.r.p. in a particular direction, and the limits for equivalent power flux-density (e.p.f.d.). Elsewhere in the Radio Regulations e.r.p limits are specified to limit the interference from broadcasting to aeronautical services and peak e.i.r.p. density is used in respect of the mobile-satellite and radiodeterminationsatellite services.

C.5 It is suggested by Cave and Webb (2003)²⁹ that property rights be established by setting appropriate power spectral density (PSD) limits at a distance from the transmitting antenna. The total average power in a periodic signal is the sum of the average powers in all the harmonic elements³⁰ of the signal. When plotted³¹ , the result is the Power Density Spectrum of the periodic signal. One limitation of this approach is that this fails to reflect the phase component of the signal. The Fourier coefficients of the signal are in general complex, and provide an alternative way to plot the power spectrum: as the magnitude and the phase of the Fourier coeffcients as a function of frequency³².

C.6 It is simple to translate power flux density (PFD) at a distance to a corresponding field strength by making some simplifying assumptions. But the measurement techniques for field strength vary considerably. High frequency (HF) field strength measurement is an example. This requires that data are taken over as wide a range of conditions as possible at a series of frequencies over paths of different lengths in all regions of the world. Measurements are needed at each hour of the day in the separate seasons and for different solar epochs.

C.7 The out-of-band spectrum of an emission is yet another key element of spectrum rights. Often prescribed by normative standards, out-of-band emissions are defined as the part of the power density spectrum (or the power spectrum when the spectrum consists of discrete components) of an emission (or emissions) which is (immediately) outside the necessary bandwidth and which results from the modulation process, with the exception of spurious emissions. It includes the effects of nonlinearity, which may cause out-of-band emissions immediately adjacent to the necessary bandwidth due to odd order intermodulation products. (See Rec. ITU-R SM.328-10)

C.8 For all of these reasons, Ofcom believes that one common measure is feasible but may be limited in its application (e.g. a spectrum mask specified as PSD may be applicable to fixed wireless access operation at microwaves). At other wavelengths and for other radiocommunication services the use of PSD may be difficult to implement practically as a tradable right with regulatory meaning or status internationally.

C.9 Whilst specifying the upper and lower bounds of an assigned band seems an obvious and simple route to defining a licensee's rights over the quantity of spectrum, the different definitions prescribed by the Radio Regulations and normative standards indicate that care is needed with defining each band such that protection afforded to existing users is not diminished.

29. M. Cave & W. Webb, Designing Property Rights for the Operation of Spectrum Markets, Papers in Spectrum Trading No.1., CMR, Warwick Business School, August 2003.

30. Where the kth harmonic element of the signal has a power.

31. We plot as a function of the product of k and the fundamental frequency, fo.

32. The concept of aperiodic signal transforms is not dealt with here.

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