Study of Market-based Exclusive Spectrum Rights
Annex 4 – The Interference Problem
To better understand the interference problem, it is worthwhile considering the ways in which radio waves propagate in a simple radio system consisting of a transmitter (Tx) and a receiver (Rx). In the simplest system, the Tx generates a signal at its antenna which propagates over the intervening space to the intended Rx. The Tx antenna size, shape, and orientation (height, angle) affects the radiated power in all directions, and similarly, the Rx antenna design affects the ability of the Rx to receive the intended signal (as opposed to signals emanating from other Tx). For example, the simplest antennas are omni-directional and are intended to radiate in all directions equally; whereas directional antennas are able to focus the signal into a specially intended radiation pattern. However, since directional antennas are imperfect some radiation typically goes in unwanted directions. By focusing the radiation better, a Tx may radiate further in the desired direction for a given emitted power levelFootnote 124 than one that also radiates in undesired directions. This includes radiating upwards and in other directions away from the desired Rx. Higher antennas (on mountains or on tall towers) radiate further and so both the angle and location of the antenna (height and terrain) affect the effective transmission distance and the radiated power density due to the Tx.
Moreover, in the path from Tx to Rx, the radio waves pass through the air and may encounter obstacles such as rain drops, leaves, or buildings that may block or reflect the radiation. Finally, there is a relationship between the frequency, wavelength and the propagation of radio waves: higher frequencies have shorter wavelengths, which means antennas may be smaller and signals tend to propagate in straight lines. Frequencies below 3 GHz support non-light-of-sight (NLOS), whereas higher frequencies require LOS. Thus, the design of the Tx and Rx antennas help determine what the pattern of radiated power is both along the intended signal path, as well as in other directions. Aspects of the terrain (hilly? tree covered? elevations?), the Tx/Rx antenna design and other characteristics of the radio system, and other environmental features (background radiation? weather?) will affect the propagation of energy due to a Tx.
The upshot of these physical characteristics of propagation is that signals follow multiple paths on the way from the Tx to the Rx (multipath) and may be distorted or blocked by the medium (air) through which they pass, increasing the problems faced by the Rx in decoding the desired signal. Additionally, the choice of modulation scheme including the bits encoded per Hz, and the level of intelligence in the system may all impact the ability of the radio system to successfully send and receive. For example, modern signal processing techniques may use knowledge about propagation characteristics and multipath to allow an Rx to use multiple inputs/multiple outputs (MIMO) to extract additional information and thereby enhance reception in noisy environments.
Thus the cause of interference is the presence of unwanted energy from alien Tx that the Rx does not know how to separate out, and hence regards as noise. This may come from background radiation (e.g., electric motors) or from other Tx operating in the same frequency band (in-band) or adjacent frequency bands (out-of-band). The goal of efficient radio system design is to allow the Tx to send sufficient energy to the Rx to allow it to successfully receive and decode the signal, while minimizing the incidence of interference causing radiation for alien Rx.
Robert Matheson and Adele Morris (2007)Footnote 125 explain how it is useful to think of characterizing radio wave propagation (and the resultant energy flux) via reference to a 7-dimensional volume, the electrospace, with the following dimensions:Footnote 126
- Frequency (1 dimension);
- Time (1 dimension);
- Spatial location (3 dimensions): longitude, latitude, elevation ;
- Direction of travel (2 dimensions): azimuth, elevation.
One can imagine the entire electrospace being divided into small 7-dimensional cells or volumes. In principle, as long as two Tx differ in at least one dimension, it is theoretically possible to design an Rx located at a cell that could differentiate between the two Tx and thus allow non-interfering sharing of the electrospace. For example, the sharing could be because the similar Tx operate in adjacent frequencies and the Rx is sufficiently frequency selective to allow it to tune to the different frequencies; or, two Tx could take turns in time using the channel; or, adequate geographic separation of the Tx could allow them to share the same frequency; and so on. However, practicalities of real-world radio system design limit the ability to differentiate between signals (nearly) co-existing in the same electrospace volume.
These practical considerations, coupled to the underlying physics of radio propagation give rise to the need to regulate adequate guard space (technical separation) between Tx to provide adequate interference protection. There are three categories of interference that are of principle concern:
- In-band interference from adjacent areas: because radio waves to not simply stop at geographic boundaries, transmissions from an alien Tx may interfere with reception for Rx that are distant from the desired Tx (i.e., when the signal at the target Rx is stronger from the alien Tx than from the desired Tx). The principal means for addressing this challenge is to adequately separate Tx and ensure that the power flux outside the licenced service area is below some maximum threshold. Limitations on Tx power can effectively address this challenge.
- In-band interference from adjacent frequencies: because Rx frequency selectivity is imperfect, energy may leak in from adjacent frequencies, once again causing interference. Analogous to the preceding, the principal means for addressing this is to ensure adequate frequency separation (guard bands) to ensure that interference from adjacent bands are limited. Once again, limitations on Tx power (out-of-band emissions) can address this.
- Out-of-band interference: once again, because of Rx imperfections, a strong signal in an adjacent band may interact with elements in the Rx front-end to produce new signals that are both hard to predict and hard to address since they can be highly non-linear. This is the hardest type of interference to address since presumably both the Tx and Rx are operating within their normal performance bands and it results when the victim Rx is near the boundary of the alien Tx's operating area.
Taken together, by properly defining the electrospace – and the size of the volumes – it is possible theoretically to specify Tx or Rx occupancy rights so that Tx/Rx must operate in different and distinct electrospace volumes to ensure non-interfering operation. Below, we discuss further some of the challenges and issues associated with defining these electrospace volumes. A Tx or Rx may occupy multiple volumes and control over these occupancy rights affords control over the interference environment. Thus, to deploy a radio system the provider needs to first acquire the rights for the electrospace that is needed to ensure adequate interference protection given the existing allotment of usage rights. A full allocation of the electrospace volumes will provide a map of the maximum energy flux allowed from the system to be deployed (so as to preclude interference to others) and to be tolerated from alien systems (to anticipate potential interference from others so that the target system may be designed appropriately).
The above considerations provide the technical foundations, in principle, for limiting interference. Before proceeding, it is worth noting that the definition and management of the electrospace framework is dependent on the physics of the frequency band and radio propagation, local conditions (terrain, weather, occupancy of adjacent channels), and radio system engineering constraints (limitations of Rx/Tx design, propagation modeling standards). These are technically complex issues which are generally beyond the understanding of almost everyone who is not a highly-trained radio engineer.
The exclusive use licence defines the rights to occupy the spectrum volume. The primary users has a presumptive right to exclude other users from occupying her electrospace. Secondary users may have the right to occupy the electrospace if they can do so without causing interference to primary users, although they have no interference protection rights of their own.
- Footnote 124
A common measure of radiated power is Equivalent Isotropically Radiated Power (EIRP) which is a theoretical construct that describes the emitted power by an isotropic antenna (one that radiates equally in all directions) to produce the peak power density observed in the direction of maximum antenna gain.
- Footnote 125
See Matheson, Robert and Adele Morris (2007), "The Technical Basis for Spectrum Rights," draft paper, May 3, 2007 (an earlier version of this paper was presented at the IEEE DYSPAN conference in April 2007, see www.ieee-dyspan.org).
- Footnote 126
Other dimensions that could be defined include modulation or polarization. Matheson & Morris (2007) do not include this since this since management of these dimensions across radio systems would be quite difficult.
- Date modified: