Broadband Propagation Aspects

Contributed by Ramjee Prasad and Luis M. Correia
Edited by Jean-Paul Linnartz

The choice of a carrier frequency for a radio system is usually dictated by

its bandwidth (imposing a minimum value)
the wave propagation characteristics.
These are related to the extension of the coverage area, and to the need to ensure coverage in shadow - non line of sight - areas, among other aspects
antenna limitations.
This because of their size, compared to the required gain.

Thus, the study of wave propagation is one of the important tasks when developing a wireless system. For broadband systems it comprises the analysis of both path loss (for the estimation of cell coverage and carrier-to-interference ratio) and impulse response (for the evaluation of the wideband radio channel characteristics). A lot of work has been done on these two areas in the past few years concerning modelling and measurements in indoor and outdoor scenarios; however, most of it has been devoted to the millimetre wave bands, and the microwave bands only recently got the attention of researchers (the infra-red band has also got some attention).

At these frequency bands, modelling of wave propagation can be done on the basis of Geometrical Optics, by using ray-theory (either the image method or the ray-launching approach); at the millimetre wave band in particular, the diffraction phenomenon can be neglected, and the sum of the direct ray (when it exists) with reflected ones is enough to describe the propagation channel behaviour with a reasonable degree of accuracy. Modelling at such high frequency bands poses the problem of accurately describing the propagation scenario at the wavelength scale wavelength is less than 10 cm and can be as low as 5 mm for the frequencies mentioned before), mainly for outdoor environments; thus, the perspective is to have a description of the major obstacles and surfaces affecting the propagation. This description is not only in terms of the geometrical parameters (dimensions, roughness, and so on), but also in terms of their electromagnetic parameters (relative dielectric constant and loss tangent), in order to enable the calculation of the reflection properties of the usual building materials; moreover, they enable as well the calculation of the transmission properties of the materials, which are essential for the evaluation of coverage and/or interference between rooms. Values for the electromagnetic parameters can be found in many handbooks for frequencies up to the micr wave band, and have been recently extended in frequency up to the 60 GHz band. A consequence of this modelling approach is that signal amplitude, or any other parameter, is calculated in terms of median or average values at certain scenarios, or their tendency is evaluated when a specific parameter is varied (like street width or room height), rather than calculating its exact value at a specific location.

Indoor

At two millimetre wave bands (40 GHz and 60 GHz) in indoor scenarios, measurements have been conducted in different types of rooms (with surfaces ranging from 80 m² to 1800 m² and heights from 3 m to 7 m), using biconical-horn antennas (with a vertical half-power beam width of 27 degree) at both ends. The transmitter was placed at random positions inside the room and receiver was placed on the ceiling at the centre of the room.

With this configuration an almost uniform coverage of the rooms can be achieved in many of the situations.
The average power decay with distance very near to zero. The delay spread showed a dependence on the room dimensions and on the reflectivity of the walls, varying between 20 ns and 75 ns (median RMS values).

Outdoor

Outdoor scenarios have also been analyzed for these frequencies, both in terms of average power decay and impulse responses:

the path loss exponent is almost as in free space, i.e., it ranges from 2 to 2.5 depending on scenario characteristics and antenna radiation pattern. It is not possible to have the "40 log d law" at these frequencies, since the break-point distance is at least of the order of 10 km, far beyond the expected cell coverage distance.
multipath is not as severe as for indoor scenarios, delay spread (median RMS) presenting measured values between 4 ns and 40 ns (the lowest for streets and the highest for squares), which can be explained by the more complex environment at outdoors (concerning reflection and obstruction of rays).

Cell planning

With the usual building thickness of walls and floors, interference between indoor and outdoor systems using the same frequency is expected only if large glass windows are present, and interference between rooms will exist only if plasterboard or glass is used as wall material; of course, the cases of low interference also mean that no proper coverage can be achieved from one scenario to the other. The particular case of the choice of the 60 GHz band for outdoor systems is due to the peak of the oxygen absorption, 15 dB/km, thus having almost no effect in cell coverage but reducing the co-channel interference; Compared to free space attenuation, oxygen and rain absorption are not important for cell coverage purposes. They lead to an increase of 3 to 5 dB in the attenuation at the limits of cells with radius of 200 m. Absorption improves carrier-to-interference ratio between cells more than 1 km apart, as the extra attenuation for the interference is about 15 dB at 1 km.

Summary for various bands

The situation for the micr wave band can be found in between the millimetre wave and the upper UHF bands. Since the latter is well studied at the present and a lot of work has been done for the former, it is not a difficult task to have an idea of the order of magnitude and of the behaviour of the several parameters characterizing wave propagation.

As frequency goes up, communication increasingly has to rely on line of sight. At the millimetre wave band, severe obstruction of direct ray may well result in loss of communication, but that at the micr wave band one can have a reliable system without the need for permanent line of sight.

At the infra-red band, communication in non line of sight conditions, thus relying on scattering from surfaces on the propagation scenario, requires very high transmitter power. Unless the ceiling is used as prime reflector, this may pose eye safety problems.