If radio propagation is an art, indoor propagation is practically magic! With today's computers and design
tools, we can ALMOST predict what's going to happen over any given radio link -- but when we add in the
complexities of structures and electrical wiring, the RF engineer still needs a bit of wizardry to make things
work. Some of the information and links on the pages below can help.
Article: An Introduction to Indoor Radio Propagation
Posted Original 12/10/98; edited 1/6/01
1.0 INTRODUCTION:
Indoor use of wireless systems poses one of the biggest design challenges, as indoor radio
(RF) propagation is essentially a Black Art.
This technical note will try to shed some light on this mysterious subject and will seek to
quantify or set boundaries for use of wireless systems inside typical buildings at 900 MHz and 2.4 GHz.
1.1 BASIC RADIO PROPAGATION:
The most basic model of radio wave propagation
involves so called "free space" radio wave propagation. In this model, radio waves
emanate from a point source of radio energy, traveling in all directions in a straight line,
filling the entire spherical volume of space with radio energy that varies in strength with
a 1/(range)^2 rule (or 20 dB per decade increase in range).
Real world radio propagation rarely follows this simple model. The three basic mechanisms of
radio propagation are attributed to reflection, diffraction and scattering. All three of these
phenomenon cause radio signal distortions and give rise to signal fades, as well as additional
signal propagation losses. Outdoors, with mobile units, movements over very small distances
give rise to signal strength fluctuations, because the composite signal is made up of a number
of components from the various sources of reflections (called "multipath signals")
from different directions as well as scattered and / or diffracted signal components. These
signal strength variations amount to as much as 30 to 40 dB in frequency ranges useful for
mobile communications and account for some of the difficulty presented to the designer of
reliable radio communications systems. The basic signal attenuation with range noticed in
the real world gives rise to what are termed "large scale" effects, while the
signal strength fluctuations with motion are termed "small scale" effects.
Indoors the situation is even worse. It is very difficult to design an "RF
friendly" building that is free from multipath reflections, diffraction around sharp
corners or scattering from wall, ceiling, or floor surfaces (let alone operate perfectly
in a randomly chosen building location). The closest one could probably get to an "RF
friendly" building would be an all wooden or all fiberglass structure -- but even
this must have a structurally solid floor of some kind and this more ideal RF building will
still have reflections, multipath and other radio propagation disturbances (as the materials
properties section below shows) which will prove to be less than ideal. Indoors then, the
simple free space model fails to account for the small and large scale fading that is
observed in real world radio links as Figure 1 (ref. 1)below readily
shows.
Figure 1 - Received RF Power plot indoors versus range in meters.
Radio wave propagation inside smooth walled metal buildings can be so bad that
radio "dead spots" can exist where the signal is virtually non-existent. These
dead spots arise because of almost perfect, lossless reflections from smooth metal walls,
ceilings or fixtures that interfere with the direct radiated signals. The dead spots
exist in 3 dimensional space within the building and motions of only a few inches can
move from no signal to full signal. It will be the main purpose of this report to try
to recommend solutions for this kind of problem.
1.2 ABOUT MULTIPATH:
In the real world, multipath occurs when there is more than one path available
for radio signal propagation. The phenomenon of reflection, diffraction and scattering
all give rise to additional radio propagation paths beyond the direct optical "line
of sight" path between the radio transmitter and receiver. As Theodore S. Rappaport
describes the phenomenon in Wireless Communications — Principles and Practice
(ref.2):
"Reflection occurs when a propagating electromagnetic wave impinges upon an object
which has very large dimensions when compared to the wavelength of the propagating wave.
Reflections occur from the surface of the earth and from buildings and walls.
Diffraction occurs when the radio path between the transmitter and receiver is
obstructed by a surface that has sharp irregularities (edges). The secondary waves
resulting from the obstructing surface are present throughout the space and even
behind the obstacle, giving rise to a bending of waves around the obstacle, even
when a line-of-sight path does not exist between transmitter and receiver. At high
frequencies, diffraction, like reflection, depends on the geometry of the object,
as well as the amplitude, phase, and polarization of the incident wave at the point
of diffraction.
Scattering occurs when the medium through which the wave travels consists of
objects with dimensions that are small compared to the wavelength, and where the
number of obstacles per unit volume is large. Scattered waves are produced by rough
surfaces, small objects, or by other irregularities in the channel. In practice,
foliage, street signs, and lamp posts induce scattering in a mobile communications
system."
In practice, not only metallic materials cause reflections, but dielectrics
(or electrical insulators) also cause reflections.
The actual signal levels reflected from insulators depends in a very complicated
way on the above characteristics as well as the geometry of the situation. Suffice it
to say, that insulators are not as good at reflecting radio signals as metal surfaces,
but even common insulating materials do cause some reflection of radio waves.
Multipath occurs when all the radio propagation effects combine in a real world
environment. In other words, when multiple signal propagation paths exist, caused
by whatever phenomenon, the actual received signal level is vector sum of all the
signals incident from any direction or angle of arrival. Some signals will aid
the direct path, while other signals will subtract (or tend to vector cancel) from
the direct signal path. The total composite phenomenon is thus called multipath.
Two kinds of multipath exist: specular multipath -- arising from discrete,
coherent reflections from smooth metal surfaces; and diffuse multipath --
arising from diffuse scatterers and sources of diffraction (the visible glint of
sunlight off a choppy sea is an example of diffuse multipath).
Both forms of multipath are bad for radio communications. Diffuse multipath
provides a sort of background "noise" level of interference, while
specular multipath can actually cause complete signal outages and radio "dead
spots" within a building. This problem is especially difficult in underground
passageways, tunnels, stairwells and small enclosed rooms. The proper functioning
of the radio communication link requires that multipath be minimized or eliminated.
2.0 MITIGATING MULTIPATH PROBLEMS:
RF multipath problems can be mitigated in a number of ways:
Radio system design
Antenna system design
Signal / waveform design
Building / environment design
Many wireless systems demand "robust" and error free radio
communications, since the overall system fails if the radio system fails.
The major features that each subsystem design will have are outlined below:
Radio system design — redundant paths for each receiver, if at all possible
Antenna system design — dual diversity antennas used at each receiver,
as a minimum
Signal / waveform design — Spread Spectrum radio design with the highest
feasible chip rate
Building / environment design — not much can be done in this area,
unless new "RF Friendly" buildings are constructed
3.0 INDOOR RF PROPAGATION:
RF propagation obstacles can be termed hard partitions if they are part of the
physical / structural components of a building. On the other hand, obstacles formed
by the office furniture and fixed or movable / portable structures that do not extend
to a buildings ceiling are considered soft partitions. Radio signals effectively
penetrate both kinds of obstacles or partitions in ways that are very hard to predict.
Remember that in free space, an additional signal loss of 20 dB is incurred for
each 10 to 1 increase in radio range. Thus, an obstacle with a measured loss of 20 dB
or more from its materials is a significant loss! The equivalent of RF transparent is
probably in the range of 3 to 6 dB loss from any obstacle's material properties.
We cannot do anything about the buildings, building materials or structures this
system will be used in, however, we must still explore the realm of overall macroscopic
signal propagation in a typical building. Electronic engineers (radio engineers, in
particular) would like to be able to predict the signal levels and range of signal
losses present in a building. To enable this prediction a number of studies and
measurements have been made which grossly characterize in building signal propagation.
Figure 2 (ref. 2), below, shows scatter plots of radio path loss as a function of
distance in a typical office building for propagation through one through four floors.
Figure 3 (ref. 3), below, shows measurements made at another "typical"
building. Observe that both sets of data were taken at 914 MHz, not 2400 MHz and
that data is shown for propagation on the same floor as well as between floors.
We can extrapolate this data for use at 2400 MHz with a fair amount of certainty,
if we add a few dB (perhaps 5 to 6 dB) to account for the higher frequency we will use
to each graph point.
Interpretation of this data provides a level of understanding of the potential
problem at hand. Figure 2 shows losses ranging from about 50 dB to over 80
dB for a transmitter-receiver separation of only 10 meters. Figure 3 shows even
higher losses, extrapolating to our frequency range gives us over 50 dB to over 90 dB
attenuation in a 10 meter separation!
Figure 2 - Path Loss Scatter Plot in a Typical Building.
Figure 3 - Path Loss Scatter Plot in Another Typical Building.
Given that the entire loss budget for the typical indoor wireless link is in
the neighborhood of 120 dB, most of our losses seem to be expected in the very
first 10 meters! However, this interpretation of the data presented in Figures 2
and 3 takes the entire realm of multi-story building propagation into account. This
is perhaps not fair in the current context. If we confine ourselves to same floor
only propagation, then losses at 10 meters can be expected to be about 60 dB.
However, going out to a 50 meter range between transmitter and receiver causes
losses as high as 110 dB -- almost all of our loss budget will disappear by 50
meters! So, can we begin to characterize losses between rooms and through the
various radio obstacles and hard / soft partitions within a typical building?
Based on the data above, as a first cut approximation to estimating indoor
path losses, if we assume that propagation follows an approximate 1/(range^3.5)
power rule, rather than 1/(range^2), we can predict propagation losses with the
following relationship (at 2.4 GHz):
Path Loss (in dB) = 40 + 35 * [LOG (D in meters)]
Thus a 10 meter path will give a loss of about 75 dB and a 100 meter path
gives a path loss of about 110 dB. If the data above are fairly accurate, we can
expect a large-scale signal flucuation of about 13 dB (or the estimated path loss
will have a variance of about 13 dB).
In the real world then, it is probably optimistic to expect our in-building
links to work well beyond about 100 meters!
5.0 CONCLUSIONS:
This report has tried to make some sense out of the mysteries of radio
propagation within buildings. We have presented measured data and come up with
an approximate formula for the average signal losses to be expected at 2.4 GHz
in typical indoor environments.
We have shown that a reasonable range of indoor radio propagation phenomenon
can be handled by a practical system at distances out to perhaps 100 meters.
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