Aalborg Universitet
A History of Radio Wave Propagation
Andersen, Jørgen Bach
Published in:
I E E E Communications Magazine
Publication date:
2017
Link to publication from Aalborg University
Citation for published version (APA):
Andersen, J. B. (2017). A History of Radio Wave Propagation: from Marconi to MIMO. I E E E Communications
Magazine, 6-10.
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History of Communications
A History of Radio Wave Propagation: From Marconi to MIMO
BY J. Bach Andersen
Abstract
Radio waves from a few kilohertz to millimeter-wave frequencies
play a key role in modern wireless communications. The development over the last 120 years is traced with an emphasis on
communication aspects and physical phenomena rather than theory. The early years were characterized by experiments with no
theory and lack of knowledge of ionospheric propagation. High
frequency (HF) propagation via the ionosphere at HF frequencies
meant global communications for thousands of kilometers. Another natural medium is the atmosphere near the Earth’s surface, the
troposphere, leading sometimes to anomalous phenomena, but it
is also important for satellite signals near the horizon.
Propagation over man-made structures like in an urban environment is covered by the simple Hata equations for first generation cellular systems. Higher generations must include delay
information to accurately describe propagation, and the Hatalike equations may be extended into the millimeter frequency
range. Indoor propagation may also be covered by a diffuse
impulse response. Finally, the promise of increased spectral efficiency is given by multiple-input multiple-output (MIMO), if
certain conditions of uncorrelated antenna signals are fulfilled.
The Early Years: Experiments and Theories
The experimental work on electromagnetic waves started in
1888, when Heinrich Hertz verified Maxwell’s theory that the
waves propagated with the velocity of light. It is interesting to
observe that Hertz had little feeling for the communication possibilities. When asked about the application of the waves in
telegraphy, he answered:
However, the vibrations of a transformer or telegraph are far
too slow; take for example a thousand in a second, which is a
high figure, then the wavelength in the ether would be 300 km
and the focal length of the mirror must be the same magnitude.
If you could construct a mirror as large as a continent, you might
succeed with such experiments, but it is impractical to do anything with ordinary mirrors, as there would not be the least effect
observable. (Quoted from [1]).
Clearly, the notion of a carrier was not understood. Only
seven years later, in 1895, a 20-year-old Italian, Guglielmo Marconi, kicked off the wireless world by establishing a 2 km link
behind a hill [2]. It took place from his parents’ home, Villa Griffone, near Bologna. The return path was acoustic, a gunshot
from an assistant. After that, events moved quickly for Marconi,
extending the range to tens of kilometers and eventually to the
famous transatlantic transmission in1902. The application was
wireless telegraphy, which of course was opposed vigorously
by the established cable companies. Other opponents were the
scientists who could prove that it was not possible to overcome
the curvature of the earth over such large distances. The presence of the ionosphere was not known at that time, so many
questioned the results in the beginning. About the same time as
Marconi, the scientist Alexander Popov from Russia established
a wireless link by improving the receiver apparatus, the so-called
coherer [3]. It is not considered fruitful to discuss who was first;
the inventions were “in the air.” Marconi directed his efforts
toward commercial success; Popov was more a scientist.
The transmitter was a multiple sparks generator coupled to a
resonant circuit and an antenna, and the following conclusions
were obtained by Marconi:
• The antenna should be a vertical wire, and the length of the
wire determined the range: the longer the wire, the larger
the range.
6
• Although the spark gap transmitter produced a wide range
of frequencies, the propagated frequency band, while still
wide, was determined by the circuit and the antenna.
It is clear that the engineer inventor was winning over the
theoreticians, who were explaining the results years later. Another interesting controversy was over the apparently simple problem of propagation from a vertical dipole over a flat finitely
conducting surface. The Norton surface wave result was published as late as 1936; see [4] for interesting details. The radiation pattern (the sky wave) is easily computed using the Fresnel
reflection coefficients.
Despite the huge success of the Marconi system, there were
some inherent serious problems related to the transmitter system. They were not apparent if you were the only one in the
world, but the incoherent pulsed signal caused the following
problems:
• The radiated energy was spread over a large range of frequencies, an inefficient spread spectrum.
• There was a lack of privacy.
• There was no means of selecting the wanted from the
unwanted signal from two or more transmitters.
R. Fessenden (Canada) is credited for introducing coherent
transmitters, audio broadcasts, and inventions of the heterodyne
principle, so gradually the sparks died out and were eventually
forbidden.
International Scientific Cooperation, URSI,
International Union of Radio Science
The first General Assembly of the Union was held in July 1922
in Brussels. At that time, only four National Committees had
been formed officially: Belgium, France, the United Kingdom,
and the United States. However, the following new Committees
adhered to the Union during the same year: Australia, Spain,
Italy, Japan, and the Netherlands. We find that, although only
as observers, two scientists from Norway participated actively in
the work of the Assembly.
The Agenda of that first Assembly had been drawn up by
Figure 1. Marconi’s lab on the second floor of Villa Griffone. From
the window one can see the Celestini Hill at a distance, the natural obstacle that obstructed the line-of-sight propagation [2].
IEEE Communications Magazine • February 2017
History of Communications
and influence of the Earth magnetic field. If the
geo-magnetic field is important, the link is no longer reciprocal, a property that is important for
6
time-division duplex systems.
Protonosphere
Maximum
electron
2000
Figure 2a shows different ray paths for a given
density
Ionosphere
frequency and various angles of departure, and
4
5
Fig. 2b the electron density and height for the
1000
Heliosphere
various layers. Due to the less-than-one index
of refraction the paths are refracted away from
3
2
1
500
the normal and eventually reflected except ray
T
6, which passes through due to the steep inciF2
Skip zone
dence, rays 1, 2, and 3 are normal long ranging
200
rays, while 4 and 5 are high-ray paths. Note that
F1
there is a certain region near the transmitter, the
E
100
skip zone, where there are no rays and covered
D
only by the surface wave. The useful range may
50
be several thousand kilometers, and the useful
1 2 3 4 5 6
frequency range will be in the HF region from 3
LogIO Ne(cm-3)
to 30 MHz. The lower frequencies from 300 kHz
to 3 MHz suffer from heavy absorption during
Figure 2. a) Ray paths in the ionosphere; b) height vs. electron density [5] (with
the day, while the still lower frequencies offer too
permission from Dover Publications).
small bandwidth.
Marconi realized the benefit of higher frequenGeneral Ferrie and Prof. Goldschmidt, to be elected later as cies in the 1920s and established good links over several thouPresident and Secretary General of the Union, respectively. sand kilometers with moderate power and simpler antennas,
Among the topics to be considered by the Commissions, Gen- with a system called the Short-Wave Beam System [6] with frequencies above 3 MHz. Today, ionospheric transmissions have
eral Ferrie cited:
limited applications due to the presence of satellites.
• Measurements of the electromagnetic field and its variations
Meteor Burst Propagation
• Study of variations in radio direction finding measurements
• Study of statics and disturbances in general
A great number of meteors constantly enters the Earth’s atmoIt was considered that it would not be desirable for URSI to sphere and thereby creates ionized trails that may be exploited
cover tubes since this might have implied a more industrial char- for communications [7]. Of course, the intermittent nature of
acter, which had to be excluded.
the link limits the applications. They operate with carrier frequenThe scientific Commissions formed in 1922 were as follows:
cies from 30 to 100 MHz with rates between a few tens and a
• Measurements Methods and Standardisation
few hundred bits per second. Maximum path length is about
• Radio Propagation, with two Sub-Commissions on the elec2000 km. Typical waiting times are between a few seconds and
tromagnetic field and on radio direction finding, respectivea few minutes. One use is communicating data from unmanned
ly (taken from www.ursi.org)
measurement stations.
URSI is still very much active; the XXXIInd URSI General
The Troposphere
Assembly and Scientific Symposium (GASS) will be held in Montreal in 2017.
The troposphere is the lowest portion of the atmosphere, up
to about 10 km. Unexpected anomalous propagation for short
The Ionosphere
radio waves beyond the horizon by radars during World War II
In 1918 G. N. Watson published a solution to the diffraction served to generate research in tropospheric propagation.
The humidity, temperature, and density of the lower atmoaround the spherical Earth with the conclusion that the resulting
fields in the shadow region were much smaller than experimen- sphere determine the index of refraction N and play an importtally observed ones, so a different explanation was necessary.
It was not until the 1920s that the presence of ionized
regions in the upper atmosphere was scientifically established,
where names like Appleton and Chapman were important. The
S-db
h
(a)
free charges like electrons are created by ionizing radiation from
the Sun at different heights, typically between 50 and 400 km
Standard diffraction
D-Mi
N
above the Earth’s surface, varying over the seasons, monthly
and diurnally. Most phenomena may be explained by the simple
T
expression
Ionosphere
ka
Height (km)
5000
2
n = ε = 1−
f p2
(b)
f2
where n is the index of refraction,  is relative permittivity, f p2
is proportional to the electron density, and f is the frequency.
fp is called the plasma frequency. The relative permeability is
one, so the medium is non-magnetic. The equation indicates an
unusual medium with propagation for f > fp (0 <  < 1) where
most normal media will have  > 1, and exponential decay for
 < 0. A more exact equation would include effect of losses
IEEE Communications Magazine • February 2017
R
h
N
(c)
S-db
Elevated layer
h
D-Mi
S-db
N
Ground based duct
D-Mi
Figure 3. Tropospheric propagation mechanisms for various values of refraction index N. D is distance, h height [8].
7
History of Communications
50
Common volume
Specific anomalies
(dB/km)
H2O
H2O
Dry air
10
Transmitter
Receiver
Dry air
Earth
1
Figure 4. Troposcatter paths [9].
ant role in communications, as sketched in the diagrams of Fig.
3 [8].
In (A) the index of refraction decreases linearly with height,
which is the standard situation, and it may be shown that this
gives rise to a downward bending of a ray, so the radio horizon
is further away than the geometrical horizon. Instead of bending
the ray, one may introduce an effective larger Earth radius, typically 4/3 of the true radius for the standard atmosphere, and
the rays may then be drawn as straight. The fading before the
horizon is due to the interference between the direct and the
ground reflected ray (third column).After the horizon the power
drops rapidly. The refraction effects have an influence on near
horizon satellites.
The second group (B) illustrates an elevated layer due to an
abrupt change in the temperature and water vapor content. The
third group(C) illustrates a waveguide effect or duct giving rise
to propagation around the surface with relatively little attenuation. It is most likely to occur over coastal regions where warm
dry air masses flow over a cooler sea[8]. The mechanisms are
infrequent and unpredictable for long-range communication systems. There is, however, one mechanism that has led to a useful
and reliable link: Troposcatter, first introduced or discovered in
the 1950s.
As indicated in Fig. 4, the link is established by scattering
from inhomogeneities in the lower atmosphere. A high gain
transmit antenna is directed slightly above the horizon and illuminates the scatterers, and a small part of the forward scattered energy is picked up by the receiver high gain antenna. A
few gigahertz are used, and data rates above 20 Mb/s may be
achieved with latency of a few milliseconds, making the technique attractive for military systems. In addition, the probability
of intercept is low [10].
The atmosphere in itself contains a number of different gases
and water particles depending on the humidity. They contribute
to the overall attenuation, as shown in Fig. 5, of relevance for
propagation from satellites and for propagation along ground.
The most severe peaks are the water resonance at 22 GHz and
the dry air resonance at 60 GHz with attenuations of 0.2 dB/km
and 15 dB/km, respectively [10].
In the satellite case rain and ice particles severely affect radio
links above 10 GHz as shown in a comprehensive study from
1982 [11] by Cox and Arnold.
It is expected that fifth generation (5G) mobile communications will utilize millimeter waves, so the peaks should be avoided. On the other hand, taking advantage of the peaks will limit
interference from neighboring cells in cellular systems.
Propagation in the Urban Environment
The first cellular networks introduced in the early 1980s were
narrowband analog systems. Examples are Advanced Mobile
Phone System (AMPS) in the USA and Nordic Mobile Telephone (NMT) in the Nordic countries, and the use was mainly
for cars: a vehicular system.
8
H2O
0.1
0.01
0
Dry
air
Dry
air
Dry air
1
10
100
350
Freq (GHz)
Figure 5. Atmospheric attenuation vs. frequency [10].
The most important propagation parameter for a wireless system is the power density, determining the coverage from a base
station. In 1968 Okumura did an extensive set of measurements
in Japan for different frequencies from150 to 1500 MHz, and
as a function of antenna height and environment. In 1980 Hata
[11] transformed the data into a set of simple equations like L =
d where L is the mean path loss, the ratio between the transmitted and received power;  is the power law exponent equal
to 2 for free space and equal to 4 for a flat lossy ground — for
the urban and suburban environments, the value lies between
3 and 4. The factor  depends on the environment, and d is
distance. It is an interesting observation that  is independent of
frequency, only dependent on the base station antenna height.
Expressed in dB, the Hata formula reads
PL(d) = PL0 + 10  log10(d/d0) + Ss
where S is a Gaussian zero mean variable with a standard deviation  depending on the environment, explaining the slow fading, and d0 a reference distance.
The simple power law has shown validity in many other situations, even indoors, and for urban structures quite different
from those in Japan. It is worth noting that delay information is
not included.
For the propagation specialists, there have been many other
results from ray tracing, diffraction theory for over roofs, and
integral equations for rural propagation, and others. They serve
more to understand the physics, but they all lack the simplicity
of the Hata model.
There is also a fine structure in the received power due to
multipath, that is, waves are arriving from a number of different directions with different amplitude and phases, and they will
sometimes enhance the signal, sometimes combine destructively
to a small value; the phenomenon is usually called fast fading.
The most severe case leads to Rayleigh fading, as illustrated in
Fig. 6, for the case of f equal to 900 MHz and a vehicle speed of
120 km/h. Less severe fading is the case if there is a direct line
of sight from the base antenna to the mobile antenna. The mean
IEEE Communications Magazine • February 2017
History of Communications
20
8
10
0
d = 0.9  s
s = 1.2  s
O = 7.1  s absolute
-10
-20
-30
-40
-50
0
0.2
0.4
t (sec)
0.6
0.8
1.0
Relative power density dB
A/rms (dB)
0
Figure 6. Rayleigh fading for f =900 MHz and for a vehicle speed
of 120 km/h.
power is measured by averaging over a few wavelengths. The fast
fading may be mitigated by using diversity, while the slow fading
is unavoidable.
The faster the speed and the higher the frequency, the more
rapid the fluctuations. We can also interpret the figure as a spatial distribution with x-axis distance instead of time, so if standing
still one might be so unfortunate as to suffer a 30 dB loss. The
remedy is to move a little.
IEEE Communications Magazine • February 2017
-20
-30
-40
0
2
4
6
8
Excess delay  s
10
12
14
Figure 7. Average power delay profile measured in New York City
[14] by Cox in 1975. The frequency is 910 MHz.
Digital Techniques
170
160
150
Path loss dB
It was clear that international cooperation was necessary to
define the next system. It started in 1982 with setting up a working group (Groupe Special Mobile, in short GSM), also known
as Global System for Mobile Communication. The cooperation
was successful, and spectrum was allocated in the 900 MHz
band and later in 1993 extended to the 1800 MHz band. The
first mobile 2G call was in 1991.
Compared to the narrowband fading discussed above, the
fading is less severe, since individual frequency fades are only
part of the total bandwidth; there will be other frequencies that
do not fade. The fading is thus frequency selective.
Combined delay-Doppler scattering function was first measured in 1973 [13]. One early work on impulse propagation
was done in 1975 by Cox [14]. The original narrow impulse is
smeared out in time due to the scattering from buildings and
other objects, and this will lead to intersymbol interference
depending on the details of the system. It is customary to use
the root mean square (rms) delay spread as a characteristic
measure of the spread. In the case of Fig. 7 it is 1.2 s.
Considerable work on propagation supporting the development of GSM under the European Union (EU) was done under
the Cooperation in Science and Technology (COST) framework
supported meetings and other collaborations.
The trend was to higher and higher carrier frequencies with
the promise of higher bandwidth, and recently the millimeter-wave frequencies were studied in various environments [15]
by Rappaport in 2013. Modeling the path loss is again done
with a Hata-like model with an example shown in Fig. 8 based
on formulas in [16]. Atmospheric attenuation is not included.
The path loss increases with the square of the frequency, so
if we have an acceptable link budget at 5 GHz as an example,
we need an additional power of 100 times or 20 dB for 50 GHz.
This path loss is an average over both the slow fading due to the
changing environment and fast fading as discussed above.
The experiments are based on static measurements and
steerable antennas searching for the maximum power[15]. In
-10
800 m
140
400 m
130
d = 200m
120
110
100
101
f GHz
102
Figure 8. Path loss vs. frequency for a non-line-of-sight urban case,
based on [16].The slow fading is not shown.
a true mobile case the antennas must deliver the missing 20
dB jointly for transmitter and receiver. This calls for an adaptive
array with many elements with the trouble of having to find the
beamforming factors, or more simply, just beam scanning. As if
this was not enough, there is an additional problem: the Doppler spread increases linearly with frequency, so the beamforming must be updated every few millimeters.
Propagation in the Indoor Environment
The growing interest in the wireless industrial environment with
machine or robot connections [17] or the office environment
has led to new models of propagation. Saleh and Valenzuela
[18] suggested in 1987 that the mean impulse response consists
of clusters with each cluster having its own response with exponential decay.
9
History of Communications
Y
-70
1
5
6
7
8
9
11
Model
-80
-90
-100
dB
-120
-130
0
50
100
 ns
150
200
Figure 9. The average power delay profiles vs. delay in nanoseconds for different positions in an 11 m  19 m furnished room.
The dashed line is the theoretical reverberation model: frequency 5.8 GHz, bandwidth 100 MHz [20].
The Hata model was also used by Ghassemzadeh et al. [19]
for different types of indoor environments, commercial buildings, and single-family homes.
An alternative theory by Andersen in 2007 [20] is to consider
the room as a cavity with lossy walls where Fig. 9 shows the
average power profile at different locations. It is noteworthy that
for a given delay on the tail, the scattered diffuse power is independent of position.
It is also interesting that radio wave propagation in a room
is similar to acoustical wave propagation for the same wavelengths.
MIMO
Winters [21] pointed out in 1987, before it was called MIMO
(Multiple Input Multiple Output), that for two arrays with M elements each, up to M independent channels can be established
in the same bandwidth.
MIMO is not really a topic only for propagation, but is intimately connected with antennas, so we need to digress slightly
from our main issue and introduce some antenna topics. The
problem may be illustrated as in Fig. 10, where two arrays are
communicating via a number of scatterers. The arrays have M
and N elements, and for convenience we assume the left array
to be transmitting. In the example, M = 5 and N = 2. The path
shown is just an example; all elements couple with all elements
via many scatterers. The interesting case is when the angular
spread is large seen from both arrays, also expressed as low
correlation between the elements. This will be the typical case
for outdoor and indoor situations when there is no direct line
of sight. The result is a number of independent parallel channels
equal to min(N,M) leading to an increase in spectral efficiency.
If the angular spread is small seen from one side, there is only
one channel [22].
An alternative situation is where one array is replaced by
many users, and the base station consists of many, maybe hundreds, of elements, called massive MIMO [23].
Concluding Remarks
We have been looking back in time, including the most recent
achievements. It is relevant to look forward and mention a
few things we have not covered. The Internet of Things, IOT,
requires widespread propagation in an environment. Wear-
10
Y
Y
Y
Y
M antennas
=
N antennas
Figure 10. Two linear arrays of M and N elements in a scattering
environment. For N = 2 there are two independent channels
where the relative gains depend on the angular spreads of the
scatterers seen from the arrays.
-110
-140
Y
able antennas need studies of propagation near the human
body. The use of optical frequencies may be a solution in certain cases. It is certain that the history of wave propagation does
not end here.
References
[1] H. Sobol,”Microwave Communications — A Historical Perspective,” IEEE Trans.
Microwave Theory and Techniques, vol. 32, no. 9, Sept. 1984, pp 1170–81.
[2] G. Falciasecca, “Marconi’s Early Experiments in Wireless Telegraphy, 1895,”
IEEE Antennas & Propagation Mag., vol. 52, no. 6, Dec. 2010, pp 220–21.
[3] O. G. Vendik, ”Contribution of Prof Alexander S Popov to the Development
of Wireless Communications,” Euro. Microwave Conf., vol 2, 1995, 895–902.
[4] J. R.Wait,”The Ancient and Modern History of EM Ground-Wave Propagation,”
IEEE Antennas and Propagation Mag., vol. 40, no. 5, Oct. 1998, pp. 7–24.
[5] K. Davies, Ionospheric Radio Propagation, Dover, 1966.
[6] W. J. Baker, A History of the Marconi Company, Methuen & Co Ltd, 1970.
[7] I. A. Glover, “Meteor Burst Propagation,” Electronics & Commun. Engineering J.,
Aug. 1991, pp. 185–92.
[8] J. Chisholm, “Progress of Tropospheric Propagation Research Related to Communications Beyond the Horizon,” IRE Trans. Commun. Systems, vol. 4, no. 1,
1956, pp. 6–16.
[9] E. Dinc and O. B. Akan, “Coherence Time and Coherence Bandwidth of
Troposcatter Links for Mobile Receivers,” IEEE Vehic. Tech. Mag., vol 18, June
2015, pp. 86–92.
[10] ITU-R Rec. P.676-3, “Attenuation by Atmospheric Gases,” 1997.
[11] D. C. Cox and H. W. Arnold, “Results from the 19- and 28-GHz COMSTAR
Satellite Propagation Experiments at Crawford Hill,” Proc. IEEE,vol. 70, no. 5,
May 1982.
[12] M. Hata, “Empirical Formula for Propagation Loss in Land Mobile Radio Services,” IEEE Trans. Vehic. Tech., vol. 29, no 3, Aug. 1980, pp. 317–25.
[13] D. C. Cox, “A Measured Delay-Doppler Scattering Function for Multipath
Propagation at 910 MHz in an Urban Mobile Radio Environment,” Proc. IEEE,
Apr. 1973.
[14] D. C. Cox and R. P. Leck, “Correlation Bandwidth and Delay Spread Multipath Propagation Statistics for 910-MHz Urban Mobile Radio Channels,” IEEE
Trans. Commun., vol. 23, 11, Nov. 1975.
[15] T .S. Rappaport et al., “Millimeter Wave Mobile Communications for 5G Cellular: It Will Work!,” IEEE Access, vol.1, 2013, pp. 335–49.
[16] S. Sun et al., “Investigation of Prediction Accuracy, Sensitivity, and Parameter
Stability of Large-Scale Propagation Path Loss Models for 5G Wireless Communications,” IEEE Trans. Vehic. Tech., vol. 65, no. 5, May 2016, pp. 2843–60.
[17] M. Cheffena, “Propagation Channel Characteristics of Industrial Wireless
Sensor Networks,” IEEE Antennas & Propagation Mag., Feb. 2016, pp. 66–73.
[18] A. A. M. Saleh and R.A. Valenzuela, “A Statistical Model for Indoor Multipath
Propagation,” IEEE JSAC, vol. 5, 2, Feb. 1987, pp 128–37.
[19] S. S. Ghassemzadeh et al., “An Empirical Indoor Path Loss Model for
Ultra-Wideband Channels,” J. Commun. and Networks, vol 5, no 4, Dec. 2003.
[20] J. Bach Andersen et al., “Room Electromagnetics,” IEEE Antennas & Propagation Mag., vol. 49, no. 2, Apr. 2007.
[21] J. H Winters, “On the Capacity of Radio Communication Systems with Diversity in a Rayleigh Fading Environment,” IEEE JSAC, vol. 3, no. 5, June 1987, pp.
871–78.
[22] J. Bach Andersen, “Antenna Arrays in Mobile Communications: Gain, Diversity, and Channel Capacity,” IEEE Antennas & Propagation Mag., vol. 42, no 2,
Apr. 2000, pp. 12–16.
[23]E. G. Larsson et al., “Massive MIMO for Next Generation Wireless Systems,”
IEEE Commun. Mag., vol. 52, no. 2, Feb. 2014, pp. 186–95.
Biography
Jørgen Bach Andersen (M’68–SM’78–F’92–LF’02) ([email protected]) received the
M.Sc. and Dr.Techn. degrees from the Technical University of Denmark (DTU),
Lyngby, Denmark, in 1961 and 1971, respectively. From 1961 to 1973, he was
with the Electromagnetics Institute, DTU, and since 1973 he has been with Aalborg University, Aalborg, Denmark. Prof. Andersen is a former Vice-President of
URSI from which he received the John Howard Dellinger Gold Medal in 2005.
IEEE Communications Magazine • February 2017