Subject: [NEXTEL-1] Tutorial: Indoor propagation issues for wireless LANs

From: Marcel <[email-masked]>
Date: Sat, 02 Nov 2002 18:09:12 -0500

-------- Original Message --------
From: "Syed Maqbool Hassan" <[email-masked]>
Subject: Tutorial: Indoor propagation issues for wireless LANs

Indoor propagation issues for wireless LANs

By Dan Dobkin
RF Design, Sep 1, 2002

Wireless local area networks (WLANs) are rapidly gaining popularity. 
These networks are primarily targeted for indoor use, and are most 
often based on either the IEEE 802.11 Ethernet-type protocols or the 
Bluetooth Special Interest Group (SIG), both using the unlicensed 
bands at 2.4 to 2.5 GHz (IEEE 802.11b — "WiFi" — and Bluetooth) or 
at 5.15 to 5.85 GHz (IEEE 802.11a — "WiFi5"). The European HiperLAN 
standard is also designed for operation around 5.2 to 5.8 GHz.

It is often said that the higher-frequency unlicensed national 
information infrastructure (U-NII) band at 5.15 to 5.85 GHz will be 
intrinsically limited to shorter ranges than the 
industrial/scientific/medical (ISM) band due to higher path loss, 
limiting the utility of 802.11a relative to that of 802.11b.

The purpose of this investigation was to examine that claim. It was 
found, at least in the test environment discussed in this article, 
to be unfounded.

Experimental method and results
 
The propagation of a continuous-wave (CW) signal at either 2.5 or 
5.2 GHz from place to place within a typical light industrial 
environment was measured. The 10 mW signal was generated using a 
sweeper in CW mode and radiated from a fixed vertically oriented ë/4 
dipole constructed from a coaxial line with exposed center conductor 
of appropriate length, soldered to a copper ground plane 15 cm on a 
side.

The signal was received by an identical dipole and monitored with a 
spectrum analyzer. The receive antenna was moved to various 
locations throughout the facility. At each nominal location, at 
least five measurements were taken, in which the exact position was 
moved randomly in 10 to 20 cm increments to sample possible signal 
variations due to shadowing and multipath fading. Nominal locations 
were the same for the 2.5 and 5.2 GHz measurements to within about 1 
meter.

The transmitter was located on the second floor and receive 
locations on both floors 1 and 2 were tested. The building is 
typical of Silicon Valley construction, with an open central area 
occupied by cubicles or engineering test benches, offices and 
conference rooms along the perimeter, and a thin steel-supported 
concrete floor. The cubicle adjacent to the transmitter contained 
several large sheet-metal bookshelves, acting as the sort of 
internal obstacle one might typically expect to encounter in an 
indoor environment.

Short distance measurements (0.6 ±0.05 and 1.2 ±0.05 meters) provide 
a rough calibration for the longer distances, because multipath 
effects are generally minimal at these distances. Distance from 
transmitter to receiver was estimated from building plans to the 
nearest meter, accounting for the 3 meter height of the floor 1 
ceiling when applicable.

Figure 3 shows the results for both frequencies and both floors. The 
dots connected by a bar represent the highest and lowest values 
measured at the same nominal location. It is seen that local fades 
of anywhere from 5 to 25 dB are common, with one instance of 45 dB. 
Average values fall as much as 30 dB below ideal free-space [1/r2] 
propagation. Finally, it is apparent that there is little 
statistically significant distinction between the path loss for the 
2.5 and 5.2 GHz signals.

To further emphasize this point, a joint fit to all of the data 
points was performed, finding a best-fit propagation model as 
defined by:

(That is, the propagation exponent is about 3.0 and the floor loss 
about 9 dB.)

The results for each measured data point from the modeled value for 
that nominal location for 2.5 and 5.2 GHz, is shown in Figures 4(a) 
and (b) as histograms. While there is a modest difference in the 
exact shape of the distributions, the difference of the averages 
(1.7 - 1.4 = 0.3) is much less than the standard deviations of the 
distributions. There is no statistically significant difference 
between 2.5 and 5.2 GHz link losses in the facility.

A bit more detail
 
Free Space — It is a common misconception that free space 
propagation is wavelength-dependent. It is timely to examine how 
this misunderstanding arises.

The signal strength received by an ideal receiving antenna from an 
ideal transmitting antenna over a free-space distance "d" can be 
expressed as:

where Ps are transmitted and received powers, gtrans is the 
directivity of the transmitting antenna, and Arec is the effective 
collecting area of the receiving antenna. Note that there is no 
explicit dependence of the propagation on wavelength1.

The received signal strength is, however, more often written in 
terms of antenna directivity2. To arrive at this form one must 
impose the reciprocity condition: transmitting from antenna 1 to 
antenna 2 should give the same result as transmitting from antenna 2 
to antenna 1 in free space (that is, space is isotropic, at least in 
the absence of an overall magnetic field and orbiting charges) by:

And, a bit of algebra shows that:

That is, the directivity is monotonically related to the effective 
collecting area of the antenna. An ideal isotropic radiator 
(directivity = 1) has an effective collecting area proportional to 
the square of the wavelength, therefore:

Substituting this in the link loss expression gives the more 
commonly observed form:

Note that, although the term in wavelength is often folded into the 
term in distance for dimensional convenience, it is actually a 
statement about antenna size. A nearly isotropic antenna must get 
smaller as the wavelength shrinks. It is the reduced collecting area 
of the receiving antenna, not any mysterious wavelength-dependent 
propagation behavior, that causes free-space link losses to increase 
with frequency when antennas of a given directivity are specified.

Therefore, higher frequencies induce one to use not shorter ranges, 
but more directional antennas with larger collection area to 
maintain a constant link budget.

Indoor propagation — The situation is, of course, more complex in 
practical indoor environments, where numerous objects may scatter, 
diffract, reflect, and absorb radiation.

Numerous experimental and theoretical studies of indoor propagation 
have been performed (3, and 6 - 11). The effects of the environment 
are often absorbed into a modified propagation exponent, ranging 
from 2.7 to 5, and a floor loss factor of 3 to 12 dB, generally in 
agreement with the empirical expression we arrived at for our data.

However, there does not seem to exist a specific comparison of 
unlicensed frequencies, under identical circumstances, such as that 
reported above, or any suggested analytic approximation for the 
effects of frequency on path loss.

Scattering — The effects of scattering and diffraction have been 
studied for many years. It is well-known that objects whose 
dimensions are small compared to a wavelength act as weak scattering 
centers, with effective scattering cross-section proportional to the 
fourth power of the impinging wavelength (Rayleigh scattering)4. 
Objects much larger than a wavelength can be treated by the familiar 
approach of phase-insensitive ray tracing learned in classical 
optics, and encountered every day in our visual world: they absorb, 
reflect and cast geometric shadows.

The dominant scatterers in indoor environments fall into neither of 
these simplistic categories: their typical sizes are from 2 to 3 cm 
to human dimensions of 2 to 3 meters vs. wavelengths of roughly 12 
cm in the 2.4 to 2.5 GHz ISM band and around 5.5 cm in the 5.2 to 
5.8 GHz UNII band. Most of these objects fall into the Mie 
scattering regime from 1 to 10 wavelengths, in which complex 
behavior depending on the shape and characteristics of the scatterer 
is expected5. Obstacles much smaller than a wavelength "disappear" 
from view. Thus FM at 100 MHz (ë = 3 meters) is undisturbed by cars 
and people, but suffers reflections and diffraction from buildings. 
This leads to multipath fade in urban environments. The situation is 
depicted qualitatively in Figure 5, showing scattering cross-section 
as a function of size for a perfectly reflecting sphere, for 
frequencies of 2.45 and 5.2 GHz.

It is expected to find a strong monotonic wavelength dependence in 
indoor propagation when there are important scatterers at sizes 
between the cutoffs of the high and low frequency waves.

For example, if the environment were populated with lots of metal 
spheres (ball bearings) of 2 cm diameter, a 5.2 GHz transmission 
would be scattered 10 times more effectively than a 2.45 GHz 
transmission. However, a quick look around an office environment 
will disclose that human beings mostly populate their world with 
larger objects; the "disappearance" of 2-cm scattering centers at 
ISM band relative to UNII band has little effect on the overall path 
loss. For larger objects (of size larger than a wavelength), the 
diffracted intensity must be examined in more detail. Investigation 
of a typical two-dimensional obstacle of short dimension 0.5 meter, 
located 5 meters from a source of either 2.4 or 5.25 GHz radiation, 
was performed using the Fresnel integrals. The diffracted minima at 
e.g. 12 meters from the obstacle, are 5 dB deeper at 5.25 GHz than 
at 2.4 GHz, though only 30 cm wide; the on-axis maxima only differ 
by about 2 dB between the high and low frequencies. Based upon this 
rationale, it can be concluded that the experimental results are 
reasonable and may be typical of indoor environments.

Multipath and delay spread — Multipath propagation leads both to 
fading and to variations in path delay of transmitted signals. Delay 
spread and number of significant paths increase gradually with 
distance in indoor environments; a typical range appears to be about 
20 to 80 nsec for 5 to 15 meter distances at 5 GHz8.

An 802.11a signal contains 48 active carriers12. At the minimum 
overall data rate of 6 Mb/s each carrier has a data rate of 125 Kb/s 
or a bit time of 8 µseconds. Even during the PLCP preamble, signal 
transmission uses 12 subcarriers for an effective rate of 500 KHz. 
It seems unlikely that the bit error rate would be extremely 
sensitive to the 10 to 30 nanosecond variations in the multipath 
delay distribution that one would expect at these ranges.

While there has been no explicit examination of the effects of 
multipath propagation, it can be noted that the magnitude of 
multipath propagation should be reflected in the variation about the 
mean of path loss (see Figures 4(a) and (b)). Similar standard 
deviations for CW propagation suggest qualitatively similar 
multipath/delay effects.

Conclusions
 
As a result of this experiment, it is suggested that no intrinsic 
impairment exists in 5.2 to 5.8 GHz propagation vs. 2.4 to 2.5 GHz 
propagation in office/light manufacturing environments. Thus, no 
intrinsic impediment to roughly equivalent deployment of 802.11a and 
802.11b wireless LAN systems exists.

Note, however, that this does not imply equivalence of practical 
systems: higher frequencies suffer higher losses in cables and 
circuit boards, and low-cost devices may suffer from reduced gain or 
lower output power at higher frequencies. Further, to achieve 
equivalent collecting area, higher-frequency antennas become more 
directional, which may be inconvenient for end-users.

References 
"Lines, Waves, and Antennas, 2nd ed.," R. Brown, R. Sharpe, W. 
Hughes and R. Post, Wiley (1973).

"Gain of Directional Antennas," J. Hill; W.J. Tech-Notes, WJ 
Communications Inc. (2001).

IS-95 and CDMA2000, V. Garg, Prentice-Hall (2000), p. 238 ff.

"Classical Electrodynamics [2nd ed.]," J. Jackson, Wiley (1974), 
chapter 9.

"Light Scattering by Small Particles," H. van de Hulst, Dover (1981).

"Efficient Ray-Tracing Acceleration Techniques for Radio Propagation 
Modeling," F. Agelet, A. Formella, J. Rabanos, F. de Vicente and F. 
Fontan, IEEE Trans Vehicular Tech, 49 #6 p. 2069.

"Outdoor/indoor propagation modeling for wireless communications 
systems," M. Iskander, Z. Yun, Z. Zhang, 2001 IEEE Antennas and 
Propagation Society International Symposium-Microstrip Antennas for 
Wireless, Boston, MA, United States, IEEE Antennas and Propagation 
Society, AP-S International Symposium (Digest) v 2 2001. p 150-153.

"Microwave indoor radio propagation measurements and modeling at 5 
GHz for future wireless LAN systems", S. Kumar, P. Santhosh, B. 
Farhang-Boroujeny, S. Uysal, C. Ng, 1999 Asia Pacific Microwave 
Conference (APMC'99) `Microwaves Enter the 21st Century', Singapore, 
Singapore, APMC v 3 1999. p 606-608, 1999.

"Improving the accuracy of ray-tracing techniques for indoor 
propagation modeling," K. Remley, H. Anderson, and A. Weisshar, IEEE 
Transactions on Vehicular Technology v 49 n 6 Nov 2000. p 2350-2358, 
2000.

"Propagation modeling for indoor wireless communication," W. Tam and 
V. Tran, Electronics & Communication Engineering Journal v 7 n 5 Oct 
1995. p 221-228, 1995.

"Effective models in evaluating radio coverage on single floors of 
multifloor building," J. Tarng and T. Liu, IEEE Transactions on 
Vehicular Technology v 48 n 3 1999. p 782-789, 1999.

IEEE 802.11 Handbook, B. O'Hara and A. Petrick, IEEE Press 1999, 
chapter 7.

About the author 
Dobkin received his B.S. degree from the California Institute of 
Technology, and a Ph.D. from Stanford University, both in Applied 
Physics. His career has spanned development of compound 
semiconductor devices and circuits, capital equipment and processes 
for semiconductor manufacturing, and expert testimony in contract 
and patent litigation. He is currently Director of Technical 
Marketing for WJ Communications. He can be reached at: 
[email-masked]

The author would like to acknowledge the assistance of colleagues 
including Eric Frick, Marc Abramson, Mike Johnston, Andy Graven, and 
Tuan Nguyen in this work.

Source:
http://rfdesign.com/ar/radio_indoor_propagation_issues/index.htm 

Best regards
Syed Hassan

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