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Chapter: Wireless Channels


Indoor Wireless RF Channels

Co-author: John S. Davis

The vehicular cellular phone systems initiated a rapid growth of wireless communication. However, with the growth of these systems cell sizes are made smaller and smaller to increase user capacity. Meanwhile the interest in indoor systems for telephony (cordless phones and wireless PABX-es) and data services (e.g. Wireless LAN's) also started. Currently, presumably more research is being conducted on indoor propagation that on outdoor propagation.

The indoor channel can less easily be captured in rough path loss exponents. While delay spreads are often much smaller than outdoors, the indoor systems often have to carry very high data rates, e.g. to support wireless multimedia computing. There are several causes of signal corruption in a wireless channel. The primary causes of attenuation are distance, penetration losses through walls and floors and multipath propagation.


Start page of video from Short Course by BMRC

Video from Wireless Communications Networks Short Course

"The main difference between indoor and outdoor propagation is that in an outdoor macro-cellular network propagation is fairly predictable. You can use a topographical database and really determine what will be the shape of a cell if you put a base station somewhere. For many reasons that's no longer feasible if you talk about indoor systems.

First of all: the data bases of the propagation environment have to be very accurate. And the models that we have now for indoor propagation do not allow us to predict everything. Signals may propagate through an elevator shaft. They may or may not propagate through the corner inside a building. ...."

See also audio interview with Dr. Daniel Davarsilvatham.


Embedded QuickTime Video

Path loss and Coverage prediction

Deterministic approach

Ray tracing allows deterministic prediction of signal level received at various indoor locations. In ray tracing, a large collection of possible propagation paths is evaluated and the amplitude and delay of each relevant path is considered. For narrowband coverage prediction an accuracy of about 2 dB has been achieved, but this requires a high-resolution 3D data base of the environment, accurate knowledge of building materials and calibration of predictions against actual measurements.

Statistical approach

Signal attenuation over distance is observed when the mean received signal power is attenuated as a function of the distance.

In addition to free space loss effects, the signal experiences decay due to ground wave loss although this typically only comes into play for very large distances (on the order of kilometers). For indoor propagation this mechanism is less relevant, but effects a wave guidance through corridors can occur. The path loss typically is of the form

power = distancen

The path loss exponent n may range from about 2 (in corridors) to 6 (for cluttered and obstructed paths). (see also: U.C. Berkeley Cory Hall 4th floor corridor).

For frequencies between 800 MHz and 1.9 GHz, COST 231 reports the following values for the path loss exponent n:

  Environment Exponent n Propagation
Mechanism 		 
  Corridors 1.4 - 1.9 Wave guidance  
  Large open rooms 2 Free space loss  
  Furnished rooms 3 FSL + multipath  
  Densely furnished rooms 4 Non-LOS, diffraction, scattering  
  Between different floors 5 Losses during floor / wall traverses  

Other models predict that the indoor path loss follows the law: 

L = LFS + c distance
where c is on the order of 0.2 to 0.6 dB per meter. This models has been proposed for metropolitan office buildings, for propagation distances from 1 to 100 meter and frequencies between 900 MHz and 4 GHz.

Multipath

Multipath results from the fact that the propagation channel consists of several obstacles and reflectors. Thus, the received signal arrives as an unpredictable set of reflections and/or direct waves each with its own degree of attenuation and delay. In indoor multipath waves tend to arrive in clusters. Within one cluster, paths have relatively small differences in delay. Delays between clusters are larger.

The Delay Spread is a parameter commonly used to quantify multipath effects. Multipath leads to variations in the received signal strength over frequency and antenna location.

The indoor channel typically behaves as a Rician channel. If the line-of-sight is blocked, Rayleigh fading becomes an appropriate model.

Rate of fading

Time variation of the channel occur if the communicating device (antenna) and components of its environment are in motion.

Java Applet: Phasor diagram of 8 scattered waves (in blue). 4 of these are considered not subject to motion and Doppler spreading. Four other scattered waves exhibit Doppler, i.e., due to (slow) motion of the reflecting object.

Closely related to Doppler shifting, time variation in conjunction with multipath transmission leads to variation of the instantaneous received signal strength about the mean power level as the receiver moves over distances on the order of less than a single carrier wavelength. Time variation of the channel becomes uncorrelated every half carrier wavelength over distance.

Fortunately, the degree of time variation within an indoor system is much less than that of an outdoor mobile system. One manifestation of time variation is as spreading in the frequency domain (Doppler spreading). Given the conditions of typical indoor wireless systems, frequency spreading should be virtually nonexistent. Doppler spreads of 0.1 - 6.1 Hz (with RMS of 0.3 Hz) have been reported.

However, this means that if the link is in a fade it only recovers very slowly.

Some researchers have considered the effects of moving people. In particular it was found by Ganesh and Pahlavan that a line of sight delay spread of 40 ns can have a standard deviation of 9.2 - 12.8 ns at 2.4 GHz. Likewise an obstructed delay spread can have a standard deviation of 3.7 - 5.7 ns.

For wireless LANs this could mean that an antenna place in a local multipath null, remains in fade for a very long time. Measures such as diversity are needed to guarantee reliable communication irrespective of the position of the antenna. Wideband transmission, e.g. direct sequence CDMA, could provide frequency diversity.

Path Loss, Wall Penetration and Cell Layout

An important issue for indoor cellular reuse systems is the possibility of interference from users in adjacent cells. In designing cells it would be convenient if natural barriers such as walls and ceilings/floors could be used as cell boundaries. 
Attenuation Factor    900 MHz   1700 MHz

Floor                  10 dB     16 dB

A signal at 1.2 GHz traversing a wall looses 3 to 8 dB of its energy.

User experience with wireless LANs is that in the 2.4 and 5GHz bands, communications signal propagate through a limited number walls and ceilings, but at higher frequencies (17 GHz) the signal is very weak after attenuation by a concrete or brick wall.

An appropriate statistical model can be to assume a building penetration loss of 12 dB with a standard deviation of 10 dB. Shadow fading with a standard deviation as large as 12 dB should be expected.

COST 231

The European COST 231 research project proposes a model for indoor office propagation. The attenuation (in dB) linearly grows the number of walls traversed. The effect of floors is non-linear. The path loss for 800-1900 MHz is calculated as: 
                                         ((n+2)/(n+1)-0.46) 
L = Lfs + 37 + 3.4 kw1 + 6.9  kw2 + 18.3 n
with 
 L   is the attenuation in dB
 Lfs  is the free space loss in dB
 n   is the number of traversed floors (reinforced concrete, but not thicker than 30 cm)
 kw1  is the number of light internal walls (e.g. plaster board), windows etc
 kw2  is the number of concrete or brick internal walls
 

Path Loss Calculator

Input parameters:
Carrier Frequency: (900 ..1800 MHz) MHz
Distance: meter
Number of floors  
Number of thick walls
e.g. brick, thicker than 10 cm		  
Number of thin walls
e.g. board, 10 cm		  

Run calculation:

  

Results:
Loss: dB

Disclaimer: Executable software programs are provided with no guarantee whatsoever.

Further reading



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