Chapter: Wireless Channels
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.
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Video from Wireless Communications Networks Short Course
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.
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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 = distancenThe 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
|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:
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.L = LFS + c distance
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.
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.
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.
((n+2)/(n+1)-0.46) L = Lfs + 37 + 3.4 kw1 + 6.9 kw2 + 18.3 nwith
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
Disclaimer: Executable software programs are provided with no guarantee whatsoever.