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Chapter: Wireless Channels


Indoor Wireless RF Channels

Co-author: John S. Davis

There are several causes of signal corruption in a wireless channel. The primary causes of corruption are signal attenuation due to distance, penetration losses through walls and floors and multipath propagation.

Effect of distance

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 for instance Cory Hall 4th floor corridor)

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.

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

Further reading



www.WirelessCommunication.NL © John Davis, II and Jean-Paul M.G. Linnartz, 1993, 1995.