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NULL DETECTOR

IP.com Disclosure Number: IPCOM000006956D
Original Publication Date: 1993-Jun-01
Included in the Prior Art Database: 2002-Feb-12
Document File: 2 page(s) / 136K

Publishing Venue

Motorola

Related People

James L. Dailing: AUTHOR

Abstract

The propagation loss between a transmitter and receiver depends on a variety of items. One of these is called local fading. Local fading is oflen called Rayleigh fading because the fading usually follows a statistically Rayleigh distribution. Tbis is predominately the case for communications inside a building.

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MOTOROLA INC. Technical Developments Volume 19 June 1993

NULL DETECTOR

by James L. Dailing

   The propagation loss between a transmitter and receiver depends on a variety of items. One of these is called local fading. Local fading is oflen called Rayleigh fading because the fading usually follows a statistically Rayleigh distribution. Tbis is predominately the case for communications inside a building.

  Figure 1 shows the real time fading when the radio was mounted at a height of 6 ft. on a non-metallic robot and moved over a 6 ft. diameter circle at the constant speed of .94 ft. per sec. at 8.50 MHz. This converts to about .81 wave lengths per second. The sample rate was 15 per second. This gives almost 17 samples per wave length. The distance the radio moved around the cir- cumference of the circle was 18.8 ft. The time for one revolution was 20 seconds. The robot made about 1.5 revolutions. One can see the close similarity between the end of one revolution and the beginning of the sec- ond revolution. The same experiment has been made at night when the building was not occupied. For that envi- ronments the similarity between the fast revolution and the second is almost perfect. The real time fading is actu- ally a standing wave caused by the many reflections throughout the building. Normally we associate a stand- ing wave with a transmission lime caused by reflections due to mis-matching. For the transmission line the stand- ing wave has a uniform shape because there is only a reflection at the end of the line. In Figure 1 there arc many reflections due to such things as walls, ceilings, furniture etc. which cause the standing wave to be an irregular shape. Actually Figure 1 should be called a quasi- standing wave since it also has reflections due to people moving in the area. The real time fading in an unoccu- pied building is more lie a standing wave even though it has a nonuniform shape.

  In Figure 1 the deepest fades have a bandwidth of 2 to 6 inches 10 dB above their minimum value. In other words, if a fade had a minimum value of -60.0 dBm, moving 1 to 3 inches in either direction would increase the signal level to -50dBm which would be an improve- ment of 10 db.

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  Rayleigh fading shows that 9.52% of the time, the signal will be at least 10 db below the mean signal in that local area. This is a serious problem for communica- tions inside buildings. The reason is that usually people who are inside buildings are not moving. For example there may be ten people attending a meeting inside a room. According to the Rayleigh distribution, one of these people will be sitting in a null where the signal level is 10 db below the average signal level of the group as a whole. For marginal signal conditions that person will miss a call or page. One solution would be to raise the output power of the transmitter but that is an over- design and expensive.

  The problem is that the transmitter does not detect the situation and the individual...