Browse Prior Art Database

METHOD OF REDUCED IM INTERFERENCE FOR HSMR SUBSCRIBERS

IP.com Disclosure Number: IPCOM000007449D
Original Publication Date: 1995-Jul-01
Included in the Prior Art Database: 2002-Mar-27
Document File: 3 page(s) / 119K

Publishing Venue

Motorola

Related People

Mark J. Marsan: AUTHOR [+2]

Abstract

We describe a method to improve the apparent IP3 of a HSMR (High Elevation Specialized Mobile Radio) mobile subscriber by adding attenuation to the front end when the receiver is above sensitivity and when the SQE algorithm detects the presence of IM interference.

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MolylRoLA Technical Developments

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METHOD OF REDUCED IM INTERFERENCE FOR HSMR SUBSCRIBERS

by Mark J. Marsan and Lawrence J. Marturano

  We describe a method to improve the apparent IP3 of a HSMR (High Elevation Specialized Mobile Radio) mobile subscriber by adding attenuation to the front end when the receiver is above sensitivity and when the SQE algorithm detects the presence of IM interference.

  The MIRS mobile IM specr was selected to reduce the probability ofthird-order IM interference from an ESMR's (Enhanced Specialized Mobile Radio) low base site into ESMR or HSMR mobile receiv- ers and was based on held measurements of talkout signal strength from low sites located close to express- ways. HSMRS are more susceptible to this kind of interference since they are single site systems which do not have the handoff capability that ESMR subscribers enjoy. In addition, they are thermal noise limited and are inherently more sensitive to increases in interference when compared to ESMRS which are cochannel interference limited. An HSMR mobile with a moderate signal level of -90 dBm will be blown away by IM interference when pass- ing by low ESMR sites which field measurements indicate can be -90 dBm or higher 0.5% ofthe time.

  Since MIRS subscritiers typically do not meet IM specs which guarantee interference-free opera- tion, an approach is next described which will improve IM performance for HSMR subscribers with signal levels above sensitivity, The basic idea is to use the SQE to instruct the AGC circuit to add atten- uation in the front end of the radio when interfer- ence is detected. If third-order IM products are the interference source, C/I+N will improve 2 dB for every 1 dB ofattenuation in the front end.

  AGC attenuation is tripped at first when the SQE's I+N estimate (24 slot sliding average) is 3 dB or greater than the thermal noise floor (about -125 dBm). If 1 is predominantly third-order IM interfer- ence, then it will drop by 3 dB for every one dB drop in front end gain. The excess interference is calcu- lated by I, = lOlog(i+n)-N, where lower case let- ters (in) are numeric values and upper case (N) are logarithmic. Then the front end attenuation is A = I,/3 + 1 > 0, where an additional 1 dB pad is added. This keeps the level of IM interference 3 dB b...