Subject: [NEXTEL-1] Motorola On- METHOD OF REDUCED IM INTERFERENCE FOR HSMR (iDEN) SUBSCRIBERS

From: [email-masked]
Date: Mon, 17 May 2004 22:25:15 -0400

METHOD OF REDUCED IM INTERFERENCE FOR (iDEN) HSMR SUBSCRIBERS

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.

Page 1 of 3

 Technical Developments

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...........................................

Full Document-  http://www.priorartdatabase.com/IPCOM/000007449/

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