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INBOUND COVERAGE METHOD BASED ON IMBE ERROR CORRECTION

IP.com Disclosure Number: IPCOM000008522D
Original Publication Date: 1997-Dec-01
Included in the Prior Art Database: 2002-Jun-19
Document File: 4 page(s) / 211K

Publishing Venue

Motorola

Related People

Don Newberg: AUTHOR [+3]

Abstract

In a digital mobile radio system, such as the APCO Project 25 system, receiver voting may be utilized to improve inbound coverage, where a com- parator is used to select the best inbound signal from a number of base station receivers. The voting in the Motorola AstroTAC comparator is performed by selecting the signal with the lowest number of errors detected in the Improved Multiband Excitation (IMBE) vocoder's most important Golay(23,12) error correction code word. A digital interface unit (DIU) is used to convert between digital and analog voice signals using the IMBE vocoder.

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MOTOROLA Technical Developments

INBOUND COVERAGE METHOD BASED ON IMBE ERROR CORRECTION

by Don Newberg, Robert LoGalbo, and Brad Hiben

1 PROBLEM STATEMENT

  In a digital mobile radio system, such as the APCO Project 25 system, receiver voting may be utilized to improve inbound coverage, where a com- parator is used to select the best inbound signal from a number of base station receivers. The voting in the Motorola AstroTAC comparator is performed by selecting the signal with the lowest number of errors detected in the Improved Multiband Excitation (IMBE) vocoder's most important Golay(23,12) error correction code word. A digital interface unit (DIU) is used to convert between digital and analog voice signals using the IMBE vocoder.

  In order to verify the coverage of such a system, inbound and outbound bit error rate (BER) mea- surements are required in the field. BER measure- ments are generally based on transmitting a prede- termined bit pattern such as V.52 and comparing the received bits to the transmitted bits. This method is sufficient when calculating BER on an outbound channel or on an inbound channel at a base station because the error rate is calculated at the point where the recovered bits are available. In a voted system, however, we wish to know the BER of the signal being de-vocoded at the DIU. Because the system is configured to error correct, parse, and vote bits that are formatted as voice information, V.52 bits will not pass through such a system to the DIU in a form that can be checked bit-by-bit for errors.

  Canned voice information, being in the form that the system knows how to process, could be sent instead of V.52 bits. At the DIU where the BER measurement is to be made, however, the only data

available is the ECC decoded voice information bits and an estimate of the number of corrected errors. Pattern matching the ECC decoded bits against their expected values will be inaccurate because the Golay(23,12) decoder will return message bits with no errors if the codeword was correctable (3 errors or less) and add errors if the codeword was uncor- rectable. The reported number of bit errors will also be inaccurate because the Colay(23,12) decoder will report 3 errors or less regardless of the actual number of errors.

2 PROPOSED SOLUTION

  An APCO Project 25 system is shown in the figure below, along with the additional equipment required to perform inbound coverage testing. An IBM PC and GPS receiver are connected to an Astro mobile running special test software and placed in the vehicle that will do the coverage test. This is the standard Fully Automated Coverage Test (FACT) platform currently used for outbound coverage testing, modified to transmit as well as receive. At the fixed end, another IBM PC and GPS receiver are connected to the wireline at the DIU to monitor the received bits. As the vehicle drives through the coverage area, the PC in the mobile platform monitors the location via the GPS receiver and compa...