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Remote Detection of Clock Frequency Errors in Cellular Infrastructure Equipment.

IP.com Disclosure Number: IPCOM000019176D
Original Publication Date: 2003-Sep-03
Included in the Prior Art Database: 2003-Sep-03
Document File: 2 page(s) / 44K

Publishing Venue

Motorola

Related People

Neil Turner: AUTHOR

Abstract

This paper describes two methods of detecting when the frequency accuracy of a cellular base station has gone out of specification. Firstly, other infrastructure equipment can be used to detect the transmissions and determine when one is out of specification. Where this is not feasible due to the effects of the cellular plan, mobiles can be used to determine the frequency accuracy of the base using the frequency correction channel (or equivalent). The information can be used to identify any base stations with inaccurate frequency references, and these can be recalibrated on-site only when necessary, reducing the requirement for regular site visits. Providing sufficient accuracy can be guaranteed, the system can be extended to provide remote frequency calibration.

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Remote Detection of Clock Frequency Errors in Cellular Infrastructure Equipment.

 

Neil Turner

Abstract

This paper describes two methods of detecting when the frequency accuracy of a cellular base station has gone out of specification. Firstly, other infrastructure equipment can be used to detect the transmissions and determine when one is out of specification. Where this is not feasible due to the effects of the cellular plan, mobiles can be used to determine the frequency accuracy of the base using the frequency correction channel (or equivalent). The information can be used to identify any base stations with inaccurate frequency references, and these can be recalibrated on-site only when necessary, reducing the requirement for regular site visits. Providing sufficient accuracy can be guaranteed, the system can be extended to provide remote frequency calibration.

Introduction

Cellular infrastructure equipment has to operate within a defined transmit frequency accuracy. In GSM this is 50 parts per billion, equating to less than 50 Hz for GSM900. Two methods can be employed to achieve this. Firstly, the frequency accuracy can be based on the data links servicing the base; this is generally the preferred method. The second is to use a free-running oven-controlled crystal oscillator (OCXO) that has been calibrated to be well within the frequency specification. This second option may be used when the data link is of insufficient quality, including microwave links.

Problem to be Solved

The free-running OCXO requires regular on-site calibration to ensure it, and hence the BTS, is within specification. This is time-consuming and expensive for the network operator. It would be preferable for the operator to know the frequency accuracy of the device without requiring on-site testing.

Solution

Two solutions are proposed. The first can be used only when at least 3 BTS sites can receive each other’s downlink transmissions. Consider the simple diagram in Figure 1, with three spatially diverse sites. It is assumed that the hardware has rf loopback functionality, whereby each can retune its receiver to demodulate downlink signals. If each site can receive the other’s transmissions, it will be possible to determine if one of the OCXOs is out of calibration.

Consider the following scenario. BTSs 1 and 2 are within frequency specification, but the OCXO in BTS 3 has drifted out of specification over time. When BTS 1 and BTS 2 demodulate the signals from BTS 3, the frequency error can be determined by the accuracy of the base band signal. This can either be demodulated pseudorandom data, or via the frequency correction channel information. The system relies on a majority-voting system, and must have a “good” frequency reference. This can be illustrated with the following simple table:

RF Link

Frequency Within Specification ?

BTS 1 – BTS 2

Yes

BTS 1 – BTS 3

No

BTS 1 – BTS 3

No

BTS 3 is the common component in the links that are out of frequency speci...