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A Method to Measure GPS Cable Delay Time Automatically

IP.com Disclosure Number: IPCOM000030659D
Original Publication Date: 2004-Aug-20
Included in the Prior Art Database: 2004-Aug-20
Document File: 5 page(s) / 4M

Publishing Venue

Motorola

Related People

Longhui Yuan: AUTHOR

Abstract

A GPS method is widely used in synchronization communication systems like the CDMA2000 BTS. For precise synchronization, the cable delay from BTS to a remote GPS receiver, which is induced by cable and interface circuit, should be calculated into the system’s synchronization reference. In this new proposed method, the function of delay measurement is based on a field bus that is a RS485 bus which is normally a communication bus carrying data from BTS to remote GPS receiver. By this new method, the delay measurement can be triggered at anytime system required and only one bus is needed for loop measurement.

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A Method to Measure GPS Cable Delay Time Automatically

Longhui Yuan

I.                    Abstract

A GPS method is widely used in synchronization communication systems like the CDMA2000 BTS. For precise synchronization, the cable delay from BTS to a remote GPS receiver, which is induced by cable and interface circuit, should be calculated into the system’s synchronization reference. In this new proposed method, the function of delay measurement is based on a field bus that is a RS485 bus which is normally a communication bus carrying data from BTS to remote GPS receiver. By this new method, the delay measurement can be triggered at anytime system required and only one bus is needed for loop measurement.

II.                 Background

A GPS receiver is a common method in communication system to accomplish time synchronization. It is widely used in cellular base stations, paging base station. For an easy installation and maintenance, most of the GPS receiver is located at the top of the building and assembled with the GPS antenna. The synchronization clock distributed from GPS receiver to BTSs is through RS485 digital bus.  However if the GPS receiver is not in close proximity to the communication system, a cable delay induced by the cable and interface circuits must be accurately calculated for precise synchronization. Accordingly, the present a new method give a new way to measure the cable delay automatically.

III.               Current Delay Measurement Methods

There are mainly two methods to measure this cable delay, these are a Manual Measurement Method (MMM) and a Power Cycle Method (PCM).

The first method of MMM requires the cable length to be measured manually when deployed on a new site.  The calculated result will be saved to the BTS data base as calibration data.  This method is very simple and does not need modifications of the system design. However the calibration at site is not easy nor is it practical. Also, this method cannot calculate the IC delay.

The second method of PCM is a more complicated one than method of MMM. The BTS method will power cycle the GPS receiver to build a loop back from the BTS_Data_Trx port to the  BTS_1PPS_Rec port. This loop back is time limited and will be cutoff by a predefined watchdog. During this loop back period the BTS will send a test pulse to the BTS_Data_Trx port and then receive it from the BTS_1PPS_Rec port through the loop back link. The time delay between sending and receiving is twice of GPS cable delay and will be calculated into the synchronization budget for this unit.

The second method of PCM measures the cable delay in an automatic way, but it also has some problems. First of all, when the BTS is in INS status it is not easy for the BTS to power cycle the GPS receiver since it will require a backup synchronization source to switch. A more important limitation is that hardware design of this method will bring a time window to software implementation. Therefore software design will be not independent of hardware design. If th...