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Method for correcting Air Mobile Radio Doppler shift for improved Communication with Base Station and improved Background Scanning

IP.com Disclosure Number: IPCOM000020010D
Original Publication Date: 2003-Oct-17
Included in the Prior Art Database: 2003-Oct-17
Document File: 5 page(s) / 117K

Publishing Venue

Motorola

Related People

Mark Rozental: AUTHOR [+3]

Abstract

In a cellular system when a Mobile Station (MS) communicates with a Base Station (BS) while moving there is a Doppler Shift added to the frequency that the MS receives from the BS. In the TETRA standard (and in many other cellular or public safety standards) the synchronization is done in a following way:

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Method for correcting Air Mobile Radio Doppler shift for improved Communication with Base Station and improved Background Scanning

By Mark Rozental, Moshe Ben Ayun and Ovadia Grossman

Problem definition

In a cellular system when a Mobile Station (MS) communicates with a Base Station (BS) while moving there is a Doppler Shift added to the frequency that the MS receives from the BS. In the TETRA standard (and in many other cellular or public safety standards) the synchronization is done in a following way:

1.     The MS receives the BS downlink signal

2.     The MS corrects its frequency standard to be 0.1 ppm accurate relative to the signal that it received from the BS. If the MS is in motion the frequency it calculates include Doppler shift.

3.     The MS transmits on the uplink using the corrected frequency reference that include Doppler shift.

In the case where MS communicates with its serving cell it receives the BS signal that is shifted by say +Fdoppler, correct its frequency reference and then transmit signal to the BS. The BS receives the signal that is MS signal shifted by +2*Fdoppler.

The above is valid when there is a line of sight between the BS and the MS. And this is always the case for Air Mobile when it is in the air.

If the Air MS is traveling at low speed the Doppler shift is relatively low the BS will not have a problem to cope with this frequency shift. But if the Air MS is traveling at high speed (this is usually the case for the MS in a helicopter or in a plane) this frequency shift becomes significant and will cause performance degradation.

Let’s look for example at a 800 MHz MS in a helicopter flying at 400 km/h

Doppler shift is

where

v- MS speed

l- wavelength

a- angle between the MS direction of move and the signal that it’s receive from the BS. Let’s assume a is 20 deg for our example.

So for our example Doppler frequency shift will be :

When this MS in the helicopter communicates with it’s serving BS, the BS will receive from MS signal that is shifted by 2*278=556 Hz!

This is problematic. It is especially problematic in TDMA system were BS has to receive slots from many Air Mobile radios.

Invention

Next generation TETRA and CGISS radios will include on board a GPS rece...