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METHOD FOR EXTENDING THE RANGE OF TETRA RADIO SYSTEMS

IP.com Disclosure Number: IPCOM000004665D
Original Publication Date: 2001-Mar-23
Included in the Prior Art Database: 2001-Mar-23
Document File: 2 page(s) / 8K

Publishing Venue

Motorola

Related People

Julian Richardson: AUTHOR [+3]

Abstract

A method is presented to assist in the range extension of a TETRA radio system in cases where the limiting factor is propagation delays causing overlap of the TDMA slots.

This text was extracted from a RTF document.
This is the abbreviated version, containing approximately 54% of the total text.

METHOD FOR EXTENDING THE RANGE OF TETRA RADIO SYSTEMS

by Julian Richardson, Rafy Carmon and Mark Shahaf

ABSTRACT

A method is presented to assist in the range extension of a TETRA radio system in cases where the limiting factor is propagation delays causing overlap of the TDMA slots.

INTRODUCTION

This paper describes a method to assist in the range extension of TETRA Radio systems. The method is particularly applicable to systems which have airborne Mobile Stations (MSs). Airborne mobiles may receive a good signal at ranges much greater than the 58km limit set by propagation delays causing overlap of the TDMA slots.

PROBLEM TO BE SOLVED

In TDMA Trunking systems such as TETRA system, the Mobile Station (MS) transmissions should be made within pre-defined time periods. This time period, named "TDMA channel", includes both actual slot length and some pre slot guard time. In TETRA the guard time is 7 symbols, or 389 microseconds, which gives ~58Km maximum cell radii. This guard time is required since the MS is travelling within a cell coverage area. This changes its distance from the Base Station (BS), thus creating variable transmission delays relatively to the BS timing, see Figure 1.

When the transmission delay exceeds the defined guard time, the transmission of certain MS may invade into adjacent TDM channels and interfere in other MS communications with the BS. For this reason, the BS must measure the path delay of each MS in order to avoid this kind of interference.

When a BS recognizes that path delay of the particular MS has exceeded the defined limit, it shall abort further illegal transmissions of that MS. In a TETRA system, when the BS recognizes that path delay of the particular MS has exceeded the defined limit, it enforces the MS to leave this cell.

This is enforced cell re-selection. In such a land mobile system, providing limited ERP, the combination of an appropriate guard time and the coverage planning strategy shall ensure that the quality of the communication link will degrade below a reasonable level, before the valid path delay is actually exceeded.

The TETRA standard is designed to fulfill among other requirements those of the public safety market, where usage of the airborne forces, especially helicopters, is common. One important difference between land and airborne mobile stations (AMS) is that a free-space propagation model applies to airborne units instead of the land propagation model which is used for system coverage planning. This will significantly increase the effective distance between the AMS and the serving BS while a reliable communication link exists (over 100 Km assuming 1 watt radio). The problem in this case is that the path delay will be greater than the permissible one, prohibiting AMS operation in the network in spite of a strong received signal.

While the AMS travels within the network, it will pe...