Browse Prior Art Database

SLOT TIMING FOR SITE STEERED SYSTEMS

IP.com Disclosure Number: IPCOM000007831D
Original Publication Date: 1996-Aug-01
Included in the Prior Art Database: 2002-Apr-29
Document File: 4 page(s) / 204K

Publishing Venue

Motorola

Related People

Al Wilson: AUTHOR [+4]

Abstract

This invention uses a time reference at each site that is coordinated with a universal time reference such as a GPS system. This time reference provides a one second tick which is the basis of the launch time encoded in digital simulcast systems. This launch time is a tick value (here afier called a launch tick) ranging from 0 to 47999, with 48000 ticks per second.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 32% of the total text.

Page 1 of 4

Technical Developments

SLOT TIMING FOR SITE STEERED SYSTEMS

by Al Wilson, David Helm, Tom Senese and Tom McGovern

  This invention uses a time reference at each site that is coordinated with a universal time reference such as a GPS system. This time reference provides a one second tick which is the basis of the launch time encoded in digital simulcast systems. This launch time is a tick value (here afier called a launch tick) ranging from 0 to 47999, with 48000 ticks per second.

  In addition to the launch tick, a new time value called the epoch number is also included. The epoch number starts at 0 for the first second and incre- ments by one on each second, up to 62. It then wraps around to 0. This gives an epoch value which is the absolute time second value modulo 63.

  Each second of time includes 133 l/3 microslots, where each microslot lasts for 7.5 msec. A total of 63 seconds therefore includes exactly 8400 microslots. These microslots are then divided into slots where each slot consists of exactly NV microslots, with an integer value for NV. Only values of Np that evenly divide 8400 are used, and these include: 1,2,3,4,5, 6,7, 8, 10,. Each site is programmed for the value of NV and it -determines the slot alignment by looking at the launch tick value and the epoch number. This system is enabled by the epoch number value which is included in the launch tick value on each trans- mission from the central site. In digital wide area systems this comes from the central comparator.

PROBLEM

  A need exists to avoid collisions on the inbound channel of a data system. For this reason, status symbols (i.e., busy bits) are embedded in outbound messages to indicate the status of the inbound chan- nel. These status symbols are typically slotted to further tune the system and minimize collisions. The optimal tuning of the slot times depends on various factors such as the speed of the subscriber equip- ment and the speed of the fixed radio equipment. For this reason, the slot times need to be adjustable for different systems.

  A further need exists to provide data in wide area systems which are served by either simulcast transmitters or site steered transmitters which are on the same frequency. The status symbol slots for each of the sites therefore needs to be synchronized so that when one site is active its slots are the same as when another site is active. The coordination of these slot times is the subject ofthis invention.

SOLUTION

  Previous digital products tried to solve this prob- lem by timing the slot born the beginning of each transmission. unfortunately, each transmission could be rather short, even less than the slot time, so that when each new transmitter is keyed in the system, its slot time is uncoordinated with the previous transmission.

  RD-LAP systems coordinate launch times for data packets at each site from a central data control- ler. The stations then insert status symbols in the outbound data packets. This works for systems wi...