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CONSECUTIVE INDEPENDENT SLOT SYNCHRONIZATION ALGORITHM FOR APCO 25 SLOTTED ALOHA

IP.com Disclosure Number: IPCOM000008857D
Original Publication Date: 1998-Sep-01
Included in the Prior Art Database: 2002-Jul-18
Document File: 2 page(s) / 110K

Publishing Venue

Motorola

Related People

Michael L. Johnson: AUTHOR

Abstract

In the APCO 25 implementation of Slotted ALOHA on a trunking control channel, a subscriber unit must synchronize to the outbound control chan- nel prior to sending any transmission on the inbound control channel. In order to maximize control chan- nel throughput, it is critical to correctly obtain syn- chronization so as to not collide with other users. It is also necessary to obtain this synchronization as quickly as possible.

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MOTOROLA Technical Developments

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CONSECUTIVE INDEPENDENT SLOT SYNCHRONIZATION

ALGORITHM FOR APCO 25 SLOTTED ALOHA

by Michael L. Johnson

INTRODUCTION

  In the APCO 25 implementation of Slotted ALOHA on a trunking control channel, a subscriber unit must synchronize to the outbound control chan- nel prior to sending any transmission on the inbound control channel. In order to maximize control chan- nel throughput, it is critical to correctly obtain syn- chronization so as to not collide with other users. It is also necessary to obtain this synchronization as quickly as possible.

by obtaining synchronization repetitively until x consecutive synchronization results are the same. That is to say, slot sizes and locations are continu- ously acquired using a low value of Niluntil a certain number, X, of consecutive synchronizations result in the same values for slot size and location, If the probability of an incorrect size sync for a given wii:is defined as f'a then the probability of an incorrect sync using the Consecutive Independent Slot Synchronization Algorithm is approximately (Pdx for x consecutive statistically independent synchronization's, Requiring more iterations will increase time required to arrive at a final value for synchronization, but significantly less time than increasing g

  More specifically, forAPC0 25, the desired reli- ability was not reached until the number of consecu- tive equally spaced slot boundary indicators ($I was five. It was quickly realized, however, that at high (but reasonable) bit error rates, the time required to acquire synchronization grew prohibitively large. It was found that searching for two consecutive syn- chronizations of three consecutive equally spaced slot boundaries resulted in a similar reliability while significantly reducing the time required to obtain final synchronization. The probability of obtaining a false synchronization methods are shown in Figure I on next page.

DESCRIPTION

  Synchronization involves monitoring status symbols in order to determine slot size and slot boundary locations. A typical solution to acquiring slot synchronization is to look for Nconsecutive slot boundary indicators spaced a constant amount (slot size) apart. Reliability of this method decreases as noise on the channel increases, but can be offset by increasing x Increasing Nhowever, causes the time required to acquire synchronization to rise exponentially due to the error prone nature of t...