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Detection Algorithm and Recovery Mechanism when Synchronized ‘Project 25 Phase 2’ TDMA Voice Channel and FDMA Control Channel get out of Sync

IP.com Disclosure Number: IPCOM000202773D
Original Publication Date: 2010-Dec-31
Included in the Prior Art Database: 2010-Dec-31
Document File: 5 page(s) / 190K

Publishing Venue

Motorola

Related People

Konjevic, Ljubomir: INVENTOR

Abstract

Project 25 Phase 2 is TDMA system which may or may not synchronize TDMA Voice Channels (VCH) to the FDMA Control Channel (CCH). The synchronized solution is preferred because it allows subscriber units (SU) to transmit sooner upon arriving on a TDMA VCH due to prior knowledge of synchronization. The problem would arise when VCH and CCH get out of sync, for whatsoever reason, and SUs still attempt to “synchronously” access TDMA VCH. In that case, obviously, the synchronization learned from CCH will not land SUs on the right VCH TDMA slot resulting in unsuccessful calls. This paper describes a detection algorithm to detect loss of synchronization between TDMA VCH and FDMA CCH followed by a description of the recovery mechanism which involves re-enabling calls, i.e. access to VCH.

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DETECTION ALGORITHM AND RECOVERY MECHANISM WHEN SYNCHRONIZED ‘PROJECT 25 PHASE 2’ TDMA VOICE CHANNEL AND FDMA CONTROL CHANNEL GET OUT OF SYNC

By Ljubomir Konjevic

Motorola Solutions

 

ABSTRACT

Project 25 Phase 2 is a TDMA system which may or may not synchronize TDMA Voice Channels (VCH) to the FDMA Control Channel (CCH). The synchronized solution is preferred because it allows subscriber units (SU) to transmit sooner upon arriving on a TDMA VCH due to prior knowledge of synchronization.

A problem arises, however, when the VCH and CCH get out of sync, for whatsoever reason, and SUs still attempt to “synchronously” access TDMA VCH. In this case, obviously, the synchronization learned from the CCH will not land SUs on the right VCH TDMA slot, resulting in unsuccessful calls.

This paper describes a detection algorithm to detect the loss of synchronization between a TDMA VCH and a FDMA CCH followed by a description of the recovery mechanism, which involves re-enabling calls, i.e., providing access to the TDMA VCH.

PROBLEM

A synchronized Phase 2 site is signaled to SUs by periodically broadcasting a SYNC_BCST Outbound Signaling Packet (OSP) message on the Control Channel. As long as a Fixed Network Equipment (FNE) is transmitting the SYNC_BCST message, the subscribers assume that they have roamed into a synchronized TDMA site and attempt to synchronously access the TDMA VCH.

The FNE calculates and populates all SYNC_BCST fields, including the micro-slot count field which is used by subscribers to determine the position of the desired VCH TDMA slot.

On the FNE side, for the synchronized TDMA site to work correctly, all Base Radios (all VCHs and CCH) have to be synced to the same micro-slot. Reference [1] describes the synchronization between FDMA CCH and TDMA VCHs.

Synchronization is essentially a single point of failure on the FNE and the problem arises if VCH(s) and the CCH get out of sync while SUs keep attempting to “synchronously” access the TDMA VCH. In that case the synchronization learned from CCH will not land the SUs on the right VCH TDMA slot, resulting in unsuccessful calls. Some of the issues that would result in VCH - CCH sync loss are:

a) VCH(s) loses track of the correct micro-slot count.

b) CCH loses track of the correct micro-slot count

c) outbound transmissions on TDMA VCH(s) are not aligned to the micro-slot.

d) outbound transmissions on the CCH are not aligned to the micro-slot, etc.

Sync loss on a single VCH would make only that channel inaccessible for TDMA calls while sync loss on CCH would make all VCHs inaccessible for TDMA SUs (as they would not be able to communicate).

SOLUTION

Since the consequences of CCH-VCH(s) sync loss might be severe (TDMA SUs not being able to communicate on a site) it is important to detect sync loss as soon as possible and to start a recovery mechanism to re-enable TDMA calls.

One solution is to start the Detection Algorithm upon the first occurrence of TDMA VCH – FDMA CCH sync issue....