Browse Prior Art Database

SIMULCAST INHIBIT CIRCUITRY FOR MOBILE REPEATERS

IP.com Disclosure Number: IPCOM000005421D
Original Publication Date: 1980-Aug-01
Included in the Prior Art Database: 2001-Oct-10
Document File: 2 page(s) / 137K

Publishing Venue

Motorola

Related People

Ron Chapman: AUTHOR

Abstract

In communication systems where a mobile repeater controlled by a plurality of portable radios (see U.S. Patent No. 4,056,779 and 4,037,158), there is the possibility that two or more mobile repeaters may be physically in close proximity with one another. It is the purpose of this inhibit circuitry to minimize the probability that these two mobile repeaters will transmit simultaneously causing interference.

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MOTOROLA Technical Disclosure Bulletin Vol. 1 No. 1 August 1980

SIMULCAST INHIBIT CIRCUITRY FOR MOBILE REPEATERS

By Ron Chapman

In communication systems where a mobile repeater controlled by a plurality of portable radios (see
U.S. Patent No. 4,056,779 and 4,037,158), there is the possibility that two or more mobile repeaters may be physically in close proximity with one another. It is the purpose of this inhibit circuitry to minimize the probability that these two mobile repeaters will transmit simultaneously causing interference.

   In such communication systems, the mobile repeats messages between the portables and a base station by using two radio frequencies. One frequency is provided between the mobile and base station, and the other frequency between the mobile and the portables. Messages transmitted from the base sta- tion on the mobile frequency may be coded with a private-line (PL) tone so as to provide privacy. The simulcast inhibit circuitry described hereinbelow effectively eliminates the possibility that two mobiles will simultaneously transmit on the portable frequency. Similar circuitry could likewise be utilized to pre- vent two mobiles from simultaneously transmitting on the mobile frequency.

   A first layer of protection against a simulcast transmission is provided by introducing a random delay between the receipt of a PL tone from the base station and the turning on of the mobile transmitter. If a mobile receives PL tone from the base station and its receiver senses that another mobile is transmitting on the portable frequency before its own random clock enables its own transmitter, an additional delay which is longer than the longest normal random delay is added before transmitting. This means that those mobiles losing the random delay race are handicapped on subsequent transmissions. However, there is a window, whose width in time is the mobile receiver's squelch response time, in which two mobiles could key up simultaneously without being able to sense the presence of each other's transmis- sion.

   For these cases, a second layer of protection is provided by a random lookback interval. A random time after the mobile begins to transmit, the mobile de-keys and its receiver looks to see if another mobile is transmitting on the portable frequency. If another mobile is transmitting, the mobile remains de-keyed and the additional handicap time is included in its random delay.

  Thus, two mobiles will simulcast only if the two random delay windows overlap and if the two lookback intervals are the same. Then, on the next transmission the random delays will be different, and one mobile will lose the race and be handicapped on further transmissions.

   If the mobile which won the race leaves the area, the remaining mobiles will compete with one another according to their handicapped random delays. The winning mobile will cause the other mobiles to handicap themselves again.

   A detailed schematic of the simulcast inhibit circuitry...