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IP.com Disclosure Number: IPCOM000007182D
Original Publication Date: 1994-Jun-01
Included in the Prior Art Database: 2002-Mar-04
Document File: 3 page(s) / 150K

Publishing Venue

Motorola

Related People

Dave Mills: AUTHOR [+3]

Abstract

Radio spectrum has become a precious resource to organizations using wireless communications. Some users may wish to operate their radios over a large geographic area, yet are only interested in com- municating with a local group of users. An example may be a countywide system in which one user wishes to communicate with another user in the same city. If available, the typical FDMA (Frequency Domain Multiple Access) solution will use several radio frequencies that may be assigned for use in the user's system. See Figure 1 below.

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Page 1 of 3

MO-LA Technical Developments Volume 22 June 1994

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by Dave Mills, Dave Wiatrowski and Anne Davies-Walsh

  Radio spectrum has become a precious resource to organizations using wireless communications. Some users may wish to operate their radios over a large geographic area, yet are only interested in com- municating with a local group of users. An example may be a countywide system in which one user wishes to communicate with another user in the same city. If available, the typical FDMA (Frequency Domain Multiple Access) solution will use several radio frequencies that may be assigned for use in the user's system. See Figure 1 below.

  User selectable private line (PL) and digital pri- vate line (DPL) were developed to address this situ- ation. (PL is a low frequency, sub-audible tone. DPL is a low baud rate FSK modulation. Both are summed with the analog voice signal and transmitted over the air.) They allow several independent repeaters to operate within a close geographic range on the same channel, while restricting access to the repeater only to the group of users that are in the repeater's desired coverage area. The repeaters are programmed to detect the PL/DPL code before allowing the radio message into the system: Thus, for a system with 1 frequency assignment and 4 local coverage regions, each with their own unique PL code, the system may be setup as shown in the Figure 2 below.

'Region 1

Freq 1 PLcodeA

0

Figure l-Multiple Frequencies and Multiple Local Zones

  This solution may be acceptable in rural areas, where spectrum is readily available, but in urban areas, the user may only be allocated a single he- quency (i.e. a single channel) for the entire radio system. One solution may be simulcast, whereby the user's radio message may be broadcast to all four regions, regardless of location. This approach has a couple of disadvantages: 1) A simulcast system is significantly more complex and costly to setup and maintain than a simple repeater system and 2) The simulcast system only allows one conversation on the channel at any instant in time. The user may require the system to support independent radio con- versations, one in each region.

Figure 2-Single Frequency and Multiple Local &es Using PL codes

  In this system the user can select the repeater they wish to use by simply specifying the correct PL/DPL code (and of course moving to the cover- age regions). Note that ~this does not improve the channel capacity (locally), it simply allows adjacent systems to operate with closer proximity to each other, while minimizing adjacent system interfer- ence with the addition of PL/DPL codes. Both the fixed equipment and subscriber equipment use PUDPL codes to ensure they are operating on the correct system.

38 0 MOtoroIa, inc. 1994

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MOlOlROLA Technical Developments Volume 22 June 1994

  Digital systems have a signal that is, for the most...