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Design and Implementation of a Practical FLEX Paging Decoder

Grayson Wireless 13

Table 2-2

Relative signal strength for 99% success rate of 80 characters alphanumeric message

Fading environment: Single Rayleigh fade with Doppler frequency = 6.85 Hz which is 5 mph @

900 Mhz.

The first part of the FLEX synchronization (Sync1) is set up such that a 10 ms fade, which is 16

bits at 1600 bps, will not effect both synchronization words in the same fade. The 16 bits of

dotting between the synchronization words ensures that at least one sync codeword will be

decoded. Thus a 10 ms fade can be tolerated during the 115 ms synchronization for a FLEX

pager to decode only one sync codeword and to continue processing the frame. The interleaving

of the data bit in a FLEX block allows up to 16 consecutive bits to be in error for 1600 bps,

because when the data is de-interleaved, each codeword will only contain 2 bit errors and the

BCH forward error correction will correct them. For 3200 bps, 32 consecutive bits can be in

error, and for 6400 bps, 64 consecutive bits can be in error. The data in a block are transmitted

by column and each column takes 5 ms to transmit, so two consecutive columns form 10 ms of

data. Thus a 10 ms fade can be tolerated during each of the 160 ms blocks.

2.5 FLEX and POCSAG mixed

The integration of FLEX with other asynchronous paging standards is quite simple. A FLEX

system must be setup to send a minimum number of frames per cycle. For example, the paging

operator wants FLEX pages to be sent every 30 seconds. The system software simply reserves

1.875 every 30 seconds for a FLEX frame. If multiple FLEX frames are required to send the

information then additional frames are transmitted until all the information is sent. During the

non-FLEX time other asynchronous paging data can be sent. If no FLEX paging data is available

at the 30 second interval then a frame does not have to be sent. Since FLEX is a synchronous

Paging Standard POCAG 512 POCSAG 1200 FLEX 1600 FLEX 6400

Gaussian reference -125.3 dBm -123.2 dBm -121.7 dBm -118.2 dBm

Fading Environment -102.2 dBm -95.9 dBm -107.2 dBm -104.2 dBm

Fading Degradation 23.1 dB 27.3 dB 14.4 dB 14.0 dB

1 2 ... 16 17 18 19 20 21 22 23 24 25 26 ... 73 74

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Motorola Pro FLX Manuel d'utilisateur Page 21