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TCH-tap-modes: document FACCH/H observations
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 22 Jul 2024 23:02:54 +0000
parents 28c1cb869d91
children
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106:28c1cb869d91 107:dfa5f99631a6
349 writes its output into the first bit position of each group of 3, but then 349 writes its output into the first bit position of each group of 3, but then
350 sometimes (and not at other times) applies another function that writes the 350 sometimes (and not at other times) applies another function that writes the
351 voting function output into the remaining bit positions, perhaps for loopback 351 voting function output into the remaining bit positions, perhaps for loopback
352 of TCH DL into TCH UL. More study is needed in this area. 352 of TCH DL into TCH UL. More study is needed in this area.
353 353
354 Appearance of FACCH in TCH/HS downlink
355 --------------------------------------
356
357 All mentions of FACCH handling in the previous description refer only to TCH/F.
358 In the case of TCH/HS, the following differences in FACCH handling are seen:
359
360 * Bits [7:5] described earlier for the case of FACCH/F are never seen set in
361 TCH/HS DL captures.
362
363 * In the first 20 ms frame position stolen for FACCH/H, status word 0 is seen
364 as 0xC007 (looks like regular BFI), status word 1 is set to 0 (instead of
365 whatever D_MACC is), and it is not clear whether or not status word 2 is
366 still filled with the bit error count from the convolutional decoding step.
367 (We've only seen 0 in that word.) The payload bits are filled with what
368 looks like garbage, in contrast with FACCH/F behavior of retaining previous
369 buffer content.
370
371 * In the second 20 ms frame position stolen for FACCH/H, status word 0 is seen
372 as 0, status word 1 looks like correct D_MACC after 456 rather than 228 bits
373 have been decoded, and status word 2 contains 0xFFFF. The "garbage" in the
374 payload bits stays the same from the first half of FACCH/H.
375
354 FreeCalypso file format for TCH DL captures 376 FreeCalypso file format for TCH DL captures
355 =========================================== 377 ===========================================
356 378
357 The file format written by fc-shell tch record command is ASCII hex, line-based, 379 The file format written by fc-shell tch record command is ASCII hex, line-based,
358 with one line for every captured 20 ms window. The new format as of 2022 is: 380 with one line for every captured 20 ms window. The new format as of 2022 is: