HRv1 UL Rx » History » Version 2
falconia, 07/22/2024 11:45 PM
1 | 1 | falconia | h1. Uplink Rx issues with HRv1 |
---|---|---|---|
2 | |||
3 | In the [[BFI criteria]] article we've looked at the problem of how a traditional E1 BTS sets the BFI flag in its TRAU-UL output. The questions raised in that article apply equally to all 3 classic GSM codecs: FRv1, HRv1 and EFR. However, in the case of HRv1 codec there are two additional questions: |
||
4 | |||
5 | # How does the BTS decide when to set the UFI flag? |
||
6 | # What thresholds are used for ternary SID classification? |
||
7 | |||
8 | h2. Frame reception status |
||
9 | |||
10 | TRAU-UL frames for FR and EFR (GSM 08.60) feature only one status bit that indicates the result of channel decoding: BFI, or Bad Frame Indicator. But the counterpart for HR (GSM 08.61) features two flag bits: BFI and UFI, the latter being Unreliable Frame Indicator. Hence the question naturally arises: what are the criteria for setting UFI? |
||
11 | |||
12 | The proposed experiments described in the [[BFI criteria]] article will need to be performed with HR as well, looking for possible effects on both BFI and UFI flags. |
||
13 | |||
14 | h2. Ternary SID classification |
||
15 | |||
16 | For all 3 codecs the TRAU-UL frame structure includes two bits that convey the ternary SID flag, classifying the received frame as non-SID speech, or as valid or invalid SID. However, the critical difference between FR/EFR and HR is that for the former two there are spec-prescribed bit counting thresholds, making this classification unambiguous and vendor-neutral, but for HR the relevant spec (GSM 06.41) leaves the details up to equipment manufacturers. |
||
17 | |||
18 | We (as in community of people who seek to bring back GSM in its full glory) will need to experiment with an E1 BTS: feed different crafted inputs over Um, constructing almost-SID frames, and see how it classifies each constructed bit pattern. Possible interactions with convolutional decoder error count will also need to be explored. |
||
19 | 2 | falconia | |
20 | h2. Clues from GSM MS DSP |
||
21 | |||
22 | Traffic channels in GSM are quite symmetric between uplink and downlink, hence the manner in which a BTS receives TCH UL is not expected to be too terribly different from the way a GSM MS receives TCH DL. We've got a recent [[HRv1_error_flags|breakthrough in understanding of TCH/HS Rx]] thanks to experiments with TI Calypso DSP - do the same principles carry over to traditional E1 BTS receiving TCH/HS UL? It would certainly be good to experiment with such BTS to gain some empirical knowledge. |