Cellular Network Infrastructure » Radio Access Network » OsmoBTS

For some inexplicable reason, osmo-bts-sysmo doesn't show any of the RACH bursts sent by a OsmocomBB layer1.

In the exact same setup, osmo-bts-trx successfully detects each and every of those RACH bursts.

the PHY in sysmobts detects energy at the channel with a sufficiently high level, but claims there is an invalid burst type present:

Rach_DecodeBurst() => Wrong burst type ! [Fn = 3003, Tn = 0, RSSI = -101.6 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3004, Tn = 0, RSSI = -101.3 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3013, Tn = 0, RSSI = -42.6 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3014, Tn = 0, RSSI = -101.3 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3023, Tn = 0, RSSI = -101.9 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3024, Tn = 0, RSSI = -101.7 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3025, Tn = 0, RSSI = -101.2 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3026, Tn = 0, RSSI = -42.5 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3027, Tn = 0, RSSI = -102.0 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3028, Tn = 0, RSSI = -105.2 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3029, Tn = 0, RSSI = -104.9 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3030, Tn = 0, RSSI = -104.7 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3031, Tn = 0, RSSI = -105.2 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3032, Tn = 0, RSSI = -42.6 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3033, Tn = 0, RSSI = -104.8 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3034, Tn = 0, RSSI = -104.2 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3035, Tn = 0, RSSI = -103.4 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3036, Tn = 0, RSSI = -105.1 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3037, Tn = 0, RSSI = -42.7 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3038, Tn = 0, RSSI = -102.7 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3039, Tn = 0, RSSI = -103.7 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3040, Tn = 0, RSSI = -103.5 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3041, Tn = 0, RSSI = -103.5 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3042, Tn = 0, RSSI = -104.4 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3043, Tn = 0, RSSI = -42.3 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3044, Tn = 0, RSSI = -103.8 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3045, Tn = 0, RSSI = -104.3 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3054, Tn = 0, RSSI = -103.9 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3055, Tn = 0, RSSI = -105.4 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3064, Tn = 0, RSSI = -104.6 dBm]
Rach_DecodeBurst() => Wrong burst type ! [Fn = 3065, Tn = 0, RSSI = -54.7 dBm]

Where the ~ -100 dBm slots are those without RACH, and the ~ -45 dBm are those where the OsmocomBB MS transmitted RACH bursts.

This is very odd. I tried with different osmo-bts-sysmo versions and could reproduce this problem always. Simply switching to osmo-bts-trx always solved the problem as reliablyy as osmo-bts-sysmo failed.

I'm 100% sure I've used OsmocomBB with sysmoBTS many times before.

This can be very well reproduced with running the BTS_Test testsuite (and particularly the TC_rach_content and TC_rach_count tests) against a physical BTS.

sysmoBTS PHY correlates against all the three synch sequences indicated in TS 05.02 Section 5.2.7 and reports this up to osmo-bts. osmo-bts-trx only seems to know the standard sequence, so it actually does less but works better.

My assumption is that somehow we're doing something wrong in OsmocomBB in generating the burst.

I've validated that the BSIC is identical between network and MS, and I've used "printf debugging" in OsmocomBB layer1 as well as osmo-bts-sysmo to ensure 0x3F was used in both cases.

I've tried with a variety of receive levels / attenuation, and even played with timing advance of OsmocomBB: no success.