[srslte-users] simulating propagation delay with shmem_rf

[J Giovatto]

Hi folks,

Not a mainline topic but may be interesting for those w/o hardware etc.

Stemming from the recent PRACH and propagation thread I'm interested in
introducing some propagation delay via the shmem_rf "device".

I first investigated the tx/rx timesatamp but realized that this would
not do it, instead I simply

added an "offest" of 10 samples to the cf_t rx_buffer and indeed this
resulted in calculated offset of 3 (see logs below).


I guess my question here is : is this a reasonable approach ?  My idea
here would be to "carry over" the time advance delta in the tx metadata,
then apply that

at the receiver in the form of an offset of the cf_t rx buffer and see
if the stream re-aligns correctly.

If so I am assuming that there is a relationship between the time
advance and the number of samples I offset my "read" into,

at 1.92 Mhz is it 1/1.92M or 0.5208 samples/usec ?

Also noted that any initial offset larger than 10 does not result in a
sync condition.

patch:

--- a/lib/src/phy/rf/rf_shmem_imp.c
+++ b/lib/src/phy/rf/rf_shmem_imp.c
@@ -348,6 +348,8 @@ static int rf_shmem_resample(double srate_in,
                              cf_t * data_out,
                              int nof_bytes)
 {
+  data_out += 10;


enb logs:

Found Cell:  PCI=1, PRB=25, Ports=1, CFO=-0.3 KHz
18:09:57.435588 [SRF.U] [I] rf_shmem_set_master_clock_rate,  rate 0.00
MHz to 23.04 MHz
18:09:57.435600 [SRF.U] [I] rf_shmem_set_rx_srate,  srate 1.92 MHz to
5.76 MHz
18:09:57.435603 [SRF.U] [I] rf_shmem_set_tx_srate,  srate 1.92 MHz to
5.76 MHz
Setting manual TX/RX offset to 0 samples
Found PLMN:  Id=00101, TAC=7
Random Access Transmission: seq=50, ra-rnti=0x2
18:09:57.526672 [SRF.U] [I] rf_shmem_set_tx_gain,  gain 0.00 to 24.00
RRC Connected
Random Access Complete.     c-rnti=0x46, ta=3
PMCH 1 extract symbols error expecting 2250 symbols but got 2400, lstart 3
Network attach successful. IP: 172.16.0.4
Software Radio Systems LTE (srsLTE)
PMCH 1 extract symbols error expecting 2250 symbols but got 2400, lstart 3

ue logs:

RACH:  tti=511, preamble=50, offset=3, temp_crnti=0x46
Illegal DCI message nCCE: 18, L: 1, nof_cce: 13, nof_bits=25
Error encoding DCI message
Error putting PUSCH 0
Illegal DCI message nCCE: 12, L: 1, nof_cce: 13, nof_bits=25
Error encoding DCI message
Error putting PUSCH 0
Illegal DCI message nCCE: 12, L: 1, nof_cce: 13, nof_bits=25
Error encoding DCI message

Best.

Joe

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