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Muhammad Awais Aslam mawais.aslam985 at gmail.comDear Neels,
Thanks for the response.
> Osmocom lives by contribution. If you have implemented useful parts that
we
> don't support yet, please submit them back to us -- that would be highly
> appreciated.
> http://osmocom.org/projects/cellular-infrastructure/wiki/C
oding_standards#Submitting-Patches
I have already uploaded the code which could be find here:
https://gerrit.osmocom.org/5490
> I'm currently working on handover, though my attention is (should be)
more on
> the HO decision making process: trigger handover due to load
considerations.
> In the process though I am touching and testing the stock handover code as
> well, and have seen some code concerned with sync and async HO.
> You say that you are able to do sync HO but not async. However, it
appeared to
> me so far that all we ever do is async HO. i.e., in handover_logic.c, we
have
> rc = rsl_chan_activate_lchan(new_lchan, RSL_ACT_INTER_ASYNC, ho->ho_ref);
> and a patch that is in the pipeline now adds even the possibility to pass
_SYNC
> instead of _ASYNC (while we're still going to pass _ASYNC anyway in all
cases,
> so far).
> So either you have SYNC and ASYNC mixed up or we're misunderstanding each
> other... You are asking about osmocom code, right?
I am talking about the code from OSMOCOM-BB and there is no file name
handover_logic.c.
I think you are talking about code from openBTS.
We took code related to handover from the osmocom-bb jolly branch by
manually adding/deleting stuff in the master branch as updating to the
latest copy was giving us issues. We added code from the “Safely change TPU
offset on TS change or sync change” commit till the “Use only sel_si for
information about the current cell” commit. Using the handover code in the
jolly branch and after making the changes we were able to obtain the
handover command from the BTS. The synchronized handover case works
sometimes though still not every time, however the non-synchronized case
doesn't work at all as we are not able to get the physical information
command from the new cell. I'll explain the changes/additions we made to
achieve this.
Firstly, we noted that in dedicated mode SB burst was not being detected.
Changes were required at three main places in order to correctly perform
FB/SB detection:
- It was seen that the results for SB were being read from DSP API location
dsp_api.db_r->a_sch which is for the idle mode. The results had to be read
from the dsp_api.ndb->a_sch26 location for the dedicated case if
TCH_SB_DSP_TASK is used.
- After reading the FB we needed to update the quarter-bit offset of the
TPU using the TOA of the FB to sync it with the start of frame of the
neighbour cell in order to catch the SB (by changing the vaule of
l1s.tpu_offset using the TOA of the FB).
- Frequency compensation needed to be performed using the afc_correct
function before reading the SB.
* We actually kind of cheated a bit by adding 3 frames to the original idle
frame in order to give us more time to perform FB/SB detection including
the synchronizations mentioned above. This was because we weren't that
proficient with the code and someone with more understanding could do this
better. The call did not get dropped. We used the initial added “idle”
frame to perform the quarter-bit and frequency compensation which was
reversed in the idle frame with the response function to tune back to the
serving cell.
These things, when added to the code did the trick and BSIC of the
neighbours was obtained.
Once the BSICs were decoded the measurement report sent to the BTS became
meaningful and the handover command was received. Upon receiving handover
command, access bursts needed to be sent on the new channel which again was
not currently being implemented properly as in order to tune to the new
cell we needed to know its quarter-bit offset for start of frame, frequency
compensation and absolute frame number which were not previously being
obtained. Now that we were able to detect FB and SB these values were
stored for the neighbours following detection of these bursts and were used
to tune to a neighbour cell in case of a handover to it before the sending
of access bursts. However, here is where we are stuck. We were expecting a
physical information message following the access bursts but were not able
to receive it due to which the handover failed. If only that could be
achieved we believe handover should be successful.
Either we are not syncing properly to the new cell or we might not be
following GSM protocol properly. We also might not be reading the FACCH
properly for physical information message after tuning to the new cell as
we couldn't really understand that bit very well. We wanted someone
expertise on this matter and were hoping our work could be of use.
Regards
M. Awais
On Mon, Jan 15, 2018 at 5:39 PM, Neels Hofmeyr <nhofmeyr at sysmocom.de> wrote:
> Hi Muhammad,
>
> On Mon, Jan 15, 2018 at 03:03:10PM +0500, Muhammad Awais Aslam wrote:
> > Hi,
> >
> > We have manged to decode the BSIC in dedicated mode which were not
> > implemented before.
>
> Osmocom lives by contribution. If you have implemented useful parts that we
> don't support yet, please submit them back to us -- that would be highly
> appreciated.
> http://osmocom.org/projects/cellular-infrastructure/wiki/
> Coding_standards#Submitting-Patches
>
> > Now we are working on the synchronize and
> > non-synchronize Handover. Once we send the fake measurement report to the
> > BTS we get a handover command from the network. That could be
> synchronized
> > or non-synchronized.
> >
> > In case of synchronized hand over, without making any changes in the TPU
> > clock offset and the frame number and frequency correction, we are able
> to
> > complete the handover process most of the time.
> >
> > In case of non-synchronized handover, a physical info is required from
> the
> > network when mobile station would send Handover access burst to the
> network
> > before the timer expires but we never get physical info during this
> period.
> > Here we are stuck.
>
> I'm currently working on handover, though my attention is (should be) more
> on
> the HO decision making process: trigger handover due to load
> considerations.
>
> In the process though I am touching and testing the stock handover code as
> well, and have seen some code concerned with sync and async HO.
>
> You say that you are able to do sync HO but not async. However, it
> appeared to
> me so far that all we ever do is async HO. i.e., in handover_logic.c, we
> have
>
> rc = rsl_chan_activate_lchan(new_lchan, RSL_ACT_INTER_ASYNC,
> ho->ho_ref);
>
> and a patch that is in the pipeline now adds even the possibility to pass
> _SYNC
> instead of _ASYNC (while we're still going to pass _ASYNC anyway in all
> cases,
> so far).
>
> So either you have SYNC and ASYNC mixed up or we're misunderstanding each
> other... You are asking about osmocom code, right?
>
> ~N
>
>
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