grgsm_Receiver block

[mazen atef]

Dears, 

I want please to ask about the cell search process in general and in the receiver block

what i understand that simply the steps are as follow and in order

1) Receive GSM signal and make the GMSK demodulation and channel decoding.

2) After demodulation , the receiver tries to find the FCCH burst by searching for the whole zero burst (Is there any decoding here ?)

3)check where is this burst and calculate the frequency offset from original expected center frequency.

4) after getting FCCH draw the multiframe structure and read directly first slot of next frame  (SCH) ( Any decoding here )

5) Get the frame numbr and what else from SCH ??? 

6)after that does it send all burst for all frames for one multiframe ? or more ?

7)Does the receiver decide if it is BCCH channel for eample or TCH ... or the demapper who decides that ? 

Thank you 

Mazen Bassiouny 

[mazen atef]

Any Explanation here please ? ;) 

[Vasil Velichkov]

Hi Mazen Atef,

On 08/17/2018 08:27 PM, mazen atef wrote:

Any Explanation here please ? ;) 
On Wednesday, August 15, 2018 at 1:51:09 PM UTC+2, mazen atef wrote:

Dears, 

I want please to ask about the cell search process in general and in the receiver block

what i understand that simply the steps are as follow and in order

1) Receive GSM signal and make the GMSK demodulation and channel decoding.

2) After demodulation , the receiver tries to find the FCCH burst by searching for the whole zero burst (Is there any decoding here ?)

I don't understand how the FCCH burst get detected but the demodulation and decoding is done after it gets synchronized.

3)check where is this burst and calculate the frequency offset from original expected center frequency.

Correct, lines 676 - 688

4) after getting FCCH draw the multiframe structure and read directly first slot of next frame  (SCH) ( Any decoding here )

Correct and yes it decodes it - see line 257.

5) Get the frame numbr and what else from SCH ???

frame number and BSIC, again on line 257.

 6)after that does it send all burst for all frames for one multiframe ? or more ?

After it gets synchronized it starts sending all bursts it successfully detects, one after another, each burst in a separate message, until it either lost synchronization, reach end of file or you stop it manually.

7)Does the receiver decide if it is BCCH channel for eample or TCH ... or the demapper who decides that ?

Neither the Receiver nor the Demapper, you are kind of manually configuring it using the various blocks and connecting them accordingly.

[Piotr Krysik]

W dniu 30.08.2018 o 14:43, Vasil Velichkov pisze:

> Hi Mazen Atef,
>
> On 08/17/2018 08:27 PM, mazen atef wrote:
>>
>> Any Explanation here please ? ;) 
>> On Wednesday, August 15, 2018 at 1:51:09 PM UTC+2, mazen atef wrote:
>>
>> Dears, 
>> I want please to ask about the cell search process in general and
>> in the receiver block
>> what i understand that simply the steps are as follow and in order
>>
>> 1) Receive GSM signal and make the GMSK demodulation and channel
>> decoding.
>> 2) After demodulation , the receiver tries to find the FCCH burst
>> by searching for the whole zero burst (Is there any decoding here ?)
>>
>
> I don't understand how the FCCH burst get detected but the demodulation
> and decoding is done after it gets synchronized.
>

Simple algorithm is looking for periods when instantaneous frequency of
the signal is constant (FCCH bursts has gsm_symbol_rate/4 frequency
where gsm_symbol_rate=1625000/6). At this point there is no GMSK
demodulation done - the algorithm is looking at raw filtered baseband
signal.

>> 3)check where is this burst and calculate the frequency offset
>> from original expected center frequency.
>>
>
> Correct, lines 676 - 688

> <https://github.com/ptrkrysik/gr-gsm/blob/592ec9b7859f544b297e9352323b45ceb450a68a/lib/receiver/receiver_impl.cc#L676-L688>

>
>> 4) after getting FCCH draw the multiframe structure and read
>> directly first slot of next frame  (SCH) ( Any decoding here )
>>
>
> Correct and yes it decodes it - see line 257.

> <https://github.com/ptrkrysik/gr-gsm/blob/592ec9b7859f544b297e9352323b45ceb450a68a/lib/receiver/receiver_impl.cc#L256-L257>
Here also GMSK demodulation with use of MLSE equalizer is done. SCH
bursts are decoded directly at the receiver, as frame number and BSIC
are needed by the receiver. Other channels are decoded outside of the
receiver.

>
>> 5) Get the frame numbr and what else from SCH ???
>
> frame number and BSIC, again on line 257.

> <https://github.com/ptrkrysik/gr-gsm/blob/592ec9b7859f544b297e9352323b45ceb450a68a/lib/receiver/receiver_impl.cc#L256-L257>

>
>>  6)after that does it send all burst for all frames for one
>> multiframe ? or more ?
>>
>
> After it gets synchronized it starts sending all bursts it successfully
> detects, one after another, each burst in a separate message, until it
> either lost synchronization, reach end of file or you stop it manually.
>
>> 7)Does the receiver decide if it is BCCH channel for eample or TCH
>> ... or the demapper who decides that ?
>>
>
> Neither the Receiver nor the Demapper, you are kind of manually
> configuring it using the various blocks and connecting them accordingly.

Yep, the demapper just sets a field with logical channel type in bursts
headers. But you are configuring it this way by doing the connection.



--
Best Regards,
Piotr Krysik