Re: [Discuss-gnuradio] Channel Model omits first 3 samples

[Tom Rondeau]

On Wed, Jul 10, 2013 at 8:42 AM, Gregory Warnes <address@hidden> wrote:
>
>
> On Jul 10, 2013, at 4:47 AM, Tom Rondeau <address@hidden> wrote:
>
>> On Tue, Jul 9, 2013 at 8:38 PM, Gregory Warnes <address@hidden> wrote:
>>> Hi All,
>>>
>>> I've been working on calculating BER for simulated transmitter/recieved pair
>>> and have noticed that the channel_model eats the first 3 samples.  Thus to
>>> correctly align the data input and output from channel_model (e.g. for
>>> calculating BER), it is necessary to account for the 3-sample offset.
>>>
>>> Perhaps it would be worth adding a note to this effect to the channel_model
>>> documentation.
>>>
>>> -Greg
>>
>> It's not that the channel model 'eat's 3 samples. This is group delay
>> of your filters that you're seeing. The transmitter, receiver, and
>> channel model may all have filters in them and each has a group delay.
>> You always have to adjust for this. Not sure we can put it into the
>> documentation except to say that you need to calculate the total group
>> delay of your system if you want to align the data.
>
> Thanks Tom,
>
> Is there a straightforward way to calculate group delay for a flow graph?
>
> Alternatively, would it be straightforward to create a block that annotates 
> the data stream with a time index and another block that uses this 
> information to align streams?
>
> -Greg


Greg,

Symmetric FIR filters have a known group delay of (N-1)/2. So what you
need to do is a) make sure all of your filters are symmetric FIRs (and
they probably are; firdes and optfir produce these types of filters)
and b) know all of the filters in the path, which includes the
transmit and receive pulse shaping filters and any filter in the
channel model (used to simulate multipath). You should be able to add
up the group delays of all filters to get your answer.

Tom