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Re: [Discuss-gnuradio] bpsk lost some packet when transmit data disconti

From: Fisheep
Subject: Re: [Discuss-gnuradio] bpsk lost some packet when transmit data discontinuously
Date: Sun, 14 Mar 2010 00:39:39 -0800 (PST)


It is really what you are said. After I transmitted packet several times, I
got the
probability of 1/2 that correctly received packet and probabilty of 1/2 that
got nothing.
I think this result is explicit when I change the carrier frequency from
900M Hz to 2.4G Hz.

It is my fault. I thought that it will work like a bpsk if I removed the
differential encoder/decoder
from dbpsk.py. But I haven't do the channel estimation to get the correct
phase rotation.

Did anyone try to implement the channel estimation into gnuradio?

I find that at the gr_mpsk_receiver_cc block there is a parameter theta that
pass into this block.
In dbpsk the theta is set to 0. Is it correct that I pass the phase rotation
from the channel estimation?
If is, how do I do the channel estimation before the synchronization...there
are two samples/symbol
before the gr_mpsk_receiver_cc block. 

Sorry for I first contact with software defined radio. And I really don't
have a complete concept of the 
transmission structure when it involves synchronization and channel


Tom Rondeau wrote:
> On Tue, Mar 9, 2010 at 3:03 AM, Fisheep <address@hidden> wrote:
>> Hi:
>> I have some problem about dbpsk and bpsk using benchmark_tx/rx.py with
>> some
>> modification.
>> My experiment set up was that the transmitter waits for a time delay (0.5
>> s)
>> after sending a packet.
>> It means that we transmit data discontinuously.
>> If we using dbpsk modulation, we can successfully receive every packet.
>> But if we removed the differential encoder/decoder (equals to bpsk
>> modulation), we uaually
>> lost some packet (almost half of the data lost).
>> However, when we transmit data continuously,the bpsk modulation seems
>> work
>> fine and we can successfully receive every packet.
>> My problem is that what reason causing the bpsk modulation can not
>> successfully receive packet when
>> transmit data discontinuously.
>> Could anyone have any idea about it, please let me know.
>> I am deeply appreciative.
>>                             Fisheep :)
> Fisheep,
> I'm wondering how you are compensating for the phase ambiguity of the
> BPSK signal in the receiver? In the current DBPSK, there is no method
> to properly rotate the constellation because the differential encoding
> doesn't care. If you are just transmitting BPSK, you'll have a 180
> degree phase ambiguity in the receiver. If that's the case, then for
> every burst of a packet, when the receiver locks to the constellation,
> it will be in one of two phase rotations, one which works and one
> which inverts every symbol. They should be distributed with equal
> probability, so half of the time you'll have the correct rotation and
> the other half you won't.
> When you transmit continuously, I would expect that you'll also see
> the same behavior. If you've got the right rotation, you'll receive
> every packet. Otherwise, you'll see nothing. Does that happen
> sometimes?
> Tom
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio

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