Embed fix sequence in payload part

hi…

i am trying to embed a fixed sequence which i want to recover during
demodulation. I have read from the previous archives that it would be
easier to embed it in the payload part.However, as soon as i add
anything in the payload part, and try to receive using rx_voice, i get
an error saying

terminate called after throwing an instance of ‘std::runtime_error’
what(): msg length is not a multiple of d_itemsize
Aborted (core dumped)

What I have understood is that the msg length changes as soon as I add
something to the payload.If I dont do anything and just run rx_voice.py
then the msg length comes to be 1687/1720

I have tried getting the msg length after adding different sequences.
If i change the code as:
payload = payload + preamble, the msg length changes to 1719

Otherwise, If i change as:
payload = payload + default_access_code
the msg length changes to 1751

where default_access_code and preamble are given in packet_utils.py.

I have not been able to find out the value of d_itemsize or what should
be its value so that the code runs.

I would be very grateful for any help.

Thanking you,
Pratik Hetamsaria

pratik hetamsaria wrote:

Aborted (core dumped)
payload = payload + default_access_code
Pratik Hetamsaria
Pratik,

I’m not entirely sure what you’re trying to do. Which example code are
you using? And also, what’s the purpose of what you are trying to do?
That might help us set you in the right direction.

Please list the code where you are making the change to add your fixed
sequence.

Tom

hi…

i am presently working on tx_voice.py and rx_voice.py for transmission
of audio files. I am interested in finding out the BER . Now, the
problem is that a few packets are dropped so i need to keep track of a
packet sequence number. I am trying to embed a packet sequence number so
that during the time of demodulation i know exactly which bits i should
compare. I have looked into the code of packet_utils.py located at
python2.4/site-packages/gnuradio and try to embed the sequence number
over there in the payload part. What i have understood is that the
preamble and access codes are dropped during the time of reception. So,
i thought it would be easier to embed it in the payload part. Please
correct me if i am wrong.

But when i try adding any sequence then during the time of reception, i
receive the errors as i have mentioned. I have seen that there is an
option of padding the payload with zeros. So, after i add a sequence
number, the length of the payload still remains the same as it was
before adding the sequence number (the number of bits padded have
changed).So, i dont know what really has changed which causes this
error.

What i have added is just one line in the make_packet function of
packet_utils.py

def make_packet(payload, samples_per_symbol, bits_per_symbol,
access_code=default_access_code, pad_for_usrp=True):

 (packed_access_code, padded) = 

conv_1_0_string_to_packed_binary_string(access_code)
(packed_preamble, ignore) =
conv_1_0_string_to_packed_binary_string(preamble)
payload = packed_preamble + payload %LINE ADDED
payload_with_crc = gru.gen_and_append_crc32(payload)
pkt = ‘’.join((packed_access_code, make_header(L),
whiten(payload_with_crc), ‘\x55’))

 if pad_for_usrp:
     pkt = pkt + (_npadding_bytes(len(pkt), samples_per_symbol, 

bits_per_symbol) * ‘\x55’)

 return pkt

I have not changed anything at the receiving end. Please tell me if i
need to do change anything during reception. I have printed the msg
length from the pkt.py code at python2.4/site-packages/gnuradio/blksimpl

For comparison of BER i am taking the payload part as reference i.e. i
am writing the payload part onto a file and during the time of reception
also, i am doing the same. Please tell if there is a better way of doing
BER.

thanks for your help
Pratik Hetamsaria
Tom R. [email protected] wrote: pratik hetamsaria wrote:

hi…

i am trying to embed a fixed sequence which i want to recover during
demodulation. I have read from the previous archives that it would be
easier to embed it in the payload part.However, as soon as i add
anything in the payload part, and try to receive using rx_voice, i
get
an error saying

terminate called after throwing an instance of ‘std::runtime_error’
what(): msg length is not a multiple of d_itemsize
Aborted (core dumped)

What I have understood is that the msg length changes as soon as I
add
something to the payload.If I dont do anything and just run
rx_voice.py then the msg length comes to be 1687/1720

I have tried getting the msg length after adding different sequences.
If i change the code as:
payload = payload + preamble, the msg length changes to 1719

Otherwise, If i change as:
payload = payload + default_access_code
the msg length changes to 1751

where default_access_code and preamble are given in packet_utils.py.

I have not been able to find out the value of d_itemsize or what
should be its value so that the code runs.

I would be very grateful for any help.

Thanking you,
Pratik Hetamsaria

Pratik,

I’m not entirely sure what you’re trying to do. Which example code are
you using? And also, what’s the purpose of what you are trying to do?
That might help us set you in the right direction.

Please list the code where you are making the change to add your fixed
sequence.

Tom

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