[Discuss-gnuradio] Need to resolve error: "Unpack requires a string argument of length 3"

[Nazmul Islam]

Hello,

This is Nazmul, a PhD student of Rutgers University. I am using a modified form of 'benchmark_Tx.py' and 'benchmark_Rx.py' for my research. I am transmitting a random binary sequence and calculating the bit error rate at the receiver (BER). Basically, I am generating a long binary sequence, grouping them into different packets and then transmitting the packets using 'benchmark_Tx' program. I receive the packets using 'benchmark_Rx.py' and then find the BER. The program works at high transmitter amplitude, i.e., when channels are good. However, when I start lowering the transmitter amplitude, i.e. create bad channels, at some point, I get the following error:

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "/usr/local/lib/python2.6/dist-packages/gnuradio/blks2impl/pkt.py", line 164, in run
    self.callback(ok, payload)
  File "./rec_randombingen.py", line 97, in rx_callback
    (ide,pktno,) = struct.unpack('!cH', payload[0:3])
error: unpack requires a string argument of length 3

After showing this error, the whole programs. Let's assume that I am transmitting 100 packets. If this error occurs at 20-th packet, the benchmark_rx does not receive the next 80 packets. This really hampers the calculation of packet loss probability and bit error rate.

I guess, for very bad channels, noise is acting as erasure, i.e., some part of the packet header is getting erased. Ideally, I would like the program to assume this packet to be 'lost' and move on to receiving the remaining packets. It is OK with me if I lose a packet due to this type of error but how can I make sure that the whole program does not stop due to this error?

Your feedback will be very appreciated.

Thanks,

Nazmul



-- 
Muhammad Nazmul Islam

Graduate Student
Electrical & Computer Engineering
Wireless Information & Networking Laboratory
Rutgers, USA.