Hi everyone:

I have already understand most part of the

tx_ofdm.grc.But the some digital i can’t calculate accurately.

1.stream_to_tagged_stream: Packet length=96 bytes.?

2.And ofdm_carrier_allocator:FFT_length=64(but 48

occupied carrier+4 pilot carriers!=64)?

Thank you in advance.Thank you.

Best regards

IIRC, the tx and rx files are just a very general implementation of the

OFDM

model.

Here’s my understanding:

- Packet length refers to the data necessary to produce n symbols (48

data

carriers, 2 symbols = 96). - The number of total carriers in the systems equals the FFT length.

In this case, 48 data carriers and 4 pilots, 52 total and the rest are

set

to 0.

I hope im sure about this, if anyone is able to confirm this just to be

precise.

http://gnuradio.org/doc/doxygen/page_ofdm.html

http://gnuradio.org/doc/doxygen/page_packet_data.html

–

View this message in context:

http://gnuradio.4.n7.nabble.com/tx-ofdm-tp48430p48432.html

Sent from the GnuRadio mailing list archive at Nabble.com.

On 05/22/2014 10:12 PM, eontool wrote:

IIRC, the tx and rx files are just a very general implementation of the OFDM

model.Here’s my understanding:

- Packet length refers to the data necessary to produce n symbols (48 data

carriers, 2 symbols = 96).

Er, no. This is the number of bytes per packet. It’s 96 because we add 4

bytes for CRC, and then the packet number is a round value.

At 100 bytes per packet, the payload is 800 bits. With BPSK, that would

be 17 OFDM symbols (plus 1 OFDM symbol for header, and 2 for preamble).

You can set whatever here, but remember that the stock equalizers don’t

do a good job on long packets. Also, the length can vary per packet.

- The number of total carriers in the systems equals the FFT length.

In this case, 48 data carriers and 4 pilots, 52 total and the rest are set

to 0.

This is correct (compare 802.11a standard).

M

Hi:

Thank you so much.

1.But from “chunks to symbols” I see you use the bpsk and qpsk

both.Right?What is the relationship between 800 bits and 17

symbols?Namely,how to calculate the 17 symbols?

17*64=1088.Can you explain?

2.I have looked at your ppt about the ofdm.You say now have two

version ofdm.Can you give me some links to them?Or some advices?Thank

you.

Best regards

On 05/23/2014 02:29 AM, xianda wrote:

Hi:

Thank you so much.

1.But from “chunks to symbols” I see you use the bpsk and qpsk

both.Right?What is the relationship between 800 bits and 17 symbols?Namely,how to

calculate the 17 symbols?

17*64=1088.Can you explain?

1 OFDM symbol @ BPSK == 48 bits in this configuration.

ceil(800/48) == 17.

`2.I have looked at your ppt about the ofdm.You say now have two version`

ofdm.Can you give me some links to them?Or some advices?Thank you.

They’re both in the tree. If you start working off of rx_ofdm an tx_ofdm

you’re already using the new stuff; no need to worry about this.

M

Hi Martin:

Thank you for your reply.

But,

1.I see the virual Source(Stream ID:Payload Bits)->Chunks

to Symbols(Symbol Table(-0.707…))(modulate:qpsk)?When I input 800 bits

to the Packet Header Generator,then chunks to symbols.How many symbols

generate in this way?When I input 800 bits to the Repack Bits,then

chunks to symbols.How many symbols generate in this way?Thank you very

much.

2.I also see another ofdm in the old version of

gnuradio.And the module structure is attached.And the connect code in

python:

self.connect((self,0), self.chan_filt) #

filter the input channel

self.connect((self,0), self.ofdm_sync) # into

the synchronization alg.

self.connect((self.ofdm_sync,0), self.nco, (self.sigmix,1)) # use

sync freq. offset output to derotate input signal

self.connect((self,0), (self.sigmix,0)) #

signal to be derotated

self.connect(self.sigmix, (self.sampler,0)) #

sample off timing signal detected in sync alg

self.connect((self.ofdm_sync,1), (self.sampler,1)) #

timing signal to sample at

```
self.connect((self.sampler,0), self.fft_demod) # send
```

derotated sampled signal to FFT

self.connect((self.sampler,1), (self.ofdm_null,0)) # send

timing signal to decoder

self.connect(self.fft_demod, (self.ofdm_null,1)) # send

freq-domain samples to decoder

```
self.connect(self.fft_demod, (self.ofdm_frame_acq,0)) # find
```

frame start and equalize signal

self.connect((self.sampler,1), (self.ofdm_frame_acq,1)) # send

timing signal to signal frame start

self.connect((self.ofdm_frame_acq,0), (self,0)) #

finished with fine/coarse freq correction,

self.connect((self.ofdm_frame_acq,1), (self,1)) #

frame and symbol timing, and equalization

And someone tell me the new version of gnuradio is just expand to the

old version.Can you tell me how much parts adjust?And the main code of

gnuradio is in gnuradio/gr-digital/examples/ofdm?And whether other code

needed?Where are they?

3.And i use two usrps to run the tx_ofdm.grc and

rx_ofdm.grc.The adjust code I also attached. But in the transmit,so many

U U U display.Why?And in the receive part,I also encouter problem.Can

you help me explain it?

Thank you very much.I see the ofdm part many days.And the

3 questions I encounter.Thank you.Sorry for my poor English if you can’t

understand.Thank you in advance.Thank you for your patience.

Best regards

On 05/23/2014 11:33 AM, xianda wrote:

Hi Martin:

Thank you for your reply.

But,

1.I see the virual Source(Stream ID:Payload Bits)->Chunks

to Symbols(Symbol Table(-0.707…))(modulate:qpsk)?When I input 800 bits

to the Packet Header Generator,then chunks to symbols.How many symbols

generate in this way?When I input 800 bits to the Repack Bits,then

chunks to symbols.How many symbols generate in this way?Thank you very much.

I’m pretty sure I’ve answered all of these questions already.

`2.I also see another ofdm in the old version of`

gnuradio.And the module structure is attached.And the connect code in

python:

Just ignore that. The new stuff is a complete rewrite.

`3.And i use two usrps to run the tx_ofdm.grc and`

rx_ofdm.grc.The adjust code I also attached. But in the transmit,so many

U U U display.Why?And in the receive part,I also encouter problem.Can

you help me explain it?

Your computer is too slow.

M