High-level OFDM question

Hi there,

before delving into the details of the OFDM implementation in GNURAdio I
wanted to ask a high-level question:

What is the method used for initial timing/frequency acquisition?
Is there one or two training OFDM symbols that precede the transmission
of data? and how many OFDM data symbols are following? Is this number
determined by the first training symbols or is it fixed?

Also, my understanding is that each data OFDM symbol has pilot tones for
continuous frequency/timing tracking, right?

Thanks
Achilleas

On Fri, Feb 12, 2010 at 10:34 AM, Achilleas A.
[email protected] wrote:

Hi there,

before delving into the details of the OFDM implementation in GNURAdio I
wanted to ask a high-level question:

What is the method used for initial timing/frequency acquisition?

We use, by default, the standard Schmidl and Cox. There are two other
methods that we wrote which you can use to varying degrees of success.
My presentation from the [email protected] symposium, 2007, has details of
all of these methods and why we use the one we do.

Is there one or two training OFDM symbols that precede the transmission of
data? and how many OFDM data symbols are following? Is this number
determined by the first training symbols or is it fixed?

We use a method that only requires 1 training symbol. The much of the
code is actually set up to allow you to put in however many you want,
but you’ll need to redesign some of the the receiver blocks to make
use of them.

The number of symbols following depends on the frame length you
specify. In the benchmark_ofdm* examples, we default to 400 bytes with
200 occupied tones per symbol, so for BPSK this is (400*8/200) = 16
symbols. So there are 16 symbols between preambles.

Also, my understanding is that each data OFDM symbol has pilot tones for
continuous frequency/timing tracking, right?

Thanks
Achilleas

While I wrote the transmitter to allow for pilot tones, we don’t
actually make use of them in the receiver. Instead, we use a decision
feedback equalizer to keep on track. Making use of the pilot tones in
the receiver is definitely something we need done, though.

Tom

Tom, thanks for the info.

So to clarify, the frame length is fixed for the transmission to a
certain number (say 400 bytes) so the system waits until it collects 400
bytes worth of data and then it generates and transmits it using a burst
of 1 training and 16 data OFDM symbols.
If the number of data in the queue at a give time is say 500 bytes it
will first process the first 400 and then wait until an additional 300
bytes (or more) comes in to generate the next burst.
Is that right?

Achilleas

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