You need to know how many of the 256 points in the FFT are populated,
data-carrying  subarriers.   Some are probably for guard band and some
are probably for pilot tones.

Once you know how many data subcarriers there are, multiply that
number by the number of bits per subcarrier per symbol (2 for DQPSK),
and divide that by the OFDM symbol duration, which you indicate is 6.4
microseconds.

So bits/subcarrier times number of subcarriers, then divide by symbol
duration.


On Mon, 10 Mar 2008 11:30:32 -0500, "Richard_K" <ngyh80@hotmail.com>
wrote:

>>On Mon, 10 Mar 2008 09:42:52 -0500, "Richard_K" <ngyh80@hotmail.com>
>>wrote:
>Hi,
>
>The details are as following:
>
>FFT size: 256
>Bandwidth 4.3MHz - 20.9MHz
>Modulation: DQPSK
>Useful duration: 5.12 microseconds
>Useful duration + Guard interval: 6.4 microseconds
>Sampling frequency: 50MHz
>
>In this regard, does the data rate of the physical layer have anything to
>do with the sampling frequency? (How the sampling frequency affects the
>data rate?) Also, may I know how to calculate the data rate of the OFDM
>system based on the above information?
>
>Many thanks.
>
>>
>>>
>>>For OFDM system with the sampling frequency of 50MHz and BPSK
>modulation
>>>scheme,what will be the data rate at physical layer? Will it be
>>>50Mbits/second as well?
>>>
>>>Please help. Thanks.
>>
>>One needs a lot more information to be able to say.
>>
>>For OFDM the raw bit rate will be the number of bits per constellation
>>symbol (one in the case of BPSK), times the number of subcarriers
>>(which is unknown), times the OFDM symbol rate (which is also
>>unspecified).
>>
>>Since the sample rate can be anything above the minimum sample rate
>>needed to support the bandwidth, there's really no way to tell from
>>the information you've given.
>>
>>Eric Jacobsen
>>Minister of Algorithms
>>Abineau Communications
>>http://www.ericjacobsen.org
>>
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org

>On Mon, 10 Mar 2008 09:42:52 -0500, "Richard_K" <ngyh80@hotmail.com>
>wrote:
Hi,

The details are as following:

FFT size: 256
Bandwidth 4.3MHz - 20.9MHz
Modulation: DQPSK
Useful duration: 5.12 microseconds
Useful duration + Guard interval: 6.4 microseconds
Sampling frequency: 50MHz

In this regard, does the data rate of the physical layer have anything to
do with the sampling frequency? (How the sampling frequency affects the
data rate?) Also, may I know how to calculate the data rate of the OFDM
system based on the above information?

Many thanks.

>
>>
>>For OFDM system with the sampling frequency of 50MHz and BPSK
modulation
>>scheme,what will be the data rate at physical layer? Will it be
>>50Mbits/second as well?
>>
>>Please help. Thanks.
>
>One needs a lot more information to be able to say.
>
>For OFDM the raw bit rate will be the number of bits per constellation
>symbol (one in the case of BPSK), times the number of subcarriers
>(which is unknown), times the OFDM symbol rate (which is also
>unspecified).
>
>Since the sample rate can be anything above the minimum sample rate
>needed to support the bandwidth, there's really no way to tell from
>the information you've given.
>
>Eric Jacobsen
>Minister of Algorithms
>Abineau Communications
>http://www.ericjacobsen.org
>

On Mon, 10 Mar 2008 09:42:52 -0500, "Richard_K" <ngyh80@hotmail.com>
wrote:

>
>For OFDM system with the sampling frequency of 50MHz and BPSK modulation
>scheme,what will be the data rate at physical layer? Will it be
>50Mbits/second as well?
>
>Please help. Thanks.

One needs a lot more information to be able to say.

For OFDM the raw bit rate will be the number of bits per constellation
symbol (one in the case of BPSK), times the number of subcarriers
(which is unknown), times the OFDM symbol rate (which is also
unspecified).

Since the sample rate can be anything above the minimum sample rate
needed to support the bandwidth, there's really no way to tell from
the information you've given.

Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org

On Mon, 10 Mar 2008 09:42:52 -0500, Richard_K wrote:

> For OFDM system with the sampling frequency of 50MHz and BPSK modulation
> scheme,what will be the data rate at physical layer? Will it be
> 50Mbits/second as well?
> 
> Please help. Thanks.

Comment 1:  Why go to the trouble to make an OFDM system and only use 
BPSK modulation?  Wouldn't you at least want to use QPSK, if not some 
more complicated constellation?

Comment 2:  If it truly is BPSK (and if I've got my head screwed on 
straight -- the caffeine hasn't hit yet this morning) then 50Mbits/sec 
would be the theoretical upper bound, and then only if you have no 
overhead and precise synchronization.  A more realistic bound would be 
significantly less than 1/2 that, with room to grow downward to an almost 
infinite extent.  Without other knowledge of the system I would hesitate 
to assign any actual bit rate just from knowledge of the sampling rate.

Comment 3:  Your post lacks in detail, which is why (caffeine aside) mine 
lacks certitude.  Tell us what you're _really_ working on, and someone 
will give you better information.

-- 
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html