ofdm RF portion doubt...

Hi all
        i am basically from GPS  doing usual Acuistion, tracking stuff for
CDMA signal...  recently moved in to ofdm... still learning..this site
doing a great favour..

           after going thorugh all basics i have few points to comment and
ask..

1.  IFFT will genrate the complex valued signal,as input is output of QAM
mapper and placement of carriers is assymetrical,   how do we go from
here..  how do we transmit a complex signal.?  will it relate to I and Q
channels in my RF transmission and then adding these signals there... or
only real part is transmiietd....

2. for 11a , 20 MHz signal is genrated in to baseband and taken to higher
band where signal has to be transmitted.  in GPS case,at reciver end
selection of IF frequncy and ADC sampling frequncy used to define where
our final digital signal fall: baseband or centred around some carrier...

if i apply same analysis to ofdm,
what are the usual sampling rate and IF signal for 11a or for any other
ofdm system for that matter...

3. if we look at one single symbol,including CP,in frq domain theye are
still impulse trains only.. they do spread only if we look over two
consecutive or more symobols(given that they hapen to tranmit differnt QAM
values) ... giving rise to that famous ofdm frequncy diagram showing
overlapping sincs.....  is it true????


Regds
and thanks in advance
sandeep
               


		
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Hi

First of all you do not transmitt a complex signal. Align the input to
your IFFT in a way called Hermitian symmetry (symetrical around DC). This
will result in only real valued output from the IFFT.

Secondly to get the famous OFDM spectrum, sum the outputs of your IFFT and
convolve this OFDM symbol value with a rectangular pulse (not sure about
the period for the pulse). This will give you the sinc functions in the
frequency domain with the DC value of each sinc equal to the OFDM symbol
value.

Hope it helps
Cheers
Jaco

>Hi all
>        i am basically from GPS  doing usual Acuistion, tracking stuff
for
>CDMA signal...  recently moved in to ofdm... still learning..this site
>doing a great favour..
>
>           after going thorugh all basics i have few points to comment
and
>ask..
>
>1.  IFFT will genrate the complex valued signal,as input is output of
QAM
>mapper and placement of carriers is assymetrical,   how do we go from
>here..  how do we transmit a complex signal.?  will it relate to I and Q
>channels in my RF transmission and then adding these signals there... or
>only real part is transmiietd....
>
>2. for 11a , 20 MHz signal is genrated in to baseband and taken to
higher
>band where signal has to be transmitted.  in GPS case,at reciver end
>selection of IF frequncy and ADC sampling frequncy used to define where
>our final digital signal fall: baseband or centred around some
carrier...
>
>if i apply same analysis to ofdm,
>what are the usual sampling rate and IF signal for 11a or for any other
>ofdm system for that matter...
>
>3. if we look at one single symbol,including CP,in frq domain theye are
>still impulse trains only.. they do spread only if we look over two
>consecutive or more symobols(given that they hapen to tranmit differnt
QAM
>values) ... giving rise to that famous ofdm frequncy diagram showing
>overlapping sincs.....  is it true????
>
>
>Regds
>and thanks in advance
>sandeep
>               
>
>
>		
>This message was sent using the Comp.DSP web interface on
>www.DSPRelated.com
>




		
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Thanks jaco for the help

i just want to elaborate few points further

>Hi
>
>First of all you do not transmitt a complex signal. Align the input to
>your IFFT in a way called Hermitian symmetry (symetrical around DC).
This
>will result in only real valued output from the IFFT.

if we place the carriers symmetrically,i believe it means same amplitude
and oppoiste phase on wither side of DC for the same index number..that
means we are forced to do so..... so as to get real output.... dont this
thing effectively halves my data transmission rate....?

>
>Secondly to get the famous OFDM spectrum, sum the outputs of your IFFT
and
>convolve this OFDM symbol value with a rectangular pulse (not sure about
>the period for the pulse). This will give you the sinc functions in the
>frequency domain with the DC value of each sinc equal to the OFDM symbol
>value.

what you said is entirely true...perhaps its another way of looking at the
same thing what i said.... say if both symbols to be transmtted are exactly
same.. each subcarrier will be continous over twice the length of
rectangular pulse.... but beacuse its 100% sure that two consecuitve
symbols will transmitt different symbols at least once over a period of
time,there will be change of phase and amplitude for each subcarrier
(equivalent of multiplication of subcarrier with that of rectangular
pulse)...in frequncy domain it becomes convoloution of each  impluse (for
subcarriers)withe the sinc function(that of rectangular pulse)....

i hope what i undersatnd is closer to TRUTH..

thansk a lot 




>
>Hope it helps
>Cheers
>Jaco
>
>>Hi all
>>        i am basically from GPS  doing usual Acuistion, tracking stuff
>for
>>CDMA signal...  recently moved in to ofdm... still learning..this site
>>doing a great favour..
>>
>>           after going thorugh all basics i have few points to comment
>and
>>ask..
>>
>>1.  IFFT will genrate the complex valued signal,as input is output of
>QAM
>>mapper and placement of carriers is assymetrical,   how do we go from
>>here..  how do we transmit a complex signal.?  will it relate to I and
Q
>>channels in my RF transmission and then adding these signals there...
or
>>only real part is transmiietd....
>>
>>2. for 11a , 20 MHz signal is genrated in to baseband and taken to
>higher
>>band where signal has to be transmitted.  in GPS case,at reciver end
>>selection of IF frequncy and ADC sampling frequncy used to define where
>>our final digital signal fall: baseband or centred around some
>carrier...
>>
>>if i apply same analysis to ofdm,
>>what are the usual sampling rate and IF signal for 11a or for any other
>>ofdm system for that matter...
>>
>>3. if we look at one single symbol,including CP,in frq domain theye are
>>still impulse trains only.. they do spread only if we look over two
>>consecutive or more symobols(given that they hapen to tranmit differnt
>QAM
>>values) ... giving rise to that famous ofdm frequncy diagram showing
>>overlapping sincs.....  is it true????
>>
>>
>>Regds
>>and thanks in advance
>>sandeep
>>               
>>
>>
>>		
>>This message was sent using the Comp.DSP web interface on
>>www.DSPRelated.com
>>
>
>
>
>
>		
>This message was sent using the Comp.DSP web interface on
>www.DSPRelated.com
>


		
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hi friends
i am amal
i am doing a project on the below topic
" power allocation for ofdm using adaptive beamforming over wireless
netowrk"
if u have any paper related to this or any article
if u know anything about this please send me
urs 

amal

1. The complex I & Q baseband signal will be upconverted to higher
frequency (being it IF or RF doesn't matter) by using sin and cos
signals as I*cos-Q*sin

2. In 11a system, we downconvert to baseband (with several stages,
usually 1 to 2), and ADC to do sampling. Usually the sampling rate is
2X (40MHz).

>1. The complex I & Q baseband signal will be upconverted to higher
>frequency (being it IF or RF doesn't matter) by using sin and cos
>signals as I*cos-Q*sin

I am trying this approach in MATLAB. after generating I and Q  signals(as
output of IFFT) i am multiplying them with

cq(i) = cos(2*pi*i*(30e06/100e06));      
ci(i) = sin(2*pi*i*(30e06/100e06));

to simulate upconversion at 30 Mhz carrier with sampling rate of 100 Mhz.

and finaly ading as u suggest 
final = I*cos-Q*sin

is it correct approach to simulate things in MATLAB....

if i want to recover signal from final after adding AWGN,how to do it..

is it again by multiplying the final+noise with cos and sin of same
frequncies in MATLAB.... where does filter etc come in to picture...


Thanks fot the help..
sandy


>2. In 11a system, we downconvert to baseband (with several stages,
>usually 1 to 2), and ADC to do sampling. Usually the sampling rate is
>2X (40MHz).
>
>


		
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