i forgot to mention
                     that after negative freq multiplication everything
strightens up.. no rotaion whatsoever after multiplying the same negative
carrier offset...

sandeep



>See comments below.
>
>sandy80 wrote:
>> Hi col
>>    Thanks for insightful information. you have done the analysis in
>> continous time domain,where, ofcourse,multiplication by exponential
will
>> shift my whole spectrum.  but there are couple of issues when u try to
do
>> things in discrete domain
>>
>
>Nope, the analysis is the same for both domains.  You just have to be
>careful where the spectral images start falling after you sample in the
>digital domain.
>
>>
>> >I believe you want the following
>> >
>> >let s(t) = Re{b(t) exp(j*W_c*t + theta_c)}
>> >
>> >where W_c = Carrier freq, theta_c = carrier phase and b(t) =
>> >a(t)exp(j*phi(t)) and represents the complex baseband information
>> >carrying signal.  At the Rx you have the following
>> >
>> >r(t) = s(t) * exp(-jW_o + theta_o)
>> >
>> >where W_o and theta_o represent the estimate of the carrier freq and
>> >phase.
>>
>>
>> here youmean to say that to get a perfect baseband signal W_c = W_o
>>
>> as well both phases.  and my carrier offset might be  abs(W_c - W_o).
>
>Correct.
>
>>
>> but in matlab i cant genrate a carrier for morethan the (f/fs) =0.5
since
>> all  looks like same. after 0.5  like 0.6 is equivalent to 0.4 etc...
>>
>
>I still don't see the problem.  Are you trying to generate carrier
>offsets that are greater than the sampling frequency?
>
>> after some jiggery pokery we end up with the following complex
baseband
>> >signal
>> >
>> >r'(t) = 0.5 * b(t) exp(j*delta*t+th)
>> >
>> >where delta = W_c - W_o and th = theta_c - theta_o representing
carrier
>> >offset and phase difference.  This means that you can simply generate
>> >this exponential signal with the offset and phase that you desire and
>> >multiply your OFDM signal b(t) after the channel filter.  I think
that
>> >covers it all.
>>
>> i have tried this thing and it is working well..
>>  following is apiece of code...
>>
>>
>>
>> recv = channel2(symbol2);   % pass the OFDM signal through channel. it
>> will add AWGN and Multipath components
>>
>>   indexx = 0:1:length(recv)-1;
>>   indexx = indexx * 0.1  ; %  carrier offset  0.1  means ratio of   
f/fs
>> .   for sampling frequncy of 20 Mhz, offset = 2Mhz.
>>
>> recv = recv.* (exp(j*2*pi*indexx));% complex exponential
multiplication
>>
>>
>>     recv = recv.* (exp(-j*2*pi*indexx));
>>    data = recved(recv);       % Recieve the signal from channel
>>
>>
>> i am getting perfect constellation after i multiply with second
>> exponential with negative frequncy....
>>
>
>Yep, this is what you would expect.
>
>>
>> why to multiply with 0.5 ,any intutive reasoning???
>
>1/2 falls out in the analysis, it doesn't change things, you just have
>to make sure that your scaling is correct at the output of the Rx low
>pass filter that's all.  In fact, you can forget about the LPF because
>you're not really performing freq translation to passband and then
>demodulate back to baseband anyway.  You just want to see what the
>baseband corruption would look like after you performed these
>operations, so that you can simulate them easily.
>
>>
>> just want to ask, say my sampling freq is 20 MHz ,exp(j*2*pi*0.001)
>> will mean offset of (10/0.5)*0.001 MHz = 20 Khz....
>>
>
>see the example from porterboy to clear up the above.
>
>>
>> also i am getting rotated points in constelation diagram.... so i am
doing
>> things right??
>>
>
>hmmm...where does the rotation come from?  You said previously that
>you're multiplying by the exp with negative freq.  Are you adding an
>arbitrary phase with this?
>
>col
>
><snipped>
>
>


		
This message was sent using the Comp.DSP web interface on
www.DSPRelated.com

For all this analysis i had switched off CP i.e Zero gaurd interval, no
multipath component ,with LOS CIR , no AWGN and data on only two succesive
subcarriers of the same amplitude for all the symbols so that 
there is no spreading of subcarriers by virtue of either CP or data.

yes adding the phase offset rotates my points in constellation diagram. 
i mean magnitude remains same but phase changes.  This results in a single
circle of points in the const diagram....

   But adding a carrier offset also rotates my points but this time i have
observed that they tend to form conecentric circles. i beleive this is
because adding a fixed carrier offset will tend to leak energy of certain
subcarriers in to other bins.  how much energy any particular subcarrier
leaks depends upon the amount of carrier offset.. after all this is
discrete domain... and certain carriers still form a perfect circle with
no DFT leakage...

sandeep


















See comments below.
>
>sandy80 wrote:
>> Hi col
>>    Thanks for insightful information. you have done the analysis in
>> continous time domain,where, ofcourse,multiplication by exponential
will
>> shift my whole spectrum.  but there are couple of issues when u try to
do
>> things in discrete domain
>>
>
>Nope, the analysis is the same for both domains.  You just have to be
>careful where the spectral images start falling after you sample in the
>digital domain.
>
>>
>> >I believe you want the following
>> >
>> >let s(t) = Re{b(t) exp(j*W_c*t + theta_c)}
>> >
>> >where W_c = Carrier freq, theta_c = carrier phase and b(t) =
>> >a(t)exp(j*phi(t)) and represents the complex baseband information
>> >carrying signal.  At the Rx you have the following
>> >
>> >r(t) = s(t) * exp(-jW_o + theta_o)
>> >
>> >where W_o and theta_o represent the estimate of the carrier freq and
>> >phase.
>>
>>
>> here youmean to say that to get a perfect baseband signal W_c = W_o
>>
>> as well both phases.  and my carrier offset might be  abs(W_c - W_o).
>
>Correct.
>
>>
>> but in matlab i cant genrate a carrier for morethan the (f/fs) =0.5
since
>> all  looks like same. after 0.5  like 0.6 is equivalent to 0.4 etc...
>>
>
>I still don't see the problem.  Are you trying to generate carrier
>offsets that are greater than the sampling frequency?
>
>> after some jiggery pokery we end up with the following complex
baseband
>> >signal
>> >
>> >r'(t) = 0.5 * b(t) exp(j*delta*t+th)
>> >
>> >where delta = W_c - W_o and th = theta_c - theta_o representing
carrier
>> >offset and phase difference.  This means that you can simply generate
>> >this exponential signal with the offset and phase that you desire and
>> >multiply your OFDM signal b(t) after the channel filter.  I think
that
>> >covers it all.
>>
>> i have tried this thing and it is working well..
>>  following is apiece of code...
>>
>>
>>
>> recv = channel2(symbol2);   % pass the OFDM signal through channel. it
>> will add AWGN and Multipath components
>>
>>   indexx = 0:1:length(recv)-1;
>>   indexx = indexx * 0.1  ; %  carrier offset  0.1  means ratio of   
f/fs
>> .   for sampling frequncy of 20 Mhz, offset = 2Mhz.
>>
>> recv = recv.* (exp(j*2*pi*indexx));% complex exponential
multiplication
>>
>>
>>     recv = recv.* (exp(-j*2*pi*indexx));
>>    data = recved(recv);       % Recieve the signal from channel
>>
>>
>> i am getting perfect constellation after i multiply with second
>> exponential with negative frequncy....
>>
>
>Yep, this is what you would expect.
>
>>
>> why to multiply with 0.5 ,any intutive reasoning???
>
>1/2 falls out in the analysis, it doesn't change things, you just have
>to make sure that your scaling is correct at the output of the Rx low
>pass filter that's all.  In fact, you can forget about the LPF because
>you're not really performing freq translation to passband and then
>demodulate back to baseband anyway.  You just want to see what the
>baseband corruption would look like after you performed these
>operations, so that you can simulate them easily.
>
>>
>> just want to ask, say my sampling freq is 20 MHz ,exp(j*2*pi*0.001)
>> will mean offset of (10/0.5)*0.001 MHz = 20 Khz....
>>
>
>see the example from porterboy to clear up the above.
>
>>
>> also i am getting rotated points in constelation diagram.... so i am
doing
>> things right??
>>
>
>hmmm...where does the rotation come from?  You said previously that
>you're multiplying by the exp with negative freq.  Are you adding an
>arbitrary phase with this?
>
>col
>
><snipped>
>
>


		
This message was sent using the Comp.DSP web interface on
www.DSPRelated.com

See comments below.

sandy80 wrote:
> Hi col
>    Thanks for insightful information. you have done the analysis in
> continous time domain,where, ofcourse,multiplication by exponential will
> shift my whole spectrum.  but there are couple of issues when u try to do
> things in discrete domain
>

Nope, the analysis is the same for both domains.  You just have to be
careful where the spectral images start falling after you sample in the
digital domain.

>
> >I believe you want the following
> >
> >let s(t) = Re{b(t) exp(j*W_c*t + theta_c)}
> >
> >where W_c = Carrier freq, theta_c = carrier phase and b(t) =
> >a(t)exp(j*phi(t)) and represents the complex baseband information
> >carrying signal.  At the Rx you have the following
> >
> >r(t) = s(t) * exp(-jW_o + theta_o)
> >
> >where W_o and theta_o represent the estimate of the carrier freq and
> >phase.
>
>
> here youmean to say that to get a perfect baseband signal W_c = W_o
>
> as well both phases.  and my carrier offset might be  abs(W_c - W_o).

Correct.

>
> but in matlab i cant genrate a carrier for morethan the (f/fs) =0.5 since
> all  looks like same. after 0.5  like 0.6 is equivalent to 0.4 etc...
>

I still don't see the problem.  Are you trying to generate carrier
offsets that are greater than the sampling frequency?

> after some jiggery pokery we end up with the following complex baseband
> >signal
> >
> >r'(t) = 0.5 * b(t) exp(j*delta*t+th)
> >
> >where delta = W_c - W_o and th = theta_c - theta_o representing carrier
> >offset and phase difference.  This means that you can simply generate
> >this exponential signal with the offset and phase that you desire and
> >multiply your OFDM signal b(t) after the channel filter.  I think that
> >covers it all.
>
> i have tried this thing and it is working well..
>  following is apiece of code...
>
>
>
> recv = channel2(symbol2);   % pass the OFDM signal through channel. it
> will add AWGN and Multipath components
>
>   indexx = 0:1:length(recv)-1;
>   indexx = indexx * 0.1  ; %  carrier offset  0.1  means ratio of    f/fs
> .   for sampling frequncy of 20 Mhz, offset = 2Mhz.
>
> recv = recv.* (exp(j*2*pi*indexx));% complex exponential multiplication
>
>
>     recv = recv.* (exp(-j*2*pi*indexx));
>    data = recved(recv);       % Recieve the signal from channel
>
>
> i am getting perfect constellation after i multiply with second
> exponential with negative frequncy....
>

Yep, this is what you would expect.

>
> why to multiply with 0.5 ,any intutive reasoning???

1/2 falls out in the analysis, it doesn't change things, you just have
to make sure that your scaling is correct at the output of the Rx low
pass filter that's all.  In fact, you can forget about the LPF because
you're not really performing freq translation to passband and then
demodulate back to baseband anyway.  You just want to see what the
baseband corruption would look like after you performed these
operations, so that you can simulate them easily.

>
> just want to ask, say my sampling freq is 20 MHz ,exp(j*2*pi*0.001)
> will mean offset of (10/0.5)*0.001 MHz = 20 Khz....
>

see the example from porterboy to clear up the above.

>
> also i am getting rotated points in constelation diagram.... so i am doing
> things right??
>

hmmm...where does the rotation come from?  You said previously that
you're multiplying by the exp with negative freq.  Are you adding an
arbitrary phase with this?

col

<snipped>

> Hi
>      i want to simulate small carrier offset in OFDM and see its effect
> on
> QAM constellations.

suppose the baseband digital signal is x in Matlab (may be FFT output,
may have cyclic prefix, or may have other features depending on your
algorithm) and suppose x has nsamp samples. Define a time index, define
a carrier offset and the rest is easy. eg.

cfo = 3e-3;                          % carrier frequency offset
relative
                                     % to the sampling frequency
cpo = pi/6;                          % carrier phase offset

n = 0:nsamp-1;                       % time index
w = 2*pi*cfo;                        % discrete CFO
ejwn = exp(sqrt(-1)*((w*n) + cpo));  % offset modulator
rx = ejwn.*x;                        % rx signal with CFO and CPO

Hi col
   Thanks for insightful information. you have done the analysis in
continous time domain,where, ofcourse,multiplication by exponential will
shift my whole spectrum.  but there are couple of issues when u try to do
things in discrete domain


>I believe you want the following
>
>let s(t) = Re{b(t) exp(j*W_c*t + theta_c)}
>
>where W_c = Carrier freq, theta_c = carrier phase and b(t) =
>a(t)exp(j*phi(t)) and represents the complex baseband information
>carrying signal.  At the Rx you have the following
>
>r(t) = s(t) * exp(-jW_o + theta_o)
>
>where W_o and theta_o represent the estimate of the carrier freq and
>phase.


here youmean to say that to get a perfect baseband signal W_c = W_o

as well both phases.  and my carrier offset might be  abs(W_c - W_o).

but in matlab i cant genrate a carrier for morethan the (f/fs) =0.5 since
all  looks like same. after 0.5  like 0.6 is equivalent to 0.4 etc...

after some jiggery pokery we end up with the following complex baseband
>signal
>
>r'(t) = 0.5 * b(t) exp(j*delta*t+th)
>
>where delta = W_c - W_o and th = theta_c - theta_o representing carrier
>offset and phase difference.  This means that you can simply generate
>this exponential signal with the offset and phase that you desire and
>multiply your OFDM signal b(t) after the channel filter.  I think that
>covers it all.

i have tried this thing and it is working well..
 following is apiece of code...



recv = channel2(symbol2);   % pass the OFDM signal through channel. it 
will add AWGN and Multipath components
   
  indexx = 0:1:length(recv)-1;
  indexx = indexx * 0.1  ; %  carrier offset  0.1  means ratio of    f/fs
.   for sampling frequncy of 20 Mhz, offset = 2Mhz.
 
recv = recv.* (exp(j*2*pi*indexx));% complex exponential multiplication


    recv = recv.* (exp(-j*2*pi*indexx));
   data = recved(recv);       % Recieve the signal from channel 


i am getting perfect constellation after i multiply with second
exponential with negative frequncy....


why to multiply with 0.5 ,any intutive reasoning???

just want to ask, say my sampling freq is 20 MHz ,exp(j*2*pi*0.001)
will mean offset of (10/0.5)*0.001 MHz = 20 Khz....


also i am getting rotated points in constelation diagram.... so i am doing
things right??



Thanks a lot for help information.

>col
>
>
>sandy80 wrote:
>> Hi
>>      i want to simulate small carrier offset in OFDM and see its
effect
>> on
>> QAM constellations.
>>                        How can is simulate carrier offset in MATLAB
for
>> OFDM signal..  is iterpolation - decimation technique helpful??
>>
>> i want to introduce this offset after passing the signal through
channel
>> filter.
>>
>> i want to simulate by noninteger factor say 0.5 i.e shift of tones by
0.5
>> so that i can see spectral leakage and its consequnse on constellation
>> diagram...
>>
>> anybody simulated such thing...?
>>
>> 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

I believe you want the following

let s(t) = Re{b(t) exp(j*W_c*t + theta_c)}

where W_c = Carrier freq, theta_c = carrier phase and b(t) =
a(t)exp(j*phi(t)) and represents the complex baseband information
carrying signal.  At the Rx you have the following

r(t) = s(t) * exp(-jW_o + theta_o)

where W_o and theta_o represent the estimate of the carrier freq and
phase.

after some jiggery pokery we end up with the following complex baseband
signal

r'(t) = 0.5 * b(t) exp(j*delta*t+th)

where delta = W_c - W_o and th = theta_c - theta_o representing carrier
offset and phase difference.  This means that you can simply generate
this exponential signal with the offset and phase that you desire and
multiply your OFDM signal b(t) after the channel filter.  I think that
covers it all.

col


sandy80 wrote:
> Hi
>      i want to simulate small carrier offset in OFDM and see its effect
> on
> QAM constellations.
>                        How can is simulate carrier offset in MATLAB for
> OFDM signal..  is iterpolation - decimation technique helpful??
>
> i want to introduce this offset after passing the signal through channel
> filter.
>
> i want to simulate by noninteger factor say 0.5 i.e shift of tones by 0.5
> so that i can see spectral leakage and its consequnse on constellation
> diagram...
>
> anybody simulated such thing...?
>
> sandeep
>
>
> This message was sent using the Comp.DSP web interface on
> www.DSPRelated.com

Hi
     i want to simulate small carrier offset in OFDM and see its effect
on
QAM constellations.
                       How can is simulate carrier offset in MATLAB for
OFDM signal..  is iterpolation - decimation technique helpful??

i want to introduce this offset after passing the signal through channel
filter.

i want to simulate by noninteger factor say 0.5 i.e shift of tones by 0.5
so that i can see spectral leakage and its consequnse on constellation
diagram...

anybody simulated such thing...?

sandeep

		
This message was sent using the Comp.DSP web interface on
www.DSPRelated.com