Modulation with a baseband signal in Matlab

Hi, 

I have a simple matlab question: I have a baseband signal with a 2MHz
bandwidth (wideband), sampled at 100MHz. I would like to use this signal to
modulate a 5Ghz carrier. What is the simplest way to proceed? I was
thinking using the ammod function, but in that case I would need to
resample my signal to a higher sampling rate (which may add some
distortion). Is there a more elegant way to go about it?

Thanks 

Nitram wrote:

> Hi, 
> 
> I have a simple matlab question: I have a baseband signal with a 2MHz
> bandwidth (wideband), sampled at 100MHz. I would like to use this signal to
> modulate a 5Ghz carrier. What is the simplest way to proceed?

The simplest way to proceed is do everything in the baseband.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

>The simplest way to proceed is do everything in the baseband.
>

My aim is to test out a frequency search algorithm where the receiver does
not know the carrier frequency of the transmitted signal. Therefore, I need
to go to RF in order to verify my search algorithm (unless there is a
better way).

Thanks

Nitram wrote:

>>The simplest way to proceed is do everything in the baseband.
>>
> 
> 
> My aim is to test out a frequency search algorithm where the receiver does
> not know the carrier frequency of the transmitted signal.

If your goal is frequency search algorithm, why bother with low level 
details in time domain?

  Therefore, I need
> to go to RF in order to verify my search algorithm (unless there is a
> better way).

Split your problem into a set of different problems at different scales 
and do everything in the baseband.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

On 11/23/2009 8:26 AM, Nitram wrote:
>> The simplest way to proceed is do everything in the baseband.
>>
>
> My aim is to test out a frequency search algorithm where the receiver does
> not know the carrier frequency of the transmitted signal. Therefore, I need
> to go to RF in order to verify my search algorithm (unless there is a
> better way).
>
> Thanks

Is the algorithm going to be applied at baseband or at RF?  If it will 
be applied to the baseband signal, why not test it there?   Frequency 
offset at RF = frequency offset at baseband.

-- 
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.abineau.com

>
>If your goal is frequency search algorithm, why bother with low level 
>details in time domain?

I guess I should explain myself in more details:

Actually I want to extract synchronization information of an OFDM signal.
I am planning to use cyclic prefix correlation to get the timing
information and then wideband filtering plus a correlation of a known
sequence in frequency domain to get the frequency correlation. Therefore, I
have to introduce a timing and frequency offset (which I was hoping could
be anywhere from 0 to 50MHz). I was planning to introduce the frequency
offset by upconverting the signal to RF and downconverting it with a
slighthly different LO, creating a frequency offset. 


>>  Therefore, I need
>> to go to RF in order to verify my search algorithm (unless there is a
>> better way).
>
>Split your problem into a set of different problems at different scales 
>and do everything in the baseband.

I am not sure if I understand correctly what are the different scales you
are thinking of. Would it be possible to elaborate?

I really appreciate your help.

>Is the algorithm going to be applied at baseband or at RF?  If it will 
>be applied to the baseband signal, why not test it there?   Frequency 
>offset at RF = frequency offset at baseband.

True, but I would need to simulate pretty large frequency offset e.g.
cases where half the bandwidth of the baseband signal is filtered out by
the RF front-end filter. What would be a simple way to achive this in
baseband?

Thanks

On 11/23/2009 9:25 AM, Nitram wrote:
>> Is the algorithm going to be applied at baseband or at RF?  If it will
>> be applied to the baseband signal, why not test it there?   Frequency
>> offset at RF = frequency offset at baseband.
>
> True, but I would need to simulate pretty large frequency offset e.g.
> cases where half the bandwidth of the baseband signal is filtered out by
> the RF front-end filter. What would be a simple way to achive this in
> baseband?
>
> Thanks

Why not model the RF filter effect at baseband?  Is there an asymmetric 
distortion that you need to model?

-- 
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.abineau.com

>Why not model the RF filter effect at baseband?  Is there an asymmetric 
>distortion that you need to model?

What I would need to do is shift my baseband signal by a certain frequency
offset, pass it through a lowpass filter to emulate a receiver RF filter(in
the graph the lowpass filter is represented by a brickwall filter) and then
attempt to find the frequency of the random shift on the receiver. ^       
                   ^  
|                           |_________         
|______                     |    _____|<--low pass filter
|      \                    |   |     |\
|       \               --> |   |     | \<--shifted signal
|________\_________>f       |___|_____|__\______>f   
<-------->                      <-------->
    BW
The shift would be a simple complex multiplication and I guess the only
thing I need to be careful about is not to shift my signal too far so that
I get aliasing before my lowpass filter (either this or do successive
shift/lowpass procedures).

What do you think?

Thanks.

On 11/23/2009 1:28 PM, Nitram wrote:
>> Why not model the RF filter effect at baseband?  Is there an asymmetric
>> distortion that you need to model?
>
> What I would need to do is shift my baseband signal by a certain frequency
> offset, pass it through a lowpass filter to emulate a receiver RF filter(in
> the graph the lowpass filter is represented by a brickwall filter) and then
> attempt to find the frequency of the random shift on the receiver. ^
>                     ^
> |                           |_________
> |______                     |    _____|<--low pass filter
> |      \                    |   |     |\
> |       \               -->  |   |     | \<--shifted signal
> |________\_________>f       |___|_____|__\______>f
> <-------->                       <-------->
>      BW
> The shift would be a simple complex multiplication and I guess the only
> thing I need to be careful about is not to shift my signal too far so that
> I get aliasing before my lowpass filter (either this or do successive
> shift/lowpass procedures).
>
> What do you think?
>
> Thanks.
>

If you're trying to simulate the effects of a frequency offset and how 
it would be affected by the frequency selectivity of the RF or IF 
filter, that looks appropriate to me.

-- 
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.abineau.com