Forums

Design of multirate filters

Started by Unknown February 24, 2009
Hi i am simulating a baseband OFDM system Digital down converter. I
have a problem regarding decimation and interpolation. In the last
stage of the DDC, i am doing low pass filtering of the data. My
problem is when i make a sharp transition filter i.e. (
Fpass = 250e3;              % Passband Frequency
Fstop = 300e3;             % Stopband Frequency

the OFDM constellation is scattered. But when i relax this transition
bandwidth, i.e.

Fpass = 250e3;              % Passband Frequency
Fstop = 400e3;             % Stopband Frequency

the OFDM constellation is better than the previous one. I really
cannot understand why it is happening like that as the previous filter
is far better than this one. Please note that after this LPF, i'm
doing down sampling by 2 as well (throwing one of two samples). If
anyone has any idea regarding this.
Thanks
Aitezaz
aitezaz.abd@gmail.com wrote:
> Hi i am simulating a baseband OFDM system Digital down converter. I > have a problem regarding decimation and interpolation. In the last > stage of the DDC, i am doing low pass filtering of the data. My > problem is when i make a sharp transition filter i.e. ( > Fpass = 250e3; % Passband Frequency > Fstop = 300e3; % Stopband Frequency > > the OFDM constellation is scattered. But when i relax this transition > bandwidth, i.e. > > Fpass = 250e3; % Passband Frequency > Fstop = 400e3; % Stopband Frequency > > the OFDM constellation is better than the previous one. I really > cannot understand why it is happening like that as the previous filter > is far better than this one. Please note that after this LPF, i'm > doing down sampling by 2 as well (throwing one of two samples). If > anyone has any idea regarding this. > Thanks > Aitezaz
Sharp filters introduce excessive ringing. Look at the impulse responses of your filters. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
On Feb 24, 8:46&#2013266080;pm, Jerry Avins <j...@ieee.org> wrote:
> aitezaz....@gmail.com wrote: > > Hi i am simulating a baseband OFDM system Digital down converter. I > > have a problem regarding decimation and interpolation. In the last > > stage of the DDC, i am doing low pass filtering of the data. My > > problem is when i make a sharp transition filter i.e. ( > > Fpass = 250e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;% Passband Frequency > > Fstop = 300e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; % Stopband Frequency > > > the OFDM constellation is scattered. But when i relax this transition > > bandwidth, i.e. > > > Fpass = 250e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;% Passband Frequency > > Fstop = 400e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; % Stopband Frequency > > > the OFDM constellation is better than the previous one. I really > > cannot understand why it is happening like that as the previous filter > > is far better than this one. Please note that after this LPF, i'm > > doing down sampling by 2 as well (throwing one of two samples). If > > anyone has any idea regarding this. > > Thanks > > Aitezaz > > Sharp filters introduce excessive ringing. Look at the impulse responses > of your filters. > > Jerry > -- > Engineering is the art of making what you want from things you can get. > &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Thanks jerry, can you please tell me what the ringing phenomenon is or refer any text, website etc. i would be thankful to you. i searched over internet but did not find it
aitezaz.abd@gmail.com wrote:
> On Feb 24, 8:46 pm, Jerry Avins <j...@ieee.org> wrote: >> aitezaz....@gmail.com wrote: >>> Hi i am simulating a baseband OFDM system Digital down converter. I >>> have a problem regarding decimation and interpolation. In the last >>> stage of the DDC, i am doing low pass filtering of the data. My >>> problem is when i make a sharp transition filter i.e. ( >>> Fpass = 250e3; % Passband Frequency >>> Fstop = 300e3; % Stopband Frequency >>> the OFDM constellation is scattered. But when i relax this transition >>> bandwidth, i.e. >>> Fpass = 250e3; % Passband Frequency >>> Fstop = 400e3; % Stopband Frequency >>> the OFDM constellation is better than the previous one. I really >>> cannot understand why it is happening like that as the previous filter >>> is far better than this one. Please note that after this LPF, i'm >>> doing down sampling by 2 as well (throwing one of two samples). If >>> anyone has any idea regarding this. >>> Thanks >>> Aitezaz >> Sharp filters introduce excessive ringing. Look at the impulse responses >> of your filters. >> >> Jerry >> -- >> Engineering is the art of making what you want from things you can get. >> &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095; > > Thanks jerry, > can you please tell me what the ringing phenomenon is or refer any > text, website etc. i would be thankful to you. i searched over > internet but did not find it
Search for "Gibbs phenomenon". Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Do you perform an equalization on the data before plotting the
constellation? 

JB.


On Feb 25, 5:27&#2013266080;am, "jbdore" <jbd...@gmail.com> wrote:
> Do you perform an equalization on the data before plotting the > constellation? > > JB.
@JB yes of course JB. do you think that filter imperfections should be handled by equalizer and i should look into equalizer algorithm? on baseband simulations, the equalizer works just fine. @ Jerry thanks for your suggestion. another thing i was suspecting was the group delay of the filter. do you think increasing filter group delay with sharper transition bandwidth can be a problem? specially when we are going to downsample the output of this filter. Thanks to both of you.
On Feb 24, 10:03&#2013266080;pm, Jerry Avins <j...@ieee.org> wrote:
> aitezaz....@gmail.com wrote: > > On Feb 24, 8:46 pm, Jerry Avins <j...@ieee.org> wrote: > >> aitezaz....@gmail.com wrote: > >>> Hi i am simulating a baseband OFDM system Digital down converter. I > >>> have a problem regarding decimation and interpolation. In the last > >>> stage of the DDC, i am doing low pass filtering of the data. My > >>> problem is when i make a sharp transition filter i.e. ( > >>> Fpass = 250e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;% Passband Frequency > >>> Fstop = 300e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; % Stopband Frequency > >>> the OFDM constellation is scattered. But when i relax this transition > >>> bandwidth, i.e. > >>> Fpass = 250e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;% Passband Frequency > >>> Fstop = 400e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; % Stopband Frequency > >>> the OFDM constellation is better than the previous one. I really > >>> cannot understand why it is happening like that as the previous filter > >>> is far better than this one. Please note that after this LPF, i'm > >>> doing down sampling by 2 as well (throwing one of two samples). If > >>> anyone has any idea regarding this. > >>> Thanks > >>> Aitezaz > >> Sharp filters introduce excessive ringing. Look at the impulse responses > >> of your filters. > > >> Jerry > >> -- > >> Engineering is the art of making what you want from things you can get. > >> &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095; > > > Thanks jerry, > > can you please tell me what the ringing phenomenon is &#2013266080;or refer any > > text, website etc. i would be thankful to you. i searched over > > internet but did not find it > > Search for "Gibbs phenomenon". > > Jerry > -- > Engineering is the art of making what you want from things you can get. > &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Jerry i read article about Gibbs Phenomenon. What i got from this is that this problem arises when we truncate the filter response in time. That means if i increase the filter length with the same sharp filters, i should get the better performance. But, when i increased the filter length of the sharp filter, the performance improved from what it was with fewer number of taps but it was still worse than the performance of filter with larger transition bandwidth. Now where can be the problem? Im using the following code to generate the filter. Fs_3 = 1e6; % Sampling Frequency Fpass = 250e3; % Passband Frequency Fstop = 500e3; % Stopband Frequency Dpass = 0.0028782312868; % Passband Ripple Dstop = 0.001; % Stopband Attenuation dens = 80; % Density Factor % Calculate the order from the parameters using FIRPMORD. [N, Fo, Ao, W] = firpmord([Fpass, Fstop]/(Fs_3/2), [1 0], [Dpass, Dstop]); % Calculate the coefficients using the FIRPM function. pfir = firpm(N, Fo, Ao, W, {dens}); i change the last line of the pfir = firpm(N*2, Fo, Ao, W, {dens}); with different N*2 values to increase the no. of the taps. Thanks for your time.
On Feb 24, 6:11&#2013266080;am, aitezaz....@gmail.com wrote:
> Hi i am simulating a baseband OFDM system Digital down converter. I > have a problem regarding decimation and interpolation. In the last > stage of the DDC, i am doing low pass filtering of the data. My > problem is when i make a sharp transition filter i.e. ( > Fpass = 250e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;% Passband Frequency > Fstop = 300e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; % Stopband Frequency > > the OFDM constellation is scattered. But when i relax this transition > bandwidth, i.e. > > Fpass = 250e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;% Passband Frequency > Fstop = 400e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; % Stopband Frequency > > the OFDM constellation is better than the previous one. I really > cannot understand why it is happening like that as the previous filter > is far better than this one. Please note that after this LPF, i'm > doing down sampling by 2 as well (throwing one of two samples). If > anyone has any idea regarding this. > Thanks > Aitezaz
First thing I'd wonder is what your OFDM signal spectrum does above 250 kHz. Is signal being removed with the sharp filter? John
On Feb 25, 3:58&#2013266080;pm, John <sampson...@gmail.com> wrote:
> On Feb 24, 6:11&#2013266080;am, aitezaz....@gmail.com wrote: > > > > > Hi i am simulating a baseband OFDM system Digital down converter. I > > have a problem regarding decimation and interpolation. In the last > > stage of the DDC, i am doing low pass filtering of the data. My > > problem is when i make a sharp transition filter i.e. ( > > Fpass = 250e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;% Passband Frequency > > Fstop = 300e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; % Stopband Frequency > > > the OFDM constellation is scattered. But when i relax this transition > > bandwidth, i.e. > > > Fpass = 250e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;% Passband Frequency > > Fstop = 400e3; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; % Stopband Frequency > > > the OFDM constellation is better than the previous one. I really > > cannot understand why it is happening like that as the previous filter > > is far better than this one. Please note that after this LPF, i'm > > doing down sampling by 2 as well (throwing one of two samples). If > > anyone has any idea regarding this. > > Thanks > > Aitezaz > > First thing I'd wonder is what your OFDM signal spectrum does above > 250 kHz. Is signal being removed with the sharp filter? > > John
not at all. the signal actually resides from 0 to 250kHz (half of the sampling frequency of 500 kHz). So nothing is being removed and this filter is behaving like an anti-aliasing filter prior to downsampling. the system is as follows ---| LPF at 1MHz |---|Down Sample by 2|--- signal at 500 kHz
aitezaz.abd@gmail.com wrote:
> On Feb 25, 5:27 am, "jbdore" <jbd...@gmail.com> wrote: >> Do you perform an equalization on the data before plotting the >> constellation? >> >> JB. > @JB > yes of course JB. do you think that filter imperfections should be > handled by equalizer and i should look into equalizer algorithm? on > baseband simulations, the equalizer works just fine. > > @ Jerry > thanks for your suggestion. another thing i was suspecting was the > group delay of the filter. do you think increasing filter group delay > with sharper transition bandwidth can be a problem? specially when we > are going to downsample the output of this filter. > > Thanks to both of you.
How is extra delay different from the signal having been transmitted a little later? If the delay is only in one path, then a compensating delay in other paths can be added. I doubt that such is the case here. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;