I'm looking for some inspiration on the following problem: I want to transmit a signal consisting of 8 QPSK-Modulated carriers over a coaxial cable, in the frequency range of 1 to 10 MHz. Since the cable has a frequency-dependent attenuation (e.g. 1dB/100m@1MHz, 3dB/ 100m@10 MHz), I need to somehow compensate for this at the receiver before it is forwarded to an optical transmitter. Since the signal consists of 8 superimposed modulated carriers, time- domain equalization is probably out of question. So I thought of a kind of frequency-domain equalization, which constantly monitors and adjusts the power of the 8 bands to the same level. Is this the way to do it? If yes, can you recommend any references to get me started? Many thanks in advance, Guy.
Cable Equalizer
Started by ●October 22, 2009
Reply by ●October 22, 20092009-10-22
On Oct 22, 7:07�am, Guy Eschemann <guy.eschem...@gmail.com> wrote:> I'm looking for some inspiration on the following problem: > > I want to transmit a signal consisting of 8 QPSK-Modulated carriers > over a coaxial cable, in the frequency range of 1 to 10 MHz. Since the > cable has a frequency-dependent attenuation (e.g. 1dB/100m@1MHz, 3dB/ > 100m@10 MHz), I need to somehow compensate for this at the receiver > before it is forwarded to an optical transmitter. > > Since the signal consists of 8 superimposed modulated carriers, time- > domain equalization is probably out of question. So I thought of a > kind of frequency-domain equalization, which constantly monitors and > adjusts the power of the 8 bands to the same level. Is this the way to > do it? If yes, can you recommend any references to get me started? > > Many thanks in advance, > Guy.Are you wanting to equalize their power levels for the purposes of demodulating the signals and then remodulating them on the optical medium, or are the QPSK signals used to directly modulate an optical carrier? If you're performing a demodulation-remodulation step, then there's no benefit to amplifying the carriers relative to each other. In that case, if the slope of the cable loss is sufficiently steep enough such that you can't consider it approximately flat across the bandwidth of one of your carriers, you could use an equalizer to attempt to compensate for the variance in loss over the width of each carrier. This could provide you some benefit in bit-error-rate at the demodulator. If you're using the attenuated signal to directly modulate an optical carrier and you need to equalize the power of each carrier, then I think what you suggested would work. I would try using a bank of 8 bandpass filters centered on each carrier. Pass the output of each filter into an energy detector and apply some lowpass filtering to smooth out the power readings. Pick one extreme as the nominal power level, then use the observed levels of the others as the inputs to a gain controller that generates weights for each of the 8 bandpass filter outputs, then perform a weighted sum of the 8 bands again to get the gain-compensated signal. Jason
Reply by ●October 22, 20092009-10-22
Hi Jason, sorry for being ambiguous. I don't want to perform a demodulation- remodulation step. I would just like to compensate for the frequency- dependent cable attenuation before transmitting the signal on an optical carrier. Thanks for your help! Guy.
Reply by ●October 22, 20092009-10-22
On Oct 22, 7:07�am, Guy Eschemann <guy.eschem...@gmail.com> wrote:> I'm looking for some inspiration on the following problem: > > I want to transmit a signal consisting of 8 QPSK-Modulated carriers > over a coaxial cable, in the frequency range of 1 to 10 MHz. Since the > cable has a frequency-dependent attenuation (e.g. 1dB/100m@1MHz, 3dB/ > 100m@10 MHz), I need to somehow compensate for this at the receiver > before it is forwarded to an optical transmitter.Do you know the attenuation? Is it fixed? If so, you could use different modulation schemes for each carrier, ie. doing rate allocation according to the channel condition. Or, you could use different transmit power for each carrier, though this is not optimal in terms of the total rate you can achieve.> > Since the signal consists of 8 superimposed modulated carriers, time- > domain equalization is probably out of question. So I thought of a > kind of frequency-domain equalization, which constantly monitors and > adjusts the power of the 8 bands to the same level. Is this the way to > do it? If yes, can you recommend any references to get me started? > > Many thanks in advance, > Guy.
Reply by ●October 22, 20092009-10-22
> Do you know the attenuation? Is it fixed? If so, you could use > different modulation schemes > for each carrier, ie. doing rate allocation according to the channel > condition. Or, you could > use different transmit power for each carrier, though this is not > optimal in terms of the total > rate you can achieve.The attenuation is fixed because it depends only on the cable type and on the cable length, so it shouldn't vary over time. Still, it's possible that the user replaces the cable by one of different length or different type at run-time, so the system should be able to adapt. Unfortunately, I have no control over the transmitter. All the carriers at transmitted with equal power. Thanks for your help, Guy.
Reply by ●October 22, 20092009-10-22
Guy Eschemann wrote:>> Do you know the attenuation? Is it fixed? If so, you could use >> different modulation schemes >> for each carrier, ie. doing rate allocation according to the channel >> condition. Or, you could >> use different transmit power for each carrier, though this is not >> optimal in terms of the total >> rate you can achieve. > > The attenuation is fixed because it depends only on the cable type and > on the cable length, so it shouldn't vary over time. Still, it's > possible that the user replaces the cable by one of different length > or different type at run-time, so the system should be able to adapt. > Unfortunately, I have no control over the transmitter. All the > carriers at transmitted with equal power.I doesn't seem unreasonable to require that the cable and its fixed equalizer be replaced together. It works that way tith tires and rims. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●October 22, 20092009-10-22
On 10/22/2009 4:07 AM, Guy Eschemann wrote:> I'm looking for some inspiration on the following problem: > > I want to transmit a signal consisting of 8 QPSK-Modulated carriers > over a coaxial cable, in the frequency range of 1 to 10 MHz. Since the > cable has a frequency-dependent attenuation (e.g. 1dB/100m@1MHz, 3dB/ > 100m@10 MHz), I need to somehow compensate for this at the receiver > before it is forwarded to an optical transmitter. > > Since the signal consists of 8 superimposed modulated carriers, time- > domain equalization is probably out of question. So I thought of a > kind of frequency-domain equalization, which constantly monitors and > adjusts the power of the 8 bands to the same level. Is this the way to > do it? If yes, can you recommend any references to get me started? > > Many thanks in advance, > Guy.How are the 8 carriers multiplexed onto the cable? Frequency division? That's the most common approach, and if that's the case then it shouldn't be too difficult to increase the transmit power on each carrier as frequency increases. I don't know why time-domain equalization wouldn't work, either. Since the channel response is known, pre-distorting (or pre-equalizing, depending on which term you prefer), shouldn't be difficult at all. Maybe I'm not seeing the difficulty, but it seems straightforward to me. -- Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com
Reply by ●October 22, 20092009-10-22
Eric Jacobsen wrote:> On 10/22/2009 4:07 AM, Guy Eschemann wrote: > >> I'm looking for some inspiration on the following problem: >> >> I want to transmit a signal consisting of 8 QPSK-Modulated carriers >> over a coaxial cable, in the frequency range of 1 to 10 MHz. Since the >> cable has a frequency-dependent attenuation (e.g. 1dB/100m@1MHz, 3dB/ >> 100m@10 MHz), I need to somehow compensate for this at the receiver >> before it is forwarded to an optical transmitter. >> >> Since the signal consists of 8 superimposed modulated carriers, time- >> domain equalization is probably out of question. So I thought of a >> kind of frequency-domain equalization, which constantly monitors and >> adjusts the power of the 8 bands to the same level. Is this the way to >> do it? If yes, can you recommend any references to get me started? >> > How are the 8 carriers multiplexed onto the cable? Frequency division? > That's the most common approach, and if that's the case then it > shouldn't be too difficult to increase the transmit power on each > carrier as frequency increases. > > I don't know why time-domain equalization wouldn't work, either. Since > the channel response is known, pre-distorting (or pre-equalizing, > depending on which term you prefer), shouldn't be difficult at all. > > Maybe I'm not seeing the difficulty, but it seems straightforward to me.May I suggest an analog RC filter? With some trivial electonics, it could be made adaptive. It is probably not qualified as the solution of the OP's ill-posed homework problem, however it does the job. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●October 22, 20092009-10-22
> How are the 8 carriers multiplexed onto the cable? �Frequency division? > � That's the most common approach, and if that's the case then it > shouldn't be too difficult to increase the transmit power on each > carrier as frequency increases. > > I don't know why time-domain equalization wouldn't work, either. � Since > the channel response is known, pre-distorting (or pre-equalizing, > depending on which term you prefer), shouldn't be difficult at all. > > Maybe I'm not seeing the difficulty, but it seems straightforward to me. > -- > Eric Jacobsen > Minister of Algorithms > Abineau Communicationshttp://www.abineau.comEric, you're right, the carriers are multiplexed onto the cable via frequency division, with each channel being approx. 600 kHz wide. Unfortunately, I have no control on the transmitter so I have to do all of the equalization on the receiver side. There's also no way I can transmit a training sequence. I'd also like to avoid having to do any demodulation at the receiving end since this is not the final receiver but just an interface to an optical transmitter. By the way, this goes into a real product so it really should be able to adapt to whatever cable type/length it is connected to. Regards, Guy.
Reply by ●October 22, 20092009-10-22
> > May I suggest an analog RC filter? With some trivial electonics, it > could be made adaptive. It is probably not qualified as the solution of > the OP's ill-posed homework problem, however it does the job. > > Vladimir Vassilevsky > DSP and Mixed Signal Design Consultanthttp://www.abvolt.comVladimir, thanks for your suggestion. I actually did my homework before posting here, but couldn't find a satisfactory solution by myself so I thought I should ask the experts. As I found in this article (http:// www.planetanalog.com/features/showArticle.jhtml?articleID=188702457), metallic cables have a frequency response which is proportional to the square root of the frequency, which seems difficult to mimic with standard analog filters. Also, as I have an FPGA, I don't mind about implementing a digital equalizer. Regards, Guy.
