> > Now I'm wondering just exactly how this is measured. When they say > "peak,," do they really mean the instantaneous peak power? Or is > it an average power over some relatively short interval? >I have seen some contraditory defenitions about this. I take it to mean the same as the old time PEP (Peak Envelope power) which is defined as the average power of a steady sine wave that has the same peak to peak voltage value as your signal. Thus no time interval is needs to be defined. Thus the PAR of a steady sine wave tone using this defenition is 0 dB. This is not the same as the true instantaneous peak power. But there are other defentions that do use the instantaneous peak and then the PAR of a steady sine wave is 3 dB, that's why I don't like those. Mark
Has The peak-to-average ratio problem of OFDM been solved?
Started by ●December 1, 2006
Reply by ●December 1, 20062006-12-01
Reply by ●December 1, 20062006-12-01
Eric Jacobsen <eric.jacobsen@ieee.org> writes:> On Fri, 01 Dec 2006 04:15:52 GMT, Randy Yates <yates@ieee.org> wrote: > >>Subject states the question. Any information is appreciated. > > To add a tiny bit to what's tranpired so far (dang, you guys move > quickly): > > The problem has been "solved" to the extent that there are quite a few > practical OFDM systems in common use, so it's definitely workable. > > My experience matches Vladimir's a bit in that for the most part > whitening alone goes a long, long way to PAPR reduction in OFDM, and > people just generally live with what's left. Other than reasonably > simple linearizing or pre-distortion technique for the PAs many > systems don't rely on highly sophisticaed tricks to mitigate PAPR in > OFDM. > > e.g., 802.11a/g uses OFDM with 48 subcarriers, and the PAPR management > techniques are pretty straightforward (whitening, etc). It's not > really specified in the standard how to manage it, just that the > transmit mask and EVM have to meet certain specs, but herculean > efforts are not needed to get there. > > FWIW, as the number of subcarriers goes up the problem stabilizes due > mostly to the central limit theorem (whitening gets easier). Beyond > FFT sizes of 64-128 points the PAPR is increasing very slowly, so it's > not hard to bound the problem. > > I did a short study a long time ago and made this comparison: 64-QAM > single-carrier with 20% RRC filtering gets a PAPR of about 5.5dB (at > about the 90% spot of the cdf). OFDM with 48 subcarriers and _only > whitening applied to mitigate PAPR_ was about 7.5dB PAPR at 90% and > OFDM with 240 subcarriers (something I was proposing at the time) was > about 8.5dB.Interesting numbers - thanks for the datapoints, Eric.> So even for an OFDM system with a fair number of subcarriers (240) the > difference to a practical single-carrier system was 3dB, without doing > anything special to reduce PAPR in the OFDM system. 3dB can be a big > deal in the rf, but it's not at all insurmountable or impractical to > deal with.But doesn't that mean that, for the same modulation type, OFDM is 3 dB less power efficient over single-carrier? So to get the same performance (i.e., SER) as single-carrier, you'd have to transmit twice the power? For single, high-power RF transmitters, e.g., 500 kW vs. 1 MW, isn't this quite a big deal, requiring significantly bigger power amps (4 times the voltage), thicker cables, thicker antennas, etc., etc.? You know where I'm going with this? -- % Randy Yates % "She has an IQ of 1001, she has a jumpsuit %% Fuquay-Varina, NC % on, and she's also a telephone." %%% 919-577-9882 % %%%% <yates@ieee.org> % 'Yours Truly, 2095', *Time*, ELO http://home.earthlink.net/~yatescr
Reply by ●December 2, 20062006-12-02
Randy Yates wrote:>>So even for an OFDM system with a fair number of subcarriers (240) the >>difference to a practical single-carrier system was 3dB, without doing >>anything special to reduce PAPR in the OFDM system. 3dB can be a big >>deal in the rf, but it's not at all insurmountable or impractical to >>deal with. > > > But doesn't that mean that, for the same modulation type, OFDM is 3 dB > less power efficient over single-carrier?No. The power efficiency of a modulation is referred to the power transmitted, not to the power consumed by the transmitter. However, the linear transmitter drains more power then the nonlinear one. But this is a different issue. So to get the same> performance (i.e., SER) as single-carrier, you'd have to transmit > twice the power?No. You may have to consume twice of electric power, however it depends on the design of the transmitter. The transmitted power is the same. The 3dB loss mentioned is for the straightforward Class A amplifier. There are other arrangements which allow for the higher efficiency linear RF amps, such as Shireux, Doherty, adjustable power supply, etc. For single, high-power RF transmitters, e.g., 500 kW> vs. 1 MW, isn't this quite a big deal, requiring significantly bigger > power amps (4 times the voltage), thicker cables, thicker antennas, > etc., etc.? > You know where I'm going with this?Global warming? Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●December 2, 20062006-12-02
On Sat, 02 Dec 2006 03:28:28 GMT, Randy Yates <yates@ieee.org> wrote:>Eric Jacobsen <eric.jacobsen@ieee.org> writes: > >> So even for an OFDM system with a fair number of subcarriers (240) the >> difference to a practical single-carrier system was 3dB, without doing >> anything special to reduce PAPR in the OFDM system. 3dB can be a big >> deal in the rf, but it's not at all insurmountable or impractical to >> deal with. > >But doesn't that mean that, for the same modulation type, OFDM is 3 dB >less power efficient over single-carrier? So to get the same >performance (i.e., SER) as single-carrier, you'd have to transmit >twice the power? For single, high-power RF transmitters, e.g., 500 kW >vs. 1 MW, isn't this quite a big deal, requiring significantly bigger >power amps (4 times the voltage), thicker cables, thicker antennas, >etc., etc.?As Vladimir pointed out, power efficiency relates to transmitted power, and with the average power transmitted the same between the two schemes there's really no difference. The real difference is that the Tx PA for the OFDM system has to have about 3dB more headroom (different than power consumption, just linear range for peak excursion), than the SC PA with no other changes. This does mean that for some linear PA technologies the power _consumption_, but not the transmitted power will be greater, and the PA with 3dB more headroom means a bigger PA if nothing else is done.>You know where I'm going with this?Yes, and I did point that out that that's now the (possibly hindsight) argument for why the US "needs" 8VSB, because we have big transmitters that cover huge rural areas. In Europe the transmitter coverage is much smaller and they tend to make the coverage bigger by adding transmitters at different locations as a "Single Frequency Network", for which OFDM is much better suited. So the difference in PA size in Europe isn't as big of a problem as it is here just from that initial look perspective. But consider that we haven't talked at all about PA linearizing techniques or the other technologies to which Vladimir alluded. At the very high Tx power outputs that you're citing the PA technology choices are very limited, but linearizing techniques can still be applied. I guess what I'm trying to say is that even the PA headroom issues could probably be mitigated using other techniques such as predistortion, etc., not to mention the DSP techniques that could be used on the OFDM signal in the first place. I think it's not a given that 8VSB has an advantage here if one if willing to push into the details a bit further, although it is certainly more work to get the needed PA headroom for OFDM at those transmit levels than it is for VSB. Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
Reply by ●January 2, 20132013-01-02
On Friday, December 1, 2006 9:45:52 AM UTC+5:30, Randy Yates wrote:> Subject states the question. Any information is appreciated.-- % Randy Yates % "And all that I can do %% Fuquay-Varina, NC % is say I'm sorry, %%% 919-577-9882 % that's the way it goes..." %%%% <yates@ieee.org> % Getting To The Point', *Balance of Power*, ELO http://home.earthlink.net/~yatescri need to understand the selection criteria for threshold of clipping technique
Reply by ●January 2, 20132013-01-02
pmjadhav85@gmail.com writes:> On Friday, December 1, 2006 9:45:52 AM UTC+5:30, Randy Yates wrote: >> Subject states the question. Any information is appreciated.-- % >> Randy Yates % "And all that I can do %% Fuquay-Varina, NC % is say >> I'm sorry, %%% 919-577-9882 % that's the way it goes..." %%%% >> <yates@ieee.org> % Getting To The Point', *Balance of Power*, ELO >> http://home.earthlink.net/~yatescr > > i need to understand the selection criteria for threshold of clipping > techniqueOh my! This must be at least 10 years old! -- Randy Yates Digital Signal Labs http://www.digitalsignallabs.com
Reply by ●January 2, 20132013-01-02
On 1/2/13 3:06 PM, Randy Yates wrote:> pmjadhav85@gmail.com writes: > >> On Friday, December 1, 2006 9:45:52 AM UTC+5:30, Randy Yates wrote: >>> Subject states the question. Any information is appreciated.-- % >>> Randy Yates % "And all that I can do %% Fuquay-Varina, NC % is say >>> I'm sorry, %%% 919-577-9882 % that's the way it goes..." %%%% >>> <yates@ieee.org> % Getting To The Point', *Balance of Power*, ELO >>> http://home.earthlink.net/~yatescr >> >> i need to understand the selection criteria for threshold of clipping >> technique > > Oh my! This must be at least 10 years old!appears to be 6 years, 1 month, and 1 day old. -- r b-j rbj@audioimagination.com "Imagination is more important than knowledge."
Reply by ●January 4, 20132013-01-04
On Saturday, December 2, 2006 5:28:28 AM UTC+2, Randy Yates wrote: <snip>> But doesn't that mean that, for the same modulation type, OFDM is 3 dB > less power efficient over single-carrier? So to get the same > performance (i.e., SER) as single-carrier, you'd have to transmit > twice the power? For single, high-power RF transmitters, e.g., 500 kW > vs. 1 MW, isn't this quite a big deal, requiring significantly bigger > power amps (4 times the voltage), thicker cables, thicker antennas,1.4 times the voltage.
