Hi, I'm new to OFDM. I read in several textbooks and papers that timing synchronization can be separated into symbol synchronization and sampling clock synchronization. The latter can be further broken down into sampling phase offset and sampling frequency offset. There is also carrier frequency synchronization (i.e., carrier frequency recovery technique), which is utilized to eliminate the carrier frequency offset caused by the mismatch of the local oscillators between the transmitter and the receiver, nonlinear characteristic of the wireless channel, as well as the Doppler shift. Sampling frequency offset, as I understand, is to mitigate the sampling clock errors due to the mismatch of the crystal oscillators. This would lead to the rob/stuff phenomenon in which the number of required samlping to represent an OFDM symbol could be +1/-1 sample. My question is what is the difference between carrier frequency synchronization and Sampling frequency offset? Any advice? Thanks.
Synchronization in OFDM systems
Started by ●September 13, 2010
Reply by ●September 13, 20102010-09-13
DerekDSP wrote:> I'm new to OFDM. I read in several textbooks and papers that timing > synchronization can be separated into symbol synchronization and sampling > clock synchronization. The latter can be further broken down into sampling > phase offset and sampling frequency offset.Sampling clock sync = symbol sync.> There is also carrier frequency synchronization (i.e., carrier frequency > recovery technique), which is utilized to eliminate the carrier frequency > offset caused by the mismatch of the local oscillators between the > transmitter and the receiver, nonlinear characteristic of the wireless > channel, as well as the Doppler shift.Understood nothing; too many smart scientific words. What is "nonlinear characteristic of the wireless channel" and how it is related to the carrier sync?> Sampling frequency offset, as I understand, is to mitigate the sampling > clock errors due to the mismatch of the crystal oscillators. This would > lead to the rob/stuff phenomenon in which the number of required samlping > to represent an OFDM symbol could be +1/-1 sample.No.> My question is what is the difference between carrier frequency > synchronization and Sampling frequency offset? Any advice? Thanks.Any advice? Think of a different profession? VLV
Reply by ●September 13, 20102010-09-13
On 09/13/2010 06:41 AM, DerekDSP wrote:> Hi, > > I'm new to OFDM. I read in several textbooks and papers that timing > synchronization can be separated into symbol synchronization and sampling > clock synchronization. The latter can be further broken down into sampling > phase offset and sampling frequency offset. > > There is also carrier frequency synchronization (i.e., carrier frequency > recovery technique), which is utilized to eliminate the carrier frequency > offset caused by the mismatch of the local oscillators between the > transmitter and the receiver, nonlinear characteristic of the wireless > channel, as well as the Doppler shift. > > Sampling frequency offset, as I understand, is to mitigate the sampling > clock errors due to the mismatch of the crystal oscillators. This would > lead to the rob/stuff phenomenon in which the number of required samlping > to represent an OFDM symbol could be +1/-1 sample. > > My question is what is the difference between carrier frequency > synchronization and Sampling frequency offset? Any advice? Thanks.The difference is that the carrier frequency and the sampling frequency are two different things, often derived from two different clocks, and affected by different processes in transmission. Draw up a block diagram of a _whole_ OFDM system, with a signal generator, a transmitter, a channel, a receiver, and a demodulator. See if it doesn't become apparent. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●September 14, 20102010-09-14
On Sep 13, 9:41�am, "DerekDSP" <DerekDSP@n_o_s_p_a_m.gmail.com> wrote:> Hi, > > I'm new to OFDM. I read in several textbooks and papers that timing > synchronization can be separated into symbol synchronization and sampling > clock synchronization. The latter can be further broken down into sampling > phase offset and sampling frequency offset. > > There is also carrier frequency synchronization (i.e., carrier frequency > recovery technique), which is utilized to eliminate the carrier frequency > offset caused by the mismatch of the local oscillators between the > transmitter and the receiver, nonlinear characteristic of the wireless > channel, as well as the Doppler shift. > > Sampling frequency offset, as I understand, is to mitigate the sampling > clock errors due to the mismatch of the crystal oscillators. This would > lead to the rob/stuff phenomenon in which the number of required samlping > to represent an OFDM symbol could be +1/-1 sample.Yes. But a more severe problem with the sampling clock offset is the carrier frequency offset that can cause ICI.> > My question is what is the difference between carrier frequency > synchronization and Sampling frequency offset? Any advice? Thanks.Carrier frequency offset can be from Doppler effect or sampling clock offset. The frequency offset is different for each sub-carrier. The frequency offset by the sampling clock offset will cause a linear phase shift that is proportional to the sampling time for the same frequency. The frequency offset by Doppler can be approximated to be the same for different sub-carrier under the assumption that the carrier frequency if much larger than the channel bandwidth.
Reply by ●September 14, 20102010-09-14
Hi Tim, From my understanding, the same crystal should be used to drive both the ADC and the LO which performs the up/downcoversion, right? So what do you mean by different clocks?>On 09/13/2010 06:41 AM, DerekDSP wrote: >> Hi, >> >> I'm new to OFDM. I read in several textbooks and papers that timing >> synchronization can be separated into symbol synchronization andsampling>> clock synchronization. The latter can be further broken down intosampling>> phase offset and sampling frequency offset. >> >> There is also carrier frequency synchronization (i.e., carrierfrequency>> recovery technique), which is utilized to eliminate the carrierfrequency>> offset caused by the mismatch of the local oscillators between the >> transmitter and the receiver, nonlinear characteristic of the wireless >> channel, as well as the Doppler shift. >> >> Sampling frequency offset, as I understand, is to mitigate the sampling >> clock errors due to the mismatch of the crystal oscillators. This would >> lead to the rob/stuff phenomenon in which the number of requiredsamlping>> to represent an OFDM symbol could be +1/-1 sample. >> >> My question is what is the difference between carrier frequency >> synchronization and Sampling frequency offset? Any advice? Thanks. > >The difference is that the carrier frequency and the sampling frequency >are two different things, often derived from two different clocks, and >affected by different processes in transmission. > >Draw up a block diagram of a _whole_ OFDM system, with a signal >generator, a transmitter, a channel, a receiver, and a demodulator. See >if it doesn't become apparent. > >-- > >Tim Wescott >Wescott Design Services >http://www.wescottdesign.com > >Do you need to implement control loops in software? >"Applied Control Theory for Embedded Systems" was written for you. >See details at http://www.wescottdesign.com/actfes/actfes.html >
Reply by ●September 14, 20102010-09-14
On 09/13/2010 08:31 PM, DerekDSP wrote: (top posting fixed)>> On 09/13/2010 06:41 AM, DerekDSP wrote: >>> Hi, >>> >>> I'm new to OFDM. I read in several textbooks and papers that timing >>> synchronization can be separated into symbol synchronization and > sampling >>> clock synchronization. The latter can be further broken down into > sampling >>> phase offset and sampling frequency offset. >>> >>> There is also carrier frequency synchronization (i.e., carrier > frequency >>> recovery technique), which is utilized to eliminate the carrier > frequency >>> offset caused by the mismatch of the local oscillators between the >>> transmitter and the receiver, nonlinear characteristic of the wireless >>> channel, as well as the Doppler shift. >>> >>> Sampling frequency offset, as I understand, is to mitigate the sampling >>> clock errors due to the mismatch of the crystal oscillators. This would >>> lead to the rob/stuff phenomenon in which the number of required > samlping >>> to represent an OFDM symbol could be +1/-1 sample. >>> >>> My question is what is the difference between carrier frequency >>> synchronization and Sampling frequency offset? Any advice? Thanks. >> >> The difference is that the carrier frequency and the sampling frequency >> are two different things, often derived from two different clocks, and >> affected by different processes in transmission. >> >> Draw up a block diagram of a _whole_ OFDM system, with a signal >> generator, a transmitter, a channel, a receiver, and a demodulator. See >> if it doesn't become apparent. >>> Hi Tim, > > From my understanding, the same crystal should be used to drive both > the ADC and the LO which performs the up/downcoversion, right? So > what do you mean by different clocks? Beware the word 'should'. It can mean "that which the manufacturer is morally obliged to provide", which easily morphs into "that which I really hope is true" ("they should honor their warranty"), or it can mean "that which can be reasonably expected" ("if I drop this stupid piece of %$@!# 20 feet onto concrete it should break in a million pieces"). Why should the same time base be used to drive both an ADC and an LO? Why should the ADC and LO be in the same box, and why should the boxes be from the same manufacturer? To be sure, if you know for a fact that the sampling rate and the carrier are locked to each (and that the transmitter and receiver aren't moving with respect to one another) then you can use this to your advantage. But there's nothing that lets you reason, from base principals, the connection between those two time bases. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
