Talk:Orthogonal frequency-division multiplexing

message: This article does a pretty poor job of explaining the orthogonality condition in OFDM. it says 
"Conceptually, OFDM is a specialized FDM, the additional constraint being: all the carrier signals are orthogonal to each other."
This makes little sense, because the concept of all FDM is that different carriers are orthogonal, and for this reason you can put different data on different carriers. In TDM/TDMA, you use orthogonal time slots; In FDM/FDMA you use orthogonal frequency slots; in CDM/CDMA you use orthogonal Spreading codes.  — Preceding unsigned comment added by 65.216.151.126 (talk) 18:41, 14 January 2016 (UTC)
thread_title:  Better description of Orthogonal in this context 

message: OFDMA links here. Should they be merged?--Gbleem 21:36, 14 December 2005 (UTC)
thread_title:  OFDMA 

message: Why does the article claim that multipath resistance exists only when combining OFDM with coding schemes? Multipath resistance is added by the fact that OFDM allows using longer symbols and therefore decreasing inefficiency caused by GI.Danielcohn 01:35, 22 June 2006 (UTC)
thread_title:  Multipath resistance only when coded? 

message: The blocks marked "ROM" are clearly meant to represent constellation mapping, but what does "ROM" stand for?
In my opinion, it would be better to replace the "impulse generator" and H_t(f) blocks with "DAC".
It's also debatable whether illustrating frequency-domain zero-padding is necessary for an "ideal" encoder.
Oli Filth 17:47, 20 August 2006 (UTC)
replies: text: All fixed. Oli Filth 13:50, 2 September 2006 (UTC)
thread_title:  Diagram 

message: In 9th October 2006, Finland has licenced the 450MHz band to an operator for building a nation-wide Flash-ODSM data network. Press release in Finnish: . Could this be added to the main article?
:I think Flash-OFDM deserves a separate article. Mange01 (talk) 06:53, 5 September 2008 (UTC)
thread_title:  Flash-OFDM 450MHZ Data Network 

message: I suggest a table that summarizes the most crucial features of common OFDM systems. I have made a similar table for two broadcasting systems in the publication http://ieeexplore.ieee.org/iel5/49/20698/00957306.pdf?arnumber=957306. 
Examples of features are:
* Standard name.
* Ratified year.
* Frequency band [GHz].
* Channel bandwidth [MHz].
* Number of subcarriers.
* Subcarrier spacing [kHz].
* Net bit rate [Mbps].
* Link spectrum efficiency [bit/s/Hz].
* Symbol length [s].
* Guard interval [s].
* Subcarriermodulation scheme. 
* Inner FEC.
* Outer FEC (if any).
* Sub-carrier adaptive transmission (if any). Yes/no.
* Multiple access scheme (if any). Example: OFDMA uplink.
* Maximum travelling speed.
* Time interleaving depth [ms] (if larger than the symbol).
* Requied carrier-to-interference ratio (for AWGN without fading). Example: 5dB for BER 10^-5.
Mange01 13:11, 13 October 2006 (UTC)
thread_title:  Suggestion: OFDM standard comparison table 

message: This is a very promising article, but it is let down by an extreme scarcity of citations, and by a number of lists that would read better if converted to prose. Once these improvements have been made, please feel free to renominate the article. MLilburne 20:20, 2 December 2006 (UTC)
thread_title: Failed GA

message: It has been suggested for a while that DAB COFDM section should be merged into the OFDM article, but none of us commented on the suggestion. Now it is accomplished, but I'm not sure that it was such a good idea. 
Anyway, some of the merged text is overlapping with the old OFDM text, or is generic, not specific for DAB, and should therefor be removed or moved up in the article. 
Secondly, Wikipedia now warns that the article has become longer than 30kB. Is that a recommended maximum length? 
Should every application of OFDM be described as detailed as DAB in this article? In case the article should be extended with something, I would prefer more illustrations, and a comparison table summarizing the features of several systems.
Whats your vote? Should the merge be reverted?
Mange01 23:37, 2 December 2006 (UTC)
replies: text: I completely agree; the verbatim insertion of the text from the DAB article completely disrupted this article, introduced a lot of repetition and redundancy, and also brought over a lot of the errors that were present in the DAB article.  I have removed this addition, and have pasted the inserted text here; I'm not going to attempt to re-edit the DAB article; it's a complete mess; I'll leave that for someone else to sort out! Oli Filth
=== Removed text ===
=====Modulation=====
The modulation used in DAB is Coded Orthogonal Frequency Division Multiplexing (COFDM). According to this acronym the three properties of COFDM are: 'C' for coding; 'O' for orthogonal modulation and 'FDM' for frequency division multiplexing. These are described here.
=====Coding=====
Coding refers to convolutional coding and means that the original data carried over the multiplex is deliberately manipulated by splitting it into small blocks and adding some intelligently designed redundant information to each, thus generating a data 'overhead'. The overhead bits added to each block are determined according to rules applied to the true data content of the block. After demodulation at the receiver the digital signal processor examines both the actually received data and overhead bits and regenerates what it believes to be the original data based on a set of statistical rules known as an algorithm. The regenerated data may include a number of data bit corrections. The algorithm used in DAB is known as a Viterbi algorithm, and is an example of a maximum likelihood algorithm. This works by maintaining a history of demodulated bit sequences, building up a view of their probabilities and then using these to finally select either a 0 or 1 for the bit under consideration. This type of coding is also known as an example of forward error correction (FEC).
To some extent the types of errors most likely to be present with DAB can be mathematically predicted and therefore corrected for. The addition of FEC requires extra information to be transmitted at the same time as the original traffic data and therefore requires an increased channel capacity, needing extra bandwidth, compared to if it had been uncoded. DAB carries different 'strengths' of FEC, a stronger one being used for the control of critical features in the receiver.
=====Orthogonal=====
Orthogonal is the mathematical term applied to two RF sinusoidal signals when their phase relationship is precisely 90 degrees. Alternatively they may be said to be in 'quadrature'. In DAB the sub-carrier frequency spacing is chosen to be the reciprocal of the active symbol period. Under this condition the DAB modulation results in successive sub-carriers having a quadrature relationship with each other. The frequency spectra components of one modulated sub-carrier will therefore integrate to zero at the corresponding components from both of the adjacent sub-carriers. This has two advantages: (a) the modulated sub-carrier spectra will efficiently occupy the allocated bandwidth with a degree of controlled overlapping and (b) simple I-Q demodulation to zero intermediate frequency (zero-IF) can be used in the receiver without needing the costly hardware overhead of many bandpass filters to extract the sub-carriers. 
=====Frequency division multiplexing=====
Frequency division multiplexing (FDM) is the process where two or more basic information channel bandwidths or basebands are shifted in frequency and added to others to form an aggregate wider bandwidth containing the information from all of the constituent basebands. To avoid mutual interference, their bandwidths would normally require shifting (translating) in frequency and no two translated basebands would occupy any part of the same frequency spectrum. In the context of DAB, FDM refers to the manner in which the modulated sub-carriers are assembled across the allocated frequency range.
=====Modulation type=====
DAB uses a digital modulation type known as differential quadrature phase shift keying (DQPSK), which is an incoherent modulation scheme. DQPSK differs from the more common quadrature phase shift keying (QPSK) in that the modulated carrier phase for the current symbol being detected depends on its phase relative to that phase detected for the previous one. In QPSK it is just the absolute phase of the modulated carrier that determines the associated symbol. A differential modulation scheme can be more resillient to the typical fading scenarios of DAB. The modulation scheme also incorporates a form of Gray coding in that only one bit changes on moving from one symbol state to an adjacent one. For a constant phase progression, the consecutive set of symbols are represented by the bit pairs 00, 01, 11 and 10. 
=====Time interleaving=====
DAB uses data buffering which enables the data symbols to be transmitted over the RF path in a different time-order than they were generated the audio source (studio). At the receiver they are re-assembled and returned to the original time-order before conversion back to analog signals to feed the receiver audio output. This process is called time interleaving. Typical multipath interference experienced in a moving vehicle is regular over time so an intelligent choice of time interleaving to some degree 'averages' out the resulting error bursts over time. This data buffering and other processing contributes to a delay, typically of a few seconds, between the studio source and the receiver. This is much longer than the equivalent delay for am FM broadcast channel which would typically be a fraction of a second. For most broadcasts such a delay would be unimportant but it does mean that, for example, real-time reference signals for setting clocks such as those re-broadcast by the BBC on DAB from their national FM service are actually quite inaccurate.
=====Frequency interleaving=====
DAB also uses frequency interleaving, a similar technique to time interleaving but applied to the sub-carriers centre frequencies in the RF spectrum instead. The data stream from the studio is deliberately not modulated serially onto sub-carriers across the frequency range, but instead in a more random way. Multipath and other forms of selective fading generally affect a relatively narrow part of the RF multiplex bandwidth at any one time so frequency interleaving would tend to average out 'bursts' of errors resulting from these.
====This is some good stuff====
This is some good text!  Put it back! -143.215.155.26 (talk) 05:17, 21 March 2009 (UTC)
thread_title:  Merge with DAB really a good idea? 

message: =====Modulation=====
The modulation used in DAB is Coded Orthogonal Frequency Division Multiplexing (COFDM). According to this acronym the three properties of COFDM are: 'C' for coding; 'O' for orthogonal modulation and 'FDM' for frequency division multiplexing. These are described here.
=====Coding=====
Coding refers to convolutional coding and means that the original data carried over the multiplex is deliberately manipulated by splitting it into small blocks and adding some intelligently designed redundant information to each, thus generating a data 'overhead'. The overhead bits added to each block are determined according to rules applied to the true data content of the block. After demodulation at the receiver the digital signal processor examines both the actually received data and overhead bits and regenerates what it believes to be the original data based on a set of statistical rules known as an algorithm. The regenerated data may include a number of data bit corrections. The algorithm used in DAB is known as a Viterbi algorithm, and is an example of a maximum likelihood algorithm. This works by maintaining a history of demodulated bit sequences, building up a view of their probabilities and then using these to finally select either a 0 or 1 for the bit under consideration. This type of coding is also known as an example of forward error correction (FEC).
To some extent the types of errors most likely to be present with DAB can be mathematically predicted and therefore corrected for. The addition of FEC requires extra information to be transmitted at the same time as the original traffic data and therefore requires an increased channel capacity, needing extra bandwidth, compared to if it had been uncoded. DAB carries different 'strengths' of FEC, a stronger one being used for the control of critical features in the receiver.
=====Orthogonal=====
Orthogonal is the mathematical term applied to two RF sinusoidal signals when their phase relationship is precisely 90 degrees. Alternatively they may be said to be in 'quadrature'. In DAB the sub-carrier frequency spacing is chosen to be the reciprocal of the active symbol period. Under this condition the DAB modulation results in successive sub-carriers having a quadrature relationship with each other. The frequency spectra components of one modulated sub-carrier will therefore integrate to zero at the corresponding components from both of the adjacent sub-carriers. This has two advantages: (a) the modulated sub-carrier spectra will efficiently occupy the allocated bandwidth with a degree of controlled overlapping and (b) simple I-Q demodulation to zero intermediate frequency (zero-IF) can be used in the receiver without needing the costly hardware overhead of many bandpass filters to extract the sub-carriers. 
=====Frequency division multiplexing=====
Frequency division multiplexing (FDM) is the process where two or more basic information channel bandwidths or basebands are shifted in frequency and added to others to form an aggregate wider bandwidth containing the information from all of the constituent basebands. To avoid mutual interference, their bandwidths would normally require shifting (translating) in frequency and no two translated basebands would occupy any part of the same frequency spectrum. In the context of DAB, FDM refers to the manner in which the modulated sub-carriers are assembled across the allocated frequency range.
=====Modulation type=====
DAB uses a digital modulation type known as differential quadrature phase shift keying (DQPSK), which is an incoherent modulation scheme. DQPSK differs from the more common quadrature phase shift keying (QPSK) in that the modulated carrier phase for the current symbol being detected depends on its phase relative to that phase detected for the previous one. In QPSK it is just the absolute phase of the modulated carrier that determines the associated symbol. A differential modulation scheme can be more resillient to the typical fading scenarios of DAB. The modulation scheme also incorporates a form of Gray coding in that only one bit changes on moving from one symbol state to an adjacent one. For a constant phase progression, the consecutive set of symbols are represented by the bit pairs 00, 01, 11 and 10. 
=====Time interleaving=====
DAB uses data buffering which enables the data symbols to be transmitted over the RF path in a different time-order than they were generated the audio source (studio). At the receiver they are re-assembled and returned to the original time-order before conversion back to analog signals to feed the receiver audio output. This process is called time interleaving. Typical multipath interference experienced in a moving vehicle is regular over time so an intelligent choice of time interleaving to some degree 'averages' out the resulting error bursts over time. This data buffering and other processing contributes to a delay, typically of a few seconds, between the studio source and the receiver. This is much longer than the equivalent delay for am FM broadcast channel which would typically be a fraction of a second. For most broadcasts such a delay would be unimportant but it does mean that, for example, real-time reference signals for setting clocks such as those re-broadcast by the BBC on DAB from their national FM service are actually quite inaccurate.
=====Frequency interleaving=====
DAB also uses frequency interleaving, a similar technique to time interleaving but applied to the sub-carriers centre frequencies in the RF spectrum instead. The data stream from the studio is deliberately not modulated serially onto sub-carriers across the frequency range, but instead in a more random way. Multipath and other forms of selective fading generally affect a relatively narrow part of the RF multiplex bandwidth at any one time so frequency interleaving would tend to average out 'bursts' of errors resulting from these.
====This is some good stuff====
This is some good text!  Put it back! -143.215.155.26 (talk) 05:17, 21 March 2009 (UTC)
thread_title:  Removed text 

message: This is some good text!  Put it back! -143.215.155.26 (talk) 05:17, 21 March 2009 (UTC)
thread_title: This is some good stuff

message: What is meant with a sub-carrier in OFDM and what is its difference compared to a normal carrier? --Abdull 10:40, 6 March 2007 (UTC)
thread_title:  Sub-carrier 

message: I was wondering about this, won't it be nice if we also show that the cyclic prefix serves to make the effect of the channel become circular convolution for the OFDM symbol? Then, we mention that circular convolution becomes just multiplication when the DFT is taken.
Something like The OFDM symbol [d_0, d_1, \ldots d_{N_c - 1}]^T is prefixed with the L - 1 length cyclic prefix and becomes [d_{N_c - L + 1}, d_{N_c - L + 2} \ldots d_{N_c - 2}, d_{N_c - 1}, d_0, d_1, \ldots d_{N_c - 1}]^T. Then, after convolution with the channel, which happens as
y[m] = \sum_{l = 0}^L h_l x[m - l] \quad 0 \le m \le N_c
which is circular convolution, as x[m - k] becomes x[(m - k) \mod N_c]. So, taking the DFT, we get:
Y[k] = H[k]\cdot X[k].
Of course, the noise etc. has to be accounted for. Note that another thign one could mention is that the distribution of the noise, if it is isotropic complex Gaussian, remains identical under the DFT operation.
Just my suggestions to make it more clear... Kumar Appaiah 12:32, 11 March 2007 (UTC)
replies: text: I suggest that some of this stuff would be well worth putting in the Cyclic prefix article, which is essentially just a stub right now.  I keep meaning to flesh that article out myself, but I never get round to it.  I don't think we want any more maths in the OFDM article, as it's already overly-long.  Oli Filth 13:05, 11 March 2007 (UTC)
replies: text:  Oli, thanks for the suggestion. I am heading to Cylic prefix to make a start! Kumar Appaiah 14:24, 11 March 2007 (UTC)
thread_title:  More explanation of the cyclic prefix purpose 

message: I think a critical clarification needed in the summary at the start of the article is that the difference between OFDM and FDM is that with OFDM we're simultaneously transmitting on all the sub-channel frequencies, whereas FDM transmits on each frequency sequentially.
Ceri Reid68.200.109.154 17:24, 29 March 2007 (UTC)
replies: text: FDM tramsits simultaneously on all frequencies as well (otherwise it would be TDM!).  Oli Filth 18:18, 29 March 2007 (UTC)
replies: text: Yes, the orthoganal part allows much closer spacing without the guardband that would be needed between conventional channels on different frequencies (such as on FM radio), without the interference being destructive. --Lindosland (talk) 16:52, 28 January 2008 (UTC)
thread_title:  '''Overview''' 

message: Can any one help of the best chipset available in the market for Baseband OFDM chipset that could be used in an OFDM modem for not that high data bit rate?, the RF part not necessary for building the OFDM modem only baseband part. Thanks Matalal 13:17, 4 April 2007 (UTC)
thread_title:  OFDM Chipset 

message: Hi, the article is good, but it is sometimes harder to perceive things for the person not already in WiFi field.
A good drawing explaining guard interval and cyclic prefix would not be obsolete, for sure.
Also one explaining orthogonality. Keep up the good work! -- Mtodorov 69 08:28, 19 April 2007 (UTC)
thread_title:  More Images!!! 

message: Section 1, "Example of applications", was originally part of the ingress, as a summary of the "Usage" section. Someone perhaps found the ingress too long, and moved it into a separate section, but now we have two sections with overlapping content. 
What is the solution to the problem?
Mange01 18:44, 7 October 2007 (UTC)
thread_title:  Overlap between "Example of applications" and "Usage" 

message: In the changes made 23 February 2008 to the article, the paragraph which explains the primary advantages of OFDM was moved down from the article lead into a separate section. I reverted those changes for the following reasons:
* It does not make sense to both have a section on "Advantages" and "Summary of advantages", which are highly overlapping with each other. Both are summaries of certain issues discussed in the "Characteristics..." section.
* I suggest that major changes to the article lead and structure should be discussed at the talk page. There has already been a discussion on this talk page, where we have tried to agree on a compromise regarding short lead. Is it desirable to make it even shorter? Perhaps it is possible, but I would prefer a discussion first.
Mange01 (talk) 17:43, 23 February 2008 (UTC)
thread_title:  New changes to the article lead and structure 

message: Please vote at Talk:Eb/N0#Survey on which unit to be used at Wikipedia for measuring  Spectral efficiency. For a background discussion, see Talk:Spectral_efficiency#Bit/s/Hz and Talk:Eb/N0#Bit/s/Hz.
Mange01 (talk) 07:21, 16 April 2008 (UTC)
thread_title:  Survey: bit/s/Hz, (bit/s)/Hz or bit·s<sup>−1</sup>·Hz<sup>−1</sup> as [[Spectral efficiency]] unit? 

message: Sections seem to be out of order or jumbled up somehow.  --KJRehberg (talk) 21:16, 23 June 2008 (UTC)
thread_title:  Cleanup of article needed 

message: Removed text that celebrated the virtues of Flash-OFDM to the point of reading like a cheap advertorial. I'm not at all sure the section should stay at all as it completely fails to explain just what sets this ``flash thing apart from the rest, beyond expanding the acronym. 85.178.66.163 (talk) 23:44, 1 October 2008 (UTC)
thread_title:  Flash-OFDM / F-OFDM section reads like an advertisement 

message: Can someone tell me what constellation mapping does with the incoming bits? If the constellation mapping was QAM, would the constellation mapping convert the incoming bits into a sinusoidal signal, with varying amplitude and phase (this signal would be digital)? Thank you in advance WielkiZielonyMelon (talk) 15:34, 6 February 2009 (UTC)
replies: text: For example, if 64QAM is used, every 6 bit data will be converted to a complex value by the constellation mapping. For example, 011101 might be converted to 5 - i2, corresponding to a vector with horizontal value 5 and vertical value -2. This is the fourier coefficient for one of the sub-carriers. After the inverse fourier transform, this coefficient will correspond to a cosine wave with amplitude 5, plus a sine wave with amplitude 2, at this specific sub-carrier frequency. Mange01 (talk) 23:09, 7 February 2009 (UTC)
replies: text: Magne01, thank you very much, I have hurried a little and forgot, that the entry of IFFT is in FREQUENCY DOMAIN, and the exit of the IFFT is in TIME DOMAIN. Anyway, thank you for explanation! WielkiZielonyMelon (talk) 10:15, 9 February 2009 (UTC)
thread_title:  Idealized system model and constellation mapping 

message: This article contains too much DAB-specific information that doesn't mesh well with the other information here.  The DAB information belongs in an article about DAB technical aspects.  If we had this much information about other specific COFDM-based services, this article would be huge and incomprehensible.
Also, none of this DAB-specific information is complemented by discussion of HD Radio (which has as many multiplexes as DAB does now!), DRM, etc.
-143.215.155.26 (talk) 05:20, 21 March 2009 (UTC)
thread_title:  DAB, HD Radio, other radio schemes 

message: There are techniques to reduce the PAPR: Active Constellation Extension (ACE), and Tone Reservation.
Could somebody comment on these points? Cheers, Nageh (talk) 12:07, 5 June 2009 (UTC)
thread_title:  Section: Linear transmitter power amplifier 

message: Would it be a good idea to move the comparison table to a template, and embed it in the end of articles about each of the compared systems? For example template:ofdm sytem comparison table. The table should be collapsible. Mange01 (talk) 19:08, 11 May 2010 (UTC)
thread_title:  Move the comparison table to a template that can be embedded in several articles? 

message: Press release here on low-cost OOFDM claims it can extend subscriber FTTH capacities by 2000 fold (e.g. 20 Gb/s vice 10 Mb/s) without significant cost increase. Seems like it might be worth some discussionin the article. See  or the Beeb for additional details. LeadSongDog come howl! 20:24, 6 November 2012 (UTC)
thread_title:  Optical OFDM 

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thread_title:  External links modified 

message: There is little mention of LTE/4G applications and significance, also new WiFi standards. Not in the table. Much that has happened within last few years, and it is a lot, is missing from discussions. Also in my opinion, DAB topics should have been kept separate, it does not make a good article all jumbled in.  — Preceding unsigned comment added by 137.71.23.54 (talk) 19:47, 9 February 2017 (UTC)
thread_title:  Needs Updates Urgently 

message: In a brief review of the article, nowhere do we say where the term COFDM comes from or what it means. We use it, but we don't define it. That's bad. We should fix that. I'm going to add a little note to the first paragraph based on a cite I found. Anyone who wants to improve it should.—chbarts (talk) 08:25, 16 April 2017 (UTC)
replies: text: This was added to the article by . It looks like it belongs here at least until it can be refined a bit. ~Kvng (talk) 14:03, 17 December 2018 (UTC)
thread_title:  OFDM vs COFDM 

message: :This was added to the article by . It looks like it belongs here at least until it can be refined a bit. ~Kvng (talk) 14:03, 17 December 2018 (UTC)
thread_title:  Caveat