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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)
I think what is important is that the subcarriers are orthogonal over a symbol period, which is by design.
A proof that the described channel spacing gives orthogonality would be helpful. 86.141.10.180 ( talk) 20:18, 4 July 2017 (UTC)HG
OFDMA links here. Should they be merged?-- Gbleem 21:36, 14 December 2005 (UTC)
NO! OFDMA is a multiple access scheme which relies on OFDM.
I removed this sentence from the text. I am not 100% sure, but is seems very unlikely to me that normal speeds (car, train, plane, etc) would affect this, especially when we talk about frequencies around 5 GHz. I wrote a small thesis about OFDM in uni and I cannot recall any major issued with moving receivers/senders. Please correct me if anyone has more accurate information. -- 83.109.152.151 22:12, 20 December 2005 (UTC)
This is a very understandable article and I'd love to see it promoted to good article. I did not pass it this time because:
Some suggestions on how to improve the article's structure/formatting:
Examples of how to do insets are available in the Columbine high school massacre article. Please renominate once the above problems are fixed.
Cedars 00:09, 14 April 2006 (UTC)
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)
OFDM uses a cyclic prefix guard interval to convert a frequency-selective channel to a set of frequency-nonselective fading channels. As a consequence, intersymbol interference is avoided. The thing is, however, OFDM does not have frequency diversity. With coding, OFDM achieves diversity and performs well in multipath. ---sct
I believe that the Ideal encoder section is in severe need of revision. I'll make these changes at some point in the near future; just wondering if anyone had any thoughts before I do.
Firstly, I don't think that it's necessary to include scrambling (shown as multiplication by in the diagram) in a hypothetical "ideal" encoder. Secondly, it is shown as occurring in the time-domain, which is completely incorrect (see section 17.3.2.1 in the 802.11a spec, for instance). Thirdly, the reason given, "in order to have a null mean value", is also incorrect.
In the second paragraph, orthogonality of the sub-carriers results in zero inter-carrier interference, not zero inter-symbol interference, and only in the case when a cyclic prefix is used, which is not mentioned or illustrated.
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 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)
Why does the "Transmitter" diagram show the IFFT block having only two outputs? Better start from the beginning again. Correct me if I'm wrong. My thoughts are --- a set of complex numbers (which could also be considered as a set of 'vectors', where each complex number can represent a 'vector', and each individual vector could be a single QAM 'symbol') is presented at the input to the IFFT. N complex numbers in parallel presented to the input of the IFFT. Even though the input to the IFFT is N complex values in parallel, we can imagine that set of numbers represent a made-up (ad-hoc) discrete frequency spectrum. A fabricated spectral picture. If we have N discrete spectral components, then the first component is expected to be the DC component, at zero frequency -- so basically the complex number for the DC value should be 0+j0 at DC (or for f = 0 Hz) because we don't want any DC component. The remainder of the components (ie. 'N - 1' of them) will be spaced evenly apart. The actual frequency spacing will later be determined by the parallel to sequential output of the IFFT system [ie. recall FFT takes N points of time-domain signal and converts to N points in frequency domain, with constant frequency spacing equal to (1/time_gap)/N or (1/sampling_period)/N or sampling_frequency/N (between components), so the time sequence spacing in the discrete time-domain will determine frequency spacing in the discrete frequency domain, and we will have full control of the time sequence spacing]. Perhaps the very first complex number (which we might purposely make to represent the DC component of our fabricated spectrum) should be zero, while the remaining 'N-1' complex-valued components should all have the same non-zero complex value (for redundancy and transmission reliability purposes. which means the set of N elements into the IFFT could be [X(0) X(1) X(2) X(3) ...... X(N-1)], with X(0) equal to zero, while X(1) = X(2) = X(3) = ..... X(N-1). X(0) is purposely set to be zero, this is to set the DC component of our ad-hoc spectrum to be zero. The rest of the components X(1), X(2) etc could all be the same complex value, which basically means that each of our non-DC fabricated spectral components are all "replicas" for purposes of redundancy. That is, if we were to transmit a time-domain equivalent of this 'spectrum', and the communications channel doesn't allow some of our spectral components to get through, then it is not a problem because we know that the same information will come through the other frequency 'channels'. So, back to the IFFT process. For N inputs to the IFFT, there should be N outputs (instead of just two outputs). The "Transmitter" diagram is currently showing just two outputs, because somebody decided to skip some important information. It should be N parallel outputs from the IFFT. And if there is meant to be N outputs for the IFFT, then what do we do with these N individual outputs? Each of these N individual outputs is typically a complex number, right? So, we then need to treat these N complex values as a complex time-domain sequence. This base-band time-domain sequence is the base-band OFDM signal sequence (in discrete time form). It's a sequence of complex numbers, which is all good and nice inside a computer. But what if we want to transmit and receive the complex number data is the fastest way possible? One way of transmitting complex numbers is by individually using the 'real part' and 'imaginary part' to modulate orthogonal carrier signals --- eg. cos(wt) carrier and sin(wt) carrier. The modulated carriers might be r(t).cos(wt) and i(t).sin(wt), where r(t) and i(t) are the real and imaginary analog values. The SUM of the two modulated carriers r(t).cos(wt) + i(t).sin(wt) is a real-valued time-domain signal (not complex signal), and it is an OFDM signal that we can transmit through a wire, or even wirelessly if the carriers cos(wt) and sin(wt) have a suitable frequency, noting that both carriers have identical frequency but are out of phase by 90 degrees. I'm really thinking that if somebody is going to explain this IFFT method of generating OFDM, then they should take a few extra steps to explain the method properly, otherwise it confuses everybody. If somebody going to teach somebody something on Wiki (or anywhere for that matter), then teach it without putting in cryptic or confusing diagrams etc. Make important areas clear for everybody to have a chance to understand.
KorgBoy (
talk)
08:53, 19 February 2018 (UTC)
The up and down mixing described would give rise to images, is OFDM actually symetrical about it's base carrier (this is implied by the article but not explicitly stated) or is it purely above the carrier (effectively SSB, and so requiring image rejection mixers and so on? Scruffy brit 12:15, 3 April 2007 (UTC)
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: [1]. Could this be added to the main article?
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:
Mange01 13:11, 13 October 2006 (UTC)
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)
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)
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 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 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 (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.
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.
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. citation needed
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 text! Put it back! - 143.215.155.26 ( talk) 05:17, 21 March 2009 (UTC)
On a somewhat related topic, i question whether the redirect to OFDM from DMT ( http://en.wikipedia.org/?title=Discrete_multitone_modulation&diff=next&oldid=10618564 ) is optimum. Certainly DMT and OFDM share many characteristics, but the common use of the DMT term in wireline, as well as in the standards reference in ANSI T1.413 http://en.wikipedia.org/wiki/ANSI_T1.413_Issue_2 suggest they should remain distinct topics. Particularly as there are non-trivial differences between the two techniques, that also in fact form some of the basis of T1.413 - such as the Cioffi/Amati patented bit loading / tone swapping algorithm which allows better throughput in copper specific interference such as the ISI found in bridged taps. Because in copper the noise is stationary and the channel is time invariant, DMT is much better able to adapt to the communication medium than OFDM would. In any case, this may not be the best place to discuss the validity of a redirect, but because it does involve technical details relevant to this article, I felt to introduce the idea for comments here first. Duedilly 23:47, 10 February 2007 (UTC)
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)
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 is prefixed with the length cyclic prefix and becomes . Then, after convolution with the channel, which happens as
which is circular convolution, as becomes . So, taking the DFT, we get:
.
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)
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 Reid 68.200.109.154 17:24, 29 March 2007 (UTC)
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) —The preceding unsigned comment was added by Matalal ( talk • contribs) 10:19, 4 April 2007 (UTC).
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)
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)
I've mostly reverted today's additions to the article lead, for the following reasons:
The example is highly relevant, since, as I understand it, a key benefit of OFDM has been the possibility of operating all transmitters across a country on one set of frequencies. 50 miles is a typical distance between such transmitters, and hence 1ms was a design criterion for such applications used in deciding on the number of carriers. This issue is something the non-engineer can readily appreciate the value of. What is transmitted on a carrier is surely 'bits' in the basic sense of 'binary digits' ie on-off representation. I don't see a need to complicate things with symbols, when the average person is familiar with bits.-- Lindosland ( talk) 20:56, 28 January 2008 (UTC)
However, I've retained a mention of SFNs. Oli Filth( talk) 19:32, 28 January 2008 (UTC)
Thanks, I'm reasonably happy with what you have done, though I still think a comparison with AM and FM in the intro would help as these are terms that have been absorbed into popular culture, while 'modulation' means nothing to many people. -- Lindosland ( talk) 21:59, 31 January 2008 (UTC)
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:
Mange01 ( talk) 17:43, 23 February 2008 (UTC)
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)
OFDM is listed under "modulation techniques", while OFDMA is listed under "multiplex techniques".
However, the OFDM article begins with "OFDM ... is a frequency-division multiplexing (FDM) scheme", which is in my opinion correct, and contrasts with the classification as a modulation technique. Fpoto ( talk) 12:55, 28 May 2008 (UTC)
Sections seem to be out of order or jumbled up somehow. -- KJRehberg ( talk) 21:16, 23 June 2008 (UTC)
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)
I further trimmed it down, rewrote in past tense a little, updated it with some sources, and removed inline links. It still seems notable, but of historical interest now that other standards seem to be battling it out. W Nowicki ( talk) 21:26, 23 July 2011 (UTC)
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)
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)
The last paragraph and sentence states that "...[Turbo codes and LDPC codes] only perform close to the Shannon limit for the AWGN channel ... [therefore] concatenated them with either RS or BCH codes to improve performance further...".
I was under the impression that RS/BCH was concatenated with LDPC/Turbo codes in order to clean up resilient errors from their error floor???
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)
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)
We know it is high, but how high? OFDM is typically 12 dB, what would it be for COFDM? I am surprised this most critical parameter for Tx design is treated so lightly. — Preceding unsigned comment added by 75.99.58.122 ( talk) 15:39, 24 January 2012 (UTC)
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 WO2011051448 A4 or the Beeb for additional details. LeadSongDog come howl! 20:24, 6 November 2012 (UTC)
Today I'm reading a paper by Martin Hoch ( "Comparison of PLC G3 and PRIME".) which mentions "t-DPSK" and "f-DPSK".
I came to this Wikipedia article looking for a better explanation of "f-DPSK" and "t-DPSK". Dear Wikipedia editors, if you know what these things are, please add them to this article. -- DavidCary ( talk) 18:07, 9 January 2013 (UTC)
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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)
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)
So far in this article, little mention has been made of the importance of "frequency synchronisation" whatever that means. Perhaps it is better to use the term "Coherency". When a (C)OFDM signal is exchanged over a radio frequency channel, approximately 95% of the resources used in the hardware, and therefore in its design, must be devoted to this function, which is not shown on any block diagram depicting the system as a whole. What must be done is the following:
The received data must be converted numerically to the best available approximation of the transmitted data. This is upstream of any error correction or demodulation, it is essentially an analog function which must be applied to the data as it is acquired from the receiver. Without this, the data is meaningless and scrambled.
- First, the start of a frame must be detected, under any reasonable condition of frequency of phase shift.
- Then, the frequency and phase of the transmitted signal must be deduced from analysis of a transmitted reference signal. The received signal is then both corrected for deviation of baseband frequency and phase. It is restored to the form in which it was transmitted.
Only at this point is it possible to proceed with the processing of the data stream as shown in the block diagrams in this article. The processing elements required to do this would, if illustrated at the same level of detail, cover about five pages.
Firstly, the detection of the beginning of a transmission. For the elaboration, downstream, of a phase reference signal, it must be detected within a certain tolerance of time. This is usually specified as a fraction of the guard interval, which, itself, might be short. The requirement is to pass a guarded version of the phase reference symbol on for further processing. The quality of detection of the beginning of a transmission required is subject to debate, but at early stages of development, it is convenient for this to be somewhat more reliable than the data itself, in the sense that data incapable of being decoded should be, nonetheless, received for analysis. Time-frequency analysis of a mixed tone frame reference symbol, possibly at a resolution of as little as a single sample of data, may be required in order to do this. A time-frequency analysis of transform length N requires N parallel channels, at delays of a single sample, of, in practice, FFT analysis.
At this point, the reference signal can be streamed to a function which calculates the channel impulse response. This compares the received reference symbol with delayed versions of itself to determine time and frequency offsets. These can then be applied to the buffered, received payload following the phase reference signal to restore it to the form in which it was transmitted. This requires further parallel FFT channels together with significant numerical elaboration of their contents.
As stated, coherency reconstruction, together with the buffering needed to accommodate its latency, is a significant element of any OFDM transceiver, and I would advise caution if budgeting hardware resources to the implementation of such a device.
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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)
I think what is important is that the subcarriers are orthogonal over a symbol period, which is by design.
A proof that the described channel spacing gives orthogonality would be helpful. 86.141.10.180 ( talk) 20:18, 4 July 2017 (UTC)HG
OFDMA links here. Should they be merged?-- Gbleem 21:36, 14 December 2005 (UTC)
NO! OFDMA is a multiple access scheme which relies on OFDM.
I removed this sentence from the text. I am not 100% sure, but is seems very unlikely to me that normal speeds (car, train, plane, etc) would affect this, especially when we talk about frequencies around 5 GHz. I wrote a small thesis about OFDM in uni and I cannot recall any major issued with moving receivers/senders. Please correct me if anyone has more accurate information. -- 83.109.152.151 22:12, 20 December 2005 (UTC)
This is a very understandable article and I'd love to see it promoted to good article. I did not pass it this time because:
Some suggestions on how to improve the article's structure/formatting:
Examples of how to do insets are available in the Columbine high school massacre article. Please renominate once the above problems are fixed.
Cedars 00:09, 14 April 2006 (UTC)
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)
OFDM uses a cyclic prefix guard interval to convert a frequency-selective channel to a set of frequency-nonselective fading channels. As a consequence, intersymbol interference is avoided. The thing is, however, OFDM does not have frequency diversity. With coding, OFDM achieves diversity and performs well in multipath. ---sct
I believe that the Ideal encoder section is in severe need of revision. I'll make these changes at some point in the near future; just wondering if anyone had any thoughts before I do.
Firstly, I don't think that it's necessary to include scrambling (shown as multiplication by in the diagram) in a hypothetical "ideal" encoder. Secondly, it is shown as occurring in the time-domain, which is completely incorrect (see section 17.3.2.1 in the 802.11a spec, for instance). Thirdly, the reason given, "in order to have a null mean value", is also incorrect.
In the second paragraph, orthogonality of the sub-carriers results in zero inter-carrier interference, not zero inter-symbol interference, and only in the case when a cyclic prefix is used, which is not mentioned or illustrated.
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 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)
Why does the "Transmitter" diagram show the IFFT block having only two outputs? Better start from the beginning again. Correct me if I'm wrong. My thoughts are --- a set of complex numbers (which could also be considered as a set of 'vectors', where each complex number can represent a 'vector', and each individual vector could be a single QAM 'symbol') is presented at the input to the IFFT. N complex numbers in parallel presented to the input of the IFFT. Even though the input to the IFFT is N complex values in parallel, we can imagine that set of numbers represent a made-up (ad-hoc) discrete frequency spectrum. A fabricated spectral picture. If we have N discrete spectral components, then the first component is expected to be the DC component, at zero frequency -- so basically the complex number for the DC value should be 0+j0 at DC (or for f = 0 Hz) because we don't want any DC component. The remainder of the components (ie. 'N - 1' of them) will be spaced evenly apart. The actual frequency spacing will later be determined by the parallel to sequential output of the IFFT system [ie. recall FFT takes N points of time-domain signal and converts to N points in frequency domain, with constant frequency spacing equal to (1/time_gap)/N or (1/sampling_period)/N or sampling_frequency/N (between components), so the time sequence spacing in the discrete time-domain will determine frequency spacing in the discrete frequency domain, and we will have full control of the time sequence spacing]. Perhaps the very first complex number (which we might purposely make to represent the DC component of our fabricated spectrum) should be zero, while the remaining 'N-1' complex-valued components should all have the same non-zero complex value (for redundancy and transmission reliability purposes. which means the set of N elements into the IFFT could be [X(0) X(1) X(2) X(3) ...... X(N-1)], with X(0) equal to zero, while X(1) = X(2) = X(3) = ..... X(N-1). X(0) is purposely set to be zero, this is to set the DC component of our ad-hoc spectrum to be zero. The rest of the components X(1), X(2) etc could all be the same complex value, which basically means that each of our non-DC fabricated spectral components are all "replicas" for purposes of redundancy. That is, if we were to transmit a time-domain equivalent of this 'spectrum', and the communications channel doesn't allow some of our spectral components to get through, then it is not a problem because we know that the same information will come through the other frequency 'channels'. So, back to the IFFT process. For N inputs to the IFFT, there should be N outputs (instead of just two outputs). The "Transmitter" diagram is currently showing just two outputs, because somebody decided to skip some important information. It should be N parallel outputs from the IFFT. And if there is meant to be N outputs for the IFFT, then what do we do with these N individual outputs? Each of these N individual outputs is typically a complex number, right? So, we then need to treat these N complex values as a complex time-domain sequence. This base-band time-domain sequence is the base-band OFDM signal sequence (in discrete time form). It's a sequence of complex numbers, which is all good and nice inside a computer. But what if we want to transmit and receive the complex number data is the fastest way possible? One way of transmitting complex numbers is by individually using the 'real part' and 'imaginary part' to modulate orthogonal carrier signals --- eg. cos(wt) carrier and sin(wt) carrier. The modulated carriers might be r(t).cos(wt) and i(t).sin(wt), where r(t) and i(t) are the real and imaginary analog values. The SUM of the two modulated carriers r(t).cos(wt) + i(t).sin(wt) is a real-valued time-domain signal (not complex signal), and it is an OFDM signal that we can transmit through a wire, or even wirelessly if the carriers cos(wt) and sin(wt) have a suitable frequency, noting that both carriers have identical frequency but are out of phase by 90 degrees. I'm really thinking that if somebody is going to explain this IFFT method of generating OFDM, then they should take a few extra steps to explain the method properly, otherwise it confuses everybody. If somebody going to teach somebody something on Wiki (or anywhere for that matter), then teach it without putting in cryptic or confusing diagrams etc. Make important areas clear for everybody to have a chance to understand.
KorgBoy (
talk)
08:53, 19 February 2018 (UTC)
The up and down mixing described would give rise to images, is OFDM actually symetrical about it's base carrier (this is implied by the article but not explicitly stated) or is it purely above the carrier (effectively SSB, and so requiring image rejection mixers and so on? Scruffy brit 12:15, 3 April 2007 (UTC)
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: [1]. Could this be added to the main article?
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:
Mange01 13:11, 13 October 2006 (UTC)
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)
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)
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 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 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 (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.
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.
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. citation needed
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 text! Put it back! - 143.215.155.26 ( talk) 05:17, 21 March 2009 (UTC)
On a somewhat related topic, i question whether the redirect to OFDM from DMT ( http://en.wikipedia.org/?title=Discrete_multitone_modulation&diff=next&oldid=10618564 ) is optimum. Certainly DMT and OFDM share many characteristics, but the common use of the DMT term in wireline, as well as in the standards reference in ANSI T1.413 http://en.wikipedia.org/wiki/ANSI_T1.413_Issue_2 suggest they should remain distinct topics. Particularly as there are non-trivial differences between the two techniques, that also in fact form some of the basis of T1.413 - such as the Cioffi/Amati patented bit loading / tone swapping algorithm which allows better throughput in copper specific interference such as the ISI found in bridged taps. Because in copper the noise is stationary and the channel is time invariant, DMT is much better able to adapt to the communication medium than OFDM would. In any case, this may not be the best place to discuss the validity of a redirect, but because it does involve technical details relevant to this article, I felt to introduce the idea for comments here first. Duedilly 23:47, 10 February 2007 (UTC)
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)
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 is prefixed with the length cyclic prefix and becomes . Then, after convolution with the channel, which happens as
which is circular convolution, as becomes . So, taking the DFT, we get:
.
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)
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 Reid 68.200.109.154 17:24, 29 March 2007 (UTC)
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) —The preceding unsigned comment was added by Matalal ( talk • contribs) 10:19, 4 April 2007 (UTC).
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)
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)
I've mostly reverted today's additions to the article lead, for the following reasons:
The example is highly relevant, since, as I understand it, a key benefit of OFDM has been the possibility of operating all transmitters across a country on one set of frequencies. 50 miles is a typical distance between such transmitters, and hence 1ms was a design criterion for such applications used in deciding on the number of carriers. This issue is something the non-engineer can readily appreciate the value of. What is transmitted on a carrier is surely 'bits' in the basic sense of 'binary digits' ie on-off representation. I don't see a need to complicate things with symbols, when the average person is familiar with bits.-- Lindosland ( talk) 20:56, 28 January 2008 (UTC)
However, I've retained a mention of SFNs. Oli Filth( talk) 19:32, 28 January 2008 (UTC)
Thanks, I'm reasonably happy with what you have done, though I still think a comparison with AM and FM in the intro would help as these are terms that have been absorbed into popular culture, while 'modulation' means nothing to many people. -- Lindosland ( talk) 21:59, 31 January 2008 (UTC)
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:
Mange01 ( talk) 17:43, 23 February 2008 (UTC)
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)
OFDM is listed under "modulation techniques", while OFDMA is listed under "multiplex techniques".
However, the OFDM article begins with "OFDM ... is a frequency-division multiplexing (FDM) scheme", which is in my opinion correct, and contrasts with the classification as a modulation technique. Fpoto ( talk) 12:55, 28 May 2008 (UTC)
Sections seem to be out of order or jumbled up somehow. -- KJRehberg ( talk) 21:16, 23 June 2008 (UTC)
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)
I further trimmed it down, rewrote in past tense a little, updated it with some sources, and removed inline links. It still seems notable, but of historical interest now that other standards seem to be battling it out. W Nowicki ( talk) 21:26, 23 July 2011 (UTC)
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)
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)
The last paragraph and sentence states that "...[Turbo codes and LDPC codes] only perform close to the Shannon limit for the AWGN channel ... [therefore] concatenated them with either RS or BCH codes to improve performance further...".
I was under the impression that RS/BCH was concatenated with LDPC/Turbo codes in order to clean up resilient errors from their error floor???
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)
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)
We know it is high, but how high? OFDM is typically 12 dB, what would it be for COFDM? I am surprised this most critical parameter for Tx design is treated so lightly. — Preceding unsigned comment added by 75.99.58.122 ( talk) 15:39, 24 January 2012 (UTC)
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 WO2011051448 A4 or the Beeb for additional details. LeadSongDog come howl! 20:24, 6 November 2012 (UTC)
Today I'm reading a paper by Martin Hoch ( "Comparison of PLC G3 and PRIME".) which mentions "t-DPSK" and "f-DPSK".
I came to this Wikipedia article looking for a better explanation of "f-DPSK" and "t-DPSK". Dear Wikipedia editors, if you know what these things are, please add them to this article. -- DavidCary ( talk) 18:07, 9 January 2013 (UTC)
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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)
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)
So far in this article, little mention has been made of the importance of "frequency synchronisation" whatever that means. Perhaps it is better to use the term "Coherency". When a (C)OFDM signal is exchanged over a radio frequency channel, approximately 95% of the resources used in the hardware, and therefore in its design, must be devoted to this function, which is not shown on any block diagram depicting the system as a whole. What must be done is the following:
The received data must be converted numerically to the best available approximation of the transmitted data. This is upstream of any error correction or demodulation, it is essentially an analog function which must be applied to the data as it is acquired from the receiver. Without this, the data is meaningless and scrambled.
- First, the start of a frame must be detected, under any reasonable condition of frequency of phase shift.
- Then, the frequency and phase of the transmitted signal must be deduced from analysis of a transmitted reference signal. The received signal is then both corrected for deviation of baseband frequency and phase. It is restored to the form in which it was transmitted.
Only at this point is it possible to proceed with the processing of the data stream as shown in the block diagrams in this article. The processing elements required to do this would, if illustrated at the same level of detail, cover about five pages.
Firstly, the detection of the beginning of a transmission. For the elaboration, downstream, of a phase reference signal, it must be detected within a certain tolerance of time. This is usually specified as a fraction of the guard interval, which, itself, might be short. The requirement is to pass a guarded version of the phase reference symbol on for further processing. The quality of detection of the beginning of a transmission required is subject to debate, but at early stages of development, it is convenient for this to be somewhat more reliable than the data itself, in the sense that data incapable of being decoded should be, nonetheless, received for analysis. Time-frequency analysis of a mixed tone frame reference symbol, possibly at a resolution of as little as a single sample of data, may be required in order to do this. A time-frequency analysis of transform length N requires N parallel channels, at delays of a single sample, of, in practice, FFT analysis.
At this point, the reference signal can be streamed to a function which calculates the channel impulse response. This compares the received reference symbol with delayed versions of itself to determine time and frequency offsets. These can then be applied to the buffered, received payload following the phase reference signal to restore it to the form in which it was transmitted. This requires further parallel FFT channels together with significant numerical elaboration of their contents.
As stated, coherency reconstruction, together with the buffering needed to accommodate its latency, is a significant element of any OFDM transceiver, and I would advise caution if budgeting hardware resources to the implementation of such a device.