DC subcarrier doubt in WiMax??

[rag_perfect]

dc subcarrier

Hello Frnds!!!
i have a deep doubt related to DC subcarrier.
i.e. As per Wimax802.16d PDF:
"DC subcarrier is the subcarrier whose frequency is equal to the RF centre frequency of the transmitting station."

what is the significance of "RF centre frequency of the transmitting station." How this information is useful at reciever side??

Also...
DC==> zero frequency. So How can a DC subcarrier contains any frequency ??

Plz clerify my doubt...!!!

 

[melc]

what is dc subcarrier

Hello Frnds!!!
i have a deep doubt related to DC subcarrier.
i.e. As per Wimax802.16d PDF:
"DC subcarrier is the subcarrier whose frequency is equal to the RF centre frequency of the transmitting station."

what is the significance of "RF centre frequency of the transmitting station." How this information is useful at reciever side??

Also...
DC==> zero frequency. So How can a DC subcarrier contains any frequency ??

Plz clerify my doubt...!!!

AFIK depending on modulation scheme, the subcarrier may use a bandwith from 1.75 to 28MHz. Meaning for a 2.5GHz center frequency would be 2.5Ghz +/-14Mhz (for a 28Mhz bandwith). The receiver must be set exactly to 2.5Ghz center frequency and the band pass filter set correct to receive the entire bandwith.

 

[rag_perfect]

importance of dc carrier in wimax

Thakx melc...
my doubt resolved a bit. but still i m confused about how a frequency can be allocated to DC or Null valued signal???

plz help.....

 

[Naveed Alam]

why is dc subcarrier not used in wimax

my doubt resolved a bit. but still i m confused about how a frequency can be allocated to DC or Null valued signal???

just see the document, may be it explain DC. DC can be direct converter or something like this. if u upload the document, we will also see it; may be we also learn something.. see end of the document, for this. good writers give abbreviations at the end.

Naveed

 

[rag_perfect]

frequency offset definition wimax

Thankx naved..
but in pdf they have nowhere given full form of DC subcarrier...
for ur reference i m attaching its link here below:



plz help...

 

[melc]

dc subcarrier meaning

I think you have missunderstood the terminology.
First you have to define which type of modulation is used. QPSK, BPSK QAM ?
When you are pointing to a standard, please link exactly to the document described and mention the page and the paragraph you are talking about.
In that way maybe someone will read it. Who the hell will read an entire standard just to find out the DC subcarrier ?

 

[alqasim]

why is dc carrier not used in wimax

Actually there are two questions in your question

The first one is "How can a DC subcarrier contains any frequency??”

& the second is ""RF centre frequency of the transmitting station." How this information is useful at receiver side?? "

The answer for the first question is the DC subcarrier is a DC without modulation (shift the channel to RF frequancy" or after doing demodulation you can find this subcarrier as a DC subcarrier.

And recommendation says that this subcarrier is null to simplify A/D & D/A operation.


The answer for second question is - I am not sure about it - as the following:

If you want to ask about how the receiver (subscriber station (SS)) can decide the center freq. and the bandwidth for the channel? I don't think that will happen from taking information from DC subcarrier may be - and again may be , I am not sure - from pilot subcarriers and it depends on if there is sub-channelization (802.16 d) or subcarrier permutation (802.16 e) .

If you ask about what is the function of DC subcarrier especially it will be null - according to the recommendation - I think they (the forum) want to keep a sort of symmetry in OFDM channel structure (for example, for 256 subcarrier channel , the remove 27 & 28 from each side for guard purpose & the remaining is 201 subcarrier ; 1 DC subcarrier (at center), 8 pilot subcarriers & 192 data subcarriers)


When I get the exact information I will let you know....

 

[rfsystem]

dc subcarrier define

The origin of the DC subcarrier exception is historical.

The analog implementation of TX path in the OFDM had in the past no special methods to suppress the carrier leakage or back referred DC offsets. Together with the situation in the past that there was no baseband controlled frequency offset correction in the TX path the easiest way was to make an exception there. Simply do not transmit information on this subcarrier.

On the receive side there was always the issue that because of the frequency offset the DC exception subcarrier from the receive TX signal and the DC offset of the receive does not coincide. That issue must be resolved by special analog and digital compensation actions.

Today for WiMAX the TX signal form the terminal station have to be frequency adjusted by digital baseband to follow the signal frequency receive from the base station. So because the synthesizer is often not able to follow quickly the TX path DC offset or carrier leakage does also not coincide with DC exception subcarrier.

In 802.11 the frequency offset is not adjusted in the TX signal. In 802.16 it is adjusted in the terminal to allow OFDMA where the carriers from different terminals have to be orthogonal and separated by about 30-40dB.

So the historical reason for the DC exception got obsolete.

The second fact which have a historical reason is the guard band definition by subcarriers. It was thought that channel filtering was implemented by sharp digital filters running at no oversampling rate. But real implementation also have to filter out the alias image and optimum oversampling is somewhere between 2x and 4x, so the filter have to run anyway at 2x oversampling or higher. It was further thought in the past that the channel separation is exactly one symbol sampling frequency. Today practical WiMAX systems allow the system vendor to pack channels closer together depending on the channel separation and interference.

Summarizing both construction principles for OFDM signals got obsolete today. I thought the last years that the standardization body had a better technical vision of a reference implementation.

 

[alqasim]

wimax dc subcarrier

rfsysystem can you explain the following

no baseband controlled frequency offset correction in the TX path

thank you in advance

 

[rfsystem]

802.16,subcarrier

The meaning should be:

"the terminal station adjust its own TX center frequency to match the received center from the base station"

The adjustment could be either in the synthesizer or the digital signal processing , typical after the IFFT. The digital frequency adjustment is the complex multiplication with an offset frequency generated by a sin/cosine-ROM and phase accumulator.

 

[Naveed Alam]

what is dc carrier wimax

my doubt resolved a bit. but still i m confused about how a frequency can be allocated to DC or Null valued signal???

Dear,
i have gone through my Masters thesis again and concluded ur question's answer,

subcarriers are made by ifft operation, when done so, all the data must have complex conjugate, and the first one must be real (as FFT process is inverse of IFFT, we set data which shows resemblance with FFT result)

DC carrier= carrier with no complex conjugate,
and this carrier is simply not used (because when u take ifft of such a data which has no complex conjugate, the output will be complex, whereas it must be real).
if i am not clear, just write here.


Naveed

 

[rfsystem]

wimax dc carriers

w/o the cyclic extension the result of the IFFT is a complex(2-valued) signal.

w/o any frequency shift in the generated TX signal the mean value over the sample is zero. Both, the real and the imaginary part, typical written as I and Q, Inphase and Quadraturephase.

Complex multiplication mean a multiplication respecting the math rules for complex number. The multiplicand is a rotating phasor with unit length. It has real and imaginary nonzero parts.

The small glimpse is here to multiply after padding the cyclic prefix of the symbol.


I think I capture the small stone one the way! If the frequency domain subcarriers are conjugate complex you will get a real signal. That mean the I(inphase) is nonzero and the Q(quadraturephase) is zero. But then the information contend in the OFDM signal is only half!

If the frequency domain is arbitary complex so it is the IFFT result!

You must process two signals after the IFFT! After the upconversion mixer the signal is real.