How is orthogonality preserved in OFDM?

[Mr Cappuccino]

orthogonality in ofdm

How is orthogonality preserved in OFDM. Factors like multipath channel fading induce phase shifts and since for orthogonality to be preserved, the phase between orthogonal signals should not vary which is not possible in a multipath environment ... ?

 

[sameerbabu]

orthogonality ofdm

I am also having same doubt. Any body can answer

 

[Willem]

ofdm orthogonality

Ortogonality can not be entirely preserved but is improved by using guard intervals and forward error correction techniques.
Also, each carrier operates at low bit rates. The equipment used in OFDM systems are also very linear to eliminate any other phase alterations.

 

[janath]

importance of orthogonality + ofdm

How is orthogonality preserved in OFDM. Factors like multipath channel fading induce phase shifts and since for orthogonality to be preserved, the phase between orthogonal signals should not vary which is not possible in a multipath environment ... ?

You add a cyclic prefix before transmission. At the receiver when we do FFT path delay will reflect only a phase shift. Hence, orthogonality willbe preserved.

 

[joinfaisal]

orthognality in ofdm

Actualy danger of loosing orthogonality is not mainly because of multipath effect...it is mainly due to doppler shift...because multipath effect is combated by using cyclic prefix addition..but it is doppler shift in frequency which can cause interference among carriers and this is removed by using some pilot carriers of known reference phase and from that phases of other carriers are adjusted to acheive the orthogonality.

 

[aomeen]

the orthogonality of ofdm

Simply, in OFDM orthogonality is preserved because the carriers are generated by taking the inverse FFT "or FFT, in fact" to the input...

now, think of the inverse of FFT as the representation in form of fourier series, in this case the signal consists of all sin and cosine components, fully orthogonal

My point is: the orthogonality comes from the definision of OFDM itself, not any extra things "cyclic prefix, pilots et, which ofcourse does help, no dought.

 

[Naveed Alam]

some ofdm explanations

hi..
using cyclic extension (guard time)..

Regards
Naveed

 

[farui]

channel estimation in ofdm

Actualy danger of loosing orthogonality is not mainly because of multipath effect...it is mainly due to doppler shift...because multipath effect is combated by using cyclic prefix addition..but it is doppler shift in frequency which can cause interference among carriers and this is removed by using some pilot carriers of known reference phase and from that phases of other carriers are adjusted to acheive the orthogonality.

I strongly agree with u, but I'm doubting that pilot carriers can work, because each subcarriers experiences an independent flat fading. The situation can be even worse in multiuser MC-CDMA system due to the MAI. anybody can give me any good idea to solve this problem?

 

[gausschen]

ofdm explanation

I agree with aomeen.The orthogonality comes from the difference of the frequency of the subcarriers

 

[jagan326]

what is orthogonality in ofdm

i will send the details of ofdm ...just chech out this site...www.skyware.com

 

[rfsystem]

Othogonality is preserved if the frequency of all subcarriers are matched. So only doppler shift, phase noise or inaccurate frequency estimates could distort the orthogonality of the subcarriers.

The orthogonality is more important for OFDMA where different subscribers send interleaved subcarriers. So maintain in a system that all subscibers to be synchron to the master to up 1-2% of the subcarrier spacing is important.

The other topic is phase&ffrequency offset estimation from a limited number of pilots. If a group of pilots have a different phase shift than another and because of fading the groups change there distribution equal effects to time changing doppler shifts happens. Buth these are rarely observed.

The total other topic is channel estimation. That is done by preamble, midamble or rolling pilots.