Difference between cyclic prefix and guard period in OFDM

[mashhur]

ofdm guard period

Hi all members! I am hesitating for a point which is about OFDM technique.

My first question is What is the differences between guard period and cyclic prefix?

Second one is does OFDM use the guard period among symbols?

we know in FDM systems, guard period is used but for OFDM i do not know.

Could somebody helps me? I really appreciate! Thanks in advance!

with king regards Mashhur! :|

 

[wadani]

ici cyclic prefix

guard time is used in ofdm to reduce isi and guard time and cp are same one anther so cp should also greater then delay spread so as to eliminate or reduce isi .......... that is wad i know if u need more information search from goagle and i hope you would get more papers that will decribe in detail

 

[Aya2002]

guard period ofdm

Cyclic Prefix / Guard Interval
The Cyclic Prefix is a periodic extension of the last part of an OFDM symbol
that is added to the front of the symbol in the transmitter, and is removed
at the receiver before demodulation.
The Cyclic Prefix has two important benefits:
² The Cyclic Prefix acts as a guard space between successive OFDM
symbols and therefore prevents Inter-symbol Interference (ISI), as long
as the length of the CP is longer than the impulse response of the
channel.
² The Cyclic Prefix ensures orthogonality between the sub-carriers by
keeping the OFDM symbol periodic over the extended symbol duration,
and therefore avoiding Inter-carrier Interference (ICI).
Mathematically, the Cyclic Prefix / Guard Interval converts the linear convolution
with the channel impulse response into a cyclic convolution. This
results in a diagonalised channel, which is free of ISI and ICI interference.
(see part ??)
The disadvantage of the Cyclic Prefix is that there is a reduction in the
Signal to Noise Ratio due to a lower efficiency by duplicating the symbol.
This SNR loss is given by:

where Tcp is the length of the Cyclic Prefix and T = Tcp +Ts is the length of
the transmitted symbol.
To minimise the loss of SNR, the CP should not be made longer than necessary
to avoid ISI and ICI.

 

[mashhur]

cyclic prefix isi cofdm

Thanks everybody for answers!

 

[shiva999]

Re: Difference between cyclic prefix and guard period in OFD

Hi Guys,
it's very interesting to see the discussions overe here. I would request you guys to elaborate some more on ISI reduction using CP,

My question is assume the 2 maultipaths received at the receiver as below with CP prefixed to Symbols on both paths:

|--CP--|--symbol data--| ---> path 1
|-td-|--CP--|--symbol data--| ----> path 2 with delay td

|------------------| integration duration (Ts - symbol dur)
|-----| this is the part of the output still gets corrupted due to ISI, since CP is the part of the OFDM symbol it will be a non zero value. So I am wondering how CP is removes the ISI.

thanks in advance.
Shiva