In MCM system there is guard band between two successive frequency bands so that the signal can be properly decoded at the receiver. But in OFDM the signals are overlapped. I found in some papers where the authors mentioned about the guard band (GB) and their work seemed to be reducing this guard band. Now, I am confused what does this GB in OFDM mean.
Also does the guard interval (GI) and cyclic prefix (CP) have the same meaning? Or GI can be achieved by inserting CP or zero padding?
Please answer.
Thanks.
First , do you really mean Orthogonal Frequency Division Mixing? Or Orthogonal Frequency Division Multiplexing? I think you mean the latter, yes?
On the cyclic prefix (CP) and the Guard Interval (GI). We are talking about one OFDM channel here, which consists of perhaps thousands of subchannels carrying orthogonal symbols:
The cyclic prefix consists of the trailing edge of the previously transmitted symbol, and that part of the previously transmitted symbol is sent, redundantly, within the guard interval. The guard interval duration is chosen ahead of time, and is based on the expected lag or lead time of worst case multipath propagation. The general idea is to send again that portion of the previous symbol that is likely to have been corrupted by intersymbol interference (otherwise called ghost image).
These describe how the CP is used:
http://www.dsplog.com/2008/02/17/cylcic-prefix-in-orthogonal-frequency-division-multiplexing/
http://rimtengg.com/coit2007/proceedings/pdfs/123.pdf
So OFDM trades off spectral efficiency for robustness, while avoiding the use of complicated time domain equalizers with many taps. The worse the multipath is likely to be, the more capacity OFDM trades off for robustness. Sometimes quite a lot of capacity. Guard intervals of 1/4 are common, where you are essentially throwing away 25% of your optimal channel capacity.
As to the guard band (GB), that's simpler.
When symbols are not guaranteed to be orthogonal, such as between two unrelated communications channels, then you have to introduce a guard band, so that receivers can adequately filter out the unrelated channels' symbols. Before going through the inverse FFT at the transmitter end, one OFDM channel is made up of perhaps even thousands of relatively low rate, very narrow subchannels. This naturally results in very steep edges to each subchannel, and hence also steep edges to the subchannels at the upper and lower extremes of the comm channel. So compared with single carrier schemes, OFDM, especially at high modes like 8K and above, will require narrower guard bands than most other modulation types, which means that these different comm channels can be more tightly packed in their frequency band.
To combat the Inter-Symbol Interference (ISI) in multipath channel, a time domain Guard Interval (GI) is added to between consecutive symbols to avoid subcarriers interference. This guards must be larger than the largest channel delay.
In the OFDM systems, GI(Guard Interval) =GB(Guard Band) =CP (Cyclic Prefix).
The OFDM guard interval can be inserted in two different ways. One is the zero padding (ZP) that pads the guard interval with zeros. The other is the cyclic extension of the OFDM symbol (for some continuity) with CP (cyclic prefix) or CS (cyclic suffix). CP is to extend the OFDM symbol by copying the last samples of the OFDM symbol into its front.
Good Luck
In order to avoid inter-symbol interference (ISI), OFDM systems append a
cyclic prefix (CP) or CS to the time domain signal. The length of the CP is set to be at least
the maximum delay spread, Nτ, of the channels response
Sorry for wrong heading. Its corrected now.
Thanks to everyone for trying to make my doubt clear. It has to some extent made my doubt clear. While some still remains.
Sir Hossein Hardani has answered that in OFDM systems, GI =GB =CP. But I got confused with this.The reason is, I found in many papers that GB as well as CP was used. If GB=CP then they would not have used that. And it was not used interchangeably in that paper.
In the answer by Sir Albert Manfredi, it was termed as a simpler aspect. So I am expecting that sir will be able to make me more clear on that.
Sir, I am not yet clear on GB. Can you explain it in a simpler way. Please do provide me some idea of it in optical domain, if you can.
Thank you sir.
OFDM is based on the well-known technique of Frequency Division Multiplexing (FDM). In FDM different streams of information are mapped onto separate parallel frequency channels. Each FDM channel is separated from the others by a frequency guard band to reduce interference between adjacent channels.
The OFDM scheme differs from traditional FDM in the following interrelated ways:
1. Multiple carriers (called subcarriers) carry the information stream,
2. The subcarriers are orthogonal to each other, and
3. A guard interval is added to each symbol to minimize the channel delay spread and intersymbol interference.
OFDM consists of multiple sub-carriers which are orthogonal (peak of one sub-carrier falls of NULLs of other sub-carriers). The sub-carriers are divided in data+pilot+null subcarriers. These null subcarriers are chosen at the edge of the band chosen, and these null sub carriers form the Guard Band of a OFDM channel. These are used to protect from Adjacent Channel interference.
Guard Interval is just OFDM symbols separated in time. It does not mean 0 airtime though. A portion of OFDM symbol is copied within the Guard Interval portion to make it resilient to ISI caused due to long delay spreads, this is called Cyclic Prefix. So longer the delay spread, more the ISI, hence we need longer GI to reduce the effect of ISI.
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