hhca
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Post by hhca on Jan 9, 2006 13:32:31 GMT 5
In communications context (OFDM wireless in particular), what are the differences between "signal" and "carrier" in the following definitions: SNR: signaltonoiseratio SINR: signaltointerferenceplusnoiseratio CNR: carriertonoiseratio CINR: carriertointerferenceplusnoiseratio
Does "carrier" in the above context mean the unmodulated carrier power? What about "signal"? The modulated carrier power?
In a Wimax base station, what are the powers that can actually measured? Is it the total power for a particular subcarrier: "the subcarrier power" + "interference power" + "noise power"?
If yes, then how can the true wanted signal power (i.e., the modulated subcarrier power) be seperated from the interference and noise?
Appreciate your help and comments.
Horace



Post by charan langton on Jan 11, 2006 17:43:13 GMT 5
The carrier is a sine/cosine wave. It is umodulated by definition but when is multiplied by the information signal becomes modulated. The power of the carrier is its amplitude squared over 2. A modulated carrier has basically the same power as an unmodulated carrier. The information does not add its power to the carrier. This is because the information is varying some aspect of the signal such as phase in PSK which does not change its amplitude. (AM signal is an exception. its power does vary) The term carrier power applies usually to the modulated carrier.
The term "signal" is more than then carrier. It includes the carrier and noise and interference. So power of S = C + N + I. At the receiver we can not sepearate these. There is no way to do that. At the receiver we are measuring the power S.The link budget done has established what this number needs to in order for the communications to take place. The measured power is desired to be within the limits established by the link budgets.
Since we know that noise is uniform across the band and interference is usually out of band, we lowpass this signal and this way remove a lot of the noise and the interference. This maximizes the C in S = C + N + I and hence S/N. Now we can apply the matched filter to this signal and do further decoding.
If we want to measure a subcarrier, then a bandpass filter would need to be tuned to that carrier frequency. This would pick out the desired subcarrier in a FDM case. If this is a CDMA case, then all carrier are mixed in one band and we need to apply a code to remove the wanted channel.



Post by ilhanna on Jan 17, 2008 23:09:48 GMT 5
Dear Charan,
I would like to know how to calculate instantaneous snr in matlab.
Let say the signal at the receiver after fading and awgn noise is given as r(t)=x(t)*fade+n(t)
Is it correct if I want to calculate instantaneous snr by taking the ratio of the signal plus fading to the noise== like this ( x(t)*fade divide with n(t)).
Hope to get some help here. Tqvm

