While doing a CWNA practice test yesterday I found yet another error. This one though, is worse than a simple typo, or lack of context. I sent a comment but didn't receive any reply so I am submitting what I found on the forum. I'd like to have the opinion of others on this...
The question asks:
"Which of the following would cause a radio frequency signal to attenuate?"
One of the choices that's supposed to be the right one is :
"A reflected signal received 70 degrees out of phase of the main signal"
Now from what I've been reading in the Sybex study guide (page 43), only a 120 or more phase difference will cause attenuation. I've also confirmed that with a signal simulator.
Anyone else thinks like me that this question/answer is completely wrong?
I have never heard of the 120 degree thing before, not to say it doesn't exist. If someone had asked me, I would have said that 70 degrees out of phase would cause some attenuation. I find it hard to believe that it wouldn't cause attenuation. Anyone else? M/Q, thoughts?
I might suggest that this is another example of an answer that has to be taken in context with the other answers.
As for a specific phase angle at which attenuation starts that is not how I view this subject. Any phase difference between two localized wave fronts will cause degradation/attenuation of the signal.
There is a need to get very specific here as to the circumstances and definitions as this is a very diverse subject that covers many different possible situations.
The fading of the signal envelope-or instantaneous signal power-can be easily pictured using a simple two-path example. The signals from each path can be of any amplitude and phase at the receiver.
If the two signals have the same amplitude, and their phase is 180 degrees apart, the interference pattern is referred to as destructive because it completely cancels both signals. Destructive interference is also sometimes called "deep fades."
If the two signals are 0 degree apart in phase, the interference pattern is considered constructive, and the signal envelope will be a percentage dB larger than the amplitudes of the individual signals.
In the two-path example, the signals rarely combine to greater than 10 dB above an individual path's power level. Destructive interference ranges from just a few decibels to fades of greater than 50 dB. The fade spacing is a function of the RF carrier frequency.
So unless some specific level of attenuation was published, the question/answer really should not even advertise a phase angle. Also in my world phase angle is not really that important. It is delay spread, which is even more critical when concerned with digital systems. This quote is from my notes of a recently attended seminar:
?¡é?€??In such cases, the propagation delays of the paths from one end of the link to the other can differ considerably. The extent of this time spreading of the signal is commonly measured by a parameter known as the delay spread of the path. One consequence of having a larger delay spread is that the reinforcement and cancellation effects will now vary more rapidly with frequency. For example, suppose we have two paths with equal attenuation and which differ in length by 300 meters, corresponding to a delay difference of 1 ???¦Ìsec. In the frequency domain, this link will have deep nulls at intervals of 1 MHz, with maxima in between. With a narrowband system, you may be lucky and be operating at a frequency near a maximum, or you may be unlucky and be near a null, in which case you lose most of your signal (techniques such as space diversity reception may help, though). The path loss in this case is highly frequency-dependent. On the other hand, a wideband signal which is, say, several MHz wide, would be subject to only partial cancellation or selective fading. Depending on the nature of the signal and how information is encoded into it, it may be quite tolerant of having part of its energy notched out by the multipath channel. Tolerance of multipath-induced signal cancellation is one of the major benefits of spread spectrum (SS) transmission techniques.
Longer multipath delay spreads have another consequence where digital signals are concerned, however: overlap of received data symbols with adjacent symbols, known as intersymbol interference or ISI. Suppose we try to transmit a 1 Mbps data stream over the two-path multipath channel mentioned above. Assuming a modulation scheme with 1 sec symbol length is used, then the signals arriving over the two paths will be offset by exactly one symbol period. Each received symbol arriving over the shorter path will be overlaid by a copy of the previous symbol from the longer path, making it impossible to decode with standard demodulation techniques. This problem can be solved by using an adaptive equalizer in the receiver, but this level of sophistication is not commonly found in amateur or WLAN modems (but it will certainly become more common as speeds separated by delays of approximately the reciprocal of the spread bandwidth, or more). These appear as separate peaks in the DSSS receiver correlator output. A diversity receiver using the RAKE principle can take advantage of some of the multipath signal power by combining it constructively before making the bit decisions. More commonly, however, only the largest correlation peak is used, and all of the other multipath energy represents continue to increase). Another way to attack this problem is to increase the symbol length while maintaining a high bit rate by using a multicarrier modulation scheme such as OFDM (Orthogonal Frequency Division Multiplex), but again, such techniques are seldom found in the wireless modem equipment available to hobbyists. For unequalized multipath channels, the delay spread must be much less than the symbol length, or the link performance will suffer greatly. The effect of multipath-induced ISI is to establish an irreducible error rate - beyond a certain point, increasing transmitter power will cause no improvement in BER, since the BER vs Eb/N0 curve has gone flat. A common rule of thumb prescribes that the multipath delay spread should be no more than about 10% of the symbol length. This will generally keep the irreducible error rate down to the order of 10-3 or less. Thus, in our two-path example above, a system running at 100K symbols/s or less may work satisfactorily. The actual raw BER requirements for a particular system will of course depend on the error-control coding technique used.
Although it is commonly believed that SS modulation schemes solve the multipath ISI problem, this is not really the case. As stated above, SS can convert a flat-faded channel into one which has selective fading, which is a good thing. In the case of Frequency Hopping (FHSS), it means that signal cancellation due to multipath will occur only a fraction of the time (i.e., only on some of the channels we hop to), and we can recover the data by means of Forward Error Correction (or by error detection and retransmission). In the case of Direct Sequence (DSSS), only a fraction of the transmitted spectrum is notched out by the multipath cancellation. This causes some degradation of the BER, but again error control coding can be used to compensate for this. In both cases, SS modulation has given us a form of frequency diversity. For DSSS, the large continuous spread bandwidth allows us to resolve many of the multipath components (those wideband interference. Regardless of whether a diversity receiver structure is used, however, ISI (and hence BER degradation) will still occur when the multipath delay spread approaches the same order of magnitude as the information symbol length.?¡é?€??
I would be interested in hearing more about the signal simulator you referred to, if you have the time could you describe what its capabilities are and how it determined the 120deg phase angle?
Also sorry for going over the top, who says us Hams are boring. Once again you have brought up a very interesting topic that is near and dear to me and IMO it is a very good question.
Wow! The sad, sad thing is, I understood a lot of what you said. Who would have guessed?
Thank you very much for the great and complete answer. I thought you might like that question!
Michael, that is a great post. Delay spread. Thank you.
David Coleman reminded me of the EMANIM application for visualizing electromagnetic waves. I checked and it agrees with the 120 degree thing.
Unfortunately it does not have a "delay spread" mode!
I hope this helps. Thanks. /criss
I get to say Wow too, I have been messing around with this RF propagation for years and did not know about that program, thank you so much.
If I understand the program mechanisms correctly you immediately see an amplitude deterioration of the resultant wave front when the phase differential is introduced. So it seems to agree with what I have tried to explain, albeit poorly.
I also was curious, when the phase difference was 180deg the resultant wave front is theoretically nulled out, the visual still shows a resultant wave front. Not complaining by any means though.
Thanks again it will be a great visual aid at our classes.
Gene, I take that as a huge compliment coming from you. We/Us amateurs take a great deal of pride in this subject matter. We teach it to anyone that will listen.
Apologies for the delay in responding to your feedback. I wanted to double-check it and I agree with you. This will be updated in the next release. Thanks for your response!
Also, thanks to CH, GT and M/Q. Great post M/Q!
Thank you all, and specially Michael for taking the time to provide us with all this information.
It seems that Criss found the simulator (EMANIM) before I had time to post my reply ;) this is the one I used to confirm what I've read.
Casey, my only concern is the possibility that errors like that appear in the real exam, please tell me that all the questions of the real test have been throughly reviewed if not triple checked !!?
I am curious to learn why 120deg is the magic number? Is there a set attenuation threshold that must be reached before it will be considered such?
I have been taught that any wave front pair that is any degree out of phase will deteriorate the resultant wave front. And unless I am not using that amazing program correctly that is what I am seeing. I hope you can help me understand this.
I realized that I haven't provided the info I got in the Sybex book which I referred to earlier, so just for the sake of being thorough, here's the part in the book talking about this which prompted me to initiate this thread in the first place :
- Downfade: This is decreased signal strength. When the multiple RF signal paths arrive at the receiver at the same time and are out of phase with the primary wave, the result is a decrease in signal strength (amplitude). Phase differences of between 121 and 179 degrees will cause downfade.
- Upfade: This is increased signal strength. When the multiple RF signal paths arrive at the receiver at the same time and are in phase or partially out of phase with the primary wave, the result is an increase in signal strength (amplitude). Smaller phase differences of between 0 and 120 degrees will cause upfade. Please understand, however, that the final received signal can never be stronger than the original transmitted signal due to free space path loss.