[digitalradio] Re: ARRL/FCC Announcement about ROS
Hi Julian, By channel coherence time do you mean time when the signal is readable? The channel choerence time is a property of a (fading) channel which gives an idea of the time interval over with the channel response is approximately *constant*. If you drive your car at 100 km/h and tune your car radio to a far and weak station in the 88/108 MHz FM broadcasting band you have probably noted that the station fades out quite fastly, say with an average rate of 10 Hz, you therefore might expect that the channel response is approximately constant for no more than a small fraction of 1/10Hz = 100 ms. This occurs because the signal you are receiving is the sum of (usually many) different scattered components, each of them coming from a random direction which is not necessary the direction you are driving through. Some of this components could come exactely from the direction you are driving through and are affected by a positive Doppler shift. Some other components could come from the direction you are coming from and they are affected by a negative Doppler shift. Other components could come from directions which form a right angle with yours and the would exhibit no doppler effect. The sum of all of these components can be treated by a stochastic ideal model which is called the (flat) Rayleigh channel model. This (ideal) channel model is essentially characterized by two parameters: 1) the maximum Doppler frequency shift (which is called the channel Doppler spread) and 2) the average channel attenuation. The Doppler spread (Fd) depends upon a velocity v (the velocity of your car) and the signal carrier frequency Fc through the formula Fd = v/c*Fc (c = light speed). If you do the calculation with v = 27.8 m/s (100 km/h) and Fc = 100 MHz you will find that the Doppler spread is approximately 10 Hz (9.3 Hz, for the sake of precision). Interestingly, the autocorrelation function R(T) (how much two samples of the process are correlated given the time interval T they are separated by) of the flat Raileigh channel model it's quite easy to compute: it's the Bessel function Jo(k*T*Fd) (k is a constant, I don't remember its value, maybe PI or something like that). For T*Fd 1, the autocorrelation function is not different from unity and this tell us that in a time interval T 1/Fd the channel response is strongly correlated. This means that if the channel response assumed a value X at time t, the probability that its amplitude does not differ so much from X at time t+T is large. I can see how this would work using widely separated frequencies. However we have all observed that when a signal goes down in QSB, it does down right across the passband. This is not always true and has to do with another parameter which is called the coherence bandwidth of the channel and which is inversely proportional to the channel time spread. In the HFs it's not unfrequent that the channel time spread is some milliseconds and that the channel coherence bandwith is few hundreds Hertz. Multiple reflections from the F and E ionosphere layers are an example in which this happens. So do you actually gain anything by spreading the transmission by only 2.2kHz, other than the ability to annoy people who consider it a selfish waste of bandwidth? Not always, sure, but you could figure out by yourself the amplitude of public crucifixions if some amateur transmission were designed to be spreaded by 10 kHz or more just because a 2.2 kHz spread is not sufficient to exploit frequency diversity as the ionosphere characteristics would require :-) In any case it is more easy to design a new communication system which copes with a given ionosphere rather than to alter the ionosphere itself. For now we just managed to alter the atmosphere with massive CO2 emissions (and it took one hundred years). For the ionosphere we need more time... 73s Nico, IV3NWV
[digitalradio] Re: ARRL/FCC Announcement about ROS
Hi Jose. You have a point too nobody had made me to stop and think about. FEC or UWB in whatever way, carried to the extremes, are two sides of the same coin. It happens, never mind. Sometimes also telecommunication engineers have not a clear vision of what they are designing :-D On crowded spectrum, efficiency certainly counts. In a message oriented and power limited fading communication system what counts is the relationship between the channel coherence time (the time interval over which the channel response can be considered almost constant) and the message duration. If the message duration is not much longer than the channel coherence time there's no other possibility than to exploit frequency diversity. In this case, transmitting your message over a narrow band channel whose coherence time were much longer than your message, you would suffer a severe message loss due to the fact that the channel attenuation is frequently larger than the average for the entire duration of your message. If instead the duration of the message were much longer than the channel coherence time, the energy of any message you would receive would be not very different from its average. In this case a clever coding system would not behave so differently from a non fading channel and would approach its capacity by few dBs. For a low-rate system which transmits messages in the range of 50 bits/message and the message length is 60 seconds or so, as i.e. both K1JT's JT65 or WSPR do, there's no need for bandwidth expansion (besides FEC of course). In these cases the channel coherence time is usually much less than the message duration and frequency diversity would be of little help. Joe designed them well :-) For communication systems with the same message information content but in which messages were required to be transmitted much faster, say in three seconds, the channel coherence time would be of the same order of magnitude of the message length and time diversity can't be exploited. In this cases, frequency diversity is mandatory whether implemented by what FCC calls a spread spectrum system or not. This is what, in my opinion, ROS has tried to address. I couldn't care less if it is legal or not, I just hope it could cohexist with the modes I'm already using. Mr. Darwin selection rules will do their job and select the better. By the way, we amateur radios already experiment daily frequency hopping spread spectrum communications. We continuously hop from the 160 m band to the 10 m band accordingly to the HF propagation conditions and, sincerely, I do not understand why FCC is so permissive with us (or better, with US amateurs). Has this to do with federal agents reaction times? ;-) 73s, Nico, IV3NWV
[digitalradio] Re: ARRL/FCC Announcement about ROS
Jose, if you are referring to me I'm not saying that theoretically it is correct to use as much bandwidth as possible. This is a conclusion you have drawn on your own. Using a 100 kHz bandwith to communicate information at a rate of 1 bit/s could by sure approach any channel capacity, but the spectral efficiency of such a communication channel would be quite questionable. Let this option to NASA deep space communications. What we need are modes which are both power AND bandwidth efficient. I think that the term spread spectrum here is misleading. What's the difference between a communication system which uses a FEC code with a very low rate, say R=0.01 (one information bit per one hundreds symbols), and a communication system which hops or spreads the modulating signal on an equivalent bandwidth? In my opinion: NONE. Both systems are using a bandwidth which is one hundreds time the bandwidth which would be used by an uncoded system. The problem is not whether a system is spread spectrum or not. The problem is how much it is bandwidth efficient. Everyone knows that an ortoghonal signalling system approaches the (AWGN) channel capacity. The legitimate question is if the whole 20 m band should be used to achieve such a result to communicate information at 3 bit/s. For what I know ROS has a really poor bandwidth efficience nor it copes with MUI (multiuser interference) issues. I do not doubt that it can achieve an exciting performance under the power efficiency point of view, but that's not all. We are called to develop systems which are efficient also in respect to bandwidth. The spread spectrum story is just a bad motivation used against true concerns. 73s Nico, IV3NWV --- In digitalradio@yahoogroups.com, Jose A. Amador ama...@... wrote: I agree with Nino, theoretically it is correct to use as much bandwidth as possible, 3 kHz in the ROS case, but due to the small spreading, the ROS signal does not have a negligble level compared to others on the channel, so it is a halfbreed, it has spread spectrum characteristics, but does not quite behave like the pure definition. ROS still had problems in version 1.6.3 and it is easy to notice that it works in a free channel, but does not stand burst errors (in fact, errors long as a packet or pactor frame length) and its ability to copy crumbles. That does not happen, at least so noticeably, with JT65 or Olivia. 73, Jose, CO2JA
[digitalradio] Re: ARRL/FCC Announcement about ROS
Julian, thanks for your comments. Yes, laws are laws. Also the Hammurabi rule If a man puts out the eye of an equal, his eye shall be put out was a law but I don't think that it would be of great help in our modern society. I agree with you that simulations should be performed prior to any other on air experiment. I think that this is already a common practice nowadays or at least that nobody interested in a serious development would omit to perform it today. I also agree that amateur bands are not just an experimenter's playground but this implicitly means that they are not exclusive to communicators. If I were an experimenter I would like to see acknowledged my right to make my experiments somewhere in our bands. I would have no interest interfering other users activity, I would just need a portion of the spectrum where me or other amateurs on the other side of the Atlantic Ocean were not considered criminals just because we are validating a model on the field. I don't agree that we should use modes which have already been invented and stop looking for new ones. Research and development in communications and in information theory are everything but dead. Turbo codes were submitted to the attention of the research community just fiftheen years ago, when many had already missed the hope that the Shannon channel capacity could be really approached. Should Berrou, Glavieux and Thitimajshima have made more use of what had been already invented instead of experimenting what had not be done yet? And what about those who dedicated their time inventing new efficient algorithms to decode LDPC (or Gallager's) codes, as David MacKay did few years later? Koetter (unfortunately passed away at a still young age), one of the two researchers who found an algebraic soft decision method to decode better than before the Reed-Solomon codes, as those used in Joe's JT65, published his work in 2003 or so. Should we have stopped our alternatives to knowledge and technologies available in 2002? I don't think so. We should better keep up with news and new modes. Nico, IV3NWV --- In digitalradio@yahoogroups.com, g4ilo jul...@... wrote: Laws are laws, whether you like them or not. And, in this particular context, is it actually necessary to go on the air to carry out experiments of this type? As has been mentioned in several posts. there are ionospheric simulators that permit the testing of different modes. The amateur bands are not just an experimenter's playground. They are also used for communication. And communication becomes increasingly difficult when you have a Tower of Babel of different, mutually incompatible modes competing for the same frequencies. There are dozens of data modes that have been developed in the last few years and most now simply lie unused because not enough people were interested in using them to make it possible to have everyday contacts. Would it not be better to make more use of the modes we already have than keep on inventing new ones? I think that before any mode is allowed off the simulator and into general use it should be proven to have benefits not provided by any pre-existing modes, as well as to justify its use of bandwidth. I think there is an argument for setting aside a small section of space for on-air experimentation with unapproved modes. But the situation where existing users of the bands suddenly have their activities disrupted when people start going mad with some flavour of the month new mode is unacceptable, and the controls the FCC exercise over amateurs in the USA do at least go some way to prevent this. Julian, G4ILO
[digitalradio] Re: ARRL/FCC Announcement about ROS
--- In digitalradio@yahoogroups.com, Dave Wright hfradio...@... wrote: http://www.arrl.org/news/stories/2010/03/04/11377/?nc=1 Quoted: The ARRL supports -- as one of the basic purposes of Amateur Radio -- the experimentation and advancing the technical skills of operators. The development and use of any new mode is exciting to many amateurs, and the League encourage amateurs to experiment within the parameters of the rules; however, the ARRL also reminds US licensees that according to Section 97.307, spread spectrum communications are only permissible in the US on frequencies above 222 MHz. Uhm, it looks like the same declaration Pontius Pilate (see i.e. http://en.wikipedia.org/wiki/Pontius_pilate) released to the community 2010 years ago. Similar things occurred to Giordano Bruno, a phylosopher which has been evaporated in a public pyre some centuries ago by our local institutions. Of course we need to regulate the access to our bands. But should we need to comply with rules that has been written tens years ago? What forbid us to take on our shoulder the weight of experimenting something more modern than a RTTY technology which is based on what has been experimented almost one century ago? Are we cows? Should we not exploit the knowledges which matured in these last years? Should we be constrained to collect vacuum tube receivers and show them proudly to our retired friends? Should we ignore that a HF channel is a smart object with its delay and doppler spread. What kind of experiments could we do if we are allowed to make experiments which pretend we are still in the '60s? How could we claim that the amateur radio service could bring innovation in communications if we are not allowed to test our ideas? Questions. I'm just asking myself these simple questions. I'd be sad if they hurt someone sensitivity. That's not my scope. I'm just trying to imagine our future. 73s Nico / IV3NWV
