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
