ct to the Terms of Use as given at the website. Radio Engineering Concepts Radio Engineering Concepts The coherent-modulation performance improvement comes at the cost of having the extra pilot signal for coherent phase alignment. Thus we usually use coherent schemes when one transmitter is sending many signals so that they can share the pilot and non- coherent schemes otherwise. This is an important difference between the forward and reverse directions in a wireless telephone system, as discussed in Secs. 8.8 and 8.9. Older digital systems used amplitude shift keying (ASK) and frequency shift keying (FSK), where amplitude and frequency changes were used instead of the phase changes we have discussed here. 1.11 Adding Radio Signals Two radio waves combine by adding amplitudes, their voltages levels. (In the same way, audio sounds combine by adding their amplitudes, atmospheric pressure.) This sounds like an easy concept, but it is not always so simple. Sometimes one one is two, sometimes four, sometimes zero, and sometimes one. To borrow a line from George Orwell's 1984, sometimes it's all of them at once. The statistical average of adding uncorrelated voltages is a power sum. To put this in plainer English, if radio signals are not closely synchronized in some way, then their combined signal is the sum of their power levels. If a receiver receives 10 pW (pico- watts) from source A, 13 pW from source B, and 32 pW from source C, then the combined signal from these three uncorrelated sources will be the total, 55 pW (10 + 13 + 32 = 55. 12 One plus one is two for uncorrelated addition. In CDMA, we find ourselves paying a lot of attention to correlation and how signals are correlated. If two waves are of the same frequency in perfect phase, then adding their voltages increases power as the square of the voltage. We call this a coherent sum. Thus a 10-V, 2-W signal coherently added to another 10-V, 2-W signal yields a 20-V, 8-W radio signal. For coherent addition, one plus one is four. Should the frequencies be the sa