Understanding Phase Shift Keying 

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Glad it helped - thanks for the feedback!

The symbol rate is simply the rate at which "symbols" are sent or interpreted (i.e. symbols per second) - there's no requirement that the symbol characteristics (frequency, amplitude, and/or phase) change between symbol times. The problem is that both sides have to agree on the symbol rate -- did I get three zeros in a row or did I get one (long) zero? One way to do this is simply by having both ends agree (beforehand) that they will use a certain symbol rate. The other way is to have a way of determining or deriving the symbol rate from the pattern of the received symbols -- e.g. using some kind of preamble, synchronization, or training sequence. In many simple BPSK based systems (like POCSAG paging), the two ends agree on the bit / symbol rate ahead of time (2400, 4800, 9600, etc.). There are advantages to having symbols change every symbol period even if the bit value doesn't change (e.g. the discussion on DPSK in this video). And in some systems, the user data bits are "scrambled" (in a predictable and recoverable way) in order to ensure a certain minimum number of bit (and therefore symbol) transitions per unit time -- i.e. avoiding long runs of ones or zeros, so to speak. Whether or not these schemes are used depends on the particular application, but they certainly aren't uncommon. It's an excellent question and one that I'm actually planning to address in an upcoming video, so thanks for asking!

In BPSK the phase shift indicates a state change from the current bit level. No phase change means new bit is the same as the last bit. The initial bit state must be defined by the RF protocol, or fixed on the back end by inverting the bitstream.

Hi John, We've received your question, and we are routing it to one of our experts to assist you. We'll get back to you soon!

Hi John -- thanks for the question. You are correct that all RF is an oscillating waveform. That said, there are numerous ways of representing a "low" state with an oscillating waveform. For example, in ASK (amplitude shift keying), a "low" state is usually the lack of the oscillating waveform (!). In FSK (frequency shift keying), the "low" or "zero" state is usually the lowest discrete frequency used, and in PSK, a "low" state is somewhat arbitrarily defined as one of the phase states. In all cases (except the on-off ASK keying), an oscillating RF waveform is always present. Hope that helps!

The cases I have worked with in regards to BPSK is in RADAR pulsed RF applications. Despite the change in phase, the amplitude & frequency at 0 and 180 degrees will be the same. Except for the brief moment that it passes near the origin. The signal(sine wave) never stops radiating until the end of the pulse period. The easiest examples to understand are barker codes. Say you had an 11 bit barker code represented as +++---+--+- and a pulse duration of 11us. The pulse is active for 11us with the first 3us as 111 next 3us as 000. And so forth.

yea same, from what i gathered from my resources, DQPSK involves differential encoding and decoding of the signal to determine phase differences between symbols, the DQPSK scheme shown in the video looks more like pi/4 QPSK

Good catch - yes, that is a typo and you are correct. Thanks!

Yes, higher bit rates generally require higher order modulation (and/or wider bandwidths). The newer "flavors" of 802.11 achieve even higher bitrates in part though even higher order modulation.