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Marcus Müller
Marcus Müller
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Obviously, we're not talking about "slow" local serial buses (thinking of, say, a 112 kbd UART connection between a microcontroller and a thermal printer, or a 12 Mb/s USB connection), because, well, there's no need to be bettersmarter (read: more expensive) than "hey, high means 1, low means 0, and here's a bit of framing" if even the worst connection suffices for that.

This is not really a definition or anything, but the moment you have to modulate onto a carrier wave, your device usually stops being called a "serial transceiver" unit or such, and becomes a Modem ("modulator-demodulator", if you wonder where that term comes from). And there's plenty of wired communications over a single wire (so, technically, "serial", but nobody would call them that) that incorporate phase to transport information. The1980 Bell 201A telephone modem at 600 Bd => 1200 b/s uses QPSK, but was to my knowledge the only telephone modem generation that did that – the channel just doesn't fit that very well; you have an SNR that is better than you need for QPSK on a telephone line, if you care enough to equalize, channel code and map with the same effort as needed for high-rate QPSK transmission. Most post-1980 telephone modems go for 16-QAM or higher modulations.

Obviously, we're not talking about "slow" local serial buses (thinking of, say, a 112 kbd UART connection between a microcontroller and a thermal printer, or a 12 Mb/s USB connection), because, well, there's no need to be better than "hey, high means 1, low means 0, and here's a bit of framing" if even the worst connection suffices for that.

This is not really a definition or anything, but the moment you have to modulate onto a carrier wave, your device usually stops being called a "serial transceiver" unit or such, and becomes a Modem ("modulator-demodulator", if you wonder where that term comes from). And there's plenty of wired communications over a single wire (so, technically, "serial", but nobody would call them that) that incorporate phase to transport information.

Obviously, we're not talking about "slow" local serial buses (thinking of, say, a 112 kbd UART connection between a microcontroller and a thermal printer, or a 12 Mb/s USB connection), because, well, there's no need to be smarter (read: more expensive) than "hey, high means 1, low means 0, and here's a bit of framing" if even the worst connection suffices for that.

This is not really a definition or anything, but the moment you have to modulate onto a carrier wave, your device usually stops being called a "serial transceiver" unit or such, and becomes a Modem ("modulator-demodulator", if you wonder where that term comes from). And there's plenty of wired communications over a single wire (so, technically, "serial", but nobody would call them that) that incorporate phase to transport information. The1980 Bell 201A telephone modem at 600 Bd => 1200 b/s uses QPSK, but was to my knowledge the only telephone modem generation that did that – the channel just doesn't fit that very well; you have an SNR that is better than you need for QPSK on a telephone line, if you care enough to equalize, channel code and map with the same effort as needed for high-rate QPSK transmission. Most post-1980 telephone modems go for 16-QAM or higher modulations.

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Marcus Müller
Marcus Müller
  • 100.1k
  • 5
  • 141
  • 262

Obviously, we're not talking about "slow" local serial buses (thinking of, say, a 112 kbd UART connection between a microcontroller and a thermal printer, or a 12 Mb/s USB connection), because, well, there's no need to be better than "hey, high means 1, low means 0, and here's a bit of framing" if even the worst connection suffices for that.

BPSK like that exists in the form NRZ; in that case, the sine wave is at f=0 Hz, but it's just a special case.

The modulation with a non-zero-frequency sine only brings complexity and no real benefits, far as I can tell:

  1. NRZ can be received with discrete-amplitude components only, trivially
  2. NRZ only has a DC component problem if you send long runs of identical data,
    • which you don't do, anyways, that's why every system that avoids DC in a local bus typically has line coding that incorporates a balancing scheme, see, for example 8b/10b
  3. You increase the bandwidth by at least a factor of 2: you really want to choose a sine wave whose frequency is higher than the symbol rate (I challenge you to build a good detector for 1/4 circle phase jumps every < 1/2 oscillation; you need to sample that at which rate? Draw the spectrum of your proposed system with an f_carrier < baud rate and sinc pulse shape in frequency / rectangular pulse shape in time domain to see the problem.)
    • this might still be desirable in incredibly specific cases where your channel has e.g. randomly appearing spectral notches in a < baud rate regime. But I'd really have to sit down, and do a lot of brain storming to come up with a problem that would result in this. And even then, it's really not obvious to me that PSK would be even remotely the solution of choice there - to me it would sound like technically, you'd just up the baud rate and add line and channel coding, in practical scenarios.

This is more prone to noise and bitrate is higher.

Only as you choose the carrier high enough to allow for orthogonal I and Q, you get twice the channel usage per unit bandwidth, but that's explicitly not what you wanted to do.

The technical reason this is commonly not done is that your receiver needs to be coherent. You can demodulate most baseband buses simply after recovering the symbol clock – no need to also recover the carrier frequency and carrier frequency phase. So, technically, PAM-4 can be easier to implement than QPSK, and gets the same bit/s/Hz. PAM good enough for Fast, Gigabit Ethernet over copper (PAM-5), 2.5, 5 and 10 Gigabit Ethernet over copper wire (PAM-16), and these are already all long-haul links, where the cost-per-lane can be a bit higher than for most serial buses. In PCI Express, gegenerations 1.0 through 5.0 use NRZ, and 6.0 uses PAM-4 – admittedly, with a lot of equalization, but still, this is incoherent reception, allowing a receiver to work relatively quickly.

This is not really a definition or anything, but the moment you have to modulate onto a carrier wave, your device usually stops being called a "serial transceiver" unit or such, and becomes a Modem ("modulator-demodulator", if you wonder where that term comes from). And there's plenty of wired communications over a single wire (so, technically, "serial", but nobody would call them that) that incorporate phase to transport information.