I have read a number of posts that seem to ask this question but have not been able to find a definitive answer. Here goes...

I understand that BPSK with NO pulse shaping would produce a signal in the time domain something like this: enter image description here

But, for wireless communication, this takes up a lot of bandwidth. Therefore the data is sampled to create a series of discrete pulses which are then used to create a 2 PAM signal (i.e. pulse shaping is applied). The 2 PAM signal is then used to modulate a continuous carrier wave. The transmitted signal therefore looks something like this (the envelope in green is the 2 PAM signal):

enter image description here

I understand that multipath effects are problematic because the receiver can have trouble distinguishing a signal from its reflections. Slowing down the symbol rate helps, and therefore, so does OFDM.

When OFDM is used with BPSK, a stream of data is split into multiple parallel streams. Each stream is BPSK modulated using a carrier wave of a slightly different frequency to the others (these are carefully calculated to ensure they are orthogonal to each other). These modulated signals are then added together and transmitted as one radio signal. Because the carriers are orthogonal, the receiver can separate the individual BPSK signals and recover the data.

Here’s my question. When it comes to wireless data transmission, are unshaped BPSK signals multiplexed using OFDM (first image), or are pulse shaped BPSK signals multiplexed using OFDM (second image). Indeed, when OFDM is used in conjunction with QPSK or QAM, are the QPSK or QAM signals pulse shaped before being multiplexed, or not?

  • $\begingroup$ Could you please refer to these posts that did not contain a definite answer? Posts I find like this, and all block diagrams of OFDM transmitters I can find are quite explicit in what they say about pulse shaping. $\endgroup$ Dec 11, 2023 at 9:45
  • $\begingroup$ I think you're hurting your own understanding with the "in OFDM, BPSK signals are modulated to "carefully chosen" subcarriers". It's really not like the input are multiple time-domain signals; it's just a vector of symbols. So, neither of your two figures is correct: there's no "wave" prior to the IDFT. $\endgroup$ Dec 11, 2023 at 9:47
  • $\begingroup$ Thanks for responding. You are quite right, I am struggling to understand. If there is no wave prior to IDFT, then in what sense is BPSK relevant when used in conjunction with OFDM? $\endgroup$
    – Drummy
    Dec 11, 2023 at 10:22
  • 2
    $\begingroup$ because it defines the kind of symbols you put into the symbol vectors. In the case of BPSK, these would be $\{-1, +1\}$. $\endgroup$ Dec 11, 2023 at 10:24


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