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The other days I read a useful article (on BER v.s. Eb/N0 curve of ofdm) whose link is as bellow. While watching the plot inside, a question was arisen in my mind. I wonder if the BER v.s. Eb/N0 curve of BPSK alone (without using OFDM) and one using OFDM could be the same in both AWGN and fading (flat and selective) channels? If so, what’s the use of applying OFDM to BPSK alone (expect the simpler equalizer for OFDM case)?

http://www.dsplog.com/2008/08/26/ofdm-rayleigh-channel-ber-bpsk/

Best Regards

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  • $\begingroup$ The BER should be the same. OFDM just maps a single high-rate modulated stream into multiple parallel lower-rate streams, so you should expect the same bit-error performance characteristics (note that for white noise, $\frac{E_b}{N_0}$ will not change between the two cases). You hit on the primary benefit: simple equalization. In addition, you gain flexibility in allocating subcarriers as needed, using different modulation schemes on different subcarriers, etc. $\endgroup$ – Jason R May 12 '14 at 12:32
  • $\begingroup$ @JasonR In the frequency selective fading case, if you don't use ofdm and instead send BPSK itself won't the error increase. The equaliser gets extremely complicated for the frequency selective fading case and due to that there could be a degradation in performance(i.e.increase in BER). Correct me if I am wrong. $\endgroup$ – Karan Talasila May 12 '14 at 12:57
  • $\begingroup$ BPSK + OFDM is no improvement of BPSK on both rayleigh channel and AWGN but ofdm + QAM 16 is an improvemnet on QAM 16 alone on both channels, can someone please explain this?? $\endgroup$ – user10733 Aug 7 '14 at 16:30
  • $\begingroup$ Welcome to dsp.se! This seems to be a question. Please post it as such (and add a reference to that statement when you do so) $\endgroup$ – Deve Aug 7 '14 at 17:58
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In a pure AWGN channel there is no benefit from OFDM because the noise will affect the received signal in the same way regardless of whether sent as a higher bandwidth serial stream or lower rate parallel streams. Similarly, there isn't much benefit in a flat fading environment either because, by definition, each constituent stream of the OFDM signal will be affected by the same amplitude change and linear phase shift.

The main benefit from OFDM comes with frequency selective fading. That is because the effects of frequency selective fading worsen as signal bandwidth increases. With OFDM, each parallel low rate signal experiences a flat fading channel instead a frequency selective channel at the full bandwidth - i.e. each individual channel can be represented by a single complex value instead a multi-tap filter. That means frequency selective equalization is easily addressed, which allows us to scale to much higher bandwidths - and everybody wants higher bandwidth.

Note that the question refers to BER only, so other OFDM related characteristics such as spectrum mask or resource allocation are left out.

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As far as I know. OFDM only helps because of the simpler equalizer. However, it is possible to structure your time domain BPSK data (in non-OFDM case) so that you can use a frequency domain equalizer and get much if not all of the advantages of OFDM. There are a few papers that basically state this. OFDM has much better advantages for >16 QAM than it would for BPSK

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