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Thread: Why OFDMA in DL is used & SC-FDMA in UL? Why OFDMA is not used in UL

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    Question Why OFDMA in DL is used & SC-FDMA in UL? Why OFDMA is not used in UL

    Can everyone explain why different Access method is used in DL (OFDMA) & UL (SC-FDMA)? Why OFDMA is not used in UL?

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    It is the vendors who decision how to allocate the resource. The difference here is that for downlink (DL), you have several ways to allocate the resource, but for uplink there is only one way to do it in LTE. For uplink you need to allocate continuous RBs to a UE (e.g. Release 8 specified 1 group. By Release 10 maximum specified 2 groups; so check Releases carefully). But for downlink, you have more ways to do it.

    As you know SC-FDMA means 'single carrier' (QAM modulation on one carrier). Assuming 64QAM can result ~7dB peak to average. However, OFDM you have N carriers each with QAM, so the peak of the combined carriers can be much higher and may even reach as high as 17dB.

    Something else to consider which helped me understand SC-FDMA/OFDM. For OFDMA transmission, the range of values for the input bits to the IFFT is the same as the output bits for DFT (which are also input bits to the IFFT) for SC-FDMA. The key difference is that the control and data bits that come from the MAC layer are not truly random, but highly correlated. This means that for OFDM, the probability that a string of bit values coming from the MAC layer requires peak power to transmit can be orders of magnitude higher than if the string of bit values was truly random. It is this expectation on transmitting at peak power that is the numerator in PAPR. We can't randomize the bit values from the MAC layer, because the bits are required to have low entropy for periods of time, due to static control and data values. Instead, the SC-FDMA solution is to prepend the IFFT with a DFT. The DFT raises the entropy of the input bits to the IFFT in a deterministic, invertible way. This reduces the expectation on peak power being required by the transmitter, and therefore reduces the PAPR.

    Last edited by Parmar; 05-08-2018 at 01:41 PM.

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