Difference between SC-FDMA and OFDMA
This page describes difference between SC-FDMA and OFDMA modulation techniques. Let us understand comparison between them with example of QPSK data symbols which need to be transmitted as shown in the figure-1. In LTE uplink, 12 contiguous subcarriers make one RB(Resource Block). In the figure-1, only 4 subcarriers across two symbols are only represented to demonstrate the difference between both the techniques.
Refer QPSK modulation to understand conversion of binary information bits which actually need to be transmitted to the complex qpsk data symbols.
Following points summarizes difference between SC-FDMA and OFDMA from the figure-1.
• OFDMA transmits 4 qpsk symbols in parallel, one data symbol per subcarrier. SC-FDMA transmits qpsk symbols in the series but at 4 time the rate compare to OFDMA. Here qpsk symbol occupy much wider bandwidth about M x 15KHz where M is no. subcarriers.
• From the figure it is imperative that OFDMA is multi-carrier system with one data symbol carried over by one subcarrier; while SC-FDMA is a single carrier system where in each qpsk symbol is carried by one much wider bandwidth subcarrier. Refer difference between SC vs OFDM page to understand concepts of Single Carrier(SC) vs OFDM.
• Though symbol length remains same in both OFDMA and SC-FDMA which is about 66.7µS; SC-FDMA symbol contains more than one sub-symbols which represents qpsk data symbols.
• Parallel multiple data symbol transmission will lead to higher PAPR(Peak to Average Power Ratio) in the OFDMA system. In SC-FDMA, PAPR is same as that of original qpsk data symbols as M qpsk data symbols are transmitted in series at M times rate compare to OFDMA. Refer PAPR article to understand Peak to Average Power Ratio.
Figure-2 depicts the block diagram of SC-FDMA and OFDMA system. Figure mentions modules unique to the SC-FDMA and in common with OFDMA system. As mentioned DFT before IFFT of OFDMA and IDFT after DFT of OFDMA system make it SC-FDMA. Refer Difference between SC-FDMA and OFDM.
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