OFDM和OFDMA的区别
OFDM（orthogonal frequency division multiplexing)，which assigns one block (in time ) to one user,
OFDMA (orthogonal frequency division multiple access), which assigns different groups of subcarriers (in frequency) to different users.
即：OFDM将所有K个子载波用于承载一个用户的数据包，在某一时隙，只有一个用户在该载波上可以发送数据，如果该载波需服务于多个用户，则需排队。
OFDMA则可以在同一时隙将不同的子载波分给不同的用户。
Now, the figures below maybe help you to understand the two communication techniques.
OFDMA与SC-OFDMA的区别
3GPP定义的LTE空中接口，在下行采用OFDMA（正交频分多址）技术，在上行采用的SC-FDMA（单载波频分多址）。
相比OFDMA，SC-FDMA的PAPR（峰值/平均功率比，peak-to-average power ratio）比低1-3dB左右（PAPR高是OFDM的多载波在频域叠加引起的），从而提高移动终端的功率发射效率，并延长电池的使用时间，降低终端成本。
SC-FDMA Scheme 比OFDMA 多了DFT@TX (或IDFT@RX). 图标黄block.因此 SC-FDMA也被叫做线性预编码OFDMA技术
从上图例子看出，发1个OFDMA Sym就并行发一次, 一次持续时间66.7us. 而发SC-FDMA Sym要串行发4次, 4次加一起也是66.7us，每一次发一个60kHz带宽的modulation symbol.也就是右图正面先发的深灰色4个峰连一起的BW，然后是灰色，然后深蓝然后蓝.
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