OFDM和OFDMA的區別
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將所有K個子載波用於承載一個用戶的數據包，在某一時隙，只有一個用戶在該載波上可以發送數據，如果該載波需服務於多個用戶，則需排隊。
OFDMA則可以在同一時隙將不同的子載波分給不同的用戶。
Now, the figures below maybe help you to understand the two communication techniques.
OFDMA與SC-OFDMA的區別
3GPP定義的LTE空中接口，在下行采用OFDMA（正交頻分多址）技術，在上行采用的SC-FDMA（單載波頻分多址）。
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的多載波在頻域疊加引起的），從而提高移動終端的功率發射效率，並延長電池的使用時間，降低終端成本。
SC-FDMA Scheme 比OFDMA 多了DFT@TX (或IDFT@RX). 圖標黃block.因此 SC-FDMA也被叫做線性預編碼OFDMA技術
從上圖例子看出，發1個OFDMA Sym就並行發一次, 一次持續時間66.7us. 而發SC-FDMA Sym要串行發4次, 4次加一起也是66.7us，每一次發一個60kHz帶寬的modulation symbol.也就是右圖正面先發的深灰色4個峰連一起的BW，然后是灰色，然后深藍然后藍.
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