OFDMA vs SC-FDMA

The multiple access of the radio channel in LTE is based on OFDMA (Orthogonal Frequency Division Multiple Access) in the DL and SC-FDMA (Single-Carrier Frequency Division Multiple Access) in the UL. These two techniques have a number of similarities but also some major differences as the UL and DL have different issues to cope with.

Both multiple access techniques aim at spreading the narrowband data signal over a higher bandwidth to provide high data rates and cope with the frequency selective radio channels. In order to achieve that the wideband channel is split into a number of narrowband subcarriers and each user is allocated subcarriers which are not fading for the user’s particular radio channel. The data for this user is then transmitted using these subcarriers only. The main advantage of OFDMA and SC-FDMA over other multicarrier techniques is that the subcarriers are orthogonal to each other allowing higher spectrum efficiency.

Let's first analyse the OFDMA technique which is a bit more standard than the SC-FDMA. The signal processing chain is shown in the next figure:




Each data symbol (QAM symbol) modulates a single narrowband subcarrier (15kHz). All these subcarriers are orthogonal to each other in order to avoid interference. All the modulated symbols are transmitted simultaneously over the air.



Now the superposition of all the modulated subcarriers is a noise-like signal according to the central limit theorem. Thus the variation of the signal amplitude is quite high leading to a high Peak-to-Average-Ratio (PAPR). This requires expensive power amplifiers and high power transmission which is acceptable for the eNB but not desirable for the UE.

Solution to this problem is the introduction of SC-FDMA which is a similar technique offering the same advantages as OFDMA without the high PAPR issue. In SC-FDMA each data symbol modulates the whole used wideband carrier instead of a narrowband one and the modulated symbols are transmitted sequentially over air. Thus the final transmitted signal is a single carrier one unlike OFDMA where the final signal is the superposition of a great amount of carriers.





It is obvious from the figures that the two techniques transmit the same amount of data symbols in the same time period and using the same bandwidth. However in SC-FDMA the UE needs to transmit only one carrier (wideband) at a time containing the information of one data symbol, while in OFDMA a number of subcarriers (narrowband) need to be transmitted at each time period. Thus the SC-FDMA technique is more power efficient.

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