Why it wasn't possible to use OFDMA for uplink in LTE? How SC-FDMA working is different from OFDMA BPSK ?

Question

PAPR is the major disadventage of OFDMA which require high resolution D/A and A/D or increasing Transmission Power.
I know that SC-FDMA used on Uplink becuase it has lower PAPR than OFDMA. On downlink direction, the high PAPR Problem of OFDMA solved by increasing eNB transmission power. This solution cannot be implemented on Uplink direction because UL is power limited link. so SC-FDMA used on UL instead of OFDMA, but why? why SC-FDMA has LOWER PAPR

Answer 1

Adding some more points in above answer.

One of the major drawbacks of an OFDMA system is that the transformation of a complex symbol mapped sequence (e.g. BPSK, QPSK, etc.) onto a small set of sub-carriers produces time sequences that have high PAPR (Peak-to-Average Power Ratio).

PAPR is the ratio between the maximum power and the average power This results in requirements for expensive transmission amplifiers and furthermore lead to high power consumption. Both effects are -particularly on terminal side unwanted.

It is thus a major design goal to limit this effect for the UL direction.

In order to reduce the PAPR a variant of OFDMA is used. It is called SC-FDMA (Single Carrier Frequency Division Multiple Access). SC-FDMA works according to the following mechanism, described for the associated transmitter structure. SC-FDMA is the method of choice for EUTRAN in the uplink direction.

Answer 2

I am not a PHY expert but let me try to explain based on what i know.

OFDM has a disadvantage that it has a high PAPR Peak to Avg Power Ratio, which makes the receiver design complex and is prone to ISI in sub carriers in case of multi path propagation. This makes it impractical to be used for the UE radio design.

In OFDM the data symbols modulate the sub carriers independently whereas in SC FDMA the modulated signal is a linear combination of all data symbols transmitted one after the other within the time instant.

Comments

Thank you...

Answer 3

OFDMA requires more power for transmission whereas SC-FDMA requires lesser power. There is a limitation at UE to send more power due to limitation of battery size. Hence SC-FDMA is used for UL by UEs.