A Survey of OFDMA and SC-FDMA in LTE System

The modern digital transmission systems offer better reliability in noise environment comparing to their analog counterparts. However, because of involvement of digital signals these systems are usually accompanied with a phenomenon known as inter symbol interference (ISI). In this phenomenon, the transmitted pulses are smeared out because of which the pulses corresponding to different symbols are not separable. The key reasons of ISI are multi-pathing and Doppler effect. In multi-pathing, the signal is received at same time through different paths of different delay which causes inter symbol interference. In addition, the wireless channel is usually dispersive. The result of data transmission over such a channel is that each received pulse is affected by adjacent pulse. This frequency-selective fading gives rise to ISI, where the received symbol over a given symbol period experiences interference from other symbols. As increasing signal power also increases the power of the ISI, this interference gives rise to an irreducible error floor that is independent of signal power. The irreducible error floor is difficult to analyze, as it depends on the ISI characteristics and the modulation format. The ISI characteristics further depend on the characteristics of the channel and the sequence of transmitted symbols.