ORTHOGONAL FREQUENCY DIVISION MULTIPLE ACCESS (OFDMA) - SINGLE-CARRIER FDMA (SC-FDMA)

ORTHOGONAL FREQUENCY DIVISION MULTIPLE ACCESS

It is a multi user version of digital modulation scheme.it works by assigning subsets of subcarriers to individual users.This technique has following advantages over CDMA:

1. It can combat multipath interference and it is less complex.
2. It can achieve a higher MIMO spectral efficiency.

Principle—  because of feedback information about the channel conditions, user to subcarrier assignment can be achieved. In this different number of subcarriers can be assigned to different users in view to support Quality of service that means to control data rate and error probability for each user.This is achieved by combining OFDM with time division multiple access. In this constant and shorter delay can be achieved. It is considered to be highly suitable for broadband wireless networks due to its advantagesincluding scalability and MIMO and it ability of channel frequency selectivity.It is used in Wimax and 3GPP LTE. High speed OFDM packet acess (HSOPA) now named as Evolved umts terrestrial radio access (E-UTRA).

  SINGLE-CARRIER FDMA (SC-FDMA)

It is a frequency- division multiple access scheme. It deals with the assignment of multiple users to a shared communication resorce.  It has drawn great attention as an attractive alternative to OFDMA. It has been adopted as the uplink multiple access scheme in 3GPP LTE or E-ULTRA ( evolved ultra). The major advantage of SC-FDMA is low PAPR (peak-to-average power ratio).

Transmission of SC-FDMA is very similar to OFDMA. In this, for each user the sequence of bits transmitted is mapped in symbols (BPSK,QPSK).

In SC-FDMA multiple access among users is made possible by assigning different users different set of sub carriers. The distinguishing feature  of SC-FDMA is that it leads to single carrier transmit signal in contrast to OFDMA which is multi carrier transmission scheme.

Advantages:

Low PAPR, low sensitivity to carrier frequency offset, greater robustness against spectra nulls, less sensitive to non linear distortion and hence, it allows the use of low cost power amplifiers.