The main advantage of SC is its lower peak-to-average power ratio than OFDM.
Single-carrier FDMA (SC-FDMA) is frequency-division multiple access scheme. It is a multi-user version of Single-carrier frequency-domain-equalization (SC-FDE) modulation scheme. SC-FDE can be viewed as a linearly precoded OFDM scheme, and SC-FDMA can as a linearly precoded OFDMA scheme, henceforth LP-OFDMA. Or, it can be viewed as a single carrier multiple access scheme. One prominent advantage over conventional OFDM and OFDMA is that the SC-FDE and LP-OFDMA/SC-FDMA signals have lower peak-to-average power ratio (PAPR) because of its inherent single carrier structure. It has been proven that perfect constant modulus transmission is achievable.
Just like in OFDM, guard intervals with cyclic repetition are introduced between blocks of symbols in view to efficiently eliminate time spreading (caused by multi-path propagation) among the blocks. In OFDM, fast Fourier transform (fft) is applied on the receiver side on each block of symbols, and inverse fft (ifft) on the transmitter side. In SC-FDE, both fft and ifft are applied on the receiver side, but not on the transmitter side. In SC-FDMA, both fft and ifft are applied on the transmitter side, and also on the receiver side.
In OFDM as well as SC-FDE and SC-FDMA, equalization is achieved on the receiver side after the fft calculation, by multiplying each Fourier coefficient by a complex number. Thus, frequency-selective fading and phase distortion can be combated. The advantage is that fft and frequency domain equalization requires less computation power than conventional time-domain equalization.
In SC-FDMA, multiple access is made possible by inserting silent fourier-coefficients on the transmitter side before the ifft, and removing them on the receiver side before the ifft. Different users are assigned to different fourier-coefficients (sub-carriers).
LP-OFDMA/SC-FDMA has drawn great attention as an attractive alternative to OFDMA, especially in the uplink communications where lower PAPR greatly benefits the mobile terminal in terms of transmit power efficiency. It is currently a working assumption for uplink multiple access scheme in 3GPP Long Term Evolution (LTE), or Evolved UTRA.
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Orthogonal frequency-division multiplexing (OFDM) — essentially identical to Coded OFDM (COFDM) and Discrete multi-tone modulation (DMT) — is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method. A large number of closely-spaced orthogonal sub-carriers are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.
OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, wireless networking and broadband internet access.
The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions — for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath — without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and eliminate intersymbol interference (ISI). This mechanism also facilitates the design of single-frequency networks, where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-carrier system.