| Passband modulation |
|---|
| Analog modulation |
| AM · SSB · QAM · FM · PM · SM |
| Digital modulation |
| FSK · MFSK · ASK · OOK · PSK · QAM MSK · CPM · PPM · TCM · SC-FDE |
| Spread spectrum |
| CSS · DSSS · FHSS · THSS |
| See also: Demodulation, modem, line coding, PAM, PWM, PCM |
Single-carrier FDMA (SC-FDMA) is a frequency-division multiple access scheme. Like other multiple access schemes (TDMA, FDMA, CDMA, OFDMA), it deals with the assignment of multiple users to a shared communication resource. SC-FDMA can be interpreted as a linearly precoded OFDMA scheme, in the sense that it has an additional DFT processing preceding the conventional OFDMA processing. Please see the diagram below. 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 SC-FDMA, multiple access among users is made possible by assigning different users, different sets of non-overlapping fourier-coefficients (sub-carriers). This is achieved at the transmitter by inserting (prior to IFFT) silent fourier-coefficients (at positions assigned to other users), and removing them on the receiver side after the FFT.
The distinguishing feature of SC-FDMA is that it leads to a single-carrier transmit signal, in contrast to OFDMA which is a multi-carrier transmission scheme. Owing to its inherent single carrier structure, a prominent advantage of SC-FDMA over OFDM and OFDMA is that its transmit signal has a lower peak-to-average power ratio (PAPR). Intuitively, the reasoning lies in the fact that while in OFDM the transmit symbols directly modulate the multiple sub-carriers, in SC-FDMA the transmit symbols are first preprocessed by a DFT block.[1]
In OFDM as well as 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 easily combated. The advantage is that FFT and frequency domain equalization requires less computation power than the conventional time-domain equalization.
A related concept is the combination of a single carrier transmission with the single-carrier frequency-domain-equalization (SC-FDE) scheme. [2] The single carrier transmission, unlike SC-FDMA and OFDM employ no IFFT or FFT at transmitter, but introduce the cyclic prefix to transform the linear channel convolution into a circular one. After removing the cyclic prefix at receiver, an FFT is applied to arrive in the frequency domain, where a simple single-carrier frequency-domain-equalization (SC-FDE) scheme can be employed, followed by the IFFT operation.
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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 and terminal costs. It has been adopted as the uplink multiple access scheme in 3GPP Long Term Evolution (LTE), or Evolved UTRA.[3][4][5]
The performance of SC-FDMA, in relation to OFDMA has been the subject of various studies.[6][7][8] Although the performance gap is not much, SC-FDMA's additional advantage of low PAPR makes it a favorite especially for uplink wireless transmission in future mobile communication systems where transmitter power efficiency is of paramount importance.
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