NDLI: Figure 2.2-7. Ofdm and Ofdma Subcarrier Allocation 2.3 Single-carrier Frequency Division Multiple Access

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Figure 2.2-7. Ofdm and Ofdma Subcarrier Allocation 2.3 Single-carrier Frequency Division Multiple Access

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Abstract	With standard OFDM the subcarrier allocations are fixed for each user and performance can suffer from narrowband fading and interference. OFDMA incorporates elements of Time Division Multiple Access (TDMA) so that the subcarriers can be allocated dynamically among the different users of the channel. The result is a more robust system with increased capacity. The capacity comes from the trunking efficiency gained by multiplexing low rate users onto a wider channel to provide dynamic capacity when needed, and the robustness comes from the ability to schedule users by frequency to avoid narrowband interference and multipath fading. The high Peak-to-Average Power Ratio (PAPR) associated with OFDM led 3GPP to look for a different modulation scheme for the LTE uplink. SC-FDMA was chosen since it combines the low PAPR techniques of single-carrier transmission systems, such as GSM and CDMA, with the multipath resistance and flexible frequency allocation of OFDMA. A mathematical description of an SC-FDMA symbol in the time domain is given in 36.211 [6] sub-clause 5.6. A brief description is as follows: data symbols in the time domain are converted to the frequency domain using a Discrete Fourier Transform (DFT); once in the frequency domain they are mapped to the desired location in the overall channel bandwidth before being converted back to the time domain using an Inverse FFT (IFFT). Finally, the CP is inserted. SC-FDMA is sometimes called Discrete Fourier Transform Spread OFDM (DFT-S-OFDM) because of this process, although this terminology is becoming less common.
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