UL Allocation (SC-FDMA) in LTE

  • SC-FDMA utilizes single carrier modulation, DFT-spread orthogonal frequency multiplexing, and frequency domain equalization.
  • LTE uses OFDMA on the DL. It allocates DL bandwidth to the user based on resource blocks, rather than individual subcarriers.
  • When the signal is a combination of subcarriers, high amplitude power peaks can occur. The base stations power amplifiers are designed to handle the resulting high peak-toaverage power ratio. 
  • Although the eNodeB is fully capable of transmitting and receiving OFDMA symbols, the subscriber units must balance reduced complexity and lower transmit power requirements with support for high data rates and good QoS at cell boundaries. OFDMA is not used on the UL. 
  • Instead, SC-FDMA was selected for the UL to meet those requirements. SC-FDMA helps with peak-to-average power ratio
    reduction by adding extra encoding steps. 
  • In OFDMA, each data stream is transmitted in a separate subcarrier. However, with SCFDMA,
    multiple data streams are transmitted in the same subcarrier and symbol time.
  • The “sub-symbols” are spread over multiple subcarriers. Each SC-FDMA symbol
    contains 12 sub-symbols transmitted at the rate of 12 times 15 KHz. The following
    diagram illustrates this.
 OFDMA vs. SC-FDMA Subcarrier Coding
  • Unlike OFDMA, the SC-FDMA signal appears to be more like a single carrier, with each data symbol represented by a wide signal spanning the 12 subcarriers of the resource block.
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