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Physical Interface

3GPP describes LTE radio access technology is described as follows:

The multiple access scheme for the LTE physical layer is based on Orthogonal Frequency Division Multiple Access (OFDMA) with a Cyclic Prefix (CP) in the downlink and a Single Carrier Frequency Division Multiple Access (SC-FDMA) with CP in the uplink.

OFDMA is particularly suited for frequency selective channel and high data rate. It transforms a wideband frequency selective channel into a set of parallel flat fading narrowband channels, thanks to the CP. This ideally allows the receiver to perform a low complexity equalization process in the frequency domain, i.e., 1 tap scalar equalization.
The baseband signal representing a downlink physical channel is defined in terms of the following steps, as shown in Figure 1:

  • Scrambling of coded bits in each of the code words to be transmitted on a physical channel
  • Modulation of scrambled bits to generate complex-valued modulation symbols
  • Mapping of the complex-valued modulation symbols onto one or several transmission layers
  • Precoding of the complex-valued modulation symbols on each layer for transmission on the antenna ports
  • Mapping of complex-valued modulation symbols for each antenna port to resource elements
  • Generation of complex-valued time-domain OFDM signal for each antenna port

Figure 1 :  Overview of Downlink Physical Channel Processing.

The baseband signal representing the physical uplink shared channel is defined in terms of the following steps, as shown in Figure 2:

  • Scrambling
  • Modulation of scrambled bits to generate complex-valued symbols
  • Transform precoding to generate complex-valued symbols
  • Mapping of complex-valued symbols to resource elements
  • Generation of complex-valued time-domain SC-FDMA signal for each antenna port

Figure 2 :  Overview of Uplink Physical Channel Processing.