st case, both terminals map 16-QAM (quadrature amplitude modulation) data signals directly onto the inner OFDM transmitter (i.e., without DFT precoding). While top and bottom traces show I1 and Q2 signals of the first and second terminals (subcarrier index on horizontal axes), respectively, the full 16-QAM constellations are typically rotated by less than 5° for each terminal at the receiver. Because the lag in the fine synchronization loop at the terminal is 0.5 ms, the residual CFO is smaller than 3 Hz under static laboratory conditions when frequency advance at both terminals is activated. The residual phase drift is a random process that must be corrected instanta- neously at the base station, for each terminal individually. As mentioned previously, the residual offset is evaluated instantaneously as a part of the uplink channel estima- tion procedure. The offset values for all resource units are delivered from the DSPs to the FPGA as a vector, where they are corrected out of the user signals after the separation in the frequency domain (i.e., immediately after the MMSE detector). 11.1.4 DFT Precoding In order to reduce the peak-to-average-power-ratio (PAPR) (especially for cell-edge users), 3GPP-LTE agreed on DFT precoding at the UE and a matching IDFT at the BS. The resulting signal envelope in the time domain shows a significantly reduced PAPR, especially for low-order modulation levels (e.g., binary phase shift keying [BPSK] and quadrature phase shift keying [QPSK]) compared to standard OFDM. For the demo system, we focused on localized subcarrier allocation [4]; nevertheless, distributed subcarrier allocation similar to interleaved frequency divi- sion multiple access (IFDMA) can also be supported with slight changes in the © 2009 by Taylor & Francis Group, LLC Early Real-Time Experiments and Field Trial Measurements F-times F repetitions up-sampling of N-DFT output N-DFT Figure 11.5 A DFT with variable size can be realized by up-sampling using a fixed-size DFT. subcarrier mapping module that, in the loc