Terminal 2 OFDM symbol index Additional pilots for phase tracking Per-antenna phase drift estimation Figure 11.3 Local downlink (left) and uplink (right) pilot grids. 11.1.2.2.1 Downlink Pilots Our local downlink pilot grid is shown in Figure 11.3 (left) for a single resource unit on both antennas. A code-multiplexed approach is used along the time axes and each fourth subcarrier is used. By this spacing, pilots get placed at the edges of resource units; this aids localized interpolation. Antennas are identified by sequences with a length of four along the time axes taken from an orthogonal set. Additional scrambling is applied along the frequency axes. Correlation over mul- tiple OFDM symbols along the time axes [2] is applied prior to the interpolation along the frequency axes. This is again carried out individually for each pair of transmit and receive antennas. 11.1.2.2.2 Frequency and Time Synchronization in the OFDM Downlink A coarse frequency estimation is performed at the user equipment (UE) before the fast Fourier transform (FFT) based on a special preamble transmitted in TTI 0. The remaining center frequency offset between the base station (BS) and the UE is measured in the frequency domain after FFT, exploiting the dedicated phase tracking pilots from the channel estimation and maximum ratio combiner (MRC) detection. Using different loop feedback, the remaining carrier frequency offset (CFO) can be reduced down to a few tens of hertz in a static environment at SNR above 10 dB. The OFDM frame synchronization is based on another special preamble using a Schmidl-Cox correlator before FFT at the UE. For the current single-user/close- range scenario, no timing advance estimation or compensation was required to be implemented. For the sake of robustness, the synchronization preambles are © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology transmitted with cyclic delay diversity (CCD) in order to sum up the power of the two transmit antennas without suffering from