subframe. The switching process between uplink and downlink configurations is also specified in [13]. The uplink-downlink configurations with both 5 ms and 10 ms downlink-to-uplink switch-point periodicity are supported. In case of 5 ms downlink-to-uplink switch-point periodicity, the special subframe exists in both half-frames. In case of 10 ms downlink-to-uplink switch-point periodicity, the special subframe exists in the first half-frame only. Subframes 0 and 5 and DwPTS are always reserved for downlink transmission. Subframe 6 in configuration with 10ms switch-point periodicity consists of DwPTS only. UpPTS and the subframe immediately following the special subframe are always reserved for uplink transmission as shown in Table 3.1. One radio frame, Tf = 307200Ts = 10 ms One half-frame, 153600 7 Ts=5 ms One slot, Tslot=15360Ts 30720Ts Subframe #0 Subframe #2 Subframe #3 Subframe #4 Subframe #5 Subframe #7 Subframe #8 Subframe #9 One subframe, 30720T S DwPTS GP UpPTS DwPTS GP UpPTS Figure 3.5 Frame structure type 2 (for 5 ms switch-point periodicity). Reproduced with permission from C 2009 3GPP [13] 3GPP Evolved Packet System (EPS) Table 3.1 Uplink-downlink configurations Downlink-to-Uplink Subframe number Uplink-downlink configuration Switch-point periodicity 10 ms 10 ms 10 ms Source: © 2009 3GPP [13] 3.5.3 Downlink Transmission The downlink transmission scheme is based on conventional OFDM using a cyclic prefix. The OFDM sub- carrier spacing is of = 15 kHz. The transmitted signal in each slot is described by a resource grid of sub- carriers and OFDM symbols. The resource grid and structure for a downlink slot is illustrated in Figure 3.6. The basic element in the resource grid is called a resource element which corresponds to a single subcarrier associated with an antenna port. One, two or four transmit antenna ports are supported. A resource block is defined as NDL symb consecutive OFDM symbols in the time domain and NRB SC consecutive sub- carriers in the frequency domain. Thus, a resource block consists of