MIMO Figure 11.24 shows the sub-6-GHz experimental system. The system uses time division duplexing (TDD) with Cyclic Prefix OFDM (CP-OFDM) modulation in both the uplink and downlink. To enable shorter scheduling latency, the system uses a bidirectional control structure for the TDD frame, enabling transmitting and receiving of schedul- ing information in every subframe [28]. Adopting CP-OFDM is also beneficial in such frequent link direction switching because CP-OFDM waveforms are localized in the time domain compared to filter-bank multi-carrier (FBMC)-based waveforms, which are localized in the frequency domain but have a pre- and post-tail in the time domain, resulting in the need for a longer guard period (GP) for link direction switching [29]. The subframe structure is symmetric in the downlink and uplink SO that each subframe can be used either for downlink data or for uplink data statically or dynamically. One subframe is formed by 11 OFDM symbols for either downlink or uplink data, followed by one OFDM symbol for downlink control, one OFDM symbol for uplink control, and one OFDM symbol for the demodulation reference signal (DM-RS) with the same link direction as the OFDM symbol for data. This is illustrated in Figure 11.25. The major radio link parameters are given in Table 11.9. The carrier bandwidth is 100 MHz, and up to four-carrier carrier-aggregation is supported as the system band- width. Eight sets of modulation and coding schemes (MCSs) are supported and can be either automatically configured based on the average error rate or configured manually. In this trial, modulation and coding are manually configured. Single-user MIMO is sup- ported up to Rank-8. Up to two transport blocks are generated and are mapped to 1, RF unit BB unit BTS (Base Transceiver Station) UE (User Equipment) Figure 11.24 Appearance of sub-6-GHz experimental system. 11.3 Field Experiments with Sub-6-GHz 5G Radio Control Data (entirely DL or entirely UL) OFDM symbol 200 us. = 0.2 ms DMRS = Demodulation Reference Signal; GP = Gu