ogy Components Downlink Demodulation Reference Uplink Figure 3.11 Self-contained subframe control Signals (DMRS) control in 5G. Slot 0 Slot 1 edges of the subframe, there is no downlink-uplink interference between the control channels. The self-contained subframe also brings other benefits including unlicensed band operation and beamforming optimization. Fast switching between the downlink and uplink helps in the case of the reciprocal channel to optimize the performance with massive MIMO antennas (see Figure 3.11). 3.4.4 Asynchronous HARQ 5G supports asynchronous Hybrid Automatic Repeat Request (HARQ) retransmission. Asynchronous HARQ allows retransmission to occur with flexible delay after the first transmission, which gives more freedom for the packet scheduler optimization. LTE uses synchronous HARQ in the uplink where the retransmission must occur with an 8 ms delay after the first transmission. The need for the retransmission is signaled in downlink by an NACK message in LTE. 5G does not use explicit ACK/NACK signaling, but the need for the retransmission is indicated with the new uplink resource alloca- tion with the same process number. The uplink retransmission schemes are shown in Figure 3.12 LTE synchronous uplink retransmission First transmission Retransmission Fixed retransmission timing in Fixed uplink with 8 ms delay Uplink ACK/NACK signaling Downlink 5G asynchronous uplink retransmission Retransmission Flexible retransmission timing First transmission Flexible No ACK/NACK signaling Uplink Retransmission indicated with new allocation with same process number Downlink allocation Figure 3.12 Uplink retransmission schemes in LTE and in 5G. 3.4 Flexible Physical Layer and Protocols 3.4.5 Lean Carrier The LTE base station transmits CRSs four times per millisecond. Reference signals are needed by the UE for cell search, mobility measurements, and channel estimation and decoding. LTE reference signals must be transmitted all the time even if there are no connected UEs in the cell. 5G system design is diff