rocess can't be re-used until the acknowledgment for that process comes in (first time or after retransmission). This defines that the round-trip time between gNB (DU + RU) and UE needs to be matched to peak throughput and maximum latency of this round-trip time. HARQ timing definitions are shown in Fig. 3-14: Key processing times in the HARQ Round Trip Time (RTT) chain are: 1. T1-Required time to transmit one TB, i.e., the physical time the TB transmission consumes-compare it to the serialization/de-serialization delay of a wireline interface. 2. T2-The time required at the UE/client between the end of the TB and the time that the acknowledgment/negative acknowledgment (ACK/NACK) for this TB is sent. It includes UE/Customer Premises Equipment (CPE) PDSCH Rx processing latency, UE/CPE UCI (PUCCH or PUSCH) Tx processing, time until next ACK/NACK transmission is available. For synchronous HARQ in LTE Uplink, this was defined to 4Ms by 3GPP standards, but 5G/NR allows more flexible timing for this time. 3. T3-Physical time to transmit one ACK/NACK. Typically, 1 (short PUCCH) to 14 (long PUCCH, PUSCH) symbols. 4. T4-The transmission delay between gNB BaseBand Unit (DU) and UE/client antenna ports. It includes physical over-the-air latency, gNB RU to DU delay (if Baseband & Analog are on different locations). Latency PDCCH/PDSCH (N)ACK < PDCCH/PDSCH PDCCH/PDSCH (N)ACK PDCCH/PDSCH Latency Latency FIGURE 3-14 Critical timing in HARQ. Chapter 3 Hardware system dimensioning 5. T5-The time required at the gNB to (re) transmit data following ACK/NACK reception. It includes ACK/NACK reception at gNB PHY, retransmission scheduling at gNB MAC and of course RU delay. Overall HARQ latency is defined as THARQ = T1 + T2 + T3 + T4 + T5 + T4. THARQ drives the minimum number of required HARQ processes to satisfy the "capability to transmit continuous stream." It can be found using the equation NMIN, HARQ X TTI THARQ. Minimizing THARQ implies reducing each T value. From the UE side, the only capability driven is T2. T5 is about gNB ca