 Low Physical Decode Encode Low Physical Receiver Transmitter Receiver Over-the-Air Transmitter Receiver Transmitter Phy Tx Phy Tx Phy Tx 4 mSec (N)Ack Receiver Encode FIGURE 2-20 LTE synchronous HARQ Timing Diagram (FDD). FDD, Frequency Division Duplexing; HARQ, Hybrid Automatic Repeat Request; LTE, Long-Term Evolution. Chapter 2 System components, requirements, and interfaces 63 Hybrid Automatic Repeat Request HARQ is a combined Physical Layer (forward error correction) and MAC layer retransmission protocol that saves the information from a previously unsuccessful decode at the Physical Layer to combine this information with incremental retransmitted (redundancy) information in subse- quent decode attempts. When the receiver fails to decode a TB [Physical Layer cyclic redundancy check (CRC) fails], it transmits a single HARQ Negative Acknowledgement (Nack) to the transmit- ter to indicate a request for additional information bits to try and do the decode operation again. HARQ is maintained by the MAC/MAC scheduler given that this entity is responsible for the time/frequency resource scheduling, including scheduling of HARQ resources. HARQ protocol in MAC is complemented by ARQ when operating in RLC AM to increase reliability of the end-to- end connection. HARQ can operate in synchronous or asynchronous manner. We first look at synchronous HARQ as used, for example, in the LTE uplink as shown in Fig. 2-20: In the LTE synchronous HARQ implementation, the HARQ process ID is reused every eight subframes (shown as the wraparound in the figure above), assuming FDD operation. Time-Division Duplex (TDD) operation is predefined using a set of standardized timing tables but operates in similar concepts. In the example shown here: The transmitter prepares (at MAC level) a new transmission at time (counted as HARQ process ID) = 2. This TB is encoded by the Physical Layer at time = 3 and transmitted (low-PHY) at time = 4. Dictated by 3GPP standards, the receiver has four slots of time to prepare and transmit the associated 