HI (HARQ indicator)

Hybrid Automatic Repeat Request (HARQ) is a mechanism that enhances the reliability of data transmission in wireless communication systems. HARQ is an error-control technique that combines Automatic Repeat Request (ARQ) and Forward Error Correction (FEC) mechanisms. HARQ helps to recover data that is lost or corrupted during transmission and improves the overall performance of the wireless network. HARQ Indicator (HI) is a signaling mechanism used by the HARQ protocol to indicate the status of a data transmission.

In wireless communication systems, data is transmitted from a sender to a receiver over a radio link. The radio link is subject to various impairments such as noise, interference, fading, and multipath propagation. These impairments can cause errors in the received data. ARQ is a mechanism that uses feedback to recover lost or corrupted data. In ARQ, the sender sends a packet of data to the receiver, and the receiver sends an acknowledgment (ACK) or negative acknowledgment (NACK) to the sender. If the sender receives an ACK, it assumes that the data was successfully received, and the next packet is sent. If the sender receives a NACK, it retransmits the packet. ARQ is a reliable method of error control, but it can be slow and inefficient, especially in wireless communication systems.

FEC is a mechanism that adds redundancy to the transmitted data to enable the receiver to recover lost or corrupted data without requiring feedback. FEC works by adding redundant bits to the transmitted data, which can be used by the receiver to correct errors. FEC is a fast and efficient method of error control, but it requires additional bandwidth to transmit the redundant bits.

HARQ is a mechanism that combines the strengths of ARQ and FEC to provide reliable and efficient error control in wireless communication systems. HARQ uses feedback to recover lost or corrupted data, but it also adds redundancy to the transmitted data to enable the receiver to recover data without requiring feedback. HARQ operates in two modes: incremental redundancy (IR) and Chase combining (CC).

In IR mode, the sender initially transmits a packet of data without redundancy. If the receiver detects errors in the packet, it sends a NACK to the sender, indicating that the packet was not successfully received. The sender then retransmits the packet with additional redundancy, which enables the receiver to recover the lost or corrupted data. The sender continues to retransmit the packet with increasing redundancy until the receiver sends an ACK or a maximum number of retransmissions is reached.

In CC mode, the sender initially transmits a packet of data with redundancy. If the receiver detects errors in the packet, it does not send a NACK but instead stores the received data and waits for the next transmission. The sender then transmits the packet again with additional redundancy. The receiver combines the two received packets to recover the lost or corrupted data. If the receiver still detects errors, it stores the received data and waits for the next transmission. The sender continues to transmit packets with increasing redundancy until the receiver successfully recovers the data.

HARQ Indicator (HI) is a signaling mechanism used by the HARQ protocol to indicate the status of a data transmission. The HI is transmitted from the receiver to the sender and indicates whether the received data was successfully decoded or not. The HI is used to trigger retransmissions in IR mode and to combine received packets in CC mode.

The HI can take two forms: positive acknowledgment (ACK) and negative acknowledgment (NACK). The ACK indicates that the received data was successfully decoded, and no retransmission is necessary. The NACK indicates that the received data was not successfully decoded, and retransmission is necessary. The NACK can be further divided into two types: soft NACK and hard NACK.

The soft NACK indicates that the received data has some errors, but it may still be possible to recover the data by adding redundancy. The hard NACK indicates that the received data has too many errors, and it is unlikely that the data can be recovered by adding redundancy. The HI is typically transmitted using a dedicated control channel, such as the Hybrid-ARQ Indicator Channel (HICH) or the Physical HARQ Indicator Channel (PHICH).

The HICH is a control channel used in the Universal Mobile Telecommunications System (UMTS) to transmit the HI in IR mode. The HICH is a shared channel that is allocated to multiple users, and each user is assigned a unique signature sequence to transmit its HI. The receiver uses a matched filter to detect the signature sequence of the intended user and decode its HI.

The PHICH is a control channel used in Long-Term Evolution (LTE) to transmit the HI in both IR and CC modes. The PHICH is a dedicated channel that is allocated to each user, and the user's HI is transmitted on its assigned PHICH. The PHICH uses a cyclic shift keying (CSK) modulation scheme to transmit the HI, and the receiver uses demodulation and decoding techniques to recover the HI.

In addition to the HI, the HARQ protocol also uses other signaling mechanisms, such as the Transport Block Size (TBS) and the redundancy version (RV), to control the transmission and retransmission of data packets. The TBS indicates the size of the data packet, and the RV indicates the level of redundancy added to the packet. The TBS and RV are used by the sender to determine the level of redundancy to add to the packet and the number of packets to transmit in IR mode.

HARQ is a critical component of modern wireless communication systems, and it plays a crucial role in enhancing the reliability and efficiency of data transmission. HARQ has been adopted by various wireless communication standards, such as UMTS, LTE, and 5G New Radio (NR), and it has become an essential feature of mobile devices and network infrastructure. The HI is a key signaling mechanism used by the HARQ protocol to indicate the status of a data transmission, and it enables the sender and receiver to communicate and coordinate the transmission and retransmission of data packets.