PHICH (physical hybrid ARQ indicator channel)
The Physical Hybrid ARQ Indicator Channel (PHICH) is an essential component in the Long Term Evolution (LTE) wireless communication system, which is designed to provide high-speed data transmission for mobile devices. In this article, we will delve into the details of PHICH, its purpose, functionality, and significance in the LTE network.
The LTE network is based on an Orthogonal Frequency Division Multiple Access (OFDMA) scheme, where the available bandwidth is divided into multiple subcarriers. These subcarriers are assigned to different users based on their requirements, and each user is allocated a specific resource block.
ARQ (Automatic Repeat Request) is an error-control technique used in communication systems to ensure reliable data transmission. It involves the retransmission of packets that are either lost or corrupted during transmission. Hybrid ARQ is an enhancement of the ARQ technique that combines both error detection and error correction mechanisms to provide improved performance.
The PHICH, as the name suggests, is responsible for carrying information related to the Hybrid ARQ process. It serves as a feedback mechanism from the receiver (UE - User Equipment) to the sender (eNodeB - Evolved NodeB) regarding the successful or unsuccessful reception of the transmitted data. This feedback allows the sender to take appropriate actions, such as retransmitting the data or moving to the next transmission.
The PHICH is transmitted on the control channel, known as the Physical Downlink Control Channel (PDCCH), in the downlink direction. It operates in the time domain and is associated with a specific resource element (RE) within the resource grid. The resource element is the basic unit of resource allocation in LTE, and it represents a specific time-frequency location within the LTE system bandwidth.
The PHICH is modulated using Binary Phase Shift Keying (BPSK), which allows the transmission of two different symbols, '0' and '1'. These symbols correspond to the acknowledgment (ACK) and negative acknowledgment (NACK) feedback signals, respectively. The ACK signal indicates that the data was successfully received, while the NACK signal indicates that the data was either lost or corrupted.
The PHICH is divided into different groups known as PHICH groups, and each PHICH group contains a set of PHICH resources. The number of PHICH groups and the number of PHICH resources per group depend on the system configuration and the number of users in the network.
The mapping of PHICH resources to users is determined by the Physical Uplink Control Channel (PUCCH) configuration, which is controlled by higher-layer signaling. The PUCCH carries uplink control information from the UE to the eNodeB, and it is crucial for the operation of the Hybrid ARQ process.
When a UE receives data on the PDSCH (Physical Downlink Shared Channel), it decodes the received data and checks for any errors using the CRC (Cyclic Redundancy Check) mechanism. If the data is error-free, the UE transmits an ACK signal on the PHICH, indicating successful reception. On the other hand, if errors are detected, the UE transmits a NACK signal.
The eNodeB monitors the PHICH for ACK/NACK signals from the UEs. When an ACK signal is received, the eNodeB considers the data successfully received by the UE and takes no further action. However, when a NACK signal is received, the eNodeB triggers a retransmission of the corresponding data. The retransmission process continues until either the data is successfully received by the UE, or the maximum number of retransmissions is reached.
It's worth noting that the PHICH operates in a contention-free manner, meaning that each UE is assigned a specific PHICH resource for its ACK/NACK transmission. This ensures that there is no interference between different UEs transmitting their feedback signals simultaneously.
In conclusion, the Physical Hybrid ARQ Indicator Channel (PHICH) is a critical component of the LTE wireless communication system. It serves as a feedback mechanism from the UE to the eNodeB regarding the successful or unsuccessful reception of data. The PHICH enables the implementation of the Hybrid ARQ technique, which ensures reliable and efficient data transmission in LTE networks.