Relay Physical Downlink Control Channel

Description

The Relay Physical Downlink Control Channel (R-PDCCH) is a specialized physical layer channel defined in 3GPP LTE specifications to support the operation of Type 1 relay nodes. A relay node is a low-power base station that extends coverage and capacity by wirelessly connecting to a Donor eNB (DeNB) via a backhaul link known as the Un interface. The R-PDCCH is a critical component of this backhaul link, specifically designed to carry downlink control information (DCI) from the DeNB to the relay node. This control information includes scheduling assignments for both downlink and uplink transmissions on the relay's backhaul link, power control commands, and other necessary signaling.

Architecturally, the R-PDCCH is transmitted by the DeNB and received by the relay node. It operates within the relay's backhaul subframes, which are time periods when the relay is configured to receive from the DeNB instead of transmitting to user equipment (UEs) on its access link. This time-division multiplexing is crucial for in-band relays to avoid self-interference. The R-PDCCH is mapped to specific resource elements within the physical resource blocks (PRBs) of these backhaul subframes. Its design differs from the conventional PDCCH used for direct UE communication; the R-PDCCH is located in the data region of the subframe (the PDSCH region) and can be transmitted using frequency-division multiplexing, allowing for more flexible resource allocation and enhanced capacity for control signaling to relays.

From a procedural perspective, the DeNB uses the R-PDCCH to dynamically allocate physical resource blocks (PRBs) on the PDSCH for data transmission to the relay (DL grants) and on the PUSCH for data reception from the relay (UL grants). The relay node continuously monitors the configured R-PDCCH search spaces in the designated backhaul subframes to decode these grants. The R-PDCCH supports both localized and distributed transmission modes to provide frequency diversity or frequency-selective scheduling gains. Its reliable operation is fundamental to the relay node's ability to forward data between the DeNB and the UEs connected to the relay, making it a key enabler for efficient network deployment in coverage holes and at cell edges.

Purpose & Motivation

The R-PDCCH was introduced to solve the specific control channel challenges associated with deploying LTE relay nodes, particularly in-band relays. In-band relaying means the relay's backhaul link (to the DeNB) and its access link (to UEs) operate on the same carrier frequency. This creates a problem: the relay cannot transmit and receive on the same frequency at the same time without severe self-interference. Therefore, the relay operates in a half-duplex mode, requiring specific, scheduled subframes for backhaul communication.

The conventional PDCCH, located in the first few symbols of every subframe, is broadcast to all UEs and is not suitable for this scheduled, relay-specific operation. The DeNB needs a dedicated mechanism to send control information (like resource grants) specifically to the relay during these allocated backhaul subframes. The R-PDCCH was created to fulfill this need. It provides a dedicated, dynamically schedulable control channel that exists only in the subframes where the relay is listening to the DeNB, ensuring reliable and efficient control signaling for the backhaul link without interfering with the relay's access link transmissions.

Historically, before standardized relays, coverage extension was achieved through additional macro sites or repeaters. Repeaters are simpler devices that amplify and forward signals but also amplify noise and interference. The 3GPP relay node, supported by channels like the R-PDCCH, is a smarter solution. It decodes, re-encodes, and retransmits signals, improving signal quality. The R-PDCCH was a necessary innovation to enable this intelligent, schedulable, and efficient in-band relay operation, which was a key feature for network densification and coverage optimization in LTE-Advanced (Rel-10).