Last OFDM Symbol index for R-PDCCH

Description

The parameter 'LL' (Last OFDM Symbol index for R-PDCCH) is a specific configuration parameter used in the context of LTE-Advanced Relay Nodes (RNs). It defines the temporal boundary of the control channel region on the backhaul downlink link between a Donor eNB (DeNB) and a Relay Node. The R-PDCCH is a specialized physical channel introduced to carry control information (like scheduling assignments) specifically for the relay backhaul link. Unlike the regular PDCCH transmitted in the first few symbols of a subframe to all UEs, the R-PDCCH can be placed in different symbol positions, including using resources traditionally reserved for the Physical Downlink Shared Channel (PDSCH).

The value of LL is an integer index (e.g., 0, 1, 2,... up to the maximum symbols in a subframe, which is typically 14 for normal cyclic prefix) that points to the last OFDM symbol occupied by the R-PDCCH within its assigned resource block pair(s). This parameter, along with the starting symbol index, defines the exact time-domain resources the Relay Node must monitor and decode to receive its control information from the DeNB. The configuration is signaled to the Relay Node via higher-layer RRC signaling, allowing for flexible allocation of the control region based on backhaul load and interference conditions.

Its role is critical for the proper operation of in-band relaying, where the backhaul link (Un interface) and the access link (Uu interface) share the same carrier frequency. To avoid the Relay Node transmitting to UEs while it is supposed to be receiving from the DeNB, specific subframes are designated as 'MBSFN subframes' on the access link. During these subframes, the Relay Node can receive the backhaul downlink. The LL parameter ensures the Relay Node knows precisely when the R-PDCCH transmission ends, after which it can either receive R-PDSCH (Relay-PDSCH) data or switch to other operations. This precise timing coordination prevents self-interference and is fundamental to the relay node's time-division operation between backhaul and access links.

Purpose & Motivation

LL was introduced to support the LTE-Advanced Relay Node feature, which was standardized in 3GPP Release 10. The purpose of in-band relaying is to extend coverage and improve cell-edge throughput without the high cost of deploying new macro sites with dedicated backhaul. A key challenge for in-band relays is that they cannot transmit and receive on the same frequency at the same time due to the immense self-interference that would occur at the relay's own receiver.

To solve this, a time-division approach is used, where the relay alternates between listening to the donor cell (backhaul downlink) and serving its own user equipments (access downlink). The R-PDCCH was created to carry the relay's specific control information, and its placement within the subframe needed to be flexible to avoid fixed resource conflicts. The LL parameter, as part of the R-PDCCH configuration, provides this necessary flexibility. It allows the network to dynamically adjust the size of the control region for the backhaul link, optimizing resource usage. Without such a configurable parameter, the relay control channel would have a fixed location, reducing scheduling flexibility and efficiency for the mixed traffic of relay and direct UEs served by the Donor eNB.