Secondary stream ETFC Offset

Description

The Secondary stream ETFC Offset (SD) is a technical parameter within the 5G New Radio (NR) physical layer specifications for uplink power control. It is applicable when the UE is configured for uplink transmission using the CP-OFDM waveform without transform precoding (i.e., not using DFT-s-OFDM) and is operating in a multi-layer (MIMO) scenario. ETFC stands for 'Effective Target received power per FFT bin per Channel'. The SD parameter, signaled by the network via RRC or MAC CE, provides an offset value in decibels (dB). This offset is used in the uplink power control formula to calculate the transmit power for a secondary data stream relative to a primary stream. The primary stream's power is determined based on open-loop and closed-loop power control mechanisms involving pathloss estimates and TPC commands. For the secondary stream, its power is derived by applying the SD offset to the primary stream's calculated power. This approach allows the gNB to finely control the relative power levels between multiple spatial layers, which is crucial for optimizing uplink MIMO performance, managing inter-layer interference, and ensuring reliable demodulation at the receiver. The parameter is part of a suite of power control adjustments that enable efficient spectral utilization and link adaptation in advanced 5G NR deployments.

Purpose & Motivation

The SD parameter was introduced to address the specific needs of uplink Multi-User MIMO (MU-MIMO) and single-user MIMO (SU-MIMO) in 5G NR, particularly when using the high-spectral-efficiency CP-OFDM waveform. In LTE, uplink MIMO was more limited and often used DFT-s-OFDM (SC-FDMA), which has different power characteristics. With 5G's support for CP-OFDM in the uplink and more advanced multi-antenna techniques, a more granular power control mechanism was required. Different spatial streams may experience different effective channels or interference conditions. Applying the same transmit power to all streams is suboptimal. The SD offset allows the gNB to independently balance the power between the primary and secondary streams, optimizing the signal-to-interference-plus-noise ratio (SINR) for each layer. This solves problems like one stream overpowering another or insufficient power for a secondary stream in a rank-2 transmission, ultimately improving uplink throughput, cell-edge performance, and overall MIMO efficiency in diverse radio conditions.