Configured Maximum UE Output Power

Description

PCMAX is a fundamental parameter in the UE's uplink transmission chain, defined per carrier and per band in 3GPP specifications for NR (New Radio) and evolved for LTE. It represents the upper limit for the UE's configured output power. The actual value of PCMAX is not a single fixed number but is determined by a combination of regulatory requirements, network configuration, and the UE's own capabilities. It operates within a defined range, PCMAX_L to PCMAX_H, where PCMAX_L is the lower bound and PCMAX_H is the upper bound. The final configured PCMAX must fall within this range.

The determination of PCMAX involves several inputs. First, regulatory limits define the absolute maximum power allowed in a specific frequency band to avoid harmful interference. Second, the network (gNB in NR, eNB in LTE) can signal a maximum allowed UE power parameter to the UE via RRC (Radio Resource Control) signaling, such as the P-Max information element in SIB1 (System Information Block 1) or in dedicated configuration. Third, the UE has its own power class capability (e.g., Power Class 3 for many handsets, defining a nominal maximum power like 23 dBm). The UE calculates the permissible PCMAX range (PCMAX_L, PCMAX_H) based on these inputs, applying formulas defined in specifications like TS 38.101-1 for NR, which account for the maximum power reduction (MPR) and additional maximum power reduction (A-MPR) needed due to factors like higher-order modulation, wide bandwidth transmission, or specific regional regulatory requirements.

In operation, the UE's uplink power control algorithm uses PCMAX as a hard ceiling. The algorithm calculates a desired transmit power based on path loss measurements, target received power at the base station, modulation and coding scheme (MCS), and the number of allocated resource blocks. The final transmit power P is set as min(calculated desired power, PCMAX). This ensures the UE never exceeds its configured maximum. The role of PCMAX is critical for managing the uplink interference landscape in a cell. By configuring PCMAX appropriately, the network can control the uplink coverage radius and manage interference to neighboring cells, especially in dense deployments or at cell edges.

Furthermore, PCMAX is vital for UE power management and thermal design. Transmitting at high power drains the battery quickly and generates heat. The network can lower the configured PCMAX for UEs close to the cell center, allowing them to save power without impacting link quality. It also plays a key role in carrier aggregation (CA) and dual connectivity (DC) scenarios. The UE has a total power limit across all transmitting chains. Therefore, a PCMAX is defined per component carrier (CC), and the UE must perform power sharing, ensuring the sum of powers across active CCs does not exceed the total UE power capability, with each CC's power capped by its respective PCMAX. This complex power management is specified in detail in TS 38.101 and test procedures in TS 38.521.

Purpose & Motivation

PCMAX exists to solve several interrelated problems in cellular uplink design: regulatory compliance, interference management, UE power efficiency, and network performance optimization. Without a well-defined and configurable maximum power limit, UEs could transmit at their absolute hardware maximum, which could violate regulatory spectral emission masks, cause excessive interference to other users in the same or adjacent bands, and create an uneven uplink interference environment that is difficult for the network to manage.

Historically, power control focused on having the UE transmit with just enough power to meet a quality target at the base station, but a simple hardware cap was insufficient. The introduction of more complex transmission schemes with variable bandwidths and high-order modulation (like 256QAM in LTE and 1024QAM in NR) meant that the UE's power amplifier needed to operate with higher linearity, often requiring a reduction in maximum output power to avoid spectral regrowth and out-of-band emissions. This is captured by the MPR/A-MPR concepts tied to PCMAX. Furthermore, as networks evolved to use carrier aggregation and multiple radio access technologies concurrently, a per-carrier configurable maximum became essential to manage the UE's total radiated power across all active transmitters.

Its creation and formalization in 3GPP specifications, particularly from Rel-15 onwards for NR, were motivated by the need for a flexible and precise framework. This framework allows the network to intelligently control the uplink power envelope of UEs. The network can set PCMAX based on deployment scenarios (e.g., lower in small cells), UE location, network load, and interference conditions. This enables optimized system capacity and coverage while ensuring UEs operate within safe thermal and battery consumption limits. It provides the necessary knob for network operators to balance between achieving high uplink data rates for edge users and maintaining overall network stability and efficiency.

Key Features

• Defines the upper limit for a UE's transmit power on a specific carrier or band.
• Determined by a range (PCMAX_L to PCMAX_H) considering regulatory limits, network configuration (P-Max), and UE power class.
• Incorporates Maximum Power Reduction (MPR) and Additional MPR (A-MPR) for signals with high peak-to-average power ratio (PAPR).
• Acts as the final cap in the uplink power control calculation (P = min(calculated power, PCMAX)).
• Essential for managing uplink interference and cell coverage in the network.
• Critical for UE power sharing and management in Carrier Aggregation (CA) and Dual Connectivity (DC) scenarios.

Evolution Across Releases

Defining Specifications