The role, enhancements, and practical applications of MIMO (Multiple-Input Multiple-Output) technology—in 3GPP Release 18

Release 18, frozen in functionality around late 2023 to mid-2024, marks the transition from foundational 5G New Radio (NR) to 5G-Advanced. MIMO has been a cornerstone of NR since Release 15; Rel-18 continues its evolution with targeted refinements rather than a wholesale redesign, focusing on higher spectral efficiency, better support for advanced devices, multi-TRP coordination, and resilience in high-mobility scenarios.

Role of MIMO in 3GPP Release 18

MIMO remains central to achieving the capacity, coverage, and reliability goals of 5G-Advanced. In prior releases (15–17), massive MIMO delivered substantial gains in spectral efficiency through spatial multiplexing, beamforming, and multi-user MIMO (MU-MIMO). Release 18 builds on this foundation to address remaining bottlenecks:

  • Uplink performance lagged downlink in many deployments, particularly for higher-layer transmission and devices with multiple antenna panels.
  • Channel State Information (CSI) accuracy degraded under medium-to-high UE mobility due to channel aging and Doppler effects.
  • Multi-Transmission Reception Point (multi-TRP) operation, introduced earlier for reliability and diversity, needed better support for coherent joint transmission (CJT) and simplified signaling.
  • Reference-signal overhead and port limitations constrained the number of simultaneous MU-MIMO layers.

The Rel-18 MIMO work item therefore concentrated on four interconnected areas: CSI enhancements, multi-TRP support, uplink capabilities, and reference-signal improvements. These changes primarily target Enhanced Mobile Broadband (eMBB) while enabling broader device types (fixed wireless access CPE, vehicles, industrial terminals) and denser multi-TRP deployments.

AI/ML techniques were studied in parallel for CSI feedback, beam management, and positioning, laying groundwork for later releases, but the core MIMO enhancements themselves rely on conventional signal-processing refinements.

Key Technical Enhancements

Channel State Information (CSI) Enhancements

CSI reporting is critical for accurate precoding. Release 18 introduced refinements particularly valuable for medium- and high-velocity UEs (approximately 30–60 km/h and beyond) and for coherent joint transmission:

  • Enhanced Type-II codebook with Doppler/time-domain compression and prediction. The UE measures multiple CSI-RS occasions, extracts Doppler characteristics, predicts future channel evolution, and compresses the predicted precoder information. This mitigates channel aging.
  • Support for Time Domain Channel Properties (TDCP) reporting in some configurations.
  • CSI acquisition enhancements tailored to CJT across up to four TRPs (assuming ideal backhaul and synchronization in FR1), including refined Type-II codebooks that account for multi-TRP spatial structure.

These features improve downlink throughput under mobility by allowing the gNB to apply more accurate, forward-looking precoding.

Multi-TRP and Unified TCI Framework

Release 17 introduced a unified Transmission Configuration Indicator (TCI) framework for single-TRP cases. Release 18 extends it to multi-TRP scenarios:

  • Joint and separate indication of multiple DL/UL TCI states.
  • Support for two Timing Advances (TAs) to handle asynchronous multi-TRP operation.
  • Improved signaling efficiency for simultaneous multi-panel or multi-TRP beam management.

For coherent joint transmission, CSI enhancements and reference-signal improvements enable constructive combining of signals from multiple TRPs, delivering significant cell-edge gains.

Uplink MIMO Evolution

Uplink received particular attention to close the performance gap with downlink and support advanced UEs:

  • Maximum number of PUSCH layers increased from 4 to 8.
  • Four new codebook types defined for 8-layer codebook-based transmission, covering fully coherent, partially coherent, and non-coherent antenna-port configurations, along with corresponding full-power transmission modes.
  • Simultaneous Transmission with Multiple Panels (STxMP): A UE can transmit from two panels using different beams toward different TRPs (or the same cell) in a single slot. This supports both Spatial Division Multiplexing (higher effective rank) and Single-Frequency Network-style repetition for reliability. Both single-DCI (ideal backhaul) and multi-DCI (non-ideal backhaul) modes are specified.
  • SRS ports increased to a maximum of 8 (with time-division multiplexing of ports across symbols to preserve per-port power).

These changes are especially relevant for Customer Premises Equipment (CPE), Fixed Wireless Access (FWA), vehicular, and industrial devices that can accommodate more antenna elements than typical smartphones.

Reference-Signal Improvements

  • Demodulation Reference Signal (DMRS) ports for both PDSCH and PUSCH expanded to a maximum of 24 orthogonal ports (from 12). This is achieved by extending the Frequency-Domain Orthogonal Cover Code (FD-OCC) length from 2 to 4. The result supports higher-order MU-MIMO while allowing reduced DMRS overhead for single-user cases through denser code-domain multiplexing.
  • SRS enhancements include better interference randomization across TRPs (e.g., cyclic-shift hopping) and support for the increased number of ports needed for 8-layer uplink.

Together these changes roughly double the number of orthogonal layers that can be spatially multiplexed in MU-MIMO scenarios under favorable conditions.

Performance Benefits

System-level evaluations and field trials reported in technical literature illustrate the gains:

  • MU-MIMO capacity can approximately double through the expanded DMRS ports and reduced overhead.
  • Coherent multi-TRP joint transmission yields substantial user-perceived throughput improvements, particularly at the cell edge (examples in literature show mean gains of 30–60 % and larger gains at the 5th percentile under various loading conditions).
  • CSI prediction for mobility scenarios delivers measurable downlink throughput improvements (on the order of 10–15 % in simulated medium/high-speed cases) by compensating for channel aging.
  • Eight-layer uplink and multi-panel transmission enable significantly higher peak and average uplink rates for capable devices, addressing a long-standing asymmetry in 5G.

Exact gains depend on deployment density, antenna configuration, traffic load, and channel conditions; the enhancements are most pronounced in dense urban multi-TRP networks and for advanced CPE/FWA terminals.

Practical Applications

The Rel-18 MIMO features translate into concrete network and service improvements:

  • Enhanced Mobile Broadband capacity: Operators can serve more simultaneous high-rate users in the same spectrum, improving cell throughput and user experience in dense areas without additional spectrum.
  • Fixed Wireless Access and CPE: Higher-order uplink MIMO and multi-panel transmission allow FWA gateways and outdoor CPE to deliver fiber-like uplink speeds, supporting residential broadband, small-business connectivity, and backhaul alternatives.
  • High-mobility scenarios: CSI prediction and unified TCI beam management improve performance for vehicles, high-speed trains, and pedestrians in urban environments, reducing throughput drops during movement.
  • Industrial and vertical use cases: Multi-TRP reliability enhancements and higher-layer support benefit factory automation, private networks, and scenarios requiring robust uplink (video surveillance, machine vision, remote control).
  • Multi-TRP densification: Operators deploying distributed antenna systems or coordinated multi-point can realize better spectral efficiency and coverage uniformity through coherent joint transmission and simplified TCI signaling.
  • Indirect support for XR and immersive services: Higher overall system capacity and improved mobility resilience help meet the stringent throughput and latency requirements of extended-reality applications, even though XR-specific optimizations appear in other Rel-18 work items.

These capabilities also interact with other Rel-18 features such as coverage enhancements, dynamic spectrum sharing refinements, and network energy-saving mechanisms, creating a more efficient overall radio access network.

Challenges and Considerations

Implementation is not without complexity. Supporting 8-layer uplink and 24 DMRS ports increases baseband processing and antenna requirements at both the UE and gNB. Phase coherence across TRPs for true CJT demands tight synchronization and ideal or near-ideal backhaul. CSI prediction algorithms add UE complexity and require careful configuration of measurement resources. Device manufacturers must balance the cost and power consumption of additional antenna panels, particularly for battery-powered or form-factor-constrained terminals. Interoperability testing across vendors remains essential, especially for multi-panel and multi-TRP features.

Looking ahead, Release 19 and subsequent work continue the trajectory toward even larger antenna arrays, deeper AI/ML integration into the CSI and beam-management loops, and further refinements for 6G-era requirements.

Role of MIMO in 3GPP Release 18

Release 18, completed in its core functionality by late 2023–2024, positions MIMO not merely as an incremental feature but as a foundational performance engine that enables the broader goals of 5G-Advanced: higher spectral efficiency, improved uplink experience, greater resilience in mobility and multi-point deployments, and support for a wider range of devices and use cases beyond traditional smartphones.

MIMO as a Continuum and Strategic Pillar

Since the introduction of 5G NR in Release 15, multi-antenna techniques have been central to achieving the high data rates and capacity promised by 5G. Massive MIMO, beamforming, and multi-user MIMO (MU-MIMO) delivered the primary spectral-efficiency gains in both frequency-division duplex (FDD) and time-division duplex (TDD) systems across FR1 and FR2.

In Release 18, MIMO retains this centrality while evolving into a more refined and versatile tool. The 3GPP RAN1 work explicitly framed MIMO evolution as one of the key topics for enhancing radio performance to a new level and enabling wider use cases. Rather than introducing an entirely new MIMO architecture, the release strengthens the existing framework in four interconnected domains—Channel State Information (CSI), multi-Transmission Reception Point (multi-TRP) operation, uplink transmission, and reference signals—so that the technology can meet the more demanding requirements of 5G-Advanced.

Core Roles of MIMO in Release 18

1. Primary driver of system capacity and spectral efficiency

By expanding the number of orthogonal DMRS ports to 24 and refining MU-MIMO operation, MIMO in Rel-18 allows more simultaneous spatial layers. This directly increases the number of users or data streams that can be served in the same time-frequency resources, addressing the growing traffic density expected in mature 5G networks.

2. Closing the uplink performance gap

Historically, uplink MIMO lagged downlink. Release 18 elevates uplink capabilities to support up to eight layers and simultaneous multi-panel transmission. This role is especially important for non-smartphone devices—fixed wireless access (FWA) customer-premises equipment, vehicles, and industrial terminals—that can physically accommodate more antennas. The result is a more balanced uplink/downlink experience and higher peak rates in the uplink direction.

3. Enabler of advanced multi-TRP and coherent joint transmission

MIMO underpins the expansion of the unified Transmission Configuration Indicator (TCI) framework from single-TRP to multi-TRP scenarios and supports coherent joint transmission (CJT) across multiple points. In this capacity, MIMO transforms coordinated multi-point operation from a reliability-focused feature into a capacity- and coverage-enhancing tool, particularly at cell edges and in dense urban deployments.

4. Mobility resilience and channel tracking

Through CSI enhancements that incorporate Doppler information and time-domain prediction, MIMO helps maintain accurate precoding even when UEs move at medium-to-high speeds. This role reduces the performance degradation caused by channel aging, making high-rate service more robust for vehicular, high-speed rail, and dense pedestrian environments.

5. Platform for device diversification and new form factors

By specifying support for higher-order uplink MIMO and multi-panel simultaneous transmission, MIMO in Rel-18 enables a broader device ecosystem. CPE, FWA gateways, and industrial equipment can now exploit advanced antenna configurations that smartphones typically cannot, expanding the commercial addressable market for 5G-Advanced.

Interaction with Other Release 18 Priorities

MIMO does not operate in isolation. Its enhancements complement and amplify other Rel-18 work areas:

  • Improved CSI frameworks interact with the study of AI/ML for air-interface optimization (particularly CSI feedback and beam management).
  • Higher spatial multiplexing efficiency contributes to overall network energy efficiency by allowing more data to be delivered with the same or fewer resources.
  • Multi-TRP and beam-management refinements support coverage enhancements, mobility improvements, and denser network topologies.
  • The capacity gains help meet the stringent throughput and latency needs of extended-reality (XR) and immersive services, even though XR-specific optimizations appear in parallel work items.

Industry and Deployment Perspective

From an operator and vendor standpoint, the role of MIMO in Release 18 is pragmatic: it maximizes the return on existing massive-MIMO hardware investments while unlocking new performance headroom through software and protocol upgrades. Networks that have already deployed large antenna arrays can realize additional capacity and coverage benefits with relatively modest radio-unit or baseband updates. For new deployments, the enhanced uplink and multi-TRP capabilities make mid-band and mmWave spectrum more attractive for both mobile and fixed services.

In summary, MIMO in 3GPP Release 18 functions as the primary radio-performance lever of 5G-Advanced. It continues the evolutionary path begun in Release 15, systematically removes remaining limitations in uplink capability, multi-point coordination, and mobility support, and provides the spatial-degree-of-freedom foundation upon which many other 5G-Advanced features depend. This positions MIMO as both a capacity workhorse and an enabler of the broader, more diverse 5G-Advanced ecosystem.

Channel State Information (CSI) Enhancements

These enhancements address two primary limitations of earlier CSI frameworks: degradation of precoding accuracy under medium-to-high UE mobility (due to channel aging and Doppler spread) and the need for more effective CSI acquisition in coherent multi-TRP joint transmission scenarios. Release 18 introduces refined Type-II codebook structures and associated reporting mechanisms that improve the freshness and accuracy of CSI available at the gNB, without fundamentally altering the underlying spatial and frequency-domain compression principles established in Releases 16 and 17.

Background: CSI Evolution Leading to Release 18

CSI enables the base station to perform accurate downlink precoding and rank adaptation. Type-I codebooks provide low-overhead, lower-resolution feedback suitable for single-user MIMO, while Type-II (and its enhanced and further-enhanced variants) deliver higher-resolution feedback optimized for multi-user MIMO.

Prior to Release 18, CSI reporting assumed relatively static or slowly varying channels. When UEs move at moderate or high speeds (approximately 30–60 km/h and above), the reported precoder becomes outdated by the time it is applied, reducing spectral efficiency. Similarly, coherent joint transmission across multiple TRPs requires precise phase-aligned CSI that earlier frameworks did not fully optimize. Release 18 directly targets these gaps.

Primary CSI Enhancements in Release 18

1. Enhanced Type-II Codebook for Predicted PMI (Doppler-Based CSI)

The most significant mobility-related enhancement is the introduction of the Enhanced Type-II codebook for predicted PMI, configured via the higher-layer parameter codebookType set to ‘typeII-Doppler-r18’ (or the port-selection variant ‘typeII-Doppler-PortSelection-r18’).

Key technical features include:

  • Channel prediction at the UE: The UE measures multiple CSI-RS resources (typically K = 4, 8, or 12 aperiodic resources, or a single periodic/semi-persistent resource) and extracts Doppler characteristics from the time-varying channel coefficients.
  • Doppler/time-domain compression: In addition to the familiar spatial-domain (SD) and frequency-domain (FD) bases of earlier Type-II codebooks, a Doppler-domain (DD) or time-domain basis is introduced. The precoder structure effectively becomes a three-dimensional compression across angle, delay, and Doppler.
  • Predicted PMI reporting: The UE reports a predicted precoder matrix indicator valid for a configured future slot interval rather than only the measurement slot. This allows the gNB to apply more up-to-date precoding despite reporting and scheduling delays.
  • Configurable parameters: Higher-layer parameters such as paramCombination-Doppler-r18, numberOfPMI-SubbandsPerCQI-Subband-Doppler-r18, and the Doppler basis length (N₄) control the trade-off between prediction accuracy, feedback overhead, and computational complexity. Rank is typically restricted to a maximum of 4.
  • CQI handling: Special rules apply for CQI calculation when multiple CSI-RS resources are used for prediction, ensuring consistent power assumptions.

These mechanisms allow the UE to exploit time-domain correlation, predict channel evolution, compress the predicted information efficiently, and report it in a form usable by the gNB. Simulation studies show measurable downlink throughput gains in medium- and high-mobility scenarios by reducing the impact of channel aging.

2. CSI Enhancements for Coherent Joint Transmission (CJT)

For multi-TRP operation, Release 18 defines Enhanced Type-II codebooks specifically for CJT, configured as ‘typeII-CJT-r18’ and ‘typeII-CJT-PortSelection-r18’.

Main characteristics:

  • Support for up to four TRPs under the assumption of ideal backhaul and synchronization (primarily targeting FR1).
  • Refinement of the Type-II structure to jointly report CSI across multiple CSI-RS resources corresponding to different TRPs.
  • Ability for the UE to select or report combinations of CSI-RS resources, enabling the gNB to construct coherent precoders that constructively combine signals from multiple points.
  • Integration with the expanded unified TCI framework so that beam and CSI indications remain consistent across TRPs.

These enhancements improve both the quality of multi-TRP CSI and the efficiency of feedback, contributing to the cell-edge and overall spectral-efficiency gains observed with coherent joint transmission.

3. Time Domain Channel Properties (TDCP) Reporting

Alongside the predicted-PMI codebooks, Release 18 supports reporting of Time Domain Channel Properties measured via CSI-RS for tracking. This provides the network with explicit information about the time-domain correlation or Doppler profile of the channel, which can be used for prediction algorithms at the gNB or for further refinement of CSI processes. TDCP complements the UE-side prediction approach and offers flexibility in how mobility-related CSI is handled.

Operational and Performance Considerations

  • Measurement and reporting overhead: Prediction requires the UE to process multiple CSI-RS occasions. This increases measurement complexity and can raise the number of active CSI processes. Configurations carefully balance the number of resources (K) against the expected Doppler spread.
  • UE capability signaling: New UE capability parameters (for example, those related to eType2Doppler support) indicate which combinations of ports, resources, rank, and Doppler-basis lengths the device supports.
  • Feedback structure: CSI reports continue to use the familiar Part 1 / Part 2 structure, with Part 1 carrying rank, CQI, and non-zero coefficient counts, while Part 2 carries the detailed predicted or multi-TRP PMI.
  • Performance impact: By delivering fresher CSI, the enhancements reduce the mismatch between the reported precoder and the actual channel at the time of data transmission. Gains are most pronounced in medium-to-high mobility and in multi-TRP CJT deployments. Exact improvements depend on speed, antenna configuration, traffic load, and prediction horizon, but literature consistently reports noticeable user throughput benefits relative to legacy Type-II reporting under comparable conditions.

Practical Implications and Multi-Angle Perspective

From a network operator perspective, these CSI enhancements allow better utilization of already-deployed massive MIMO arrays in environments with significant user mobility (urban vehicular corridors, high-speed rail corridors, dense pedestrian areas) and in multi-TRP densification scenarios. They improve the effectiveness of MU-MIMO and coherent multi-point transmission without requiring proportional increases in reference-signal overhead.

From a device and chipset perspective, the features demand additional baseband processing for Doppler extraction, prediction, and three-dimensional compression. This is manageable for higher-tier UEs and is particularly relevant for FWA CPE, vehicular modules, and industrial terminals that can support the associated power and computational budgets.

From a standardization and research viewpoint, the Release 18 CSI framework also serves as a foundation for subsequent AI/ML-based CSI enhancements studied in parallel and further developed in later releases. The explicit support for time-domain and Doppler information creates a natural interface for data-driven prediction models.

Summary

The CSI enhancements in 3GPP Release 18 represent a targeted evolution of the Type-II codebook family. By adding Doppler-domain compression and UE-side channel prediction for medium- and high-mobility scenarios, and by refining multi-TRP CSI acquisition for coherent joint transmission, the standard significantly improves the accuracy and usefulness of CSI available for downlink precoding. These changes strengthen the overall MIMO performance of 5G-Advanced networks, particularly where user mobility or multi-point coordination previously limited spectral efficiency.

Multi-TRP and Unified TCI Framework

These features address the growing need for flexible, low-overhead beam management and coordinated transmission/reception across multiple geographically separated points in dense or reliability-critical deployments. Release 18 extends the Rel-17 unified TCI framework—originally designed primarily for single-TRP operation—into multi-TRP scenarios, while introducing supporting mechanisms such as dual Timing Advances (TAs) to handle timing differences across TRPs.

Background: From Single-TRP Unified TCI to Multi-TRP Support

In Release 17, the unified TCI framework simplified beam indication by allowing a single (or jointly indicated) TCI state to apply across multiple downlink and uplink channels and reference signals. It supported both joint DL/UL TCI states and separate DL and UL TCI states, reducing signaling overhead compared with the earlier per-channel TCI/spatial-relation approach.

Release 18 builds directly on this foundation. The primary goal is to enable efficient multi-TRP operation—both intra-cell and inter-cell—while preserving the low-overhead benefits of the unified framework. This supports single-DCI (S-DCI) and multi-DCI (M-DCI) based multi-TRP schemes, coherent joint transmission (CJT), simultaneous multi-panel uplink transmission, and improved reliability or capacity through spatial diversity and coordinated precoding.

Key Enhancements to the Unified TCI Framework in Release 18

Extension to Multi-TRP Operation

The framework now supports indication and application of multiple TCI states corresponding to different TRPs:

  • S-DCI based multi-TRP: A single DCI can indicate joint, DL, or UL TCI state(s) applicable to one or both TRPs in a given bandwidth part or component carrier. RRC configuration can define how indicated states map to CORESETs or groups of CORESETs (for example, applying the first, second, both, or none of the indicated states).
  • M-DCI based multi-TRP: The existing TCI field in DCI formats 1_1 or 1_2 (with or without DL assignment), associated with a specific CORESET pool index, indicates TCI state(s) specific to that pool/TRP. One joint TCI state (or a pair of DL/UL states) can be mapped to a TCI codepoint.
  • Up to two joint TCI states can be indicated (via MAC-CE or DCI) and applied to CJT-based PDSCH reception. Support for one or two indicated states is subject to UE capability.
  • Combinations of joint and separate states are supported within a single codepoint in certain configurations (for example, two joint states or up to four separate states in S/S mode), with a maximum of eight codepoints typically available.
  • Dynamic association of indicated TCI states with specific TRPs, CORESETs, search-space sets, or power-control/path-loss reference signals is enabled through RRC, MAC-CE, or fixed mapping rules.

These mechanisms allow the network to update beams for multiple TRPs with reduced latency and overhead compared with pre-unified approaches.

Support for Two Timing Advances

A critical practical enhancement is the introduction of two Timing Advances for uplink multi-DCI multi-TRP operation:

  • Applicable to both FR1 and FR2, and to both intra-cell and inter-cell multi-TRP scenarios.
  • The network can configure up to two n-TimingAdvanceOffset values (or equivalent TRP-specific TA parameters) per serving cell.
  • TCI states (or groups of states) can be associated with specific TA values so that uplink transmissions toward different TRPs use the appropriate timing advance.
  • This resolves the timing misalignment that arises when a UE communicates simultaneously or near-simultaneously with non-collocated TRPs that have different propagation delays.

The dual-TA support integrates with both the legacy TCI frameworks and the extended unified TCI framework, as well as with spatial-relation-based uplink beam indication.

Additional Supporting Features

  • Association of indicated joint or UL TCI states with uplink power-control parameters (P0, alpha, closed-loop index) and path-loss reference signals, ensuring consistent application for PUSCH, PUCCH, and related transmissions (including S-DCI based repetition with TDM).
  • Support for TRP-specific beam failure recovery within the unified framework.
  • Compatibility with simultaneous multi-panel uplink (STxMP) schemes, where different panels may point toward different TRPs and apply different TCI states or beams.
  • Provisions for inter-cell multi-TRP, including potential association of TCI states with different Physical Cell Identities (PCIs).

Operational Modes and Use Cases

AspectSingle-TRP (Rel-17 baseline)Multi-TRP (Rel-18 extension)
TCI indication1 joint or 1 DL + 1 ULMultiple (up to 2 joint or combinations of separate)
DCI typePrimarily S-DCIS-DCI and M-DCI
Timing AdvanceSingle TAUp to two TAs
Primary benefitLow-overhead common beamCoordinated multi-point beams + timing alignment
Typical target channelsPDCCH, PDSCH, PUCCH, PUSCH, CSI-RS, SRSSame, plus CJT PDSCH and multi-panel UL

These enhancements enable:

  • Higher reliability through spatial diversity or repetition across TRPs.
  • Increased capacity via coherent joint transmission (constructive combining of signals from multiple TRPs).
  • Improved coverage and cell-edge performance in dense urban or indoor deployments.
  • Flexible uplink multi-panel operation for advanced devices (CPE, FWA, vehicles, industrial terminals).
  • Reduced signaling overhead for beam management in multi-TRP networks compared with independent per-TRP TCI management.

Practical Implications and Considerations

From a network perspective, the extended unified TCI framework allows operators to densify TRPs (for example, distributed antenna systems or coordinated multi-point clusters) while keeping beam indication efficient. The dual-TA support is particularly valuable in non-ideal backhaul or geographically separated TRP deployments where propagation delays differ.

From a UE perspective, support is capability-dependent. Devices must handle simultaneous or near-simultaneous application of multiple TCI states, maintain multiple timing advances, and correctly map indicated states to the appropriate channels, panels, and power-control sets. Higher computational and RF complexity is expected for full multi-TRP and multi-panel support, making these features more relevant for higher-tier or specialized devices.

Challenges include ensuring robust QCL assumptions across TRPs, managing power limitations (especially in FR2 for multi-panel simultaneous transmission), and maintaining interoperability for both intra-cell and inter-cell multi-TRP scenarios. Dynamic switching between single-TRP and multi-TRP modes is also facilitated by the number of indicated TCI states.

Summary

In 3GPP Release 18, the multi-TRP and unified TCI framework enhancements transform beam management from a primarily single-point capability into a scalable multi-point tool. By extending joint and separate TCI indication to multiple TRPs, supporting dual Timing Advances, and integrating with CJT and multi-panel uplink schemes, the standard enables more efficient, reliable, and capacity-oriented multi-TRP deployments. These features form a critical part of the MIMO evolution in 5G-Advanced, bridging earlier single-TRP designs with the denser, more coordinated networks required for future performance targets.

Uplink MIMO Evolution

Uplink performance has historically lagged downlink in 5G NR. Release 18 prioritizes closing this gap by enabling higher-order spatial multiplexing, multi-panel simultaneous transmission, and supporting reference-signal and codebook structures tailored to advanced devices. These changes primarily target non-smartphone form factors such as Fixed Wireless Access (FWA) customer-premises equipment, vehicles, and industrial terminals that can accommodate more antenna elements and higher transmit power.

Core Motivations for Uplink MIMO Evolution

Prior to Release 18, PUSCH supported a maximum of four layers. This limited peak rates and multi-user MIMO capacity on the uplink, especially in mid-band and mmWave deployments where uplink coverage and throughput are critical. Advanced UEs with eight or more transmit antennas could not fully exploit their hardware. Multi-TRP and multi-panel coordination further required mechanisms for simultaneous transmission from different panels toward different points.

Release 18 addresses these limitations through three interconnected pillars: expansion of the maximum number of layers, simultaneous multi-panel transmission, and supporting enhancements to reference signals and codebooks.

Maximum Layers Increased to Eight

The maximum number of layers for PUSCH is extended from four to eight. This applies to both codebook-based and non-codebook-based transmission.

Codebook-based transmission introduces four new codebook types for eight antenna ports:

  • Codebook 1: Assumes all eight ports are fully coherent. Uses DFT-vector precoders similar to the downlink Type-I codebook.
  • Codebook 2: Groups ports into coherent subsets (for example, two groups of four).
  • Codebook 3: Further partitioning into smaller coherent groups.
  • Codebook 4: Fully non-coherent port selection.

These designs accommodate different levels of antenna coherence that real devices can achieve. Full-power transmission modes (0, 1, and 2) originally defined in Release 16 are extended to support eight layers so that maximum UE transmit power can still be reached under the various coherence assumptions.

For ranks of five or higher, two codewords are used (mirroring downlink behavior for high-rank PDSCH). MCS, NDI, and RV indications, as well as codeword-to-layer mapping, follow rules analogous to those already established for downlink.

Simultaneous Transmission with Multiple Panels (STxMP)

Release 18 specifies Simultaneous Transmission with Multi-Panel (STxMP), allowing a UE to transmit from two antenna panels using different beams in the same slot. This feature is particularly valuable in multi-TRP scenarios and for mmWave operation where panel-specific beams help combat blockage.

Two primary schemes are supported:

  • Spatial Division Multiplexing (SDM): Different layers or data streams are sent from different panels, effectively increasing rank and throughput.
  • Single-Frequency Network (SFN)-style transmission: The same layer is repeated across panels/beams to improve reliability through spatial diversity.

Scheduling supports both single-DCI (ideal backhaul, coordinated scheduling) and multi-DCI (non-ideal backhaul, independent scheduling) modes. Frequency-domain resource allocations between the two transmissions can be fully overlapping, partially overlapping, or non-overlapping. Time-domain overlap can also be full or partial.

STxMP improves uplink spectral efficiency and reliability without requiring sequential beam switching across multiple slots.

Reference-Signal Enhancements Supporting Higher-Order Uplink MIMO

Sounding Reference Signal (SRS)

The maximum number of SRS ports is increased to eight to enable channel sounding for eight-layer transmission. Because transmitting eight ports in a single OFDM symbol would reduce per-port power, Release 18 allows time-division multiplexing of ports across two symbols (maximum of four ports per symbol) in addition to existing repetition techniques. For non-codebook-based transmission, the number of SRS resources per resource set is also expanded.

Demodulation Reference Signal (DMRS)

The enhanced DMRS design (extended Frequency-Domain Orthogonal Cover Code length of four) applies to PUSCH as well as PDSCH. This doubles the number of orthogonal DMRS ports (up to 24), enabling higher-order multi-user MIMO on the uplink while potentially reducing DMRS overhead for single-user cases.

Practical Applications and Performance Impact

These uplink MIMO enhancements deliver the greatest benefit for:

  • Fixed Wireless Access and CPE — Higher-layer and multi-panel transmission raise uplink peak and average rates, making FWA more competitive with wired alternatives.
  • Vehicular and industrial devices — Multi-panel STxMP improves reliability under mobility and blockage, while eight-layer support boosts throughput for high-data-rate uplink applications (video, sensor data, machine vision).
  • Multi-TRP deployments — Coordinated or independent multi-panel transmission toward different TRPs enhances both capacity and coverage uniformity.
  • Overall network capacity — Expanded orthogonal ports and higher per-UE ranks increase the number of simultaneous uplink streams that can be spatially multiplexed.

System-level evaluations and vendor analyses indicate substantial gains in uplink user-perceived throughput and cell-edge performance, particularly when advanced UEs are present in the network.

Implementation Considerations

Supporting eight-layer uplink and STxMP increases UE baseband processing, RF complexity (multiple panels, power amplifiers, and coherence requirements), and power consumption. Capability signaling allows the network to configure features only for devices that support them. Full-power modes and careful SRS design help mitigate power-per-port reductions. Interoperability testing across vendors remains important, especially for multi-panel and multi-DCI multi-TRP combinations.

Summary

Uplink MIMO evolution in 3GPP Release 18 represents a decisive step toward balanced 5G-Advanced performance. By raising the maximum PUSCH rank to eight, introducing simultaneous multi-panel transmission, and enhancing the supporting SRS, DMRS, and codebook structures, the standard unlocks higher uplink throughput and reliability for advanced device classes. These features complement the downlink and multi-TRP improvements discussed in other aspects of the Release 18 MIMO work item, collectively strengthening the radio interface for the next phase of 5G deployment.

Reference-Signal Improvements

Reference signals are fundamental to accurate channel estimation, demodulation, and sounding. In earlier releases, the number of orthogonal ports and certain interference-management capabilities limited the scalability of multi-user MIMO and higher-rank transmission. Release 18 specifically enhances the Demodulation Reference Signal (DMRS) and Sounding Reference Signal (SRS) to support more spatial layers, denser multi-user multiplexing, multi-TRP operation, and advanced uplink capabilities.

Motivations for Reference-Signal Enhancements

Massive MIMO performance depends heavily on the quality and quantity of orthogonal reference signals. Prior to Release 18:

  • DMRS port counts constrained the maximum number of simultaneous MU-MIMO layers.
  • SRS configurations limited effective channel sounding for higher-rank uplink and multi-TRP scenarios.
  • Interference among TRPs and power limitations per port reduced estimation accuracy in denser deployments.

Release 18 addresses these constraints while controlling overhead and maintaining compatibility with existing designs.

Demodulation Reference Signal (DMRS) Enhancements

The primary DMRS improvement is the expansion of orthogonal ports for both PDSCH and PUSCH.

Key technical change The length of the Frequency-Domain Orthogonal Cover Code (FD-OCC) is extended from 2 to 4. This doubles the number of orthogonal DMRS ports relative to Releases 15–17:

  • Up to 24 orthogonal ports are supported (previously limited to 12 in the highest configurations).
  • Applies to both DMRS Type 1 and Type 2.
  • Ports 0 and 1 retain the same channel configuration as earlier releases for backward compatibility.

Benefits

  • Enables higher-order MU-MIMO by allowing more simultaneous spatially multiplexed layers with orthogonal demodulation references.
  • In single-user scenarios, denser code-domain multiplexing can reduce the time-frequency resources occupied by DMRS, lowering overhead and increasing the resources available for data.
  • Supports both downlink and uplink higher-rank transmission (including the new eight-layer uplink capability).
  • Improves multi-TRP operation by providing more orthogonal ports for coherent or non-coherent joint reception/transmission scenarios.

The enhancement is realized through length-4 FD-OCC applied across adjacent resource elements within or across physical resource blocks, with specific generation methods (Walsh codes for PDSCH, cyclic shifts for PUSCH). Scheduling constraints for basic UEs versus advanced UEs are defined to ensure practical deployment.

Sounding Reference Signal (SRS) Enhancements

SRS improvements focus on supporting higher-rank uplink transmission and multi-TRP interference management.

Port expansion and power management

  • Maximum number of SRS ports increased to eight to enable channel sounding for up to eight-layer PUSCH.
  • Transmitting all eight ports in a single OFDM symbol would reduce per-port power. Release 18 therefore supports time-division multiplexing of ports across two symbols (maximum of four ports per symbol), in addition to existing repetition techniques.
  • For non-codebook-based transmission, the number of SRS resources per resource set is expanded (up to eight in relevant configurations).

Interference randomization for multi-TRP SRS designs incorporate improved randomization of interference among different TRPs (for example, via cyclic-shift hopping or related techniques). This is particularly valuable in TDD multi-TRP coherent joint transmission scenarios, where cross-TRP SRS interference can degrade channel estimation quality.

Additional supporting features These changes work in conjunction with the expanded uplink codebooks and simultaneous multi-panel transmission, allowing the network to obtain accurate CSI for advanced UEs and multi-TRP deployments.

Combined Impact on MIMO Performance

Reference SignalPre-Rel-18 LimitationRel-18 ImprovementPrimary Benefit
DMRSMax ~12 orthogonal portsUp to 24 ports via length-4 FD-OCCHigher MU-MIMO capacity; reduced overhead in SU cases
SRSMax 4 ports; limited multi-TRP handlingUp to 8 ports; TDM option; better interference randomizationSupport for 8-layer UL; improved multi-TRP CSI

Together, the DMRS and SRS enhancements enable:

  • Approximately doubled MU-MIMO layer capacity under favorable conditions.
  • Accurate channel estimation for the new eight-layer uplink and multi-panel simultaneous transmission features.
  • Better spectral efficiency and reliability in multi-TRP deployments through cleaner orthogonal references and reduced inter-TRP interference.
  • More efficient use of resources by balancing denser multiplexing against overhead.

Practical Considerations

Implementation requires UE and gNB support for the new OCC lengths, port configurations, and scheduling behaviors. Capability signaling allows the network to enable enhanced DMRS/SRS only for capable devices. Power control and resource allocation must account for the TDM SRS option to maintain adequate per-port energy. In multi-TRP networks with non-ideal backhaul, the improved randomization helps maintain estimation quality without excessive coordination overhead.

These reference-signal improvements form a foundational enabler for the other Release 18 MIMO advances—CSI enhancements for mobility, multi-TRP unified TCI operation, and higher-order uplink MIMO—ensuring that the spatial degrees of freedom introduced by those features can be effectively utilized.

Summary

In 3GPP Release 18, reference-signal improvements center on expanding DMRS orthogonality to 24 ports and enhancing SRS to support eight ports with better multi-TRP interference handling and power-efficient transmission. These changes directly increase the scalability of multi-user and higher-rank MIMO while controlling overhead and supporting the denser, more coordinated deployments characteristic of 5G-Advanced networks.

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