---
title: Intelligent Transmitting Surface (ITS)
url: https://www.emergentmind.com/topics/intelligent-transmitting-surface-its
type: topic
---

# Intelligent Transmitting Surface (ITS)

An Intelligent Transmitting Surface (ITS) is a dynamically programmable two-dimensional array of subwavelength elements designed to impose independently tunable amplitude and phase transformations on incident electromagnetic waves, predominantly for transmission (refractive) functionality, thereby enabling full-space coverage and advanced channel reconfigurability in electromagnetic environments. ITS technology generalizes the conventional reconfigurable intelligent surface (RIS) paradigm from reflection-only to transmission, and, in its most general form (the STAR-RIS or hybrid mode), supports simultaneous transmission and reflection with per-element-independent control, leading to rich degrees of freedom for beamforming, coverage, and spatial diversity [2101.09663][2504.09618][2212.01097].

## 1. ITS Hardware Models and Element Design

An ITS element is typically modeled as a passive or weakly active two-port network characterized by complex transmission ($T_m$) and reflection ($R_m$) coefficients:
\[
T_m = \sqrt{\beta^T_m}\,e^{j\phi^T_m}, \quad R_m = \sqrt{\beta^R_m}\,e^{j\phi^R_m}
\]
where $|T_m|^2 + |R_m|^2 \leq 1$ ensures energy conservation. For pure transmission-mode ITS, $R_m = 0$ and $|T_m| = 1$. Advanced architectures such as the Beyond-Diagonal RIS (BD-RIS) feature independent control of $T_m$ and $R_m$ per cell using hybrid topologies: each cell comprises back-to-back phase-reconfigurable antennas (one for transmission, one for reflection) interconnected via a lossless, varactor-based two-port splitter, enabling power splitting over $-20$ to $+20$~dB in the prototype [2504.09618].

Implementation options include:
- **PIN-diode patch arrays**: sub-GHz/low-GHz, coarse phase/amplitude quantization, low cost.
- **Varactor/delay-line structures**: continuous control, higher insertion loss at large phase shifts.
- **Dielectric/graphene metasurfaces**: high-frequency operation, fine/continuous control.
- **Active amplifying ITS elements**: add gain for outdoor-to-indoor or penetration loss scenarios [2206.13801].

A summary of hardware architectures and their degree of phase/amplitude control:

| Architecture          | Amplitude/Phase Control           | Operational Frequency   |
|-----------------------|-----------------------------------|------------------------|
| PIN-diode patch array | 1-2 bits, coupled T/R            | <10 GHz                |
| Delay-line/Varactor   | Continuous, partly decoupled      | 1-60 GHz               |
| Graphene metasurface  | Continuous, near-independent      | 100s GHz–THz           |
| BD-RIS hybrid cell    | 2-bit phase, full power splitting | 2-3 GHz (demo), scalable |

## 2. Physical and Analytical Channel Models

ITSs modify the propagation environment by coupling incident waves via subwavelength-distributed, phase- and amplitude-programmable transmission coefficients. Analytical models treat each element as a S-parameterized two-port scatterer, yielding discrete and continuous-space end-to-end channel expressions. Key models:

- **Far-field (planewave) approximation**: The composite channel between transmitter, surface, and user is $g_k^T = (\mathbf{r}_k^T)^H\,\mathrm{diag}(T_1,\ldots,T_M)\,\mathbf{h}_\mathrm{Tx \to ITS}$, modulated by large/small-scale fading and path-loss [2101.09663][2212.01097].
- **Near-field/Fresnel diffraction**: Applicable for large-aperture ITS or users in the Fresnel region, integrating nonuniform path length and element factor [2101.09663]. 
- **Huygens–Fresnel/Kirchhoff, polarizability tensor (GSTC), and multiport circuit network**: Allow for high-fidelity simulation of beam propagation, near-field effects, and element coupling [2108.06233].
  
Path-loss laws, beamforming patterns, and transmission gains must consider the 3D geometry, orientation, and electromagnetic efficiency, including element insertion loss and mutual coupling [2101.09663][2212.01097].

## 3. Diversity, Beamforming, and Communication Performance

ITS architectures substantially increase communication diversity and beamforming flexibility versus reflect-only RIS:
- **Diversity Order**: STAR-RIS/ITS structures with $M$ elements achieve a diversity order of $M+1$ per side (for both reflection and transmission), enabling outage and BER slopes unattainable by separate transmit/reflect arrays of the same total aperture. The sum diversity is $2(M+1)$, outstripping the $M+2$ of a conventional transmit/reflect split RIS [2101.09663].
- **Full-space (360°) coverage**: By steering transmission and reflection beams independently, ITS achieves full-space beamforming [2504.09618][2212.01097].
- **MIMO capacity and precoding**: ITS enables hybrid architectures with a small number of active feeds and many passive phase-tunable elements, achieving high spatial multiplexing and array gain with greatly reduced hardware complexity. Precoder design involves non-convex optimization due to unit-modulus and physical constraints, solvable via alternating optimization, SCA, semidefinite relaxation, or manifold-based methods [2212.01097][1902.07670].
- **Zero-forcing/ML-based precoding for ITS**: For massive arrays in single-RF-chain scenarios, spatial Sigma–Delta modulation provides low-complexity multiuser precoding—approaching unquantized ZF with only quantized per-element phases [2310.16347].

Key performance metrics under practical scenarios:
- **Beam steering**: BD-RIS prototype demonstrates ±45° scan range, HPBW ≈ 8–14°, transmitted/received gain ≈ 6–7 dBi, and up to 50 dB continuous power splitting [2504.09618].
- **Outage/BER**: ITS achieves orders-of-magnitude lower outage/BER at a given SNR compared to half-aperture split RIS for the same element count [2101.09663].
- **Energy efficiency**: ITS-aided transmitters maintain constant system power as $M$ increases, enabling energy efficiency to rise without additional RF chains [1902.07670].

## 4. Optimization, Control, and Channel Estimation

ITS-based communication requires joint optimization and estimation routines accounting for the unique constraints:
- **Element-wise optimization**: Joint amplitude and phase control under energy conservation constraints is addressed via alternating optimization (AO), successive convex approximation (SCA), and semidefinite programming (SDP). For coupled transmission/reflection phases in purely passive designs, the per-element phases must satisfy $|\phi^T_m - \phi^R_m| = \pi/2$ or $3\pi/2$ when both $\beta^T_m$ and $\beta^R_m$ are nonzero [2110.02374].
- **Operating protocols**: ITSs/STAR-RISs support energy-splitting (ES), mode-switching (MS), and time-switching (TS) modes; resource allocation and switching can be optimized for multi-sector, multi-user, and full-duplex operation [2212.01097][2110.05311].
- **Channel estimation**: Lack of RF chains at the surface necessitates upstream estimation. Techniques include model-based pilot cycling, compressed sensing (for sparse mmWave), PARAFAC tensor decomposition, deep learning, and reinforcement learning-driven beam training. Efficient two-pilot MAP channel tracking leverages temporal correlation for near-perfect channel state information (CSI) with minimal overhead [2601.19660].
- **Scattering matrix and dynamic grouping**: In BD-RIS (beyond-diagonal) architectures, dynamic grouping of elements and optimized block-diagonal scattering enhances multiuser sum-rate, implemented by fractional programming and manifold optimization [2210.02499].

## 5. Prototyping, Practical Realizations, and Limitations

ITS prototypes have validated theoretical predictions in reflection, transmission, and hybrid modes:
- **BD-RIS/ITS testbeds**: 4×4 BD-RIS at 2.4 GHz achieves dual-mode beamforming, independent transmission/reflection control, switchable in <100 ns via FPGA, with insertion losses <0.8 dB (pure modes), ~4 dB in split mode [2504.09618].
- **Active ITS**: Power-amplified transmissive elements, block-grouped for cost reduction, provide high-throughput outdoor-to-indoor penetration at mmWave, with half to an order-of-magnitude fewer elements needed for given sum-rate versus passive designs [2206.13801].
- **Wireless power transfer (WET) ITS**: Digital beamforming feeder arrays illuminating passive ITSs enable scalable, energy-efficient RF WET to IoT devices, with SCA-based joint optimization of digital and ITS config [2507.06805].
- **Channel estimation for mobility/penetration**: ITSs embedded in high-speed railways (transparent windows) are shown to facilitate tractable estimation of Doppler shifts, path gains, and spatial AoDs/AoAs with minimal pilot overhead, outperforming reflection-based solutions in challenging environments [2208.00455].

Identified practical limitations and non-idealities:
- **Phase-amplitude coupling**: Passive metasurface elements do not support fully independent amplitude and phase control—actual transfer functions must be characterized empirically per hardware.
- **Quantization and mutual coupling**: Element discretization and near-field mutual coupling require high-order EM modeling and quantization-aware robust design.
- **Insertion loss, complexity, and control**: Splitter/varactor and phase shifter insertion loss, finite phase resolution, and large I/O requirement pose implementation challenges, mitigated by block/group amplifying, complexity-reducing protocols, and higher efficiency components.

## 6. Emerging Applications and Open Research Issues

ITSs are poised for broad impact across upcoming 6G and sub-THz wireless networks:
- **Full-duplex and MIMO systems**: Simultaneous transmission/reflection with independent beam control enables in-band full-duplex, spatial multiplexed links, and 360° coverage for dense user deployments [2203.05411][2504.09618].
- **Integrated sensing and communication (ISAC)**: ITSs support joint radar–communication functions (sensing-assisted comms, vehicle echo localization, dynamic object tracking) using split power and phase control for concurrent echo enhancement and high-rate communication [2211.04200].
- **Wireless power and IoT**: ITS-illuminated WET power beacons outcompete conventional analog/digital/hybrid beamforming in scaling, efficiency, and hardware cost for massive IoT deployments [2507.06805].
- **Resource allocation and security**: Dynamic ATA assignment (grouping), secure programmable transmission, and robust channel estimation for multi-ITS, multi-cell systems remain active research frontiers [2210.02499][2212.01097].

Major open challenges include:
- **Physically compliant modeling for large surfaces at mmWave/THz bands**, encompassing practical nonidealities, delay, and coupling.
- **Ultra-low-overhead and real-time channel estimation/beam training**, exploiting machine learning while respecting physical and hardware constraints.
- **Physical-layer security and reconfigurable jamming** in 360° programmable environments.
- **Scalable control/management protocols** suitable for distributed, federated, and rapidly reconfigurable ITS deployments.
- **Active/passive hardware hybridization** to approach the theoretical performance bounds of fully adaptive, multi-modal surfaces.

## 7. Summary Table: Key Properties of ITS/STAR-RIS

| Property               | ITS/STAR-RIS                  | Conventional RIS       | BD-RIS Prototype          |
|------------------------|-------------------------------|-----------------------|---------------------------|
| Coverage               | 360° (dual-sided T & R)       | ≤180° (reflect-only)  | 360°, steered in both domains |
| Per-element control    | $|T_m|,\,\angle T_m$; $|R_m|,\,\angle R_m$ (subject to $|T_m|^2+|R_m|^2\le 1$)  | $|R_m|,\,\angle R_m$ | 2-bit phase, continuous power-splitting |
| Diversity order        | $M+1$ per side                | $M_\chi+1$ on one side| Up to $2(M+1)$           |
| Hardware complexity    | Moderate–High (T/R splitters, per-bit phase) | Low–Moderate             | Moderate; demo: 4×4 cells = 160 I/Os   |
| Application domains    | Full-duplex, XR/VR, outdoor-to-indoor, WET, ISAC | Coverage extension only | Penetrating glass/wall, FD communication |

ITSs, building on reconfigurable surface theory and hybrid transmission/reflection meta-atom design, constitute a foundational enabling platform for the next generation of "smart radio environments," with experimentally validated advantages in spectral efficiency, energy efficiency, and network coverage [2101.09663][2504.09618][2212.01097].

Source: https://www.emergentmind.com/topics/intelligent-transmitting-surface-its