Inter-Band Beam Configuration (IBBC)

• Inter-Band Beam Configuration (IBBC) is a design paradigm that leverages beam parameters from one band to optimize beamforming in another, addressing issues like beam squint.
• It encompasses techniques such as subband phase compensation, out-of-band beam selection, and hierarchical reinforcement learning for joint band assignment and beam management.
• IBBC reduces training overhead and enhances network performance, as demonstrated by significant SINR recovery in simulations and practical dual-band RIS implementations.

Searching arXiv for the cited IBBC-related papers and closely related terminology. arxiv_search(query="all:(\"Inter-Band Beam Configuration\" OR \"beam squinting compensation\" OR \"joint band assignment and beam management\" OR \"coordinated beam selection out-of-band\" OR \"dual-band reconfigurable intelligent surface\" OR \"sub-THz radio unit selection\" )", max_results=10, sort_by="submittedDate")

Inter-Band Beam Configuration (IBBC) denotes a family of cross-frequency beam-control mechanisms in which beam parameters, beam states, or beam geometry in one band or subband are used to configure, compensate, infer, or coordinate beam operation in another. In the literature represented here, the term appears in several technically distinct but related forms: subband beam-squint compensation for network-controlled repeaters (NCRs), hierarchical control for joint band assignment and beam management, out-of-band (OOB) beam selection from sub-6 GHz to mmWave, shared-aperture dual-band reconfigurable intelligent surface (RIS) operation, and user-equipment (UE) orientation modeling through the angle between low-band and high-band broadside directions (Sousa et al., 2024, Kim et al., 2023, Maschietti et al., 2019, Rao et al., 2024, Gupta et al., 12 Jul 2025). Taken together, these usages indicate that IBBC is not a single algorithm but a cross-band design paradigm for maintaining beam consistency, reducing training overhead, and exploiting structural relations between bands.

1. Terminological scope and formal definitions

In a subband beamforming setting, IBBC is realized by taking each beamformer designed at the “measurement” band $f_c$, calculating a simple per-antenna phase-shift vector that exactly cancels the extra delay $\Delta f \cdot R_n \cdot U^*/c$, and then applying these corrected weights whenever transmitting in band $f_c+\Delta f$ (Sousa et al., 2024). In that usage, the central problem is beam squinting: signaling related to measurements is transmitted in one subband centered at frequency $f_c$, while data transmission is performed at a different frequency $f_c+\Delta f$, so a frequency-dependent array radiation pattern can lead to beam misalignment.

In a dual-band UE formulation, Inter-Band Beam Configuration is defined as the angle between the broadside vectors of the low-band array and the high-band array. With $\mathbf{u}_{\mathrm{LB}}$ and $\mathbf{u}_{\mathrm{HB}}$ denoting the unit-vectors pointing along the broadside of the low-band and high-band arrays, respectively, the IBBC angle is written as

$$\theta_{\mathrm{IBBC}}=\arccos\!\bigl(\mathbf{u}_{\mathrm{LB}}^{T}\mathbf{u}_{\mathrm{HB}}\bigr).$$

When the high-band beam is one of a finite steering grid, $\beta$ can be two-dimensional, written as

$$\beta=[\,\theta_{\mathrm{IBBC}}^{\mathrm{az}},\;\theta_{\mathrm{IBBC}}^{\mathrm{el}}\,]^T$$

(Gupta et al., 12 Jul 2025). In that setting, IBBC captures the UE’s orientation and the relative beamforming direction at low- versus high-band.

In OOB-aided multi-user mmWave MIMO, the same cross-band idea appears as an inter-band mapping from sub-6 GHz beams to mmWave candidate beams. Because sub-6 GHz beams are wider, one sub-6 GHz beam pair $\Delta f \cdot R_n \cdot U^*/c$0 “covers” several mmWave beam pairs through a 3 dB mapping set $\Delta f \cdot R_n \cdot U^*/c$1 (Maschietti et al., 2019). In hierarchical reinforcement learning (HRL) for multi-band communication, IBBC is cast as a sequential decision problem in which a high-level policy selects the band and a low-level policy sets beam-management thresholds in the selected band (Kim et al., 2023). In dual-band RIS design, IBBC refers to independently reconfigurable beam steering in sub-6 GHz and mmWave within a single aperture (Rao et al., 2024).

These formulations are not identical. A plausible implication is that IBBC should be understood as an umbrella concept covering cross-band beam coupling, rather than a uniquely standardized protocol primitive.

2. Array-theoretic basis and subband beam-squint compensation

The most explicit signal model appears in the NCR-assisted subband framework. A planar antenna array of $\Delta f \cdot R_n \cdot U^*/c$2 elements is located at positions $\Delta f \cdot R_n \cdot U^*/c$3, $\Delta f \cdot R_n \cdot U^*/c$4. In many examples, a Uniform Linear Array (ULA) along $\Delta f \cdot R_n \cdot U^*/c$5 is used: $\Delta f \cdot R_n \cdot U^*/c$6 A plane wave departing toward azimuth $\Delta f \cdot R_n \cdot U^*/c$7 and optionally zenith $\Delta f \cdot R_n \cdot U^*/c$8 at frequency $\Delta f \cdot R_n \cdot U^*/c$9 induces the element-wise phase

$f_c+\Delta f$0

with

$f_c+\Delta f$1

and steering vector

$f_c+\Delta f$2

Given complex weights $f_c+\Delta f$3, the array factor is

$f_c+\Delta f$4

The wide band is split into two or more subbands, with $f_c+\Delta f$5 for measurement and $f_c+\Delta f$6 for data (Sousa et al., 2024).

If the beamformer $f_c+\Delta f$7 is designed to point at true direction $f_c+\Delta f$8 at $f_c+\Delta f$9, then at $f_c$0 the peak shifts to $f_c$1, where for an ideal ULA

$f_c$2

For $f_c$3, and for small $f_c$4 in radians, this becomes

$f_c$5

with stated validity for $f_c$6 and $f_c$7 (Sousa et al., 2024).

The compensation mechanism uses the phase-deviation view

$f_c$8

where the extra per-element phase

$f_c$9

produces the squint. Let $f_c+\Delta f$0 be the intended beam direction found by maximizing $f_c+\Delta f$1 at $f_c+\Delta f$2. Define the compensation vector

$f_c+\Delta f$3

The compensated weights at $f_c+\Delta f$4 are then

$f_c+\Delta f$5

or, in vector form,

$f_c+\Delta f$6

This closed-form update is directly realizable in analog or hybrid beamforming because only phase shifts are applied, and it also supports a codebook approach in which a compensated version of each nominal beam is pre-stored for each subband (Sousa et al., 2024).

3. Control, performance metrics, and implementation constraints

In the NCR setting, downlink performance is measured through

$f_c+\Delta f$7

where $f_c+\Delta f$8 includes path loss, shadowing and small-scale fading. The instantaneous rate is

$f_c+\Delta f$9

or it is mapped to discrete MCS based on a 10% BLER target (Sousa et al., 2024).

The reported numerical result is specific: in a 3GPP-style system-level simulation at 28 GHz with a ULA of 256 elements, without compensation and $\mathbf{u}_{\mathrm{LB}}$0, the 10%-ile user’s SINR fell by $\mathbf{u}_{\mathrm{LB}}$1 versus the $\mathbf{u}_{\mathrm{LB}}$2 reference. With compensation, the SINR CDF nearly collapses onto the $\mathbf{u}_{\mathrm{LB}}$3 curve, recovering the $\mathbf{u}_{\mathrm{LB}}$4 loss and restoring 10%-ile throughput (Sousa et al., 2024). This supports the narrower claim that subband IBBC can make measurement-band beam decisions usable for data-band transmission without material performance loss when the compensation model is available.

The operational cost is also tightly specified. Per subband, the computational burden is one complex-vector multiplication of length $\mathbf{u}_{\mathrm{LB}}$5, or codebook corrections can be pre-computed off-line. The only side information needed is $\mathbf{u}_{\mathrm{LB}}$6, the beam’s pointing direction, already known from beam management. No new pilot signals or feedback are required; the method uses existing beam-report procedures such as SSB/CSI-RS $\mathbf{u}_{\mathrm{LB}}$7 RSRP $\mathbf{u}_{\mathrm{LB}}$8 best-beam index. Control-plane impact is described as negligible: IBBC requires tagging each subband’s beam index or weight set in the scheduling grant, and the NCR control link simply forwards the compensated beam index (Sousa et al., 2024).

For multiple subbands, compensation generalizes by computing

$\mathbf{u}_{\mathrm{LB}}$9

for subband $\mathbf{u}_{\mathrm{HB}}$0 at $\mathbf{u}_{\mathrm{HB}}$1, then forming

$\mathbf{u}_{\mathrm{HB}}$2

For multi-beam or multi-sector operation, the same process is repeated for each beam direction $\mathbf{u}_{\mathrm{HB}}$3 (Sousa et al., 2024).

Several limitations are explicit. Endfire ambiguity arises when codebook entries produce symmetric beams and user-side direction feedback is required to disambiguate. For large $\mathbf{u}_{\mathrm{HB}}$4 or elevated $\mathbf{u}_{\mathrm{HB}}$5 tilt, compensation is exact only for the main lobe; side lobes may still shift. Finite-bit phase shifters introduce quantization error in $\mathbf{u}_{\mathrm{HB}}$6. Under fast dynamic mobility, if $\mathbf{u}_{\mathrm{HB}}$7 changes faster than the time required to update $\mathbf{u}_{\mathrm{HB}}$8, residual squint may degrade performance (Sousa et al., 2024).

4. Cross-band beam selection and hierarchical decision frameworks

Cross-band IBBC is also used to reduce beam-training overhead by transferring information from a low-frequency band to a high-frequency band. In coordinated OOB beam selection, the sub-6 GHz channel $\mathbf{u}_{\mathrm{HB}}$9 is projected into a spatial spectrum

$$\theta_{\mathrm{IBBC}}=\arccos\!\bigl(\mathbf{u}_{\mathrm{LB}}^{T}\mathbf{u}_{\mathrm{HB}}\bigr).$$0

and the side-information is the expectation $$\theta_{\mathrm{IBBC}}=\arccos\!\bigl(\mathbf{u}_{\mathrm{LB}}^{T}\mathbf{u}_{\mathrm{HB}}\bigr).$$1, where the $$\theta_{\mathrm{IBBC}}=\arccos\!\bigl(\mathbf{u}_{\mathrm{LB}}^{T}\mathbf{u}_{\mathrm{HB}}\bigr).$$2-th entry gives the average sub-6 GHz gain seen by UE $$\theta_{\mathrm{IBBC}}=\arccos\!\bigl(\mathbf{u}_{\mathrm{LB}}^{T}\mathbf{u}_{\mathrm{HB}}\bigr).$$3 with transmit beam $$\theta_{\mathrm{IBBC}}=\arccos\!\bigl(\mathbf{u}_{\mathrm{LB}}^{T}\mathbf{u}_{\mathrm{HB}}\bigr).$$4 and receive beam $$\theta_{\mathrm{IBBC}}=\arccos\!\bigl(\mathbf{u}_{\mathrm{LB}}^{T}\mathbf{u}_{\mathrm{HB}}\bigr).$$5 (Maschietti et al., 2019). No extra training overhead is required because this expectation is obtained from standard sub-6 GHz CSI measurements.

A central mapping is the 3 dB relation between coarse sub-6 GHz beams and candidate mmWave beams. For each sub-6 beam pair $$\theta_{\mathrm{IBBC}}=\arccos\!\bigl(\mathbf{u}_{\mathrm{LB}}^{T}\mathbf{u}_{\mathrm{HB}}\bigr).$$6, the set

$$\theta_{\mathrm{IBBC}}=\arccos\!\bigl(\mathbf{u}_{\mathrm{LB}}^{T}\mathbf{u}_{\mathrm{HB}}\bigr).$$7

contains all mmWave beam pairs whose 3 dB lobes overlap the chosen sub-6 GHz beams (Maschietti et al., 2019). The fine mmWave search is then restricted to that candidate set. In the uncoordinated version, each UE chooses $$\theta_{\mathrm{IBBC}}=\arccos\!\bigl(\mathbf{u}_{\mathrm{LB}}^{T}\mathbf{u}_{\mathrm{HB}}\bigr).$$8 to maximize its own average single-user SNR. In the hierarchical coordinated version, UEs are ordered, and each UE overhears via low-rate device-to-device links the chosen sub-6 receive beam indices of lower-ranked UEs, then solves a constrained optimization to avoid receive-beam conflicts (Maschietti et al., 2019).

The key analytical statement is Proposition 1: in the large-array limit, after ZF combining at the base station, the average SINR of user $$\theta_{\mathrm{IBBC}}=\arccos\!\bigl(\mathbf{u}_{\mathrm{LB}}^{T}\mathbf{u}_{\mathrm{HB}}\bigr).$$9 is

$\beta$0

(Maschietti et al., 2019). The practical interpretation in the source is that any two UEs sharing the same mmWave BS beam cause zero effective SINR for one of them under ZF, described there as irreducible co-beam interference. The coordination rule therefore forbids candidates whose receive beam conflicts with already selected beams.

The reported performance summary is again explicit. For $\beta$1 UEs, $\beta$2, $\beta$3, mmWave at 28 GHz, and sub-6 GHz at 3 GHz, coordinated IBBC yields up to $\beta$4–$\beta$5 sum-rate gain over uncoordinated selection at moderate SNR $\beta$6–$\beta$7 dB and small inter-UE distances $\beta$8 m. The D2D exchange requires only $\beta$9 bits per UE at beam-update intervals, and beam coherence times are stated to be much larger than channel coherence times (Maschietti et al., 2019).

A different cross-band control view is developed in the HRL formulation for joint band assignment and beam management. Time is indexed by $$\beta=[\,\theta_{\mathrm{IBBC}}^{\mathrm{az}},\;\theta_{\mathrm{IBBC}}^{\mathrm{el}}\,]^T$$0, each $$\beta=[\,\theta_{\mathrm{IBBC}}^{\mathrm{az}},\;\theta_{\mathrm{IBBC}}^{\mathrm{el}}\,]^T$$1 being one OFDM time frame. The state

$$\beta=[\,\theta_{\mathrm{IBBC}}^{\mathrm{az}},\;\theta_{\mathrm{IBBC}}^{\mathrm{el}}\,]^T$$2

captures currently installed analog and digital beams and codebook indices in each band, together with recent spectral-efficiency feedbacks (Kim et al., 2023). The high-level action or goal is

$$\beta=[\,\theta_{\mathrm{IBBC}}^{\mathrm{az}},\;\theta_{\mathrm{IBBC}}^{\mathrm{el}}\,]^T$$3

indicating the selected band, with $$\beta=[\,\theta_{\mathrm{IBBC}}^{\mathrm{az}},\;\theta_{\mathrm{IBBC}}^{\mathrm{el}}\,]^T$$4 sub-6 GHz and $$\beta=[\,\theta_{\mathrm{IBBC}}^{\mathrm{az}},\;\theta_{\mathrm{IBBC}}^{\mathrm{el}}\,]^T$$5 mmWave. The low-level action is

$$\beta=[\,\theta_{\mathrm{IBBC}}^{\mathrm{az}},\;\theta_{\mathrm{IBBC}}^{\mathrm{el}}\,]^T$$6

two continuous thresholds in the chosen band. Reward is built from the instantaneous spectral efficiency

$$\beta=[\,\theta_{\mathrm{IBBC}}^{\mathrm{az}},\;\theta_{\mathrm{IBBC}}^{\mathrm{el}}\,]^T$$7

and

$$\beta=[\,\theta_{\mathrm{IBBC}}^{\mathrm{az}},\;\theta_{\mathrm{IBBC}}^{\mathrm{el}}\,]^T$$8

where $$\beta=[\,\theta_{\mathrm{IBBC}}^{\mathrm{az}},\;\theta_{\mathrm{IBBC}}^{\mathrm{el}}\,]^T$$9 during beam training and $\Delta f \cdot R_n \cdot U^*/c$00 when data is sent (Kim et al., 2023).

The hierarchy separates a slow-timescale upper-level policy $\Delta f \cdot R_n \cdot U^*/c$01 from a fast-timescale lower-level policy $\Delta f \cdot R_n \cdot U^*/c$02. Both levels use DDPG with experience replay and slow-moving target networks, and off-policy correction is done either by direct importance sampling or action-relabeling (Kim et al., 2023). The evaluation uses a Manhattan grid at $\Delta f \cdot R_n \cdot U^*/c$03, vehicle density $\Delta f \cdot R_n \cdot U^*/c$04, QuaDRiGa “3GPP UMi” ray-tracing, $\Delta f \cdot R_n \cdot U^*/c$05 Tx/$\Delta f \cdot R_n \cdot U^*/c$06 Rx mmWave hybrid arrays with $\Delta f \cdot R_n \cdot U^*/c$07 RF chains and $\Delta f \cdot R_n \cdot U^*/c$08 streams, and $\Delta f \cdot R_n \cdot U^*/c$09 fully digital sub-6 GHz arrays with $\Delta f \cdot R_n \cdot U^*/c$10 streams. The metric is ensemble-averaged cumulative data rate over $\Delta f \cdot R_n \cdot U^*/c$11 channel realizations. At $\Delta f \cdot R_n \cdot U^*/c$12, HRL achieves $\Delta f \cdot R_n \cdot U^*/c$13 versus $\Delta f \cdot R_n \cdot U^*/c$14 for DRL and $\Delta f \cdot R_n \cdot U^*/c$15 for a greedy mmWave-only policy, and HRL converges in $\Delta f \cdot R_n \cdot U^*/c$16 episodes while DRL takes $\Delta f \cdot R_n \cdot U^*/c$17 episodes for $\Delta f \cdot R_n \cdot U^*/c$18 (Kim et al., 2023).

These two strands—OOB beam transfer and hierarchical band/beam control—address different problems, but both instantiate IBBC as a mechanism for exploiting inter-band structure to reduce the effective cost of beam management.

5. IBBC as a latent geometry variable in RU selection

In sub-THz radio unit selection, IBBC is elevated from a beam-control primitive to a latent geometry variable. The end-to-end mapping for a UE at unknown 3-D position $\Delta f \cdot R_n \cdot U^*/c$19 is written as

$\Delta f \cdot R_n \cdot U^*/c$20

where $\Delta f \cdot R_n \cdot U^*/c$21 is a sub-10 GHz channel feature vector, $\Delta f \cdot R_n \cdot U^*/c$22 is the UE’s 3-D orientation, $\Delta f \cdot R_n \cdot U^*/c$23 is the IBBC, and $\Delta f \cdot R_n \cdot U^*/c$24 is the best sub-THz RU index (Gupta et al., 12 Jul 2025). If the network ignores $\Delta f \cdot R_n \cdot U^*/c$25 and $\Delta f \cdot R_n \cdot U^*/c$26, the learned function $\Delta f \cdot R_n \cdot U^*/c$27 suffers from “one-to-many” ambiguity.

The framework uses three supervised deep-learning classifiers. With $\Delta f \cdot R_n \cdot U^*/c$28 as a measured sub-THz feature vector from the coarsely presumed-best RU, the inputs are

$\Delta f \cdot R_n \cdot U^*/c$29

corresponding respectively to IBBC inference, RU inference with IBBC, and RU inference without IBBC (Gupta et al., 12 Jul 2025). The IBBC label is quantized into one of $\Delta f \cdot R_n \cdot U^*/c$30 classes, and the RU index is chosen among $\Delta f \cdot R_n \cdot U^*/c$31 RUs. All three classifiers share a fully connected architecture with three hidden layers of sizes $\Delta f \cdot R_n \cdot U^*/c$32, ReLU hidden activations, a Softmax output, categorical cross-entropy loss, SGDM with learning rate $\Delta f \cdot R_n \cdot U^*/c$33, batch size typically $\Delta f \cdot R_n \cdot U^*/c$34, and an $\Delta f \cdot R_n \cdot U^*/c$35 training-validation split (Gupta et al., 12 Jul 2025).

The inference loop is staged. Algorithm 3 first produces a coarse RU estimate without IBBC. The network then measures $\Delta f \cdot R_n \cdot U^*/c$36 on that RU, forms $\Delta f \cdot R_n \cdot U^*/c$37, applies Algorithm 1 to infer $\Delta f \cdot R_n \cdot U^*/c$38, and finally refines the RU estimate by running Algorithm 2 on $\Delta f \cdot R_n \cdot U^*/c$39 (Gupta et al., 12 Jul 2025). This procedure is explicitly motivated by the observation that IBBC indicates beamforming information or UE orientation, which is typically not shared with the network as part of signalling.

The simulation uses an $\Delta f \cdot R_n \cdot U^*/c$40 m office room, $\Delta f \cdot R_n \cdot U^*/c$41 ceiling-mounted RUs, one wall-mounted sub-10 GHz AP, UEs on a $\Delta f \cdot R_n \cdot U^*/c$42 m grid, one of $\Delta f \cdot R_n \cdot U^*/c$43 IBBCs assigned at random, $\Delta f \cdot R_n \cdot U^*/c$44 URAs at UE and RU/AP, carriers at $\Delta f \cdot R_n \cdot U^*/c$45 GHz and $\Delta f \cdot R_n \cdot U^*/c$46 GHz, and ray-traced instantaneous power-delay profiles as features (Gupta et al., 12 Jul 2025). The reported SNR CDFs show median UE SNR $\Delta f \cdot R_n \cdot U^*/c$47 dB for RU inference without IBBC, $\Delta f \cdot R_n \cdot U^*/c$48 dB for perfect IBBC, and $\Delta f \cdot R_n \cdot U^*/c$49 dB for inferred IBBC. At the $\Delta f \cdot R_n \cdot U^*/c$50th percentile, inferred IBBC improves by $\Delta f \cdot R_n \cdot U^*/c$51–$\Delta f \cdot R_n \cdot U^*/c$52 dB over the no-IBBC baseline. Final classification accuracies are $\Delta f \cdot R_n \cdot U^*/c$53 for RU inference without IBBC, $\Delta f \cdot R_n \cdot U^*/c$54 with true IBBC, and $\Delta f \cdot R_n \cdot U^*/c$55 for the $\Delta f \cdot R_n \cdot U^*/c$56-class IBBC inference task (Gupta et al., 12 Jul 2025).

This usage differs conceptually from subband compensation. Here IBBC is not a phase-correction vector but a hidden state variable linking low-band channel fingerprints to high-band beam or RU choice.

6. Hardware embodiment in dual-band RISs

A hardware realization of independent inter-band beam configuration appears in a shared-aperture dual-band sub-6 GHz and mmWave RIS (Rao et al., 2024). The mmWave RIS element at 28 GHz is a double-layer microstrip patch aperture-fed through a ground slot and loaded with a 1-bit reflection-type phase shifter using an MA4GP907 PIN diode. The two states are “off” with $\Delta f \cdot R_n \cdot U^*/c$57 and “on” with $\Delta f \cdot R_n \cdot U^*/c$58, and the measured $\Delta f \cdot R_n \cdot U^*/c$59 is approximately $\Delta f \cdot R_n \cdot U^*/c$60 dB to $\Delta f \cdot R_n \cdot U^*/c$61 dB. The reflection coefficient is

$\Delta f \cdot R_n \cdot U^*/c$62

The sub-6 GHz RIS element at 3.5 GHz reuses an $\Delta f \cdot R_n \cdot U^*/c$63 block of 28 GHz patches. The central $\Delta f \cdot R_n \cdot U^*/c$64 patches are selectively interconnected with $\Delta f \cdot R_n \cdot U^*/c$65 PIN-diode RF switches, giving $\Delta f \cdot R_n \cdot U^*/c$66 reconfigurable states. A multi-port model describes the internal connections through an adjacency matrix $\Delta f \cdot R_n \cdot U^*/c$67, a switching vector $\Delta f \cdot R_n \cdot U^*/c$68, and an inter-port loading matrix $\Delta f \cdot R_n \cdot U^*/c$69. The scattered field per state is

$\Delta f \cdot R_n \cdot U^*/c$70

(Rao et al., 2024).

Beam steering is based on the continuous phase law

$\Delta f \cdot R_n \cdot U^*/c$71

At 28 GHz, the desired phase is quantized to $\Delta f \cdot R_n \cdot U^*/c$72. At 3.5 GHz, each of the $\Delta f \cdot R_n \cdot U^*/c$73 states yields a distinct reflection phase, and a phase-entropy metric

$\Delta f \cdot R_n \cdot U^*/c$74

is used to ensure uniform spacing of the states (Rao et al., 2024).

Independent operation is achieved through physical and control isolation. A planar spiral inductor (PSI) is optimized so that at 28 GHz it is near-invisible to the patch, with $\Delta f \cdot R_n \cdot U^*/c$75 dB, while at 3.5 GHz it acts as a low-impedance metal link. A suspended electromagnetic band gap (EBG) structure is used to suppress surface waves, with a simulated bandgap of approximately 27–29 GHz. The FPGA writes sub-6 and mmWave registers over separate SPI buses or multiplexes, and the source states that no cross-coupling was measured: mmWave beam patterns remained unchanged over all 8 sub-6 states (Rao et al., 2024).

The experimental ranges are also explicit. The fabricated sub-6 GHz RIS with $\Delta f \cdot R_n \cdot U^*/c$76 sub-6 elements achieves beam steering from $\Delta f \cdot R_n \cdot U^*/c$77 to $\Delta f \cdot R_n \cdot U^*/c$78. The $\Delta f \cdot R_n \cdot U^*/c$79 mmWave RIS achieves beam steering from $\Delta f \cdot R_n \cdot U^*/c$80 to $\Delta f \cdot R_n \cdot U^*/c$81. At 28 GHz, reported gain is about $\Delta f \cdot R_n \cdot U^*/c$82 dBi with side-lobe level at most $\Delta f \cdot R_n \cdot U^*/c$83 dB; at 3.5 GHz, efficiency is about $\Delta f \cdot R_n \cdot U^*/c$84 with circa $\Delta f \cdot R_n \cdot U^*/c$85 dBi broadside gain (Rao et al., 2024). This hardware instantiation shows that IBBC can be realized not only algorithmically but also as an independently reconfigurable electromagnetic structure.

7. Limitations, misconceptions, and synthesis across the literature

A recurring misconception would be to treat IBBC as a single universally accepted mathematical object. The cited works do not support that simplification. In one line of work, IBBC is a per-element phase compensation across subbands (Sousa et al., 2024). In another, it is an HRL decomposition of band selection and beam management (Kim et al., 2023). In OOB multi-user MIMO, it is a cross-band beam-candidate restriction and coordination mechanism (Maschietti et al., 2019). In dual-band RIS design, it is independent beam steering in two bands on a shared aperture (Rao et al., 2024). In sub-THz RU selection, it is the angle between low-band and high-band broadside vectors, or a two-dimensional azimuth-elevation quantity capturing UE orientation (Gupta et al., 12 Jul 2025).

The limitations are equally heterogeneous. Subband compensation is exact only for the main lobe under large $\Delta f \cdot R_n \cdot U^*/c$86 or elevated $\Delta f \cdot R_n \cdot U^*/c$87 tilt, can be affected by finite-bit phase shifter quantization, and can degrade under fast mobility if $\Delta f \cdot R_n \cdot U^*/c$88 changes too quickly (Sousa et al., 2024). Coordinated OOB selection depends on exchange of beam-related information over low-rate D2D links and on the large-array approximation underlying the zero-SINR co-beam result (Maschietti et al., 2019). HRL performance depends on the choice of goal horizon $\Delta f \cdot R_n \cdot U^*/c$89, off-policy correction strategy, and beam-training overhead model (Kim et al., 2023). The dual-band RIS relies on PSI resonance, EBG tuning, and separate bias/control networks to preserve electrical decoupling (Rao et al., 2024). RU selection with inferred IBBC remains limited by imperfect IBBC classification, with final accuracy around $\Delta f \cdot R_n \cdot U^*/c$90 on the $\Delta f \cdot R_n \cdot U^*/c$91-class task (Gupta et al., 12 Jul 2025).

Across these variants, a consistent technical pattern nevertheless emerges. Beam behavior in one band or subband is treated as informative for another band, but only after accounting for geometric mismatch, hardware constraints, training overhead, or latent orientation variables. This suggests that the core significance of IBBC lies in exploiting cross-band structure without assuming that beams are naively transferable. In that narrower and evidence-supported sense, IBBC functions as a unifying design principle for multi-band and multi-subband beam management in mmWave, sub-THz, and dual-band RIS systems (Sousa et al., 2024, Kim et al., 2023, Maschietti et al., 2019, Rao et al., 2024, Gupta et al., 12 Jul 2025).