FlexLink: Decoupling Control and Data Beams for Next-Generation Wideband Networks Ish Kumar Jain
Rohith Reddy Vennam
Dinesh Bharadia
Rensselaer Polytechnic Institute Troy, NY, USA
University of California San Diego La Jolla, CA, USA
University of California San Diego La Jolla, CA, USA
CCS Concepts • Hardware → Wireless devices; • Networks → Physical links; Wireless access points, base stations and infrastructure.
Keywords Multi-beamforming, Delay-phased arrays, Control–data decoupling, Millimeter-wave and mid-band networks, Hardware prototyping ACM Reference Format: Ish Kumar Jain, Rohith Reddy Vennam, and Dinesh Bharadia. 2025. FlexLink: Decoupling Control and Data Beams for Next-Generation Wideband Networks . In International Symposium on Theory, Algorithmic Foundations, and Protocol Design for Mobile Networks and Mobile Computing (MobiHoc ’25), October 27–30, 2025, Houston, TX, USA. ACM, New York, NY, USA, 10 pages. https://doi.org/10.1145/ 3704413.3764470
This work is licensed under a Creative Commons Attribution 4.0 International License. MobiHoc ’25, Houston, TX, USA © 2025 Copyright held by the owner/author(s). ACM ISBN 979-8-4007-1353-8/25/10 https://doi.org/10.1145/3704413.3764470
5G NR SSB Control SSB Signal
Wasted Resources
Useful Data
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(a) Traditional 5G NR
Frequency
The next generation of 6G networks aims to utilize ultrawideband spectrum and massive antenna arrays to serve multiple users with both control and data channels at low latency and high efficiency. However, phased arrays at mmWave and mid-bands are fundamentally constrained to a single beam or suffer sharp beamforming loss when split across directions, limiting simultaneous control-data support. In FlexLink, we introduce and prototype a novel delay-phased array architecture that overcomes this limitation by redistributing energy jointly across frequency and space, enabling multiple narrow beams without sacrificing per-beam gain or requiring additional power. We design and prototype FlexLink on a custom 4-7 GHz hardware testbed, demonstrating for the first time that control and data beams can be decoupled in practice, achieving nearly double spectral efficiency compared to conventional phased arrays.
Decoupled Ctrl and Data support
Only Data
Only Control
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arXiv:2606.01454v1 [eess.SP] 31 May 2026
Abstract
Useful Data
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(b) FlexLink
Figure 1: FlexLink designs a novel radio architecture that can decouple control and data signaling through flexible frequency resource distribution of control and data to different directions with narrow pencil beams, much like a flexible configurable prism.
1
Introduction
Wideband and Multi-antenna systems such as current Millimeter wave (mmWave) bands in Frequency Range 2 (24.25 GHz to 52.6 GHz) and upcoming mid-bands in Frequency Range 3 (7.125 GHz to 24.25 GHz) are vital for next-generation 6G and beyond networks to support high throughput and low latency applications such as autonomous vehicles, industrial IoT, XR streaming. To facilitate communication at these bands, 3GPP adopts OFDMA to optimize time and frequency resources (in the form of resource blocks or RB) with various control and data signals. However, current mmWave systems suffer from low spectrum utilization and cannot fill all orthogonal RBs with control and data signals due to a constraint posed by directional links. A phased array creates a single beam to radiate all RBs in a single direction, meaning all the control and data are radiated in one single direction. For instance, the SSB (Secondary Synchronization Block) control signal beam has to cover all 360 space since a new user may appear at any angle around the base station. However, the data beam focuses on an active user direction, which may not be located along the SSB control beam direction (Figure 1). Additionally, since these control signals occupy a small number of resource blocks, the remaining RBs could go unused/wasted resources. SSB control signal requires only 7% of the entire 400 MHz band; the remaining 93% RBs are unused and wasted as they cannot serve any
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IK Jain, RR Vennam, D Bharadia
user with data communication [1]. This frequency-direction problem with a non-linear constraint and solving it to a constraint leads to the coupling of control and data signals. closed-form expression. The closed-form formula allows us The specs for mid-bands are not yet designed, but they are to obtain per-antenna delay and phase values as a function expected to follow a similar pattern to 5G NR [2], facing of the desired number of beams, beam angles, and the fracsimilar issues of coupled data and control beams. tion of bandwidth per beam. The computation is O(𝐾) for To decouple control and data signals, multi-beams archiphases and O(𝐾 2 ) for delays, for 𝐾 beams, independent of tectures are proposed, that can support multiple directions the number of antennas 𝑁 or number of subcarriers 𝑀. In simultaneously, i.e., one direction for control signals and practice, we require only a few beams for control and data the remaining directions for data communication in a sintraffic (e.g., one beam for control beam scan and 1-2 beams gle time slot. Traditional phased arrays can be used to crefor data), so the complexity is close to O(1), for a few fixed ate multi-beams, but since they utilize an antenna-splitting beams scenarios. To the best of our knowledge, this is the mechanism [3, 4], they compromise performance in terms of first work to achieve a closed-form formula for delay and lower throughput and coverage. Splitting a beam into two phase values to program DPA. reduces beamforming gain by half for each split beam to 2) Supporting a wide range of bandwidth parts: Secpreserve the total radiated power. TTD-based architecture is ond, the multi-beams should support both control and data proposed to create infinitely many beams at the same time signals with a wide range of bandwidth parts. The control by spreading each frequency subcarrier to different direcsignal may require as small as 7% of bandwidth, while the tions like a prism [5–9]. But this extreme of infinitely many data may need a large 93%. So, the hardware should support beams radiates in all directions, even those that may not have such diverse bandwidth parts. We show that the traditional an active user, thus wasting spectrum resources. Moreover, solution is limited to a minimum of 20% bandwidth part each direction receives signals from a tiny fraction of bandsupport. Lower than 20% bandwidth part, the beamforming width, which is not enough to schedule a wideband signal gain degrades exponentially and cannot be supported. In transmission for data signals. Therefore, there is a gap in the contrast, we optimize the framework to support this wide literature to provide support for decoupling control and data range of bandwidth-parts while supporting less than 10% signals without wasting precious spectrum resources. bandwidth-part for the control beam, while the remaining In this paper, we propose FlexLink, a novel multi-beamforming 90% goes to the data beam. system to decouple the control and data beams in a flexi3) Designing hardware prototype for FlexLink: Fible, 5G-compliant manner that achieves high spectrum utinally, we are the first to build a hardware prototype for DPA lization. FlexLink uses an antenna array architecture called and demonstrate the performance of frequency-dependent delay-phased array (DPA) [8, 9]. The DPA architecture conmulti-beams in real-world settings with over-the-air experisists of both delays and phases, unlike traditional phased arments. Our prototype operates in the 4-7 GHz range, which rays, which only have phase elements, or TTD arrays, which can be extended to mid-band frequencies. We discuss various only have delay elements. Recent work on DPA shows the design choices in building this hardware prototype, including ability to create flexible frequency-dependent multi-beamforming, the impact of range and resolution of delay and phase values with the ability to stream a subset of frequencies in one diper antenna, the methodology to achieve a wideband delay rection and another subset in another direction until all freelement using a switched array architecture, and system inquency resources are utilized without wastage. While this tegration. The code and artifacts for FlexLink are available architecture has been previously used for multi-user comonline1 . munication with frequency multiplexing, we propose a new FlexLink Evaluation Overview: We verify the perforapplication for decoupling control and data signals, while mance of FlexLink through both system and circuit-level meeting more stringent requirements for this application. simulations and over-the-air hardware measurements. We We particularly make three important contributions: compare antenna gain and throughput performance with two 1) Fast configuration of multi-beams: First, we develop baselines: One DPA-based heuristic algorithm, which suffers a new optimization framework to estimate delays and phases from high computational complexity, and another phased per antenna to create a desired multi-beam response, for inarray multi-beam baseline, which suffers from low SNR and stance, one beam for control and another for data at the throughput. We show that FlexLink can create any arbitrary same time, using orthogonal frequency RBs. Unlike previous multi-beam response with a configurable bandwidth part iterative methods, which suffer from high computation comwith O(1) complexity, improving the spectral efficiency of plexity (𝑂 (𝑀𝑁 log(𝑀𝑁 )) for M directions and N frequency both control and data beams simultaneously. subcarriers), our formulation delivers the same result in one shot. We achieve this fast estimation by approximating an 1 Artifact link wcsng.ucsd.edu/dpa NP-hard optimization problem as an L2 norm minimization
FlexLink: Decoupling Control and Data Beams for Next-Generation Wideband Networks
2
Related Work
Multi-beams with phased arrays: Conventional phased arrays can generate multiple concurrent beams, but only by splitting power, which reduces effective gain and throughput [3, 4, 10, 11]. Quasi-omni designs [12] perform even worse, spreading energy broadly with low SNR. These methods also radiate the entire bandwidth in all directions. In contrast, FlexLink leverages frequency-dependent multi-beams that concentrate gain in targeted directions, simultaneously supporting control and data without sacrificing performance. Fixed true-time delay (TTD) and related arrays: Architectures based on TTD [6, 7, 13], frequency-scanning [14, 15], and leaky-wave antennas [16, 17] create frequency-dependent patterns by dispersing signals prism-like across directions. However, each beam carries only a small bandwidth fraction (< 6% [7, 13]), wasting spectrum when no active users are present. Moreover, such designs often require modifications at both base stations and user devices, making them incompatible with 5G NR. By combining delay and phase, FlexLink avoids these pitfalls and remains fully 5G NR compliant while flexibly assigning bandwidth per beam. Delay-phased arrays (DPA): Recent work on DPA [8] and joint phase-time arrays [9, 18] shows the potential of frequency-selective multi-beams. Prior studies explored beam squint mitigation [19, 20], 2D arrays [21, 22], near-field effects [23], and codebook design [24]. Yet, none derived closedform delay/phase expressions or addressed bounded delay requirements for hardware. FlexLink is the first to provide closed-form analysis, apply DPA to control–data decoupling, and validate performance with an open-source over-the-air prototype. A recent Samsung demonstration [18] further confirms practicality, but FlexLink uniquely demonstrates end-to-end feasibility for simultaneous control and data. Hardware TTD implementations: TTD hardware spans CMOS/SiGe RFICs with ns-scale ranges [25–27], FR2/D-band silicon with ps-scale delays [28], and photonic beamformers offering ultrabroad bandwidths [29]. These approaches trade delay range against operating frequency. Unlike prior device-level efforts, FlexLink provides a system-level insight: the delay needed for multi-beam combining is bounded and independent of array size. We exploit this analytically and experimentally, enabling a practical 4–7 GHz prototype with only a 1 ns delay range using the Extreme-Waves unit [30].
3
Design for FlexLink
Problem: Coupled control and data signaling In conventional phased arrays, all subcarriers are radiated in the same beam direction due to a single RF-chain analog front-end. As shown in Figure 2(a), three disjoint subcarrier groups 𝑓1 , 𝑓2 , and 𝑓3 are steered together toward an SSB beam at 𝜃 2 , even when active users are located at 𝜃 1 and 𝜃 3 . This coupling
Wasted resources
MobiHoc ’25, October 27–30, 2025, Houston, TX, USA SSB Control
𝑓$ 𝑓# 𝑓% Bandwidth OFDMA DSP/ symbol IFFT Digital Domain
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UE1 SSB beam
𝑓$ , 𝑓# , 𝑓%
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𝑓!"
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(a) Traditional: Coupled control and data to one fixed direction Useful Data
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(b) FlexLink: Decoupled control and data to multiple unique directions
Figure 2: Delay-phased array (DPA) is an analog front-end architecture with programmable delay and phase elements.
wastes time–frequency resources and prevents simultaneous service of users in different directions. A naive fix is to widen or split beams, but this reduces beamforming gain [3], degrading throughput and coverage. FlexLink with Delay-Phased Array: FlexLink overcomes this limitation using a delay-phased array (DPA) architecture that integrates programmable delays and phase shifts. The delay element 𝜏𝑛 at antenna 𝑛 induces a frequency-dependent phase rotation 2𝜋 𝑓 𝜏𝑛 , enabling different frequency bands to radiate in different directions. This property allows FlexLink to decouple the SSB control beams from the data beams, for example, directing the SSB toward 𝜃 2 while simultaneously steering the data beams to 𝜃 1 and 𝜃 3 (Figure 2(b)). Unlike split-beam methods, this approach preserves beamforming gain and unlocks higher throughput and lower latency. The remainder of this section is organized as follows. Section 3.1 derives closed-form expressions for computing delay and phase settings to realize multi-beam patterns. Section 3.2 introduces a bandwidth-splitting strategy that supports arbitrary resource allocation per beam. Section 3.3 discusses insights from our eight-antenna DPA prototype, validating the practicality of the approach.
3.1
Closed-form computation of delay and phase in DPA
Our goal is to program the DPA delay and phase values to generate a desired frequency-dependent multi-beam response. The number of beams in our multi-beams can be arbitrary but finite. The direction of each beam can be anywhere in the field-of-view of DPA, ideally at different angles. Importantly, each beam should carry a fraction of the bandwidth (as a set of contiguous subcarriers or bandwidth parts), such that the total bandwidth parts sum to the system
MobiHoc ’25, October 27–30, 2025, Houston, TX, USA
IK Jain, RR Vennam, D Bharadia
𝑤 dpa (𝑛, 𝑓 ) = 𝑒 𝑗Φ𝑛 +𝑗2𝜋 𝑓 𝜏𝑛
(1)
which is a function of antenna index 𝑛 and frequency 𝑓 . The beamforming gain 𝐺 (𝑓 , 𝜃 ) of DPA as a function of the weight vector is given by: 𝐺 (𝑓 , 𝜃 ) =
𝑁 −1 ∑︁
𝑤 dpa (𝑛, 𝑓 )𝑒 − 𝑗𝑛𝜋 sin(𝜃 )
(2)
𝑛=0
which is a function of the beamforming angle 𝜃 and frequency 𝑓 , much like a 2D energy map shown in Figure 2(d). Note, we assume the antenna spacing is approximately 𝜆/2 and ignore the effect of beam squint for simplicity [9]. Our objective is to maximize the beamforming gain along the desired multibeam directions as: max 𝐺 (𝑓 , 𝜃 )
2
, 𝑠.𝑡 .(𝜃, 𝑓 ) ∈ {(𝜃 1, 𝑓1 ), . . . , (𝜃 𝐾 , 𝑓𝐾 )}
𝜏𝑛 ,Φ𝑛
(3)
where the desired 𝐾 multi-beam directions are represented by set {(𝜃 1, 𝑓1 ), . . . , (𝜃 𝐾 , 𝑓𝐾 )}. Ideally, we do not want to radiate any energy in the undesired bands/angles, but it is inevitable due to the undesired side-lobes that appear due to a finite number of antennas. Now, we ask what set of delays and phases per antenna would give us the beamforming gain pattern with the desired bandwidth part and beam direction. We first consider a simple case of two beams with equal bandwidth parts of 𝐵/2 each, where 𝐵 is the total system bandwidth. We assume the two beams are directed along (−𝜃 0, 𝜃 0 ) respectively as shown in Figure 3(a). We will later discuss a general case with an arbitrary bandwidth part and beam direction. We formulated it as an optimization problem and solved it to a closed-form expression for the set of delays 𝜏𝑛 and phases Φ𝑛 for each antenna 𝑛 (𝑛 = 0, 1, . . . , 𝑁 − 1) that would generate the given beamforming response as follows: ■ Theorem 1. (2-beam symmetric case) The closed-form expression for the set of delays 𝜏𝑛 and phases Φ𝑛 for each antenna 𝑛 (𝑛 = 0, 1, . . . , 𝑁 − 1) that would generate a given two-beam response with equal bandwidth part 𝐵/2 and angles ±𝜃 0 respectively is as follows: 3 3 3 𝜏𝑛 = 𝑛 sin(𝜃 0 ) + mod (4) 2𝐵 4𝐵 2𝐵 Φ𝑛 = round(𝑛 sin(𝜃 0 ))𝜋 mod 2𝜋
(5)
Before proving the theorem, we want to draw two important insights from this closed-form formula. First, the
𝒉 𝒏, 𝒇 = Φn + 2𝜋𝑓𝜏𝑛
Frequency Band
bandwidth. These flexible multi-beams are achieved via DPA hardware by programming the delay and phase elements appropriately. Mathematically, we model DPA as a uniform linear antenna array with the phase value Φ𝑛 and delay value 𝜏𝑛 at antenna index 𝑛. The antenna weight vector 𝑤 dpa (𝑛, 𝑓 ) is then given by:
𝐵/2
𝑛𝜋sin(𝜃0 ) −𝐵/2
0
𝐵/2
−𝑛𝜋sin(𝜃0 ) −𝐵/2
−𝜃0
Best fit line Desired step function
𝒉 𝒏, 𝒇
Frequency
𝜃0 Direction
(a) Desired multi-beam response with given beam-bandwidth
(b) Solution for delay and phase is obtained from the best fit line
Figure 3: Proof of estimating closed-form delay and phase values for a given two-beam response at ±𝜃 0 .
computation of delays and phases is like a plug-and-play model by putting the desired angle-frequency pairs into the formula. Unlike past methods, this does not require iterative optimization, thus reducing the computational complexity to O(1), allowing fast computation in FPGAs. Second, similar to how phase is bounded by 2𝜋, the delays are also bounded by a max value of 3/2𝐵, independent of the number of antennas. This shows the promise of scaling the DPA architecture to arbitrarily large antenna arrays without requiring longer delay lines that are very hard to achieve in practice (discussed in Section 3.3). Proof of Theorem 1: Two-beam case We drive the expression of delays and phases per antenna that would give us the beamforming gain pattern with the desired bandwidth part and beam direction for the two-beam case. We reformulate the optimization in (3) to include twobeam constraints as:
max
𝑁 −1 ∑︁
2
𝑒
𝑗ℎ (𝑛,𝑓 ) − 𝑗Φant (𝑛,𝑓 )
𝑒
𝜏𝑛 ,Φ𝑛 𝑛=0
𝑠.𝑡 . ℎ(𝑛, 𝑓 ) = Φ𝑛 + 2𝜋 𝑓 𝜏𝑛 ( −𝑛𝜋 sin(𝜃 0 ) ant and Φ (𝑛, 𝑓 ) = 𝑛𝜋 sin(𝜃 0 )
(6) 𝑓 ∈ [ −𝐵 2 , 0] 𝑓 ∈ (0, 𝐵2 ]
where ℎ(𝑛, 𝑓 ) is a function of variable phase Φ𝑛 and delay 𝜏𝑛 at antenna 𝑛 and the function Φant (𝑛, 𝑓 ) represents the constraints from the desired frequency-direction response. We propose an optimization framework that can help to find a closed-form expression for delays and phases. Our optimization problem is formulated in a way that finds the line ℎ that best fits the given step function Φant . We achieve this by approximating our optimization problem in (7) to the best line-fitting on a per-antenna basis: min ||ℎ(𝑛, 𝑓 ) − Φant (𝑛, 𝑓 )|| 2
𝜏𝑛 ,Φ𝑛
(7)
We can visualize this optimization in Figure 5(a), where the line ℎ(𝑛, 𝑓 ) is fit over the step function Φant (𝑛, 𝑓 ). The slope of the best-fit line gives the delay value, and the y-intercept gives the phase value. In this way, we can estimate both delay and phase values by solving for the best-fit line.
FlexLink: Decoupling Control and Data Beams for Next-Generation Wideband Networks
MobiHoc ’25, October 27–30, 2025, Houston, TX, USA
Desired step function 𝚽 𝐚𝐧𝐭 (𝒏, 𝒇)
Frequency Band
Phase
Best fit line 𝒉(𝒏, 𝒇) = Φ𝑛 + 2𝜋𝒇𝜏𝑛
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𝐵/2
2𝜋 − (𝑛 + 1)𝜋sin(𝜃0 ) 𝑛𝜋sin(𝜃0 )
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𝐵/2
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−𝑛𝜋sin(𝜃0 )
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−𝐵/2
𝑓
𝐵/2
𝑓
−(𝑛 + 1)𝜋sin(𝜃0 )
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(a) Best fit line for antenna 𝑛
Figure 4: Desired frequency-space beam response for 2 users at directions −𝜃 0 and 𝜃 0 .
24 3 dB Threshold
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0
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1000
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26 Oracle
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Figure 5: Sketch of proof: We show the phase response for each antenna is a step function with a variable step size that depends on the antenna index, 𝑛.
26
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(b) Poor fit line for antenna 𝑛 + 1
Oracle
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Φ𝑛 + 2𝜋𝑚Δ𝑓 𝜏𝑛 = Φant (𝑛, 𝑚Δ𝑓 ) ∀𝑚 ∈ [−𝑀/2, 𝑀/2]
Avg. of Two Users User 1 only
0
500
𝐵 = (𝑀 + 1)Δ𝑓 . Note that there are 𝑀 frequency bins that can be a large number, i.e., 𝑀 → ∞ for creating a continuous frequency axis. We then formulate a set of linear equations for each frequency term to solve for the variable delay 𝜏𝑛 and phase Φ𝑛 for each antenna 𝑛, given by:
1000
Bandwidth Part of User 1 (MHz)
Bandwidth Part of User 1 (MHz)
(a) Without optimization
(b) With FlexLink optimization
Figure 6: Impact of low bandwidth fraction and mitigation.
■ Impact of linear approximation: However, as antenna index 𝑛 increases, the error in line fitting also increases due to the linear increase in the step size with 𝑛, as shown in Figure 5(b). This could lead to high error for large antenna arrays and limit our solution to scale with antennas. ■ Minimizing linear approximation error: We have an innovative and simple solution to address this issue. To address this issue, we utilize the concept of wrapping the phase of a signal by 2𝜋, i.e., adding an integer multiple of 2𝜋 to the phase does not change the signal. We use this idea to strategically add a phase of multiple of 2𝜋 to a specific set of frequencies in order to minimize the error in line fitting as shown in Figure 5(c). With this insight, we redefine the step function Φant as: ( 𝑘2𝜋 − 𝑛𝜋 sin(𝜃 0 ) 𝑓 ∈ [ −𝐵 2 , 0] Φant (𝑛, 𝑓 ) = (8) 𝑛𝜋 sin(𝜃 0 ) 𝑓 ∈ (0, 𝐵2 ] where 𝑘 is a constant integer. A natural question is how do we estimate this integer to minimize the error in line fitting? Our solution is a two-step process: we solve for the delays and phases as a function of 𝑘 and then find the optimal value of 𝑘 to minimize the error. To solve for per-antenna delays and phases, we form a system of linear equations. We discretize the frequency as 𝑓 = 𝑚Δ𝑓 for 𝑚 ∈ [−𝑀/2, 𝑀/2], where the bandwidth is
(9)
which is in the form of a system of linear equations (𝐴𝑥 = 𝑏), with unknown 𝑥 = [𝜏𝑛 , Φ𝑛 ]. Solving this system of linear equations gives the desired estimate of delay and phase values. ■ Generalization to an arbitrary number of beams: We also show generalized beamforming response to an arbitrary number of beams with arbitrary beam directions and arbitrary bandwidth parts. ■ Theorem 2. (Generalized case): Let there are 𝐷 beam directions with beam angles 𝜃𝑑 and bandwidth part 𝛼𝑑 𝐵 for Í 𝑑 𝛼𝑑 = 1. We define 𝜙𝑑 = 𝑛𝜋 sin(𝜃 𝑑 ) for simplicity. The per-antenna phases and delays in realizing such a generalized beamforming response is given by: Φ𝑛 =
𝐷 ∑︁
𝛼𝑑 (𝜙𝑑 + 2𝑘𝑑 𝜋)
(10)
𝑑=1
𝜏𝑛 =
𝐷 ∑︁ 3
(𝜙𝑑 + 2𝜋𝑘𝑑 )𝛼𝑑 (
𝑑 ∑︁
2𝛼 ℓ − 𝛼𝑑 − 1)
(11)
𝜋𝐵 𝑑=1
ℓ=1
where, 𝜙𝑑 = 𝑛𝜋 sin(𝜃𝑑 ) (12) and the constant integer 𝑘𝑑 for beam 𝑑 and antenna 𝑛 is: ( 0 𝑑 =1 (13) 𝑘𝑑 = 𝑘𝑑 −1 +round( 𝑛 sin(𝜃𝑑 −1 )2−𝑛 sin(𝜃𝑑 ) ) 𝑑 ≥ 2 The proof follows similarly to the two-beam case2 . Also, note that the number of multiplications in the computation 2 Visit wcsng.ucsd.edu/dpa for details.
MobiHoc ’25, October 27–30, 2025, Houston, TX, USA
IK Jain, RR Vennam, D Bharadia Ribbon connector to supply power and SPI lines with FPGA
FPGA Control Board
Phase shifter IC
1 RF chain
(a) True-time delay unit
Splitter
8 Phase shifters
8 True-time delays
(b) Phase shift unit
of phase in (10) is O(K) and delay (11) is O(𝐾 2 ), which is independent of the number of antennas or system bandwidth, making it scalable to larger arrays.
Flexible bandwidth part with DPA
A key requirement of FlexLink is supporting arbitrary bandwidth parts. However, line-fitting errors limit DPA multibeamforming, especially when a user’s bandwidth part is very small. Figure 6(a) shows that while overall two-user gain remains high, users with less than 20% of system bandwidth suffer more than 10 dB loss due to skewed step functions and poor line fitting. To mitigate this, we introduce a threshold mechanism that prevents any beam’s gain from dropping more than 3 dB below optimal. This is enforced by setting a minimum bandwidth threshold of 20% (corresponding to 3 dB loss), though the value can be adjusted (e.g., 25% for 1.5 dB, 15% for 6 dB). If a beam requests less than 20%, we reassign delays and phases as if in a 20%–80% split. As shown in Figure 6(b), this caps the low-bandwidth beam’s loss at 3 dB, while the other beam experiences less than 1 dB loss, an acceptable tradeoff. This heuristic thus enables support for arbitrarily small bandwidth parts without severe performance degradation.
3.3
Rotating Platform
(c) Switched delay architecture
Figure 7: Implementing 8 antenna delay- Figure 8: Hardware components of TTD unit and phase shift unit. phased array at 4-7 GHz band.
3.2
Delay Phased Array
DPA Hardware Design
To demonstrate the practical performance of FlexLink, we designed a custom prototype of DPA hardware in the lab using commercial true-time delay chips and phase shifter chips operating at 4 GHz–6 GHz frequencies as shown in Figure 7. Although generally, there are differences between sub-6 GHz and mmWave systems, the generation of desired beam patterns is agnostic to center frequency and can be validated with our sub-6 setup. Here, we discuss the DPA hardware requirements in terms of range, resolution, wideband design of delay and phase circuits, and implementation setup.
Figure 9: Beam pattern measurement in an anechoic chamber.
DPA Hardware Overview: We describe the hardware shown in Figure 7 from a downlink perspective and emphasize that the uplink path follows a similar reverse order. The input to DPA hardware is a single analog radio frequency waveform at the desired center frequency, which can be generated from a single DAC and a series of mixers (e.g., a signal generated from a USRP). We then split this RF signal into 8 equal parts using a 1:8 splitter and pass the 8 copies of the signal to 8 phase shifters. The phase-shifted signal then passes through 8 true-time delay modules, which are finally connected to 8 antennas. Thus, the hardware resembles the concept diagram we presented in Figure 2. ■ Range of delay and phase elements in DPA: The range of phase values is a constant 2𝜋 that covers all scenarios because the exponential phase always wraps around 2𝜋. However, the range of delay values is not straightforward because, unlike phase, delays don’t generally wrap around a certain value. In fact, prior works on using a delay element for multibeamforming showed an unbounded delay that increases linearly with the number of antennas [5], thus making it hard to build and scale in a practical circuit design because of large size, bandwidth, and matching constraints [31]. In contrast, we designed FlexLink to have shorter and bounded delay lines that do not scale with the number of antennas. 3 For the two-beam case, the delay range for FlexLink is 2𝐵 , which is 1.5𝑛𝑠 for 1 GHz bandwidth. the range of delay can be further reduced with a smaller field of view of the array. A 120𝑜 angular field-of-view requires only 1𝑛𝑠 delay range. ■ Wideband Delay and Phase Units: To meet the ultrahigh bandwidth of 5G and the next generation of cellular networks, we need to design a wideband circuit for delay and phase units. Delay lines can be implemented in an integrated circuit using techniques such as passive elements [27], all-pass filter [25], and Switched Capacitor design [31, 32] depending on applications and requirements. Our true-time delay unit is built in-house by Extreme Waves [30], which
4 Evaluation 4.1 Hardware Results We obtained over-the-air signal measurements with two beams at various angles and bandwidth parts. Figure 10 shows example beam patterns across frequency and angle with varying bandwidth parts. The two beams point towards 30𝑜 and −30𝑜 , as can be visualized through the beam patterns. We reduce the bandwidth of one of the beams from 50% to 20% (consequently increasing it for the other beam from 50% to 80%). For higher split scenarios such as 30-70, 40-60, we drew similar conclusions as 20-80 and 50-50 splits, so we omitted them for brevity. The gain across the frequency band for the smaller bandwidth part is capturing a smaller bandwidth, and vice versa. The gain is high in the desired frequency-angle bin shown by a black box and low elsewhere, as expected. We compared the measured beam patterns with simulated ones to show the similarities. We ensured our simulation plots consider the same quantization levels and
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implemented delays using a switched delay architecture [33] as shown in Figure 8 because of its advantage over wideband operation. It deploys a series of SPDT switches connected to two transmission lines of variable length, switching to the longer transmission line would increase delay by a certain unit along the path. With 𝑚 switches, we can generate 𝑚 bit quantized delay with 2𝑚 possible delay values. The phase shift can also be implemented using transmission lines, but transmission line-based phase shift supports only narrowband frequencies. We choose a varactor-based phase shift design for wideband operation. ■ Design of FPGA Control Board PCB: We designed a PCB board that hosts a CMOD A7 FPGA [34] to program the delay and phase values in our hardware. The PCB also contains a power distribution module with appropriate LDOs that takes a single 12V input and distributes different voltage levels (e.g., 5V, -5V, and 3.3V) to the phase shifter and TTD chips via a ribbon cable. ■ Beam Pattern Measurement Setup: The whole setup was laid over a motor that was controlled via an automated software program, as shown in Figure 9. To measure the DPA beam pattern, we rotate the motor 1◦ at a time in the ±60◦ range. We connected the Tx antenna to the VNA’s port 1 and the output of the combiner to its port 2. We saved the S21 parameters for each angle over the operating frequency range. Then, we compute the received power using the measured S21 parameters and plot the power versus frequency and angle. We verified the setup in an anechoic chamber and then conducted over-the-air experiments in a lab environment. We used radiation-absorbent materials to isolate the setup to reduce channel effects, which can be misleading and affect the resulting images.
MobiHoc ’25, October 27–30, 2025, Houston, TX, USA
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FlexLink: Decoupling Control and Data Beams for Next-Generation Wideband Networks
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beam squint effect we face in our hardware. Figure 11(a) shows that the beamforming gain at the desired angle from hardware matches the simulation. However, the hardware patterns suffer from additional degradation due to various factors such as multipath (we did over-the-air experiments in a conference room), interference with other devices such as WiFi at 5 GHz, and hardware artifacts (e.g., the matching effect of wideband transmission lines, PCB substrate, and delay/phase IC). Nonetheless, the general behavior suggests that the desired frequency-angle patterns are achievable with our hardware. Spectral Efficiency Improvement: FlexLink enhances spectral efficiency by enabling high-gain beams for both control and data signals, unlike traditional phased arrays that form a single control beam. As shown in Figure 11(b), the baseline achieves high efficiency only for control, while FlexLink provides high efficiency for both, nearly doubling data efficiency in the worst 10% cases. Although control efficiency
MobiHoc ’25, October 27–30, 2025, Houston, TX, USA
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drops slightly due to linear approximation and hardware artifacts, the overall efficiency remains balanced. Importantly, while the baseline uses only 7% of spectrum for control (SSB), FlexLink fully utilizes 100% of the spectrum with both control and data.
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We use the ANSYS HFSS (High-Frequency Structure Simulator) tool to simulate a FlexLink so that all the non-ideal effects involving substrate waves, material losses, coupling between adjacent elements, and individual antenna element patterns are modeled by providing the appropriate delay and phase values. The element is modeled using a simple inset-fed patch antenna on a Rogers RO4350 substrate with a variable delay + phase applied at the input port (see Figure (12a)). The results of the simulation are shown in Figure (12b) with the main lobe along 0◦ and -30◦ .
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Due to the limited 8 antenna array and 1ns max range of delay unit, we cannot perform hardware measurements for a large-scale system. We performed simulations to show the impact of the number of antennas (up to 64 antennas) and bandwidth parts. Scaling to 3 or more beams: We extensively evaluate FlexLink using a 32-element array and compare it with the high-complexity FSDA algorithm [8]. As shown in Figure 13(a), FlexLink produces accurate multi-beam patterns for arbitrary beam counts, directions, and bandwidth parts. The antenna gain is high in desired frequency–direction bins, verifying that our linear approximation of the per-antenna phase profile is effective. Only at the frequency edges do we observe gradual transitions, while the beams remain sharp in angular space without leakage to unintended directions. Low computation complexity: We show that FlexLink achieves the same high performance with low computational
IK Jain, RR Vennam, D Bharadia
complexity compared to the heuristic algorithm [8]. Figure 13(b) shows the corresponding delay and phase values obtained by FlexLink and mmFlexible baseline optimization are the same values. These delay and phase values are used to create the beamforming pattern in Figure 13(a). FlexLink achieves this high performance with a simple O(1) complexity in estimating the delay and phase values, while the baseline requires high 𝑂 (𝑁 𝑀 log(𝑁 𝑀)) complexity. For instance, 𝑁 = 64 antennas and 𝑀 = 256 directions, the baseline requires 69k multiplications, while FlexLink requires less than 10 multiplications, which makes it efficient to program in FPGAs. Impact of bandwidth part allocation: Applications such as concurrent communication, control, or scenarios involving both high and low-bandwidth users require consistently high beam gains to ensure low latency and reliability. We show that FlexLink achieves this by maintaining high gain across all users, independent of bandwidth allocation. To demonstrate, we used a linear 16-element array with 𝜆2 spacing and formed two beams in separate directions. If beam 1 (for UE 1) is assigned 𝑥% of sub-carriers, beam 2 (for UE 2) receives the remaining (100 − 𝑥)%. We varied 𝑥 from 5% to 95% and compared gains. Unlike the split antenna method, which offers constant but lower gain by spanning the full bandwidth in each user direction, FlexLink maintains high and balanced gain, as shown in Figure 13(c). Even at extremes—5% or 95% allocation—both users experience similar, strong gains. Error bars show gain variation due to different beam angle separations. These results confirm that FlexLink supports flexible bandwidth allocation without compromising per-user gain. Scaling with number of antennas: Here, we present gain evaluations with an increase in the number of antennas for both DPA and split antenna techniques. We evaluate both approaches in supporting 3 users in different directions and varying the number of antennas from 3 to 64, as illustrated in Figure 13(d), with an increase in the number of antennas, both DPA and the split technique gain increase with a similar pattern. Note that for the three antennas scenario (to support three user directions), the split antennas approach radiates the same as quasi-Omni. Hence, it has a lower gain than the rest. The error bars in black indicate the variations in gain accommodating different user directions (Monte-Carlo simulations varying user directions). Split antennas as high error bars indicate high gain variations compared to DPA. Significance of phase-wrapping in FlexLink: Recall that FlexLink uses phase-wrapping to bound the step size in the antenna phase response (Figure 5). To highlight its impact, we compare against a baseline without wrapping, where step size grows with antenna index. Figure 14 shows that FlexLink maintains bounded error (∼3 radians), while the
FlexLink: Decoupling Control and Data Beams for Next-Generation Wideband Networks
MobiHoc ’25, October 27–30, 2025, Houston, TX, USA
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baseline error grows unbounded (8 radians at 8 antennas, 64 radians at 64 antennas). In fact, the resultant beam pattern with this baseline resembles a rainbow beam response [5], which spreads all frequencies in all directions uniformly. Thus, phase-wrapping is essential for FlexLink to achieve bounded error independent of array size.
5
Conclusion and Future work
We presented FlexLink, a system for decoupling control and data signaling through a novel delay-phased array (DPA) antenna architecture. We presented a detailed mathematical analysis of the DPA and derived a closed-form mathematical expression for delay and phase values. We built a hardware prototype for DPA using wideband delay and phase units and showed the feasibility of multi-beam patterns in over-the-air testing and demonstrated its benefits in improving spectrum utilization for control and data signals. As a first prototype, FlexLink demonstrates promising capabilities in multi-beamforming and frequency-dependent PHY design. This section discusses remaining system-level challenges and outlines future directions, including mobility support, MAC layer integration, and extension to mmWave frequencies.
■ Mobility and dynamic beam switching: In FlexLink, we focused on a static user with multi-beamforming. Other system-level issues and algorithm designs, such as beam tracking for mobile users, can be adopted from extensive literature in this area. We will discuss how fast FlexLink can perform a beam scan through the control beam while it co-exists with a data beam and compare with traditional approaches, which only use a control beam without any data beam. ■ Integration with MAC and higher layers: FlexLink introduces physical layer optimization tools that incorporate novel hardware and algorithmic approaches for frequencydependent beamforming. This architectural shift necessitates a reexamination of the end-to-end protocol stack. In particular, a MAC scheduler capable of frequency-directional beam splitting is required to support simultaneous multi-user transmission across distinct spatial directions. While OFDMAbased schedulers have already been adapted to accommodate directional constraints in single-beamforming scenarios, extending these mechanisms to support multi-beamforming remains an important direction for future work. ■ Extension to mmWave frequencies: Implementing a circuit for delay in mmWave frequencies is challenging due to non-linearity, bandwidth, and matching constraints. Therefore, most available delay circuits in mmWave bands are limited to a few picoseconds, which does not meet the requirements of a 1-10 ns delay in DPA. Alternative architectures can be used for mmWave bands where delay elements are implemented at sub-6 frequencies and then upconverted at each antenna using a series of mixers. We leave this exploration for the future.
6
Acknowledgment
We thank YungYi Sun, Sonny Cao, and Nagarjun Bhat for assistance with prototyping and experiments, and Prof. Gabriel Rebeiz and Qian Ma of Extreme Waves for support with the true-time delay unit. We also thank the WCSNG lab at UC San Diego for discussions and proofreading. This work was supported by NSF awards #2211805 and #2232481.
MobiHoc ’25, October 27–30, 2025, Houston, TX, USA
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