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Hybrid Beamforming for Millimeter Wave MIMO Integrated Sensing and Communications
Chenhao Qi, Wei Ci, Jinming Zhang, Xiaohu You
TL;DR
The paper addresses transmit-beam design for mmWave MIMO ISAC under communication-user SINR and base-station power constraints. It adds a phase vector and uses alternating minimization to design the transmit beam and hybrid beamformers. Simulations report better radar beam quality with larger antenna arrays under the same SINR constraint.
Problem
The paper studies how to design an mmWave MIMO ISAC transmit beam that approaches an objective radar pattern while satisfying communication-user SINR and total-power constraints.
Source: §I · p. 2Method
A phase vector and alternating minimization are used to optimize the transmit beam, followed by analog and digital beamformer design under the phase-shifter constant-envelope constraint.
Source: §I · pp. 2–3, §III · pp. 8–9Results
Under the same communication-user SINR constraint, larger antenna arrays can achieve better radar beam quality.
Source: §IV · p. 11, §V · pp. 12–13Takeaways & Limitations
A larger antenna array is a design lever for improving radar beam quality while maintaining the same communication-user SINR constraint.
Source: §IV · p. 11, §V · pp. 12–13
Abstract
from arXiv · showhide
In this letter, we consider hybrid beamforming for millimeter wave (mmWave) MIMO integrated sensing and communications (ISAC). We design the transmit beam of a dual-functional radar-communication (DFRC) base station (BS), aiming at approaching the objective radar beam pattern, subject to the constraints of the signal to interference-plus-noise ratio (SINR) of communication users and total transmission power of the DFRC BS. To provide additional degree of freedom for the beam design problem, we introduce a phase vector to the objective beam pattern and propose an alternating minimization method to iteratively optimize the transmit beam and the phase vector, which involves second-order cone programming and constrained least squared estimation, respectively. Then based on the designed transmit beam, we determine the analog beamformer and digital beamformer subject to the constant envelop constraint of phase shifter network in mmWave MIMO, still using the alternating minimization method. Simulation results show that under the same SINR constraint of communication users, larger antenna array can achieve better radar beam quality.
I. INTRODUCTION
The introduction frames mmWave MIMO ISAC beamforming as a radar-pattern design problem constrained by communication-user SINR and transmit power. It proposes phase-augmented alternating optimization, then designs analog and digital beamformers for the hybrid architecture.
- Earlier work considered hybrid mmWave MIMO ISAC through a weighted communication-and-radar objective with constant-modulus and total-power constraints.Source: §I · p. 2
- The transmit beam is designed to approach an objective radar pattern subject to communication-user SINR and total base-station power constraints.Source: §I · p. 2
- A phase vector adds design freedom, and alternating minimization optimizes it with the transmit beam using SOCP and constrained least-squares estimation.Source: §I · pp. 2–3
- The designed transmit beam is decomposed into analog and digital beamformers while respecting the phase-shifter network's constant-envelope constraint.Source: §I · p. 3
II. SYSTEM MODEL
The system model uses a DFRC base station and a shared antenna array for communication and radar sensing. Communication streams use dedicated RF chains, while radar sensing may rely on dedicated chains or communication resources.
- The DFRC base station serves single-antenna communication users and detects targets using an NBS-element uniform linear array with half-wavelength spacing.Source: §II · p. 3
- Each of Nc users receives an independent stream through Nc communication RF chains; dedicated radar RF chains provide additional target-detection resources.Source: §II · p. 3
- With no dedicated radar RF chains, target detection relies entirely on communication resources, making system design more challenging.Source: §II · p. 3
DFRC BS
The DFRC base station designs a transmit beam to approach an objective radar pattern while satisfying communication-user SINR requirements and a total-power constraint. The formulation models a single time instance and focuses on spatial beamforming.
DFRC BS
- The transmitted signal x is divided into a front portion for communication users and a rear portion containing the radar waveform; the design considers one time instance and omits time-domain waveform design.Source: DFRC BS · pp. 4–5
- Users may have different service-quality needs, so each user's SINR constraint uses its own threshold Γn.The passage gives video and text services as examples of higher and lower SINR requirements, respectively.Source: DFRC BS · p. 5
- To detect multiple targets, the DFRC BS needs various radar beams; scanning different angles with DFT codewords is one example.Source: DFRC BS · p. 5
- The angle space of interest is sampled at M points, and increasing M gives finer sampling and a more precise beam description.Source: DFRC BS · p. 5
- The transmit-beam pattern is formed by projecting the beam onto steering vectors at sampled angles and taking the entrywise absolute values.Source: DFRC BS · p. 6
- The DFRC BS designs a transmit beam to approximate a nonnegative objective radar pattern while meeting communication users’ SINR constraints and the total transmission-power limit.The objective can weight angle-sampling points differently, with larger weights indicating higher requirements for matching the target pattern there.Source: DFRC BS · p. 6
III. HYBRID BEAMFORMING DESIGN
The design first optimizes a transmit beam to approach the objective radar pattern, using a phase vector for additional design freedom. It then factors that beam into analog and digital beamformers under the phase-shifter constraint.
III. HYBRID BEAMFORMING DESIGN
- A phase vector is added to the objective beam pattern to provide additional freedom in transmit-beam design.The phase vector has unit-modulus entries.Source: §III · pp. 6–7
- A nonnegative diagonal matrix is defined with diagonal entries taken from the corresponding entries of the objective beam pattern.The designed transmit beam is then represented using the resulting beamforming vectors.Source: §III · pp. 6, 8
- Alternating minimization optimizes the transmit-beam vectors for a fixed phase vector, then the phase vector for fixed beam vectors.The beam-vector step is an SOCP; the phase-vector step is constrained least-squares estimation.Source: §III · pp. 7–8
- The hybrid-beamformer design alternates between a least-squares digital-beamformer update and a Riemannian-manifold analog-beamformer update.The analog entries satisfy the constant-envelope phase-shifter constraint, and the digital beamformer is normalized to meet total transmission power.Source: §III · pp. 9–10
IV. SIMULATION RESULTS
At the same communication-user SINR constraint, larger antenna arrays achieve better radar beam quality. Higher SINR requirements worsen radar beam quality, while hybrid beamforming slightly degrades the beam pattern relative to the design without HBF.
IV. SIMULATION RESULTS
- The DTB with HBF has a slightly worse beam pattern than the DTB without HBF because the phase-shifter network's constant-envelope constraint causes energy leakage.Both patterns fall slightly below the objective beam pattern within its two bands because energy is also used for the three communication-user peaks.Source: §IV · p. 11
- As the SINR constraint increases, beam-pattern MSE grows, indicating worse radar beam quality at higher communication requirements.The comparison considers DTB with and without HBF across different SINR constraints.Source: §IV · p. 11, Figure 3 · p. 12
- At the same communication-user SINR constraint, a larger antenna array achieves better radar beam quality.The authors attribute this to the additional degrees of freedom for beam design.Source: §IV · p. 11
- The MSE gap between DTB with and without HBF becomes larger as the number of BS antennas increases.The authors attribute this to more HBF phase shifters imposing more constraints and increasing the error between the designed and hybrid beam patterns.Source: §IV · p. 11
V. CONCLUSION
The paper proposes alternating minimization to optimize the transmit beam and phase vector, then designs analog and digital beamformers under the phase-shifter network's constant-envelope constraint. Its simulations show better radar beam quality with a larger antenna array at the same communication-user SINR.
V. CONCLUSION
- Alternating minimization iteratively optimizes the transmit beam and phase vector, then determines analog and digital beamformers subject to the phase-shifter network's constant-envelope constraint.The beamformers are determined based on the designed transmit beam.Source: §V · p. 12
- At the same communication-user SINR constraint, a larger antenna array achieves better radar beam quality.Source: §V · pp. 12–13
- Future work will focus on performance optimization for mmWave.Source: §V · p. 13