Movable antenna (MA), which can flexibly change the position of antenna in three-dimensional (3D) continuous space, is an emerging technology for achieving full spatial performance gains. In this paper, a prototype of MA communication system with ultra-accurate movement control is presented to verify the performance gain of MA in practical environments. The prototype utilizes the feedback control to ensure that each power measurement is performed after the MA moves to a designated position. The system operates at 3.5 GHz or 27.5 GHz, where the MA moves along a one-dimensional horizontal line with a step size of 0.01λ and in a two-dimensional square region with a step size of 0.05λ, respectively, with λ denoting the signal wavelength. The scenario with mixed line-of-sight (LoS) and non-LoS (NLoS) links is considered. Extensive experimental results are obtained with the designed prototype and compared with the simulation results, which validate the great potential of MA technology in improving wireless communication performance. For example, the maximum variation of measured power in the considered scenario reaches over 40 dB and 23 dB at 3.5 GHz and 27.5 GHz, respectively, thanks to the flexible antenna movement. In addition, experimental results indicate that the power gain of MA system relies on the estimated path state information (PSI), including the number of paths, their elevation and azimuth angles of arrival (AoAs), as well as the complex gain of each path.

For integrated sensing and communication (ISAC) systems, channel information that is essential for communication and sensing tasks fluctuates at different timescales. Specifically, the composite channel state information (CSI) for wireless communication is static during channel coherence time. However, this concept is less appropriate for describing the wireless channel for sensing. To this end, in this paper, we first introduce a new timescale to study the real-time variations of the path state information (PSI) (e.g., delay, angle, and Doppler) of individual multi-path, termed path-invariant time, during which the PSI remains constant. As the goal of environment sensing for PSI essentially aligns with the channel estimation for the recently proposed delay-Doppler alignment modulation (DDAM) technique, we introduce a novel framework for a bi-static ISAC system, which refers to as DDAM-based ISAC. To acquire the PSI, in this paper, by capitalizing on the dual timescales of wireless channels, we propose a novel algorithm, termed as adaptive simultaneously orthogonal matching pursuit algorithm with support refinement (ASOMP-SR). The performance of DDAM with the imperfectly sensed PSI is analyzed, where the signal-to-interference-plus-noise ratio (SINR) and the achievable spectral efficiency are derived. Numerical results unveil that the proposed ASOMP-SR algorithm achieves better sensing performance than the conventional orthogonal matching pursuit (OMP) algorithm, in terms of the normalized mean squared error (NMSE) and the number of multi-paths resolved. In addition, DDAM-based ISAC can achieve superior spectral efficiency and a reduced peak-to-average power ratio (PAPR) compared to standard orthogonal frequency division multiplexing (OFDM).

Delay alignment modulation (DAM) is a novel transmission technique for wireless systems with high spatial resolution by leveraging delay compensation and path-based beamforming, to mitigate the inter-symbol interference (ISI) without resorting to complex channel equalization or multi-carrier transmission. However, most existing studies on DAM consider a simplified sce-nario by assuming that the channel multi-path delays are integer multiples of the signal sampling interval. This paper investigates DAM for the more general and practical scenarios with fractional multi-path delays. We first analyze the impact of fractional multi-path delays on the existing DAM design, termed integer DAM (iDAM), which can only achieve delay compensations that are integer multiples of the sampling interval. It is revealed that the existence of fractional multi-path delays renders iDAM no longer possible to achieve perfect delay alignment. To address this issue, we propose a more generic DAM design called fractional DAM (fDAM), which achieves fractional delay pre-compensation via upsampling and fractional delay filtering. By leveraging the Farrow filter structure, the proposed approach can eliminate ISI without real-time computation of filter coefficients, as typically required in traditional channel equalization techniques. Simulation results demonstrate that the proposed fDAM outperforms the existing iDAM and orthogonal frequency division multiplexing (OFDM) in terms of symbol error rate (SER) and spectral efficiency, while maintaining a comparable peak-to-average power ratio (PAPR) as iDAM, which is considerably lower than OFDM.

The sixth generation (6G) mobile communication networks are expected to offer a new paradigm of cellular integrated sensing and communication (ISAC). However, due to the intrinsic difference between wireless sensing and communication in terms of coverage requirement, current cellular networks that are deliberately planned mainly for communication coverage are difficult to achieve seamless sensing coverage. Therefore, this paper studies the coverage issue for cellular ISAC systems, which aims to concurrently sense a prescribed region while serving a group of communication users equipment (UEs). Towards this end, the radar sensing signal processing procedures and communication signal models are presented in a general multi-static cellular ISAC system with coordinated multi-point joint transmission (CoMP-JT), and an optimization problem is formulated to maximize the worst-case sensing signal-to-noise ratio (SNR) in the prescribed sensing coverage region, subject to the signal-to-interference-plus-noise ratio (SINR) requirement for each communication UE. To gain useful insights, we first investigate the basic bi-static ISAC system, for which a closed form expression of the optimal beamforming is obtained for the special case with one UE and one sensing point. Then, the general case with multiple communication UEs and contiguous regional sensing coverage is further studied, in which the mesh grid approach and direction discretization approach are proposed for ease of solving the optimization problem. Afterward, we further investigate the beamforming optimization in the multi-static ISAC system to maximize the probability of detection of the prescribed sensing coverage region. We show that the problem is equivalent to maximize the sum of sensing SNR from the different transmit BSs. The formulated problems are non-convex, and we propose an efficient algorithm based on successive convex approximation (SCA) technique. Numerical results demonstrate that the proposed ISAC design is able to achieve seamless sensing coverage in the prescribed region while guaranteeing the communication requirements of the UEs.

For integrated sensing and communication (ISAC) systems, the desired channel variables by communication and sensing tasks vary with different timescales. For sensing, one is mainly interested in the state information (e.g., delays, angles, Doppler frequencies, etc.) of individual multi-path channel components, which evolves much more slowly than the composite channel state information (CSI) required by communications. In this paper, by exploiting the double timescales for sensing and communication, a novel technique termed as delay-Doppler alignment modulation (DDAM) is investigated, which is an appealing technique for ISAC systems, since the sensing result of resolvable multi-paths can be directly exploited for delay-Doppler compensation and path-based beamforming of DDAM. We first show that with perfect CSI, as long as the number of base station (BS) antennas is no smaller than that of resolvable multi-paths, the proposed DDAM is able to transform the time-frequency double selective-fading channel into a simple additive white Gaussian noise (AWGN) channel for inter-symbol interference (ISI)-free communication without requiring the conventional channel equalization or multi-carrier transmission. We then present the DDAM-based signal processing for ISAC, and the resulting communication performance with imperfectly sensed CSI is studied. Simulation results demonstrate that the proposed DDAM-based ISAC can achieve higher communication rate compared to orthogonal frequency division multiplexing (OFDM) and DFT-spread(s)-OFDM, while guaranteeing high sensing performance.

Delay alignment modulation (DAM) has been recently proposed to enable manipulable channel delay spread for efficient single- or multi-carrier communications. In particular, with perfect delay alignment, inter-symbol interference (ISI) can be eliminated even with single-carrier (SC) transmission, without relying on sophisticated channel equalization. The key ideas of DAM aredelay pre-compensationandpath-based beamforming, so that all multi-path signal components may arrive at the receiver simultaneously and be superimposed constructively, rather than causing the detrimental ISI. Compared to the classic orthogonal frequency division multiplexing (OFDM) transmission, DAM-enabled SC communication has several appealing advantages, including low peak-to-average-power ratio (PAPR) and high tolerance for Doppler frequency shift, which renders DAM also appealing for radar sensing. Therefore, in this paper, DAM is investigated for integrated sensing and communication (ISAC) systems. We first study the output signal-to-noise ratios (SNRs) for ISI-free SC communication and radar sensing, respectively, and then derive the closed-form expressions for DAM-based sensing in terms of the ambiguity function (AF) and integrated sidelobe ratio (ISR). Furthermore, we study the beamforming design problem for DAM-based ISAC to maximize the communication SNR while guaranteeing the sensing performance in terms of the sensing SNR and ISR. Finally, we provide performance comparison between DAM and OFDM for ISAC, and it is revealed that DAM signal may achieve better communication and sensing performance, thanks to its low PAPR, reduced guard interval overhead, as well as higher tolerance for Doppler frequency shift. Simulation results are provided to show the great potential of DAM for ISAC.

Effective beamforming is essential for multi-antenna based integrated sensing and communication (ISAC), where multiple beams are usually required to concurrently direct signal power towards both the communication user equipment (UE) and the sensing target. This paper studies millimeter wave (mmWave) ISAC system, where an ISAC node with large antenna arrays wishes to simultaneously communicate with an UE and sense a target using the cost-effective analog beamforming. We first propose a subarray-based double-beam codebook design, which includes the conventional single-beam codebook as a special case. With the proposed codebook design, for all possible combinations of the UE and target directions, signal power can be effectively directed towards them concurrently, as long as the beam is appropriately selected. To this end, we further propose a novel beam sweeping protocol, for which the beam searching processes for communication and sensing are carried out simultaneously, with either single-beam or double-beam sweeping that achieves different balances between the required sweeping time and beamforming gain. Simulation results are provided to show the effectiveness of the proposed codebook design and simultaneous beam sweeping methods.

Delay alignment modulation (DAM) has been recently proposed to enable inter-symbol interference (ISI)-free single-carrier (SC) communication without relying on sophisticated channel equalization. The key idea of DAM is to pre-introduce deliberate symbol delays at the transmitter side, so that all multi-path signal components may arrive at the receiver simultaneously and be superimposed constructively, rather than causing the detrimental ISI. Compared to the classic orthogonal frequency division multiplexing (OFDM) transmission, DAM has several appealing advantages, including low peak-to-average-power ratio (PAPR) and high tolerance for Doppler frequency shift, which makes DAM also appealing for radar sensing. Therefore, in this paper, DAM is investigated for the emerging integrated sensing and communication (ISAC) setup. We first derive the output signal-to-noise ratios (SNRs) for ISI-free communication and radar sensing, respectively, and then propose an efficient beamforming design for DAM-ISAC to maximize the communication SNR while guaranteeing the sensing performance. The comparison analysis of DAM versus OFDM for ISAC is developed, and it is revealed that DAM enables higher sensing SNR and larger Doppler frequency estimation. Simulation results are provided to show the great potential of DAM for ISAC.

Integrated sensing and communication (ISAC) is a promising technology to fully utilize the precious spectrum and hardware in wireless systems, which has attracted significant attentions recently. This paper studies ISAC for the important and challenging monostatic setup, where one single ISAC node wishes to simultaneously sense a radar target while communicating with a communication receiver. Different from most existing schemes that rely on either radar-centric half-duplex (HD) pulsed transmission with information embedding that suffers from extremely low communication rate, or communication-centric waveform that suffers from degraded sensing performance, we propose a novel full-duplex (FD) ISAC scheme that utilizes the waiting time of conventional pulsed radars to transmit communication signals. Compared to radar-centric pulsed waveform with information embedding, the proposed design can drastically increase the communication rate, and also mitigate the sensing eclipsing and near-target blind range issues, as long as the self-interference (SI) is effectively suppressed. On the other hand, compared to communication-centric ISAC waveform, the proposed design has better auto-correlation property as it preserves the classic radar waveform for sensing. Performance analysis is developed by taking into account the residual SI, in terms of the probability of detection and ambiguity function for sensing, as well as the spectrum efficiency for communication. Numerical results are provided to show the significant performance gain of our proposed design over benchmark schemes.