In millimeter wave cellular communication, fast and reliable beam alignment via beam training is crucial to harvest sufficient beamforming gain for the subsequent data transmission. In this paper, we establish fundamental limits in beam-alignment performance under both the exhaustive search and the hierarchical search that adopts multi-resolution beamforming codebooks, accounting for time-domain training overhead. Specifically, we derive lower and upper bounds on the probability of misalignment for an arbitrary level in the hierarchical search, based on a single-path channel model. Using the method of large deviations, we characterize the decay rate functions of both bounds and show that the bounds coincide as the training sequence length goes large. We go on to characterize the asymptotic misalignment probability of both the hierarchical and exhaustive search, and show that the latter asymptotically outperforms the former, subject to the same training overhead and codebook resolution. We show via numerical results that this relative performance behavior holds in the non-asymptotic regime. Moreover, the exhaustive search is shown to achieve significantly higher worst-case spectrum efficiency than the hierarchical search, when the pre-beamforming signal-to-noise ratio (SNR) is relatively low. This study hence implies that the exhaustive search is more effective for users situated further from base stations, as they tend to have low SNR.

Swift and accurate alignment of transmitter (Tx) and receiver (Rx) beams is a fundamental design challenge to enable reliable outdoor millimeter-wave communications. In this paper, we propose a new Optimized Two-Stage Search (OTSS) algorithm for Tx-Rx beam alignment via spatial scanning. In contrast to one-shot exhaustive search, OTSS judiciously divides the training energy budget into two stages. In the first stage, OTSS explores and trains all candidate beam pairs and then eliminates a set of less favorable pairs learned from the received signal profile. In the second stage, OTSS takes an extra measurement for each of the survived pairs and combines with the previous measurement to determine the best one. For OTSS, we derive an upper bound on its misalignment probability, under a single-path channel model with training codebooks having an ideal beam pattern. We also characterize the decay rate function of the upper bound with respect to the training budget and further derive the optimal design parameters of OTSS that maximize the decay rate. OTSS is proved to asymptotically outperform state-of-the-art beam alignment algorithms, and is numerically shown to achieve better performance with limited training budget and practically synthesized beams.

We propose a new method of modeling the signal transmission in underwater acoustic communications when the transmitter and receiver are moving. The motion-induced channel time variations can be modeled by sampling the transmitter/receiver trajectory at the signal sampling rate and calculating, for each position, the channel impulse response from the acoustic-field computation. This approach, however, would result in high complexity. To reduce the complexity, the channel impulse response is calculated for fewer (waymark ) positions and then interpolated by local splines to recover it at the signal sampling rate. To allow higher distances between waymarks and, thus, further reduction in the complexity, the multipath delays are appropriately adjusted before the interpolation. Because, for every time instant, this method only requires local information from the trajectory, the impulse response can recursively be computed, and therefore, the signal transmission can be modeled for arbitrarily long trajectories. An approach for setting the waymark sampling interval is suggested and investigated. The proposed method is verified by comparing the simulated data with data from real ocean experiments. For a low-frequency shallow-water experiment with a moving source that transmits a tone set, we show that the Doppler spectrum of the received tones is similar in the simulation and experiment. For a higher frequency deep-water experiment with a fast-moving source that transmits orthogonal frequency-division multiplexing (OFDM) communication signals, we investigate the detection performance of a receiver and show that it is similar in the simulation and experiment.

MmWave communications applied to small cells has been recognized recently as an important means to break the spectrum gridlock and to dramatically scale up the system capacity for the evolving 5G mobile networks. However, significant technical challenges must be resolved to fully exploit the potential of mmWave small cells. Among them, base station discovery is particularly challenging due to the severe propagation loss at mmWave bands. Beam alignment arises as another critical problem that must be solved adaptively for 5G mobile access. This article discusses protocols and techniques for these two aspects, and presents important system design insights. Some open research problems are also highlighted in base station discovery, multi-user beam alignment, and network management.

In this paper, we consider covert communications for bistatic backscatter systems, where a tag transmits information passively to a reader by reflecting incident carrier signals with artificial noise (AN) generated from a dedicated carrier emitter (CE) under the supervision of a warden. By exploiting the channel uncertainty introduced by CE, we analytically derive the warden's minimum sum of error probabilities to evaluate communication covertness. We find that communication covertness can be improved by reducing the tag's reflection coefficient, but is not affected by the CE's transmit power. Therefore, in order to achieve energy-efficient design, we optimize the tag's reflection coefficient to minimize the CE's transmit power under the constraints of communication covertness and reliability. Our analysis shows that the minimum CE's transmit power can be further reduced with a minor loss of the achievable covert rate.

Locating multiple underwater acoustic sources is a problem that can be solved using antenna array beamforming based on the matched field (MF) processing. However, known MF beamforming techniques fail to provide good performance for multiple sources, a high noise power, and/or when the sources are close to each other. This paper proposes an MF technique for solving the localization problem. The proposed technique exploits formulation of the localization problem in terms of sparse representation of a small number of source positions among a much larger number of potential positions. The sparse representation is formulated as the basis pursuit de-noising (BPDN) problem for complex-valued variables. The solution is found as a joint solution to a set of BPDN problems corresponding to the set of source frequencies subject to the joint support. The joint BPDN problem is efficiently solved using the Homotopy approach and coordinate descent search. For further reduction in the complexity, a position grid refinement method is applied. Using simulated and real experimental data, it is shown that the technique can provide accurate source localization for multiple sources. The proposed technique outperforms other MF techniques in resolving sources positioned closely to each other, tolerance to the noise and capability of locating multiple sources.