RESEARCH RESULTS

In millimeter wave communications, narrow beams will be employed in order to support high capacity and mitigate the propagation loss at these frequencies. This poses the need to maintain alignment between the beams at the transmitter and receiver as the mobile user moves around, which can be a source of significant communication overhead and may degrade the communication performance. We have been looking at the design of beam alignment protocols that optimize the trade-off between the communication overhead incurred for beam alignment and the communication performance in terms of delay and throughput.

Our analysis provides insights for the design of energy efficient beam-alignment protocols. In particular, in our Journal paper published on IEEE TWC (link) we demonstrate the optimality of a decoupled fractional search policy, which decouples over time the alignment of angle of departure (AoD) and of arrival (AoA), and iteratively scans a fraction of their region of uncertainty. Additionally, we demonstrate the optimality of a beam-alignment procedure of fixed duration (as opposed to an online scheme which switches to data communication based on the beam-alignment feedback). Our numerical results using analog beams (shown in the figure) demonstrate the superior performance of our scheme (indicated as DFS) over a state-of-the-art bisection method (BiS) and exhaustive search schemes (IES and CES).

In addition to its overhead, the beam-alignment procedure may fail due to detection errors. We proposed a coded energy-efficient beam-alignment scheme, which is robust against detection errors. Specifically, the beam-alignment sequence is designed such that the error-free feedback sequences are generated from a codebook with the desired error correction capabilities. Therefore, in the presence of detection errors, the error-free feedback sequences can be recovered with high probability. The numerical results show superior performance of the proposed scheme, depicting up to 4dB and 8dB performance gain compared to the exhaustive and uncoded energy-efficient beam-alignment, respectively.

The beam alignment protocols proposed rely on perfect ACK/NACK mechanism without any false-alarm/mis-detection. However, in reality, non-zero false-alarm and mis-detection rates occur due to imperfect beam shape and thermal noise at the receiver. To enable the operation of the beam alignment schemes and assuming a phased antenna array architecture, we investigated the design of codewords (which assign the amplitude and phase of each antenna array element) to achieve minimum mis-detection and false-alarm rate. The proposed beam design shows superior performance compared to the state-of-art beam-design, demonstrating an improvement of up to 33% in mis-detection performance.

Millimeter-wave communications incur a high beam alignment cost in mobile scenarios such as vehicular networks. Therefore, an efficient beam alignment mechanism is required to mitigate the resulting overhead. We investigated analytically a one-dimensional mobility model where a mobile user (MU), such as a vehicle, moves along a straight road with time-varying and random speed, and communicates with the base stations (BSs) located on the roadside over the mm-wave band (as shown in Fig. 1).

To compensate for location uncertainty, the BS widens its transmission beam and, when a critical beamwidth is achieved, it performs beam-sweeping to refine the MU position estimate, followed by data communication over a narrow beam. We derived the average rate and average transmission power in closed form and the optimal beamwidth for communication, number of sweeping beams, and transmission power allocation so as to maximize the average rate under an average power constraint. We proved structural properties of the optimal design, and designed a bisection algorithm to determine the optimal sweeping -- communication parameters.

A key question in mm-wave vehicular networks is how to optimize the trade-off between directive Data Transmission (DT) and directional Beam Training (BT), which enables it. We have optimized the trade-off between DT and BT in a mobile scenario based on a Partially Observable (PO) Markov Decision Process (MDP) formulation, where the system state corresponds to the position of the MU within the road link. The goal is to maximize the number of bits delivered by the BS to the MU over the communication session, under a power constraint. The resulting optimal policies (shown in Fig. 3) reveal that adaptive BT/DT procedures significantly outperform common-sense heuristic schemes, and that specific mobility features, such as user position estimates, can be effectively exploited to enhance the overall system performance and optimize the use of system resources.

We performed millimeter-wave propagation measurements at 28 GHz for a typical suburban environment using a 400-megachip-per-second custom-designed broadband sliding correlator channel sounder and highly directional 22-dBi (15 degree half-power beamwidth) horn antennas. With a 23-dBm transmitter installed at a height of 27 m to emulate a microcell deployment, the receiver obtained more than 5000 power delay profiles over distances from 80m to 1000m at 50 individuals sites and on two pedestrian paths. We compared the resulting basic transmission losses with predictions of the over-rooftop model in recommendation ITU-R P.1411-9. Our analysis reveals that the traditional channel modeling approach may be insufficient to deal with the varying site-specific propagations of millimeter waves in suburban environments. For line-of-sight measurements, the path loss exponents obtained for the close-in (CI) free space reference distance model and the alpha-beta-gamma (ABG) model are 2.00 and 2.81, respectively, which are close to the recommended site-general value of 2.29. The root mean square errors (RMSEs) for these two reference models are 9.93 dB and 9.70 dB, respectively, which are slightly lower than that for the ITU site-general model (10.34 dB). For non-line-of-sight measurements, both reference models, with the resulting path loss exponents of 2.50 for the CI model and 1.12 for the ABG model, outperformed the site-specific ITU model by around 14 dB RMSE.