A reconfigurable hybrid beamforming (HBF) system is presented to dynamically exploit the channel diversities and antenna array directivity in millimeter wave (mmWave) channels. To develop an effective method for HBF architecture adaptation in wideband mmWave channels, the performance of an architecture is evaluated based on its average frequency-domain channel capacity. According to our analysis and simulations, the spatial covariance of the frequency-domain channels of using HBF can still be approximately modeled in a Kronecker form. Furthermore, beamforming directivity and channel spatial diversity have different advantages on capacity improvement in different transceiving conditions. Based on the results of channel modeling, system analysis, and simulations, a reconfigurable HBF architecture is thus proposed to enable flexible adjustments of the active subarrays, the antennas and polarizations of each subarray, and the active analog front-end and radio frequency chains linked to the active subarrays. A training procedure is meanwhile developed for the HBF system to learn proper architectures according to the system constraints, the channel statistics, the baseband precoder structure, and the channel capacity between the transmitter and the receiver. Compared with a fixed HBF system, such a reconfigurable system provides 3-15-dB power gains in channel capacity under various simulated channel conditions.

A hybrid architecture of radio frequency (RF) beamforming (BF) and baseband (BB) signal processing is presented for downlink (DL) multi-user (MU) multiple input multiple output (MIMO) systems operating in millimeter wave (mmWave) channels. To avoid the strong spatial correlation and the transmission or reception dead zones caused by the use of single-polarized antenna arrays in mmWave radio, the hybrid architecture is implemented on dual-polarized (DP) planar antenna arrays (PAA). To balance the quality of each data stream of each user under a limited power consumption, an iterative algorithmfor jointly designing the RF beamformers (BFer), BB precoder, BB equalizers, and the power allocation is proposed for the DL MU-MIMO system of using the hybrid architecture. Simulation results show that the signal to interference-plus-noise ratio (SINR) achieved with the MU-MIMO system that uses the proposed hybrid architecture is only 5~6 dB lower than that of the conventional one which uses 8 times more BB modules for the same 4 × 4 DP-PAA. Furthermore, the RF BFers obtained by the proposed iterative algorithm provide a better SINR than that of directly steering the RF BFs towards the line-of-sight transmission path.

The efficacy of channel modeling and training for multiple-input multiple-output (MIMO) time-varying flat faded Rayleigh channels is studied herein from the information-theoretical perspective. To characterize the channel dynamics in wide-sense stationary uncorrelated scattering wireless environments, proper autoregressive (AR) channel models for different fading speeds are discussed from the viewpoints of the mean squared error (MSE) and Bayesian Cramér-Rao lower bound (BCRB) of channel estimation. Furthermore, the training efficacy is examined with the achievable capacity when the MSE of channel estimation attains the BCRB. Our numerical simulations show that neither is the first-order AR model enough, nor is a large-order AR model needed for modeling time-varying fading channels. The analysis on BCRB also shows that the influence of the multiple access interference among transmit antennas is not negligible for channel estimation in time-varying fading channels even if using orthogonal sequences for training. As channel tracking can utilize the current and all past training symbols, the optimal training lengths for each data packet may be less than the number of transmit antennas. These results help re-examine the efficacy of model complexity and training overhead, and characterize the achievable rate for MIMO systems that use more practical methods in estimating Rayleigh fading channels.

A hybrid architecture is presented for downlink beamforming (BF) with phased antenna arrays (PAA) in indoor 60 GHz spatial division multiple access (SDMA) channels. To manage the multiple access and inter-symbol interferences (MAI/ISI) encountered in SDMA with limited feedbacks, a cost-effective time-domain hybrid BF (HBF) method is presented to exploit the directivity provided by PAA in radio frequency (RF) beam patterns and the spatial diversity offered by multiple baseband processing modules. To maintain signal qualities under unpredictable MAI/ISI in wireless multimedia streaming to which indoor 60 GHz radio mainly applies, robust beamformers are designed to maintain the signal to interference-plus-noise ratio (SINR) for each user with minimum total transmit power. The percentages in which the target SINRs can be satisfied with the proposed HBF schemes are found sensitive to uncertainties in the phase shifters of PAA. Two kinds of robust formulations are thus proposed to jointly combat the MAI, ISI and phase uncertainties. Robust beamformers with semi closed-form expressions can be obtained with a nonlinear kind of them, whose SINR satisfaction ratio can attain 80% or more by extensive simulations in an indoor two-user 60 GHz environment if RF beam patterns of the users do not highly overlap in space.

In this paper, we study the achievable rate of the training-based multi-input multi-output (MIMO) systems over time-varying flat fading channels modeled with the L-th order autoregressive, AR(L), process. Using the Bayesian Cramer-Rao lower bound (BCRB) to characterize the mean squared error of channel estimation, the achievable rate of the MIMO systems is investigated from the information-theoretical perspective. The optimum lengths for the training and the data blocks are determined to maximize the achievable rate. Besides, by modeling the time-varying wireless fading channel with a proper AR(L) model for the specific normalized Doppler frequency, fdTs, the aforementioned results can be extended to characterize the achievable rate in a more realistic time-varying wireless fading channel.

A cost-effective architecture is studied for a joint radio frequency (RF) and baseband hybrid beamforming (HBF) method that uses planar antenna arrays (PAA) in 60GHz radio transmission. Under the hybrid architecture, a space-time precoded HBF scheme is presented to exploit the spatial diversity for non line-of-sight transmission in 60GHz radio. Furthermore, an efficient one-bit feedback (OBF) beamforming (BF) scheme is proposed to search for the pointing directions of each RF beamformer (BFer) of the HBF architecture. Simulation results show that the proposed HBF architecture can provide a 5~10dB gain in the signal to noise ratio compared to an architecture that uses the RF BF only. In addition, the OBF BF scheme converges close to the LOS transmission in 15 iterations, which is much more efficient than exhaustively searching for the BF directions.

An effective joint radio-frequency (RF) and baseband (BB) hybrid beamforming (HBF) scheme is presented for two-user spatial division multiple access (SDMA) in 60GHz radio using planar antenna arrays (PAA). Taking into account the physical constraints of the phase shifters of PAA, we study the design of downlink HBF from two perspectives of conic optimization: transmit power minimization subject to a lower limit on the signal-to-interference plus noise ratio (SINR), and the maximization of the minimum SINR subject to an upper limit on the transmit power. For the power minimization problem, we obtain closed-form expressions for the BB beamforming (BF) weights. And for the SINR maximization, a simple iterative scheme is obtained based on the closed-form expressions for the first problem. Simulation results show that the bit error rates with HBF can significantly outperform the ones with RF BF only when the desired user's signal is severely jammed by the interfering user's signal.

The modified Bayesian Cramer-Rao bound (MBCRB) to the mean squared error (MSE) of multi-input and multi-output (MIMO) channel estimation under discrete nuisance parameters are studied in this work. A recursive formula is provided for the computation of the MBCRB in time-varying MIMO Rayleigh fading channels. Under the assumption that channel coefficients of the MIMO system are independent and identically distributed, the MBCRB for MIMO channel estimation is shown to be the same to the single-input and single-output (SISO) system. Based on the MBCRB, the MSE of Kalman channel tracking is investigated for various MIMO channel configurations with different orders of the channel model.

A radio-frequency and baseband hybrid beam-forming (HBF) scheme is presented for spatial division multiple access (SDMA) of 60 GHz applications using planar antenna arrays (PAA). PAA with phase shifters for each element antenna is partitioned into sub-blocks and each block is applied a common baseband beamforming weight. To suppress the grating lobes of beam pattern and to maintain the signal to interference plus noise ratio (SINR) of users in the SDMA system, we study the design of this HBF using convex optimization. Two design criteria are considered herein: one minimizes the transmit power subject to a SINR constraint, and the other maximizes the worst case SINR subject to a total power constraint. Simulation results show that both the SINR and directivity of HBF can be significantly improved by making use of the convex optimization technique.

A radio-frequency and baseband beamforming scheme is presented for spatial division multiple access (SDMA) of 60 GHz applications using planar antenna arrays (PAA). PAA with phase shifters for each element antenna is partitioned into sub-blocks and each block or multiple blocks are applied to a user in the SDMA system. According to the analysis for the signal to interference plus noise ratios (SINRs) of the users in the SDMA system, it is shown that different users may have different permissible operating regions in order to maintain the SINRs for all users in the system. A procedure is proposed to identify the permissible operating regions for each user in the SDMA system. Based on these results, more flexible hybrid beamforming schemes are currently studied to support SDMA in a more efficient way for 60 GHz millimeter wave applications.