Lin-Kai Chiu

Robust Hybrid Beamforming with Phased Antenna Arrays for Downlink SDMA in Indoor 60 GHz 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.

Planar arrays hybrid beamforming for SDMA in millimeter wave applications

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.

Hybrid beamforming using convex optimization for SDMA in millimeter wave radio

A radio-frequency and baseband hybrid beam-forming (HBF) scheme is presented for spatial division multiple access (SDMA) of 60GHz 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.

Hybrid beamforming for two-user SDMA in millimeter wave radio

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 users signal is severely jammed by the interfering users signal.

Hybrid radio frequency beamforming and baseband precoding for downlink MU-MIMO mmWave channels

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 Modified Bayesian Cramer-Rao Bound for MIMO Channel Tracking

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.

Diversity and directivity tradeoff in indoor 60 GHz MIMO channels: A case study with dual-polarized hybrid planar antenna arrays

The channel capacity of indoor 60 GHz radio is studied in this work from the diversity and the directivity viewpoints of multiple input multiple output (MIMO) wireless systems. To characterize the diversity and directivity (DnD) tradeoff of using planar antenna arrays (PAA) in indoor 60 GHz channels, a hybrid architecture for dual-polarized PAA (DP-PAA) is employed to exploit the potential advantages between the two extremes of this DnD tradeoff. A DP-PAA under this hybrid architecture can either be configured to apply the spatial multiplexing (SM) gain of MIMO channel diversity, or to enhance the array gain with beamforming directivity. Furthermore, the polarization diversity of DP-PAA can provide the system with a minimum degree of SM gain and compensate the coverage hole of a single-polarized PAA. Through theoretical investigations and numerical simulations, we show that the indoor 60 GHz channel of using a hybrid DP-PAA pair at both the transmitter and the receiver can approximately be modeled as a Kronecker MIMO channel. In addition, using the directivity of DP-PAA can lead to a higher channel capacity than using its spatial diversity under FCCs 40 dBm equivalent isotropically radiated power limit for indoor unlicensed 60 GHz links. This prevents the use of expensive high-dimensional MIMO to improve the channel capacity of indoor 60 GHz radio.

An Effective Approach to Evaluate the Training and Modeling Efficacy in MIMO Time-Varying Fading Channels

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 space-time precoded hybrid beamforming architecture for broadband transmissions in 60GHz radio

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.

Planar Antenna Array Hybrid Beamforming for SDMA in Millimeter Wave WPAN

The increasing demands on bandwidth for personal and indoor wireless multimedia applications have driven the research and development for a new generation of broadband wireless personal area network (WPAN) Alliance (n.d.); IEEE 802.11 WLAN Very High Throughput in 60GHz (n.d.); IEEE 802.15 WPAN Millimeter Wave Alternative PHY Task Group (TG3c) (n.d.); WirelessHD (n.d.). This new WPAN is intended to support data rate up to 5Gbps or more and allows for wireless interconnection among devices, such as laptops, camcorders, monitors, DVD players and cable boxes, etc. Moreover, it could also serve as a wireless alternative to the High-Definition Multimedia Interface (HDMI). In addition to its very large bandwidth, the new WPAN also demands for short-range and secure wireless connections. The characteristics of broad unlicensed bandwidth FCC (2004), high penetration loss Smulders (1995; 2002) and significant oxygen absorption Anderson & Rappaport (2004) at 60GHz radio make it an ideal wireless interface for the next generation WPAN. Furthermore, the millimeter wavelength of 60GHz radio also makes it possible to use tens of tiny antennas to steer radio signals with high directivity to the intended receivers. This feature of high-directivity beam pattern not only improves the wireless link quality Balanis & Ioannides (2007) but also increases the spatial reuse factor, allowing for multiple users to gain access to the wireless channel at the same frequency and time. In view of the great potential of 60GHz radio on WPAN and the advantages of beamforming (BF) for millimeter wave (mmWave) applications, we study in this article a simple hybrid beamforming (HBF) technique for spatial division multiple access (SDMA) using planar antenna arrays (PAAs). Digital BF has been used to compensate the rather fixed radiation patterns of switch-beam or beam-selection antennas to create more flexible hybrid beam patterns Celik et al. (2006); Rezk et al. (2005); Zhang et al. (2003). In conjunction with the phase shifters of the element antennas of PAAs, a hybrid type of BF is considered in Smolders & Kant (2000) which exploits the advantage of BF both in the baseband and the radio-frequency (RF) ranges. Motivated by the above results and taking into account the practical limitation and implementation cost of the full digital BF, we study herein a special type of baseband and RF HBF for SDMA that only requires four digital processing paths to support HBF on a 8 × 8 PAA illustrated in Fig. 1. The entire PAA of Fig. 1 is partitioned into four blocks of patch antennas. Each block is driven by a digital BF weight, while each element patch antenna in a block is equipped with 4