Abstract
In conventional hybrid beamforming approaches, the number of radio-frequency (RF) chains is the bottleneck on the achievable spatial multiplexing gain. Recent studies have overcome this limitation by increasing the update-rate of the RF beamformer. This paper presents a framework to design and evaluate such approaches, which we refer to as agile RF beamforming, from theoretical and practical points of view. In this context, we consider the impact of the number of RF-chains, phase shifters' speed, and resolution to design agile RF beamformers. Our analysis and simulations indicate that even an RF-chain-free transmitter, which its beamformer has no RF-chains, can provide a promising performance compared with fully-digital systems and significantly outperform the conventional hybrid beamformers. Then, we show that the phase shifter's limited switching speed can result in signal aliasing, in-band distortion, and out-of-band emissions. We introduce performance metrics and approaches to measure such effects and compare the performance of the proposed agile beamformers using the Gram-Schmidt orthogonalization process. Although this paper aims to present a generic framework for deploying agile RF beamformers, it also presents extensive performance evaluations in communication systems in terms of adjacent channel leakage ratio, sum-rate, power efficiency, error vector magnitude, and bit-error rates.
Highlights
Fully-digital beamformers, with a dedicated radio-frequency (RF) chain per antenna, provide a high level of flexibility and accuracy to control the amplitude and phase of the signal at each antenna element in multiple-input multiple-output (MIMO) systems
We focus on the impact of TSW on adjacent channel leakage ratio (ACLR), achievable sum-rates and power efficiency compared to the conventional hybrid beamformers, root mean squared (RMS) EVM of 64QAM constellation and the bit error rates (BER)
We presented a framework to design and evaluate agile RF beamformers from theoretical and practical perspectives in wireless systems
Summary
Fully-digital beamformers, with a dedicated radio-frequency (RF) chain per antenna, provide a high level of flexibility and accuracy to control the amplitude and phase of the signal at each antenna element in multiple-input multiple-output (MIMO) systems. Hybrid beamformers can be used in both sub-6 GHz [8], [10], [11] and mmWave frequencies [12]. MmWave systems, generally, need a larger number of antennas and the cost and complexity of digital beamformers for such systems is relatively higher than sub-6 GHz technologies. Based on the angular properties of mmWave channels, [14] proposes codebook based approaches for fully-connected and subconnected hybrid beamformers in a single-user scenario where the quantization bits are nonuniformly assigned to different coverage angles. The authors in [15] treat the hybrid beamforming problem for mmWave systems as a matrix factorization problem and employ alternating minimization algorithms to propose lowcomplexity solutions. Considering a wideband multiuser scenario, [17] exploits the angle-of-arrival properties and covariance matrix of mmWave to calculate the beamforming weights
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