Abstract
Most of the previous work on hybrid transmit and receive beamforming focused on narrowband channels. Because the millimeter wave channels are expected to be wideband, it is crucial to propose efficient solutions for frequency-selective channels. In this regard, this paper proposes an iterative analog–digital multi-user equalizer scheme for the uplink of wideband millimeter-wave massive multiple-input-multiple-output (MIMO) systems. By iterative equalizer we mean that both analog and digital parts are updated using as input the estimates obtained at the previous iteration. The proposed iterative analog–digital multi-user equalizer is designed by minimizing the sum of the mean square error of the data estimates over the subcarriers. We assume that the analog part is fixed for all subcarriers while the digital part is computed on a per subcarrier basis. Due to the complexity of the resulting optimization problem, a sequential approach is proposed to compute the analog phase shifters values for each radio frequency (RF) chain. We also derive an accurate, semi-analytical approach for obtaining the bit error rate (BER) of the proposed hybrid system. The proposed solution is compared with other hybrid equalizer schemes, recently designed for wideband millimeter-wave (mmWave) massive MIMO systems. The simulation results show that the performance of the developed analog–digital multi-user equalizer is close to full-digital counterpart and outperforms the previous hybrid approach.
Highlights
The underutilized millimeter-wave frequency spectrum has been explored for future wideband cellular communication networks because there is an overcrowding of conventional sub-6 GHz bands [1]
The authors of [27] designed a joint spatio–radio resource and three hybrid precoding algorithms for systems with limited feedback: (1) a hybrid precoder with user-beam selection to maximize the sum proportional rate; (2) a low complexity suboptimal solution using limited statistical channel state information (CSI) feedback; (3) a k-mean algorithm based on an unsupervised machine learning scheme
In the forward path, the signal first passes through a linear filter W used at subcarrier k and of the equalizer for the kth subcarrier is the solution of the followingd,k convex optimization problem follows single-carrier frequency-division multiple access (SC-FDMA) decoding and data demodulation
Summary
The underutilized millimeter-wave (mmWave) frequency spectrum has been explored for future wideband cellular communication networks because there is an overcrowding of conventional sub-6 GHz bands [1]. The combination of mmWave with mMIMO is very attractive because, when compared to the current communication systems, it has a smaller wavelength, and more antennas can be compacted in the same volume [4]. This combination offers more degrees of freedom, but it leads to more correlated channels [5], and new and efficient beamforming techniques and spatial multiplexing for both the transmitter and the receiver sides must be exploited [6]. In this paper we adopt a hybrid analog–digital architecture, to achieve a good tradeoff between performance and complexity and we design a fully iterative analog–digital multiuser equalizer for wideband mmWave mMIMO SC-FDMA systems
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