318 research outputs found
Discrete-Phase Constant Envelope Precoding for Massive MIMO Systems
We consider downlink of a multiuser massive multiple-input multiple-output (MIMO) system and focus on reducing the hardware costs by using a single common power amplifier and separate phase shifters (PSs) for antenna front-ends. In the previous literature, the use of analog PSs in this setup has been considered. Here, we study the use of practical digital PSs, which only support a limited set of discrete phases. Considering the sum of interference powers as a metric, we formulate the corresponding nonlinear discrete optimization problem and solve for the phases to be used during transmission. We devise a low-complexity algorithm, which employs a trellis structure providing suboptimal, but efficient and effective solutions. We demonstrate via examples that the proposed solutions have comparable performance to conventional analog PS-based algorithms. Furthermore, we prove that by utilizing discrete-phase constant envelope precoding, the interference can be made arbitrarily small by increasing the number of antennas. Therefore, the asymptotic gains promised by massive MIMO systems are preserved. We also obtain closed-form expressions for the rate loss due to errors in the phase and amplitude of the PSs, for both low and high SNR regimes. © 1972-2012 IEEE
Waveforms for the Massive MIMO Downlink: Amplifier Efficiency, Distortion and Performance
In massive MIMO, most precoders result in downlink signals that suffer from
high PAR, independently of modulation order and whether single-carrier or OFDM
transmission is used. The high PAR lowers the power efficiency of the base
station amplifiers. To increase power efficiency, low-PAR precoders have been
proposed. In this article, we compare different transmission schemes for
massive MIMO in terms of the power consumed by the amplifiers. It is found that
(i) OFDM and single-carrier transmission have the same performance over a
hardened massive MIMO channel and (ii) when the higher amplifier power
efficiency of low-PAR precoding is taken into account, conventional and low-PAR
precoders lead to approximately the same power consumption. Since downlink
signals with low PAR allow for simpler and cheaper hardware, than signals with
high PAR, therefore, the results suggest that low-PAR precoding with either
single-carrier or OFDM transmission should be used in a massive MIMO base
station
Efficient Quantized Constant Envelope Precoding for Multiuser Downlink Massive MIMO Systems
Quantized constant envelope (QCE) precoding, a new transmission scheme that
only discrete QCE transmit signals are allowed at each antenna, has gained
growing research interests due to its ability of reducing the hardware cost and
the energy consumption of massive multiple-input multiple-output (MIMO)
systems. However, the discrete nature of QCE transmit signals greatly
complicates the precoding design. In this paper, we consider the QCE precoding
problem for a massive MIMO system with phase shift keying (PSK) modulation and
develop an efficient approach for solving the constructive interference (CI)
based problem formulation. Our approach is based on a custom-designed
(continuous) penalty model that is equivalent to the original discrete problem.
Specifically, the penalty model relaxes the discrete QCE constraint and
penalizes it in the objective with a negative -norm term, which leads
to a non-smooth non-convex optimization problem. To tackle it, we resort to our
recently proposed alternating optimization (AO) algorithm. We show that the AO
algorithm admits closed-form updates at each iteration when applied to our
problem and thus can be efficiently implemented. Simulation results demonstrate
the superiority of the proposed approach over the existing algorithms.Comment: 5 pages, 5 figures, submitted for possible publicatio
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