Abstract
Herein, we consider constant envelope precoding in a multiple-input multiple-output orthogonal frequency division multiplexing system (CE MIMO-OFDM) for frequency selective channels. In CE precoding the signals for each transmit antenna are designed to have constant amplitude regardless of the channel realization and the information symbols that must be conveyed to the users. This facilitates the use of power-efficient components, such as phase shifters (PS) and nonlinear power amplifiers, which are key for the feasibility of large-scale antenna array systems because of their low cost and power consumption. The CE precoding problem is firstly formulated as a least-squares problem with a unit modulus constraint and solved using an algorithm based on coordinate descent. The large number of optimization variables in the case of the MIMO-OFDM system motivates the search for a more computationally efficient solution. To tackle this, we reformulate the CE precoding design into an unconstrained nonlinear least-squares problem, which is solved efficiently using the Gauss-Newton algorithm. Simulation results underline the efficiency of the proposed solutions and show that they outperform state of the art techniques.
Highlights
T HE use of large-scale antennas at the base station (BS) is crucial for the future of wireless communications, as they have shown to offer much needed improvements in average spectral as well as energy efficiency, [3]– [5]
We propose the use of a post-processing scalar factor which changes on a per OFDM block basis, and partially compensates for the lack of amplitude control in the transmitter because of constant envelope (CE) precoding
Before we describe the system model of the proposed CE MIMO-OFDM scheme, let us first briefly consider the downlink of a classic multiuser MIMO-OFDM, using a linear precoding scheme (i.e. Zero Forcing precoding), system operating over a frequency selective channel
Summary
T HE use of large-scale antennas at the base station (BS) is crucial for the future of wireless communications, as they have shown to offer much needed improvements in average spectral as well as energy efficiency, [3]– [5]. Amplitude of the discrete time signal that is transmitted by each BS antenna remains constant regardless of the channel realization or the information symbols that are conveyed to the users As a result these transmit signals have low peakto-average ratio (PAR) and make possible the use of power efficient nonlinear power amplifiers. We propose the use of a post-processing scalar factor which changes on a per OFDM block basis, and partially compensates for the lack of amplitude control in the transmitter because of CE precoding This scalar factor facilitates the exploitation of the array gain provided by the large-scale antenna array by scaling the signal at the receivers . Notations: ı denotes the imaginary unit, || · ||F denotes the Frobenius norm, || · ||2 denotes the L-2 norm, E[·] denotes the expectation operator, R denotes the set of real numbers, denotes the real part of a complex number, denotes the imaginary part of a complex number, [·]T denotes the transpose of a matrix and [·]H denotes the conjugate transpose of a matrix
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