Power efficient digital transmission for MIMO Systems using spherical codes

In this work, we investigate whether so-called spherical codes, i. e., codes with code words that are distributed over the surface of a multidimensional hypersphere in a Euclidean space, are suited as modulation alphabets for digital transmission over multiple-antenna systems. Using spherical codes as a modulation alphabet is called Phase Shift Keying on the Hypersphere (PSKH), because it can be understood as a generalization of conventional Phase Shift Keying (PSK). If PSKH is used in combination with suitable amplifiers in the output stage of a transmitter, so-called load modulated amplifiers, the power efficiency of a digital transmission system can be improved. This work begins by comparing different methods to generate spherical codes with respect to power efficiency in the case of coded and uncoded transmission. It turns out that large minimum distance does not necessarily result in high power efficiency in the coded case when combined with Bit-Interleaved Coded Modulation (BICM). Furthermore, methods to reduce the Peak-to-Average-Sum-Power-Ratio (PASPR) are analyzed. Achieving a low PASPR is vital to fully exploit load modulated transmitters. The PASPR reduction can be achieved in two ways: Either by modifying the pulse shaping at the transmitter, or by using rotated versions of a base constellations. The latter method is termed pi/2-shifted PSKH. In this work, pi/2-shifted PSKH is thoroughly analyzed both in terms of capacity and power efficiency, whereby different receiver algorithms are introduced and compared with respect to performance and computational complexity.