Low complexity soft sphere decoding via optimal radius control

In this paper, we present a low-complexity list sphere search algorithm for achieving near-optimal a posteriori (APP) detection in iterative detection and decoding (IDD). Motivated by the fact that the list sphere decoding searching fixed number of lattice points is inefficient in many scenarios, we design a criterion to search lattice points with non-vanishing likelihood and derive the optimal sphere radius satisfying this requirement. Through simulations on realistic IDD systems, we show that the proposed method provides considerable complexity savings while maintaining near-optimal performance.