A decoder for decoding a signal received through a transmission channel represented by a channel matrix using a search sphere radius. The decoder comprises a radius determination device (307) for determining a search sphere radius from a preliminary radius. The radius determination device (307) is configured to: i. apply a machine learning algorithm to input data derived from the received signal, the 10 channel matrix and a current radius, the current radius being initially set to the preliminary radius, which provides a current predicted number of lattice points associated with the current radius; ii. compare the current predicted number of lattice points to a given threshold; iii. update the current radius if the current predicted number of lattice points is strictly higher 15 than the given threshold, the current radius being updated by applying a linear function to the current radius; Steps i to iii are iterated until a termination condition is satisfied, the termination condition being related to the current predicted number, the radius determination device (307) being configured to set the search sphere radius to the current radius in response to the 20 termination condition being satisfied.