This paper is of a tutorial nature, introducing the array of antennas and the paradigm of software radio in next-generation mobile communications. To the conventional time, frequency, and code diversity, the use of array of antennas adds another dimension, namely space diversity, which enhances the performance of mobile networks. The concept of Software-Defined Radio (SDR) refers to a programmable hardware for implementing receiver functionalities either by different resident software or by software downloaded from the network. Both approaches require advanced, state-of-the-art digital signal processing (DSP) techniques and technologies. This paper provides a basic overview of these two approaches and discusses the challenges they present.