I. Introduction
The design of high-capacity wireless communication systems aims at efficient use of the limited spectrum available for transmission. To this end, the use of multiple antennas plays an important role: A multiple-input multiple-output (MIMO) system increases spectral efficiency by allowing for spatial multiplexing—the simultaneous transmission of several data streams using multiple antennas [1]. Alternatively, the additional propagation paths can be used to provide diversity and increased link reliability [2]. In the context of a multi-user MIMO uplink, a receiving base station (BS) further has the capability to separate simultaneously transmitting users by spatial multiple access [3], [4]. While promising high spectral efficiency, the computational burden of jointly decoding multiple data streams on a narrowband carrier is a major downside of spatial multiplexing and multiple access: The complexity of highly effective schemes such as maximum likelihood scales exponentially in the number of data streams [5]. For the BS in a MIMO uplink, which typically deals with a number of narrowband carriers, joint data-stream decoding on each carrier may not be practically viable.