Joint channel estimation and activity detection for multiuser communication systems

We consider overloaded (non-orthogonal) code division multiple access multiuser wireless communication systems with many transmitting users and one central aggregation node, a typical scenario in e.g. machine-to-machine communications. The task of the central node is to detect the set of active devices and separate their data streams, whose number at any time instance is relatively small compared to the total number of devices in the system. We introduce a novel two-step detection procedure: the first step involves the simultaneous transmission of a pilot sequence used for identification of the active devices and the estimation of their respective channel coefficients. In the second step the payload is transmitted by all active devices and received synchronously at the central node. The first step reduces to a compressed sensing (CS) problem due to the relatively small number of simultaneously active devices. Using an efficient CS recovery scheme (approximate message passing), joint activity detection and channel estimation with high reliability is possible, even for extremely large-scale systems. This, in turn, reduces the data detection task to a simple overdetermined system of linear equations that is then solved by classical methods in the second step.