Coded Random Access in Cell-Free Massive MIMO Networks with Access Point Signal Combining

Grant-free uplink from a massive number of machine-type devices in cell-free massive MIMO networks is considered. A new approach is introduced that relies on coded random access, on the device side, with combining of signals received at properly selected access points (APs) and successive interference cancellation, on the network side. The gain potentially achievable via AP signal combining is evaluated using a genie-aided scheme, where the network is assisted by a genie in the selection of the cluster of APs for each active device. Then, two practical AP selection algorithms are proposed, working in grant free conditions. These algorithms can substantially enhance the system performance despite absence of any prior information. Under specific conditions, one of them even performs very close to the genie-aided scheme.