Analysis of Fronthaul Delays in Uplink of Cell-free Massive MIMO and Small Cell Networks

In the uplink of cell-free massive MIMO (CF-mMIMO), the data from user equipment (UE) reaches the central processing unit (CPU) via all the access points (APs), whereas it is only via a single serving AP in a small-cell (SC) network. Due to the finite capacities of fronthaul (FH) links from APs to CPU, packet transfer over them leads to queuing delays. Thus, the FH network delay in CF-mMIMO is the maximum of the FH delays on the links from all the APs to the CPU. However, the FH network delay in the SC network is only the FH delay on the link from the serving AP. We analyze the FH network delays in both the networks by modeling the queuing of FH packets at any AP. This yields tractable, closed-form expressions for the probability distributions of the delays and conditions for the stability of the queues at APs, which in turn yields conditions for the stability of the networks. Through extensive simulations, we validate the analytical results and see that FH delays are significantly lower in CF-mMIMO compared to the SC network. Furthermore, the FH delays in SC networks are impacted more by variations in system parameters compared to those in CF-mMIMO.