Joint Optimization of Cloud and Edge Processing for Fog Radio Access\n Networks

This work studies the joint design of cloud and edge processing for the\ndownlink of a fog radio access network (F-RAN). In an F-RAN, as in cloud-RAN\n(C-RAN), a baseband processing unit (BBU) can perform joint baseband processing\non behalf of the remote radio heads (RRHs) that are connected to the BBU by\nmeans of the fronthaul links. In addition to the minimal functionalities of\nconventional RRHs in C-RAN, the RRHs in an F-RAN may be equipped with local\ncaches, in which frequently requested contents can be stored, as well as with\nbaseband processing capabilities. They are hence referred to as enhanced RRH\n(eRRH). This work focuses on the design of the delivery phase for an arbitrary\npre-fetching strategy used to populate the caches of the eRRHs. Two\nfronthauling modes are considered, namely a hard-transfer mode, whereby\nnon-cached files are communicated over the fronthaul links to a subset of\neRRHs, and a soft-transfer mode, whereby the fronthaul links are used to convey\nquantized baseband signals as in a C-RAN. Unlike the hard-transfer mode in\nwhich baseband processing is traditionally carried out only at the eRRHs, the\nsoft-transfer mode enables both centralized precoding at the BBU and local\nprecoding at the eRRHs based on the cached contents, by means of a novel\nsuperposition coding approach. To attain the advantages of both approaches, a\nhybrid design of soft- and hard-transfer modes is also proposed. The problem of\nmaximizing the delivery rate is tackled under fronthaul capacity and per-eRRH\npower constraints. Numerical results are provided to compare the performance of\nhard- and soft-transfer fronthauling modes, as well as of the hybrid scheme,\nfor different baseline pre-fetching strategies.\n