  The question of whether the degrees of freedom (DoF) of multi-user networks
can be enhanced even under isotropic fading and no channel state information
(or output feedback) at the transmitters (CSIT) is investigated. Toward this
end, the two-user MIMO (multiple-input, multiple-output) broadcast and
interference channels are studied with no side-information whatsoever at the
transmitters and with receivers equipped with full-duplex radios. The
full-duplex feature allows for receiver cooperation because each receiver, in
addition to receiving the signals sent by the transmitters, can also
simultaneously transmit a signal in the same band to the other receiver. Unlike
the case of MIMO networks with CSIT and full-duplex receivers, for which DoF
are known, it is shown that for MIMO networks with no CSIT, full-duplex
receiver cooperation is beneficial to such an extent that even the DoF region
is enhanced. Indeed, for important classes of two-user MIMO broadcast and
interference channels, defined by certain relationships on numbers of antennas
at different terminals, the exact DoF regions are established. The key to
achieving DoF-optimal performance for such networks are new retro-cooperative
interference alignment schemes. Their optimality is established via the DoF
analysis of certain genie-aided or enhanced version of those networks.
