Abstract : We investigate the degrees of freedom (DoF) for the two-user multiple-input multiple-output interference channel (MIMO IC) with parallel multicasting channels. Specifically, in addition to the MIMO IC, each transmitter is also connected to both receivers via an out-of-band multicast channel. Our main contribution lies in the characterization of the optimal sum DoF when the channel state information (CSI) on the MIMO IC is available to the transmitters with some delay (delayed CSIT). We show that jointly coding over the parallel multicast channels can achieve higher DoF than channel aggregation does. Furthermore, as long as the rate of the multicast channels is above a certain threshold, delayed CSIT is enough to achieve the same DoF performance as with instantaneous CSIT. I. INTRODUCTION Today's wireless networks are heterogeneous, consisting of multiple radios with different capabilities, protocol stacks, and spectrum allocations. Such heterogeneous networks offer several advantages such as flexible resource allocation, better coverage, and higher capacity. In heterogeneous networks, transmitters can send information to users through parallel channels operating over different systems, such as a cellular system and a WiFi system. While parallel channels have been studied extensively in the literature [1]–[8], these works typically considered parallel realizations of the same physical channel, e.g., over different time or frequency slots. Recently in [9], we studied heterogeneous parallel channels, where a transmitter sends information to receivers through two parallel channels: a multiple-input single-output broadcast channel (MISO BC) and a rate-limited multicast channel. We showed that jointly coding over the two channels can strictly outperform simple channel aggregation and can even achieve the same sum degrees of freedom (DoF) performance as with instantaneous CSIT when the CSIT on the MISO BC is completely stale, given that the multicast rate of the second channel is larger than a certain threshold. The main idea was to send information over the MISO BC at a rate above its capacity and use the second channel to send additional side information to allow for reliable decoding at both receivers (which we coined the overload-multicast strategy). Two key questions that remain open are whether the overload-multicast strategy can be optimally applied to other settings of heterogeneous parallel channels, and whether the insights obtained in [9] still hold, in particular that jointly
