Degrees of Freedom of Three-User SISO-X Networks with Hybrid CSI at the Transmitters

In this letter, we consider the degrees of freedom (DoF) of the three-user memoryless X networks with independent and identically distributed (i.i.d.) fading across antennas and channel uses and under hybrid channel state information (CSI) at the transmitters (CSIT). Hybrid CSI implies that some part of the CSI available to the transmitters may be delayed while some part may be perfect. We show that with the help of some local instantaneous CSIT, in addition to the delayed CSIT, we can obtain a DoF greater than that achievable using existing schemes based on completely delayed CSIT. In our first result, we show that with local instantaneous CSIT available to every transmitter through its own receiver only, we can achieve a DoF of 4/3. In the second result, we show that with the help of local CSIT available to the transmitter only through its local links to all the receivers, a higher DoF of 3/2 can be obtained at the expense of some more feedback compared to the first result.