Degrees of Freedom of Interference Channels With Hybrid Beamforming

We study the sum degrees of freedom (DoF) of interference channels with hybrid beamforming in which each transmitter i uses M'
_i
antennas and M
_i
RF chains and each receiver i uses N'
_i
antennas and N
_i
RF chains, where M
_i
≤ M'
_i
and N
_i
≤ N'
_i
, ∀i = 1, 2,..., K, and hybrid beamforming composed of analog and digital precoders is employed at each node. For the two-user case, we completely characterize the sum DoF for an arbitrary number of antennas and RF chains by developing an achievable scheme optimized for the hybrid beamforming structure and deriving its matching upper bound. For a general K-user case, we focus on a symmetric case where M
_i
= M, N
_i
= N, M'
_i
= M , and N'
_i
= N', ∀i = 1, 2, . . ., K, and obtain lower and upper bounds on the sum DoF, which are tight when max{M',N'}/min{M',N'} is an integer. The results show that with a fixed number of RF chains, employing more antennas can increase the sum DoF of interference channel under certain conditions while this cannot improve the sum DoFs of point-to-point channel, multiple access channel, and broadcast channel.