Finite rate of innovation channel models and DoF of MIMO multi-user systems with delayed CSIT feedback

Abstract

Channel State Information at the Transmitter (CSIT) is of utmost importance in multiuser wireless networks, in which transmission rates at high SNR are characterized by Degrees of Freedom (DoF, the rate prelog). In recent years, a number of ingenious techniques have been proposed to deal with delayed and imperfect CSIT. However, we show that the precise impact of these techniques in these scenarios depends heavily on the channel model (CM). We introduce the use of linear Finite Rate of Information (FRoI) signals (which could also be called Basis Expansion Model (BEM)) to model time-selective channel coefficients. The FRoI dimension turns out to be well matched to Degree of Freedom (DoF) analysis since the FRoI CM allows compressed feedback (FB) and captures the DoF of the channel coefficient time series. Both the block fading model and the stationary bandlimited channel model are special cases of the FRoI CM. However, the fact that FRoI CMs model stationary channel evolutions allows to exploit one more dimension: arbitrary time shifts. In this way, the FroI CM allows to maintain the DoF unaffected in the presence of CSIT feedback (FB) delay, by increasing the FB rate. We call this Foresighted Channel FB (FCFB). FRoI CM relates also to (predictive) filterbanks and we work out the optimization details in the biorthogonal case (different analysis and synthesis filters). The FRoIC model with multiple basis functions accommodates FB delay beyond the coherence time and handling of users with unequal coherence times.

Foresighted delayed CSIT feedback for finite rate of innovation channel models and attainable NetDoFs of the MIMO interference channel

“…The linear FroI channel model was introduced in [6]. The main characteristic of FRoI CMs is that they closely approximate stationary (BL) signals.…”

“…Using a sophisticated variation of the MAT scheme, [5] was able to propose an improved scheme for the case where the FB delay T f b is less than the channel coherence time T c (defined as the inverse of the Doppler bandwidth (BW)). We use FRoI channel models [6] and exploit their approximately stationary character to propose a simple IA scheme based on Foresighted Channel FB (FCFB) for the MIMO IC as was done in [7] for ZF in the MISO BC.…”

Distributed Space–Time Interference Alignment With Moderately Delayed CSIT

“…Later, the characterization of the delayed CSIT effects was extended to the case where the feedback delay is less than the channel coherence time, i.e., moderately-delayed CSIT regime [13]- [15], [19]- [22]. This regime is particularly interesting because, in practice, the feedback delay can be less than the channel coherence time depending on the user mobility.…”