Multiuser Millimeter Wave Beamforming Strategies With Quantized and Statistical CSIT

Abstract

To alleviate the high cost of hardware in mmWave systems, hybrid analog/digital precoding is typically employed. In the conventional two-stage feedback scheme, the analog beamformer is determined by beam search and feedback to maximize the desired signal power of each user. The digital precoder is designed based on quantization and feedback of effective channel to mitigate multiuser interference. Alternatively, we propose a one-stage feedback scheme which effectively reduces the complexity of the signalling and feedback procedure. Specifically, the second-order channel statistics are leveraged to design digital precoder for interference mitigation while all feedback overhead is reserved for precise analog beamforming. Under a fixed total feedback constraint, we investigate the conditions under which the one-stage feedback scheme outperforms the conventional twostage counterpart. Moreover, a rate splitting (RS) transmission strategy is introduced to further tackle the multiuser interference and enhance the rate performance. Consider (1) RS precoded by the one-stage feedback scheme and (2) conventional transmission strategy precoded by the two-stage scheme with the same firststage feedback as (1) and also certain amount of extra secondstage feedback. We show that (1) can achieve a sum rate comparable to that of (2). Hence, RS enables remarkable saving in the second-stage training and feedback overhead.

Rate-Splitting Unifying SDMA, OMA, NOMA, and Multicasting in MISO Broadcast Channel: A Simple Two-User Rate Analysis

“…A lower ρ indicates that both private streams effectively operate at a lower SNR. According to (12), for a given t, a low ρ favors power allocation to a single private stream (NOMA/OMA/Multicasting regime) over a wider range of P , and also leads to a smaller interference power (and therefore a higher rate) for the common stream. A higher ρ leads to a higher effective SNR and therefore a better capability to support two private streams (RS/SDMA regime).…”

“…We assume n t = 2, and channel vectors given by h 1 = 1/ √ 2 [1, 1] H and h 2 = γ/ √ 2 [1, e jθ ] H . Assuming the precoding strategies in Section III and the WF power allocation (12), the colors in Fig. 3(a) and (b) illustrate the optimum value (obtained from exhaustive search whenever not available in closed form) of t that maximizes the sum-rate and the corresponding preferred communication strategy (RS, SDMA, NOMA, OMA) as a function of ρ = 1 − hH 1h 2 2 (ranging from 0 to 1) and γ dB = 20 log 10 (γ) (ranging from 0 to -20dB), i.e., user-1 and user-2 have a long-term SNR of 20dB and 0dB ≤ 20dB + γ dB ≤ 20dB, respectively.…”

“…1) OMA/NOMA/Multicasting Regime: If µ ≤ 1 h2 2 ρ , i.e., tP ≤ Γ, we set P 2 = 0 and P 1 = tP according to (12), and RS specializes to multicasting for t = 0, NOMA for 0 < t < 1, and OMA for t = 1. In this regime, t needs to be adjusted so as to identify the best strategy among OMA, NOMA, and multicasting, and therefore efficiently allocate power across the common stream s c and the private stream s 1 .…”

“…Though originally introduced for the two-user Single-Input Single-Output Interference Channel (IC) in [2], RS has become an underpinning communication-theoretic strategy to tackle modern interference-related problems and has recently been successfully investigated in several Multiple-Input Single-Output (MISO) Broadcast Channel (BC) settings, namely, unicast-only transmission with perfect Channel State Information at the Transmitter (CSIT) [3], [4] and imperfect CSIT [5]- [13], (multigroup) multicast-only transmission [14], as well as superimposed unicast and multicast transmission [15]. Results highlight that RS provides significant benefits in terms of spectral efficiency [3], [6], [7], [9], [13]- [15], energy efficiency [4], robustness [8], and CSI feedback overhead reduction [6], [12] over conventional strategies used in LTE-A/5G that rely on fully treating interference as noise (e.g. conventional multiuser linear precoding and Space Division Multiple Access -SDMA) or fully decoding interference (e.g.…”

Cooperative Rate Splitting for MISO Broadcast Channel With User Relaying, and Performance Benefits Over Cooperative NOMA

“…In contrast to NOMA that relies on some users to fully decode the messages of other users (i.e. fully decode interference), Rate-Splitting (RS) also exploits SIC but exploits a more flexible framework of non-orthogonal transmission, which enables to partially decode interference and partially treat remaining interference as noise, and hence provide significant benefits in terms of spectral efficiency [13], [15]- [20], energy efficiency [21], robustness [22], and CSI feedback overhead reduction [15], [23]. In addition, on the standpoint of encoding structure, the combination of a common stream decoded by all users and private streams dedicated to each user also provides a fundamental compatibility with other advanced techniques [24].…”