The time-domain signal in orthogonal frequency division multiplexing (OFDM) has a large peak-to-average power ratio (PAPR). Most existing solutions for this problem are designed for OFDM systems operating with a fully-digital MIMO architecture. However, these solutions cannot be used for an OFDM system operating with a hybrid beamforming architecture. Therefore, in this work we address PAPR reduction in OFDM with hybrid beamforming. We propose to transmit a PAPR reduction signal along with the precoded information signal. For an efficient design of the PAPR reduction signal, we propose to use extra radio frequency (RF) chains. The design of the PAPR reduction signal and the analog beamformers corresponding to the extra RF chains, is written as a constrained optimization problem. An iterative algorithm based on successive convex approximation is proposed to solve the optimization problem. We show that the proposed technique achieves a considerable end-to-end performance gain in the presence of a non-linear power amplifier.

The single carrier modulation is a potential candidate at the sub-THz band, because of the channel sparsity and the necessity for an analog front-end friendly modulation. However, the peak-to-average-power ratio (PAPR) of the single carrier modulation in a multi-user MIMO downlink scenario is still a concern. In this work, we address this problem, by transmitting a PAPR reduction signal (PRS) together with the precoded data symbols. The goal of the PRS is to reduce the PAPR of the continuous-time signal entering the power amplifier attached to each antenna. We propose two different methods to design the PRS. In the first method, we restrict the PRS to the channel null space, so that it does not cause interference with the users’ information symbols at the receiver. As such, this method has limited degrees of freedom for the PAPR reduction. Considering this, we propose a second method for the PRS design, where the PRS can interfere with the users’ information symbols in a constrained manner. This relaxation enables an increased PAPR reduction without reducing the data detection performance. The design of the PRS is formulated as an optimization problem and the performance of the proposed schemes is studied semi-analytically. Our analysis shows that the proposed scheme is able to reduce the PAPR by about 9 dB at the cost of only 0.5 dB SINR. Consequently, the bit-error-rate performance of the proposed scheme is negligibly impacted by non-linear power amplifier distortion.

Analog beamforming (ABF) is a key enabler for high-throughput mmWave communication to harvest sufficient beamforming gain from large antenna arrays. It relies on beam alignment and the radio frequency (RF) architecture. In multiuser scenarios, beam alignment becomes challenging because of the inter-beam-interference between different users. Motivated by the large number of available antennas at the base station, we propose a downlink sparse ABF architecture in which only a subset of antennas per RF chain can be scheduled for the beamforming. For this purpose, we model the RF chain-antenna association as a binary association matrix (BAM) and jointly optimize the transmit beams and the BAM subject to RF hardware constraints. To alleviate the complexity of the resulting mixed–integer programming problem, we propose an iterative beam alignment and BAM optimization where the beam alignment and BAM are successively optimized until convergence. The proposed sparse-connected RF architecture performs significantly better than the traditional fully and partially connected architectures in terms of signal-to-interference-plus-noise ratio and bit error rate. The simulation results indicate that optimizing the BAM profiles relaxes the need for optimized beam allocation beyond beamforming along the line-of-sight.

The channel sparsity and the necessity for an analog front-end friendly modulation have made single carrier a considerable candidate for sub-THz communication. However, the peak-to-average-power ratio (PAPR) of the single carrier modulation in a multi-user MIMO scenario is still a concern. In this work, we reduce the PAPR of the single carrier scheme, by transmitting an additional PAPR reduction signal (PRS). The PRS is constrained to lie in the null space of the channel so that it does not cause interference with the users’ information symbols at the receiver. Moreover, the additional transmit power consumed by the PRS is studied. An optimization problem is formulated to design the PRS, and the optimal power allocation for the PRS is analyzed. Our analysis shows that by trading off only 1 % of the total transmitted power, a PAPR reduction of about 3 dB can be achieved. Further, our simulations reveal that when an optimal power is allocated to the PRS, the proposed scheme achieves a significant performance gain compared to the classical single carrier system in the presence of power amplifier distortion.

Orthogonal Chirp Division Multiplexing (OCDM), a chirp based waveform, has gathered interest because of its robustness to time and frequency selective channels. In this work, we study the phase noise effect in OCDM, which has not been investigated in the literature yet. We analytically show that phase noise causes a common phase error (CPE) rotation of data symbols and inter-symbol interference (ISI) in OCDM. We investigate the influence of OCDM system parameters on the phase noise effect. For this purpose, a system parameter, chirp frequency offset is defined and its influence on the CPE and ISI is studied. Depending on the value of this chirp frequency offset, either CPE or ISI becomes the dominant phase noise effect. Our findings also show that an OCDM system can be made robust to phase noise at the cost of spectral efficiency by reducing the number of employed chirps. Our performance analysis demonstrates that OCDM has a gain over OFDM and Single Carrier (SC) in a fading channel with phase noise because of the frequency diversity and the different distortion caused by the phase noise generated ISI terms respectively. We also investigate the performance of OCDM in the presence of narrowband interference with phase noise and observe that OCDM has a benefit over OFDM and SC at medium and low interference power.