Abstract-In this paper, we propose a simple polynomial linearisation technique for nonlinear phased arrays including amplitude control. Due to the large number of antennas and thus power amplifiers in the array, it is inefficient to linearise each power amplifier individually. Therefore, it is demonstrated that the array can be linearised over-the-air using single polynomial. The simulations show that the linearisation is achieved by first linearising the higher driven PAs at the precompression region and then cancelling the compression by the heavily expanding lower driven PAs. The proposed approach offers an alternative way of re-thinking the concept of array linearisation over multiple PAs.Index Terms-digital predistortion, hybrid beamforming, polynomial model, power amplifiers, 5G.

This paper shows how digital predistortion of a phased array can benefit from the parametric variations over parallel power amplifiers (PAs). Different antenna configurations are simulated by varying the PA input drive levels by the Monte-Carlo method. The error vector magnitude (EVM) at the steering angle and total radiated adjacent channel power ratio (TRACPR) are used as performance metrics. The simulation results indicate that array predistortion can benefit from the variations between the PAs to improve the EVM significantly. However, at the same time, the TRACPR performance is reduced. This gives a new trade-off to balance between in-band and outoff-band distortion in the fifth generation beamforming systems.

Abstract-Wideband fifth generation (5G) systems utilizing high carrier frequency and multiple-input multiple-output (MIMO) raise major challenges for the system design. Wave propagation and practical hardware tradeoffs at higher frequency ranges provide new boundary conditions for the implementation. This paper addresses system performance boundaries and the analysis method towards multibeam communications at mmW. We combine analysis from antennas and propagation to the RF transceiver specifications and beamforming requirements. Realistic propagation model and antenna implementation is used to generate beam-specific path gains and provide a wide variety of user scenarios. Using this approach, system level interdependencies and RF performance boundaries can be verified with different antenna configurations in various propagation environments. As an example, we present MIMO link budget analysis targeting 10 Gbits/s for multiple devices in the office scenario at 27 GHz.

support only one-stream transmission and is not viable for multi-stream case [6]. To exploit the full potential of both spatial multiplexing and beamforming in a MIMO system, fully digital beamforming architecture is leveraged [7], [8]. Implementing a fully digital MIMO beamformer requires one radio frequency (RF) chain per antenna element. Due to the high number of power-hungry RF components, such as digital-to-analog converters (DACs), in conventional digital beamforming, the practical implementation of a massive MIMO system requires new approaches to be realized [2], [6]. As a solution, the combination of both digital and analog beamforming, using a limited number of RF chains, known as hybrid beamforming (HBF) has been considered.HBF splits the whole beamforming process into digital and analog parts, that enables multi-stream transmissions with a reduced complexity [6], [7]. Various HBF methods are proposed for fully and partially connected RF architectures in the literature [9]-[13], ranging from theoretical rate maximizing solutions to heuristic algorithms. In a fully connected HBF architecture, each RF chain is connected to all antennas [7], [10]. For a practical implementation of a mm-wave massive MIMO system, the fully connected structure may be too complex to be used due to very challenging and lossy RF signal division and combining processes. A more practical solution is to use partially connected RF architecture, in which each RF chain is connected only to a subset of the antenna array, called a sub-array [14], [15].Partially connected RF architecture itself can be divided into two categories, i.e., full-array-based and sub-array-based processing structures. In full-array-based processing, all data streams are conveyed to all sub-arrays. Thus, each stream is transmitted via its corresponding beam which is generated by using all sub-arrays and their antennas. This implies that full beamforming gain is potentially available. However, the directions of different beams are interdependent due to the partial connectivity. This sets restrictions to the beam generation and leads to suboptimal beam directions. In sub-array-based processing, each sub-array transmits only a single data stream. This leads to a more efficient and flexible beam design process. However, the beamforming gain is limited by the number of antennas per sub-array.Most of the HBF works in the literature use only phase shifting in the analog beamforming process [10]- [13]. However, employing analog amplitude control in addition to phase Abstract-Hybrid analog-digital beamforming has been recognized as a promising solution for a practical implementation of massive multiple-input multiple-output (MIMO) systems based on millimeter-wave technology. In this paper, three hybrid beamforming algorithms are proposed for single-user MIMO systems with partially connected radio frequency (RF) architecture, including a singular value decomposition (SVD) matching algorithm, an iterative orthogonalization algorithm, and a transmitreceive zero forcing (ZF...

Digital predistortion (DPD) of a phased array requires that multiple transmit paths must be measured by a feedback (FB) receiver (Rx). In this paper, we propose a FB concept for DPD in a time-division-duplex (TDD) phased arrays. We use a single FB line to collect the waveform samples from the parallel transmit paths to the FB Rx. The TDD switches are used to enable and disable individual transmit paths. The feedback is calibrated by comparing the FB outputs from individual PAs to over-the-air (OTA) measurement reference performed with a frequency modulated continuous wave (FMCW) signal. The individual PA measurements are post-equalized before the DPD training to model the far-field signal. Three alternative strategies are considered for training the DPD through the calibrated FB line and compared with the OTA DPD. The performance is verified by OTA measurements of a 28 GHz phased array transmitter and with fifth generation New Radio waveform in terms of total radiated (TR) adjacent channel power ratio (ACPR), cumulative absolute ACP (CACP), and main lobe error vector magnitude (EVM). The best EVM and ACPR performance is achieved by the strategy where the individual PA responses are treated independently. The methods were comparable to the OTA DPD performance, achieving all < 37 dB TRACPR, −29 dBm/MHz CACP, and ≤ 7 % EVM.