In this paper, we introduce a novel amplitude and phase reciprocity calibration protocol for distributed multi-antenna systems to enable coherent transmission from multiple access points (APs). The proposed scheme significantly enhances reciprocity calibration by resolving both phase and amplitude mismatches between APs, achieving near-optimal synchronization performance at practical signal-to-noise ratio (SNR) regimes. The protocol provides a closed-form solution for reciprocity calibration performed locally at each AP, maintaining low computational complexity while ensuring scalability and high-fidelity coherent transmission in large and dense distributed networks. The numerical results validate the robustness of the proposed technique, demonstrating accurate calibration and efficient transmission across various system configurations, including variations in the number of antennas and SNR values.

The frequency and spatial characteristics of nonlin-ear distortion from a MIMO-OFDM (multiple-input multiple-output orthogonal frequency-division multiplexing) basestation with memoryful nonlinear amplifiers (modeled by a generalized memory polynomial) is studied herein. We prove a theorem about a linear representation of the amplified signal from which the nonlinear distortion is obtained. Our measurement results demonstrate that the generalized memory polynomial can provide better accuracy of the nonlinear distortion than the memoryless and memory polynomial models. Furthermore, it is shown that in contrast to the memoryless amplifier, the amplifier with memory can distort the useful linear signal and introduce interference in a wider range of sub carriers. The derived theory is useful to predict how the nonlinear distortion will behave, to analyze the in-band and out-of-band radiation, and to schedule users in the frequency plane to minimize the effect of nonlinear distortion.

Although signal distortion-based peak-to-average power ratio (PAPR) reduction is a feasible candidate for orthogonal frequency division multiplexing (OFDM) to meet standard/regulatory requirements, the error vector magnitude (EVM) stemming from the PAPR reduction has a deleterious impact on the performance of high data-rate achieving multiple-input multiple-output (MIMO) systems. Moreover, these systems must constrain the adjacent channel leakage ratio (ACLR) to comply with regulatory requirements. Several recent works have investigated the mitigation of the EVM seen at the receivers by capitalizing on the excess spatial dimensions inherent in the large-scale MIMO that assume the availability of perfect channel state information (CSI) with spatially uncorrelated wireless channels. Unfortunately, practical systems operate with erroneous CSI and spatially correlated channels. Additionally, most standards support user-specific/CSI-aware beamformed and cell-specific/non-CSI-aware broadcasting channels. Hence, we formulate a robust EVM mitigation problem under channel uncertainty with nonconvex PAPR and ACLR constraints catering to beamforming/broadcasting. To solve this formidable problem, we develop an efficient scheme using our recently proposed three-operator alternating direction method of multipliers (TOP-ADMM) algorithm and benchmark it against two three-operator algorithms previously presented for machine learning purposes. Numerical results show the efficacy of the proposed algorithm under imperfect CSI and spatially correlated channels.

Spectral precoding is a promising technique to suppress out-of-band emissions and comply with leakage constraints over adjacent frequency channels and with mask requirements on the unwanted emissions. However, spectral precoding may distort the original data vector, which is formally expressed as the error vector magnitude (EVM) between the precoded and original data vectors. Notably, EVM has a deleterious impact on the performance of multiple-input multiple-output orthogonal frequency division multiplexing-based systems. In this paper we propose a novel spectral precoding approach which constrains the EVM while complying with the mask requirements. We first formulate and solve the EVM-unconstrained mask-compliant spectral precoding problem, which serves as a springboard to the design of two EVM-constrained spectral precoding schemes. The first scheme takes into account a wideband EVM-constraint which limits the average in-band distortion. The second scheme takes into account frequency-selective EVM-constraints, and consequently, limits the signal distortion at the subcarrier level. Numerical examples illustrate that both proposed schemes outperform previously developed schemes in terms of important performance indicators such as block error rate and system-wide throughput while complying with spectral mask and EVM constraints.

Although spectral precoding is a propitious technique to suppress out-of-band emissions, it has a detrimental impact on the system-wide throughput performance, notably, in high data-rate multiple-input multiple-output (MIMO) systems with orthogonal frequency division multiplexing (OFDM), because of (spatially-coloured) transmit error vector magnitude (TxEVM) emanating from spectral precoding. The first contribution of this paper is to propose two mask-compliant spectral precoding schemes, which mitigate the resulting TxEVM seen at the receiver by capitalizing on the immanent degrees-of-freedom in (massive) MIMO systems and consequently improve the system-wide throughput. Our second contribution is an introduction to a new and simple three-operator consensus alternating direction method of multipliers (ADMM) algorithm, referred to as TOP-ADMM, which decomposes a large-scale problem into easy-to-solve subproblems. We employ the proposed TOP-ADMM-based algorithm to solve the spectral precoding problems, which offer computational efficiency. Our third contribution presents substantial numerical results by using an NR release 15 compliant simulator. In case of perfect channel knowledge at the transmitter, the proposed methods render similar block error rate and throughput performance as without spectral precoding yet meeting out-of-band emission (OOBE) requirements at the transmitter. Further, no loss on the OOBE performance with a graceful degradation on the throughput is observed under channel uncertainty.

Several new features are added for the High Speed Packet Access (HSPA) evolution in order to meet the requirements set by the International Mobile Telecommunications Advanced (IMT-A). The main objective of these new features is to increase the average spectral efficiency. One possible technique for improving downlink spectral efficiency would be to introduce support for four branch multiple input multiple output (MIMO), i.e. utilize up to four transmit and receive antennas to enhance the spatial multiplexing gains and to offer improved beamforming capabilities. Four branch MIMO provides up to 84 Mbps per 5 MHz carrier for high signal to noise ratio (SNR) users and improves the coverage for low SNR users. This paper discusses a few design considerations related to physical layer aspects of four branch MIMO. In the first part of the paper we outline a few design options and show that four branch MIMO can be introduced with modest impact on the existing standards and networks. In the second part of the paper we show by means of system level simulation that significant gains in average user throughput and cell edge user throughput can be achieved with four branch MIMO. Based on these evaluations we conclude that four branch MIMO is an important component in the continued evolution of the HSPA standard.

In this paper, the impact of transmit antenna separations at the base station on the downlink in a cellular OFDM system with per antenna rate control (PARC) is investigated. For the minimum mean square error (MMSE) with successive interference cancellation (SIC) receiver, the highly correlated interference resulting from closely spaced antennas is more easily suppressed than the independent interference resulting from widely separated antenna installations assuming the same number of interfering source(s). In the region where the interference from co-located source is dominating, the larger the transmit antenna separation, the better performances by the MMSE-SIC receiver. But in the region where interference from other cells is dominating, no gain is found when enlarging the transmit antenna separation, since it will only increase the number of uncorrelated interference sources. This means that for cell edge users it is beneficial with base stations with closely separated antenna installation in order to maximize the cancellation of inter-cell interference since the gain by multi-stream transmission for these users are small anyway.

In this paper we present the main concepts of high speed packet access evolution currently being standardized in 3GPP. In general HSPA evolution consists of introduction of MIMO, higher order modulation, and protocol optimizations and optimizations for voice over IP. We describe these improvements in detail and show that HSPA Evolution can reach performance comparable to those of long term evolution of UMTS terrestrial radio access network in a 5 MHz deployment.

This paper reports on a set of channel models that have been proposed for the evaluation of system concepts for the long-term evolution of third generation mobile communications systems. Conflicting requirements between simplicity of implementation and accuracy in reproducing the radio channel are presented, and the resulting modeling choices are discussed. The channel models have been derived by combining a spatial channel model with several different antenna arrangements that are envisioned to be typical for the systems in question. These antenna arrangements include both spatial separation of elements and polarization. The channel models are supplied as tapped delay lines with correlation parameters for simulating full MIMO channels of size up to 4times4

Antenna arrays at the base stations allow capacity enhancement in cellular WCDMA networks. In this contribution we compare the relative downlink capacity between a system employing standard three sector site configurations with a system employing four fixed beam antenna systems. Since different packet services are expected to contribute largely to the traffic demand within a few years after initial deployment, this comparison is made in a mixed traffic environment. By realistic computer simulations it is found that a substantial gain can be expected by the introduction of adaptive (smart) antenna base stations. In this contribution a fixed multi beam system is considered, which is a fairly low complex adaptive antenna implementation. The simulations show that generally, a threefold capacity increase can be gained by employing a 4 beam antenna system. For some scenarios even higher gains can be achieved.

The capacity of W-CDMA systems in which base stations are equipped with adaptive antennas (AA) is investigated in mixed service scenarios. The simulations show that generally, a threefold downlink (DL) capacity increase can be gained by employing a fixed or steered beam antenna system. For some scenarios, DL capacity gains of 4 to 6 times can be achieved. Further, a DL capacity comparison between the steered beam concept and the fixed (multi) beam AA concept is presented.

This contribution presents some link and system level simulation results obtained by a WCDMA base station equipped with an adaptive antenna system. The results show that the performance gain obtained by an advanced antenna system could be substantial compared to an ordinary sector covering system. The link simulations show that for a given SIR target in a single cell system, the capacity is increased considerably compared to a conventional single sector covering antenna system. It is also shown that the difference between the different adaptive antenna algorithms is marginal. The system level simulations have shown that the capacity performance is basically proportional to the number of orthogonal beams used in the base station. In cases where the conventional three sector reference system is not truly interference limited, the performance gain can be even higher. The system simulations have also shown that using more beam positions than those defining the set of orthogonal fixed beams slightly improves the performance. However the number of beam positions can be kept relatively small, in the order of twice the number of orthogonal beams, without significant loss.

In a previous paper we presented a novel method for spatial and temporal frequency estimation assuming that the sources are uncorrelated. The current paper analyzes this method in the case of spatial frequency estimation. In particular an optimal weighting matrix is derived and it is shown that the asymptotic variance of the frequency estimates coincides with the relevant Cramer-Rao lower bound. This means that the estimator is in large samples an efficient subspace-based spatial frequency estimator. The proposed method thus utilizes the a priori knowledge about the signal correlation as opposed to previously known subspace estimators. Moreover, when a uniform linear array is employed, it is possible to implement the estimator in a non-iterative fashion.