In this paper, we present a novel method that detects appearing targets and improves their detection probability in a known static environment. Ray tracing-based channel model is used to calculate the radio signal paths using a 3D model of the environment. This preliminary information and the measured radar image are delivered to the convolutional neural network (CNN) based autoencoder (AE). The output image provides an improved representation of appearing targets, as the redundancy coming from known static environment is modeled and cancelled from the radar image. The method does not need the traditional constant false alarm rate (CFAR) threshold setting, which is an advantage, especially in heterogenous environments. The functionality of the method is tested with radar measurements in a laboratory corridor. Our results show that environmental clutter decreases significantly in the radar image and new targets are more clearly distinguished.

A reconfigurable intelligent surface (RIS) can be used to control the propagation of electromagnetic waves (EM). By deploying the RIS units in radio environment allows to steer the transmitted EM waves to areas that are otherwise shadowed by buildings or other geographic formations resulting in coverage enhancement. The gain offered by the RIS is due to its ability to focus the impinging signal to the desired direction. Hence, errors in the beamforming degrade the performance of a RIS-assisted link. The effect of the errors in the beamforming at the RIS are studied in this paper. Simulations show that the errors especially in the zenith angle can have significant effect on the uplink performance. With a RIS based on a 16 × 16 element reflect array errors greater than 1° start to limit the achievable throughput. After the RIS is configured based on the direction information, the user equipment must use a proper precoder to capitalize the benefit of the path via the RIS. The usability of existing precoders from the 5G NR are also studied in this paper. In the simulated use case, utilizing a RIS with 5G NR precoders resulted in 3.6 dB gain in the uplink direction.

Configuration of a reconfigurable intelligent surface (RIS) as a solution to an optimization problem for spectral or energy efficiency requires the estimation of the channels between the transmitter and the RIS and between the RIS and the receiver. The channel estimation and the RIS configuration by optimization are computationally intensive processes in RIS assisted wireless links. The approach proposed in this paper simplifies and speeds up the RIS configuration process by relaxing the demand to find the optimal solution but instead aims to reach a predetermined channel quality measured with the signal-to-noise ratio. It is also shown that when a single dominant path exists between a mobile user and the RIS and also between the RIS and a base station, the RIS configuration calculated as a solution to a spectral efficiency maximization problem and configuration based on conventional antenna array beam steering give the same result.

5G system design is a complex process due to a great variety of applications and their diverse requirements. This article describes our experiences in developing a centimeter waves mobile broadband concept satisfying future capacity requirements. The first step in the process was the radio channel measurement campaign and statistical modeling. Then the link level design was performed tightly together with the radio frequency (RF) implementation requirements to allow as large scalability of the air interface as possible. We started the concept development at 10 GHz frequency band and during the project World Radiocommunication Conference 2015 selected somewhat higher frequencies as new candidates for 5G. Thus, the main learning was to gain insight of interdependencies of different phenomena and find feasible combinations of techniques and parameter combinations that might actually work in practice, not only in theory.

We consider a multi-operator small cell network where mobile network operators are sharing a common pool of radio resources. The goal is to ensure long term fairness of spectrum sharing without coordination among small cell base stations. It is assumed that spectral allocation of the small cells is orthogonal to the macro network layer, and thus, only the small cell traffic is modeled. We develop a decentralized control mechanism for base stations using the Gibbs sampling based learning technique, which allocates a suitable amount of spectrum for each base station. Five algorithms are compared addressing co-primary multi-operator resource sharing under heterogeneous traffic requirements and the performance is assessed through extensive system-level simulations. The main performance metrics are user throughput and fairness between operators. The numerical results demonstrate that the proposed Gibbs sampling based learning algorithm provides about tenfold cell edge throughput gains compared to state-of-the-art algorithms, while ensuring fairness between operators.

With increasing popularity of high speed trains and traffic forecast for future cellular networks, the need to provide improved data rates using higher frequency bands (HFBs) for train passengers is becoming crucial. In this paper, we modify the OFDM frame structure for HST, taking into account the increasing sensitivity to speed at HFBs. A lower bound on the SNR/SINR for a given rate for reliable communication was derived considering the physical layer parameters from the OFDM frame. We also analyze different pathloss models in the context of examining the required gain needed to achieve the same performance as with microwave bands. Finally, we present a time-based analogue beamforming selection approach for HST. We observed that, irrespective of the pathloss models used, the required gains are within the same range. For the same SNR/SINR at different frequency bands, the achievable data rate varies with respect to the frequency bands. Our results show the potential of the use of HFBs. However, due to the increased sensitivity of some channel parameters, a maximum frequency band of 38 GHz is suggested. Evaluation of our proposed beamforming scheme indicates a close performance to the optimal SVD scheme with a marginal rate gap of less than 2 b/s/Hz.

To tackle the challenge of providing higher data rates within limited spectral resources we consider the case of multiple operators sharing a common pool of radio resources. Four algorithms are proposed to address co-primary multi-operator radio resource sharing under heterogeneous traffic in both centralized and distributed scenarios. The performance of these algorithms is assessed through extensive system-level simulations for two indoor small cell layouts. It is assumed that the spectral allocations of the small cells are orthogonal to the macro network layer and thus, only the small cell traffic is modeled. The main performance metrics are user throughput and the relative amount of shared spectral resources. The numerical results demonstrate the importance of coordination among co-primary operators for an optimal resource sharing. Also, maximizing the spectrum sharing percentage generally improves the achievable throughput gains over non-sharing.

We study the joint subcarrier allocation and pre-coder design (JSAPD) problem to maximize the sum rate of downlink orthogonal frequency division multiple access (OFDMA) systems under a sum power constraint. Naturally, this problem belongs to a class of combinatorial optimization problems which are difficult to solve in general. Based on the concept of big-M formulation, and by exploiting its specific structure, we can transform the JSAPD problem into a mixed integer second order cone program (MI-SOCP), which then offers two advantages. Firstly, when the number of subcarriers/users is small, the design problem can be solved to global optimum in reasonable time by dedicated solvers. Secondly, when the number of subcarriers/users is large, near-optimal solutions of the JSAPD problem can be found by considering the continuous convex relaxation of the MI-SOCP. Numerical experiments are carried out to demonstrate the improved performance of the proposed designs compared to known solutions.

We investigate the spectral efficiency of full-duplex small cell wireless systems, in which a full-duplex capable base station (BS) is designed to send/receive data to/from multiple half-duplex users on the same system resources. The major hurdle for designing such systems is due to the self-interference at the BS and co-channel interference among users. Hence, we consider a joint beamformer design to maximize the spectral efficiency subject to certain power constraints. The design problem is first formulated as a rank-constrained optimization problem, and the rank relaxation method is then applied. However, the relaxed problem is still nonconvex, and thus, optimal solutions are hard to find. Herein, we propose two provably convergent algorithms to obtain suboptimal solutions. Based on the concept of the Frank-Wolfe algorithm, we approximate the design problem by a determinant maximization program in each iteration of the first algorithm. The second method is built upon the sequential parametric convex approximation method, which allows us to transform the relaxed problem into a semidefinite program in each iteration. Extensive numerical experiments under small cell setups illustrate that the full-duplex system with the proposed algorithms can achieve a large gain over the half-duplex system.

Self organizing network techniques are needed to manage serious two-tier interference in an ad hoc operation of dense femtocell deployment. This paper considers downlink frequency domain inter-cell interference coordination (ICIC) in femtocell deployment. The studied resource partitioning method works such that each active femtocell may adapt, determine, and advertise a so-called binary resource partition sequence indicating which physical resource blocks (PRBs) the femtocells may occupy for their current operation. The main idea is to avoid interference from neighbor femto access points (FAPs) and try to achieve guaranteed bit-rate to femto users. The considered coordination method requires only low rate, infrequent updates in stationary phase and enables ICIC in distributed self organizing manner. The resource partitioning strategies are simulated in an LTE compliant system level simulator. The system level results show that different resource partitioning methods provide substantial gain when MRC receiver is used. When LMMSE receiver is used gains are smaller but at least one of the resource partitioning methods provided a gain over the case without ICIC. Furthermore, it is shown that proposed method works in two different femto environments.

In this paper we present system level simulation results for a cooperative moving relay node (MRN) system deployed on a high speed train (HST). Recently interest has grown in using MRNs to provide enhanced cellular coverage to users in public transport, particularly HSTs, of which the modern construction materials and techniques cause high vehicle penetration loss (VPL) when signals propagate into the train. MRNs utilising antenna arrays on the exterior and interior of the train are a promising solution to overcoming this VPL in order to provide onboard users with improved service. We show that a cooperative system of 8 MRNs onboard a HST is able to provide significant improvements to achievable throughput of onboard users when compared to direct transmission, as well as indirectly improving the throughput of other users located in cells the HST is passing through.

This paper addresses energy efficiency and throughput performance of joint LTE-A macrocell and femtocell deployment. We consider two techniques discontinuous transmission (DTX) and offloading of users from macrocells to lower power femtocells. When the DTX is applied base stations delay transmissions until sufficient user data is available to transmit using 40 percent or more of the system bandwidth. This allows the base stations to remain inactive for larger periods of time, and therefore reducing energy consumption. The target is to reduce the overall network energy consumption per area with minimal impact to the throughput. The numerical results show that when the majority of macrocell users can be offloaded to be served by femtocells the mean power per area consumption will decrease by up to 34.7 percent and at the same time a larger portion of users are able to meet the intended throughput target. When DTX is applied energy consumption is reduced further still to 45.3 percent.

We introduce a full duplex multiuser multiple-input multiple-output (FD MU-MIMO) system, and consider the total throughput maximization problem under a sum power constraint in the downlink (DL) channel and per-user power constraints in the uplink (UL) channel. Due to the nature of asymmetric DL/UL capacity, a trivial method to this problem is to optimize the DL and UL channels sequentially. However, when the self-interference (SI) is large, the sum rate of the UL channel in this sequential design is dramatically degraded. Herein, a joint design is proposed, in which the DL and UL channels are optimized simultaneously. Since the objective function of the throughput maximization problem is non-convex, it is difficult to find the optimal solution. Thus, we propose a joint iterative algorithm to find a suboptimal design, using a local optimization strategy. Simulation results demonstrate that the iterative joint design outperforms the sequential design, and the FD MU-MIMO system is superior to the conventional half duplex (HD) system in terms of the total system throughput when the SI is sufficiently small. This makes the FD MU-MIMO techniques promising for small cell deployments where the transmit power is relatively small.

Spatial cellular topologies and link distance geometries are studied for generic direct sequence UWB system multi-access performance. Compared cellular configurations are based on circular, square, and hexagonal cells. Link distance statistics are derived and/or simulated for single and multiple cell configurations with centralised and distributed topologies. Conditional outage probability in correlated lognormal multipath fading is used as a performance measure in numerical illustrations. The numerical results indicate that the link distance statistics in circular, square, and hexagonal cell geometries closely resemble each other. In the presence of correlated multipaths the simulated outage probability performance deviates considerably from the results generated by Fenton-Wilkinson and Schwartz-Yeh lognormal sum approximations

In this paper we analyze the impact of system imperfections on the overall cellular code-division multiple access (CDMA) radio network performance. The theory is general and some examples of practical sets of channel and system parameters are used as illustration. A flexible signal model, based on the complex signal format, is used enabling us to model all wideband CDMA standards. For such a signal, we first derive a complex decorrelator structure. In the next step imperfections in the operation of a decorrelating space-time Rake combiner are modeled and analyzed. Relative capacity losses and the network sensitivity function are used as performance measures. Simulations are also performed to confirm some of the key assumptions made in the analysis. Numerical results show that the user capacity varies significantly depending on the multipath profile, diversity order, fading rate, and code crosscorrelations. It is shown that up to 97 % of the system capacity can be lost due to the system imperfections. More, advanced and robust parameter estimators and/or multiuser detectors are needed to alleviate these degradations at the cost of increased complexity

Ultra wideband (UWB) spread spectrum system capacity is studied. User capacity is evaluated analytically against the conditional outage criterion. The probability of outage varies depending on the multipath channel intensity profile, severity of shadowing, signal-to-interference ratio target, number of users, timing errors, and the number of fingers combined in the RAKE receiver. The sum of lognormally distributed signals is mainly modelled by a Fenton-Wilkinson approximation. Numerical results show that the outage performance is quite sensitive to the variation of several key parameters.

We analyze capacity losses in advanced wideband code division multiple access (WCDMA) networks due to imperfections in the estimation of the system and channel parameters. Key components in the analysis are multiple access intracell interference (MAI), intercell interference, efficiency of the interference cancellation, multipath intensity profile, efficiency of the RAKE combiner, inefficiency of signal and channel parameter estimation and additive white Gaussian noise. Receiver structures are based on a conventional matched filter RAKE combiner that can be supported by either a linear or nonlinear interference canceler. Both the equal gain and maximal ratio combining techniques are considered. Results show that significant capacity gains can be achieved by interference cancellation in comparison to the conventional techniques. However, the capacity gain may be completely lost in fast fading channels due to the estimation imperfections.

We analyze capacity losses in code division multiple access (CDMA) networks due to imperfections in the operation of the system components. The main result of this work is a systematic mathematical framework for capacity evaluation of CDMA-based networks in fading channels. This should be considered as an alternative tool to extensive simulations being used for these purposes at the moment. The intention is to provide simple approximate relations that simultaneously take into account: multiple access intracell interference (MAI), intercell interference, and near-far effect, including different sources of power control imperfections. In addition to this, the efficiencies of the major receiver components are also included in the analysis. A flexible complex signal format is used which enables us to model at currently interesting proposals for wideband CDMA (W-CDMA) standards. The theory is general, and some examples of practical set of channel and system parameters are used as illustration. Further elaboration of these results, including extensive numerical analysis based on the offered analytical framework, would provide enough background for the understanding of W-CDMA system performance in a realistic environment.