Risto Wichman

• PhD
• Professor at Aalto University

This letter proposes a three-dimensional (3D) satellite-terrestrial communication network assisted with reconfigurable intelligent surfaces (RISs). Using stochastic geometry models, we present an original framework to derive tractable yet accurate closed-form expressions for coverage probability and ergodic capacity in the presence of fading. A hom...

In this article, we review the key concepts and the progress in the design of physical-layer aspects of in-band full-duplex (IBFD) communications. One of the fundamental challenges in realizing IBFD is self-interference that can be up to 100 dB stronger than signals of interest. Thus, we start by reviewing state-of-the-art research in self-interfer...

Wireless Inertial Measurement Units provide motion capture data with a low hardware cost while offering a lot of mobility for the user. The current solutions rely on Wi-Fi or custom radio protocols, which are usually access point-centre having all the traffic routed through a single access point, limiting direct interaction capabilities with the su...

In several practical networks the environment is changing fast; however, acquiring channel state information (CSI) continuously is often infeasible. In such cases, it is often feasible to infer statistical CSI at the transmitters (CSIT). In this paper, we study the mode selection problem for a cooperative network, consisting of a source, a buffer-a...

Full-duplex relaying is an enabling technique of sixth generation (6G) mobile networks, promising tremendous rate and spectral efficiency gains. In order to improve the performance of full-duplex communications, power control is a viable way of avoiding excessive loop interference at the relay. Unfortunately, power control requires channel state in...

In this paper, we are interested in the joint estimate of user equipment (UE) location and orientation for millimeter-wave multi-input-multi-output (mmWave MIMO) systems. In practice, mmWave signals suffer from small-scale fading, which degrades UE localization. Moreover, mmWave MIMO-based UE localization is a non-convex optimization problem, and t...

Millimeter-wave (mmWave) massive multiple input multiple input (MIMO) has shown great potential in user equipment (UE) localization of 5G wireless communication systems. However, mmWave signals usually suffer from non-line-of-sight (NLOS) propagation, which will affect mmWave MIMO-based UE localization performance. Hence, it is non-trivial to revea...

Mobile data traffic grew by more than 50 percent between the first quarters of 2019 and 2020 and is projected to grow by close to a factor of 5 to reach 164 exabytes per month by 2025. This insatiable demand for mobile broadband communication services, including video, social networking, web browsing, audio, software updates, and peer-to-peer file...

Full-duplex relaying is an enabling technique of sixth generation (6G) mobile networks, promising tremendous rate and spectral efficiency gains. In order to improve the performance of full-duplex communications, power control is a viable way of avoiding excessive loop interference at the relay. Unfortunately, power control requires channel state in...

In recent years, wireless node density has increased rapidly, as more base stations, users, and machines coexist. Exploiting this node density, cooperative relaying has been deployed to improve connectivity throughout the network. Such a configuration , however, often demands relay scheduling, which comes with increased channel estimation and signa...

This paper proposes the integration of non-orthogonal multiple access (NOMA) and broadcasting with buffer-aided (BA) full-duplex (FD) relaying to improve the spectral efficiency of a single-source two-user network. Furthermore , opportunistic relay selection is employed, activating one or two FD relays to support the end-to-end communication. The n...

This work focuses on the interplay between rotation angle, transmit power, fading severity, and noise impairment severity in signal space diversity-based three time-slot decode-and-forward two-way relaying networks. Specifically, we develop a joint design for rotation angle selection and transmit power allocation, while taking into account the perf...

Networks with device-to-device (D2D) technology allow for two possible communication modes: traditional communication via the base station, and direct communication between the users. Recent studies show that in-band full-duplex (IBFD) operations can be advantageously combined with D2D communication to improve the spectral efficiency. However, no a...

Buffer-aided (BA) relaying has been proposed as an efficient way to improve the diversity of multi-hop networks. Nonetheless, it results in increased end-to-end delay, as packets tend to reside in the relays’ buffers for many time-slots. In order to enable the adoption of BA relays in delay-intolerant applications, the combination with successive r...

We analyze the performance of amplify-and-forward (AF) automatic repeat request (ARQ) for a two-hop cooperative system with reliability constrains. For this setup, we first derive the closed-form outage probability expression. Next, we present a power allocation scheme that allows us to achieve a target outage probability, while minimizing the outa...

Massive antenna arrays are capable of cancelling out the loop interference (LI) at the relay station in multipair full-duplex (FD) relay networks even without LI channel knowledge if the number of antennas is allowed to grow without a bound. For large but finite number of antennas, however, channel estimation based LI cancellation is required. In t...

Non-orthogonal multiple access (NOMA) aims to increase the spectral efficiency of fifth generation (5G) networks by relaxing the orthogonal use of radio-resources. In this work, a network with multiple half-duplex (HD) buffer-aided (BA) relays is considered, where the source transmits with fixed rate towards two users. The users might demand the sa...

In networks with multiple wireless nodes and especially in fast changing environments, the continuous acquisition of Channel State Information (CSI) may become infeasible. In this paper, we study the relay selection problem for a cooperative network, where Full-Duplex (FD) relays are equipped with buffers and only statistical CSI is available at th...

Learn about the key technologies and understand the state of the art in research for full-duplex communication networks and systems with this comprehensive and interdisciplinary guide. Incorporating physical, MAC, network, and application layer perspectives, it explains the fundamental theories on which full-duplex communications are built, and lay...

Machine-to-machine (M2M) communication represents a new paradigm for mobile cellular networks, where a massive number of low-cost devices request the transfer of small amounts of data without human intervention. One option to tackle this problem is obtained by combining random beamforming (RBF) with opportunistic scheduling. RBF can be used to indu...

This paper considers the transmit power balancing trade-off between the pilot and data signals in a two-hop full-duplex decode-and-forward relay link when the feedforward data channels are Rayleigh-fading and the residual feedback self-interference channel follows Rician-fading. The study concentrates on time-multiplexed orthogonal pilot sequences...

In this paper, we consider the problem of channel and phase noise estimation for an orthogonal frequency division multiplexing (OFDM) radio link. We solve this problem by first investigating the subspace in which the phase noise spectral vector lies and then exploiting this information during estimation. Building upon earlier works, the phase noise...

Spatially distributed transmissions in coordinated multipoint (CoMP) systems can lead to mean channel power imbalance (CPI) at the receiver. Similar imbalance also occurs in distributed antenna and co-located multi-antenna systems due to inaccurate antenna calibration. This paper studies performance impact of power imbalance on some practical CoMP...

This work focuses on implementation of dynamic spectrum access in a scenario where existing wireless infrastructure is shared by a secondary cellular system. We study the optimization of the physical antenna configuration of the secondary system base stations, assuming that these antennas are deployed at the transmitter sites of a primary broadcast...

In this letter we study a precoding matrix (PM) interpolation in the context of user-specific channel estimation. We interpolate the fed-back PMs at the transmitter by the means of geodesic to enable continuous channel estimation across continuously scheduled frequency clusters in the receiver. The proposed scheme is compared to the case where a si...

This paper studies the impact of two implementa-tion imperfections, namely, transceiver frequency-response non-reciprocity and antenna mutual coupling mismatches at the base-station and user equipment on the channel reciprocity assumption in TDD systems. Comprehensive signal models are first developed to analyze the joint effects of these two imper...

The articles in this special issue focus on the technology and applications supported by in-band full duplex wireless services.

Hardware impairments in radio-frequency components of a wireless system cause unavoidable distortions to transmission that are not captured by the conventional linear channel model. In this paper, a 'binoisy' single-user multiple-input multiple-output (SU-MIMO) relation is considered where the additional distortions are modeled via an additive nois...

This paper addresses the modeling and digital cancellation of self-interference in in-band full-duplex (FD) transceivers with multiple transmit and receive antennas. The self-interference modeling and the proposed digital canceller structure takes into account, by design, the effects of I/Q modulator imbalances and power amplifier (PA) nonlineariti...

Cognitive radio systems aim to take advantage of the spatiotemporal empty spectrum without causing harmful interference towards the primary network by utilizing knowledge of the prevailing radio environment. The radio environment is typically modeled with propagation models or by interpolating spatially distributed field measurement data. This pape...

We develop a method for self-interference mitigation in wideband full-duplex multiple-input multiple-output regenerative relays, taking into account transmitter impairments and limited receiver dynamic range. The method combines a cancellation and suppression architecture by incorporating feedforward filters at both sides of the relay. The design c...

User performance on a wireless network depends on whether a neighboring cochannel interferer applies a single (spatial) stream or a multi stream transmission. This work analyzes the impact of interference rank on a beamforming and orthogonal space-time block coded (OSTBC) user transmission. We generalize existing analytical results on signal-to-int...

This paper studies the spectrum sharing of femtocell base station (FBS) and macrocell base station (MBS) in a heterogeneous network setting. We allow femtocell users (FUEs) to re-use the resource blocks of certain macrocell users (MUEs) who are categorized as outdoor users. We study the design tradeoff between MUE spectral diversity and the need to...

Despite the intensive recent research on wireless single-channel full-duplex communications, relatively little is known about the transceiver chain nonidealities of full-duplex devices. In this paper, the effect of nonlinear distortion occurring in the transmitter power amplifier (PA) and the receiver chain is analyzed, alongside with the dynamic r...

In an in-band full-duplex system, radios transmit and receive simultaneously in the same frequency band at the same time, providing a radical improvement in spectral efficiency over a half-duplex system. However, in order to design such a system, it is necessary to mitigate the self-interference due to simultaneous transmission and reception, which...

In-band full-duplex (IBFD) operation has emerged as an attractive solution for increasing the throughput of wireless communication systems and networks. With IBFD, a wireless terminal is allowed to transmit and receive simultaneously in the same frequency band. This tutorial paper reviews the main concepts about in-band full-duplex wireless .A key...

This letter analyzes the impact of imperfect timing synchronization of cellular base stations on feasibility of almost blank subframes, an interference management concept for heterogeneous networks. We claim that the most significant impact of timing synchronization mismatch lays in potential disturbance of a control channel that is located in the...

Simultaneous reception and transmission in the same frequency, so-called full-duplex operation, causes an infinite feedback loop in an amplify-and-forward relay. The unwanted echoes may result in oscillation at the relay, making it unstable, and distorting the spectrum. This paper presents an adaptive MIMO filtering method for full-duplex amplify-a...

In a cognitive radio system, awareness of the prevailing radio environment of the primary network, such as digital terrestrial television, is important since it governs the emission limits for the secondary system devices. Field strength estimates of the primary network are typically computed using terrain based radio propagation models, or by usin...

We propose a scheme for interference management in wireless heterogeneous networks. We, specifically, consider the problem of downlink interference of a Home eNB (HeNB) to nearby macrocell user equipments (MUEs) served by a macrocell base station. We propose a distributed, self-learning channel access mechanism that enables the HeNB to adaptively c...

High capacity requirements in wireless systems can be met, at the network level, by using dense small cell deployments and, at the link level, by improving spectral efficiency via spectrum reuse. In this context, we consider a small-area radio system, e.g. a pico-or femtocell, where a full-duplex access point serves simultaneously two half-duplex d...

We develop and analyze a time-domain bi-level power control scheme using Almost Blank Subframe (ABS) for downlink cross-tier interference mitigation. Even for only one femto cell and macro cell, the optimization problem given rate constraint is nonconvex. We prove that by relaxing the integer requirement for subframe division, the optimal strategy...

Full-duplex relays suffer from self-interference due to simultaneous reception and transmission in the same frequency. This relay-induced interference signal can be of significant power and considerably distort the information-bearing signal, leading to unacceptable performance degradation. In this paper we present a method to solve the problem of...

In heterogeneous cellular scenarios with macrocells, femtocells or picocells users may suffer from significant co-channel cross-tier interference. To manage this interference 3GPP introduced almost blank subframe (ABSF), a subframe in which the interferer tier is not allowed to transmit data. Vulnerable users thus get a chance to be scheduled in AB...

A new approach to relay and user selection is proposed using threshold-based transmission to reduce outage probability and save transmit power. By exploiting properties of dual-hop transmission using amplify-and-forward (AF) relaying, new threshold-based relay and user (TRU) selection protocols that employ one-bit channel state information (CSI) fe...

White spaces are locally or temporally available frequencies that are principally reserved for primary use such as TV broadcasting. The currently prevalent view to the utilization of such spectrum resources is that white space devices must query a geolocation database to obtain information about available frequencies and related maximum transmissio...

We analyze the performance of bidirectional communication links between two multi-antenna full-duplex transceivers, explicitly taking into account the effect of unavoidable self-interference. The main theme of this paper is to compare spatial-domain suppression (implemented in the form of null-space projection) and subtractive time-domain cancellat...

Wireless communication systems typically employ a repetitive preamble in each slot which is used for parameter acquisition. The repetitive preamble is useful for estimating the carrier frequency offset (CFO), usually based on the autocorrelation of the received signal. In this paper, we derive a family of novel data-aided CFO estimators. The propos...

We study in this work CFO compensation methods for two multicarrier multiple access techniques in a high mobility scenario. In particular, we consider orthogonal frequency division multiple access (OFDMA) and filter bank multicarrier multiple access (FBMC-MA). The main motivation for this study is not only the different sensitivity these multicarri...

We propose iterative least-squares based compensation algorithms for the in-band phase noise distortion of an orthogonal frequency division multiplexing (OFDM) signal. In particular, we impose constraints on the least-squares estimates of the spectral components of the phase noise process which are subsequently used for phase noise compensation. Th...

We analyze the performance of full-duplex MIMO-OFDM transceivers with subtractive self-interference cancellation in analog and/or digital domain, i.e., before and/or after analog-to-digital converters (ADCs). In particular, the non-ideal ADCs are modeled by assuming uniform b-bit quantization which allows us to derive closed-form expressions for th...

The fifth chapter, deals with the Impact and Digital Suppression of Oscillator Phase Noise in Radio Communications, and is authored by Mikko Valkama, and Ville Syrjälä, from Tampere University of Technology, Finland, and Risto Wichman, and Pramod Mathecken from Aalto University, Finland. The design of compact and low-cost radios with high performan...

We derive the average capacity for an OFDM radio link that is impaired by phase noise and carrier frequency offset at both transmitter and receiver side. It is well known that, for OFDM, phase noise and frequency offset cause intercarrier interference (ICI). We first notice the dependency of the instantaneous capacity on the ICI power and on the fa...

We propose a discrete-time model for phase noise processes occurring in phase locked loop (PLL) based oscillators. By using linear time-invariant analysis in the phase domain, we arrive at a parallel auto-regressive moving-average phase noise model that captures the major noise sources and corresponding loop transfer functions of the PLL circuitry....

In this paper , we propose a resource allocation scheme for interference mitigation in heterogeneous networks to simultaneously serve both Macrocell User Equipments (MUEs) and Home User Equipments (HUEs). In the proposed Shadow Chasing scheme, a Home eNB (HeNB) uses Downlink Control Information (DCI) together with ACK/NAK feedback to assign its HUE...

We propose novel direct and indirect learning predistorters (PDs) that employ a new baseband simplicial canonical piecewise linear (SCPWL) function. The performance of the proposed PDs is easily controlled by varying the number of segments of the SCPWL function. When comparing to polynomial-based PDs, our SCPWL-based PDs are more robust for modelin...

We consider a two-hop downlink hybrid orthogonal frequency-division multiple-access [OFDM(A)] communication system in which a fixed infrastructure-based relay node is deployed to serve multiple user equipment beyond the coverage area of a base station. The key factor defining the performance and complexity of such systems is the relaying protocol,...

A currently prevalent view of the operation of white space cognitive radios is that they should use geolocation databases to determine their frequency band and maximum transmission power in any given location so that interference radiated to broadcast receivers is negligible. To estimate the level of such interference, physical reference geometries...

According to the European Electronic Communications Committee a practical approach for constructing a cognitive radio network operating in TV white spaces is based on geolocation databases. The practical communication rates achievable for the secondary users and the interference caused to the primary system are fundamentally limited by the informat...

This work considers in-band interference that appears in two-tier macro/femto cellular networks when macrocells and femtocells operate on the same frequency band. We focus on the time division duplex (TDD) mode where uplink-downlink interference occurs if time sharing between uplink and downlink are different in the overlaying macrocell and underly...

Full-duplex relaying is more spectrally efficient than half-duplex relaying as only one channel use is needed per two hops. However, it is crucial to minimize relay self-interference to render full duplex feasible. For this purpose, we analyze a broad range of multiple-input multiple-output (MIMO) mitiga- tion schemes: natural isolation, time-domai...

This chapter analyzes the potential fro TV White Spaces adoption in Europe. We describe the TV digitalization background to understand the historical basis. We carry out two analyses: a capacity analysis based on signal propagation and an innovation diffusion estimate. By applying the country-specific capacity and parameter values in the technology...

We develop transmit power control schemes for a regenerative full-duplex relay link. The focus is on MIMO scenarios with multiple antennas at all communicating nodes. As a new aspect for the optimization problem, we note that practical full-duplex relays are subject to unavoidable self-interference propagating from the transmit antenna array to the...

Phase noise compensation for OFDM based wireless systems is a necessary task to be performed in order to achieve good performance using cheap components in the transceiver electronics. In this paper, we propose a decision-feedback-frequency-domain compensation method that utilizes Bayesian a priori information of the phase noise process. The a prio...

Precoding codebook design for limited feedback MIMO systems is known to reduce to a discretization problem on a Grassmann manifold. The case of two-antenna beamforming is special in that it is equivalent to quantizing the real sphere. The isometry between the Grassmannian ${\rm G}^{{\BBC}}_{2,1}$ and the real sphere $S^2$ shows that discretization...

Focusing on two-antenna infrastructure relays em- ployed for coverage extension, we develop hybrid techniques that switch opportunistically between full-duplex and half-duplex relaying modes. To rationalize the system design, the classic three-node full-duplex relay link is first amended by explicitly modeling residual relay self-interference, i.e....

Modern cellular communication systems are based on frequency reuse 1. Although reuse 1 promises high peak data rates and significantly simplified network planning, the cell edge or cell overlap areas suffer from excessive inter-cell interference. In this work we address interference management for downlink part of these systems. Part of the radio r...

Mobile communication systems have evolved over the past decades and each new generation brought new experience to the users enabled by technology innovations, while keeping some well established principles from previous generations. This trend continued up to LTE (Long Term Evolution) Advanced, the predominant th generation system which has just be...

This work presents an iterative receiver cancellation technique for mitigating the inband distortion introduced by a nonlinear wideband transmitter power amplifier (PA). The proposed decision-based technique employs a Wiener–Hammerstein model that accounts for the nonlinear transfer function and memory of the PA as well as for the wireless propagat...

The design of predistortion techniques for broadband multiple input multiple output-OFDM (MIMO-OFDM) systems raises several implementation challenges. First, the large bandwidth of the OFDM signal requires the introduction of memory effects in the PD model. In addition, it is usual to consider an imbalanced in-phase and quadrature (IQ) modulator to...

Predistortion techniques are implemented in OFDM systems to improve power amplifier efficiency while keeping in-band and out-of-band distortion at acceptable levels. Predistorter (PD) design for broadband MIMO-OFDM systems introduces several implementation challenges. Coupling effects cannot be avoided in MIMO transceivers where the paths are imple...

Instructor: Taneli Riihonen, M.Sc. (Tech.) This thesis studies the effect of Wiener phase noise on the performance of orthogonal frequency division multiplexing (OFDM) systems. The main performance metrics used in the analysis are capacity and signal-to-interference-plus-noise ratio (SINR). OFDM is a multi-carrier modulation technique in which data...

We present a novel channel estimator/predictor for OFDM systems over time-varying channels using a recursive for- mulation of a basis expansion model (BEM) based on an approxi- mated discrete cosine transform (DCT). We derive a recursive im- plementation of the approximated DCT-BEM for tracking time- varying channels based on a filter bank. The rec...

We present a prediction-based resource allocation algorithm (RA) for OFDMA downlink over mobile wireless chan- nels, where prediction errors are incorporated in the problem formulation. A solution based on the empirical characterization of the prediction error is proposed. We test the proposed scheme on a realistic scenario using a practical low co...

We address the main technical challenge encoun- tered in the practical implementations of full-duplex MIMO relays, i.e., how to mitigate efficiently the self-interference that loops back from the relay's transmit antenna array to its receive antenna array. In particular, we consider spatial-domain suppression using optimal eigenbeamforming that min...

We consider infrastructure-based amplify-and-forward (AF) relaying for extending downlink and uplink coverage areas of a cellular base station. The base station serves multiple mobile users via a multi-hop backhaul relay link by sharing out access link channel resources with maximum signal-to-noise ratio (SNR) scheduling. We analyze the performance...

We present a prediction-based resource allocation algorithm (RA) for orthogonal frequency-division multiple-access (OFDMA) down- link, where inaccuracies in the wireless channel predictions are accounted for in the problem formulation. As the prediction quality significantly degrades with the prediction horizon, we propose a solution based on the h...

We propose Eigen quantization as an alternative to normalized channel covariance matrix Direct quantization. The normalized covariance matrix is decomposed into the eigenspace and the eigenvalues that are quantized separately. We provide theoretical cumulative density function of normalized ordered eigenvalues for case of two eigenvalues. In additi...

We investigate inter-cell interference management based on two methods: instantaneous signal-to-interference plus noise (SINR) prediction and reverse reporting. Reverse reporting, a concept where vulnerable receivers announce themselves, is a common approach. SINR prediction is an alternative solution which brings the decision responsibility to the...

We present an analysis of a femtocellular communications network and the impact of cochannel interference on link performance. Furthermore, we propose a method whereby user terminals can maintain a controlonly connection to an adjacent femtocell for interference mitigation purposes. Specifically, we provide an emphasis on suboptimal but practical m...

Nonlinear effects of a power amplifier (PA) in communications systems include spectral spreading, which causes adjacent channel interference. For predistorter design, it is crucial to employ a model that can sufficiently model the intermodulation distortion (IMD) caused by the PA. The order of IMD generated by a polynomial is determined by its high...

Transmit diversity techniques have received a lot of attention recently, and open-loop and closed-loop downlink transmit diversity modes for two transmit antennae have been included into universal terrestrial radio access (UTRA) frequency division duplex (FDD) specification. Closed-loop modes provide larger system capacity than open-loop modes, but...