Yngve Selén

Sensor Selection for Cooperative Spectrum Sensing

This article considers spectrum-on-demand in a cellular system. A communication system that wants to access spectrum to which it does not own a license must perform spectrum sensing to identify spectrum opportunities, and to guarantee that it does not cause unacceptable interference to the license owner. Because a single sensor may be in a fading dip, cooperative sensing among multiple sensors which experience uncorrelated fading is required to guarantee reliable sensing performance. At the same time, as few sensors as possible should be used to reduce the battery consumption, while still employing enough many for the sensing to be reliable. Since shadow fading is correlated for closely spaced sensors, it is desired to select sensors which are sufficiently spatially separated. The present article addresses the problem of selecting appropriate sensors from a candidate set to engage in cooperative sensing, using different degrees of knowledge about the sensor positions. Three different algorithms for sensor selection are presented and evaluated by means of simulation. It is shown that all algorithms outperform random selection of the sensors.

Ultra-dense networks in millimeter-wave frequencies

Demands for very high system capacity and end-user data rates of the order of 10 Gb/s can be met in localized environments by Ultra-Dense Networks (UDN), characterized as networks with very short inter-site distances capable of ensuring low interference levels during communications. UDNs are expected to operate in the millimeter-wave band, where wide bandwidth signals needed for such high data rates can be designed, and will rely on high-gain beamforming to mitigate path loss and ensure low interference. The dense deployment of infrastructure nodes will make traditional wire-based backhaul provisioning challenging. Wireless self-backhauling over multiple hops is proposed to enhance flexibility in deployment. A description of the architecture and a concept based on separation of mobility, radio resource coordination among multiple nodes, and data plane handling, as well as on integration with wide-area networks, is introduced. A simulation of a multi-node office environment is used to demonstrate the performance of wireless self-backhauling at various loads.

Evolving Wireless Communications: Addressing the Challenges and Expectations of the Future

The wireless-access networks of today will have to evolve in several ways in order to address the challenges and expectations of the future. New technology components will be introduced as part of the evolution of current wireless-access technologies, such as high-speed packet access (HSPA) and long-term evolution (LTE). However, additional components may also constitute future new wireless-access technologies, which may complement the evolved technologies. Examples of such new technology components are new ways of accessing spectrum and substantially higher frequency ranges, the introduction of massive antenna configurations, direct device-to-device communication, and ultradense deployments.

Spectrum sharing scenarios and resulting technical requirements for 5G systems

Cellular networks today are designed for and operate in dedicated licensed spectrum. At the same time there are other spectrum usage authorization models for wireless communication, such as unlicensed spectrum or, as widely discussed currently but not yet implemented in practice, various forms of licensed shared spectrum. Hence, cellular technology as of today can only operate in a subset of the spectrum that is in principle available. Hence, a future wireless system may benefit from the ability to access also spectrum opportunities other than dedicated licensed spectrum. It is therefore important to identify which additional ways of authorizing spectrum usage are deemed to become relevant in the future and to analyze the resulting technical requirements. The implications of sharing spectrum between different technologies are analyzed in this paper, both from efficiency and technology neutrality perspective. Different known sharing techniques are outlined and their applicability to the relevant range of future spectrum regulatory regimes is discussed. Based on an assumed range of relevant (according to the views of the authors) future spectrum sharing scenarios, a toolbox of certain spectrum sharing techniques is proposed as the basis for the design of spectrum sharing related functionality in future mobile broadband systems.

3G LTE Simulations Using Measured MIMO Channels

In this article we present downlink simulation results for a realistic implementation of the LTE (Long Term Evolution) 3G standard. In contrast to previous studies, actual measured channels (as opposed to computer generated artificial channels) have been used in the simulation. The used 2 times 2 MIMO channels were measured using two realistic receiver mockups, one laptop and one handset, as well as a pair of reference dipole antennas. The results suggest that LTE is able in practice to support multi stream transmission with very high data rates, even for small hand held terminals. Also, the improvements of 2 times 2 MIMO over SISO transmission are clearly shown.

Optimizing power limits for white space devices under a probability constraint on aggregated interference

This paper presents a solution to the problem of setting power limits for white space devices sharing a spectrum band. It is desired to utilize the available white space efficiently while also protecting the primary system from harmful interference. Power limits are set individually for each white space device by maximizing a joint utility measure, e.g., sum capacity. The aggregated interference caused by the white space devices to the primary system is controlled by constraining the probability of harmful aggregated interference to be below a defined threshold. First, the problem of single white space channel sharing is given a mathematical formulation in the form of an optimization problem. Under the common assumption of lognormal fading the distribution of the aggregate interference is unknown and the optimization problem becomes infeasible to solve. A computationally feasible approximation of the initial optimization problem is formulated in which the distribution of the aggregated interference is modeled using the Fenton-Wilkinson approximation. We derive the expressions needed for efficiently solving the simplified optimization problem with a numerical solver, including the gradients of the constraint and objective functions. We show by means of simulations that the solutions to the simplified optimization problem typically fulfill the original probability constraints with good precision. Further, the resulting sum-capacity values are higher than what can typically be obtained by using fixed margins for coping with the aggregate interference. We also discuss multi channel extensions which are able to handle not only interference to primary systems operating on adjacent channels, but also the joint problem of selecting the channels for white space operation and deciding the associated power limits.

Cooperative Detection of Programme Making Special Event Devices in Realistic Fading Environments

In this article the effect of licensed non-standardized low power transmitters, i.e., PMSE (programme making special event) devices such as wireless microphones, on secondary usage of TV white space is considered. In particular, the performance for energy detection of these devices is studied under realistic fading and interference situations. One motivation for using energy detection is that PMSE devices are not standardized and, hence, there are few common signal properties that can be exploited when trying to detect them. Further, a novel semi-analytical approach based on single sensor detection performance as a function of distance is developed and shown to accurately predict cooperative sensing performance. The conclusion from the study is that it is sometimes very challenging to accurately detect the presence of PMSE devices. Many spectrum opportunities at times needs to be sacrificed in order to guarantee a low probability of interference.

A short feasibility study of a cognitive TV black space system

This paper evaluates the feasibility of overlaying a secondary system on a digital TV (DTV) system. We denote this type of approach TV black space (in contrast to TV white space). In this type of operation the secondary transmitter transmits on the same spectrum resources as the DTV system. Particularly, we consider the case where the secondary transmitter is collocated with the DTV transmitter and uses the same antennas. The fraction of the total transmitted power that is used for the secondary signal is set low enough so that the DTV receivers are always able to decode the DTV signals. The black space receivers decode the TV signal component and cancel it from the total received signal which results in an improved SINR of the secondary signal in the residual. Numerical evaluations show that this type of overlay in the DTV spectrum is feasible only when the secondary system is satisfied with relatively low SINR values of around 5 to 10 dB. A critical parameter when determining the secondary signal SINR after DTV signal cancellation is the SINR requirements by the primary system: DVB-T systems typically require SINR values above 20 dB and then overlay becomes less feasible since the fraction of power left for the secondary signal becomes very low. Additionally, realistic levels of TX EVM noise (around 6% to 8%) at the transmitter causes low achievable SINR values for the secondary signal even after DTV signal cancellation at the receiver side.

Future Wireless Communications

The wireless-access networks of today will have to evolve in several dimensions in order to address the challenges and expectations of the future. New technology components will be introduced as part of the evolution of current wireless-access technologies, such as HSPA and LTE. However, additional components may also constitute future new wireless-access technologies, which may complement the evolved technologies. Examples of such new technology components are new ways of accessing spectrum and substantially higher frequency ranges, the introduction of massive antenna configurations, direct device-to-device communication, and ultra-dense deployments.