Gabriel Porto Villardi

Enabling coexistence of multiple cognitive networks in TV white space

We address the problem of coexistence among wireless networks in TV white space. We present a standard independent framework to enable exchange of information relevant for coexistence based on two mechanisms: centralized and distributed. Both mechanisms introduce the use of multiradio cluster-head equipment (CHE) as a physical entity that acquires relevant information, identifies coexistence opportunities, and implements autonomous coexistence decisions. The major conceptual difference between them lies in the fact that the centralized mechanism utilizes coexistence database(s) as a repository for coexistence related information, where CHEs need to access before making coexistence decisions. On the other hand, the distributed mechanism utilizes a broadcast channel to distribute beacons and directly convey coexistence information between CHEs. Furthermore, we give a concise overview of the current activities in international standardization bodies toward the realization of communications in TVWS along with measures taken to provide coexistence between secondary cognitive networks.

Overview of TV White Spaces: Current regulations, standards and coexistence between secondary users

TV White Spaces (TVWS) refer to segments of the TV spectrum not used by licensed users in a given location. In recent years, regulation agencies throughout the world consider the possibility to open TVWS to unlicensed use, which is followed by standardization activities to realize it. One of the important aspects of unlicensed operation in TVWS is coexistence between heterogenous networks. This paper provides an overview of current regulations, standardization activities related to TVWS, while the main focus is IEEE P802.19.1 standardization project to enable coexistence in TVWS.

Efficiency of dynamic frequency selection based coexistence mechanisms for tv white space enabled cognitive wireless access points

We investigate the efficiency of dynamic frequency selection (DFS) in mitigating interference among neighboring low-power cognitive wireless portable networks operating in the TV white space. We derive an interference model to predict the range and level of interference generated in the TV bands by portable low-height antenna cognitive wireless access points in suburban and urban areas. Based on the aforementioned model, we provide an analysis of the spectral availability for either the scenarios where DFS coexistence is employed or not. The steps of our analysis are introduced in a tutorial fashion, and a coexistence case study of TVWS enabled low-power cognitive wireless portable APs in Japan is presented. Our analysis demonstrates the intrinsic relationship SA holds with the TVWS channel set as well as statistical information (e.g., household density of wards and cities, Internet penetration, and white space radio AP market penetration).

Coexistence of TV White Space enabled cognitive wireless access points

We address the problem of coexistence between neighboring cognitive wireless networks operating in the TV White Space (TVWS). We derive an interference model to predict the range and level of interference generated in the TV bands by portable low-heigh antenna cognitive wireless access points (AP) in suburban and urban areas. Based on our model, we provide an analysis of the spectral availability (SA) for either the scenarios where dynamic frequency selection (DFS) coexistence technique is employed or not. We derive the steps of our analysis and provide, as an illustrative example, the expected coexistence performance of TVWS enabled cognitive wireless APs in Japan. Our analysis demonstrates the intrinsic relationship SA holds with the TVWS channel set as well as statistical information, e.g., house-hold density of wards and cities, Internet penetration, White Space (WS) radio AP market penetration.

Channel direction information probing for multi-antenna cognitive radio system

This work studies the problem of channel direction information (CDI) probing for multi-antenna cognitive radio system. The CDI of the channel from the secondary transmitter (ST) to primary receiver (PR) is elementary information in designing the beamforming at the ST for mitigating the interference to the PR. However, lacking the explicit cooperation between primary and secondary systems, the CDI has to be acquired by probing at the ST, which is challenging. To solve this, we consider the line of sight (LoS) channel between the ST and the PR, and propose one CDI probing scheme for the ST. Specifically, the ST sends two types of probing signals by beamforming towards an interested region where both the secondary receiver (SR) and the PR are located and then actively learns the hidden feedback information from the primary system to acquire the CDI. The proposed scheme has a closed-form solution, and avoids the iteration between the probing and acquisition, which is desirable for practical system. Moreover, we show that the proposed probing scheme can be extended for primary systems working under multi-access channel and broadcasting channel. Simulation results demonstrate that the proposed scheme can improve the accuracy of the acquired CDI at the ST in cognitive ratio system remarkably.

Coexistence of secondary user networks under primary user constraints in TV white space

Moving forward based on the current spectrum regulations and industrial standardization for unlicensed usage of TV white space, we postulate a potential deployment scenario and describe the management procedure of coexisting secondary user (SU) networks under the interference constraints to the primary user (PU) receivers. For the quantitative evaluation of coexistence we first introduce a new parameter named quality of coexistence (QoC), which is utilized to measure the benefits achieved by a novel coexistence scheme also proposed in this paper, namely, interference-constrained time slot splitting and allocation. Furthermore, on the basis of a system model we obtain the optimal SU networks time slot allocation ratios by maximizing the aforementioned QoC, therefore, leading to an efficient spectrum usage by SUs without incurring harmful interference to the PUs. Finally, we simulate a coexistence scenario in TV white space in order to compare the proposed scheme with other schemes that select a single SU network to operate based on simplistic criteria. It can be seen that when the SU network employs adaptive transmit power control (TPC), the medium value of QoC is doubled by using the optimal time slot allocation, resulting in significant improvement in coexistence.

Multilevel Protection to Broadcaster Contour and Its Impact on TV White Space Availability

In light of recent regulations from the Federal Communications Commission (FCC) governing the very-high-frequency/ultrahigh-frequency band sharing, we consider the database as a tool to plan the utilization of this spectrum by unlicensed wireless devices. Our approach, however, differs from that adopted by the FCC in the sense that it exploits the ability that white space (WS) devices have in controlling their effective isotropic radiated power through the transmit power control technique when concomitantly operating in a cochannel or in an adjacent channel to a TV station. We derive equations for the expected interference level generated by WS devices with antenna heights ranging from a few meters (portable devices) up to a 1000 m (fixed devices) based on very recent TV white space (TVWS) propagation literature and regulatory requirements. Additionally, we also provide exact and numerical methods to calculate the critical distance that prevents interference to primary contours, thus enabling fast identification of regions where operation is harmless to incumbents in the presence of shadowing and multipath fading. These equations reveal the intrinsic relationship between system design parameters, environmental parameters, and country regulations with TVWS availability. Finally, we provide an analysis that indicates notable enhancements in the expected TVWS availability through the use of a multilevel method while obtaining the same desirable degree of protection required by the current FCC regulation based on the single-level methodology. The analysis is divided in two parts: theoretical evaluations to obtain insights into the potential of multilevel protection in recovering TVWS-less regions from a single exclusion zone and extensive computations to obtain the expected TVWS availability yielded by both protection methodologies and primary-contour determination techniques across the Japanese territory. Specifically, multilevel protection is found to have the effect of doubling the TVWS bandwidth, as compared with single-level protection. On the other hand, diffraction methods used to determine the primary contour nearly triples the bandwidth, as compared with the height above average terrain (HAAT) technique. The combined effect of multilevel protection and diffraction-based primary contours is TVWS availability on 98.76% of the Japanese territory with a bandwidth expansion six times of that inherent to single-level protection and HAAT-based contours.

Heterogeneous Networks in Shared Spectrum Access Communications

We investigate an advanced two-phase shared spectrum access communication scheme as an efficient approach to enhance the spectral utilization of a network. In the first phase, we devise a spectrum-sharing policy based on demands, fairness, and so on, which utilizes a priority scheme in fulfilling operators’ demands, and envision a secure operator-specific information sharing policy where no critical information is exchanged between the operators. In the second phase, a macro cell network (MCN) benefits through offloading services offered by small cell network (SCN). This allows the MCN to satisfy its users’ capacity demands, improve its quality of services and coverage under Nakagami fading channel. As a repayment, the SCN is rewarded with licenses to share and operate on the spectrum originally owned by the MCN. We devise a density division-based shared spectrum access model, where the density of the licensee’s SCN deployment is exploited as network resources. A fair division of the densities of the licensee operator’s small cell base stations into fractions of licensed small cell base stations serving its own users and offloading small cells is presented. Unlike most of the previous research works that considered Poisson point process (PPP) to model the distribution of the network entities even when PPP modeling is not accurate for the networks, where the number of MCN/SCN base stations is definite and the number of MCN/SCN base stations in disjoint areas is not independent, we employ a more realistic network model known as binomial point process to perform an analytical analysis of the cumulative interference and performance of the system. Furthermore, we analyze the rate coverage and outage performances considering a wide range of values for path-loss exponent and fading severity parameter of Nakagami fading.

Throughput Enhancement of Multicarrier Cognitive M2M Networks: Universal-Filtered OFDM Systems

We consider a cognitive radio (CR) network consisting of a primary cellular system and a secondary cognitive machine-to-machine (M2M) system, and study the throughput enhancement problem of the latter system employing universal-filtered orthogonal frequency division multiplexing (UF-OFDM) modulation. The downlink transmission capacity of the cognitive M2M system is thereby maximized, while keeping the interference introduced to the primary users (PUs) below the prespecified threshold, under total transmit power budget of the secondary base station (SBS). The performance of UF-OFDM-based CR system is compared to the performances of OFDM-based and filter bank multicarrier (FBMC)-based CR systems. We also propose a near-optimal resource allocation method separating the sub-band and power allocation. The solution is less complex compared to optimization of the original combinatorial problem. We present numerical results that show that for given interference thresholds of the PUs and maximum transmit power limit of the SBS, the UF-OFDM-based CR system exhibits intermediary performance in terms of achievable capacity compared to OFDM- and FBMC-based CR systems. Interestingly, for a certain degree of robustness of the PUs, the UF-OFDM performs equally well as FBMC. Furthermore, the percentage rate-gain of UF-OFDM-based CR system increases by a large amount when UF-OFDM modulation with lower sidelobes ripple is employed. Numerical results also show that the proposed throughput enhancing method despite having lower computational complexity compared to the optimal solution achieves near-optimal performance.

Outage Analysis of Offloading in Heterogeneous Networks: Composite Fading Channels

Small cells deployment is one of the most significant long-term strategic policies of the mobile network operators. In heterogeneous networks (HetNets), small cells serve as offloading spots in the radio access network to offload macro users (MUs) and their associated traffic from congested macrocells. In this paper, we perform analytical analysis and investigate how the radio channel propagation impairments such as multipath fading, shadowing, and small cell base station density affect MUs’ offloading to small cells network (SCN). In particular, we exploit composite fading channels in our evaluation when an MU is offloaded to SCN with varying small cell base station density in the stochastic geometry HetNets framework. We derive the expressions for service outage probability (equivalently service coverage probability) of the MU in macro cell network and SCN under two different composite fading scenarios, viz., Nakagami-Lognormal channel fading and time-shared (combined) shadowed/unshadowed channel fading. We propose efficient approximations for the probability density functions of the channel fading (power) for the aforementioned composite fading distributions that do not have closed-form expressions employing Gauss–Hermite integration and finite exponential series, respectively. Finally, the service outage probability performance of MU with and without offloading services is analyzed under various system parameters and channel fading conditions.