Sudeep Bhattarai

An Overview of Dynamic Spectrum Sharing: Ongoing Initiatives, Challenges, and a Roadmap for Future Research

We are in the midst of a major paradigm shift in how we manage radio spectrum. This paradigm shift is necessitated by the growth of wireless services of all types and the demand pressure imposed on limited spectrum resources under legacy management regimes. The shift is feasible because of advances in radio and networking technologies that make it possible to share spectrum dynamically in all possible dimensions-i.e., across frequencies, time, location, users, uses, and networks. Realizing the full potential of this shift to Dynamic Spectrum Sharing will require the co-evolution of wireless technologies, markets, and regulatory policies; a process which is occurring on a global scale. This paper provides a current overview of major technological and regulatory reforms that are leading the way toward a global paradigm shift to more flexible, dynamic, market-based ways to manage and share radio spectrum resources. We focus on current efforts to implement database-driven approaches for managing the shared co-existence of users with heterogeneous access and interference protection rights, and discuss open research challenges.

Multi-tier exclusion zones for dynamic spectrum sharing

Reducing the size of exclusion zones (EZs) in spectrum sharing is vital for efficient utilization of fallow spectrum as well as for the economic viability of spectrum sharing itself. In this paper, we explore two approaches for reducing the size of EZs. We show that multi-tiered EZs can be used to improve spectrum utilization efficiency by implementing the concept of differential spectrum access hierarchy. Also, we provide quantitative results that show the impact of using a point-to-point mode terrain profile in calculating an EZs contour. Such a terrain profile captures the effects of propagation losses due to area-specific topography, which are not considered by the F-curves, a common method of calculating an EZs boundary. Our results indicate that the use of such a terrain profile results in a noticeable decrease in the size of an EZ.

Defining incumbent protection zones on the fly: Dynamic boundaries for spectrum sharing

In spectrum sharing, a spatial separation region is defined around primary users (PUs) to protect them from secondary user (SU)-induced interference. This protection region - referred to by a number of names, such as an exclusion zone (EZ) or a protection zone (PZ) - has a static boundary, and this boundary is determined very conservatively to provide an additional margin of protection for the PUs. This legacy notion of interference protection is overly rigid, and often results in poor spectrum utilization efficiency. In this paper, we propose a novel framework for prescribing interference protection for the PUs that addresses some of the limitations of legacy EZs. Specifically, we introduce the concept of Multi-tiered Incumbent Protection Zones (MIPZ), and show that it can be used to dynamically adjust the PUs protection boundary based on the radio environment, network conditions, and the PU interference protection requirement. MIPZ can serve as an analytical framework for quantitatively analyzing a given PZ to gain insights on and determine the tradeoffs between interference protection and spectrum utilization efficiency. It allows a number of SUs, say N, to operate closer to the PU, and improves the overall spectrum utilization efficiency while ensuring a probabilistic guarantee of interference protection to the PU. We leverage the combined power of database-driven spectrum sharing and stochastic optimization theory for dynamically computing the zone boundary and the value of N. Using extensive simulation results, we demonstrate that the proposed framework improves spectrum utilization efficiency by adapting to the changing interference environment through dynamic adjustments of the zone boundary.

Incentivizing spectrum sensing in database-driven dynamic spectrum sharing

The legacy concept of exclusion zones (EZs) is inept at enabling efficient utilization of fallow spectrum by secondary users (SUs), since legacy EZs are static and overly-conservative. The notion of a static EZ implies that it has to protect incumbent users (IUs) from the union of likely interference scenarios, leading to a worst-case, conservative solution. In this paper, we propose the concept of dynamic, multi-tier EZs, which takes advantage of participatory spectrum sensing carried out by SUs to support efficient database-driven spectrum sharing while protecting IUs against SU-induced aggregate interference. Specifically, the database directly incentivizes SUs to participate in spectrum sensing, which augments geolocation database by defining smaller EZs with dynamic boundaries and creating additional spectrum access opportunities for SUs. We propose an incentive mechanism based on a two-level game-theoretic model, in which the database conducts dynamic pricing in a first-level Stackelberg game in the presence of SUs who strategically contribute to spectrum sensing in a second-level stochastic game. The existence of an equilibrium solution is proven. According to our findings, the proposed incentive mechanism for the concept of dynamic, multi-tier EZs is effective to improve spectrum utilization efficiency while guaranteeing incumbent protection.

Robust transmit beamforming against steering vector uncertainty in cognitive radio networks

A robust transmit beamforming scheme is proposed for green cognitive radio networks in presence of incorrect steering vector estimations. The beamformer is designed using a stochastic optimization approach. The proposed beamforming scheme achieves three objectives: a) Probability for large-than-a-threshold for secondary users received power in the steering direction is maximized; b) Probabilities for less-than-a-threshold interferences to the primary users in different locations are bounded; and c) Transmitted power along the directions of the moving secondary receiver is maintained as a constant. In addition, we present an algorithm to calculate the inverse Marcum Q function, and use it to solve the optimization problem for proposed beamforming scheme. Simulation results demonstrate that the proposed robust beamformer can achieve significant BER reductions for primary users while ensuring secondary users BER is not sacrificed.

Robust cooperative beamforming against steering vector uncertainty in cognitive radio networks

Transmit beamforming is one of the promising ways of accessing spectrum in an underlay cognitive radio network (CRN). In this paper, a robust cooperative transmit beamforming scheme is proposed for a mobile (or stationary) secondary receiver in a CRN where steering vector available at the distributed transmitters is error prone. The solution to the beamforming problem is based on stochastic optimization theory. Specifically, the proposed beamforming scheme achieves the following three objectives: i) probability of large-than-a-threshold for received power in the direction of the secondary receiver is maximized; ii) probabilities of less-than-a-threshold for received interference power in the direction of primary receivers are bounded; and iii) transmit power along the directions of the moving secondary receiver is forced to be uniform so as to ensure the same quality of service. Our simulation results demonstrate that the proposed robust cooperative beamformer can achieve noticeable bit-error-rate reductions for primary users as compared to the non-robust approach, where inaccuracy in the estimation of the steering vector is not considered, while ensuring almost the same quality of communication for the secondary user.

Location Privacy of Non-Stationary Incumbent Systems in Spectrum Sharing

Although using geolocation databases for spectrum sharing has many pragmatic advantages, it also raises potentially serious operational security (OPSEC) issues. OPSEC is especially a paramount consideration in the light of recent calls in the U.S. for spectrum sharing between federal government (including military) systems and non- government systems (e.g., cellular service providers). In this paper, we explore the OPSEC, location privacy in particular, of incumbent radars in the 3.5 GHz band. First, we show that adversarial secondary users can easily infer the locations of incumbent radars by making seemingly innocuous queries to the database. Then, we propose several obfuscation techniques that can be implemented by the database for countering the inference attacks. We also investigate the inherent tradeoff between the degree of obfuscation and spectrum utilization efficiency. Finally, we validate our discussions by providing results from extensive simulations.

TESSO: An analytical tool for characterizing aggregate interference and enabling spatial spectrum sharing

Radio propagation models play a crucial role in realizing effective spectrum sharing. Unlike propagation models that do not use the exact details of terrain, terrain-based propagation models are effective in identifying spatial spectrum sharing opportunities for the secondary users (SUs) around an incumbent user (IU). Unfortunately, terrain-based propagation models, such as the Irregular Terrain Model (ITM) in point-to-point (PTP) mode, are computationally expensive, and they require precise geo-locations of the SUs. Such requirements render them challenging, if not impractical, to implement in real-time applications, such as geolocation database (GDB)-driven spectrum sharing. To address this problem, we propose a pragmatic approach called Tool for Enabling Spatial Spectrum Sharing Opportunities (TESSO). TESSO characterizes the aggregate interference caused by the SUs and identifies spatial spectrum sharing opportunities effectively. It is computationally efficient, and does not require precise geo-locations of the SUs. Our results show that TESSO provides the same level of interference protection guarantee to the IU as that offered by the terrain-based models. TESSO can be implemented in GDB-driven spectrum sharing ecosystems for effectively exploiting spatial spectrum sharing opportunities.

A computationally efficient node-selection scheme for cooperative beamforming in Cognitive Radio enabled 5G systems

Cooperative transmit beamforming (CTB) is a practical approach for addressing the challenging problem of spectrum scarcity in broadband 5G wireless communication systems. It is a technique that allows a group of secondary users (SUs), each equipped with a single omni-directional antenna, to collaborate and steer the signal towards the intended receiver. CTB allows SUs to co-exist with primary users in the same spectrum, which helps to significantly improve the efficiency of spectrum utilization. One of the key factors that affect the performance of CTB is the selection of participatory nodes. In this paper, we first formulate the CTB as an optimization problem, and then investigate the impact of different node-selection schemes on the performance of CTB. Our findings illustrate that exhaustive search based optimal node-selection scheme is computationally infeasible for real-time systems, while simple random-selection and highest-channel-state based selection often result in poor performance. Motivated by these findings, we propose a computationally efficient node-selection scheme for CTB that achieves a near-optimal performance. The proposed scheme is based on iterative node-replacement and is computationally scalable to large system size. Results from extensive simulations show that the proposed scheme asymptotically approaches the exhaustive search based optimal system performance. In our example, the performance of the proposed scheme is approximately 98.5% of the optimal system performance while limiting the required computations to only 1.67%.

Robust beamforming for cognitive radio based vehicular communication

This paper proposes a robust beamforming scheme under steering vector uncertainty for cognitive radio based vehicular communication. The service probability of secondary users (SU) is maximized, the SUs transmission power is uniform within the target mobility range, and the probability of interference level to primary users (PU) is constrained. Simulation results show significant BER improvement for PU without trading off the SUs performance when there are errors in steering vectors.