Aditya V. Padaki

Practical Issues for Spectrum Management With Cognitive Radios

The policy of permanently assigning a frequency band to a single application has led to extremely low utilization of the available spectrum. Cognitive radio, with its ability to be both intelligent and frequency agile, is thought to be one of the prime contenders to provide the necessary capabilities needed for dynamic spectrum access systems. With this in mind, this paper discusses the practical issues inherent to the deployment of spectrum management systems utilizing cognitive radios.

On the Co-Existence of TD-LTE and Radar Over 3.5 GHz Band: An Experimental Study

This letter presents a pioneering study based on a series of experiments on the operation of commercial Time-Division Long-Term Evolution (TD-LTE) systems in the presence of pulsed interfering signals in the 3550-3650 MHz band. TD-LTE operations were carried out in channels overlapping and adjacent to the high power SPN-43 radar with various frequency offsets between the two systems to evaluate the susceptibility of LTE to a high power interfering signal. Our results demonstrate that LTE communication using low antenna heights was not adversely affected by the pulsed interfering signal operating on adjacent frequencies irrespective of the distance of interfering transmitter. Performance was degraded only for very close distances (1-2 km) of overlapping frequencies of interfering transmitter.

Role of receiver performance data in efficient spectrum utilization

This paper elaborates on the importance of including receiver performance characteristics to optimize for efficient spectral usage. Futuristic heterogeneous wireless networks are expected to be diverse and dynamic. Knowledge of receiver spectral performance enables the spectrum management system not only to provide adequate protection from harmful interference, but also provides an effective way to maximize the network level spectrum efficiency for networks with diverse and dynamic environments. We formulate the problem for such an inclusive optimization with several constraints and demonstrate using numerical simulations that using receiver performance characteristics for spectrum management has the potential to considerably improve the network level spectrum efficiency.

Impact of intermodulation distortion on spectrum preclusion for DSA: A new figure of merit

This paper discusses the importance of understanding the vulnerability of receivers towards intermodulation distortion. Knowledge of the RF front end non-linearity from a spectrum consumption point of view is of crucial importance in efficient spectrum management. A specific case of intermodulation distortion for Low Noise Amplifier (LNA) is studied to quantify the spectrum consumption. We show that the traditional single point IIP3 definition would render insufficient information about the extent of vulnerability to adjacent channel interference. Consequently we propose a new metric as a Figure of Merit with the inclusion of variation of IIP3 with tone spacing to quantify the LNA performance or tolerance to adjacent channel interference. Example measurements of this new FOM are presented along with the discussion on its utility and importance for efficient cognitive spectrum access.

Receiver Non-Linearity Aware Resource Allocation for Dynamic Spectrum Access Systems

In this paper we propose a novel resource allocation framework for Dynamic Spectrum Access (DSA) systems which accounts for nonlinear distortions due to adjacent channel interference in receivers. We first discuss the potential reasons for RF pollution resulting in elevation of noise floor for wireless systems employing DSA, which compel receivers to operate in the nonlinear region, which produce distortions. We then develop a framework for resource allocation in dynamic spectrum access considering the receiver nonlinearity. We use this framework for maximizing sum data rate with minimum rate constraints. Distortion aware resource allocation will yield a considerably higher network data rate while ensuring the minimum required Quality of Service for all users.

Efficient Spectrum Sharing with RF Diversity: Adapting to Nonlinearity of Front Ends

RF front-end characteristics significantly impact the performance of the receiver. Receivers are vulnerable to harmful adjacent channel interference, especially in shared spectrum environments, which allow a free-style of spectrum access with diverse receiver technologies accessing the same band of spectrum. Channel assignments agnostic to receiver characteristics can have a severe detrimental effect on network level performance of a wireless network. In this paper, we develop a novel dynamic channel assignment framework which accounts for the vulnerabilities posed by RF front end, in particular, receiver pre-selector bandwidth and front-end nonlinearity. We further propose an approximate, heuristic, greedy algorithm for channel assignment which is computationally efficient and provides a near-optimal solution. We demonstrate through simulations that the developed framework of receiver characteristics aware dynamic channel assignment will substantially improve the network-wide data rate, and thereby the overall spectrum efficiency of the wireless network. We further demonstrate that the approximate algorithm provides a near-optimal solution for channel assignment in the statistical sense.

On scalability and interference avoidance in nonlinear adjacent channel interference networks

Adjacent channel interference caused by intermodulation distortion adversely affects the network operations in next generation heterogeneous and dynamic spectrum access networks. Multitudes of radio access technologies make the receivers susceptible to harmful interference due to nonlinear RF front ends. In this paper we analyze the intermodulation distortion arising from pairwise interactions of adjacent channel signals from a spectrum centric point of view and develop frameworks to ascertain the adjacent channel signals causing interference at a given desired channel. We further propose achievable schemes for interference avoidance and assess the scalability of the next generation Nonlinear Adjacent Channel Interference Networks. We further propose schemes for complete interference protection of incumbents with sensitive receiver requirements from secondary operations in adjacent channels in the spatio-temporal vicinity. This paper presents valuable insights on scalability and schemes for nonlinear adjacent channel interference avoidance in next generation shared spectrum networks.

Network aware spectrum efficiency metric for heterogeneous and dynamic radio environments

In this paper, we formalize a new definition for spectrum efficiency, with the specific goal of addressing the diverse needs and requirements of various technologies and users. Existing metrics for spectrum efficiency do not account for changing radio environments. Hence, they are insufficient for future systems which employ dynamic allocation schemes. We introduce a heuristic parameterized definition for spectrum efficiency dependent on the network dynamics, radio environment and diverse requirements of technologies. This metric accounts for the frequency use and reuse, interference footprint of a user, and has a parameter to specify priority/importance for users/bits (e.g. public safety). We study the behavior and the optimal transmit power to maximize the efficiency for a point-to-point link. We numerically evaluate the efficiency for a two user case, and discuss the trade-offs involved. Overall, the proposed metric accounts for the interplay between users in a network to evaluate efficiency of spectrum utilization.