Prasanth Karunakaran

Sensing for spectrum sharing in cognitive LTE-A cellular networks

In this work we present a case for dynamic spectrum sharing between different operators in systems with carrier aggregation (CA) which is an important feature in 3GPP LTE-A systems. Cross-carrier scheduling and sensing are identified as key enablers for such spectrum sharing in LTE-A. Sensing is classified as Type 1 sensing and Type 2 sensing and the role of each in the system operation is discussed. The more challenging Type 2 sensing which involves sensing the interfering signal in the presence of a desired signal is studied for a single-input singleoutput system. Energy detection and the most powerful test are formulated. The probability of false alarm and of detection are analyzed for energy detectors. Performance evaluations show that reasonable sensing performance can be achieved with the use of channel state information, making such sensing practically viable.

On simultaneous sensing and reception for cognitive LTE-A systems

We consider the application of cognitive radio technology in future LTE-A systems. The carrier aggregation (CA) features of LTE-A can be exploited for enabling a cognitive operation in a subset of component carriers. Spectrum sharing between different network operators, device-to-device communications and the use of unlicensed bands for LTE-A are potential applications for such an operation. Sensing is expected to play an important role in such systems. To this end we explore the possibilities of spectrum sensing, especially the sensing while receiving a desired signal, in LTE-A systems. Specifically, an energy detector based on reference signal cancellation is introduced and compared to a beamforming based energy detector under practical channel conditions and realistic channel estimation assumptions. The results are expected to play an important role in the development of protocols for cognitive operation in LTE-A systems.

On Interference Rejection Combining for LTE-A Systems: Analysis of Covariance Estimators and an Iterative Algorithm for Frequency-Selective Channels

Spatial suppression of interference using interference rejection combining (IRC) is an important feature considered in LTE-A systems for improving the reception performance of users, especially at the cell-edges. The efficiency of interference suppression depends on the accuracy of the channel estimation and the spatial covariance estimation. In this work, we first analyze two different approaches of covariance estimation, namely data based covariance estimation and reference signal based covariance estimation. It is shown that the reference signal based covariance estimation is superior, even for a block flat fading channel and the reasons behind this behavior are investigated. Secondly, we propose an iterative IRC receiver with soft iterative channel estimation and covariance estimation for highly frequency-selective channels. The performance results demonstrate that the iterative IRC receiver can provide a significant gain.

Energy and delay efficient cooperative media content download

The advent of smartphones has brought about a new change in the traffic pattern of mobile users, for instance, there is a growing demand for high quality media content. In this paper, we consider a group of users within proximity of each other, who are interested in downloading the same content. Instead of the conventional cellular method of downloading the same content independently from the base station (BS), the group of users can exploit cooperative downlink strategies in a framework that involves both cellular and short-range communications. A few of such two-stage cooperative downlink strategies are studied in this paper. The results show that a significant energy saving of approximately 60% and a maximum possible delay reduction of 57% on an average, can be achieved by the proposed cooperative strategies.

Coverage and achievable rate analysis for indoor terahertz wireless networks

With the emergence of numerous novel dataintensive applications, the demand on fast wireless access is experiencing an unprecedented growth. Following this trend, the “Terabit era” is expected to become a reality in the near future. Terahertz technology is promising as an enabler due to its feature of an extremely high bandwidth. However, due to the high carrier frequency along with a high molecular absorption, the transmission distance is limited to a few meters only. In this work, the coverage problem and the achievable data rate performance of indoor THz wireless networks (THz-WNs) are investigated. In order to overcome the severe propagation loss and to improve the transmission range, a single frequency network (SFN) is advocated. The minimum individual user rate for different resource allocation schemes is analyzed, taking into account the effects of inter-symbol interference due to channel dispersion in the THz band and as a consequence of the SFN transmit protocol. Results demonstrate that the proposed SFN scheme is able to provide a high minimum achievable user data rate. The coverage probability increases from 25% when only a single access point (AP) is employed up to 95% when 20 APs are considered, for an output power of 1 W.

A reference signal based GLRT for simultaneous sensing and reception in cognitive LTE-A systems

We explore the application of cognitive radio (CR) principles to 3GPP LTE-A. In such a system, a cognitive radio (CR), synchronized to the macro base station (BS), exploits the spectral holes in the orthogonal frequency division multiple access (OFDMA) time-frequency grid. Spectrum sharing between different network operators and device-to-device (D2D) communications in LTE-A are presented as potential applications. Sensing is expected to play a prominent role in such systems. We focus on simultaneous sensing and reception (SSR) that can avoid the overhead in sensing before transmission (SBT) protocols. Firstly, for a flat fading channel, we develop generalized likelihood ratio tests (GLRTs) based on the demodulation reference signals (DMRSs) of transmission mode 9 in LTE-A that do not require the knowledge of signal powers, channel statistics and noise parameters. Secondly, using a polynomial fitting, we extend the algorithm to frequency-selective fading scenarios. Simulation results are presented to compare the different algorithms which indicate that good performance can be achieved for SSR with full knowledge of the DMRSs.

Exploiting molecular absorption in the THz band for low latency wearable wireless device communications

Wearable wireless devices (WWDs) have been emerging as an important tool for future communication and control systems. When multiple uncoordinated WWD bearers exist in close proximity, the issue of interference between cochannel transmissions presents a major challenge. In this work, we exploit the molecular absorption (MA) regions of the Terahertz band (0.1-10 THz) for deploying WWDs efficiently. While such regions are not suitable for relatively long-range transmission schemes, the fast exponential decay of signal power with distance due to MA proves to be beneficial for WWD systems in limiting the cochannel interference. Our results for a symbol rate of 1 Gsymbols/s show that 95% of the users achieve a signal-to-interference-plus-noise ratio greater than 3 dB for a typical application, even in high-density user scenarios.

An Analysis of WiFi Cochannel Interference at LTE Subcarriers and Its Application for Sensing

LTE-Unlicensed (LTE-U) involving the deployment of LTE in unlicensed bands has been gaining significant interest lately. The standardization of LTE-U has been proposed for Release 13 of 3GPP LTE. The two main requirements mandated for such an operation are the coexistence mechanisms with the WiFi systems and the sensing before transmission by an LTE-U device. Often in literature the interference in the LTE-U scenario is modeled as white Gaussian noise. There is a need to understand the properties of interference in such scenarios more accurately by taking into account the physical layer specifications of the LTE and the WiFi standards. To this end, we analyze the interference generated by a WiFi transmitter at an LTE receiver and characterize its correlation properties. We show that the interference powers across the LTE subcarriers exhibit a periodic behavior and that this can be exploited to develop sensing schemes selective to WiFi signals.