Salil Kashyap

Antenna selection in LTE: from motivation to specification

Transmit antenna selection technology has been adopted for the uplink by the next generation Long Term Evolution wireless standard in order to harness the spatial diversity offered by multiple antennas at the mobile transmitter, while keeping the hardware complexity and cost of a mobile low. In TAS, the number of RF chains for processing/up-conversion is smaller than the number of available antenna elements, so at any time the signals can only be transmitted from a (dynamically optimized) subset of antenna elements. As a result, the training procedure for AS needs to be carefully engineered. In LTE, this is accomplished by reusing the wideband sounding reference signal for the purpose of AS training. Furthermore, new mechanisms are required to facilitate feedback from the receiver (the base station) to the transmitter about which subset is optimal and should thus be used by the mobile. In LTE, this is accomplished by employing a unique masking technique on the downlink control channel that eliminates the feedback overhead at the expense of a minor increase in complexity at the mobile. This article provides an in-depth and systematic overview of all physical and higher layer features in the LTE standard that enable TAS. Also highlighted are the variety of technical and standardization challenges that drove the specification of AS in LTE, and the aspects of the LTE standard that are impacted by AS.

On the Feasibility of Wireless Energy Transfer Using Massive Antenna Arrays

In this paper, we examine the feasibility of wireless energy transfer (WET) using arrays with multiple antennas. Specifically, we compute the probability of outage in energy transfer over a Rician fading channel when the base station (BS) with multiple antennas transfers energy to a wireless sensor node (WSN). Through our analytical and numerical results, we prove that by deploying more antennas at the BS, the range of WET can be increased while maintaining a target outage probability. We observe that the use of massive antenna arrays at the BS results into huge savings of radiated energy. We show that for typical energy levels used in WET, the outage performance with imperfect channel state information (CSI) is essentially the same as that obtained based on perfect CSI. We also observe that a strong line-of-sight component between the BS and the WSN lowers the probability of outage in energy transfer.

SEP-Optimal Transmit Power Policy for Peak Power and Interference Outage Probability Constrained Underlay Cognitive Radios

In underlay cognitive radio (CR), a secondary user (SU) can transmit concurrently with a primary user (PU) provided that it does not cause excessive interference at the primary receiver (PRx). The interference constraint fundamentally changes how the SU transmits, and makes link adaptation in underlay CR systems different from that in conventional wireless systems. In this paper, we develop a novel, symbol error probability (SEP)-optimal transmit power adaptation policy for an underlay CR system that is subject to two practically motivated constraints, namely, a peak transmit power constraint and an interference outage probability constraint. For the optimal policy, we derive its SEP and a tight upper bound for MPSK and MQAM constellations when the links from the secondary transmitter (STx) to its receiver and to the PRx follow the versatile Nakagami-m fading model. We also characterize the impact of imperfectly estimating the STx-PRx link on the SEP and the interference. Extensive simulation results are presented to validate the analysis and evaluate the impact of the constraints, fading parameters, and imperfect estimates.

Can wireless power transfer benefit from large transmitter arrays

In this paper, we illustrate the potential benefits of using large transmitter arrays for wireless power transfer. Specifically, we analyze the probability of outage in energy transfer over fading channels when the base station (BS) with multiple antennas beamforms energy to a wireless sensor node. Our analytical and numerical results show that by using large transmitter arrays, the range of wireless power transfer can be increased while maintaining a target outage probability. We also observe and quantify that by using multi-antenna arrays at the BS, a lower downlink energy is required to get the same outage performance.

Optimal Binary Power Control for Underlay CR With Different Interference Constraints and Impact of Channel Estimation Errors

Adapting the power of secondary users (SUs) while adhering to constraints on the interference caused to primary receivers (PRxs) is a critical issue in underlay cognitive radio (CR). This adaptation is driven by the interference and transmit power constraints imposed on the secondary transmitter (STx). Its performance also depends on the quality of channel state information (CSI) available at the STx of the links from the STx to the secondary receiver and to the PRxs. For a system in which an STx is subject to an average interference constraint or an interference outage probability constraint at each of the PRxs, we derive novel symbol error probability (SEP)-optimal, practically motivated binary transmit power control policies. As a reference, we also present the corresponding SEP-optimal continuous transmit power control policies for one PRx. We then analyze the robustness of the optimal policies when the STx knows noisy channel estimates of the links between the SU and the PRxs. Altogether, our work develops a holistic understanding of the critical role played by different transmit and interference constraints in driving power control in underlay CR and the impact of CSI on its performance.

Novel scheme of video steganalysis for detecting antipodal watermarks

In this paper we present a frame by frame steganalysis scheme for digital video sequences based on average filtering of frame. The goal of steganalysis is to detect and/or estimate potentially hidden information from observed data with little or no knowledge about steganography algorithm and/or its parameters. Our blind scheme is successful in detecting hidden information of antipodal nature.

On the feasibility of wireless energy transfer using massive antenna arrays in Rician channels

In this paper, we examine the feasibility of wireless energy transfer (WET) using arrays with multiple antennas. Specifically, we compute the probability of outage in energy transfer over a Rician fading channel when the base station (BS) with multiple antennas transfers energy to a wireless sensor node (WSN). Through our analytical and numerical results, we prove that by deploying more antennas at the BS, the range of WET can be increased while maintaining a target outage probability. We observe that the use of massive antenna arrays at the BS results into huge savings of radiated energy. We show that for typical energy levels used in WET, the outage performance with imperfect channel state information (CSI) is essentially the same as that obtained based on perfect CSI. We also observe that a strong line-of-sight component between the BS and the WSN lowers the probability of outage in energy transfer.

Power Gain Estimation and Its Impact on Binary Power Control in Underlay Cognitive Radio

We propose a novel minimum mean square error estimator that estimates the channel power gain of the link from the secondary transmitter to the primary receiver (PRx). It lowers the root mean square error compared to several other estimators used in the underlay cognitive radio literature that first estimate the channel amplitude. We then analyze its system impact for two types of interference constraints. To this end, for the optimal binary transmit power control policy, we derive closed-form expressions for the average interference and the probability that the interference at the PRx violates a peak interference constraint with the proposed estimator. We show that the proposed estimator performs closer to the perfect channel state information scenario compared to the other estimators.

Joint Antenna Selection and Frequency-Domain Scheduling in OFDMA Systems with Imperfect Estimates from Dual Pilot Training Scheme

Transmit antenna selection (AS) has been adopted in contemporary wideband wireless standards such as Long Term Evolution (LTE). We analyze a comprehensive new model for AS that captures several key features about its operation in wideband orthogonal frequency division multiple access (OFDMA) systems. These include the use of channel-aware frequency-domain scheduling (FDS) in conjunction with AS, the hardware constraint that a user must transmit using the same antenna over all its assigned subcarriers, and the scheduling constraint that the subcarriers assigned to a user must be contiguous. The model also captures the novel dual pilot training scheme that is used in LTE, in which a coarse system bandwidth-wide sounding reference signal is used to acquire relatively noisy channel state information (CSI) for AS and FDS, and a dense narrow-band demodulation reference signal is used to acquire accurate CSI for data demodulation. We analyze the symbol error probability when AS is done in conjunction with the channel-unaware, but fair, round-robin scheduling and with channel-aware greedy FDS. Our results quantify how effective joint AS-FDS is in dispersive environments, the interactions between the above features, and the ability of the user to lower SRS power with minimal performance degradation.

Frequency-domain interpolation of the zero-forcing matrix in massive MIMO-OFDM

We consider massive multiple input multiple output (MIMO) systems with orthogonal frequency division multiplexing (OFDM) that use zero-forcing (ZF) to combat interference. To perform ZF, large dimensional pseudo-inverses have to be computed. In this paper, we propose a discrete Fourier transform (DFT)-interpolation-based technique where substantially fewer ZF matrix computations have to be done with very little deterioration in data rate compared to computing an exact ZF matrix for every subcarrier. We claim that it is enough to compute the ZF matrix at L(≪ N) selected subcarriers where L is the number of resolvable multipaths and N is the total number of subcarriers and then interpolate. The proposed technique exploits the fact that in the massive MIMO regime, the ZF impulse response consists of L dominant components. We benchmark the proposed method against full inversion, piecewise constant and linear interpolation methods and show that the proposed method achieves a good tradeoff between performance and complexity.