Awais Khawar

Spectrum sharing between S-band radar and LTE cellular system: A spatial approach

Spectrum sharing is a new way forward to solve spectrum scarcity problem. In this paper, we propose a spatial approach for spectrum sharing between a MIMO radar and an LTE cellular system with NBS base stations (BS). The MIMO radar and LTE share NBS interference channels i.e. Hi, i = 1,2, ..., NBS. We propose projecting the radar signal onto the null space of interference channel between the MIMO radar and LTE using our proposed interference-channel-selection algorithm, in order to have zero-interference from the MIMO radar. We select interference channel with the maximum null space i.e. arg max1≤i≤NBS dim [N(Hi)] and project the radar signal onto the null space of this channel. Our proposed spatial spectrum-sharing algorithm is radar-centric such that it causes minimum loss in radar performance by carefully selecting the interference channel and at the same time protects the ith LTE BS from the radar interference. Through our analytical and simulation results we show that the loss in the radar performance is less when the proposed interference-channel-selection algorithm is used to select the channel onto which radar signals are projected.

A projection based approach for radar and telecommunication systems coexistence

We propose projecting radar waveform onto the null space of an interference channel matrix between the radar and a communication system as a solution for coexistence of radar and communication systems in the same band. This approach assumes that the cognitive radar has full knowledge of the interference channel and tries to modify its signal vectors in such a way that they fall in the null space of the channel matrix. We investigate the effects of null space projections on radar performance and target parameter identification both analytically and quantitatively by using maximum likelihood and Cramer-Rao bound performance bounds to estimate target direction in the two cases of no null space projection and null space projection. Through simulation we demonstrate that by optimal choice of the number of antennas, the performance and target identification capabilities of radar in our method are competitive with that of traditional radar waveforms, while simultaneously guaranteeing coexistence between radar and communication systems.

MIMO radar waveform design for coexistence with cellular systems

The design of multiple-input multiple-output (MIMO) radar waveforms with specified properties has a number of applications including clutter suppression and interference mitigation. In this paper, we design radar waveforms with the aim to mitigate radar interference to communication system. We design constant envelope (CE) transmit beampattern of a MIMO radar, when it is sharing its spectrum with a cellular system, with the constraint that the waveform is in the null space of interference channel between radar and communication system. We design the desired beampattern by unconstrained nonlinear optimization of the desired covariance matrix. We consider waveform design problem for a stationary maritime MIMO radar when interference channels are changing slowly and thus they can be included in the beampattern matching optimization problem. In addition, we also consider the case when the maritime MIMO radar is moving and thus experiences interference channels that are changing fast enough to be not included in the optimization problem. For this case, we design the CE waveform first and then project it onto the null space of interference channel, before transmission, in order to have zero interference to the communication system. We demonstrate, through simulations, the effect of including the constraint of spectrum sharing or the waveform be in the null space of interference channel on the CE waveform design.

Target Detection Performance of Spectrum Sharing MIMO Radars

Future wireless communication systems are envisioned to share radio frequency spectrum with radars in order to meet the growing spectrum demands. In this paper, we address the problem of target detection by radars that project waveform onto the null space of interference channel in order to mitigate interference to cellular systems. We consider a multiple-input multiple-output (MIMO) radar and an MIMO cellular communication system with X base stations (BSs). We consider two spectrum sharing scenarios. In the first scenario, the degrees of freedom (DoF) available at the radar are not sufficient enough to simultaneously detect target and mitigate interference to X BSs. For this case, we select one BS among X BSs for waveform projection on the basis of guaranteeing minimum waveform degradation. For the second case, the radar has sufficient DoF to simultaneously detect target and mitigate interference to all X BSs. We study target detection capabilities of null-space projected (NSP) waveform and compare it with the orthogonal waveform. We derive the generalized likelihood ratio test for target detection and derive detector statistic for NSP and orthogonal waveform. The target detection performance for both waveforms is studied theoretically and via Monte Carlo simulations.

Beampattern analysis for MIMO radar and telecommunication system coexistence

In this paper, we present a beampattern analysis of the MIMO radar coexistence algorithm proposed in [1]. We extend the previous work and analyze the performance of MIMO radars by projecting finite alphabet constant-envelope waveforms onto the null-space of interference channel matrix. First, we compare and analyze the Cramér-Rao bound (CRB) on angle direction estimation. Second, we compare and analyze beampatterns of the original radar waveform and the null-projected radar waveform. Analytical and simulation results show minimal degradation of a radars angle estimation of a target and transmit-receive beampattern. We also propose methods to substantially improve angle estimation and beampatterns of a null projected radar waveform which will not only guarantee optimal performance of the radar but at the same time guarantee coexistence of the radar and communication systems.

On The Impact of Time-Varying Interference-Channel on the Spatial Approach of Spectrum Sharing between S-band Radar and Communication System

Spectrum sharing is a new approach to solve the congestion problem in RF spectrum. A spatial approach for spectrum sharing between radar and communication system was proposed, which mitigates the radar interference to communication by projecting the radar waveform onto null space of interference channel, between radar and communication system [1]. In this work, we extend this approach to maritime MIMO radar which experiences time varying interference channel due to the oscillatory motion of ship, because of the breaking of sea/ocean waves. We model this variation by using the matrix perturbation theory and the statistical distribution of the breaking waves. This model is then used to study the impact of perturbed interference channel on the spatial approach of spectrum sharing. We use the maximum likelihood (ML) estimate of targets angle of arrival to study the radars performance when its waveform is projected onto the null space of the perturbed interference channel. Through our analytical and simulation results, we study the loss in the radars performance due to the null space projection (NSP) of its waveform on the perturbed interference channel.

Coexistence Analysis Between Radar and Cellular System in LoS Channel

Sharing spectrum with incumbents such as radar systems is an attractive solution for cellular operators in order to meet the ever-growing bandwidth requirements and ease the spectrum crunch problem. In order to realize efficient spectrum sharing, interference mitigation techniques are required. In this letter, we address techniques to mitigate multiple-input-multiple-output (MIMO) radar interference at MIMO cellular base stations (BSs). We specifically look at the amount of power received at BSs when radar uses null space projection (NSP)-based interference mitigation method. NSP reduces the amount of projected power at targets that are in close vicinity to BSs. We study this issue and show that this can be avoided if radar employs a larger transmit array. In addition, we compute the coherence time of channel between radar and BSs and show that the coherence time of channel is much larger than the pulse repetition interval of radars. Therefore, NSP-based interference mitigation techniques that depend on accurate channel state information (CSI) can be effective as the problem of CSI being outdated does not occur for most practical scenarios.

A mathematical analysis of cellular interference on the performance of S-band military radar systems

In the United States, the 3500-3650 MHz band is a potential candidate for spectrum sharing between military radars and commercial cellular systems. This paper presents a framework for the analysis of radar performance under cellular interference. The impact on the performance of radar due to cellular interference is studied by deriving bounds on the probability of detection and probability of miss detection. For this purpose, we first derive the distribution of aggregate cellular interference, in a correlated shadow fading environment, at the radar receiver. We prove that the sum of interference signals from a cellular system has a log-normal distribution with probability 1. We then derive a lower bound on the probability of miss target where we consider our target to be a ship and target returns are modeled by a log-normal distribution. Along with the analytical results we also provide the corresponding simulation results showing degradation in radar performance due to interference from cellular systems.

Recognizing FM, BPSK and 16-QAM using supervised and unsupervised learning techniques

In this paper, we explore the use of supervised and unsupervised machine learning for signal classification in the joint presence of AWGN, carrier offset, asynchronous sampling and symbol intervals and correlated fast fading. Three simple features are studied to classify frequency modulation, binary phase shift keying and 16 point quadrature amplitude modulation. Support vector machines and self-organizing maps are used to classify the signals.

Spectral Coexistence of MIMO Radar and MIMO Cellular System

This paper details designing the precoder of a MIMO-radar spectrally-coexistent with a MIMO cellular system. Spectrum sharing with zero or minimal interference is achieved by using, respectively, the conventional switched null space projection (SNSP) or the newly proposed switched small singular value space projection (SSSVSP). Loss in radar target localization capability due to precoding can be compensated by using SSSVSP instead of SNSP to some extent but increasing the number of radar antenna elements is more effective.