Spectral Nulling Research Articles

Spectral fitting method for designing radar sequence with spectral nulling and correlation constraints

In this study, the authors consider the radar sequence generation problem, where the sequence is required to possess the spectral nulling and low-correlation levels in crowded electromagnetic environments. The design problem of the sequence with the desired spectrum and correlation properties is formulated as the minimisation of the difference between the true ones and the desired ones, respectively. Then a spectral fitting method is proposed to solve it, by applying the time-frequency transformation. The proposed method is based solely on fast Fourier transform operations, thus it is computationally efficient. An extension to sequence sets design is also presented. Numerical simulations indicate that, compared with the state-of-the-art algorithms, the proposed method can achieve better or identical results with greatly reduced running time.

TDCS-IDMA System for Cognitive Radio Networks With Cloud

Cognitive radio (CR) is a novel technology for improving the utilization of radio electromagnetic spectrum. Recently, a new model called cognitive radio networks with cloud (CRNC) has been proposed to eliminate the constraints (pertinent to traditional CR) on power, memory space, and computational capacity. To enable multiple access of CRNC system, orthogonal frequency division multiplexing-interleave division multiple access (OFDM-IDMA) has been investigated. Traditional OFDM-IDMA may not be applied to CRNC in a straightforward manner. This is because the spectral nulling in CR and the subsequent spectral sensing mismatch (between CR transmitter and receiver) could lead to unacceptable data error performance. This paper presents a new multiple access communication system, called transform domain communication system IDMA communication system, to deal with spectral nulling problems. Two improved schemes based on BPSK and cyclic code shift keying modulations are proposed. The proposed system structure integrates the advantages of both OFDM-IDMA and CRNC, which can make full use of other users’ information while satisfying CR constraints. Simulation results demonstrate that the two proposed system architectures can achieve significant improvement on BER performance, multiple access capability, and anti-interference ability with spectral nulling.

Colocated MIMO radar waveform-design based on two-step optimizations in spatial and spectral domains

We propose an efficient colocated multiple-input multiple-output radar waveform-design method based on two-step optimizations in the spatial and spectral domains. First, a minimum integrated side-lobe level strategy is adopted to obtain the desired beam pattern with spatial nulling. By recovering the hidden convexity of the resulting fractional quadratically constrained quadratic programming non-convex problem, the global optimal solution can be achieved in polynomial time through a semi-definite relaxation followed by spectral factorization. Second, with the transmit waveforms obtained via spatial optimization, a phase changing diagonal matrix is introduced and optimized via power method-like iterations. Without influencing the shape of the optimized beam pattern, the transmit waveforms are further optimized in the spectral domain, and the desired spectral nulling is formed to avoid radar interference on the overlaid licensed radiators. Finally, the superior performance of the proposed method is demonstrated via numerical results and comparisons with other approaches to waveform design.

Mitigating interference via spatial and spectral nulling

Phase-only transmit nulling may help the next generation of radar systems operate in an over-crowded radio frequency spectrum. The reiterative uniform weight optimisation (RUWO) algorithm generates constant modulus radar waveforms with spatial, frequency or space-frequency nulls for multiple-input multiple-output (MIMO) transmit arrays by utilising the maximum signal-to-interference plus noise ratio framework in a reiterative fashion. This study presents loop-back and open air experimental results, obtained using an eight-channel X-band coherent MIMO radar test bed, that demonstrate the use of RUWO to generate constant modulus waveforms that possess spatial and/or frequency nulls. The RUWO algorithm is first used to deterministically generate phase-only weights that produce spatial, frequency or space-frequency nulls at baseband in a loop-back configuration. The algorithm is then utilised to generate weights to produce spatial nulls, both deterministically and adaptively, for the full eight-channel test bed in an open air configuration. The deterministic and adaptive strategies are compared; results demonstrate that RUWO can be used to create spatial nulls and highlight the need for careful calibration of both the transmitter and the receiver.

Spectral Nulling on Transmit via Nonlinear FM Radar Waveforms

Modern radars operate in electromagnetic environments crowded with other RF users. Adaptive spectral nulling on transmit can reduce interference from and to these other RF users. An analytical theory is developed for spectral nulling by a minimal adjustment, or offset, to the phase of the radar pulse that maintains constant pulse amplitude. Numerical examples show that a small time-varying phase offset designed by the proposed method can produce deep spectral nulls at one or more chosen frequencies in the radar's spectral sidelobes with little other effect on the pulse spectrum or ambiguity function.

Channel noise robust transmit power optimized linear precoder for ISI channel with minimal complexity equalizer

A special class of linear precoder-equalizer pair with equalizers formed by simple tapped delay lines is proposed. With this special equalizer structure, the equalizer of this system not only can enjoy the minimal implementation complexity, but also can avoid the noise amplification effect. Moreover, using nonmaximally decimated filterbank structure, this precoder-equalizer pair is proven to be able to guarantee intersymbol interference (ISI)-free communication by zero-forcing approach, even if spectral nulling exists in the physical channel. Another advantage of this system is that the length of the guard period introduced by the precoder can be any positive integer values, thus the guard period can be designed to maximize the spectral efficiency by selecting it to be as short as one symbol per block of transmitted signal. A systematic method for designing a minimal transmit power finite-impulse response (FIR) precoder under the proposed precoder-equalizer pair structure is presented. Under the ISI channel with independent additive white Gaussian noise, the design of this minimal transmit power FIR precoder is shown to be a minimum mean square error precoder design. Simulation results are presented to illustrate the performance of the proposed precoder-equalizer structure and that of the transmit power minimized precoder.

An adaptive algorithm for stable decision-feedback filtering

A method of decision-feedback filtering that is particularly well suited for data that have undergone spectral nulling is introduced. The lack of spectral content causes a large eigenvalue spread of the data correlation matrix, with deleterious effects on the performance of conventional adaptive algorithms. An adaptive decision-feedback equalizer update algorithm that does not have these undesirable properties is presented. The algorithm calculates an iterative regularized inverse solution for the decision-feedback equalizer coefficients and applies appropriate attenuation (regularization) at frequencies lacking signal energy. This automatic estimation of the significant signal components provides an improvement over other methods that use matrix arithmetic to perform the same task. The other features of the algorithm are linear computational complexity and fast tracking capability. Simulation results that compare the algorithm to the conventional LMS and RLS algorithms are presented. >