Tractable Radar Waveform Design Under Practical Constraints

Open Access
Author:
Aldayel, Omar Saad
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 18, 2017
Committee Members:
  • Vishal Monga, Dissertation Advisor
  • Vishal Monga, Committee Chair
  • Julio Urbina, Committee Member
  • Kenneth Jenkins, Committee Member
  • Necdet Serhat Aybat, Outside Member
Keywords:
  • Radar
  • waveform design
  • convex optimization
  • Ambiguity Function Shaping
  • SQR
  • SCF
  • BIC
  • ASR
  • beampattern design
  • constant modulus constraint
  • similarity constraint
  • spectral constraint
Abstract:
Waveform design is one of the central aspects of radar systems. It can determine many of the radar properties. A well-designed waveform can improve the signal-to-interference-plus-noise ratio (SINR), enable suitable delay (range) resolution, and utilize the spectrum efficiently. Moreover, for multiple array radar system, waveform diversity can be employed to enhance the flexibility of the transmit beampattern design and enable efficient management of radar radiation power in directions of interest. While unconstrained waveform design is straightforward, a key open challenge is to enforce some of the significant practical constraints of constant modulus, waveform similarity and spectral interference constraints. Incorporating these constraints in an analytically tractable manner is a longstanding open challenge. This is due to the fact that the optimization problem subject to these constraints is a hard non-convex problem. Decades of the past work have shown a stiff trade-off between analytical tractability (achieved by relaxations to manageable constraints) and realistic design that exactly obeys these practical constraints but is computationally troublesome. In this dissertation, we propose a new framework that breaks this classical trade-off. In the first part, we address the problem of a joint transmit waveform and receive filter design for radar systems under the important practical constraints of constant modulus and waveform similarity. We develop a new analytical approach that involves solving a sequence of convex Quadratically Constrained Quadratic Programing (QCQP) problems, which we prove converges to a sub-optimal solution. Because an improvement in SINR results via solving each problem in the sequence, we call the method Successive QCQP Refinement (SQR). We evaluate SQR against other candidate techniques with respect to SINR performance, beampattern and pulse compression properties in a variety of scenarios. Results show that SQR outperforms state of the art methods that also employ constant modulus and/or similarity constraints while being computationally less burdensome. In the second part, we address the problem of designing a beampattern for Multiple-Input-Multiple-Output (MIMO) radar, which in turn is determined by the transmit waveform. A new approach is proposed in our work, which involves solving the hard non-convex problem of beampattern design using a sequence of convex equality constrained Quadratic Programs (QP), each of which has a closed form solution. The converged solution achieves constant modulus and satisfies the Karush–Kuhn–Tucker (KKT) optimality conditions, which we prove formally is possible under realistic assumptions. We evaluate the proposed successive closed forms (SCF) algorithm against state-of-the-art MIMO beampattern design techniques and show that SCF breaks the trade-off between desirable performance and the associated computation cost. Furthermore, we address the problem of designing a beampattern for MIMO radar under a spectral interference constraint, which in turn is determined by the transmitted waveform, as well as the constant modulus constraint. One key challenge when jointly enforcing the spectral interference constraint and the constant modulus constraint is to ensure feasibility of the optimization problem. A new approach is proposed in our work, which also involves solving a sequence of constrained quadratic programs, each of which results in a closed form solution. We formally prove that feasible set of each QP problem in the proposed beampattern optimization under interference constraint and the constant modulus constraint (BIC) algorithm is always non-empty. We evaluate BIC algorithm against the state-of-the-art MIMO beampattern design techniques under the constant modulus constraint and show that BIC achieves a higher performance while maintaining a low spectral interference level in the desired bands. In the final part of this dissertation, we address the problem of radar ambiguity function shaping. Ambiguity function shaping continues to be one of the most challenging open problems in cognitive radar. Analytically, a complex quartic function should be optimized as a function of the radar waveform code. We develop a new approach called Adaptive Sequential Refinement (ASR) to suppress the clutter returns for a desired range-Doppler, i.e. ambiguity function response. ASR solves the aforementioned optimization problem in a unique iterative manner such that the formulation is updated depending on the iteration index. We establish formally that: 1.) the problem in each step of the iteration has a closed form solution, and 2.) monotonic decrease of the cost function until convergence is guaranteed. Experimental validation shows that ASR perform better than state of the art alternatives even as its computational burden is orders of magnitude lower. In summary, this dissertation develop a new sequence of convex problems approaches towards solving the hard non-convex problems that appear in practical radar waveform design. Finally, we present some ideas for future research which address waveform design for cooperative radar-communication systems optimization, MIMO ambiguity function shaping and waveform design for autonomous vehicle.