The Dual-function radar communication (DFRC) design aims to exploit the tractability and reusability of both radar and communication systems’ components, parameters, and spectrum to achieve an integrated system. In this work, we propose a novel DFRC signal design by leveraging the phenomenon of the MIMO beampattern invariance, which was comprehensively explored in our previous work. It is demonstrated that the flexibility afforded by this beampattern invariance can be used to construct an integrated waveform while maintaining the required beam. A beampattern-preserving unitary transformation of the MIMO weight matrix is proposed to realize a desired information-bearing symbol at the communication receiver. The proposed method has the following promising features: 1) low-complexity, closed-form solution for the transmitted signals; 2) flexibility of modulation scheme; 3) independence from the type of orthonormal MIMO waveforms; 4) independence from the beampattern design; 5) adaptability for multi-target and multi-user scenarios. Using this generic signal design method, a number of modulation schemes are implemented. Furthermore, the impact of this operation on the estimation accuracy of the target parameters is analyzed by computing their respective Cramer-Rao Lower Bounds (CRLBs). Simulation results show that modulation schemes yielding a range of symbol error rate (SER) performances can be implemented while keeping the radar parameter estimation unaffected.

The conventional method of designing the desired beampattern in MIMO systems involves the computation of the waveform covariance and/or weight matrices. In this work, through some simple derivations, it is demonstrated that there are infinitely many covariance matrices that yield the same beampattern. Furthermore, when beam is created using weighted sums of orthogonal waveforms, there are infinitely many weight matrices that also generate the same beampattern. Thus, the conditions for the beampattern invariance are formulated with respect to the covariance and the weight matrices. This theoretical foundation allows the transmitted waveform to be altered, without changing the beampattern and the orthogonal radar waveforms. Methods for the computation of beampattern-invariant covariance and weight matrices are proposed. Consequently, it is demonstrated that there is additional degrees of freedom in the design space of many applications, such as the dual-function radar communication (DFRC), in which information can be embedded in radar transmissions while keeping the radar function intact. Other potential applications are peak-to-average power ratio (PAPR) reduction at the radar transmitter and deceptive jamming avoidance.

In this paper, we comprehensively investigate the achievable rates of selected band-limited intensity modulation schemes, which are important for optical wireless communication applications, while accounting for the specific nature of their signal construction (non-negative, real, and baseband), and imposing identical bandwidth and average optical power constraints. Furthermore, we identify/devise methods to effectively trade between these parameters. Three variants of orthogonal frequency division multiplexing (OFDM), namely, asymmetrically clipped optical OFDM (ACO-OFDM), spectrally and energy efficient OFDM (SEE-OFDM), and dc-biased optical OFDM (DCO-OFDM), and single-carrier pulse amplitude modulation are studied. The clipping noise in ACO-OFDM and SEE-OFDM is found to consume a large excess bandwidth. The detrimental effects of this excess bandwidth on the achievable rate are evaluated. For SEE-OFDM, the problem of optimal power allocation among its components is formulated and solved using the Karush-Kuhn-Tucker method. For DCO-OFDM, the clipping noise is modeled and incorporated in the analysis. Among the existing schemes, DCO-OFDM yields the best overall performance, due to its compact spectrum. In order to improve the achievable rate, we propose and analyze two improved distortionless variants, filtered ACO-OFDM and filtered SEE-OFDM (FSEE-OFDM), which yield better spectral efficiency than ACO-OFDM and SEE-OFDM, respectively. FSEE-OFDM, being the most spectrally efficient, outperforms all schemes.

Information-theoretic and signal processing aspects of some modulation schemes designed for intensity modulation/direct detection (IM/DD) optical wireless communication (OWC) systems are studied. Due to the constraints of IM/DD signals (non-negative real and baseband signals), the construction of these signals along with their time and frequency characteristics differ from their RF counterparts. This necessitates a careful study of such schemes under practical constraints. Three schemes are studied in this paper, namely, single carrier pulse amplitude modulation (SC-PAM), asymmetrically clipped optical OFDM (ACO-OFDM), and DC biased optical OFDM (DCO-OFDM). Our aim is to carry out a comparative study of these schemes in the presence of identical constraints on bandwidth and average optical power. The study reveals that the clipping operation required in ACO-OFDM significantly reduces its information rate, and as a result, it is outperformed by SC-PAM. Such a limitation does not apply to DCO-OFDM which has a higher information rate, even though part of the available optical power is expended in the non-information-bearing DC bias.

Approximate multipliers are gaining importance in energy-efficient computing and require careful error analysis. In this paper, we present the error probability analysis for recursive approximate multipliers with approximate partial products. Since these multipliers are constructed from smaller approximate multiplier building blocks, we propose to derive the error probability in an arbitrary bit-width multiplier from the probabilistic model of the basic building block and the probability distributions of inputs. The analysis is based on common features of recursive multipliers identified by carefully studying the behavioral model of state-of-the-art designs. By building further upon the analysis, Probability Mass Function (PMF) of error is computed by individually considering all possible error cases and their inter-dependencies. We further discuss the generalizations for approximate adder trees, signed multipliers, squarers and constant multipliers. The proposed analysis is validated by applying it to several state-of-the-art approximate multipliers and comparing with corresponding simulation results. The results show that the proposed analysis serves as an effective tool for predicting, evaluating and comparing the accuracy of various multipliers. Results show that for the majority of the recursive multipliers, we get accurate error performance evaluation. We also predict the multipliers' performance in an image processing application to demonstrate its practical significance.

Approximate adders are widely being advocated as a means to achieve performance gain in error resilient applications. In this paper, a generic methodology for analytical modeling of probability of occurrence of error and the Probability Mass Function (PMF) of error value in a selected class of approximate adders is presented, which can serve as performance metrics for the comparative analysis of various adders and their configurations. The proposed model is applicable to approximate adders that comprise of subadder units of uniform as well as non-uniform lengths. Using a systematic methodology, we derive closed form expressions for the probability of error for a number of state-of-the-art high-performance approximate adders. The probabilistic analysis is carried out for arbitrary input distributions. It can be used to study the dependence of error statistics in an adder's output on its configuration and input distribution. Moreover, it is shown that by building upon the proposed error model, we can estimate the probability of error in circuits with multiple approximate adders. We also demonstrate that, using the proposed analysis, the comparative performance of different approximate adders can be correctly predicted in practical applications of image processing.

Approximate adders are widely being advocated for developing hardware accelerators to perform complex arithmetic operations. Most of the state-of-the-art accuracy configurable approximate adders utilize some integrated Error Detection and Correction (EDC) circuitry. Consequently, the accumulated area overhead due to the EDC (integrated within individual adders) is significant. In this paper, we propose a low-cost Consolidated Error Correction (CEC) unit, that essentially corrects the accumulated error at the accelerator output. The proposed CEC is based on a mathematical model of approximation error. We integrate our CEC unit in approximate hardware accelerators deployed in different applications to demonstrate its area savings and speed enhancement compared to state-of-the-art.