Zhiping Zhang

Mixed signal detection and carrier frequency estimation based on spectral coherent features

Signal detection and RF parameter estimation have received strong interest in recent years due to the need of spectrum sensing in rapidly growing cognitive radio network research. In most of existing work, the target signal is often assumed to be a single primary user signal without overlap in spectrum with other signals. However, in a spectrally congested environment such as cognitive radio network, or in a spectrally contested environment such as a battlefield, multiple signals are often mixed together with significant overlap in spectrum. In our previous work, we have demonstrated the feasibility of using second order spectrum correlation function (SCF) cyclostationary feature to perform mixed signal detection. In this paper, we extend our work to employ a robust algorithm to detect mixed signals and estimate their carrier frequencies via spectral coherence function (SOF) features. We also evaluate the detection and estimation performances of the proposed algorithm in various channel conditions and signal mixture scenarios. Simulation results confirm the effectiveness of the proposed scheme.

The practical use of Ill-posed theory: Improved dynamic subcarrier coordinated interleaving OFDM system with pre-equalization

The ultimate goal of the security of wireless communication is to ensure the normal communication of legitimate receivers and prevent the eavesdroppers from the information interception. In this paper, based on the ill-posed theory, we present a novel pre-equalization aided scheme to improve the performance of dynamic subcarrier coordinated interleaving orthogonal frequency division multiplexing (DSCI-OFDM) system. We firstly build up the mathematical model based on the ill-posed theory, which translates the randomized signal transmission into solving ill-posed equation. Then we derive the equivalent channel for the legitimate receivers and eavesdroppers respectively. Further, we present a novel pre-equalization scheme combining the feedback channel information with OFDM modulation in the transmitter. The channel estimation error is removed and hence with our scheme, the symbol error rate (SER) performance is improved for the legitimate users. Simulations are performed to compare the SER and security performance of the proposed scheme and DSCI-OFDM system. It is demonstrated that this practical use of ill-posed theory improves the SER performance for the OFDM system, and enhance the security of DSCI-OFDM system with the proposed pre-equalization scheme.

Dual-Use Multicarrier Waveform for Radar Detection and Communication

The use of multicarrier waveforms, such as orthogonal frequency division multiplexing (OFDM) as used in radio communication, is gaining interest within the radar community. This paper considers the optimization of radar performance within the structure imposed by a coded OFDM format required to achieve an acceptable communication link. The dual goal of achieving both satisfactory radar and communication performance raises challenges that can be substantively addressed by combining phase coding and modulation techniques to provide the temporal and spectral structure necessary to implement simultaneous radar and communication operations.

RF Steganography via LFM Chirp Radar Signals

A novel radio frequency (RF) steganography scheme is proposed to hide digital communication in linear frequency modulation (LFM) radar signals. This joint radar/communication waveform serves two purposes simultaneously: it performs as the original radar waveform, and it provides a covert communication to legitimate receivers. The proposed RF steganography scheme hides digitally modulated communication information inside an LFM radar signal to prevent enemy from detecting the existence of such hidden information via a new modulation and variable symbol duration design.

Noise-suppressing chaos generator to improve BER for DCSK systems

In this paper, we propose a noise suppressing chaos generator to improve the bit error rate (BER) performances for the high-order differential chaos shift keying (DCSK) systems. The presented chaos generator outputs the overlapping chaotic sequences for the information modulation, which use a buffer for temporary storage, and the stored chaotic sequence is used to modulate the information bit while the second element of the chaotic sequence is fed back to serve as the initial value to generate chaotic sequence for the modulation of the next information bit. Namely, the different information bit may share the same chaotic segments of the whole chaotic sequence. Thus, the repetitive transmission of the same reference chaotic segments suppress the channel noise, and the BER performance of high-order DCSK systems are improved. We derive the theoretical BER expression for the proposed scheme over additive white Gaussian noise (AWGN) channel. The simulation results match the theoretical BER, and demonstrate that the BER performances have been improved with the use of noise-suppressing chaos generator.

Software Defined Radio based mixed signal detection in spectrally congested and spectrally contested environment

In a spectrally congested environment or a spectrally contested environment which often occurs in cyber security applications, multiple signals are often mixed together with significant overlap in spectrum. This makes the signal detection and parameter estimation task very challenging. In our previous work, we have demonstrated the feasibility of using a second order spectrum correlation function (SCF) cyclostationary feature to perform mixed signal detection and parameter estimation. In this paper, we present our recent work on software defined radio (SDR) based implementation and demonstration of such mixed signal detection algorithms. Specifically, we have developed a software defined radio based mixed RF signal generator to generate mixed RF signals in real time. A graphical user interface (GUI) has been developed to allow users to conveniently adjust the number of mixed RF signal components, the amplitude, initial time delay, initial phase offset, carrier frequency, symbol rate, modulation type, and pulse shaping filter of each RF signal component. This SDR based mixed RF signal generator is used to transmit desirable mixed RF signals to test the effectiveness of our developed algorithms. Next, we have developed a software defined radio based mixed RF signal detector to perform the mixed RF signal detection. Similarly, a GUI has been developed to allow users to easily adjust the center frequency and bandwidth of band of interest, perform time domain analysis, frequency domain analysis, and cyclostationary domain analysis.

High performance phase rotated FD-MC-CDMA to exploit full diversity

FD-MC-CDMA is an attractive frequency domain CDMA system that provides high performance in multi-path fading channels by exploiting both diversity gain and multi-user detection (MUD) gain. In FD-MC-CDMA, by decomposing the entire subcarrier set into multiple non-contiguous subcarrier sets, the number of interfering users within each subcarrier set is significantly reduced. As a direct result, an optimal maximum likelihood MUD receiver can be implemented at low complexity. In this paper, we first revisit previously developed phase rotation spreading code design to bring signal space diversity to conventional MC-CDMA systems. Similarly, due to the binary nature of the spreading codes, full diversity is not always exploited in FD-MC-CDMA system. Then we combine the phase rotated spreading code design scheme with FD-MC-CDMA system to exploit full diversity. With the phase rotated spreading codes and the exploitation of full diversity, the new FD-MC-CDMA system offers significant performance enhancement compared with the original FD-MC-CDMA system along with conventional MC-CDMA system. Simulation results over multi-path fading channels confirm the performance gain of the proposed scheme.

High performance phase rotated spreading codes for MC-CDMA

In conventional multi-carrier CDMA (MC-CDMA) systems, binary spreading codes such as Hadamard-Walsh codes are employed to spread user information across all subcarriers to exploit frequency diversity in frequency selective fading channels. We first recap that because of the binary nature of the spreading codes, transmission power is not distributed evenly across all subcarriers. Oftentimes, certain subcarriers have zero transmission power, leading to less diversity to be exploited at receiver. We employ a phase rotated spreading code design and derive the corresponding combining scheme for the phase rotated codes. As a direct result, MC-CDMA system now exploits full diversity available in the channel at all times, leading to significant performance gain. Simulation results over various multi-path fading channels confirm the performance gain of the scheme.