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.

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.

This chapter focuses on the application of spectral diversity via transform domain (TD) waveform design instead of the traditional time domain formulation. The basic idea behind TD waveform design is to address interference at the transmitter and thereby minimize signalprocessing complexities at the receiver [3]. This approach is generalized within the spectrally-modulated-spectrally-encoded (SMSE) framework and includes all multicarrier waveform designs such as orthogonal frequency division multiplexing (OFDM) and multicarrier code-division multiple access (MC-CDMA) [4,5] as special cases. These multicarrier SMSE waveforms are utilized in the design of overlay and underlay cognitive radio waveforms. The vast majority of existing research on physical layer spectrum coexistence addresses cognitive radio systems, where the focus is mainly on the sharing of spectrum between multiple communication systems [2,6,7]. In this chapter, we introduce the foundational work for a transform domain communication system (TDCS) and SMSE framework, and then discuss the design of multicarrier communication and radar waveforms based on SMSE.

The use of multi-carrier waveforms, such as Orthogonal Frequency Division Multiplexing (OFDM) as used in radio communication, for radar operations has gained strong interest recently. While the authors have completed research on using multi-carrier waveforms for simultaneous radar and communications operations, this study focuses on the recent research on using these waveforms for communications. Namely, they demonstrate how to employ OFDM to modulate Multi-Frequency Complementary Phase Coded sequences for wireless communications.

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.

Inspired by the success of multi-carrier waveforms such as Orthogonal Frequency Division Multiplexing (OFDM) in radio communication, multi-carrier based radar waveform designs have gained strong interest recently. Reasons for this include their resistance to multipath fading, ability to overcome the limitations of a congested frequency spectrum, ability to exploit frequency diversity gains, and potential to perform radar and communication functions simultaneously within the same hardware using the same waveform. This paper provides recent research on multi-carrier waveform design for joint radar and communication systems. First, we employ OFDM to modulate Multi-Frequency Complementary Phase Coded (MCPC) sequences in order to improve radar range resolution performance. Next, it is shown that even though these MCPC sequences produce periodic autocorrelation sidelobes with deep nulls, their overall sidelobe level is higher than that of other traditional pulse-compression radar waveforms such as Linear Frequency Modulated (LFM) waveforms. A method, termed Polar Signal Detection, is introduced as a means of overcoming these large autocorrelation sidelobes without sacrificing range measurement resolution.

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.

Signal detection and RF parameter estimation have received great interest in recent years due to the need for spectrum sensing in rapidly growing cognitive radio and cyber security research. In most conventional signal detection and RF parameter estimation 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 or a spectrally contested environment which often occurs in cyber security applications, multiple signals are often mixed together with significant overlap in spectrum. 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, but the detection was confined to BPSK modulation. In this paper, we extend our work to QPSK modulation by using a robust algorithm to detect mixed signals and estimate their symbol rate via spectral coherence function (SOF) features. We also evaluate the detection and estimation performance of the proposed algorithm in various channel conditions and signal mixture scenarios. Simulation results confirm the effectiveness of the proposed scheme.

FD-MC-CDMA is an attractive candidate for next generation high speed aerial vehicle communication for its high spectrum efficiency and excellent BER performance. Similar to other multi-carrier transmission technologies, FD-MC-CDMA suffers significant performance degradation resulting from intercarrier interference (ICI) in high mobility environments. Particularly, because the ICI is observed from all other subcarriers, the benefit of decomposing subcarriers into non-contiguous sets diminishes. In our previous work, a parallel processing based blind frequency offset estimation and ICI cancellation method to significantly improve the BER performance in high mobility environments has been proposed. However, the proposed blind frequency offset estimation and ICI cancellation scheme relies on the assumption that there will be one global minimum to the cost function that is generated from the Euclidean distances between the true received signal vector and synthesized signal vectors. In this paper, we theoretically analyze the cost function in this blind estimation and cancellation algorithm and verify this assumption. Furthermore, by thoroughly examining the cost function in various scenarios, we also prove that this cost function is actually statistically symmetric. This cost function analysis not only help us understand the underlying principle of the effectiveness of the proposed blind frequency offset estimation and ICI cancellation scheme, it points to more sophisticated high efficiency algorithms which can significantly reduce the computational complexity.

Orthogonal Frequency Division Multiplexing (OFDM) has been considered as a strong candidate for next generation high speed aerial vehicle communication systems. However, OFDM systems suffers severe performance degradation due to inter-carrier interference (ICI) in high mobility channel, if no ICI cancellation is performed. Traditionally, training symbols have been employed in one packet to help the OFDM receiver to estimate the multi-path channel and the carrier frequency offset (CFO) between the transmitter local oscillator and the receiver local oscillator. However, in aerial vehicle communication, the relative transmitter-receiver speed changes so rapidly that it is unreasonable to assume a constant speed (and CFO) during the entire packet transmission. Hence, to accurately estimate the CFO, training symbols need to be transmitted for every OFDM symbol. Obviously, this significantly reduces OFDM throughput while adding complexity due to repeated CFO estimation. In this paper, we extend our previous work to propose a joint channel/CFO estimation and ICI cancellation algorithm. Specifically, in our previous work, we have proposed a total ICI cancellation algorithm using parallel processing for OFDM system which offers the excellent ICI cancellation and BER performance. However, in this work, perfect channel information was assumed. In this paper, we combine the channel estimation with the ICI cancellation together. The proposed general total ICI cancellation algorithm has the ability to jointly estimate the carrier frequency offset and channel information, and improve the performance significantly. Meanwhile, a serial processing is proposed to reduce the computation complexity. Simulation results in different scenarios confirm the performance of the proposed scheme in multipath fading channels for high speed aerial vehicle communication.

FD-MC-CDMA is an attractive candidate for next generation high speed aerial vehicle communication for its high spectrum efficiency and excellent BER performance. Similar to other multi-carrier transmission technologies, FD-MC-CDMA suffers significant performance degradation resulting from intercarrier interference (ICI) in high mobility environments. Particularly, because of the ICI is observed from all other subcarriers, the benefit of decomposing subcarriers into non-contiguous sets diminishes. In this paper, we propose a parallel processing based blind frequency offset estimation and ICI cancelation method for FD-MC-CDMA system to significantly improve the BER performance in high mobility environment. Specifically, by exploiting frequency offset quantization, the proposed scheme takes advantage of the orthogonality of the ICI matrix and offers excellent ICI cancelation and significant BER improvement. Moreover, the proposed scheme does not lower the transmission rate or reduce the network capacity. It is important to note that the proposed ICI cancelation scheme maintains the low complexity of optimum multi-user detection (MUD) receiver and achieves the ICI cancelation and excellent BER performance at linearly growing cost. Simulation results in AWGN channel and multi-path fading channel confirm the performance of the proposed scheme in the presence of frequency offset and in mobile channel.