By precoding multiple data bocks before cyclic-prefixed orthogonal frequency-division multiplexing (CP-OFDM) modulation, the rectangularly-pulsed CP-OFDM frame modulation structure has been adopted to achieve better error performance than CP-OFDM over linear time-varying multipath (LTV) channels. Typical embodiments include cyclic-preflxed orthogonal time frequency space (CP-OTFS), cyclic-preflxed orthogonal time sequency multiplexing (CP-OTSM), cyclic-preflxed affine frequency division multiplexing (CP-AFDM), cyclic-preflxed single carrier (CP-SC), and cyclic-preflxed anchor-aided single carrier (CP-ASC). These CP-OFDM frame modulations show various desirable but not simultaneously existing features, including resistance to double channel dispersion, low peak-to-average power ratio (PAPR), and high spectral compactness. By incorporating anchor-symbol insertion, selected mapping, and uniform data spreading into the CP-OFDM frame modulation structure, cyclic-preflxed anchor-aided time sequency multiplexing (CP-ATSM) is proposed in this paper. CP-ATSM performs comparably with CP-OTFS and CP-OTSM and outperforms CP-AFDM, CP-SC, and CP-ASC in uncoded error performance over LTV channels, while offering reduced PAPR compared to CP-OTFS, CP-OTSM, and CP-AFDM. Besides, CP-ATSM performs comparably with CP-ASC and outperforms CP-OTFS, CP-OTSM, CP-AFDM, and CP-SC significantly in spectral efficiency.

In this paper, a novel analytical model for resource allocation is proposed for a device- to-device (D2D) assisted cellular network. The proposed model can be applied to underlay and overlay D2D systems for sharing licensed bands and offloading cellular traffic. The developed model also takes into account the problem of unlicensed band sharing with Wi-Fi systems. In the proposed model, a global system state reflects the interaction among D2D, conventional cellular, and Wi-Fi packets. Under the standard traffic model assumptions, a threshold-based flow control is proposed for guaranteeing the quality-of-service (QoS) of Wi-Fi, The packet blockage probability is then derived. Simulation results show the proposed scheme sacrifices conventional cellular performance slightly to improve overlay D2D performance significantly while maintaining the performance for Wi-Fi users. Meanwhile, the proposed scheme has more flexible adjustments between D2D and Wi-Fi than the underlay scheme.

In uplink cellular environments, random access preamble detection (RAPD) is achieved by identifying possibly multiple received requesting orthogonal frequency-division multiplexing (OFDM) signals carrying arbitrarily-chosen constant-amplitude (CA) preamble sequences through threshold comparison. Such threshold-based RAPD is analytically studied in terms of false alarm, outage, and false identification probabilities for the receiver operating under independent Rician/Rayleigh multipath channels and in the presence of time offsets among multiple received requesting OFDM signals. The RAPD system is shown to operate under less influence of inter-sequence interference when the preamble sequence set consists of mutually orthogonal CA sequences and contains as many ϖmin-permissible cyclically-shiftable constant-amplitude (CSCA) sequences as possible, where assigning (MPMA) and Zadoff-Chu sequence families, which aremin-permissible CSCA sequences are cyclically shifted from each other with a minimum cyclic-shift-distance ϖminin time domain. The performance characteristics are thus demonstrated for the RAPD systems adopting the modified phase-model-assigning (MPMA) and Zadoff-Chu sequence families, which are comprised of orthogonal and nonorthogonal, respectively, ϖmin-permissible CSCA sequence subfamilies. While constraining upper false-alarm and outage probability bounds, the system adopting the MPMA sequence family is shown to provide much smaller false-identification probability than the system adopting the Zadoff-Chu sequence family.

Constant-amplitude (CA) sequences are efficacious in the construction of multicarrier sounding waveforms in order to facilitate simultaneous channel estimation (SCE) in uplink multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The permissible single-root length-N Zadoff-Chu sequences that are cyclically shiftable with a minimum cyclic shift distance (CSD) ${\varpi _{{\text{min }}}}$ in time domain are widely adopted to enable the separation of simultaneously received sounding OFDM waveforms and thus achieve accurate SCE when at most $\left\lfloor {N/{\varpi _{\min }}} \right\rfloor $ MIMO channels are estimated simultaneously. To enable the large-size SCE of more than $\left\lfloor {N/{\varpi _{\min }}} \right\rfloor $ MIMO channels, nonorthogonal multiple-root Zadoff-Chu sequences have to be adopted and this degrades the SCE performance remarkably due to heavy inter-sequence interference (ISI). In this paper, various CA sequence families are thus investigated for performance prevalence in the large-size SCE over independent Rician multipath channels. It is analytically shown that the large-size SCE can be achieved with less influence from ISI when the adopted CA sequence family is composed of orthogonal sequence subfamilies, each containing as many permissible cyclically-shiftable CA sequences as possible.

In this paper, coarse initial time synchronization (CITS) is investigated for orthogonal time frequency space systems operating over the linear time-varying multipath channel exhibiting real-valued delay and Doppler shifts on the delay-Doppler grid. By acquiring a comb-type preamble frame waveform carrying the discrete-Fourier-transform sequence, a particular CITS system is developed to estimate the start time of a received preamble frame on the leading channel path. Based on the cross-ambiguity between the received signal and local discrete-frequency-shifted comb-type preamble signals for discrete time shifts, the CITS system can operate over a wide timing acquisition range and reach a coarse accuracy scale under the influence of data self-interference. When the channel exhibits a strong leading path and wide ranges of Doppler shifts, the comb-type CITS system is shown to outperform the conventional CITS systems acquiring a linear frequency modulated preamble waveform and a hybrid data/preamble frame waveform, significantly in estimation accuracy and robustness.

Iterative decision-feedback equalization is commonly employed in single-carrier frequency-division-multiplexing to suppress the intersymbol interference (ISI) caused during the equalized symbol decision process while avoiding significant noise enhancement, and thereby provides satisfactory error performance. Conventional algorithms have been designed based on the same equalization criterion in all iterative steps and thus leave the room for performance enhancement in that the reduced levels of residual ISI in ensuing iterative steps are not taken into account. In this paper, an iterative equalization algorithm is proposed based on various equalization criteria in different iterative steps and shown to improve error performance in equalized symbol decision over Rician multipath channels.

Rectangularly-pulsed orthogonal time frequency space (OTFS) can be efficiently signaled by a frame of orthogonal frequency-division multiplexing (OFDM) block signals and outperforms OFDM in error performance over doubly selective channels. However, due to the discontinuity in rectangular pulse, the rectangularly-pulsed OTFS suffers large power spectral sidelobes decaying as $f^{-2}$, with f being the frequency in Hertz. In this paper, spectral precoding is integrated with rectangularlypulsed OTFS to make the compact spectrum rendering small sidelobes decaying as $f^{-2I-2}$ or faster. The characteristics of peak-to-average power ratio, achievable data rate, spectral efficiency, and error performance are also investigated for the spectrally-precoded OTFS.

In practical orthogonal frequency division multiplexing standards, preamble waveforms are adopted to facilitate timing synchronization, frequency synchronization and system parameter acquisition. Due to implementation simplicity, the crosscorrelation-based method is adopted to determine the transmitted preamble sequence in the literature. However, because of the lack of channel estimation, the crosscorrelation-based method suffers severe performance degradation in large delay spread channels. To solve the problem, a preamble sequence detection method based on the generalized likelihood ratio test (GLRT) principle is proposed and analyzed in the paper. Due to embedded channel estimation, the GLRT method is shown to provide the decision error performance robust to channel dispersion and outperform the conventional crosscorrelation-based method significantly in large delay spread channels, at cost of increased implementation complexity.

Cancellation symbols (CSs) are inserted into guard subcarriers to suppress the sidelobe power of the OFDM pilot waveform for channel estimation. Different from the conventional schemes where CSs are designed to suppress the sidelobe power of composite OFDM waveforms conveying data and pilot symbols, the inserted CSs depend on pilot symbols only and are shown to make the spectrally compact pilot waveform at the cost of less cancellation symbol power. The composite OFDM waveforms conveying spectrally precoded data symbols, optimum pilot symbols, and pilot-dependent CSs are shown to exhibit highly compact spectrum, while maintaining the channel estimation optimality on the quasi-static multipath channel.

The spectrally compact pilot waveform that exhibits small power spectral sidelobes and thus high spectral compactness while achieving accurate channel estimation is desirable for orthogonal frequency division multiplexing systems. However, existing pilot waveforms either suffer large power spectral sidelobes or can achieve high spectral compactness only when the length of the carried pilot sequence is limited to a constrained set of non-prime numbers, particularly those containing a large positive integer power of two as a factor. In this paper, new procedures are proposed for designing more pilot sequences with a broader choice of sequence lengths, so that spectrally compact pilot waveforms can be constructed to achieve optimal channel estimation on quasi-static multipath channels.

In practical orthogonal frequency division multi-plexing systems and standards, preamble waveforms are adopted to facilitate initial timing and frequency synchronization. These preamble waveforms are constructed from properly designed preamble sequences which possess exact or near zero periodic autocorrelation (ZAC) property to achieve robust synchronization and consist of exact or near constant amplitude (CA) symbols to achieve low waveform peak-to-average power ratio (PAPR). Unfortunately, preamble waveforms carrying conventional CAZAC sequences (which have ZAC property and consist of CA symbols) or near-CAZAC sequences result in large power spectral sidelobes and thus are not spectrally compact. In this paper, new near-CAZAC sequences are designed to construct spectrally compact OFDM preamble waveforms with robust timing and frequency synchronization and very low waveform PAPR comparable to those constructed from the conventional CAZAC sequences.

Pilot-aided channel estimation is essential to orthogonal frequency division multiplexing systems and standards. Under the system setup that data and pilot waveforms are processed individually for enhancing the spectral efficiency of the composite waveform, pilot patterns are designed here to jointly optimize channel estimation on quasistatic multipath channels and provide extremely high spectral compactness by suppressing spectral sidelobes. Specifically, a general constraint on pilot pattern is developed to endow the pilot waveform with very small spectral sidelobes and thus compact spectrum. Using the constraint, systematic multiple-stage design procedures are proposed to develop the desired pilot patterns achieving Cramér Rao bound for channel estimation on quasistatic multipath channels while providing fast decaying spectral sidelobes. Among the demonstrated pilot patterns achieving optimum channel estimation, the pilot waveforms using AI- and BI-typed pilot patterns developed from the procedures are shown to exhibit higher spectral compactness than conventional pilot waveforms. Besides, CI-typed pilot pattern that exploits extremely high spectral compactness without resort to channel estimation optimization is also developed. By sacrificing a slight loss in channel estimation, the pilot waveforms using CI-typed pilot pattern are shown to exhibit spectrums approaching the Nyquist spectrum.

In this paper, the specific-band power minimization precoding technique is incorporated with spectrally precoded OFDM to suppress near spectral sidelobes more efficiently and thus improve the spectral compactness. When the total number of redundant symbols introduced by both spectral precoder and specific-band power minimization precoder is fixed, the proposed OFDM system can find a balance between near sidelobe power suppression capability and sidelobe decaying capability to meet the desired spectral performance by properly allocating redundancy to the two precoders.

Spectrally precoded orthogonal frequency-division multiple access (OFDMA) using cyclic prefix and interleaved subcarrier allocation (IOFDMA) is a useful rectangularly pulsed OFDMA signaling format for it exploits high spectral compactness and frequency diversity characteristics. In this paper, two new spectral precoders of the largest precoding rate are developed for IOFDMA in order to provide improved spectral compactness and reduce implementation complexity, respectively. Specifically, a new correlative precoder is developed and shown to minimize the dominant spectral coefficient among all correlative precoders of the largest precoding rate and thus can construct IOFDMA signals providing extremely high spectral compactness. Besides, a multiple-stage spectral precoding technique is proposed to construct a low-complexity orthogonally precoded IOFDMA with power spectral sidelobes decaying as f^{-2L-2} for L>1, where L is the number of stages. The corresponding orthogonally precoded IOFDMA signals conveying independent zero-mean data are shown to provide the same power spectrum as existing orthogonally precoded IOFDMA signals of the largest precoding rate while requiring much less complexity.

In this letter, the orthogonal multirate (MR) modulation signal is constructed by the orthogonal multiplexing of block-precoded component modulation waveforms occupying the Nyquist bandwidth and shown to outperform the conventional nonorthogonal MR modulation signal in error performance, while giving the same bandwidth efficiency.

In this paper, a new spectral precoding scheme is developed for constant-envelope orthogonal frequency-division multiplexing (CE-OFDM) signals to provide very small power spectral sidelobes. It is shown that the proposed spectrally precoded CE-OFDM scheme can provide higher spectral compactness than previously designed spectrally precoded CE-OFDM block scheme and continuous-phase CE-OFDM scheme.

Spectrally precoded orthogonal frequency-division multiplexing with cyclic prefix (SP-CP-OFDM) and SP-CP-OFDM multiple access (SP-CP-OFDMA) are investigated in this paper. A general constraint on spectral precoding is developed for both SP-CP-OFDM and SP-CP-OFDMA signal formats with unconstrained guard ratios and arbitrary input data statistics to ensure very small power spectral sidelobes decaying asymptotically as f-2J-2, with J a predesigned integer-valued parameter. In the SP-CP-OFDM signaling format, new spectral precoders are devised to meet the constraint. It is shown that the proposed SP-CP-OFDM signals can provide much smaller spectral sidelobes than the previously designed SP-CP-OFDM signals with block partitioning and constrained guard ratios. In the SP-CP-OFDMA signaling format, a general procedure is proposed to facilitate the precoder design for SP-CP-OFDMA signals with arbitrary subcarrier allocation for each user and some spectral precoders are accordingly devised. It is shown that the proposed SP-CP-OFDMA signals can suppress sidelobe powers effectively and thus achieve higher spectral compactness than nonprecoded rectangularly-pulsed CP-OFDMA.

In this letter, a spectral sidelobe-decaying property of the Walsh-Hadamard code employed in multicarrier code-division multiple access (MC-CDMA) systems is exploited. It is shown that the sidelobe-decaying characteristic of the MC-CDMA signal can be properly described by the code index of the underlined Walsh-Hadamard codeword.