In this paper, the problem of reducing the peak-to-average power ratio (PAPR) of a discrete Fourier transform-spread orthogonal frequency-division multiplexing signal is considered for the transmission of constellation-rotated QPSK symbols. In particular, a joint optimization of the constellation-rotation angle and the pulse-shaping vector is conducted, where the number of allocated subcarriers is larger than the number of QPSK symbols. To avoid the exponential complexity required to evaluate the exact PAPR, a computationally efficient upper bound on the worst-case PAPR is presented and adopted as the objective function. The signal-to-interference-plus-noise ratio (SINR) at the output of a constellation-derotating matched filter is also presented. Then, joint optimizations are conducted through numerical search for various target SINRs. Surprisingly, the optimal rotation angle is close to but not exactly equal to π/4. Numerical results show that the proposed technique is not only applicable to QPSK symbols but also to QAM symbols to significantly reduce the PAPR.

In this paper, the problem of further reducing the peak-to-average power ratio (PAPR) of a discrete Fourier transform-spread orthogonal frequency-division multiplexing signal is considered for the transmission of π/2-BPSK symbols in the 5G New Radio specifications. In particular, a joint optimization of the constellation-rotation angle and the pulse-shaping vector is conducted. To avoid the exponential complexity required to evaluate the PAPR, a computationally efficient upper bound on the worst-case PAPR is presented and adopted as the objective function. The signal-to-interference-plus-noise ratio (SINR) at the output of a constellation-derotating matched filter is also presented. Then, joint optimizations are conducted through numerical search for various target SINRs. Surprisingly, the optimal rotation angle is close to but not exactly equal to π/2. It is shown that the proposed combinations significantly outperform the existing combinations of the rotation angle and the shaping vector when the signal is amplified by a nonlinear PA.

Recently, as asynchronous heterogeneous network scenario becoming one of the key features for next generation wireless communications, superior spectrum confinement as well as higher spectral efficiency compared to cyclic prefixed orthogonal frequency division multiplexing (CP-OFDM) has been taken into consideration for future radio access technologies. In this paper, we propose a new quadrature amplitude modulation based filter-bank multi-carrier (QAM-FBMC) system that provides with an inherent spectral efficiency gain against CP-OFDM, which comes from reduction of the redundancies such as cyclic prefix (CP) and guard-band, while keeping the symbol rate same as in CP-less OFDM. We propose a new transceiver structure consisting of at least two different filter-bank bases at both transmitter and receiver sides. Practical algorithms including channel estimation and equalization to mitigate multi-path fading channel without CP are proposed. Various evaluation results show that the proposed system performs comparable to the CP-OFDM system even without CP and guard-band reduction is also available from the well-confined spectrum.

As a promising candidate to replace cyclic prefixed orthogonal frequency division multiplexing (CP-OFDM), a new quadrature amplitude modulation filter-bank multi-carrier (QAM-FBMC) has been proposed for enabling spectral efficiency increase and spectrum confinement while keeping near orthogonality. QAM transmission can combat intrinsic interference of conventional offset-QAM (OQAM)-FBMC, which is main bottleneck in channel estimator and multiple antenna transceiver design. Compared to existing single filter based QAM-FBMC, the new QAM-FBMC has more optimization freedom in multiple prototype filters to increase self signal-to-interference ratio (SIR). In this paper, the possibility of per-tone multiple input multiple output (MIMO) reception in the new QAM-FBMC system is exploited. Our MIMO receiver is constructed with per-tone MIMO equalization in oversampled signal domain and its soft-demapper in symbol domain. Not only the separate log likelihood ratio (LLR) calculation per stream but also joint LLR calculation (i.e., soft maximum likelihood (ML) detection) is implemented with pre-processing. Simulation results show that the per-tone MIMO reception in QAM-FBMC is comparable to that of CP-OFDM.

In this paper, the sum-rate optimal restricted FDMA system is investigated for asymmetric-power users, when they have individual upper limits on the bandwidths in addition to a constraint on total system bandwidth. A convex optimization problem is formulated to optimally allocate the system bandwidth to the users. First, the users are classified into oversized and non-oversized users, based on the strength of their minimal power spectral densities (PSDs). Then, the maximum sum rate and the unique optimal solution in a closed algorithmic expression are derived by using the Karush-Kuhn-Tucker optimality conditions. An iterative algorithm to construct the optimal solution is also presented that provides insights into the restricted FDMA system. Interestingly, the non-oversized users end up with an equal PSD among one another, which is always less than the PSDs of the oversized users, if they exist. It turns out that the sum-rate optimal restricted FDMA system achieves the sum capacity of the band-limited multiple-access channel if and only if there is no oversized user.

In this paper, the sum rate of a multicode code-division multiple-access (CDMA) system with asymmetric-power users is maximized, given a processing gain and the power profile of users. Unlike the sum-rate maximization for a single-code CDMA system, the optimization requires the joint optimal distribution of each user's power to its multiple data streams as well as the optimal design of signature sequences. The crucial step is to establish an equivalence of the multicode CDMA system to restricted frequency-division multiple-access (FDMA) and time-division multiple-access (TDMA) systems. The CDMA system has upper limits on the numbers of multicodes of users, whereas the FDMA system has upper limits on the bandwidths, and the TDMA system has upper limits on the duty cycles of users. The equivalence facilitates the complete characterization of the maximum sum rate of the multicode CDMA system and also provides new insights into single- and multicode CDMA systems in terms of the parameters of the equivalent FDMA and TDMA systems.

In this paper, the design of an overlay system is considered that operates on a channel already occupied by a legacy system. It is assumed that the legacy transmitter generates a cyclostationary signal, and that the legacy receiver is equipped with a linear filter followed by a uniform sampler. Under a zero-interference constraint at the legacy receiver, the meansquared error at the overlay receiver is minimized to find the optimal overlay system that employs linear modulation and demodulation. For the trade-off between performance and rate of the overlay system, the problem is formulated to allow a transmission rate that is a fraction of the fundamental cycle frequency of the legacy signal. The notion of a virtual legacy receiver is devised to incorporate cases with non-identical frequency bands for the transmission and reception of the overlay signal. Using the vectorized Fourier transform technique, the problem is reformulated, the solution is derived, and a necessary and sufficient condition for the existence of the optimal solution is found, all in the frequency domain. Numerical results show that the proposed scheme significantly improves the spectral efficiency by effectively exploiting the cyclostationarity of the legacy signal.

In this paper, we consider overlaying a secondary-user signal over a satellite communication channel occupied by a primary user. The objective is to find the jointly optimal transmitter and receiver pair of a secondary system to minimize the mean squared error (MSE) at the output of the secondary receiver, subject to zero interference induced at the output samples of the primary receiver. It is assumed that both the primary and the secondary transmitters employ linear modulation and the receivers employ linear filter front-ends. Unlike prior work, the secondary user is allowed to transmit data symbols at a fraction of the primary user's symbol rate. This rate reduction technique enables the secondary user to achieve the additive white Gaussian noise (AWGN) bound or the performance close to the AWGN bound even when the excess bandwidth of the primary user is very small but positive. Numerical results show that the proposed scheme significantly improves the spectral efficiency by exploiting the cyclostationarity of the primary-user signal.

In this paper, the jointly optimal transmitter and receiver pair of an overlay system is derived to minimize the mean squared error (MSE) at the output of the linear receiver, subject to zero interference induced to legacy systems already existing in the frequency band of interest. It is assumed that the overlay and the legacy systems all employ linear modulation with the same symbol transmission rate but they are not necessarily symbol- synchronized. The symbol sequences are modeled as wide-sense stationary (WSS) colored random processes and the channels are modeled as strictly band-limited linear time-invariant systems with frequency selective impulse responses. Using the vectorized Fourier transform (VFT) technique, the optimal transmit and receive waveforms are derived in the frequency domain. Numerical results show that the proposed system significantly improves the bit error rate (BER) performance over the systems with receiver-only optimization and the systems with no transmitter and receiver optimization.