Non-orthogonal multiple access (NOMA) is a novel multi-user multiplexing scheme that improves spectral efficiency for LTE enhancements and 5G. By overlapping radio resources (e.g., time/frequency/code), but assigning different powers to the multiplexed users, NOMA is able to improve the system performance and fairness. To assess the performance of downlink NOMA under realistic conditions and confirm its feasibility with real hardware, we conducted experimental trials in outdoor environments using smartphone-sized NOMA chipset-embedded devices. This paper introduces our trial setup and trial results of NOMA combined with closed-loop 4×2 SU-MIMO.

In this paper, we consider superposition coding (SC) scheme in a multiple-input multiple-output (MIMO) broadcast channel when the transmitter obtains the channel state information (CSI) through user's quantized feedback. The algorithms for the base station (BS) scheduling, including users selection, power allocation among co-scheduled users, and precoding matrix assignment are investigated. We also derive bounds for the achievable rates of the SC scheme with quantized CSI feedback and imperfect user measurement being taken into account.

In this paper, two issues in assessing the performance of superposition coding (SC) in the downlink Long Term Evolution (LTE) are investigated. The first issue is the prediction of user's signal-to-interference-plus-noise ratio (SINR) at the scheduler based on the user's channel state information (CSI) feedback, when the actual transmission scheme is different from the scheme used for CSI computation. Prediction of the SINR value is needed for the assignment of the modulation order and the code rate. The second issue is with regards to the physical layer abstraction used for modeling the system-level performance of the SC scheme on the basis of the link-level simulation results. With the physical layer abstraction, the complexity of the system level simulation is reduced significantly.

This paper consists of two parts. In the first part, asymptotic theorems about the product of certain structured random matrices are developed by means of the moment convergence theorem (MCT) and the free probability theory. This product of random matrices is a generalization of the product of a sample covariance matrix and an arbitrary Hermitian matrix. In the second part, the theoretical results obtained in the first part are applied to analyze a randomly spread asynchronous direct sequence-code-division multiple-access (DS-CDMA) system with both the number of usersKand the number of chips per symbolNapproaching infinity but the ratioK/Nkept as a finite constant. Two levels of asynchronism are considered; one is symbol-asynchronous but chip-synchronous, and the other is chip-asynchronous. Asymptotic spectral distribution (ASD) of cross-correlation matrix and asymptotic spectral efficiency are investigated. Conditions under which CDMA systems with various synchronism levels (synchronous and two levels of asynchronism) have the same performance are also established.

In cognitive radio (CR), a cognitive user must cease transmission or reduce the transmit power at the band where a primary system (PS) is detected. To this end, a popular method for orthogonal frequency division multiplexing (OFDM) based CR is the active interference cancellation (AIC) which tunes the modulation of some subcarriers to minimize the interference to PS. This paper focuses on complexity reduction and performance enhancement of AIC. The former relies on the findings that (i) many matrices in AIC depend only on the PS bandwidth and are irrelevant to where PS resides, and (ii) statistical relation among the data carried by subcarriers for interference suppression is available. For the latter, performance metric of AIC that minimizes the total residual power at the PS band is adjusted to minimizing the largest residual power, avoiding the undesirable situation that, although the total power is the least, part of the band is severely interfered. Spectrum overshoot problem occurred in an ultra-wideband (UWB)-CR system is also addressed. Simulation results demonstrate that complexity reduction of AIC induces negligible performance loss, and the new performance metric significantly reduces the number of subcarriers required to suppress the interference below a certain level.

In this paper, detection of the primary user (PU) signal in an orthogonal frequency division multiplexing (OFDM) based cognitive radio (CR) system is addressed. According to the prior knowledge of the PU signal, three detection algorithms based on the Neyman-Pearson philosophy are proposed. In the first case, a Gaussian PU signal with completely known probability density function (PDF) except for its received power is considered. The frequency band that the PU signal resides is also assumed known. Detection is performed individually at each OFDM sub-carrier possibly interfered by the PU signal, and the results are then combined to form a final decision. In the second case, the sub-carriers that the PU signal resides are known. Observations from all possibly interfered sub-carriers are considered jointly to exploit the fact that the presence of a PU signal interferers all of them simultaneously. In the last case, it is assumed no PU signal prior knowledge is available. The detection is involved with a search of the interfered band. The proposed detector is able to detect an abrupt power change when tracing along the frequency axis.

In this paper, we consider the design of a reduced- rank minimum mean-squared error (MMSE) combiner in an asynchronous multicarrier-direct sequence-code division multiple access (MC-DS-CDMA) system. It is shown that the combiner weights are related to quantities whose expressions resembles the eigenvalue moments of the interference autocorrelation matrix. To lift the burden of recomputing the weights from time to time, the weights are evaluated in a large system regime and averaged over random parameters including asynchronous delays, correlated fading coefficients and spreading sequences. Explicit expressions for the asymptotic weights of the MMSE combiner are derived. The large system setting is with the spreading gain N and user number K approaching infinity, but K/N = beta and the number of sub-carriers both finite.

In this paper, the avoidance problem in detect and avoid (DAA) technology of an orthogonal frequency division multiplexing (OFDM) based cognitive radio (CR) is addressed. The focus is on reducing the computational complexity of the active interference cancellation (AIC) technique. Three methods are proposed. These methods mainly rely on the properties that: (i) many matrices occurred in AIC depend only on the number of tones to be avoided and are irrelevant to where these tones reside, and (ii) there exists a statistical relation between the tones to be avoided and the auxiliary tones employed to aid nulling the transmitted sidelobes.

Asymptotic spectral distribution (ASD) of the cross-correlation matrix is derived for a large-system asynchronous direct sequence-code division multiple access (DS-CDMA) with random spreading sequences. The crosscorrelation matrix is formed by an infinite input symbol length. Two levels of asynchronism are considered. One is symbol-asynchronous but chip-synchronous, and the other is chip-asynchronous. The results are applicable to arbitrary chip waveforms. The existence of a nonrandom ASD is proved by moment convergence theorem, where the focus is on the existence of asymptotic eigenvalue moments (AEM) of all orders. A combinatorics approach based on noncrossing partition of set partition theory is adopted for AEM computation.

The carrier-frequency offset effect on the performance of asynchronous multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) systems with aperiodic random spreading and correlated Rayleigh fading is studied in this paper. We obtain the optimum combining filter that maximizes the signal-to-interference-plus-noise ratio (SINR) of the combined statistics and exploits correlated information among subchannels. A closed-form expression for the unconditional covariance matrix of the interference-plus-noise vector, which forms the basis of our theoretical analysis of the maximum SINR and the average bit error probability formula, is derived by averaging several random parameters including asynchronous delays, correlated Rayleigh fading, and signature sequences. The analytic results obtained are applicable to MC-CDMA with appropriate modifications. Furthermore, we show that the MC-CDMA system with a common random signature sequence over all subcarriers for a given user outperforms that with distinct sequences over different subcarriers. Finally, the performance of MC-CDMA systems using the optimum combining technique is compared with that of different combining filters in the simulation.

This paper considers a large chip-synchronous (CS) random spreading CDMA system. It is shown that, conditioned on all deterministic asynchronous delays, its asymptotic eigenvalue moments of the correlation matrix are equal to those of the symbol-synchronous (SS)-CDMA. Moreover, for a receiver in the family proposed in D. Guo, S. Verdu and L. K. Rasmussen, IEEE Inform. Theory, Dec. 2002, the output has the same distribution in large CS- and SS-CDMA, i.e. a Gaussian with the same conditional mean and variance. Furthermore, although the linear successive interference cancellation (LSIC) is not a member of receivers mentioned above, the asymptotic distribution of the output is the same Gaussian in SS- and CS-CDMA

The effects of inter-carrier interference (ICI) and aperiodic random spreading sequences on the performance of asynchronous multicarrier code-division multiple-access (CDMA) systems with correlated fading between sub-carriers are investigated in this research. To conduct theoretical analysis for the maximal ratio combining (MRC) receiver, random parameters including asynchronous delays, correlated Rayleigh fading, and spreading sequences are averaged to find the unconditional covariance matrix of the interference-plus-noise vector. We demonstrate that the ICI in the system proposed by Kondo and Milstein (1996) can be mitigated by assigning a common random spreading sequence over all sub-carriers for each user, rather than using a set of distinct spreading sequences, where code sequences are assumed perfectly time-synchronized. Moreover, the analytic expression for the bit error probability (BEP) can be obtained with the Gaussian approximation. Simulation results are used to demonstrate the accuracy of our analysis. Various design tradeoffs including the number of sub-carriers, fading correlations, ICI and multipath effect are presented in simulation. We conclude that, when properly choosing a sufficiently large number of sub-carriers, the benefit of mitigating multipath interference and exploiting potential hidden frequency diversity shall prevail the impairment brought by increased ICI.

The effects of frequency offsets and aperiodic random spreading sequences on the performance of asynchronous multicarrier code division multiple access (CDMA) systems with correlated fading are investigated in this research. Random parameters including asynchronous delays, correlated Rayleigh fading and spreading sequences are averaged to determine the covariance matrix of the interference-plus-noise vector. An analytic expression for the average signal-to-interference-plus-noise ratio (SINR) is obtained. Furthermore, average bit error probability (BEP) based on the Gaussian approximation is also derived. We show that the system degrades significantly if the frequency offset of the desired user is present, while the system is insensitive to interferers' frequency offsets. In consequence, it is crucial to estimate and compensate the desired user's synchronization imperfection. Finally, design tradeoff among the number of subcarriers, fading correlations, and inter-carrier interference (ICI) are presented in simulation results.

The effect of inter-carrier interference (ICI) and aperiodic random spreading sequences on the performance of asynchronous multicarrier code division multiple access (CDMA) systems with correlated fading is investigated in this research. Random parameters including asynchronous delays, correlated Rayleigh fading and spreading sequences are averaged to determine the unconditional covariance matrix of the interference-plus-noise vector. An analytic expression for the bit error probability is obtained based on the Gaussian approximation. Then, we extend the result by considering situations where eigenvalues are not necessarily all identical or distinct. Finally, design tradeoffs among the number of sub-carriers, fading correlations, ICI and multipath effect are presented in simulation results.

The performance of the multistage linear groupwise successive interference canceller (GSIC) in long-code direct sequence-code division multiple access (DS-CDMA) systems is analyzed in this research. We adopt a graphical approach to calculate the conditional mean and variance of the output signal of the receiver at an arbitrary stage. The computation of such statistics involves the evaluation of high order moments of large size matrices and, therefore, has a very high complexity, which increases exponentially with the number of stages of the receiver. The solution of this problem via graph theory allows very accurate evaluation of the conditional mean and variance. Simulation results show that the proposed method is able to estimate the bit error probability (BEP) with high accuracy.

The signal-to-interference-plus-noise-ratio performance of the multistage linear parallel and successive interference cancellers (LPIC and LSIC) in a long-code code-division multiple-access system is analyzed using a graphical approach. The decision statistic is modeled as a Gaussian random variable, whose mean and variance can be expressed as functions of moments of R for the LPIC and L for the LSIC, respectively, where R is the correlation matrix of signature sequences and L is the strict lower triangular part of R. Since the complexity of calculating these moments increases rapidly with the growth of the stage index, a graphical representation of moments is developed to facilitate the computation. Propositions are presented to relate the moment calculation problem to several well-known problems in graph theory, i.e., the coloring, the graph decomposition, the biconnected component finding, and the Euler tour problems. It is shown that the derived analytic results match well with simulation results.

The bit error probability of the multistage linear successive interference canceller (LSIC) in a long-code code division multiple access (CDMA) system is analyzed with a graphic approach in this research. The receiver decision statistic is modelled as a Gaussian random variable, whose conditional mean and variance are related to the moments of functions of matrix L, where L is the strict lower triangular part of the correlation matrix of the signature sequences. The complexity of calculating these moments increases rapidly with the growth of the stage index. A graphical representation of moments is developed here to facilitate their computation. For the multistage linear parallel interference canceller (LPIC), we showed previously that its moment calculation can be decomposed into several well known problems in graph theory. Besides graphic tools developed by Hwang, Kim and Jay Kuo (see Proc. IEEE Int. Conf. Commun., 2002) we introduce a new tool, i.e. the chromatic polynomial of a directed graph, to carry out the current task. It is shown that analytic results match well with simulation results in the bit error probability of the LSIC.

The bit error probability of the multistage linear parallel interference canceller in a long-code code division multiple access (CDMA) system is analyzed with a graphic approach in this research. The decision statistic is modelled as a Gaussian random variable, whose mean and variance can be expressed as functions of moments of (R-I), where R is the correlation matrix of the signature sequences. Since the complexity of calculating these moments increases rapidly with the growth of the stage index, a graphical representation of moments is developed to facilitate the computation. Propositions are presented to relate the moment calculation problem to several well known problems in graph theory, i.e., the coloring, the graph decomposition, the biconnected component, and the Euler tour problems. It is shown that the analytic results of the bit error probability match well with simulation results.