Abstract Reflecting intelligent surface (RIS) is one of the key enabling technologies for beyond fifth generation wireless networks to further improve coverage, spectral‐ and energy‐efficiency of wireless networks. Recently, a novel simultaneous transmission and reflection RIS (STAR‐RIS) technology is introduced which enables more degrees‐of‐freedom in simultaneously serving users in reflection and transmission regions of RIS. This paper investigates a STAR‐RIS assisted two‐user full‐duplex communication system, wherein multi‐antenna base station (BS) and single‐antenna users are subject to maximum power constraints. Firstly, weighted sum rates are computed for three well‐known STAR‐RIS protocols, e.g. energy splitting, mode selection, and time splitting. Then, for each of the cases, weighted sum rate optimization problem is investigated to find optimal resource allocations, i.e. base station's precoding and combining matrices, coefficients of STAR‐RIS, and power allocations. The optimization problem is transferred into multiple convex sub‐problems using equivalent weighted minimum mean‐square‐error forms. Also, by use of the successive convex approximation method, tunable parameters of the STAR‐RIS are optimized. Although the original problem is a non‐convex one, proposed iterative alternating technique achieves an acceptable sub‐optimal performance. Finally, performance of the system is analysed and compared to some baselines to highlight superiority of the STAR‐RIS compared to the conventional RIS schemes.

This paper proposes the class of generalized least-square-error (GLSE) precoders for multiuser massive multiple-input multiple-output (MIMO) systems. For a generic transmit constellation, the GLSE precoders minimize the interference at user terminals assuring that some given constraints on the transmit signals are satisfied. The general form of these precoders enables us to impose multiple restrictions at the transmit signal, such as limited peak power and restricted number of active transmit antennas. The performance of these precoders is analyzed in the large-system limit. It is shown that the output symbols are identically distributed, and their statistics are described with an equivalent scalar GLSE precoder. To demonstrate the applications of the proposed framework, we employ the GLSE precoding to form transmit signals over a discrete alphabet and to select an effective subset of transmit antennas. Our investigations show that a computationally efficient GLSE precoder requires 41% less active transmit antennas than the conventional selection protocols in order to achieve a given level of input-output distortion.

This paper proposes nonlinear least square error (LSE) precoders for multiuser MIMO broadcast channels. The LSE precoders are designed such that the discrete output signals are from a predefined set. This predefined set allows us to model several signal constraints such as peak power constraint, constant envelope, and discrete constellations. We study the large-system performance of these precoders via the replica method from statistical physics, and derive a closed-form expression for the asymptotic distortion. Our results demonstrate that an LSE precoder with the output peak-to-average power ratio of 3 dB can perform similar to the regularized zero forcing (RZF) precoder. As the peak-to-average power ratio reduces to one, the constant envelope precoder is recovered. The investigations show that the performance of the RZF precoder is achieved by a constant envelope precoder with 20% additional transmit antennas. For M-phase shift keying constellations, our analysis gives a lower bound on the asymptotic distortion which is tight for moderate antenna-to-user ratios and deviates as the ratio grows. We improve this bound by deriving the replica solution under one-step of replica symmetry breaking. Our numerical investigations for this case show that the bound is tight for antenna-to-user ratios less than 5.

User pairing in non-orthogonal multiple-access (NOMA) uplink is investigated considering some predefined power allocation schemes. The base station divides the set of users into disjunct pairs and assigns the available resources to these pairs. The combinatorial problem of user pairing to achieve the maximum sum rate is analyzed in the large system limit for various scenarios, and some optimum and sub-optimum algorithms with a polynomial-time complexity are proposed. In the first scenario, 2 M users and the base station have a singleantenna and communicate over M subcarriers. The performance of optimum pairing is derived for M → ∞ and shown to be superior to random pairing and orthogonal multiple access techniques. In the second setting, a novel NOMA scheme for a multi-antenna base station and single carrier communication is proposed. In this case, the users need not be aware of the pairing strategy. Furthermore, the proposed NOMA scheme is generalized to multi-antenna users. It is shown that for the considered power allocation scheme, random and optimum user pairing perform similarly in the large system limit, but optimum pairing is significantly better in finite dimensions. It is shown that NOMA with the proposed user pairing scheme outperforms a previously proposed NOMA with signal alignment.

We introduce π/2-shifted Phase Shift Keying on the Hypersphere (π/2-PSKH), a new modulation scheme for Multiple-Input Multiple-Output (mIMO) systems. π/2-PSKH is a multidimensional extension of shifted QAM which allows to use load-modulated MIMO transmitters with a highly efficient power amplifier with reduced backoff. We propose π/2-PSKH constellations and explain how a π/2-PSKH signal is generated. The signal point sequence for π/2-PSKH is time-varying and as such has an inherent code. For the receiver, three algorithms are introduced and numerical simulations are used to examine their performance. These algorithms are further modified in order to reduce their complexity. We discuss the effect of this complexity reduction and the trade-off between receiver complexity and performance. It is shown that due to its inherent code, π/2-PSKH shows excellent performance in terms of power efficiency even if used with conventional amplifiers.

A general class of nonlinear Least Square Error (LSE) precoders in multi-user multiple-input multiple-output systems is analyzed using the replica method from statistical mechanics. A single cell downlink channel with N transmit antennas at the base station and K single-antenna users is considered. The data symbols are assumed to be iid Gaussian and the precoded symbols on each transmit antenna are restricted to be chosen from a predefined set X. The set X encloses several well-known constraints in wireless communications including signals with peak power, constant envelope signals and finite constellations such as Phase Shift Keying (PSK). We determine the asymptotic distortion of the LSE precoder under both the Replica Symmetry (RS) and the one step Replica Symmetry Breaking (1-RSB) assumptions. For the case of peak power constraint on each transmit antenna, our analyses under the RS assumption show that the LSE precoder can reduce the peak to average power ratio to 3dB without any significant performance loss. For PSK constellations, as N/K grows, the RS assumption fails to predict the performance accurately and therefore, investigations under the 1-RSB assumption are further considered. The results show that the 1-RSB assumption is more accurate.

This paper studies the large-system performance of Least Square Error (LSE) precoders which minimize the input-output distortion over an arbitrary support subject to a general penalty function. The asymptotics are determined via the replica method in a general form which encloses the Replica Symmetric (RS) and Replica Symmetry Breaking (RSB) ansätze. As a result, the “marginal decoupling property” of LSE precoders for b-steps of RSB is derived. The generality of the studied setup enables us to address special cases in which the number of active transmit antennas are constrained. Our numerical investigations depict that the computationally efficient forms of LSE precoders based on “li-norm” minimization perform close to the cases with “zero-norm” penalty function which have a considerable improvements compared to the random antenna selection. For the case with BPSK signals and restricted number of active antennas, the results show that RS fails to predict the performance while the RSB ansatz is consistent with theoretical bounds.

We introduce phase modulation on the hypersphere (PMH) for load-modulated multiple-input multiple-output (MIMO) transmitters with a single central power amplifier. In PMH, the peak to average ratio of the sum power before pulse shaping is 1; thus, the central power amplifier of load-modulated MIMO transmitters does not require any back-off. We derive the capacity of PMH on an additive white Gaussian noise channel and show that the input signal should be uniformly distributed on a hypersphere. The mutual information of uniformly distributed PMH input is derived in an uplink multiple-access independent identically distributed Gaussian MIMO channel using the replica method from statistical physics. Furthermore, discrete PMH is introduced using spherical codes and also generalizing minimum shift keying from the complex unit circle to the hypersphere. We investigate different pulse shaping methods for PMH including a novel spherical filtering. Using spherical pulse shaping, the signal stays on the hypersphere. Various filters are investigated, and a tradeoff between spectral shape and peak-to-average-sum-power ratio (PASPR) is found. For as few as four antennas, good spectral properties (similar to root-raised cosine pulses) can be achieved at very low PASPR. Both former and latter further improve with increasing the number of antennas.

We extend the result by Tse and Verdú on the optimum asymptotic multiuser efficiency of randomly spread code division multiple access (CDMA) with binary phase shift keying input. Random Gaussian and random binary antipodal spreading are considered. We obtain the optimum asymptotic multiuser efficiency of a K-user system with spreading gain N when K and N → ∞ and the loading factor, (K/N), grows logarithmically with K under some conditions. It is shown that the optimum detector in a Gaussian randomly spread CDMA system has a performance close to the single user system at high signal-to-noise ratio when K and N → ∞ and the loading factor, (K/N), is kept less than (log3K/2). Random binary antipodal matrices are also studied and a lower bound for the optimum asymptotic multiuser efficiency is obtained. Furthermore, we investigate the connection between detecting matrices in the coin weighing problem and optimum asymptotic multiuser efficiency. We obtain a condition such that for any binary input, an N × K random matrix, whose entries are chosen randomly from a finite set, is a detecting matrix as K and N → ∞.

We design a new precoding scheme for massive multiple-input multiple-output (MIMO) systems subject to an instantaneous total power constraint. Unlike the classical way of designing precoders, we use a more realistic model for power amplifiers by considering their maximum power. The considered power constraint is applicable in the single-RF MIMO transmitter which has been proposed recently for massive MIMO systems. For sake of analysis, i.i.d. Gaussian signals are considered. It is shown that the designed precoding scheme results in a good performance in terms of signal to interference plus noise ratio (SINR) at the receivers and power efficiency at the transmitter. It is shown that the power efficiency utilizing this precoding scheme is almost equal to the case of constant envelope signals. Since the proposed precoding limits the total transmit power, there is no need for peak-to-average power ratio (PAPR) reduction algorithms.

We derive some bounds on the Optimum Asymptotic Multiuser Efficiency (OAME) of randomly spread CDMA as extensions of the result by Tse and Verdú. To this end, random Gaussian and random binary antipodal spreading are considered. Furthermore, the input signal is assumed to be Binary Phase Shift Keying (BPSK). It is shown that in a CDMA system with K-user and N chips when K and N → 8 and the loading factor, K over N, grows logarithmically with K, the OAME converges to 1 almost surely under some condition. It is also shown that a Gaussian randomly spread CDMA system has a performance close to the single user system at high Signal to Noise Ratio (SNR) when the loading factor is kept less than log3K over 2. Moreover, for random binary antipodal matrices, we show that the loading factor cannot grow faster than equation.

In this study, the performance of a spectral‐amplitude‐coding optical code division multiple access (SAC‐OCDMA) system in the asynchronous regime is evaluated using a Gaussian approximation of the decision variable for codes with fixed cross‐correlation used in SAC‐OCDMA systems. The authors consider the effect of phase‐induced intensity noise (PIIN), thermal noise and shot noise. Moreover, the validity of the Gaussian approximation is confirmed by a Kolmogorov–Smirnov fitness test. For sake of comparison, the bit error rate (BER) of the asynchronous SAC‐OCDMA system is also plotted numerically in comparison with the BER of the synchronous SAC‐OCDMA. They show that a SAC‐OCDMA system without any time management for the users, that is, the asynchronous regime, has a better performance than the synchronous SAC‐OCDMA when PIIN effect exists.

This paper analyzes a multiplicative data hiding scheme, where the watermark bits are embedded within frames of a Gaussian host signal by two different, but arbitrary, embedding matrices. A closed form expression for the bit error rate (BER) of the optimal detector is derived when the frame sizes tend to infinity. Furthermore, a structure is proposed for the optimal detector which divides the detection process into two main blocks: host signal estimation and decision making. The proposed structure preserves optimality, and allows for a great deal of flexibility: The estimator can be selected according to the a priori knowledge about host signal. For example, if the host signal is an Auto-Regressive (AR) process, we argue that a Kalman filter may serve as the estimator. Compared to a direct implementation of the Neyman-Pearson detector, this approach results in significantly reduced complexity while keeping optimal performance.

In this study, the authors investigate the performance of optical code‐division multiple access (OCDMA) systems with belief propagation (BP)‐based receivers. They propose three receivers for the optical fibre channel that provide a trade‐off between detecting complexity and system performance. The first proposed receiver achieves a performance very close to the so‐called known interference lower bound. The second receiver exhibits a considerably less complexity at the expense of a slight degradation in performance. They show that the third BP‐based receiver, which is a simplified version of the second receiver, is surprisingly the same as the so‐called multistage detector in OCDMA systems. They then study the problem of finding proper spreading codes for the proposed receivers. BP‐based receivers perform well if the graph corresponding to the spreading matrix has no short cycles. The probability of existence of short cycles directly depends on the sparsity of the spreading matrix. Therefore they look for sparse spreading matrices that are also uniquely detectable, that is, the corresponding input data vectors and the output spread vectors are in one‐to‐one correspondence. The existence of random uniquely detectable matrices (for which the elements are binary with equal probability) has already been proved by Edrös and Rényi when the dimensions of matrix tend to infinity. In this study, they prove the existence of sparse uniquely detectable spreading matrices in the large system limit, when the number of users and the number of chips approach infinity and their ratio is kept constant. For finite length systems, they propose to use optical codes with one chip interference between codes and show that they exhibit a better performance than random sparse codes.

We consider the total capacity of a Gaussian multiple-access MIMO channel with a linear array of R receive antennas and T distributed transmit antennas. If the spatial distribution of transmit antennas weighed by their path loss and marginalized to a sphere around the receive array is proportional to the ratio of receive antenna directivity to transmit antenna directivity, the capacity is shown to approach Rlog(1 + SNR) as T → ∞, irrespective of the inter-element distance at the receiver.We further show that the capacity for other distributions of transmit antennas can be even greater, as the inter-element distance approaches zero.