In response to growing demands for both sensing and communication performance from future applications of wireless networks, simultaneous transmitting and reflecting intelligent reflecting surface (STAR-IRS), sparse code multiple access (SCMA), and integrated sensing and communication (ISAC) have been introduced. In this paper, we investigate the joint beamforming and resource allocation optimization problem (OP) for STAR-IRS-Aided SCMA for an ISAC System by considering ISAC, SCMA, beamforming, quality of service (QoS), and STAR-IRS constraints in the THz band. The investigated OP is a type of NP-hard OP that cannot be solved by conventional methods. Moreover, the possible solution must have the ability to adapt to different situations. Then to solve this problem efficiently, we employ a new meta deep reinforcement learning (MDRL). The simulation results demonstrate the effectiveness of the proposed solution for the resource allocation problem across different system model and MDRL parameters, as well as multiple access methods. Compared to non-orthogonal multiple access (NOMA) and orthogonal frequency-division multiple access (OFDMA), the proposed system model improves energy efficiency (EE) by 15% and 20%, respectively. The suggested MDRL approach achieves a 123% enhancement over classic deep deterministic policy gradient (DDPG). Additionally, a scenario involving a coupled phase-shift model was conducted for the proposed system model. To investigate the effect of channel state information (CSI) estimation on our proposed system model, we considered a simulation with both imperfect and perfect CSI. Additionally, when solving the optimization problem using a convex optimization approach, MDRL shows a 10% improvement.

Abstract This paper examines a new beamforming method, and detector for a multi‐relay system is proposed, in which the signals from relays, and direct link are combined to achieve optimal performance. Relays employ link‐adaptive regeneration relaying method to scale their decoded symbols before retransmitting them in order to alleviate the error propagation. The scaling is adapted with the signal‐to‐noise ratio of the source‐relay, and the intended relay‐destination links. In the proposed beamforming scheme, two‐bit quantized phase information of the channel gain between the relay, and destination is used to retransmit decoded symbols at relays. It is analytically shown that the proposed method achieves full diversity order. Numerical results show a close match between simulation, and analytical derivations.

Analogue only beamforming generally refers to a structure of analogue phase shifters that have no control over the amplitude of the signals and are utilised in millimetre‐wave massive multi‐input multi‐output systems to reduce the cost and power consumption. Conventional analogue only beamforming methods are based on beam‐training and subspace sampling where the transmitter and receiver select the best beamformer and combiner from a list of predefined beam patterns to maximise the beamforming gain. In this study, first, the beam‐alignment problem is formulated as a quadratic optimisation problem with quadratic constraints. Then, a practical solution for analogue only beamforming is proposed to maximise beamforming gain. The proposed method has no assumption on the array structure or isotropic antenna elements in the array. Also, a new method is proposed to design sub‐codebooks to reduce the construction time of the codebook. Simulation results also corroborate the analytical results and performance of the proposed method.

In a conventional cellular network underlaid with femtocells, the authors employ interference alignment (IA) to restrict cross‐layer co‐channel interference (X‐interference). In their scenario, locally obtained channel state information is allowed to be conveyed between heterogeneous nodes to construct a decentralised IA‐based scheme. Furthermore, they provide closed‐form formulas for the upper bounds of X‐interference in both uplink and downlink directions and characterise how they asymptotically approach zero for big enough channel dimensions used by individual nodes. Simulation results demonstrate that X‐interference is always bounded by the above‐mentioned limits and prove accuracy of their analytical formulas.

Successive relaying is a bandwidth‐efficient scheme in cooperative networks. In this study, a novel distributed space‐time coding (DSTC) transmission scheme for a two‐path successive relay network is proposed that enjoys threshold‐based selection relaying protocol. A new beamforming method is also employed alongside the proposed DSTC transmission scheme. Three scenarios are considered in this study. In the first scenario, single‐input–single‐output (SISO) relays are considered. In the second and third scenarios, multiple‐input–multiple‐output (MIMO) relays are used, but in the second case a transmission scheme named as quasi‐DSTC scheme alongside employing space‐time coding at each relay is proposed. A closed‐form expression for the end‐to‐end bit error rate (BER) of the system is derived and it is shown that regardless of the fading in inter‐relay channel, the proposed scheme achieves full diversity order even when there are decoding errors at the relay nodes. Simulation results show a significant saving in power of about 2.5, 3, and 4 dB at BER compared with other conventional methods in the case of using SISO relays, MIMO relays in the second and third scenarios, respectively.

An efficient spectrum sensing scheme is proposed for detecting a primary signal corrupted by a Generalized Gaussian Noise (GGN). It is assumed that the power of the primary signal is much smaller than the noise power, and secondary users are equipped with multiple antenna receivers. The proposed scheme, referred to as sigmoid detector, uses a sigmoid soft limiter. The sigmoid limiter is characterized by a parameter which controls the softness of its limiter. The sigmoid parameter is optimized to achieve the best performance(probability of detection) using the chernoff bound for probability of detection. It is shown through simulations that the sigmoid detector outperforms well known detectors such as SL-PCA (soft-limited polarity coincidence array) with soft limiting function, PCA (polarity coincidence array) and the energy detection (ED) scheme in the presence of heavy-tailed GGN. Also, there is a match between the simulation and analytical results.

Cellular networks are of particular interest when communication takes place. The main barrier of employing cellular networks to reach broad access everywhere is interference. Although interference alignment has by theory raised hopes to leap this last hurdle, however, no operator will show interest for practical implementation of the complicated algorithms proposed so far, especially considering the need for global channel knowledge. Besides, there is no measure to appraise the performance of existing algorithms. In this study, the authors study blind cellular networks to characterise their achievable degrees of freedom. As it is very hard – if not impossible – to obtain its exact value, they provide related upper and lower bounds for this special scenario when power control mechanism is applied in the network. They also propose constructive proof for the lower bound and offer their seminal blind resource allocation algorithm in which they utilise concepts of both frequency alignment and fractional frequency reuse. Analytical upper bounds are also proposed and proved to be tight with respect to simulated results.

In this study, two diversity achieving schemes in two‐way relay channel (TWRC) under Nakagami‐ m fading are proposed. A general network of N r relays is considered where each relay is equipped with N a antennas. It is also assumed that the relays operate in decode‐and‐forward (DF) mode. The data transmission is divided into two phases, multiple access (MAC) phase and broadcast (BC) phase. In the MAC phase, the sources send their data to the relays simultaneously and after decoding, one relay is selected with MAX–MIN relay selection criterion and then in the BC phase retransmit the decoded symbols using the proposed transmission schemes at the selected relay. With evaluating the asymptotic behaviour of the bit error rate (BER) of the proposed schemes, it is proved that the diversity order of them is N r N a m . As a result, the proposed schemes achieve full diversity order.

Cooperative communication through relaying can improve the performance of communication systems under multipath fading conditions. Increasing the number of relays will also boost the performance of the system, but since relays transmit through orthogonal channels, this improvement is at the cost of bandwidth or rate of the system. A two-hop multi-relay decode-and-forward cooperative system is proposed in which selected relays transmit simultaneously in the same band, and therefore, there is no rate or bandwidth loss. To mitigate the error propagation due to incorrect decoding of symbols at the relays, we use a selection relaying method in which a relay transmits in the second hop if the channel quality between the relay and the source exceeds a minimum threshold regardless of the relay-destination link. In our proposed beamforming scheme, the phase of selected relays for transmission in the second hop is adjusted to compensate for the channel between the relay and the destination and achieve the best performance at the destination using only a simple MRC receiver. The relays use only two-bit quantized phase information of the channel gain between the relay and the destination, which can be sent to the relays by the destination. The general family of M-QAM modulation constellations is considered in the performance and diversity analysis. It is proved that the scheme achieves full diversity order, and closed-form expressions for the thresholds that achieve full diversity are found.

A cooperative network consisting of a source, one relay, and a destination is considered. The source and destination are both equipped with a single antenna. The relay is equipped with N antennas and operates in decode-and-forward (DF) mode. At the relay, the detected symbols are retransmitted using a novel beamforming method proposed in this paper that assumes the relay has 2-bit quantized information about the phase of all links from the relay to the destination. At the destination, the detector combines the received signals from the source and the relay in the first and second phases of transmission, respectively. An upper bound for the bit error rate (BER) performance of the detector is derived, and it is shown that the proposed beamforming and detector achieve full diversity order. The detector is extended to a multicast system with one single antenna source, one N-antenna DF relay and M single antenna destinations. It is assumed that the relay has 2-bit quantized phase information and exact signal-to-noise ratio (SNR) information at all the links from the relay to the destinations. An antenna selection algorithm based on a MAX-MIN criterion at the relay is proposed, and after beamforming, the detector is used at all destinations. By deriving a rigorous upper bound for BER performance at all destinations, it is shown that the proposed scheme achieves full diversity order at all destinations. Simulation results show that the BER performance of the proposed antenna selection algorithm outperforms the well known MAX-MIN antenna selection scheme.

Cooperative communication can improve the performance of communication systems under multi‐path fading conditions through relaying. Increasing the number of relays will also boost the performance of the system, but since relays transmit through orthogonal channels, this improvement is obtained at the cost of bandwidth or rate of the system. Best relay selection is a well‐known technique to achieve full order of diversity and use bandwidth efficiently in a cooperative two‐hop multi‐relay network. A new beamforming at the relay is proposed that selects the best two among several relays and then the selected relays forward their detected symbols in a common channel to the destination. For achieving full diversity order at the destination, a modified maximal ratio combining detector is proposed. In the proposed beamforming scheme, the phase of the selected relays is adjusted in such a way to compensate the channel between the relays and the destination and achieve acceptable performance at the receiving end. The selected relays require two‐bit quantised phase information of the channel gain between the relay and the destination, which can be sent to the relay by the destination. It is shown that the proposed method achieves full diversity order and has considerable performance gain in comparison with the best relay selection method.

We consider the downlink of a multiple‐input multiple‐output broadcast channel where the transmitter uses per user unitary and rate control (PU2RC) to perform the user scheduling and beamforming. The main shortcoming of the PU2RC scheme is in networks where the number of users is less than or comparable with the codebook size. In these networks, PU2RC cannot properly exploit the available channel dimensions. To deal with this problem, they propose two different approaches to enhance the throughput performance of the system. In the first approach, they try to find and assign unscheduled users to the unused channel dimensions. This is done through a signal‐to‐interference plus noise ratio (SINR) estimation method in which they approximate the SINR of the users along new beamforming vectors. In this approach, they usually achieve the maximum multiplexing order of the system. In the second approach, they adjust the SINR feedbacks according to the number of scheduled users. When the number of scheduled users is less than the number of transmitting antennas, the feedback SINRs are in fact underestimated. In this case, they correct the SINRs and improve the average throughput. The simulation results show that both proposed methods can effectively improve the performance of the PU2RC scheme.

One of the main drawbacks of cooperative communication systems with half‐duplex relaying is inefficient use of spectrum. Incremental relaying is an effective technique that overcomes this limitation. In this study, for a single‐relay regenerative system, the authors propose a spectrally‐efficient incremental relaying scheme that eliminates redundant relay transmissions such that the error propagation from the relay to the destination is effectively mitigated and the full diversity is achieved. The authors examine the impact of the error propagation on the performance of the system and derive closed‐form expressions for the end‐to‐end symbol error probability (SEP) for different modulation schemes. They also derive an approximate closed‐form expression for the optimal threshold that minimises the end‐to‐end SEP. The high‐signal‐to‐noise ratio (SNR) analysis demonstrates that this threshold achieves full diversity whereas neither the bandwidth‐consuming cyclic redundancy check code nor the availability of the instantaneous source–relay channel state information at the destination is required in the system. Furthermore, the authors purpose a class of thresholds that are independent of the network geometry and achieve full diversity. The asymptotic behaviour of the system shows that the spectral efficiency for both the optimal and geometry‐independent thresholds in the high‐SNR regime tends to the maximum achievable value.

The optimization of pilot symbol parameters can improve the spectral efficiency of adaptive modulation for orthogonal frequency division multiplexing (OFDM) systems, since pilot symbols impose an overhead on the system consuming power and bandwidth. An optimal pilot symbol assisted adaptive modulation (PSAAM) scheme for OFDM systems is proposed that maximizes spectral efficiency by adapting the power and constellation size of each subcarrier based on employing imperfect channel state information (CSI) at the transmitter. The pilot symbol power and spacing is also optimized in this scheme. A suboptimum scheme that decreases computational complexity without perceivable loss in performance is also presented. The optimality of minimum mean square error (MMSE) channel prediction for OFDM systems expressed in terms of a lower bound on spectral efficiency is approached. It is proved that the rectangular pilot pattern with equi-spaced and equal power pilot tones achieves the minimum MSE of the channel prediction in addition to having the advantage of simplifying PSAAM design. Numerical results show the importance of optimal pilot parameter adjustment for rapidly fading channels.

Spectrum sensing is one of the main features of cognitive radio that enables secondary users (SUs) to detect spectrum holes. In this study, the authors propose a more realistic measure of interference which is based on the collision between secondary and primary signals. Considering the single SU scenario, the authors find optimal sensing time, transmission time and energy detector threshold to maximise its efficiency while keeping its interference on primary users below a certain threshold. Then, cooperative sensing schemes in a centralised network are investigated in which, the authors derive the optimal cooperation method in the Neyman–Pearson framework and subsequently propose simpler suboptimal cooperative schemes. Furthermore, the authors formulate the limited feedback scheme that uses a two bit quantiser for energy detectors, and compare its performance with other cooperation schemes and it will be shown to have the closest performance to the optimal scheme among all other mentioned methods.

We consider the multiple-input multiple-output broadcast channels and the per user unitary and rate control (PU2RC) transmission and we characterize the performance of this transmission scheme for arbitrary number of active users in the network. In this paper we first introduce the method of computing the probability distribution of the feedback SINR in the PU2RC and will show that unlike the common assumption, the maximum of SINR in the different orthogonal sets are not independent of each other. In the next step we introduce a method to evaluate an upper and lower bound over the achievable rates in the PU2RC transmission scheme that can characterize its throughput performance and the results can be used in the optimal feedback designs and Multimode switching.

Error propagation from the relay to the destination is a major drawback of digital relaying protocols that substantially diminishes the cooperative diversity gain of the system. Appropriately designed selection relaying schemes can effectively mitigate this phenomenon and improve the diversity order, especially when the source-relay channel state information (CSI) is not available at the destination. In this study, the authors investigate the SNR-based selection relaying scheme in a multi-relay cooperative network (with R potential relay nodes) and examine the impact of the error propagation on the end-to-end performance of the system. The authors derive closed-form expressions for the end-to-end symbol error probability (SEP) of the system for both identically distributed and non-identically distributed Rayleigh fading environments. In the analysis, the authors consider M-ary phase shift keying (M-PSK), M-ary pulse amplitude modulation (M-PAM) and M-ary quadrature amplitude modulation (M-QAM) schemes. They also propose a class of geometry-independent thresholds that effectively mitigate the error propagation and provide full diversity of order R+1. Furthermore, for the special case of single-relay network, the authors derive closed-form expressions for the optimal thresholds that minimise the end-to-end SEPs.

In this study, the authors investigate the hybrid decode-amplify-forward (HDAF) relaying protocol for the multiple parallel relays over independent and non-identical distribution flat Rayleigh fading channels using maximum ratio combining at the destination. They derive closed-form tight bounds for the outage probability and bit error rate (BER) of the HDAF relaying protocol. They also show that the outage probability performance of this protocol outperforms decode-forward (DF) protocol, and is very close to amplify-forward (AF) scheme. On the other hand, the BER of the HDAF scheme is superior to AF protocol, and is close to BER of DF scheme. From practical point of view, when taking into account outage and BER performance of a system together, the HDAF protocol performs better than AF and DF schemes by combining the merits of both. Also, the authors investigate power allocation among the relay nodes to minimise the system outage probability and BER under total and individual power constraints. Numerical results are provided to show the correctness of our analytical expressions.

Adaptive modulation exhibits the advantage of great spectral efficiency for high data rate wireless transmission, where the transmission power and rate of subchannels in orthogonal frequency division multiplexing (OFDM) systems are matched to the channel state information (CSI). CSI can be acquired at the receiver by MMSE channel estimation based on pilot symbol assisted modulation (PSAM), which is a widely used technique, and fed back to the transmitter. While more accurate channel estimation will result in more reliable transmission, pilot symbols don''t carry information and they impose an overhead on the system consuming power and bandwidth. An optimal adaptive PSAM scheme, that maximizes spectral efficiency by optimizing pilot power and spacing simultaneously is proposed. A rectangular pilot pattern is adopted to simplify the adaptive scheme.

Error propagation from the relay to the destination is a major drawback of digital relaying protocols that substantially diminishes the cooperative diversity gain of the system. Appropriately designed selection relaying schemes can effectively mitigate this phenomenon and improve the diversity order, especially when the source-relay channel state information (CSI) is not available at the destination. In this study, the authors investigate the SNR-based selection relaying scheme in a multi-relay cooperative network that employs the distributed space–time coding (DSTC) strategy. The authors present a closed-form expression for the end-to-end bit error probability (BEP) of the system. It is demonstrated that the orthogonality of the space–time coded signals transmitted by the relays is not preserved under the error propagation from the relays to the destination and this gives rise to intersymbol interference at the destination. The SNR loss and intersymbol interference caused by the error propagation substantially diminish the diversity order of the system. The error propagation can be effectively mitigated by employing appropriate thresholds at the relays. The authors derive a closed-form expression for the optimal threshold that minimises the end-to-end BEP of the system. The authors also analyse the asymptotic behaviour of the BEP and demonstrate that by employing the optimal threshold the full diversity order is achieved. Furthermore, the authors propose a class of thresholds that are independent of the network geometry and also provide full diversity order. By employing these thresholds, each relay node can decide to forward or remain silent independent of other relay nodes.

In cyclic-prefixed communication systems, if the delay spread of the channel is longer than the cyclic prefix (CP) a channel-shortening equalizer (CSE) can be used to restore the desired operation of such systems. Since in time-varying environment we are interested in fast adaptive equalizer with tracking capability, the aim of this paper is to propose RLS-type algorithm for channel shortening. In this paper, we first propose an RLS-type algorithm to estimate the eigenvector corresponding to the smallest eigenvalue of a matrix and based on this algorithm we develop an RLS-type blind channel shortener. We also, based on PAST algorithm, propose an RLS-type update rule to shorten the channel under MMSE criterion. Simulations show the speed advantage of the proposed algorithms.

In this paper a new sub-space method for estimating the direction of arrival (DOA) of coherent signals at the base station of a direct-sequence code division multiple access (DS-CDMA) communication system is proposed. In this method, the signal subspace of the desired user is decomposed into proper smaller sub-spaces to resolve the spatial temporal signatures of the coherent received signals. The idea is based on exploiting the spreading code sequence of the user to derive a set of vectors constructing a subspace where a part of the desired spatial temporal signature lies. Using a step by step procedure, all DOAs of different paths of the desired user are obtained by the well-known MUSIC algorithm.

We introduce a new algorithm for decoding of space-time codes in a wireless system with an arbitrary number transmit and receive antennas. This algorithm benefits from the lattice structure of space-time codes, and the algorithm reaches maximum-likelihood performance while it shows a much lower complexity. The simulation results of this algorithm are presented.