One of the most promising technologies for 6G is the use of millimeter waves (mmWaves) because these waves provide wide bandwidth and high data rates. However, these waves are highly sensitive to blockage and hence have short range. Thanks to the relay-aided multi-hop transmission, we can overcome this limitation and improve the range of mmWaves. In this paper, we investigate different types of relay selection schemes and routing for the cluster-based multi-hop (CBMH) mmWave network. These schemes are classified into bypass-based and without bypass, where bypass refers to the direct links from the intermediate clusters to the destination. We analyze and simulate these schemes in terms of the blockage probability, the end-to-end (e2e) normalized energy consumption (NEC), the e2e average hop count (AHC) and the spectral efficiency. The analytical results and the simulations show that the bypass-based relay selection schemes outperform their counterparts without bypass in terms of the blockage probability, the e2e AHC and the spectral efficiency. However, in terms of the e2e NEC, the situation is reversed. The simulation results also show that the proposed relay selection schemes perform very close to the benchmark in terms of the spectral efficiency.

In this paper, we design a NOMA-based spectrum leasing scheme by considering the following three assumptions for the successive interference cancellation (SIC): (i) the primary network performs the SIC (PSIC), (ii) the secondary network performs the SIC (SSIC), (iii) either the primary or secondary network adaptively performs the SIC depending on the channel conditions (ASIC). We first analyze the achievable rate regions of these schemes. Then, we optimize the power allocation to different layers of the superimposed signal by maximizing the achievable rate of the secondary network under a QoS constraint for the primary network. Finally, we analyze these schemes in terms of the outage probability. The analysis shows that: (i) In terms of the primary network, the PSIC and ASIC schemes perform the same and these schemes outperform the SSIC scheme. (ii) In terms of the secondary network, the ASIC scheme outperforms the other two schemes. (iii) In terms of the primary network, the NOMA-based and OMA-based schemes perform the same, provided that the SIC is performed either adaptively or in a fixed manner at the primary network. (iv) In terms of the secondary network, the NOMA-based scheme outperforms the OMA-based scheme, provided that the SIC is performed adaptively.

In this paper, we propose an asymmetric adaptive modulation scheme for a heterogeneous two-way relay network (TWRN). In this scheme, due to the heterogeneity of the TWRN, different modulation schemes with different bit rates are assigned to the source nodes. Due to the mismatch between the bit rates of the two opposite traffic flows, the conventional XOR-based network coding is not applicable to this scheme. To cope with this heterogeneity at the relay node, we propose a network coding scheme in which the two data streams are combined at the level of symbol. We optimize the pair of modulation schemes and bit rates such that the instantaneous transmission rate is maximized under the constraint that a quality-of-service requirement in terms of the end-to-end BER is satisfied. To find the solution of this multi-dimensional problem, we propose a simple procedure with very low computational complexity by reducing the problem dimension to one. We analyze the system performance in terms of the average end-to-end BER, the average transmission rate, and the outage probability over Nakagami fading channel. Our analysis shows that this scheme not only achieves high spectral efficiency but also mitigates error propagation from the relay to the transceivers.

In this paper, for an overlay cognitive radio network in which the primary user’s transmission is assisted by a cognitive multi-hop multi-branch network, we propose and analyze a spectrum leasing scheme that achieves full diversity in both the primary and secondary networks. Among the available branches, the best one is selected to establish a multi-hop relay link between the primary users. In return, the secondary users of the selected branch are rewarded with a fraction of spectrum to transmit their own signals over a nonorthogonal multiple-access channel. We jointly optimize the branch selection and the spectrum allocation such that the sum-rate of the secondary users is maximized under the constraint that a minimum quality-of-service (QoS) requirement in terms of the achievable rate is satisfied in the primary network. We also optimize the power allocation to the secondary users such that the asymptotic outage probability of the primary network is minimized. The analytical results show that this spectrum leasing scheme guarantees the full diversity of order$M$in both the primary and secondary networks, where$M$is the number of cognitive branches. The simulation results also show that optimizing the allocation of power significantly decreases the risk of spectrum leasing failure.

In this paper, we investigate the signal-to-noise ratio-based incremental relaying scheme for a two-way relay channel (TWRC) with symbol-based network coding. Under the assumption of error propagation from the relay to the destination, we analyze the performance of the system in terms of the end-to-end symbol error rate (SER), the average spectral efficiency (ASE), the outage probability, the asymptotic behavior, and the diversity order for M-ary phase-shift keying modulation. All of these performance measures depend on a single threshold employed by the destination. By optimizing this threshold with respect to two different performance measures, we attain two different incremental relaying schemes: 1) the SER-optimal scheme that minimizes the end-to-end SER and 2) the ASE-constrained scheme that minimizes the end-to-end SER under an ASE constraint for the TWRC. In all cases, we derive closed-form expressions for the performance measures and the optimal thresholds. The asymptotic analysis of the system shows that both the SER-optimal and ASE-constrained schemes achieve the full diversity gain at both transceivers. Moreover, both of the schemes are optimal in the sense of the asymptotic ASE. We also propose a class of thresholds that are independent of the network geometry and asymptotically perform the same as the optimal thresholds.

In this study, for a network‐coded two‐way relay channel (TWRC), the authors propose three different relaying schemes that satisfy certain quality of service requirements at both transceivers. In these schemes, the relay decides independently whether to help each transceiver or not based on the instantaneous signal‐to‐noise ratios received from the transceivers. Under the assumption of error propagation, they analyse the performance of the system in terms of the end‐to‐end bit error rate (BER), the outage probability, the average spectral efficiency (ASE) and the diversity order. All of these performance measures depend on two thresholds employed by the relay node. By optimising these thresholds under three different criteria, they achieve three different relaying schemes: (i) the BER­ optimal scheme that minimises the end‐to‐end BER; (ii) the outage‐constrained scheme that minimises the end‐to­ end BER under an outage probability constraint; and (iii) the BER‐constrained scheme that maximises the ASE under an end‐to‐end BER constraint. The analysis of the system shows that the BER‐optimal scheme guarantees the full diversity gain at both transceivers; the outage‐constrained scheme results in a bidirectional relaying scheme with a constant outage probability; and the BER‐constrained scheme makes use of spectrum as efficiently as possible while maintaining the target BER.

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.

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.

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.