Training-Based Joint Antenna and Relay Selection in Multiuser Downlink Cellular Network with RF Impairments

This paper presents the performance analysis of decode-and-forward (DF) relaying system in presence of both channel estimation error (CEE) and radio frequency impairments (RFI). First, the end-to-end signal-to-noise-plus-distortion-and-error ratio (SNDER) expression is derived, followed by an exact closed-form outage probability (OP) expression for Nakagami-m fading channel. As a special case, the OP analysis for Rayleigh fading channel is also provided. From the derived expressions, the relation amongst CEE and RFI is analyzed. For a complete study, the high SNR analysis of the derived equations is also carried out. The analytical results have been verified using the Monte Carlo simulations. Finally, intriguing results are presented with the help of plots shown in the numerical analysis.

Multi-input multi-output (MIMO) systems are often realized with low cost front-end architectures, e.g. the so called direct conversion architectures. However, such systems are very sensitive to imperfections in the analog front-end, resulting in radio frequency (RF) impairments such as in-phase/quadrature-phase (IQ) imbalance and carrier frequency offset (CFO). The RF impairments in such low cost front-end systems are unavoidable and can result in a severe performance degradation. In this paper we propose a generally applicable equalization technique for space-time block coded (STBC) MIMO orthogonal frequency division multiplexing (OFDM) communication systems. The Alamouti based STBC scheme is examined in detail. It is shown that the compensation scheme can be easily extended to other higher order STBC systems. We consider a digital compensation scheme for joint transmitter and receiver IQ imbalance along with front-end filter mismatch, CFO and frequency selective channel distortions.

This study investigates the performance analysis of a transmit antenna selection–maximal ratio combining relay system over non-identical channel estimation error. The proposed system selects both relay and user terminal with a maximum signal-to-noise ratio (SNR) using feedback information. The exact closed-form expression of the proposed system for the outage probability and symbol error rate (SER) is derived in the presence of non-identical channel estimation error. Using the approximated probability density function, the authors quantify the diversity order for the SER and outage probability. From the above results, the diversity order of the proposed system is proportional to the number of relay nodes and user terminals. Thus, the diversity order of the proposed system is superior to that of the previous selection systems. The diversity order of the proposed system is independent of the number of receive antennas at the relay and user terminal and the channel correlation coefficient. Furthermore, the SNR gain of the proposed system is in proportion to the sum of the channel correlation coefficients and the number of receive antennas at the relay and user terminal.

In this paper, we investigate the performance of asymmetric radio frequency (RF) and free space optical (FSO) dual-hop cooperative relay network along with a direct RF link between source and destination. The FSO link experiences double generalized Gamma turbulence in the presence of the generalized non-zero boresight pointing errors, and RF links experience non-identically distributed Nakagami-m fading. Moreover, considering both cognitive radio and non-cognitive scenarios, a partial relay selection (PRS) strategy has been employed. Furthermore, both relay selection and underlay power restriction are governed with the outdated channel state information (CSI). We also assume both heterodyne and intensity modulation/direct detection methods in the FSO receiver. Under the assumption of amplify-and-forward relaying and PRS, we derive closed-form expressions for outage probability (OP) of both scenarios, while bit-error probability and ergodic capacity of the non-cognitive scenario are also obtained. The asymptotic expressions of OP and diversity order of this network are derived for both perfect CSI and outdated CSI cases. It is demonstrated that the diversity order is a function of the fading severity of the RF links, turbulence parameters of the FSO link, and pointing error, regardless of interference channel parameter of the primary user.

We study the performance of multi-hop networks composed of millimeter wave (MMW)-based radio frequency (RF) and free-space optical (FSO) links. The results are obtained in the cases with and without hybrid automatic repeat request (HARQ). Taking the MMW characteristics of the RF links into account, we derive closed-form expressions for the network outage probability. We also evaluate the effect of various parameters such as power amplifiers efficiency, number of antennas as well as different coherence times of the RF and the FSO links on the system performance. Finally, we present mappings between the performance of RF- FSO multi-hop networks and the ones using only the RF- or the FSO-based communication, in the sense that with appropriate parameter settings the same outage probability is achieved in these setups. The results show the efficiency of the RF-FSO setups in different conditions. Moreover, the HARQ can effectively improve the outage probability#x002F;energy efficiency, and compensate the effect of hardware impairments in RF-FSO networks. For common parameter settings of the RF-FSO dual- hop networks, outage probability 10^{-4} and code rate 3 nats-per-channel-use, the implementation of HARQ with a maximum of 2 and 3 retransmissions reduces the required power, compared to the cases with no HARQ, by 13 and 17 dB, respectively.

In this paper, we investigate the outage performance of decode-and-forward (DF) based opportunistic cooperative communications with channel estimation errors at the receivers under Rayleigh fading channels, in which the effect of the direct link has been explicitly evaluated. To be specific, the exact expression of outage probability has been derived for arbitrary values of signal-to-noise ratio (SNR) when pilot power is fixed. Through asymptotic analysis, the closed-form expressions of outage probability in the high SNR region and the diversity order are obtained. The analysis shows that the channel estimation errors due to limited pilot power impose a significant impact on the outage performance, leading to the diversity order be 0 even when the direct link is available. Although deploying more relays can mitigate the estimation errors effectively, the performance limit caused by the estimation errors becomes more obvious as SNR grows. For specific pilot power configuration and relay deployment, the outage probability remains unchanged even if data power approaches to infinity. Furthermore, simulation results are presented to verify our analysis.

This paper investigates the impact of relay selection (RS) on the performance of cooperative non-orthogonal multiple access (NOMA), where relays are capable of working in either full-duplex (FD) or half-duplex (HD) mode. A number of relays (i.e., K relays) are uniformly distributed within the disc. A pair of RS schemes are considered insightfully: 1) single-stage RS (SRS) scheme; and 2) two-stage RS (TRS) scheme. In order to characterize the performance of these two RS schemes, new closed-form expressions for both exact and asymptotic outage probabilities are derived. Based on analytical results, the diversity orders achieved by the pair of RS schemes for FD/HD cooperative NOMA are obtained. Our analytical results reveal that: 1) the FD-based RS schemes obtain a zero diversity order, which is due to the influence of loop interference at the relay; and 2) the HD-based RS schemes are capable of achieving a diversity order of K , which is equal to the number of relays. Finally, simulation results demonstrate that: 1) the FD-based RS schemes have better outage performance than HD-based RS schemes in the low signal-to-noise ratio (SNR) region; 2) as the number of relays increases, the pair of RS schemes considered are capable of achieving the lower outage probability; and 3) the outage behaviors of FD/HD-based NOMA SRS/TRS schemes are superior to that of random RS and orthogonal multiple access based RS schemes.

Energy harvesting (EH), which refers to replenishing energy from the environment, is demonstrated to be a promising approach in reducing the operational expenses (OPEX) and increasing the network lifetime of emerging wireless communication technologies by eliminating the need for frequent battery replacing or recharging of wireless devices. Among various ambient energy sources, the focus of this dissertation is on wireless power transfer (WPT) considering the unique characteristic of radio frequency (RF) signals; that is, they inherently carry both information and energy. A particularly interesting scenario arises when sources support simultaneous wireless information and power transfer (SWIPT) to provide a remote, controllable, and on-demand energy source. In this context, one of the promising deployment approaches is SWIPT-enabled cognitive radio networks (CRNs), which offer significant gains in terms of spectral and energy efficiencies. One of the practical scenarios of RF-powered CRN is deploying a number of distributed cognitive sensor nodes, equipped with RF EH modules, to sense a specific area and send the sensed data to an access point while coexisting with a primary licensed network. However, exploiting the full potentials of SWIPT-enabled CRNs is subject to carefully aligning the requirements of the EH unlicensed secondary receivers (SRs) with those of the legitimate primary receivers (PRs). In light of this, the first contribution of this thesis focuses on investigating the problem of beamforming for the downlink of multi-user multiple-input single-output (MU-MISO) CRNs. With the objective of minimising the transmission power of the secondary base station (SBS), optimal and suboptimal solutions for the formulated optimisation problem are provided by jointly optimising the transmit beamforming vector at the SBS and adjusting the parameters of the energy harvesters at the SRs. It is shown that the obtained solutions are efficient in meeting the EH and quality-of-service (QoS) requirements of the SRs and the levels of interference accepted by the PRs. Apart from SWIPT-enabled CRNs, relay-assisted SWIPT networks are envisioned to be a promising framework offering extended coverage, diversity gains, and enhanced energy efficiency. In this case, the relay network itself can benefit from the relayed transmissions in terms of saving energy, and the harvested energy can be used to charge relay nodes and extend their lifetime as compensation for their role of data forwarding. Nonetheless, a concrete performance analysis on the impact of the involved system parameters such as, the energy conversion efficiency and the location of the EH relay terminal, on the trade-off between the achievable information transfer efficiency and the harvested energy level is crucial for the successful implementation of SWIPT in this context. This inspired the research work in the next two contributions with the main focus on developing novel comprehensive analytical frameworks for the investigation and evaluation of SWIPT relaying systems. Specifically, the second contribution is dedicated to examining the application of noncoherent modulation, which is recognised as an energy efficient modulation scheme for SWIPT, due to its ability to eliminate the need of instantaneous channel state information (CSI) estimation/tracking. Through adopting a moments-based approach, novel expressions are derived for the outage probability, achievable throughput, and average symbol error rate (ASER) of dual-hop SWIPT relaying systems. Furthermore, new asymptotic analytical results are derived for the high SNR regime and are then utilised to analytically quantify the achievable diversity order. The proposed mathematical tools are demonstrated to be an accurate and efficient means by which one can conduct a thorough analysis on the system performance without the burden of Monte Carlo simulations. Finally, the third contribution focuses on SWIPT relaying systems operating in the presence of impulsive non-Gaussian noise, which is typical in several practical scenarios. Several studies show sufficient evidences that impulse man-made noise is encountered in various metropolitan, indoor, and underwater wireless applications. Examples of these sources include automotive ignition, electronic devices, household appliances, medical equipment, and industrial equipment. To characterise the behavior of the system performance under this type of noise, an efficient analytical framework is developed where novel closed-form xpressions for the pairwise error probability (PEP) are derived for two relaying schemes, namely, blind relaying and CSI-assisted relaying, employed under two assumptions imposed on the deployed EH process; namely, instantaneous EH (IEH) and average EH (AEH). Apart from being accurate, the derived expressions are shown to be efficient in quantifying the diversity order of the system and providing a comprehensive study on the impact of the severity of impulsive noise on the behavior of the system.

Building compact and low-cost yet flexible and reconfigurable radios for future wireless systems is generally a challenging task. On one hand, the needs for flexibility and re-configurability prevent using dedicated hardware particularly designed and optimized for only a single application or part of the radio spectrum. And on the other hand, to keep the overall size and cost of the radio equipment feasible, especially in multi-antenna multi-radio scenarios, the cost and size of individual radios are strongly limited. As a result, various imperfections and impairments are expected to take place in the used radio transceivers, especially in the radio frequency (RF) analog electronics. Good examples of such imperfections are, e.g., mirror-frequency interference due to I/Q imbalance, non-linear distortion due to mixer and amplifier nonlinearities, timing jitter and non-linearities in sampling and analog-to-digital (A/D) converter circuits, and oscillator phase noise. These impairments, if not properly understood and taken into account, can easily become a limiting factor to the quality and performance of the radio device and thereon of the whole wireless link. This is even more so, when more complex and more sensitive high-order modulated wideband communications waveforms are being deployed in the future systems. This article gives an overview of the essential RF impairments in modern radio communication context. Furthermore, different conceptual alternatives for reducing the effects of RF impairments in radio transmitters and receivers utilizing digital signal processing are described.

In this paper, we investigate the outage performance of an amplify-and-forward (AF) multiple input multiple output (MIMO) opportunistic relay selection system under time-varying channels. A practical scenario is considered, where both the relay selection and the beamforming transmission are performed with outdated channel state information (CSI), due to channel fluctuations and delays involved in relay selection and CSI feedback. To provide efficient means of evaluating the outage performance, a closed-form expression for the outage probability is given. The effects of parameters, such as the correlation between the outdated and current CSI, the number of antennas are also analyzed. All the analytical results are validated by Monte Carlo simulations.

The author investigates the performance of transmitter space shift keying (TSSK) with maximum-ratio combining (MRC) and receiver spatial modulation (RSM) with maximum-ratio transmission (MRT) in an unbalanced number of antennas at the transmitter and receiver. The author shows that RSM-MRT exhibits a superior performance under the same data rate, transmit power, and bandwidth in downlink cellular network systems where the number of transmit antennas at the base station is much larger than the number of receiver antennas at the mobile station. The author identifies a figure of metric as fading parameters and finds their probability density function (PDF) and cumulative distribution function (CDF). The performance gap between TSSK-MRC and RSM-MRT expands as the number of antennas increases while maintaining the same ratio of the number of antennas at the transmitter and receiver for a chosen modulation.

Direct-conversion radio receivers can offer highly integrated low-cost hardware solutions for spectrum sensing in cognitive radio systems. However, these receivers are susceptible to radio frequency (RF) impairments, such as in-phase and quadrature-phase imbalance, low-noise amplifier nonlinearities and phase noise, which limit the spectrum sensing capabilities. In this paper, we study the joint effects of RF impairments on energy detection based spectrum sensing in multi-channel environments. In particular, we provide an analytical framework for the evaluation of the probabilities of detection and false alarm, assuming Rayleigh fading and approximating the joint effects of RF impairments by a Gaussian distribution. Numerical results illustrate the detrimental effects of RF imperfections on the spectrum sensing performance, which bring significant losses in the spectrum utilization.

In this study, the outage performance of a multi-relay cooperative network operating in a spectrum sharing environment is analysed. Focusing on the secondary communication process, a relay selection procedure is performed prior to the communication process, where the selected relay is the one maximising the instantaneous end-to-end signal-to-noise ratio (SNR) and, at the same time, satisfying the power restrictions imposed by the primary receivers. The secondary destination employs a selection combining strategy, selecting the best path (direct link or relaying link) with the highest instantaneous SNR. Two kinds of relaying protocols are considered: decode-and-forward and amplify-and-forward. For the former, an exact closed-form expression for the outage probability (OP) is derived, whereas for the latter an accurate approximation for the OP is obtained. Then, an asymptotic analysis is carried out and reveals that the system diversity order is N + 1 for both relaying protocols, with N denoting the number of relays. Monte Carlo simulations assess the accuracy of the authors' results.

In this study, the performance of wirelessly powered relay‐aided non‐orthogonal multiple access networks is investigated in terms of outage probability. Specifically, two relay selection strategies, i.e. two‐stage relay selection (TRS) and maximum energy harvesting relay selection (MEHS), and two energy harvesting scenarios, i.e. time switching (TS) and power splitting (PS) are considered. In each setup, outage probabilities' analytical expressions and their asymptotics are derived. Monte‐Carlo simulations are also carried out to verify the correctness of the analysis. The results show that regardless of relay selection and energy harvesting strategies, increasing transmit power can improve the proposed system performance. However, TRS can achieve a full diversity order, while MEHS has a unit diversity order. Besides, the results recommend parameter selections of PS and TS coefficients for optimal performance in term of outage probability.

Multiple-input-multiple-output (MIMO) receivers with direct-conversion radio-frequency (RF) architecture are investigated in this paper. Direct conversion is a low-cost RF design that requires fewer external components in chip implementation. Nevertheless, it introduces extra RF impairments such as I-Q imbalance and dc offset in addition to frequency offset that is commonly encountered in all RF architectures. This paper proposes to do the joint least squares (LS) estimation of frequency, dc offset, I-Q imbalance, and a channel in MIMO receivers to improve performance; frequency-dependent and frequency-independent I-Q imbalances are included. Previously, RF impairments were separately estimated in MIMO receivers, which leads to inferior performance. In particular, a receiver architecture that facilitates the joint estimation of the frequency, the dc offset, the I-Q imbalance, and the channel is proposed. The LS criterion is then applied to obtain the joint estimators, with a special training-sequence design to reduce complexity. Simplified estimators on the frequency and dc offset are also proposed with almost no loss in performance. Finally, the estimators are shown through analysis to be unbiased and approach the Cramer-Rao lower bound (CRLB) for signal-to-noise ratios (SNRs) of interest. The analysis is verified by computer simulations.