Performance analysis of NOMA in 5G systems with HPA nonlinearities

LISBOA-01-0145-FEDER-0307095-PTDC/EEITEL/30709/2017 PTDC/EEI-TEL/30588/20 UIDB/50008/2020In this paper, we provide an analytical performance assessment of downlink non-orthogonal multiple access (NOMA) systems over Nakagami-m fading channels in the presence of nonlinear high-power amplifiers (HPAs). By modeling the distortion of the HPA by a nonlinear polynomial model, we evaluate the performance the NOMA scheme in terms of outage probability (OP) and ergodic sum rate. Hence, we derive a new closed-form expression for the exact OP, taking into account the undesirable effects of HPA. Furthermore, to characterize the diversity order of the considered system, the asymptotic OP in the high signal-to-noise (SNR) regime is derived. Moreover, the ergodic sum rate is investigated, resulting in new upper and lower bounds. Our numerical results demonstrate that the performance loss in presence of nonlinear distortions is very substantial at high data rates. In particular, it is proved that in presence of HPA distortion, the ergodic sum rate cannot exceed a determined threshold which limits its performance compared to the ideal hardware case. Monte-Carlo simulations are conducted and their results agree well with the analytical results.publishersversionpublishe

Full-duplex wireless communications: challenges, solutions and future research directions

The family of conventional half-duplex (HD) wireless systems relied on transmitting and receiving in different time-slots or frequency sub-bands. Hence the wireless research community aspires to conceive full-duplex (FD) operation for supporting concurrent transmission and reception in a single time/frequency channel, which would improve the attainable spectral efficiency by a factor of two. The main challenge encountered in implementing an FD wireless device is the large power difference between the self-interference (SI) imposed by the device’s own transmissions and the signal of interest received from a remote source. In this survey, we present a comprehensive list of the potential FD techniques and highlight their pros and cons. We classify the SI cancellation techniques into three categories, namely passive suppression, analog cancellation and digital cancellation, with the advantages and disadvantages of each technique compared. Specifically, we analyse the main impairments (e.g. phase noise, power amplifier nonlinearity as well as in-phase and quadrature-phase (I/Q) imbalance, etc.) that degrading the SI cancellation. We then discuss the FD based Media Access Control (MAC)-layer protocol design for the sake of addressing some of the critical issues, such as the problem of hidden terminals, the resultant end-to-end delay and the high packet loss ratio (PLR) due to network congestion. After elaborating on a variety of physical/MAC-layer techniques, we discuss potential solutions conceived for meeting the challenges imposed by the aforementioned techniques. Furthermore, we also discuss a range of critical issues related to the implementation, performance enhancement and optimization of FD systems, including important topics such as hybrid FD/HD scheme, optimal relay selection and optimal power allocation, etc. Finally, a variety of new directions and open problems associated with FD technology are pointed out. Our hope is that this treatise will stimulate future research efforts in the emerging field of FD communication

Identification of key research topics in 5G using co-word analysis

Project Work presented as the partial requirement for obtaining a Master's degree in Information Management, specialization in Knowledge Management and Business IntelligenceThe aim of this research is to better understand the field of 5G by analyzing the more than 10000 publications found in the Web of Science database. To achieve this, a co-word analysis was performed to identify research topics based on the author keywords and a strategic diagram was used to measure their level of maturity and relevance to the field. In total this analysis identified that all the articles can be grouped into seven topics, from which, two are mature but peripheral, one is both well developed and central to the field, and the rest are central, but underdeveloped. The value of this research, was the usage of a well-established technique that has been used in many fields, but never in the field of 5G which is growing in relevance

Quantifying Potential Energy Efficiency Gain in Green Cellular Wireless Networks

Conventional cellular wireless networks were designed with the purpose of providing high throughput for the user and high capacity for the service provider, without any provisions of energy efficiency. As a result, these networks have an enormous Carbon footprint. In this paper, we describe the sources of the inefficiencies in such networks. First we present results of the studies on how much Carbon footprint such networks generate. We also discuss how much more mobile traffic is expected to increase so that this Carbon footprint will even increase tremendously more. We then discuss specific sources of inefficiency and potential sources of improvement at the physical layer as well as at higher layers of the communication protocol hierarchy. In particular, considering that most of the energy inefficiency in cellular wireless networks is at the base stations, we discuss multi-tier networks and point to the potential of exploiting mobility patterns in order to use base station energy judiciously. We then investigate potential methods to reduce this inefficiency and quantify their individual contributions. By a consideration of the combination of all potential gains, we conclude that an improvement in energy consumption in cellular wireless networks by two orders of magnitude, or even more, is possible.Comment: arXiv admin note: text overlap with arXiv:1210.843

Nonlinear amplifier distortion in cooperative OFDM systems

OFDM (Orthogonal frequency division multiplexing) on lupaava langattoman tietoliikenteen teknologia johtuen sen hyvästä suorituskyvystä monitieympäristössä. Yhteistoiminnallisen tiedonvälityksen tekniikka on nykyisin jatkuvan tutkimuksen kohteena. Se hyödyntää muiden päätteiden antenneja virtuaalisen moniantennijärjestelmän luomiseen mahdollistaen moniantennijärjestelmille ominaisia kapasiteettihyötyjä. Tässä diplomityössä tutkitaan epälineaarista vahvistussäröä, kun näitä molempia tekniikoita käytetään yhdessä. Ensimmäiset kappaleet käsittelevät OFDM-järjestelmien ja epälineaaristen OFDM-järjestelmien särön sekä yhteistoiminnallisen tiedonvälityksen taustoja. Yhteistoiminnallisten OFDM-järjestelmien suorituskykyä mitataan simulaatioiden avulla epälineaarisen särön vaikuttaessa. Suorituskykyä mitataan bittivirhesuhteena käyttäen epäyhteistoiminnallista ja lineaarista yhteistoiminnallista järjestelmää vertailukohteena. Lisäksi särötermi myös analysoidaan. Systeemimalli sisältää epälineaarisen vahvistuksen välittimessä, jota mallinnetaan elektronisella tehovahvistimella. Lopuksi esitellään ja testataan tekniikka järjestelmän suorituskyvyn parantamiseen optimoimalla maksimisuhdeyhdistintä. Se optimoidaan mallintamalla vahvistussäröä normaalijakaumalla. Lisäksi esitellään ja testataan yhteistoiminnallisille järjestelmille sopiva tehovahvistimen epälineaarisuuden poistotekniikan muunnelma, jolla saadaan lähellä lineaarista tapausta olevia tuloksia.Orthogonal frequency division multiplexing (OFDM) is a promising technique for wireless communications because of its good performance under multipath environments. The concept of cooperative communications is currently under constant research. It uses antennas of other terminals to create virtual multiple input multiple output (MIMO) systems, providing capacity gains similar to those of MIMO systems. This thesis studies the issue of nonlinear amplifier distortion when these two techniques are used together. The first chapters give a background on OFDM systems, nonlinear distortion in OFDM systems, and Cooperative Communications. The performance of OFDM cooperative systems under nonlinear distortion are measured by simulations. The performance is measured in terms of BER using a non-cooperative system and a linear cooperative system as references. In addition, the distortion term is also analysed. The system model includes a non-linear amplifier at the relay, modelled as a solid state power amplifier (SSPA). A technique for improving the performance of the system, by optimising the maximum ratio combiner (MRC), is introduced and tested. The MRC is optimised by modelling the distortion noise as Gaussian. Also, a modification to the power amplifier nonlinearity cancellation (PANC) technique, suitable to cooperative systems, is introduced and tested, showing results close to the linear case

Diversity techniques for broadband wireless communications: performance enhancement and analysis

The diversity techniques have been proven to be effective for next generation broadband wireless communications, and are the focus of this thesis. The diversity techniques can be broadly categorized into three types: Space, Time, and Frequency. In this thesis, we are mainly concerned with frequency and space diversity techniques. Orthogonal Frequency Division Multiplexing (OFDM) is a frequency diversity technique which offers several benefits such as easier digital implementation, immunity to multipath channels, low complexity channel equalization, etc. Despite these desirable features, there are few inherent problems in OFDM such as high peak-to-average power ratio (PAPR). High PAPR demands large dynamic range in the transmitted chain such as digital to analog converter (DAC) and power amplifier (PA). Unless pre-processed, the transmitted signal gets distorted due to quantization errors and inter-modulation. In the initial stage of PhD candidature, the author focused on PAPR reduction techniques. A simple modification on conventional iterative clipping and filtering (ICF) technique was proposed which has less computational complexity. The power savings achievable from clipping and filtering method was considered next. Furthermore the ICF is compared with another distortion-less PAPR reduction technique called Selective Mapping (SLM) based on power savings. Finally, impact of clipping and filtering on the channel estimation was analyzed. Space diversity seeks to exploit the multi-path characteristics of wireless channels to improve the performance. The simplest form of the space diversity is the receive diversity where two or more antennas with sufficient spacing collect independent copies of the same transmitted signal, which contributes to better signal reception. In this thesis new analytical expressions for spectral efficiency, capacity, and error rates were presented for adaptive systems with channel estimation error. Beamforming (steering signal towards desired receiver) is another useful technique in multiple-antenna systems to further improve the system performance. MRT (Maximal Ratio Transmission) or MIMO-MRC is such system where the transmitter, based on channel feedback from the receiver, uses weighting factors to steer the transmitted signal. Closed form expressions for symbol error rates were derived for MRT system with channel estimation error. The results were extended to evaluate closed form expressions of error rates for Rectangular QAM. Antenna correlation was considered in another contribution on MRC systems. Relay and Cooperative networks represent another form of spatial diversity and have recently attracted significant research attention. These networks rely on intermediate nodes called "relays" to establish communication between the source and the destination. In addition to coverage extension, the relay networks have shown to offer cooperative diversity when there is a direct link or multiple relays. The first contribution is to analyze a dual-hop amplify-forward relay networks with dissimilar fading scenarios. Next error rates of Rectangular QAM for decode-forward selection relay system are derived. Multiple antenna at relay is included to analyze the benefits of dual spatial diversity over Rayleigh and Nakagami fading channels. Antenna selection is a cost-effective way to exploit the antenna diversity. General Order Antenna Selection (GOAS), based on Ordered Statistics, is used to evaluate signal statistics for a MIMO relay network