How Much Do Downlink Pilots Improve Cell-Free Massive MIMO?

In this paper, we analyze the benefits of including downlink pilots in a cell-free massive MIMO system. We derive an approximate per-user achievable downlink rate for conjugate beamforming processing, which takes into account both uplink and downlink channel estimation errors, and power control. A performance comparison is carried out, in terms of per-user net throughput, considering cell-free massive MIMO operation with and without downlink training, for different network densities. We take also into account the performance improvement provided by max-min fairness power control in the downlink. Numerical results show that, exploiting downlink pilots, the performance can be considerably improved in low density networks over the conventional scheme where the users rely on statistical channel knowledge only. In high density networks, performance improvements are moderate.Comment: 7 pages, 5 figures. IEEE Global Communications Conference 2016 (GLOBECOM). Accepte

Downlink Training in Cell-Free Massive MIMO: A Blessing in Disguise

Cell-free Massive MIMO (multiple-input multiple-output) refers to a distributed Massive MIMO system where all the access points (APs) cooperate to coherently serve all the user equipments (UEs), suppress inter-cell interference and mitigate the multiuser interference. Recent works demonstrated that, unlike co-located Massive MIMO, the \textit{channel hardening} is, in general, less pronounced in cell-free Massive MIMO, thus there is much to benefit from estimating the downlink channel. In this study, we investigate the gain introduced by the downlink beamforming training, extending the previously proposed analysis to non-orthogonal uplink and downlink pilots. Assuming single-antenna APs, conjugate beamforming and independent Rayleigh fading channel, we derive a closed-form expression for the per-user achievable downlink rate that addresses channel estimation errors and pilot contamination both at the AP and UE side. The performance evaluation includes max-min fairness power control, greedy pilot assignment methods, and a comparison between achievable rates obtained from different capacity-bounding techniques. Numerical results show that downlink beamforming training, although increases pilot overhead and introduces additional pilot contamination, improves significantly the achievable downlink rate. Even for large number of APs, it is not fully efficient for the UE relying on the statistical channel state information for data decoding.Comment: Published in IEEE Transactions on Wireless Communications on August 14, 2019. {\copyright} 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other use

Self-Learning Detector for the Cell-Free Massive MIMO Uplink: The Line-of-Sight Case

The precoding in cell-free massive multiple-input multiple-output (MIMO) technology relies on accurate knowledge of channel responses between users (UEs) and access points (APs). Obtaining high-quality channel estimates in turn requires the path losses between pairs of UEs and APs to be known. These path losses may change rapidly especially in line-of-sight environments with moving blocking objects. A difficulty in the estimation of path losses is pilot contamination, that is, simultaneously transmitted pilots from different UEs that may add up destructively or constructively by chance, seriously affecting the estimation quality (and hence the eventual performance). A method for estimation of path losses, along with an accompanying pilot transmission scheme, is proposed that works for both Rayleigh fading and line-of-sight channels and that significantly improves performance over baseline state-of-the-art. The salient feature of the pilot transmission scheme is that pilots are structurally phase-rotated over different coherence blocks (according to a pre-determined function known to all parties), in order to create an effective statistical distribution of the received pilot signal that can be efficiently exploited by the proposed estimation algorithm.Comment: Paper accepted for presentation in IEEE SPAWC 2020 - 21st IEEE International Workshop on Signal Processing Advances in Wireless Communications. {\copyright} 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other use

Ubiquitous Cell-Free Massive MIMO Communications

Since the first cellular networks were trialled in the 1970s, we have witnessed an incredible wireless revolution. From 1G to 4G, the massive traffic growth has been managed by a combination of wider bandwidths, refined radio interfaces, and network densification, namely increasing the number of antennas per site. Due its cost-efficiency, the latter has contributed the most. Massive MIMO (multiple-input multiple-output) is a key 5G technology that uses massive antenna arrays to provide a very high beamforming gain and spatially multiplexing of users, and hence, increases the spectral and energy efficiency. It constitutes a centralized solution to densify a network, and its performance is limited by the inter-cell interference inherent in its cell-centric design. Conversely, ubiquitous cell-free Massive MIMO refers to a distributed Massive MIMO system implementing coherent user-centric transmission to overcome the inter-cell interference limitation in cellular networks and provide additional macro-diversity. These features, combined with the system scalability inherent in the Massive MIMO design, distinguishes ubiquitous cell-free Massive MIMO from prior coordinated distributed wireless systems. In this article, we investigate the enormous potential of this promising technology while addressing practical deployment issues to deal with the increased back/front-hauling overhead deriving from the signal co-processing.Comment: Published in EURASIP Journal on Wireless Communications and Networking on August 5, 201

On the Performance of Cell-Free Massive MIMO with Short-Term Power Constraints

In this paper we consider a time-division duplex cell-free massive multiple-input multiple-output (MIMO) system where many distributed access points (APs) simultaneously serve many users. A normalized conjugate beamforming scheme, which satisfies short-term average power constraints at the APs, is proposed and analyzed taking into account the effect of imperfect channel information. We derive an approximate closed-form expression for the per-user achievable downlink rate of this scheme. We also provide, analytically and numerically, a performance comparison between the normalized conjugate beamforming and the conventional conjugate beamforming scheme in [1] (which satisfies long-term average power constraints). Normalized conjugate beamforming scheme reduces the beamforming uncertainty gain, which comes from the users' lack of the channel state information knowledge, and hence, it improves the achievable downlink rate compared to the conventional conjugate beamforming scheme.Comment: 6 pages, 4 figures, 21st IEEE International Workshop on Computer Aided Modelling and Design of Communication Links and Networks (CAMAD). Special Session - 5Gwireless: Innovative Architectures, Wireless Technologies and Tools for High Capacity and Sustainable 5G Ultra-Dense Cellular Network

Reinventing geopolitical codes in the post-Cold War world with special reference to international terrorism

Through a study of geopolitical codes, this thesis examines the condition of the Westphalian sovereign state in the post-Cold War world. Focusing primarily on the events of September 11th 2001 and their aftermath, the research questions the sustainability of the state as conceived by (neo)realists in the context of new regional and global actors and the processes underpinning these. From a critical realist perspective the study uses a comparison between Europe, where regionalization is particularly noticeable, and the hegemonic United States, in order to explore how the non-state global terrorist actor and the regional European actor impact upon responses, characterizations and therefore geopolitical codes relating to terrorism. In so doing the plausibility of emerging common European geopolitical codes is considered. The thesis is structured around the discussion of the codes of the United States, Britain and France, in addition to a more limited examination of the European Union. This (neo)realist component is complemented by the use of discourse analysis, a technique more common in critical geopolitics. The analysis is applied to government documents from each of the sample states (and the EU). From this analysis the research determines that each state retains unique geopolitical codes while sharing many components that contribute to their reproduction as sovereign states. Furthermore, although common European codes appear to be unlikely in these circumstances, the European context and imaginations apparent in Britain and France points to a regional dimension. The thesis concludes that the Westphalian sovereign state remains the dominant geopolitical actor, although other actors impinge upon it. This is more apparent in Europe where the regional dimension constitutes an added layer of governance and may signify a move away from the ‘modern’ character of the Westphalian state that continues to be more persistent in hegemonic America.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Multicarrier Modulation and Coding for Multichannel Systems

ii The use of Rate Compatible Punctured Convolutional codes (RCPC-codes) for rate matching in multichannel systems is also presented. A DS-CDMA system is used for evaluating purposes. The RCPC-codes are shown to provide a flexible and efficient coding scheme which is superior compared to repetition encoding schemes. Keywords: Multicarrier modulation, OFDM, Channel estimation, Decision directed coherent detection, Parallel combinatory signaling, RCPC-codes, rate matching, multichannel systems, multi-code DS-CDMA. iii ABSTRACT i CONTENTS iii ACKNOWLEDGMENTS vii 1 INTRODUCTION 1 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Short Introduction to OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Applications of OFDM OFDM Techniques 1.3 Multi-Code DS-CDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.4 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.5 Cont..

Multirate Codes and Multicarrier Modulation for Future Communication Systems

In this thesis, new detection and modulation methods for multicarrier systems are presented. The proposed multicarrier detectors are based on feedback of previously detected symbols and the idea is to exploit the correlation, both in time between consecutive symbols, and in frequency between neighboring sub-carriers. This enables us to calculate a reliable channel estimate, to be used for coherent detection. The proposed detectors have significantly lower irreducible bit error probabilities compared to a conventional dioeerential detector. Furthermore, a Parallel Combinatory - Orthogonal Frequency Division Multiplexing (PC-OFDM) system is proposed and analyzed. Here, some of the information bits select, in each symbol interval, a subset of the available sub-carriers. The selected subcarriers are then modulated by points from an M-PSK signal constellation. PC-OFDM systems can be designed to have lower bit error probability on Gaussian channels, lower Peak-to-Average Power Ratio (PAPR) ..

Turbo decoding on Rayleigh fading channels with noisy channel estimates

A new metric for Turbo decoding on Rayleigh fading channels with noisy channel estimates is derived. By including the error variance of the channel estimate in the decoder metric derivation, we calculate the correct channel reliability factor for this case. Simulations show that the SNR gain of using this new metric, compared to if we ignore the uncertainty of the channel estimate, may be as large as 1 dB at a bit error probability of 10(-3). This gain is obtained without any complexity increase