Automatic Distributed MIMO Testbed for beyond 5G Communication Experiments

This paper demonstrates an automated testbed suitable for beyond-5G distributed MIMO experiments, where bandwidth, number of transmitters and precoding methods are flexible and configured through a central unit. This is based on an all-digital radio-over-fiber approach to communication through 12 fully coherent, low-complexity remote radio transmitters. An automated robot receiver is implemented to facilitate efficient communication data collection in realistic environments. Using the proposed system, co-located and distributed MIMO communication antenna configurations are compared in a real in-door environment. The results show that distributed MIMO provides more significantly more uniform power distribution and better overall MIMO capacity compared to co-located MIMO

On the sum rate of ZF detectors in correlated K fading MIMO channels

This paper presents a detailed sum rate investigation of Zero-Forcing (ZF) detectors over composite multiple-input multiple-output (MIMO) channels. To this end, we consider the generic K distribution (Rayleigh/gamma distribution) to model the composite fading fluctuations and also assume the general case of semi-correlated small-scale fading. Novel exact analytical expressions are derived for the achievable sum rate followed by asymptotic expressions in the low Signal-to-Noise ratio (SNR) regime. In parallel, new, closed-form upper and lower bounds on the sum rate are derived that remain tight for all SNRs. The theoretical analysis is validated via a set of Monte-Carlo simulations

Experimental analysis of concentrated versus distributed massive MIMO in an indoor cell at 3.5 GHz

This paper presents a measurement-based comparison between distributed and concentrated massive multiple-input multiple-output (MIMO) systems, which are called D-mMIMO and C-mMIMO systems, in an indoor environment considering a 400 MHz bandwidth centered at 3.5 GHz. In both cases, we have considered an array of 64 antennas in the base station and eight simultaneously active users. The work focuses on the characterization of both schemes in the up-link, considering the analysis of the sum capacity, the total spectral efficiency (SE) achievable by using the zero forcing (ZF) combining method, as well as the user fairness. The effect of the power imbalance between the different transmitters or user terminal (UT) locations, and thus, the benefits of performing an adequate power control are also investigated. The differences between the C-mMIMO and D-mMIMO channel performances are explained through the observation of the structure of their respective measured channel matrices through parameters such as the condition number or the power imbalance between the channels established by each UT. The channel measurements have been performed in the frequency domain, emulating a massive MIMO system in the framework of a time-domain duplex orthogonal frequency multiple access network (TDD-OFDM-MIMO). The characterization of the MIMO channel is based on the virtual array technique for both C-mMIMO and D-mMIMO systems. The deployment of the C-mMIMO and D-MIMO systems, as well as the distribution of users in the measurement environment, has been arranged as realistically as possible, avoiding the movement of people or machines.This work has been supported by the Spanish Ministry of Economy, Industry and Competitiveness (TEC2017-86779-C2-1-R)

Reverse Link Analysis and Modeling of CDMA based Distributed Antenna Systems

Distributed Antenna Systems (DAS) are gaining widespread popularity in the indoor wireless market. Most of the existing research literature on this subject has focused on comparing the advantages of DAS systems with traditional macro cells. The goal of this thesis is to define a performance platform for CDMA based distributed antenna systems which can serve as essential design guidelines for future DAS deployments. One of the major issues presented by DAS systems is the noise floor rise in the reverse link caused by the DAS noise figure. This noise rise becomes a limiting factor in determining capacity and coverage of the DAS based network. The common practice of adding an attenuator (padding) in front of the Base Station receiver can bring down the noise floor, but at the same time force the mobile units to transmit at higher power. We establish an operating platform for the DAS so that the system can be effectively operated within the limits of mobile transmit power and base station receive power, maximizing the system capacity

Near-capacity fixed-rate and rateless channel code constructions

Fixed-rate and rateless channel code constructions are designed for satisfying conflicting design tradeoffs, leading to codes that benefit from practical implementations, whilst offering a good bit error ratio (BER) and block error ratio (BLER) performance. More explicitly, two novel low-density parity-check code (LDPC) constructions are proposed; the first construction constitutes a family of quasi-cyclic protograph LDPC codes, which has a Vandermonde-like parity-check matrix (PCM). The second construction constitutes a specific class of protograph LDPC codes, which are termed as multilevel structured (MLS) LDPC codes. These codes possess a PCM construction that allows the coexistence of both pseudo-randomness as well as a structure requiring a reduced memory. More importantly, it is also demonstrated that these benefits accrue without any compromise in the attainable BER/BLER performance. We also present the novel concept of separating multiple users by means of user-specific channel codes, which is referred to as channel code division multiple access (CCDMA), and provide an example based on MLS LDPC codes. In particular, we circumvent the difficulty of having potentially high memory requirements, while ensuring that each user’s bits in the CCDMA system are equally protected. With regards to rateless channel coding, we propose a novel family of codes, which we refer to as reconfigurable rateless codes, that are capable of not only varying their code-rate but also to adaptively modify their encoding/decoding strategy according to the near-instantaneous channel conditions. We demonstrate that the proposed reconfigurable rateless codes are capable of shaping their own degree distribution according to the nearinstantaneous requirements imposed by the channel, but without any explicit channel knowledge at the transmitter. Additionally, a generalised transmit preprocessing aided closed-loop downlink multiple-input multiple-output (MIMO) system is presented, in which both the channel coding components as well as the linear transmit precoder exploit the knowledge of the channel state information (CSI). More explicitly, we embed a rateless code in a MIMO transmit preprocessing scheme, in order to attain near-capacity performance across a wide range of channel signal-to-ratios (SNRs), rather than only at a specific SNR. The performance of our scheme is further enhanced with the aid of a technique, referred to as pilot symbol assisted rateless (PSAR) coding, whereby a predetermined fraction of pilot bits is appropriately interspersed with the original information bits at the channel coding stage, instead of multiplexing pilots at the modulation stage, as in classic pilot symbol assisted modulation (PSAM). We subsequently demonstrate that the PSAR code-aided transmit preprocessing scheme succeeds in gleaning more information from the inserted pilots than the classic PSAM technique, because the pilot bits are not only useful for sounding the channel at the receiver but also beneficial for significantly reducing the computational complexity of the rateless channel decoder