23 research outputs found
Analysis and Design of Cell-Free Massive MIMO Systems under Spatially Correlated Fading Channels
Mención Internacional en el título de doctorWireless communications have become a key pillar in our modern society. It can be hard to
think of a service that somehow does not rely on them. Particularly, mobile networks are one of
the most necessary technologies in our daily life. This produces that the demand for data rates
is by no means stopping from increasing. The cellular architecture is facing a crucial challenge
under limited performance by interference and spectrum saturation. This involves cell-edge
users experiencing poor performance due to the close vicinity of base stations (BSs) using
the same carrier frequency. Based on a combination of the coordinated multi-point (CoMP)
technique and traditional massive multiple-input multiple-output (MIMO) systems, cell-free
(CF) massive MIMO networks have irrupted as a solution for avoiding inter-cell interference
issues and for providing uniform service in large coverage areas. This thesis focuses on the
analysis and design of CF massive MIMO networks assuming a spatially correlated fading
model. A general-purpose channel model is provided and the whole network functioning is
given in detail.
Despite the many characteristics a CF massive MIMO system shares with conventional colocated
massive MIMO its distributed nature brings along new issues that need to be carefully
accounted for. In particular, the so-called channel hardening effect that postulates that the variance
of the compound wireless channel experienced by a given user from a large number of
transmit antennas tends to vanish, effectively making the channel deterministic. This critical
assumption, which permeates most theoretical results of massive MIMO, has been well investigated
and validated in centralized architectures, however, it has received little attention in the
context of CF massive MIMO networks. Hardening in CF architectures is potentially compromised
by the different large-scale gains each access point (AP) impinges on the transmitted
signal to each user, a condition that is further stressed when not all APs transmit to all users as
proposed in the user-centric (UC) variations of CF massive MIMO. In this document, the presence
of channel hardening in this new architecture scheme is addressed using distributed and
cooperative precoders and combiners and different power control strategies. It is shown that
the line-of-sight (LOS) component, spatially correlated antennas, and clustering schemes have
an impact on how the channel hardens. In addition, we examine the existent gap between the
estimated achievable rate and the true network performance when channel hardening is compromised. Exact closed-form expressions for both a hardening metric and achievable downlink
(DL) and uplink (UL) rates are given as well.
We also look into the pilot contamination problem in the UL and DL with different degrees
of cooperation between the APs. The optimum minimum mean-squared error (MMSE) processing
can take advantage of large-scale fading coefficients for canceling the interference of
pilot-sharing users and thus achieves asymptotically unbounded capacity. However, it is computationally
demanding and can only be implemented in a fully centralized network. Here,
sub-optimal schemes are derived that provide unbounded capacity with much lower complexity
and using only local channel estimates but global channel statistics. This makes them suited for
both centralized and distributed networks. In this latter case, the best performance is achieved
with a generalized maximum ratio combiner that maximizes a capacity bound based on channel
statistics only.Programa de Doctorado en Multimedia y Comunicaciones por la Universidad Carlos III de Madrid y la Universidad Rey Juan CarlosPresidente: Rui Dinis.- Secretario: María Julia Fernández-Getino García.- Vocal: Carmen Botella Mascarel
Joint Design of Power Control and Access Point Scheduling for Uplink Cell-Free Massive MIMO Networks
This work proposes a joint power control and access points (APs) scheduling
algorithm for uplink cell-free massive multiple-input multiple-output
(CF-mMIMO) networks without channel hardening assumption. Extensive studies
have done on the joint optimization problem assuming the channel hardening.
However, it has been reported that the channel hardening may not be validated
in some CF-mMIMO environments. In particular, the existing Use-and-then-Forget
(UatF) bound based on the channel hardening often seriously underestimates user
rates in CF-mMIMO. Therefore, a new performance evaluation technique without
resorting to the channel hardening is indispensable for accurate performance
estimations. Motivated by this, we propose a new bound on the achievable rate
of uplink CF-mMIMO. It is demonstrated that the proposed bound provides a more
accurate performance estimate of CF-mMIMO than that of the existing UatF bound.
The proposed bound also enables us to develop a joint power control and APs
scheduling algorithm targeting at both improving fairness and reducing the
resource between APs and a central processing unit (CPU). We conduct extensive
performance evaluations and comparisons for systems designed with the proposed
and existing algorithms. The comparisons show that a considerable performance
improvement is achievable with the proposed algorithm even at reduced resource
between APs and CPU.Comment: 30 pages, 7 Figures. This work has been submitted to the IEEE for
possible publication. Copyright may be transferred without notice, after
which this version may no longer be accessibl
A Tutorial on Extremely Large-Scale MIMO for 6G: Fundamentals, Signal Processing, and Applications
Extremely large-scale multiple-input-multiple-output (XL-MIMO), which offers
vast spatial degrees of freedom, has emerged as a potentially pivotal enabling
technology for the sixth generation (6G) of wireless mobile networks. With its
growing significance, both opportunities and challenges are concurrently
manifesting. This paper presents a comprehensive survey of research on XL-MIMO
wireless systems. In particular, we introduce four XL-MIMO hardware
architectures: uniform linear array (ULA)-based XL-MIMO, uniform planar array
(UPA)-based XL-MIMO utilizing either patch antennas or point antennas, and
continuous aperture (CAP)-based XL-MIMO. We comprehensively analyze and discuss
their characteristics and interrelationships. Following this, we examine exact
and approximate near-field channel models for XL-MIMO. Given the distinct
electromagnetic properties of near-field communications, we present a range of
channel models to demonstrate the benefits of XL-MIMO. We further motivate and
discuss low-complexity signal processing schemes to promote the practical
implementation of XL-MIMO. Furthermore, we explore the interplay between
XL-MIMO and other emergent 6G technologies. Finally, we outline several
compelling research directions for future XL-MIMO wireless communication
systems.Comment: 38 pages, 10 figure
Digital and Mixed Domain Hardware Reduction Algorithms and Implementations for Massive MIMO
Emerging 5G and 6G based wireless communications systems largely rely on multiple-input-multiple-output (MIMO) systems to reduce inherently extensive path losses, facilitate high data rates, and high spatial diversity. Massive MIMO systems used in mmWave and sub-THz applications consists of hundreds perhaps thousands of antenna elements at base stations. Digital beamforming techniques provide the highest flexibility and better degrees of freedom for phased antenna arrays as compared to its analog and hybrid alternatives but has the highest hardware complexity.
Conventional digital beamformers at the receiver require a dedicated analog to digital converter (ADC) for every antenna element, leading to ADCs for elements. The number of ADCs is the key deterministic factor for the power consumption of an antenna array system. The digital hardware consists of fast Fourier transform (FFT) cores with a multiplier complexity of (N log2N) for an element system to generate multiple beams. It is required to reduce the mixed and digital hardware complexities in MIMO systems to reduce the cost and the power consumption, while maintaining high performance.
The well-known concept has been in use for ADCs to achieve reduced complexities. An extension of the architecture to multi-dimensional domain is explored in this dissertation to implement a single port ADC to replace ADCs in an element system, using the correlation of received signals in the spatial domain. This concept has applications in conventional uniform linear arrays (ULAs) as well as in focal plane array (FPA) receivers.
Our analysis has shown that sparsity in the spatio-temporal frequency domain can be exploited to reduce the number of ADCs from N to where . By using the limited field of view of practical antennas, multiple sub-arrays are combined without interferences to achieve a factor of K increment in the information carrying capacity of the ADC systems. Applications of this concept include ULAs and rectangular array systems. Experimental verifications were done for a element, 1.8 - 2.1 GHz wideband array system to sample using ADCs.
This dissertation proposes that frequency division multiplexing (FDM) receiver outputs at an intermediate frequency (IF) can pack multiple (M) narrowband channels with a guard band to avoid interferences. The combined output is then sampled using a single wideband ADC and baseband channels are retrieved in the digital domain. Measurement results were obtained by employing a element, 28 GHz antenna array system to combine channels together to achieve a 75% reduction of ADC requirement.
Implementation of FFT cores in the digital domain is not always exact because of the finite precision. Therefore, this dissertation explores the possibility of approximating the discrete Fourier transform (DFT) matrix to achieve reduced hardware complexities at an allowable cost of accuracy. A point approximate DFT (ADFT) core was implemented on digital hardware using radix-32 to achieve savings in cost, size, weight and power (C-SWaP) and synthesized for ASIC at 45-nm technology
Cell-Free Massive MIMO and Millimeter Wave Channel Modelling for 5G and Beyond
Huge demand for wireless throughput and number of users which are connected to the base station (BS) has been observed in the last decades. Massive multiple-input multiple-output (MIMO) is a promising technique for 5G for the following reasons; 1) high throughput; 2) serving large numbers of users at the same time; 3) energy efficiency. However, the low throughput of cell-edge users remains a limitation in realistic multi-cell massive MIMO systems. In cell-free massive MIMO, on the other hand, distributed access points (APs) are connected to a central processing unit (CPU) and jointly serve distributed users. This thesis investigates the performance of cell-free Massive MIMO with limited-capacity fronthaul links from the APs to the CPU which will be essential in practical 5G networks. To model the limited-capacity fronthaul links, we exploit the optimal uniform quantization. Next, closed-form expressions for spectral and energy efficiencies are presented. Numerical results investigate the performance gap between limited fronthaul and perfect fronthaul cases, and demonstrate that exploiting a relatively few quantization bits, the performance of limited-fronthaul cell-free Massive MIMO closely approaches the perfect fronthaul performance. Next, the energy efficiency maximization problem and max-min fairness problems are considered with per-user power and fronthaul capacity constraints. We propose an iterative procedure which exploits a generalized eigen vector problem and geometric programming (GP) to solve the max-min optimization problem. Numerical results indicate the superiority of the proposed algorithms over the case of equal power allocation. On the other hand, the performance of communication systems depends on the propagation channel. To investigate the performance of MIMO systems, an accurate small scale fading channel model is necessary. Geometry-based stochastic channel models (GSCMs) are mathematically tractable models to investigate the performance of MIMO systems