262 research outputs found
Link abstraction models based on mutual information for LTE downlink
Postprint (author’s final draft
Efficient DSP and Circuit Architectures for Massive MIMO: State-of-the-Art and Future Directions
Massive MIMO is a compelling wireless access concept that relies on the use
of an excess number of base-station antennas, relative to the number of active
terminals. This technology is a main component of 5G New Radio (NR) and
addresses all important requirements of future wireless standards: a great
capacity increase, the support of many simultaneous users, and improvement in
energy efficiency. Massive MIMO requires the simultaneous processing of signals
from many antenna chains, and computational operations on large matrices. The
complexity of the digital processing has been viewed as a fundamental obstacle
to the feasibility of Massive MIMO in the past. Recent advances on
system-algorithm-hardware co-design have led to extremely energy-efficient
implementations. These exploit opportunities in deeply-scaled silicon
technologies and perform partly distributed processing to cope with the
bottlenecks encountered in the interconnection of many signals. For example,
prototype ASIC implementations have demonstrated zero-forcing precoding in real
time at a 55 mW power consumption (20 MHz bandwidth, 128 antennas, multiplexing
of 8 terminals). Coarse and even error-prone digital processing in the antenna
paths permits a reduction of consumption with a factor of 2 to 5. This article
summarizes the fundamental technical contributions to efficient digital signal
processing for Massive MIMO. The opportunities and constraints on operating on
low-complexity RF and analog hardware chains are clarified. It illustrates how
terminals can benefit from improved energy efficiency. The status of technology
and real-life prototypes discussed. Open challenges and directions for future
research are suggested.Comment: submitted to IEEE transactions on signal processin
Técnicas de pré-codificação para sistemas multicelulares coordenados
Doutoramento em TelecomunicaçõesCoordenação Multicélula é um tópico de investigação em rápido
crescimento e uma solução promissora para controlar a interferência entre
células em sistemas celulares, melhorando a equidade do sistema e
aumentando a sua capacidade. Esta tecnologia já está em estudo no LTEAdvanced
sob o conceito de coordenação multiponto (COMP). Existem
várias abordagens sobre coordenação multicélula, dependendo da
quantidade e do tipo de informação partilhada pelas estações base, através
da rede de suporte (backhaul network), e do local onde essa informação é
processada, i.e., numa unidade de processamento central ou de uma forma
distribuída em cada estação base.
Nesta tese, são propostas técnicas de pré-codificação e alocação de
potência considerando várias estratégias: centralizada, todo o
processamento é feito na unidade de processamento central; semidistribuída,
neste caso apenas parte do processamento é executado na
unidade de processamento central, nomeadamente a potência alocada a
cada utilizador servido por cada estação base; e distribuída em que o
processamento é feito localmente em cada estação base. Os esquemas
propostos são projectados em duas fases: primeiro são propostas soluções
de pré-codificação para mitigar ou eliminar a interferência entre células,
de seguida o sistema é melhorado através do desenvolvimento de vários
esquemas de alocação de potência. São propostas três esquemas de
alocação de potência centralizada condicionada a cada estação base e com
diferentes relações entre desempenho e complexidade. São também
derivados esquemas de alocação distribuídos, assumindo que um sistema
multicelular pode ser visto como a sobreposição de vários sistemas com
uma única célula. Com base neste conceito foi definido uma taxa de erro
média virtual para cada um desses sistemas de célula única que compõem
o sistema multicelular, permitindo assim projectar esquemas de alocação
de potência completamente distribuídos.
Todos os esquemas propostos foram avaliados em cenários realistas,
bastante próximos dos considerados no LTE. Os resultados mostram que
os esquemas propostos são eficientes a remover a interferência entre
células e que o desempenho das técnicas de alocação de potência
propostas é claramente superior ao caso de não alocação de potência. O
desempenho dos sistemas completamente distribuídos é inferior aos
baseados num processamento centralizado, mas em contrapartida podem
ser usados em sistemas em que a rede de suporte não permita a troca de
grandes quantidades de informação.Multicell coordination is a promising solution for cellular wireless systems
to mitigate inter-cell interference, improving system fairness and
increasing capacity and thus is already under study in LTE-A under the
coordinated multipoint (CoMP) concept. There are several coordinated
transmission approaches depending on the amount of information shared
by the transmitters through the backhaul network and where the
processing takes place i.e. in a central processing unit or in a distributed
way on each base station.
In this thesis, we propose joint precoding and power allocation techniques
considering different strategies: Full-centralized, where all the processing
takes place at the central unit; Semi-distributed, in this case only some
process related with power allocation is done at the central unit; and Fulldistributed,
where all the processing is done locally at each base station.
The methods are designed in two phases: first the inter-cell interference is
removed by applying a set of centralized or distributed precoding vectors;
then the system is further optimized by centralized or distributed power
allocation schemes. Three centralized power allocation algorithms with
per-BS power constraint and different complexity tradeoffs are proposed.
Also distributed power allocation schemes are proposed by considering
the multicell system as superposition of single cell systems, where we
define the average virtual bit error rate (BER) of interference-free single
cell system, allowing us to compute the power allocation coefficients in a
distributed manner at each BS.
All proposed schemes are evaluated in realistic scenarios considering LTE
specifications. The numerical evaluations show that the proposed schemes
are efficient in removing inter-cell interference and improve system
performance comparing to equal power allocation. Furthermore, fulldistributed
schemes can be used when the amounts of information to be
exchanged over the backhaul is restricted, although system performance is
slightly degraded from semi-distributed and full-centralized schemes, but
the complexity is considerably lower. Besides that for high degrees of
freedom distributed schemes show similar behaviour to centralized ones
Frequency and space precoded MIMO OFDM with substream adaptation
A new frequency and space precoding scheme for multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) systems is presented. For frequency precoding, the data symbols to be transmitted are divided into multiple substreams, and a predefined unitary matrix is applied to each substream to obtain different linear combinations of data symbols in the substream to gain frequency diversity. For space precoding, different precoding matrices selected from a predefined orthogonal matrix are used to allocate each frequency precoded data symbol to all transmit antennas to gain spatial diversity. The number of substreams and the corresponding data symbol mapping scheme are also adaptively determined at the receiver under varying received signal strength and MIMO channel conditions, and are made available to the transmitter through a low-rate feedback channel. Simulation results show that the proposed MIMO OFDM system with adaptive substream selection can effectively exploit both frequency and spatial diversity, and deliver the maximum system throughput. © 2009 IEEE
Unified Framework for Multicarrier and Multiple Access based on Generalized Frequency Division Multiplexing
The advancements in wireless communications are the key-enablers of new applications with stringent requirements in low-latency, ultra-reliability, high data rate, high mobility, and massive connectivity. Diverse types of devices, ranging from tiny sensors to vehicles, with different capabilities need to be connected under various channel conditions. Thus, modern connectivity and network techniques at all layers are essential to overcome these challenges. In particular, the physical layer (PHY) transmission is required to achieve certain link reliability, data rate, and latency. In modern digital communications systems, the transmission is performed by means of a digital signal processing module that derives analog hardware. The performance of the analog part is influenced by the quality of the hardware and the baseband signal denoted as waveform. In most of the modern systems such as fifth generation (5G) and WiFi, orthogonal frequency division multiplexing (OFDM) is adopted as a favorite waveform due to its low-complexity advantages in terms of signal processing. However, OFDM requires strict requirements on hardware quality.
Many devices are equipped with simplified analog hardware to reduce the cost. In this case, OFDM does not work properly as a result of its high peak-to-average power ratio (PAPR) and sensitivity to synchronization errors. To tackle these problems, many waveforms design have been recently proposed in the literature. Some of these designs are modified versions of OFDM or based on conventional single subcarrier. Moreover, multicarrier frameworks, such as generalized frequency division multiplexing (GFDM), have been proposed to realize varieties of conventional waveforms. Furthermore, recent studies show the potential of using non-conventional waveforms for increasing the link reliability with affordable complexity. Based on that, flexible waveforms and transmission techniques are necessary to adapt the system for different hardware and channel constraints in order to fulfill the applications requirements while optimizing the resources.
The objective of this thesis is to provide a holistic view of waveforms and the related multiple access (MA) techniques to enable efficient study and evaluation of different approaches. First, the wireless communications system is reviewed with specific focus on the impact of hardware impairments and the wireless channel on the waveform design. Then, generalized model of waveforms and MA are presented highlighting various special cases. Finally, this work introduces low-complexity architectures for hardware implementation of flexible waveforms. Integrating such designs with software-defined radio (SDR) contributes to the development of practical real-time flexible PHY.:1 Introduction
1.1 Baseband transmission model
1.2 History of multicarrier systems
1.3 The state-of-the-art waveforms
1.4 Prior works related to GFDM
1.5 Objective and contributions
2 Fundamentals of Wireless Communications
2.1 Wireless communications system
2.2 RF transceiver
2.2.1 Digital-analogue conversion
2.2.2 QAM modulation
2.2.3 Effective channel
2.2.4 Hardware impairments
2.3 Waveform aspects
2.3.1 Single-carrier waveform
2.3.2 Multicarrier waveform
2.3.3 MIMO-Waveforms
2.3.4 Waveform performance metrics
2.4 Wireless Channel
2.4.1 Line-of-sight propagation
2.4.2 Multi path and fading process
2.4.3 General baseband statistical channel model
2.4.4 MIMO channel
2.5 Summary
3 Generic Block-based Waveforms
3.1 Block-based waveform formulation
3.1.1 Variable-rate multicarrier
3.1.2 General block-based multicarrier model
3.2 Waveform processing techniques
3.2.1 Linear and circular filtering
3.2.2 Windowing
3.3 Structured representation
3.3.1 Modulator
3.3.2 Demodulator
3.3.3 MIMO Waveform processing
3.4 Detection
3.4.1 Maximum-likelihood detection
3.4.2 Linear detection
3.4.3 Iterative Detection
3.4.4 Numerical example and insights
3.5 Summary
4 Generic Multiple Access Schemes 57
4.1 Basic multiple access and multiplexing schemes
4.1.1 Infrastructure network system model
4.1.2 Duplex schemes
4.1.3 Common multiplexing and multiple access schemes
4.2 General multicarrier-based multiple access
4.2.1 Design with fixed set of pulses
4.2.2 Computational model
4.2.3 Asynchronous multiple access
4.3 Summary
5 Time-Frequency Analyses of Multicarrier
5.1 General time-frequency representation
5.1.1 Block representation
5.1.2 Relation to Zak transform
5.2 Time-frequency spreading
5.3 Time-frequency block in LTV channel
5.3.1 Subcarrier and subsymbol numerology
5.3.2 Processing based on the time-domain signal
5.3.3 Processing based on the frequency-domain signal
5.3.4 Unified signal model
5.4 summary
6 Generalized waveforms based on time-frequency shifts
6.1 General time-frequency shift
6.1.1 Time-frequency shift design
6.1.2 Relation between the shifted pulses
6.2 Time-frequency shift in Gabor frame
6.2.1 Conventional GFDM
6.3 GFDM modulation
6.3.1 Filter bank representation
6.3.2 Block representation
6.3.3 GFDM matrix structure
6.3.4 GFDM demodulator
6.3.5 Alternative interpretation of GFDM
6.3.6 Orthogonal modulation and GFDM spreading
6.4 Summary
7 Modulation Framework: Architectures and Applications
7.1 Modem architectures
7.1.1 General modulation matrix structure
7.1.2 Run-time flexibility
7.1.3 Generic GFDM-based architecture
7.1.4 Flexible parallel multiplications architecture
7.1.5 MIMO waveform architecture
7.2 Extended GFDM framework
7.2.1 Architectures complexity and flexibility analysis
7.2.2 Number of multiplications
7.2.3 Hardware analysis
7.3 Applications of the extended GFDM framework
7.3.1 Generalized FDMA
7.3.2 Enchantment of OFDM system
7.4 Summary
7 Conclusions and Future work
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