68 research outputs found
Multi-user linear equalizer and precoder scheme for hybrid sub-connected wideband systems
Millimeter waves and massive multiple-input multiple output (MIMO) are two promising
key technologies to achieve the high demands of data rate for the future mobile communication
generation. Due to hardware limitations, these systems employ hybrid analog–digital architectures.
Nonetheless, most of the works developed for hybrid architectures focus on narrowband channels,
and it is expected that millimeter waves be wideband. Moreover, it is more feasible to have a
sub-connected architecture than a fully connected one, due to the hardware constraints. Therefore,
the aim of this paper is to design a sub-connected hybrid analog–digital multi-user linear equalizer
combined with an analog precoder to efficiently remove the multi-user interference. We consider
low complexity user terminals employing pure analog precoders, computed with the knowledge
of a quantized version of the average angles of departure of each cluster. At the base station,
the hybrid multi-user linear equalizer is optimized by using the bit-error-rate (BER) as a metric over
all the subcarriers. The analog domain hardware constraints, together with the assumption of a flat
analog equalizer over the subcarriers, considerably increase the complexity of the corresponding
optimization problem. To simplify the problem at hand, the merit function is first upper bounded,
and by leveraging the specific properties of the resulting problem, we show that the analog equalizer
may be computed iteratively over the radio frequency (RF) chains by assigning the users in an
interleaved fashion to the RF chains. The proposed hybrid sub-connected scheme is compared with a
fully connected counterpart.publishe
Reduced complexity detection in MIMO systems with SC-FDE modulations and iterative DFE receivers
This paper considers a Multiple-Input Multiple-Output (MIMO) system with P transmitting
and R receiving antennas and different overall noise characteristics on the different receiver antennas
(e.g., due to nonlinear effects at the receiver side). Each communication link employs a Single-Carrier
with Frequency-Domain Equalization (SC-FDE) modulation scheme, and the receiver is based on
robust iterative frequency-domain multi-user detectors based on the Iterative Block Decision Feedback
Equalization (IB-DFE) concept. We present low complexity efficient receivers that can employ low
resolution Analog-to-Digital Converters (ADCs) and require the inversion of matrices with reduced
dimension when the number of receive antennas is larger than the number of independent data
streams. The advantages of the proposed techniques are particularly high for highly unbalanced
MIMO systems, such as in the uplink of Base Station (BS) cooperation systems that aim for
Single-Frequency Network (SFN) operation or massive MIMO systems with much more antennas at
the receiver side.publishe
Hybrid multi-user equalizer for massive MIMO millimeter-wave dynamic subconnected architecture
This paper proposes a hybrid multi-user equalizer for the uplink of broadband millimeterwave massive multiple input/multiple output (MIMO) systems with dynamic subarray antennas. Hybrid
subconnected architectures are more suitable for practical applications since the number of required phase
shifters is lower than in fully connected architectures. We consider a set of only analog precoded users
transmitting to a base station and sharing the same radio resources. At the receiver end, the hybrid multi-user
equalizer is designed by minimizing the sum of the mean square error (MSE) of all subcarriers, considering
a two-step approach. In the first step, the digital part is iteratively computed as a function of the analog
part. It is considered that the digital equalizers are computed on a per subcarrier basis, while the analog
equalizer is constant over the subcarriers and the digital iterations due to hardware constraints. In the second
step, the analog equalizer with dynamic antenna mapping is derived to connect the best set of antennas to
each radio frequency (RF) chain. For each subset of antennas, one antenna and a quantized phase shifter are
selected at a time, taking into account all previously selected antennas. The results show that the proposed
hybrid dynamic two-step equalizer achieves a performance close to the fully connected counterpart, although
it is less complex in terms of hardware and signal processing requirements.publishe
Esquemas de pré-codificação e equalização para arquiteturas hÃbridas sub-conectadas na banda de ondas milimétricas
In the last years, the demand for high data rates increased substantially and the mobile communications are currently a necessity for our society. Thus, the number of users to access interactive services and applications has increased. The next generation of wireless communications (5G) is expected to be released in 2020 and it is projected to provide extremely high data rates for the users. The millimeter wave communications band and the massive MIMO are two promising keys technologies to achieve the multi Gbps for the future generations of mobile communications, in particular the 5G. The conjugation of these two technologies, allows packing a large number of antennas in the same volume than in the current frequencies and increase the spectral efficiency. However, when we have a large number of antennas, it is not reasonable to have a fully digital architecture due to the hardware constrains. On the other hand, it is not feasible to have a system that works only in the analog domain by employing a full analog beamforming since the performance is poor. Therefore, it is required a design of hybrid analog/digital architectures to reduce the complexity and achieve a good performance. Fully connected and sub-connected schemes are two examples of hybrid architectures. In the fully connected one, all RF chain connect to all antenna elements while in the sub-connected architecture, each RF chain is connected to a group of antennas. Consequently, the sub-connected architecture is more attractive due to the low complexity when compared to the fully connected one. Also, it is expected that millimeter waves be wideband, however, most of the works developed in last years for hybrid architectures are mainly focused in narrowband channels.
Therefore, in this dissertation it is designed a low complex analog precoder at the user terminals and a hybrid analog-digital multi-user linear equalizer for broadband sub-connected millimeter wave massive MIMO at the base station. The analog precoder at the transmitter considers a quantized version of the average angle of departure of each cluster for its computation. In order to remove the multi-user interference, it is considered a hybrid sub-connected approach that minimizes the bit error rate (BER). The performance results show that the proposed hybrid sub-connected scheme is close to the hybrid full-connected design. However, due to the large number of connections, the full-connected scheme is slightly better than the proposed sub-connected scheme but with higher complexity. Therefore, the proposed analog precoder and hybrid sub-connected equalizer are more feasible to practical applications due to the good trade-off between performance and complexity.Nos últimos anos, a necessidade por elevadas taxas de transmissão de dados tem vindo a aumentar substancialmente uma vez que as comunicações móveis assumem cada vez mais um papel fundamental na sociedade atual. Por isso, o número de utilizadores que acedem a serviços e aplicações interativas tem vindo a aumentar. A próxima geração de comunicações móveis (5G) é esperada que seja lançada em 2020 e é projetada para fornecer elevadas taxas de transmissão de dados aos seus utilizadores. A comunicação na banda das ondas milimétricas e o MIMO massivo são duas tecnologias promissoras para alcançar os multi Gb/s para as comunicações móveis futuras, em particular o 5G. Conjugando essas duas tecnologias, permite-nos colocar um maior número de antenas no mesmo volume comparativamente à s frequências atuais, aumentando assim a eficiência espectral. No entanto, quanto se tem um grande número de antenas, não é viável ter uma arquitetura totalmente digital devido à s restrições de hardware. Por outro lado, não é viável ter um sistema que trabalhe apenas no domÃnio analógico. Assim sendo, é necessária uma arquitetura hÃbrida analógica-digital de modo a remover a complexidade geral do sistema. É esperado que os sistemas de comunicação baseados em ondas milimétricas sejam de banda larga, no entanto, a maioria dos trabalhos feitos para arquiteturas hÃbridas são focados em canais de banda estreita. Dois exemplos de soluções hÃbridas são as arquiteturas completamente conectada e sub-conectada. Na primeira, todas as cadeias RF estão ligadas a todas as antenas enquanto na arquitetura sub-conectada cada cadeia RF é ligada apenas a um grupo de antenas. Consequentemente, a arquitetura sub-conectada é mais interessante do ponto de vista prático devido à sua menor complexidade quando comparada à arquitetura completamente conectada.
Nesta dissertação é projetado um pré-codificador analógico de baixa complexidade no terminal móvel, combinado com um equalizador multiutilizador desenhado para uma arquitetura hÃbrida sub-conectada, implementado na estação base. O pré-codificador no transmissor assume um conhecimento parcial da informação do canal e, de modo a remover eficientemente a interferência multiutilizador, é proposta também uma arquitetura hÃbrida sub-conectada que minimiza a taxa média de erro. Os resultados de desempenho mostram que o esquema hÃbrido sub-conectado proposto está próximo da arquitetura hÃbrida completamente conectada. No entanto, devido ao grande número de conexões, a arquitetura hÃbrida completamente conectada é ligeiramente melhor que a arquitetura sub-conectada proposta à custa de uma maior complexidade. Assim sendo, o pré-codificador analógico e o equalizador sub-conectado hÃbrido proposto são mais viáveis para aplicações práticas devido ao compromisso entre o desempenho e a complexidade.Mestrado em Engenharia Eletrónica e Telecomunicaçõe
Interference Alignment Techniques for Multi-User MIMO Systems at Millimeter-Wave
In this work a review of the state-of-the-art of modern multi-user MIMO systems is given, presenting various algorithms that use interference alignment techniques to allocate multiple users over the same physical channel. In particular, the performance achieved with these methods over the millimeter-wave channel are evaluated. Finally, the work is completed with the description of a novel frequency domain non-linear equalizer for wideband channel
Técnicas de equalização para MIMO massivo com amplificação não linear
The dawn of the new generation of mobile communications and the trafic
explosion that derives from its implementation pose great challenge. The
milimeter wave band and the use of massive number of antennas are technologies
which, when combined, allow the transmission of high data rate,
functioning in zones of the electromagnetic spectrum that are less explored
and with capability of allocation of dozens of GHz of bandwidth.
In this dissertation we consider a massive MIMO millimeter wave system
employing a hybrid architecture, i.e., the number of transmit and receive
antennas are lower than the number of radio frequency chains. As consequence,
the precoder and equalizers should be designed in both digital and
analog domains. In the literature, most of the proposed hybrid beamforming
schemes were evaluated without considering the effects of nonlinear amplifications. However, these systems face non-avoidable nonlinear effects due
to power amplifiers functioning in nonlinear regions. The strong nonlinear
effects throughout the transmission chain will have a negative impact on the
overall system performance and thus its study and the design of equalizers
that take into account these effects are of paramount importance.
This dissertation proposes a hybrid iterative equalizer for massive MIMO millimeter
wave SC-FDMA systems. The user terminals have low complexity,
just equipped with analog precoders based on average angle of departure,
each with a single radio frequency chain. At the base station it is designed
an hybrid analog-digital iterative equalizer with fully connected architecture
in order to eliminate both the multi-user interference and the nonlinear distortion
caused by signal amplification during the transmission. The equalizer
is optimized by minimizing the bit error rate, which is equivalent to minimize
the mean square error rate. The impact of the saturation threshold of the
amplifiers in the system performance is analysed, and it is demonstrated that
the iterative process can efficiently remove the multi-user interference and
the distortion, improving the overall system performance.O surgimento de uma nova geração de comunicações móveis e a explosão
de tráfego que advém da sua implementação apresenta grandes desafios. A
banda de ondas milimétricas e o uso massivo de antenas são tecnologias que,
combinadas, permitem atingir elevadas taxas de transmissão, funcionando
em zonas do espectro electromagnético menos exploradas e com capacidade
de alocação de dezenas de GHz para largura de banda.
Nesta dissertação foi considerado um sistema de MIMO massivo de ondas
milimétricas usando uma arquitectura hÃbrida, i.e., o número de antenas para
transmissão e recepção é menor que o número de cadeias de radiofrequência.
Consequentemente, o pré-codificador e equalizadores devem ser projectados
nos domÃnios digital e analógico. Na literatura, a maioria dos esquemas
hÃbridos de beamforming são avaliados sem ter em conta os efeitos de não linearidade
da amplificação do sinal. No entanto, estes sistemas sofrem
inevitavelmente de efeitos não lineares devido aos amplificadores de potência
operarem em regiões não lineares. Os fortes efeitos das não-linearidades ao
longo da cadeia de transmissão têm um efeito nefasto no desempenho do
sistema e portanto o seu estudo e projecto de equalizadores que tenham em
conta estes efeitos são de extrema importância.
Esta dissertação propõe um equalizador hÃbrido para sistemas baseados em
ondas milimétricas para MIMO massivo com modulação SC-FDMA. Os terminais
de utilizador possuem baixa complexidade, equipados apenas com
pré-codificadores analógicos baseados no ângulo médio de partida, cada um
com uma única cadeia de radiofrequência. Na estação base é projectado
um equalizador iterativo hÃbrido analógico-digital com arquitectura completamente
conectada de modo a eliminar a interferencia multi-utilizador e a
distorção causada pela amplificação do sinal aquando da transmissão. O
equalizador é optimizado minimizando a taxa de erro de bit, o que é equivalente
a minimizar a taxa de erro quadrático médio. O impacto do limiar
de saturação dos amplificadores no desempenho do sistema é analisado, e é
demonstrado que o processo iterativo consegue eliminar de modo eficiente
a interferência multi-utilizador e a distorção, melhorando o desempenho do
sistema.Mestrado em Engenharia Eletrónica e Telecomunicaçõe
Multiuser equalizer for hybrid massive MIMO mmWave CE-OFDM systems
This paper considers a multiuser broadband uplink massive multiple input multiple
output (MIMO) millimeter-wave (mmWave) system. The constant envelope orthogonal frequency
division multiplexing (CE-OFDM) is adopted as a modulation technique to allow an efficient power
amplification, fundamental for mmWave based systems. Furthermore, a hybrid architecture is
considered at the user terminals (UTs) and base station (BS) to reduce the high cost and power
consumption required by a full-digital architecture, which has a radio frequency (RF) chain per
antenna. Both the design of the UT’s precoder and base station equalizer are considered in this work.
With the aim of maximizing the beamforming gain between each UT and the BS, the precoder analog
coefficients are computed as a function of the average angles of departure (AoD), which are assumed
to be known at the UTs. At the BS, the analog part is derived by assuming a system with no multi-user
interference. Then, a per carrier basis nonlinear/iterative multi-user equalizer, based on the iterative
block decision feedback equalization (IB-DFE) principle is designed, to explicitly remove both the
multi-user and residual inter carrier interferences, not tackled in the analog part. The equalizer
design metric is the sum of the mean square error (MSE) of all subcarriers, whose minimization is
shown to be equivalent to the minimization of a weighted error between the hybrid and the full
digital equalizer matrices. The results show that the proposed hybrid multi-user equalizer has a
performance close to the fully digital counterpart.publishe
Iterative analog-digital multi-user equalizer for wideband millimeter wave massive MIMO systems
Most of the previous work on hybrid transmit and receive beamforming focused on narrowband channels. Because the millimeter wave channels are expected to be wideband, it is crucial to propose efficient solutions for frequency-selective channels. In this regard, this paper proposes an iterative analog-digital multi-user equalizer scheme for the uplink of wideband millimeter-wave massive multiple-input-multiple-output (MIMO) systems. By iterative equalizer we mean that both analog and digital parts are updated using as input the estimates obtained at the previous iteration. The proposed iterative analog-digital multi-user equalizer is designed by minimizing the sum of the mean square error of the data estimates over the subcarriers. We assume that the analog part is fixed for all subcarriers while the digital part is computed on a per subcarrier basis. Due to the complexity of the resulting optimization problem, a sequential approach is proposed to compute the analog phase shifters values for each radio frequency (RF) chain. We also derive an accurate, semi-analytical approach for obtaining the bit error rate (BER) of the proposed hybrid system. The proposed solution is compared with other hybrid equalizer schemes, recently designed for wideband millimeter-wave (mmWave) massive MIMO systems. The simulation results show that the performance of the developed analog-digital multi-user equalizer is close to full-digital counterpart and outperforms the previous hybrid approach.publishe
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