144 research outputs found
Per-antenna Constant Envelope Precoding for Large Multi-User MIMO Systems
We consider the multi-user MIMO broadcast channel with single-antenna
users and transmit antennas under the constraint that each antenna emits
signals having constant envelope (CE). The motivation for this is that CE
signals facilitate the use of power-efficient RF power amplifiers. Analytical
and numerical results show that, under certain mild conditions on the channel
gains, for a fixed , array gain is achievable even under the stringent
per-antenna CE constraint (essentially, for a fixed , at sufficiently large
the total transmitted power can be reduced with increasing while
maintaining a fixed information rate to each user). Simulations for the i.i.d.
Rayleigh fading channel show that the total transmit power can be reduced
linearly with increasing (i.e., an O(N) array gain). We also propose a
precoding scheme which finds near-optimal CE signals to be transmitted, and has
O(MN) complexity. Also, in terms of the total transmit power required to
achieve a fixed desired information sum-rate, despite the stringent per-antenna
CE constraint, the proposed CE precoding scheme performs close to the
sum-capacity achieving scheme for an average-only total transmit power
constrained channel.Comment: Submitted to IEEE Transactions on Communication
Performance Comparison of Constant Envelope and Zero-forcing Precoders in Multiuser Massive MIMO
In this article, the adoption and performance of a constant envelope (CE)
type spatial precoder is addressed in large-scale multiuser MIMO based cellular
network. We first formulate an efficient computing solution to obtain the
antenna samples of such CE precoder. We then evaluate the achievable CE
precoder based multiuser downlink (DL) system performance and compare it with
the corresponding performance of more ordinary zero-forcing (ZF) spatial
precoder. We specifically also analyze how realistic highly nonlinear power
amplifiers (PAs) affect the achievable DL performance, as the individual PA
units in large-array or massive MIMO systems are expected to be small, cheap
and operating close to saturation for increased energy-efficiency purposes. It
is shown that the largely reduced peak-to-average power ratio (PAPR) of the PA
input signals in the CE precoder based system allows for pushing the PA units
harsher towards saturation, while allowing to reach higher
signal-to-interference-plus-noise ratio (SINRs) at the intended receivers
compared to the classical ZF precoder based system. The obtained results
indicate that the CE precoder can outperform the ZF precoder by up to 5-6 dBs,
in terms of the achievable SINRs, when the PA units are pushed towards their
saturating region. Such large gains are a substantial benefit when seeking to
improve the spectral and energy-efficiencies of the mobile cellular networks.Comment: Accepted for publication at IEEE WCNC 201
Joint precoding and antenna selection in massive mimo systems
This thesis presents an overview of massive multiple-input multiple-output (MIMO) systems and proposes new algorithms to jointly precode and select the antennas. Massive MIMO is a new technology, which is candidate for comprising the fifth-generation (5G) of mobile cellular systems. This technology employs a huge amount of antennas at the base station and can reach high data rates under favorable, or asymptotically favorable, propagation conditions, while using simple linear processing. However, massive MIMO systems have some drawbacks, such as the high cost related to the base stations. A way to deal with this issue is to employ antenna selection algorithms at the base stations. These algorithms reduce the number of active antennas, decreasing the deployment and maintenance costs related to the base stations. Moreover, this thesis also describes a class of nonlinear precoders that are rarely addressed in the literature; these techniques are able to generate precoded sparse signals in order to achieve joint precoding and antenna selection. This thesis proposes two precoders belonging to this class, where the number of selected antennas is controlled by a design parameter. Simulation results show that the proposed precoders reach a lower bit-error rate than the classical antenna selection algorithms. Furthermore, simulation results show that the proposed precoders present a linear relation between the aforementioned design parameter that controls the signals’ sparsity and the number of selected antennas. Such relation is invariant to the number of base station’s antennas and the number of terminals served by this base station.Esta dissertação apresenta uma visão geral sobre MIMO (do termo em inglês, multiple-input multiple-output) massivo e propõe novos algoritmos que permitem a pré-codificacão de sinais e a seleção de antenas de forma simultânea. MIMO massivo é uma nova tecnologia candidata para compor a quinta geração (5G) dos sistemas celulares. Essa tecnologia utiliza uma quantidade muito grande de antenas na estação-base e, sob condições de propagação favorável ou assintoticamente favorável, pode alcançar taxas de transmissão elevadas, ainda que utilizando um simples processamento linear. Entretanto, os sistemas MIMO massivo apresentam algumas desvantagens, como por exemplo, o alto custo de implementação das estações-bases. Uma maneira de lidar com esse problema é utilizar algoritmos de seleção de antenas na estação-base. Com esses algoritmos é possível reduzir o número de antenas ativas e consequentemente reduzir o custo nas estações-bases. Essa dissertação também apresenta uma classe pouco estudada de pré-codificadores não-lineares que buscam sinais pré-codificados esparsos para realizar a seleção de antenas conjuntamente com a pré-codificação. Além disso, este trabalho propõem dois novos pré-codificadores pertencentes a essa classe, para os quais o número de antenas selecionadas é controlado por um parâmetro de projeto. Resultados de simulações mostram que os pré-codificadores propostos conseguem uma BER (do termo em inglês, bit-error rate) menor que os algoritmos clássicos usados para selecionar antenas. Além disso, resultados de simulações mostram que os pré-codificadores propostos apresentam uma relação linear com o parâmetro de projeto que controla a quantidade de antenas selecionadas; tal relação independe do número de antenas na estação-base e do número de terminais servidos por essa estação
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