Christopher Mollén
Doktorand
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Linköpings universitet,
Institutionen för systemteknik (ISY)
581 83 Linköping
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Hus B, en trappa, korridor A mellan ingångarna 27 och 29.
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christopher.mollen@liu.se
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013-282643
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013-139282
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Presentation
Som doktorand på avdelningen Kommunikationssystem är mitt forskningsintresse massiv MIMO – en trådlös transmissionsteknik som använder uppställningar av hundratals antenner för att sända information till flera användare samtidigt och över samma frekvens. Denna teknik kommer potentiellt att kunna öka datatakten hos framtida kommunikationssystem, och samtidigt göra dem mer miljövänliga genom att minska deras strömförbrukning.
Jag undersöker tekniker för lågtoppvärdesförkodning, vilka är metoder för att göra signalerna mer hårdvaruvänliga. Sådana förkodningsmetoder skulle kunna göra massiv MIMO enklare att implementera och sänka strömförbrukningen hos framtida basstationer ytterligare.
Följande kurser har jag undervisat eller undervisar jag i: CDIO kursen, Signalteori och Signaler, information och kommunikation.
Aktuellt
Den 2 september, 2016, försvarade jag min licentiatavhandling Om massiv-MIMO-basstationer med enkel hårdvara. Vänligen kontakta mig för ett exemplar av avhandlingen om intresse föreligger. Den finns även tillgänglig på universitetsbibiloteket.
Om Du vill skriva ett examensarbete inom massiv MIMO, så har jag ett par idéer som jag skulle vilja testa. Kontakta mig!
Besök gärna vår nya blogg och läs mer om massiv MIMO.
Massiv MIMO på svenska
När forskning inom nya områden bedrivs, stöter man ofta på nya begrepp och fenomen som ännu saknar etablerad nomenklatur. För att underlätta eventuell diskurs inom massiv MIMO, presenterar jag nedan en tabell över de svenska massiv-MIMO-termer som jag använder. Om inte annat, kan den användas för att tolka mina skrifter. Dessa termer är (ännu) inte etablerade, åsikter och förslag på förbättringar tas därför gärna emot via mejl.
Term |
Definition |
Engelsk motsvarighet |
Kinesisk motsvarighet |
antennförstärkning |
förhållandet mellan den utsända effekten från en riktantenn och från en enda rundstrålande antenn när den mottagna effekten i båda fallen är densamma |
antenna gain |
天線益 |
enbärartransmission |
transmission, där data sänds på en bärvåg |
single-carrier transmission |
單載波傳輸 |
fleranvändarförkodning |
koordineringen av signalerna som sänds från basstationsantennerna med avsikt att flera användare samtidigt skall ta emot för dem ämnade individuella meddelanden |
multiuser precoding |
多用戶預編碼 |
fördelaktig utbredning |
en utbredningsmiljö som tillåter hög rumsmultiplexering |
favourable propagation |
有利傳播 |
förkodning |
tekniken att styra en signal från en antennuppställning till en eller flera punkter i rummet baserat på kännedom om utbredningsvägen till punkten |
precoding |
預編碼 |
gruppantenn |
en samling samarbetande, individuellt styrbara antenner |
antenna array |
天線陣列 |
gruppantennsförstärkning |
förhållandet mellan den utsända effekten från en uppställning av antenner och från en enda antenn när den mottagna effekten i båda fallen är densamma |
array gain |
陣增益 |
inversförkodning |
en förkodare som ämnar minimera interanvändarinterferensen |
zero-forcing precoding |
迫零預編碼 |
kanalförstelning |
fenomenet att egenskaperna hos en stokastisk kommunikationskanal blir determinisktiska |
channel hardening |
信道固化 |
kanalkännedom |
det som är känt om kanalen |
channel state information |
信道狀態信息 |
konstantenveloppförkodning |
en förkodare som producerar sändsingaler, vars envelopp är konstanta |
constant-envelope precoding |
恆定包絡預編碼 |
lobformning |
tekniken att rikta en signal från en antennuppställning i en eller flera riktningar enbart baserat på antennuppställningens geometri |
beamforming |
波束賦形 |
lågtoppvärdesförkodning |
en förkodare som producerar sändsignaler med lågt toppvärde |
low-PAR precoding |
低峯均比預編碼 |
maximieffektförkodning |
en förkodare som maximerar den mottagna nyttoeffekten hos varje användare |
maximum-ratio precoding |
最大比預編碼 |
multiplexeringsvinst |
antalet dataströmmar som sänds parallellt |
multiplexing gain |
複用增益 |
pilotkontaminering |
det fel som uppstår i en pilotbaserad kanalskattning på grund av interferens från andra användare |
pilot contamination |
導頻污染 |
rak sikt |
en oblockerad utbredningsväg mellan sändare och mottagare |
line-of-sight |
視距 |
samordnad förkodning |
förkodning, där symbolerna förkodas centralt baserat på kännedom om kanalen från alla antenner till alla användare |
coordinated precoding |
協調預編碼 |
(mottagar-)sammanställning |
tekniken att plocka upp en signal med en antennuppställning från en eller flera punkter i rummet baserat på kännedom om utbredningsvägen från punkten |
(receive) combining |
(接收)合併 |
skymd sikt |
en blockerad utbredningsväg mellan sändare och mottagare |
non-line-of-sight |
非視距 |
spridningspunkt |
en punkt, ur vilken en infallande signal sprids i alla riktningar |
scatterer |
散射者 |
toppvärde |
förhållandet mellan högsta toppen i en signal och signalens medelamplitud |
peak-to-average ratio, crest factor |
峯均比 |
total takt |
summan av datatakterna som varje individuell användare betjänas med |
sum-rate |
總信息率 |
utombandsstrålning |
energin hos en utsändsignal, vilken kan fångas upp utanför det allokerade frekvensbandet |
out-of-band radiation |
帶外泄露 |
utspridd förkodning |
förkodning, där symbolerna förkodas lokalt vid varje antenn baserat på kännedom om kanalen från den lokala antennen till alla användare |
distributed precoding |
分調預編碼 |
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Publications
Show/hide year headlines.
Journal papers
2017
Abstract
Analog-to-digital converters (ADCs) stand for a significant part of the total power consumption in a massive multiple-input multiple-output (MIMO) base station. One-bit ADCs are one way to reduce power consumption. This paper presents an analysis of the spectral efficiency of single-carrier and orthogonal-frequency-division-multiplexing (OFDM) transmission in massive MIMO systems that use one-bit ADCs. A closed-form achievable rate, i.e., a lower bound on capacity, is derived for a wideband system with a large number of channel taps that employ low-complexity linear channel estimation and symbol detection. Quantization results in two types of error in the symbol detection. The circularly symmetric error becomes Gaussian in massive MIMO and vanishes as the number of antennas grows. The amplitude distortion, which severely degrades the performance of OFDM, is caused by variations between symbol durations in received interference energy. As the number of channel taps grows, the amplitude distortion vanishes and OFDM has the same performance as single-carrier transmission. A main conclusion of this paper is that wideband massive MIMO systems work well with one-bit ADCs.
Keywords
MIMO, Channel estimation, Wideband, Quantization (signal), Distortion, Fading channels, One-bit ADCs, Engineering and Technology
BIBTEX
@article{diva2:1083712,
author = {Moll\'{e}n, Christopher and Choi, Junil and Larsson, Erik G. and Heath, Robert W.},
title = {{Uplink Performance of Wideband Massive MIMO With One-Bit ADCs}},
journal = {IEEE Transactions on Wireless Communications},
year = {2017},
volume = {16},
number = {1},
pages = {87--100},
}
2016
Abstract
In massive multiple-input multiple-output (MIMO), most precoders result in downlink signals that suffer from high peak-to-average ratio (PAR), independently of modulation order and whether single-carrier or orthogonal frequency-division multiplexing (OFDM) transmission is used. The high PAR lowers the power efficiency of the base-station amplifiers. To increase the power efficiency, low-PAR precoders have been proposed. In this paper, we compare different transmission methods for massive MIMO in terms of the power consumed by the amplifiers. It is found that: 1) OFDM and single-carrier transmission have the same performance over a hardened massive MIMO channel and 2) when the higher amplifier power efficiency of low-PAR precoding is taken into account, conventional and low-PAR precoders lead to approximately the same power consumption. Since downlink signals with low PAR allow for simpler and cheaper hardware, than signals with high PAR, therefore, the results suggest that low-PAR precoding with either single-carrier or OFDM transmission should be used in a massive MIMO base station.
Keywords
Engineering and Technology
BIBTEX
@article{diva2:952121,
author = {Moll\'{e}n, Christopher and Larsson, Erik G. and Eriksson, Thomas},
title = {{Waveforms for the Massive MIMO Downlink: Amplifier Efficiency, Distortion and Performance}},
journal = {IEEE Transactions on Communications},
year = {2016},
volume = {46},
number = {12},
pages = {5050--5063},
}
Conference papers
2017
Salil Kashyap, Christopher Mollén, Björnson Emil, Erik G. Larsson,
"Performance Analysis of (TDD) Massive MIMO with Kalman Channel Prediction",
Acoustics, Speech and Signal Processing (ICASSP), 2017 IEEE International Conference on, International Conference on Acoustics Speech and Signal Processing ICASSP,
3554-3558,
2017.
Abstract
In massive MIMO systems, which rely on uplink pilots to estimate the channel, the time interval between pilot transmissions constrains the length of the downlink. Since switching between up- and downlink takes time, longer downlink blocks increase the effective spectral efficiency. We investigate the use of low-complexity channel models and Kalman filters for channel prediction, to allow for longer intervals between the pilots. Specifically, we quantify how often uplink pilots have to be sent when the downlink rate is allowed to degrade by a certain percentage. To this end, we consider a time-correlated channel aging model, whose spectrum is rectangular, and use autoregressive moving average (ARMA) processes to approximate the time-variations of such channels. We show that ARMA-based predictors can increase the interval between pilots and the spectral efficiency in channels with high Doppler spreads. We also show that Kalman prediction is robust to mismatches in the channel statistics.
Keywords
channel aging, channel estimation, channel prediction, Kalman estimation, massive MIMO, Engineering and Technology
BIBTEX
@inproceedings{diva2:1083742,
author = {Kashyap, Salil and Moll\'{e}n, Christopher and Emil, Björnson and Larsson, Erik G.},
title = {{Performance Analysis of (TDD) Massive MIMO with Kalman Channel Prediction}},
booktitle = {Acoustics, Speech and Signal Processing (ICASSP), 2017 IEEE International Conference on},
year = {2017},
series = {International Conference on Acoustics Speech and Signal Processing ICASSP},
pages = {3554--3558},
}
Christopher Mollén, Junil Choi, Erik G. Larsson, Robert W. Heath,
"Achievable Uplink Rates for Massive MIMO with Coarse Quantization",
2017 IEEE InternationalConference on Acoustics, Speech,and Signal Processing Proceedings, Acoustics, Speech and Signal Processing (ICASSP),
Vol. 2017,
6488-6492,
2017.
Abstract
The high hardware complexity of a massive MIMO base station, which requires hundreds of radio chains, makes it challenging to build commercially. One way to reduce the hardware complexity and power consumption of the receiver is to lower the resolution of the analog-to-digital converters (ADCs). We derive an achievable rate for a massive MIMO system with arbitrary quantization and use this rate to show that ADCs with as low as 3 bits can be used without significant performance loss at spectral efficiencies around 3.5 bpcu per user, also under interference from stronger transmitters and with some imperfections in the automatic gain control.
Keywords
ADC, channel estimation, low resolution, massive MIMO, quantization, Engineering and Technology
BIBTEX
@inproceedings{diva2:1083735,
author = {Moll\'{e}n, Christopher and Choi, Junil and Larsson, Erik G. and Heath, Robert W.},
title = {{Achievable Uplink Rates for Massive MIMO with Coarse Quantization}},
booktitle = {2017 IEEE InternationalConference on Acoustics, Speech,and Signal Processing Proceedings},
year = {2017},
series = {Acoustics, Speech and Signal Processing (ICASSP)},
volume = {2017},
pages = {6488--6492},
}
2016
Abstract
We consider massive multiple input multiple output (MIMO) systems with orthogonal frequency division multiplexing (OFDM) that use zero-forcing (ZF) to combat interference. To perform ZF, large dimensional pseudo-inverses have to be computed. In this paper, we propose a discrete Fourier transform (DFT)-interpolation-based technique where substantially fewer ZF matrix computations have to be done with very little deterioration in data rate compared to computing an exact ZF matrix for every subcarrier. We claim that it is enough to compute the ZF matrix at L(amp;lt;amp;lt; N) selected subcarriers where L is the number of resolvable multipaths and N is the total number of subcarriers and then interpolate. The proposed technique exploits the fact that in the massive MIMO regime, the ZF impulse response consists of L dominant components. We benchmark the proposed method against full inversion, piecewise constant and linear interpolation methods and show that the proposed method achieves a good tradeoff between performance and complexity.
Keywords
Massive MIMO; interpolation; zero-forcing, Engineering and Technology
BIBTEX
@inproceedings{diva2:1048138,
author = {Kashyap, Salil and Moll\'{e}n, Christopher and Björnson, Emil and Larsson, Erik G},
title = {{Frequency-Domain Interpolation of the Zero-Forcing Matrix in Massive MIMO-OFDM}},
booktitle = {2016 IEEE 17TH INTERNATIONAL WORKSHOP ON SIGNAL PROCESSING ADVANCES IN WIRELESS COMMUNICATIONS (SPAWC)},
year = {2016},
series = {IEEE International Workshop on Signal Processing Advances in Wireless Communications},
publisher = {IEEE},
}
Abstract
The power consumption of analog-to-digital converters (ADCs) grows linearly in the number of antennas in massive MIMO base stations. To reduce power consumption, one-bit ADCs can be used. It is believed that the nonlinear distortion of onebit ADCs makes channel estimation and symbol equalization in such systems computationally complex and resource demanding. In this paper, it is shown that low-complexity linear channel estimation and symbol equalization are feasible in massive MIMO with one-bit ADCs when the number of channel taps is large. The effective SINR of the received symbol estimates of a maximum-ratio combiner with estimated channel state information is 4 dB lower in a system with one-bit ADCs than in an equivalent unquantized system.
Keywords
Engineering and Technology
BIBTEX
@inproceedings{diva2:952128,
author = {Moll\'{e}n, Christopher and Larsson, Erik G. and Choi, Junil and Heath, Robert W.},
title = {{Performance of Linear Receivers for Wideband Massive MIMO with One-Bit ADCs}},
booktitle = {WSA 2016 - 20th International ITG Workshop on Smart Antennas 03/09/2016 - 03/11/2016 at München, Deutschland},
year = {2016},
pages = {7--},
}
Abstract
The spatial characteristics of the out-of-band radiation that a multiuser MIMO system emits, due to its power amplifiers (modeled by a polynomial model) being nonlinear, are studied by deriving an analytical expression for the continuous-time cross-correlation of the transmit signals. It is shown that, at any spatial point and on any frequency, the received power averaged over many channel realizations from a MIMO base station is the same as from a SISO base station when the two radiate the same amount of power. For a specific channel realization however, the received power can deviate from this average. We show that the deviations from the average are small in a MIMO system with multiple users and that the deviations can be significant with only one user. Using an ergodicity argument, we conclude that out-of-band radiation is less of a problem in massive MIMO, where precoding and array gain let us reduce the total radiated power compared to SISO systems. The requirements on spectral regrowth can therefore be relaxed in MIMO systems without causing more total out-of-band radiation.
Keywords
Engineering and Technology
BIBTEX
@inproceedings{diva2:952120,
author = {Moll\'{e}n, Christopher and Gustavsson, Ulf and Eriksson, Thomas and Larsson, Erik G.},
title = {{Out-of-Band Radiation Measure for MIMO Arrays with Beamformed Transmission}},
booktitle = {2016 IEEE International Conference on Communications (ICC)},
year = {2016},
series = {IEEE International Conference on Communications},
pages = {1--6},
publisher = {IEEE},
}
Abstract
We investigate the performance of wideband massive MIMO base stations that use one-bit ADCs for quantizing the uplink signal. Ourmain result is to show that the many taps of the frequency-selective channel make linear combiners asymptotically consistent and the quantization noise additive and Gaussian, which simplifies signal processing and enables the straightforward use of OFDM . We also find that single-carrier systems and OFDM systems are affected in the same way by one-bit quantizers in wideband systems because the distribution of the quantization noise becomes the same in both systems as the number of channel taps grows.
Keywords
Engineering and Technology
BIBTEX
@inproceedings{diva2:897045,
author = {Moll\'{e}n, Christopher and Choi, Junil and Larsson, Erik G. and Heath, Robert W.},
title = {{One-Bit ADCs in Wideband Massive MIMO Systems with OFDM Transmission}},
booktitle = {2016 IEEE INTERNATIONAL CONFERENCE ON ACOUSTICS, SPEECH AND SIGNAL PROCESSING PROCEEDINGS},
year = {2016},
series = {International Conference on Acoustics Speech and Signal Processing ICASSP},
pages = {3386--3390},
publisher = {Institute of Electrical and Electronics Engineers (IEEE)},
}
2015
Abstract
A transmission scheme for the multiuser MIMO downlink, where the transmit signal from each antenna has constant envelope and a limited bandwidth, is proposed in order to enable the use of highly efficient, nonlinear amplifiers at the base station. To evaluate its performance, an achievable rate is derived and the necessary transmit power of the proposed scheme is computed for a system with 40 antennas that serves 4 users at data rates around 1 bpcu. For this system and 40% excess 30 dB-bandwidth, approximately 3 dB more transmit power is required to achieve the same sum-rate as without the constantenvelope constraints.
Keywords
Engineering and Technology
BIBTEX
@inproceedings{diva2:842435,
author = {Moll\'{e}n, Christopher and Larsson, Erik G},
title = {{Multiuser MIMO Precoding with Per-Antenna Continuous-Time Constant-Envelope Constraints}},
booktitle = {2015 IEEE 16th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC)},
year = {2015},
series = {IEEE International Workshop on Signal Processing Advances in Wireless Communications},
pages = {261--265},
publisher = {Institute of Electrical and Electronics Engineers (IEEE)},
}
2014
Abstract
Conventional downlink signals in massive MIMO suffer from high PAR, which requires the power amplifiers of the base station to be backed off to avoid signal distortion, which lowers power efficiency. To improve efficiency, there are precoding schemes that produce signals with low PAR. To compare different precoding schemes, this article estimates their power consumptions through simulations, in which in-band distortion and amplifier efficiency effects are taken into consideration. It is found that, when only in-band distortion and the data rate requirement determine the back-off, the inband distortion is negligible and the power amplifiers can be operated at peak efficiency. It also turns out that low-PAR and conventional, high-PAR, precoding schemes consume approximately the same amount of power when the out-of-band part of the distortion is disregarded.
Keywords
massive MIMO; multiuser precoding, crest-factor reduction, low-PAR precoding, Engineering and Technology
BIBTEX
@inproceedings{diva2:765917,
author = {Moll\'{e}n, Christopher and Larsson, Erik G. and Eriksson, Thomas},
title = {{On the Impact of PA-Induced In-Band Distortion in Massive MIMO}},
booktitle = {Proceedings of European Wireless 2014},
year = {2014},
pages = {201--206},
publisher = {VDE Verlag GmbH},
address = {Berlin, Offenbach, Germany},
}
Theses (Licentiate, PhD)
2017
Abstract
Massive MIMO (multiple-input–multiple-output) is a multi-antenna technology for cellular wireless communication, where the base station uses a large number of individually controllable antennas to multiplex users spatially. This technology can provide a high spectral efficiency. One of its main challenges is the immense hardware complexity and cost of all the radio chains in the base station. To make massive MIMO commercially viable, inexpensive, low-complexity hardware with low linearity has to be used, which inherently leads to more signal distortion. This thesis investigates how the degenerated linearity of some of the main components—power amplifiers, analog-to-digital converters (ADCs) and low-noise amplifiers—affects the performance of the system, with respect to data rate, power consumption and out-of-band radiation. The main results are: Spatial processing can reduce PAR (peak-to-average ratio) of the transmit signals in the downlink to as low as 0B; this, however, does not necessarily reduce power consumption. In environments with isotropic fading, one-bit ADCs lead to a reduction in effective signal-to-interference-and-noise ratio (SINR) of 4dB in the uplink and four-bit ADCs give a performance close to that of an unquantized system. An analytical expression for the radiation pattern of the distortion from nonlinear power amplifiers is derived. It shows how the distortion is beamformed to some extent, that its gain never is greater than that of the desired signal, and that the gain of the distortion is reduced with a higher number of served users and a higher number of channel taps. Nonlinear low-noise amplifiers give rise to distortion that partly combines coherently and limits the possible SINR. It is concluded that spatial processing with a large number of antennas reduces the impact of hardware distortion in most cases. As long as proper attention is paid to the few sources of coherent distortion, the hardware complexity can be reduced in massive MIMO base stations to overcome the hardware challenge and make massive MIMO commercial reality.
Keywords
Engineering and Technology, Engineering and Technology, Engineering and Technology, Engineering and Technology, Natural Sciences
BIBTEX
@phdthesis{diva2:1163832,
author = {Moll\'{e}n, Christopher},
title = {{High-End Performance with Low-End Hardware:
Analysis of Massive MIMO Base Station Transceivers}},
school = {Linköping University},
type = {{Linköping Studies in Science and Technology. Dissertations No. 1896}},
year = {2017},
address = {Sweden},
}
2016
Abstract
Massive MIMO (Multiple-Input Multiple-Output) base stations have proven, both in theory and in practice, to possess many of the qualities that future wireless communication systems will require. They can provide equally high data rates throughout their coverage area and can concurrently serve multiple low-end handsets without requiring wider spectrum, denser base station deployment or significantly more power than current base stations. The main challenge of massive MIMO is the immense hardware complexity and cost of the base station—each element in the large antenna array needs to be individually controllable and therefore requires its own radio chain. To make massive MIMO commercially viable, the base station has to be built from inexpensive simple hardware. In this thesis, it is investigated how the use of low-end power amplifiers and analog-to-digital converters (ADCs) affects the performance of massive MIMO. In the study of the signal distortion from low-end amplifiers, it is shown that in-band distortion is negligible in massive MIMO and that out-of-band radiation is the limiting factor that decides what power efficiency the amplifiers can be operated at. A precoder that produces transmit signals for the downlink with constant envelope in continuous time is presented to allow for highly power efficient low-end amplifiers. Further, it is found that the out-of-band radiation is isotropic when the channel is frequency selective and when multiple users are served; and that it can be beamformed when the channel is frequency flat and when few users are served. Since a massive MIMO base station radiates less power than today's base stations, isotropic out-of-band radiation means that low-end hardware with poorer linearity than required today can be used in massive MIMO. It is also shown that using one-bit ADCs—the simplest and least power-hungry ADCs—at the base station only degrades the signal-to-interference-and-noise ratio of the system by approximately 4 dB when proper power allocation among users is done, which indicates that massive MIMO is resistant against coarse quantization and that low-end ADCs can be used.
Keywords
massive MIMO, hardware, ADC, power amplifier, out-of-band radiation, peak-to-average ratio, Engineering and Technology
BIBTEX
@phdthesis{diva2:952130,
author = {Moll\'{e}n, Christopher},
title = {{On Massive MIMO Base Stations with Low-End Hardware}},
school = {Linköping University},
type = {{Linköping Studies in Science and Technology. Thesis No. 1756}},
year = {2016},
address = {Sweden},
}
Student theses (BSc, MSc)
2013
Abstract
Storskaliga fleranvändar-MIMO-system, med hundratals basstationsantenner, studeras med allt större intresse både inom akademin och industrin. En anledning är att sådana system kan betjäna flera enantennsanvändare samtidigt över samma tids-frekvens-resurs med fleranvändarförkodning. Det innebär högre datahastigheter och bättre spektral effektivitet. En annan anledning är att basstationens energiförbrukning förväntas avta linjärt med antalet antenner tack vare den ökade antennförstärkningen. För att möjliggöra den stora ökningen av antalet antenner, måste priset per antenn, med dess sändtagarkedja, vara lågt. Vore det möjligt att tillverka basstationsantenner av billiga, massproducerade mobiltelefonskomponenter, som effektförstärkare utan avancerad linearisering, då skulle storskalig fleranvändar-MIMO kunna bli verklighet.
Effektförstärkare i mobiltelefoner är generellt anpassade att ha hög verkningsgrad och har, i och med detta, kraftigt olinjära överföringsegenskaper. Det är fördelaktigt att sända signaler med lågt toppvärde genom sådana effektförstärkare, för att undvika svår distortion och för att maximera verkningsgraden genom att endast använda en liten avbackning från arbetspunkten. Konventionellt förkodade signaler har tyvärr högt toppvärde (ca. 10 dB). Detta arbete har undersökt en av Mohammed m.fl. (2013a) föreslagen förkodning för storskalig MIMO som resulterar i sändarsignaler med lågt toppvärde. Det visas att denna förkodning ger signaler med ett toppvärde på 4 dB, och att toppvärdet kan göras godtyckligt litet genom att dessutom begränsa fasvariationen. Ju mer fasen begränsas, desto lägre blir emellertid antennförstärkning. Till exempel om fasvariationen begränsas till π/2, sänks toppvärdet till 2,6 dB, men 2-3 dB högre sändareffekt behövs för att bibehålla samma prestanda eller, likvärdigt, så måste basstationen utrustas med 1,6-2,0 gånger fler antenner. Kontinuerlig fasmodulering som ett sätt att få sändarsignaler med konstant envelopp har studerats kort. Lågtoppvärdesförkodning, där sändarsignalerna ligger innanför en cirkel, föreslås som ett sätt att minska den erfodrade sändareffekten utan att öka toppvärdet märkvärt (<4,5 dB) relativt Mohammeds m.fl. förkodning. Förkodningsalgoritmen som utvecklades för detta fastnade dock i lokala minima, vilket försämrade dess prestanda. Sändareffekten kunde därför endast minskas lite grand (<1 dB) vid höga datahastigheter.
En preliminär länkbudget baserad på en enkel effektförstärkarmodell har visat att, med fullständig kanalkännedom och i frekvensplatt fädning, skulle lågtoppvärdesförkodning kunna minska energiförbrukningen med 33 % jämfört med konventionell, linjär förkodning i en basstation med 100 antenner. Analysen antyder att olineariserade klass AB mobiltelefonseffektförstärkare kan vara ett alternativ för storskalig fleranvändar-MIMO-basstationer.
Keywords
MIMO, PAR Reduction, Precoding, Multi-User, Engineering and Technology, Communication Systems
BIBTEX
@mastersthesis{diva2:633750,
author = {Moll\'{e}n, Christopher},
title = {{Low-PAR Precoding for Very-Large Multi-User MIMO Systems}},
school = {Linköping University},
type = {{LiTH-ISY-EX--13/4671--SE}},
year = {2013},
address = {Sweden},
}
Sidansvarig:
Christopher Mollén
Senast uppdaterad: 2017 10 11 17:19