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On Massive MIMO Base Stations with Low-End Hardware
Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0002-2093-5466
2016 (English)Licentiate thesis, comprehensive summary (Other academic)Alternative title
Om massiv-MIMO-basstationer med enkel hårdvara (Swedish)
Abstract [en]

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.

Abstract [sv]

Massiv-MIMO-basstationer (eng: Multiple-Input Multiple-Output) har visats, både i teori och praktik, besitta många av de egenskaper som framtida trådlösa kommunikationssystem kommer att behöva.  De kan tillhandahålla enhetligt höga datatakter i hela täckningsområdet och simultant betjäna flera enkla mobilenheter utan att använda bredare spektrum, tätare basstationsplacering eller betydligt mer effekt än dagens basstationer.  Huvudutmaningen med massiv MIMO är basstationens enorma hårdvarukomplexitet och -kostnad – varje element i den stora gruppantennen skall kunna kontrolleras individuellt och kräver sålunda sin egen radiokedja.  För att massiv MIMO skall bli kommersiellt attraktiv, måste basstationen byggas av billig, enkel hårdvara.  I denna avhandling undersöks hur enkla effektförstärkare och analog-till-digital-omvandlare (AD-omvandlare) påverkar massiv-MIMO-systemets prestanda.  I studien av signaldistorsionen från enkla förstärkare visas det att inband-distorsionen är försumbar i massiv MIMO och att utombandsstrålningen är den begränsande faktorn som bestämmer vid vilken verkningsgrad förstärkarna kan arbeta.  En förkodare som åstadkommer nerlänks-sändsignaler som har konstant envelopp i kontinuerlig tid presenteras för att möjliggöra användandet av enkla förstärkare med hög verkningsgrad.  Vidare konstateras det att utombandsstrålningen är isotrop när kanalen är frekvensselektiv och när flera användare betjänas; och att den kan lobformas när kanalen är frekvensflat och när få användare betjänas.  Eftersom en massiv-MIMO-basstation utstrålar mindre effekt än dagens basstationer, betyder isotrop utombandsstrålning att enkel hårdvara med sämre linearitet än vad som krävs idag kan användas i massiv MIMO.  Det visas även att användandet av enbits-AD-omvandlare – de enklaste och mest strömsnåla AD-omvandlarna – i basstationen endast minskar signal-till-interferens-och-brus-förhållandet med 4 dB när tillbörlig effektallokering mellan användarna utförs, vilket indikerar att massiv MIMO är motståndskraftig mot grov kvantisering och att enkla AD-omvandlare kan användas.

Abstract [zh]

大規模多輸入多輸出基站,無論從理論上或實際上,皆已經證明具有許多未來無線通訊系統所需的特質。比如:在其整個覆蓋區域均一地提供高數據傳輸速率、在同一時間頻率資源上服務多個簡單的終端設備,而無需佔用更多頻譜資源或更密集地部署基站,亦無需提高基站的功耗。實現大規模多入多出系统的主要挑戰在於硬件複雜度及基站成本——大規模天線陣列中的每一個天線元必須單獨可控,因此需要其自身的射頻鏈路。爲使大規模多入多出基站有商業吸引力,基站必須以簡單低成本的硬件來建造。本論文探討簡單的功率放大器與模擬數字轉換器對大規模多入多出性能的影響。對低端功放信號失真的研究表明,帶內失真對大規模多入多出的性能影響幾乎可以忽略,而帶外泄露是限制功放效率的決定因素。爲使用高功率效率低端功放,本文提出能產生具有恆定包絡連續時間信號的預編碼。本文指出,在頻率選擇性衰落信道上服務多個用戶時,帶外泄露呈現各向同性;而在平坦衰落信道上服務少數用戶時,帶外泄露可呈現波束賦形。由於大規模多入多出基站比現用基站輻射較少功率,帶外泄露各向同性意味著大規模多入多出基站可使用低端硬件,其線性要求不比現有基站的高。另外表明,如果進行合理的多用戶功率分配,基站使用單比特模擬數字轉換器——最簡單低耗的轉換器——僅使系統的信干噪比降低約4分貝。以此可見,大規模多入多出系統對非精確量比較穩定,低端模擬數字轉換器可於此類系統中使用。

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. , 53 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1756
Keyword [en]
massive MIMO, hardware, ADC, power amplifier, out-of-band radiation, peak-to-average ratio
National Category
Communication Systems
Identifiers
URN: urn:nbn:se:liu:diva-130516DOI: 10.3384/lic.diva-130516ISBN: 9789176857076 (Print)OAI: oai:DiVA.org:liu-130516DiVA: diva2:952130
Presentation
2016-09-02, Algortimen, Hus B, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2016-08-23 Created: 2016-08-11 Last updated: 2016-08-31Bibliographically approved
List of papers
1. Waveforms for the Massive MIMO Downlink: Amplifier Efficiency, Distortion and Performance
Open this publication in new window or tab >>Waveforms for the Massive MIMO Downlink: Amplifier Efficiency, Distortion and Performance
2016 (English)In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, no 99, 1-1 p.Article in journal (Refereed) Epub ahead of print
Abstract [en]

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.

National Category
Communication Systems
Identifiers
urn:nbn:se:liu:diva-130514 (URN)10.1109/TCOMM.2016.2557781 (DOI)
Funder
EU, FP7, Seventh Framework Programme, ICT-619086Swedish Research CouncileLLIIT - The Linköping‐Lund Initiative on IT and Mobile Communications
Available from: 2016-08-11 Created: 2016-08-11 Last updated: 2016-08-31
2. Multiuser MIMO Precoding with Per-Antenna Continuous-Time Constant-Envelope Constraints
Open this publication in new window or tab >>Multiuser MIMO Precoding with Per-Antenna Continuous-Time Constant-Envelope Constraints
2015 (English)In: 2015 IEEE 16th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), Institute of Electrical and Electronics Engineers (IEEE), 2015, 261-265 p.Conference paper (Refereed)
Abstract [en]

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.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2015
Series
, IEEE International Workshop on Signal Processing Advances in Wireless Communications, ISSN 2325-3789
National Category
Communication Systems
Identifiers
urn:nbn:se:liu:diva-120251 (URN)10.1109/SPAWC.2015.7227040 (DOI)000380547100053 ()978-1-4799-1931-4 (ISBN)
Conference
SPAWC 2015 The 16th IEEE International Workshop on Signal Processing Advances in Wireless Communications, June 28 – July 1, 2015, Stockholm, Sweden
Available from: 2015-07-20 Created: 2015-07-20 Last updated: 2016-09-19Bibliographically approved
3. Out-of-Band Radiation Measure for MIMO Arrays with Beamformed Transmission
Open this publication in new window or tab >>Out-of-Band Radiation Measure for MIMO Arrays with Beamformed Transmission
2016 (English)In: 2016 IEEE International Conference on Communications (ICC), 2016, 1-6 p.Conference paper (Refereed)
Abstract [en]

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.

National Category
Communication Systems
Identifiers
urn:nbn:se:liu:diva-130513 (URN)10.1109/ICC.2016.7511629 (DOI)9781479966646 (ISBN)
Conference
2016 IEEE International Conference on Communications (ICC), 22-27 May 2016, Kuala Lumpur, Malaysia
Funder
EU, FP7, Seventh Framework Programme, ICT-619086Swedish Research Council
Available from: 2016-08-11 Created: 2016-08-11 Last updated: 2016-08-31Bibliographically approved

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