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.

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.

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.

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.

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.

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.

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.