| 1. |
- Kashyap, Salil, et al.
(författare)
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Performance Analysis of (TDD) Massive MIMO With Kalman Channel Prediction
- 2017
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Ingår i: 2017 IEEE INTERNATIONAL CONFERENCE ON ACOUSTICS, SPEECH AND SIGNAL PROCESSING (ICASSP). - : IEEE. - 9781509041176 ; , s. 3554-3558
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Konferensbidrag (refereegranskat)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.
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| 2. |
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| 3. |
- Mollén, Christopher, 1987-, et al.
(författare)
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Achievable Uplink Rates for Massive MIMO with Coarse Quantization
- 2017
-
Ingår i: 2017 IEEE InternationalConference on Acoustics, Speech,and Signal Processing Proceedings. - 9781509041176 - 9781509041169 - 9781509041183 ; , s. 6488-6492
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Konferensbidrag (refereegranskat)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.
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| 4. |
- Mollén, Christopher, 1987-
(författare)
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High-End Performance with Low-End Hardware : Analysis of Massive MIMO Base Station Transceivers
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
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| 5. |
- Mollén, Christopher, 1987-, et al.
(författare)
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Multiuser MIMO Precoding with Per-Antenna Continuous-Time Constant-Envelope Constraints
- 2015
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Ingår i: 2015 IEEE 16th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC). - : Institute of Electrical and Electronics Engineers (IEEE). - 9781479919314 ; , s. 261-265
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Konferensbidrag (refereegranskat)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.
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| 6. |
- Mollén, Christopher, 1987-
(författare)
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On Massive MIMO Base Stations with Low-End Hardware
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)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.
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| 7. |
- Mollén, Christopher, 1987-, et al.
(författare)
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On the Impact of PA-Induced In-Band Distortion in Massive MIMO
- 2014
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Ingår i: Proceedings of European Wireless 2014. - Berlin, Offenbach, Germany : VDE Verlag GmbH. - 9783800736218 ; , s. 201-206
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Konferensbidrag (refereegranskat)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.
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| 8. |
- Mollén, Christopher, 1987-, et al.
(författare)
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One-Bit ADCs in Wideband Massive MIMO Systems with OFDM Transmission
- 2016
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Ingår i: 2016 IEEE INTERNATIONAL CONFERENCE ON ACOUSTICS, SPEECH AND SIGNAL PROCESSING PROCEEDINGS. - : Institute of Electrical and Electronics Engineers (IEEE). - 9781479999880 ; , s. 3386-3390
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Konferensbidrag (refereegranskat)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.
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| 9. |
- Mollén, Christopher, 1987-, et al.
(författare)
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Out-of-band radiation measure for MIMO arrays with beamformed transmission
- 2016
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Ingår i: 2016 IEEE International Conference on Communications, ICC 2016, Kuala Lumpur, Malaysia, 22-27 May 2016. - : IEEE. - 1550-3607. - 9781479966646 ; , s. Art. no. 7511629-
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Konferensbidrag (refereegranskat)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.
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| 10. |
- Mollén, Christopher, 1987-, et al.
(författare)
-
Performance of Linear Receivers for Wideband Massive MIMO with One-Bit ADCs
- 2016
-
Konferensbidrag (refereegranskat)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.
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| 11. |
- Mollén, Christopher, 1987-, et al.
(författare)
-
Uplink Performance of Wideband Massive MIMO With One-Bit ADCs
- 2017
-
Ingår i: IEEE Transactions on Wireless Communications. - : Institute of Electrical and Electronics Engineers (IEEE). - 1536-1276 .- 1558-2248. ; 16:1, s. 87-100
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Tidskriftsartikel (refereegranskat)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.
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| 12. |
- Mollén, Christopher, 1987-, et al.
(författare)
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Waveforms for the Massive MIMO Downlink: Amplifier Efficiency, Distortion and Performance
- 2016
-
Ingår i: IEEE Transactions on Communications. - : IEEE. - 0090-6778 .- 1558-0857. ; 64:12, s. 5050 - 5063
-
Tidskriftsartikel (refereegranskat)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.
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| 13. |
- Van Chien, Trinh, 1989-, et al.
(författare)
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Large-Scale-Fading Decoding in Cellular Massive MIMO Systems With Spatially Correlated Channels
- 2019
-
Ingår i: IEEE Transactions on Communications. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 0090-6778 .- 1558-0857. ; 67:4, s. 2746-2762
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Tidskriftsartikel (refereegranskat)abstract
- Massive multiple-input-multiple-output (MIMO) systems can suffer from coherent intercell interference due to the phenomenon of pilot contamination. This paper investigates a two-layer decoding method that mitigates both coherent and non-coherent interference in multi-cell Massive MIMO. To this end, each base station (BS) first estimates the channels to intra-cell users using either minimum mean-squared error (MMSE) or element-wise MMSE estimation based on uplink pilots. The estimates are used for local decoding on each BS followed by a second decoding layer where the BSs cooperate to mitigate inter-cell interference. An uplink achievable spectral efficiency (SE) expression is computed for arbitrary two-layer decoding schemes. A closed form expression is then obtained for correlated Rayleigh fading, maximum-ratio combining, and the proposed large-scale fading decoding (LSFD) in the second layer. We also formulate a sum SE maximization problem with both the data power and LSFD vectors as optimization variables. Since this is an NP-hard problem, we develop a low-complexity algorithm based on the weighted MMSE approach to obtain a local optimum. The numerical results show that both data power control and LSFD improve the sum SE performance over single-layer decoding multi-cell Massive MIMO systems.
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| 14. |
- Van Chien, Trinh, 1989-, et al.
(författare)
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Two-Layer Decoding in Cellular Massive MIMO Systems with Spatial Channel Correlation
- 2019
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Ingår i: ICC 2019 - 2019 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC). - : IEEE. - 9781538680889 - 9781538680896
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Konferensbidrag (refereegranskat)abstract
- This paper studies a two-layer decoding method that mitigates inter-cell interference in multi-cell Massive MIMO systems. In layer one, each base station (BS) estimates the channels to intra-cell users and uses the estimates for local decoding on each BS, followed by a second decoding layer where the BSs cooperate to mitigate inter-cell interference. An uplink achievable spectral efficiency (SE) expression is computed for arbitrary two-layer decoding schemes, while a closed-form expression is obtained for correlated Rayleigh fading channels, maximum-ratio combining (MRC), and large-scale fading decoding (LSFD) in the second layer. We formulate a non-convex sum SE maximization problem with both the data power and LSFD vectors as optimization variables and develop an algorithm based on the weighted MMSE (minimum mean square error) approach to obtain a stationary point with low computational complexity.
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