Lund University Publications
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Lund University Lund University Publications2000-01-01T00:00+00:001dailyDecentralized Massive MIMO Systems : Is There Anything to be Discussed?
https://lup.lub.lu.se/search/publication/e1ca883c-f64f-46bb-b68f-9e8dc38f1589
Sanchez, Jesus RodriguezVidal Alegria, JuanRusek, Fredrik2019Algorithms for Massive MIMO uplink detection are typically based on a centralized approach, by which baseband data from all antenna modules need to be routed to a central node for further processing. In the case of Massive MIMO, where hundreds or thousands of antennas are expected in the base-station, such architecture requires high interconnection bandwidth between antennas and the central node. Recently, decentralized architectures have been proposed to maintain low interconnection bandwidth, where channel-state-information (CSI) is obtained locally in each antenna node and not shared. Further, Massive MIMO performance is sensitive to CSI quality. However, in the literature, ideal CSI is typically assumed in decentralized systems, which is not only far from reality but also limits the generality of the analysis.This paper proposes a decentralized (a term that will be defined in the main body of the paper) architecture with the following main features: (i) the channel matrix is not made available at any single node, (ii) there is no inter-communication among antennas, (iii) the architecture used during the payload data phase, is reused to provide a certain statistic to a processing node, (iv) A non-standard channel estimation problem based on said statistic arises, (v) a matrix inversion is needed (in case of zero-forcing) at said processing node.A hefty share of the paper is devoted to (iv).http://lup.lub.lu.se/record/e1ca883c-f64f-46bb-b68f-9e8dc38f1589http://dx.doi.org/10.1109/ISIT.2019.8849465ISBN: 9781538692912scopus:85073149812engKommunikationssystemdecentralizeddetectionMAPMassive MIMOMLMMSEwishartzero-forcingDecentralized Massive MIMO Systems : Is There Anything to be Discussed?contributiontobookanthology/conferenceinfo:eu-repo/semantics/conferencePapertextCapacity Degradation with Modeling Hardware Impairment in Large Intelligent Surface
https://lup.lub.lu.se/search/publication/a80db21c-5996-4c7d-a7d4-e56444a52b6b
Hu, ShaRusek, FredrikEdfors, Ove2019-02-21In this paper, we consider capacity degradations stemming from potential hardware impairments (HWI) of newly proposed Large Intelligent Surface (LIS) systems. Without HWI, the utility of surface-area (the first-order derivative of the capacity with respect to surface-area) is shown to be proportional to the inverse of it. With HWI, the capacity as well as the utility of surface-area are both degraded, due to a higher effective noise level caused by the HWI. After first modeling the HWI in a general form, we derive the effective noise density and the decrement of utility in closed-forms. With those the impacts of increasing the surface-area can be clearly seen. One interesting but also natural outcome is that both the capacity and utility can be decreased when increasing the surface-area in the cases with severe HWI. The turning points where the capacity and the utility start to decrease with HWI can be evaluated from the derived formulas for them. Further, we also consider distributed implementations of a LIS system by splitting it into multiple small LIS-Units, where the impacts of HWI can be significantly suppressed due to a smaller surface-area of each unit.http://lup.lub.lu.se/record/a80db21c-5996-4c7d-a7d4-e56444a52b6bhttp://dx.doi.org/10.1109/GLOCOM.2018.8647606ISBN: 9781538647271scopus:85063522362engKommunikationssystemCapacity Degradation with Modeling Hardware Impairment in Large Intelligent Surfacecontributiontobookanthology/conferenceinfo:eu-repo/semantics/conferencePapertextSpatial Separation of Closely-Located Users in Measured Massive MIMO Channels
https://lup.lub.lu.se/search/publication/82e55a25-787f-4b58-b8e5-724b47fe8725
Flordelis, JoseRusek, FredrikGao, XiangDahman, GhassanEdfors, OveTufvesson, Fredrik2018-07-08We investigate the ability of Massive multiple-input multiple-output (MIMO) systems to spatially separate up to eighteen users located close to one another in line-of-sight (LOS) propagation conditions, in both indoor and outdoor environments. For that, we use fully-synchronous measured channels at 2.6 GHz of single-antenna users moving within a small area and concurrently communicating with a base station (BS) equipped with a compact 128-port array. To quantify the degree of spatial user separability, we use three scalar metrics, namely, the achievable sum-rates, the condition number of the channel matrix, and the angle to interference factor. Our results show that Massive MIMO with zero-forcing (ZF) or regularized ZF (RZF) can spatially separate nine, even eighteen, concurrent users at practical SNR values even in the challenging case of dominant LOS propagation. In particular, signal-to-noise ratio losses relative to ideal (non-interfering and equally strong) channels can be reduced dramatically compared with standard multiuser MIMO systems, which typically have the same number of users as BS antennas. Our findings suggest that with RZF or ZF the ratio of BS antennas to number of served users should be at least three to four, to harvest most of the available spatial gains that the environment can offer. Although orthogonality and array gains complement each other, for the suggested ratios of antennas to users, the main contribution to improving system performance, measured in sum-rates, comes from the orthogonality gain.http://lup.lub.lu.se/record/82e55a25-787f-4b58-b8e5-724b47fe8725http://dx.doi.org/10.1109/ACCESS.2018.2854307scopus:85049797888engIEEE Access; 6, pp 40253-40266 (2018)ISSN: 2169-3536KommunikationssystemChannel measurementsMassive MIMOspatial separationSpatial Separation of Closely-Located Users in Measured Massive MIMO Channelscontributiontojournal/articleinfo:eu-repo/semantics/articletextMassive MIMO Performance - TDD Versus FDD : What Do Measurements Say?
https://lup.lub.lu.se/search/publication/58cd02bd-dd1d-4b3f-bf92-fe276feb54be
Flordelis, JoseRusek, FredrikTufvesson, FredrikLarsson, Erik G.Edfors, Ove2018-04Downlink beamforming in Massive MIMO either relies on uplink pilot measurements--exploiting reciprocity and time-division duplexing (TDD) operation, or on the use of a predetermined grid of beams with user equipments reporting their preferred beams, mostly in frequency-division duplexing (FDD) operation. Massive MIMO in its originally conceived form uses the first strategy, with uplink pilots, whereas there is currently significant commercial interest in the second, grid-of- beams. It has been analytically shown that with isotropic scattering (independent Rayleigh fading) the first approach outperforms the second. Nevertheless, there remains controversy regarding their relative performance in practical channels. In this contribution, the performances of these two strategies are compared using measured channel data at 2.6 GHz.http://lup.lub.lu.se/record/58cd02bd-dd1d-4b3f-bf92-fe276feb54behttp://dx.doi.org/10.1109/TWC.2018.2790912scopus:85041185733engIEEE Transactions on Wireless Communications; 17(4), pp 2247-2261 (2018)ISSN: 1536-1276Kommunikationssystemchannel measurementsFDDMassive MIMOperformanceTDDMassive MIMO Performance - TDD Versus FDD : What Do Measurements Say?contributiontojournal/articleinfo:eu-repo/semantics/articletextMultiuser Bandwidth Minimization with Individual Rate Requirements for Non-Orthogonal Multiple Access
https://lup.lub.lu.se/search/publication/49829fea-c95d-4ec5-bd64-4775cb198a8d
Rusek, FredrikChitti, KrishnaChaitanya, Tumula2017-06-04Non-Orthogonal Multiple Access (NOMA) for a multi-user single- input single-output (SISO) setup is studied in the power and the frequency domains simultaneously. The problem of sum-bandwidth minimization under perfect channel state information is solved for various combinations of rate requirements and user pairings. In this process, an iterative Tabu-search based algorithm is applied to avoid an exhaustive search over all possible user pairing combinations. It is assumed that each constituent user of the pair has a required quality of service and a flat power spectrum. Further, for each user pair, the NOMA operating region is defined by dividing the allocated bandwidth into two non-overlapping Orthogonal Multiple Access (OMA) sub-bands and an overlapping NOMA sub-band. Such an assumption achieves a greater capacity region when compared to the conventional power domain only NOMA. Also for most of the rate requirements, only one of the OMA sub-bands is active in addition to the NOMA sub-band. This simplifies the NOMA user rate expressions and power allocation process. To verify the performance gain, NOMA is compared to an OMA technique.http://lup.lub.lu.se/record/49829fea-c95d-4ec5-bd64-4775cb198a8dhttp://dx.doi.org/10.1109/VTCSpring.2017.8108686ISBN: 978-1-5090-5932-4 ISBN: 978-1-5090-5933-1scopus:85040592539engKommunikationssystemMultiuser Bandwidth Minimization with Individual Rate Requirements for Non-Orthogonal Multiple Accesscontributiontobookanthology/conferenceinfo:eu-repo/semantics/conferencePapertextThe Potential of Using Large Antenna Arrays on Intelligent Surfaces
https://lup.lub.lu.se/search/publication/61d4e6ec-7895-43b4-9ba5-6b83e16704f2
Hu, ShaRusek, FredrikEdfors, Ove2017-11-16In this paper, we consider capacities of single-antenna terminals communicating to large antenna arrays that are deployed on surfaces. That is, the entire surface is used as an intelligent receiving antenna array. Under the condition that the surface area is sufficiently large, the received signal after matched-filtering (MF) can be well approximated by an intersymbol interference (ISI) channel where channel taps are closely related to a sinc function. Based on such an approximation, we have derived the capacities for both one-dimensional (terminals on a line) and high dimensional (terminals on a plane or in a cube) terminal-deployments. In particular, we analyze the normalized capacity $\bar{\mathcal{C}}$, measured in nats/s/Hz/m$^2$, under the constraint that the transmit power per m$^2$, $\bar{P}$, is fixed. We show that when the user-density increases, the limit of $\bar{\mathcal{C}}$, achieved as the wavelength $\lambda$ approaches 0, is $\bar{P}/(2N_0)$ nats/s/Hz/m$^2$, where $N_0$ is the spatial power spectral density (PSD) of noise. In addition, we also show that the number of signal dimensions is $2/\lambda$ per meter deployed surface for the one-dimensional case, and $\pi/\lambda^2$ per m$^2$ deployed surface for two and three dimensional terminal-deployments.http://lup.lub.lu.se/record/61d4e6ec-7895-43b4-9ba5-6b83e16704f2http://dx.doi.org/10.1109/VTCSpring.2017.8108330ISBN: 978-1-5090-5932-4 scopus:85040542986engKommunikationssystemThe Potential of Using Large Antenna Arrays on Intelligent Surfacescontributiontobookanthology/conferenceinfo:eu-repo/semantics/conferencePapertextA Simulation Framework for Multiple-Antenna Terminals in 5G Massive MIMO Systems
https://lup.lub.lu.se/search/publication/8f74d937-6bd6-4962-8010-8ad221184dbb
Bengtsson, Erik L.Rusek, FredrikMalkowsky, SteffenTufvesson, FredrikKarlsson, Peter C.Edfors, Ove2017-11-18The recent interest in massive MIMO has spurred intensive work on massive MIMO channel modeling in contemporary literature. However, current models fail to take the characteristics of terminal antennas into account. There is no massive MIMO channel model available that can be used for evaluation of the influence of different antenna characteristics at the terminal side. In this paper, we provide a simulation framework that fills this gap. We evaluate the framework with antennas integrated into Sony Xperia handsets operating at 3.7 GHz as this spectrum is identified for the 5G new radio standard by 3GPP. The simulation results are compared with measured terminal performance when communicating with the Lund University&#x2019;s massive MIMO testbed under the same loading conditions. Expressions are derived for comparison of the gain obtained from different diversity schemes computed from measured far-field antenna patterns. We conclude that the simulation framework yields results close to the measured ones and that the framework can be used for antenna evaluation for terminals in a practical precoded massive MIMO system.http://lup.lub.lu.se/record/8f74d937-6bd6-4962-8010-8ad221184dbbhttp://dx.doi.org/10.1109/ACCESS.2017.2775210scopus:85035793841engIEEE Access; 5, pp 26819-26831 (2017)ISSN: 2169-3536Kommunikationssystem5GAntenna measurementschannel modelChannel modelsDirective antennasDiversitymassive MIMOMIMONRpilot signalReceiving antennasSRSterminal antennatransmission schemeTransmitting antennasA Simulation Framework for Multiple-Antenna Terminals in 5G Massive MIMO Systemscontributiontojournal/articleinfo:eu-repo/semantics/articletextBeyond Massive-MIMO : The Potential of Data-Transmission with Large Intelligent Surfaces
https://lup.lub.lu.se/search/publication/552776ea-89af-4ec4-953d-a0d9716117ad
Hu, ShaRusek, FredrikEdfors, Ove2018-05-15In this paper, we consider the potential of data-transmission in a system with a massive number of radiating and sensing elements, thought of as a contiguous surface of electromagnetically active material. We refer to this as a Large Intelligent Surface (LIS). The "LIS" is a newly proposed concept, which conceptually goes beyond contemporary massive MIMO technology, and arises from our vision of a future where man-made structures are electronically active with integrated electronics and wireless communication making the entire environment "intelligent". Firstly, we consider capacities of single-antenna autonomous terminals communicating to the LIS where the entire surface is used as a receiving antenna-array in a perfect line-of-sight (LOS) propagation environment. Under the condition that the surface-area is sufficiently large, the received signal after a matched-filtering (MF) operation can be closely approximated by a sinc-function-like intersymbol interference (ISI) channel. Secondly, we analyze a normalized capacity measured per unit-surface, for a fixed transmit power per volume-unit with different terminal-deployments. As terminal-density increases, the limit of the normalized capacity [nats/s/Hz/volume-unit] achieved when wavelength <formula><tex>$\lambda$</tex></formula> approaches zero is equal to half of the transmit power per volume-unit divided by noise spatial power spectral density (PSD). Thirdly, we show that the number of independent signal dimensions that can be harvested per meter deployed surface is <formula><tex>$2/\lambda$</tex></formula> for one-dimensional terminal-deployment, and <formula><tex>$\pi/\lambda^{2}$</tex></formula> per square meter for two and three dimensional terminal-deployments. Lastly, we consider implementations of the LIS in the form of a grid of conventional antenna-elements, and show that the sampling lattice that minimizes the surface-area and simultaneously obtains one independent signal dimension for every spent antenna is the hexagonal lattice.http://lup.lub.lu.se/record/552776ea-89af-4ec4-953d-a0d9716117adhttp://dx.doi.org/10.1109/TSP.2018.2816577scopus:85044028460engIEEE Transactions on Signal Processing; 66(10), pp 2746-2758 (2018)ISSN: 1053-587XKommunikationssystem2D samplingHankel transformhexagonal latticeindependent signal dimensionintersymbol interference (ISI)Large intelligent surface (LIS)LatticesLensesline-of-sight (LOS)massive multiinput multi-output (MIMO)MIMO communicationnormalized capacityReceiving antennasSurface treatmentSurface wavesBeyond Massive-MIMO : The Potential of Data-Transmission with Large Intelligent Surfacescontributiontojournal/articleinfo:eu-repo/semantics/articletextBeyond Massive MIMO: The Potential of Positioning with Large Intelligent Surfaces
https://lup.lub.lu.se/search/publication/2b8a8240-0f1a-4ef2-8828-46b70b2627f8
Hu, ShaRusek, FredrikEdfors, Ove2018-04-01We consider the potential for positioning with a system where antenna arrays are deployed as a large intelligent surface (LIS), which is a newly proposed concept beyond massive multi-input multi-output (MIMO) where future man-made structures are electronically active with integrated electronics and wireless communication making the entire environment "intelligent". In a first step, we derive Fisher-information matrix (FIM) and Cramer-Rao lower bound (CRLB) in closed-form for positioning a terminal located perpendicular to the center of the LIS, whose location we refer to as being on the central perpendicular line (CPL) of the LIS. For a terminal that is not on the CPL, closed-form expressions of the FIM and CRLB seem out of reach, and we alternatively find approximations which are shown to be accurate. Under mild conditions, we show that the CRLB for all three Cartesian dimensions ($x$, $y$ and $z$) decreases quadratically in the surface-area of the LIS, except for a terminal exactly on the CPL where the CRLB for the $z$-dimension (distance from the LIS) decreases linearly in the same. In a second step, we analyze the CRLB for positioning when there is an unknown phase $\varphi$ presented in the analog circuits of the LIS. We then show that the CRLBs are dramatically {\color{red}degraded} for all three dimensions but decrease in the third-order of the surface-area. Moreover, with an infinitely large LIS the CRLB for the $z$-dimension with an unknown $\varphi$ is 6 dB higher than the case without phase uncertainty, and the CRLB for estimating $\varphi$ converges to a constant that is independent of the wavelength $\lambda$. At last, we extensively discuss the impact of centralized and distributed deployments of LIS, and show that a distributed deployment of LIS can enlarge the coverage for positioning and improve the overall performance.http://lup.lub.lu.se/record/2b8a8240-0f1a-4ef2-8828-46b70b2627f8http://dx.doi.org/10.1109/TSP.2018.2795547scopus:85040932814engIEEE Transactions on Signal Processing; 66(7), pp 1761-1774 (2018)ISSN: 1053-587XKommunikationssystemBeyond Massive MIMO: The Potential of Positioning with Large Intelligent Surfacescontributiontojournal/articleinfo:eu-repo/semantics/articletextImproving the Performance of OTDOA based Positioning in NB-IoT Systems
https://lup.lub.lu.se/search/publication/8240e864-8fee-4917-ab43-75c59bff90e9
Hu, ShaBerg, AxelLi, XuhongRusek, Fredrik2018In this paper, we consider positioning with observed-time-difference-of-arrival (OTDOA) for a device deployed in long-term-evolution (LTE) based narrow-band Internet-of-things (NB-IoT) systems. We propose an iterative expectation- maximization based successive interference cancellation (EM-SIC) algorithm to jointly consider estimations of residual frequency- offset (FO), fading-channel taps and time-of- arrival (ToA) of the first arrival-path for each of the detected cells. In order to design a low complexity ToA detector and also due to the limits of low-cost analog circuits, we assume an NB-IoT device working at a low-sampling rate such as 1.92 MHz or lower. The proposed EM-SIC algorithm comprises two stages to detect ToA, based on which OTDOA can be calculated. In a first stage, after running the EM-SIC block a predefined number of iterations, a coarse ToA is estimated for each of the detected cells. Then in a second stage, to improve the ToA resolution, a low-pass filter is utilized to interpolate the correlations of time-domain PRS signal evaluated at a low sampling-rate to a high sampling-rate such as 30.72 MHz. To keep low-complexity, only the correlations inside a small search window centered at the coarse ToA estimates are upsampled. Then, the refined ToAs are estimated based on upsampled correlations. If at least three cells are detected, with OTDOA and the locations of detected cell sites, the position of the NB-IoT device can be estimated. We show through numerical simulations that, the proposed EM-SIC based ToA detector is robust against impairments introduced by inter-cell interference, fading-channel and residual FO. Thus significant signal-to-noise (SNR) gains are obtained over traditional ToA detectors that do not consider these impairments when positioning a device.http://lup.lub.lu.se/record/8240e864-8fee-4917-ab43-75c59bff90e9http://dx.doi.org/10.1109/GLOCOM.2017.8254510scopus:85046373371engKommunikationssystemImproving the Performance of OTDOA based Positioning in NB-IoT Systemscontributiontobookanthology/conferenceinfo:eu-repo/semantics/conferencePapertextCramer-Rao Lower Bounds for Positioning with Large Intelligent Surfaces
https://lup.lub.lu.se/search/publication/22d6df59-efbe-41df-8f42-31835607d73c
Hu, ShaRusek, FredrikEdfors, Ove2018-02-12We consider the potential for positioning with a system where antenna arrays are deployed as a large intelligent surface (LIS). We derive Fisher-informations and Cram\'{e}r-Rao lower bounds (CRLB) in closed-form for terminals along the central perpendicular line (CPL) of the LIS for all three Cartesian dimensions. For terminals at positions other than the CPL, closed-form expressions for the Fisher-informations and CRLBs seem out of reach, and we alternatively provide approximations (in closed-form) which are shown to be very accurate. We also show that under mild conditions, the CRLBs in general decrease quadratically in the surface-area for both the x and y dimensions. For the z-dimension (distance from the LIS), the CRLB decreases linearly in the surface-area when terminals are along the CPL. However, when terminals move away from the CPL, the CRLB is dramatically increased and then also decreases quadratically in the surface-area. We also extensively discuss the impact of different deployments (centralized and distributed) of the LIS.http://lup.lub.lu.se/record/22d6df59-efbe-41df-8f42-31835607d73chttp://dx.doi.org/10.1109/VTCFall.2017.8288263scopus:85045284667engKommunikationssystemCramer-Rao Lower Bounds for Positioning with Large Intelligent Surfacescontributiontobookanthology/conferenceinfo:eu-repo/semantics/conferencePapertextTransmission schemes for Multiple Antenna Terminals in Real Massive MIMO systems
https://lup.lub.lu.se/search/publication/6daa2654-3111-448d-be8b-30f0ee3bc2f3
Bengtsson, Erik Lkarlsson, Peter C.Tufvesson, FredrikVieira, JoaoMalkowsky, SteffenLiu, LiangRusek, FredrikEdfors, Ove2016-12-13In massive MIMO performance evaluations it is often assumed that the terminal has a single antenna. The combination of multiple antennas in a terminal and massive MIMO precoding at the base station side can further improve overall system performance. We present measurement results for multi antenna terminals operating in different transmission schemes and how they perform under varying loading conditions. Gain expressions are derived that enable easy comparison between the transmission schemes. The evaluation is performed on realistic antennas integrated into Sony Xperia handsets tuned to 3.7&#126;GHz and operated together with the Lund University massive MIMO (LuMaMi) test bed. It is concluded that the approach used in today's mobile systems, where up link and down link are addressed independently, will not provide the best performance. The performance can be improved by the selection of transmission schemes optimized for massive MIMO.http://lup.lub.lu.se/record/6daa2654-3111-448d-be8b-30f0ee3bc2f3http://dx.doi.org/10.1109/GLOCOM.2016.7842355ISBN: 978-1-5090-1328-9 ISBN: 978-1-5090-1329-6 scopus:85015444185engKommunikationssystemTransmission schemes for Multiple Antenna Terminals in Real Massive MIMO systemscontributiontobookanthology/conferenceinfo:eu-repo/semantics/conferencePapertextA Soft-Output MIMO Detector with Achievable Information Rate based Partial Marginalization
https://lup.lub.lu.se/search/publication/4a1a9cd3-578e-44c8-b184-55bb1afc0caa
Hu, ShaRusek, Fredrik2017-03-15In this paper, we propose a soft-output detector for multiple-input multiple-output (MIMO) channels that utilizes achievable information rate (AIR) based partial marginalization (PM). The proposed AIR based PM (AIR-PM) detector has superior performance compared to previously proposed PM designs and other soft-output detectors such as K-best, while at the same time yielding lower computational complexity, a detection latency that is independent of the number of transmit layers, and straightforward inclusion of soft input information. Using a tree representation of the MIMO signal, the key property of the AIRPM is that the connections among all child layers are broken. Therefore, least-square (LS) estimates used for marginalization are obtained independently and in parallel, which have better quality than the zero-forcing decision feedback (ZF-DF) estimates used in previous PM designs. Such a property of the AIRPM detector is designed via a mismatched detection model that maximizes the AIR. Furthermore, we show that the chain rule holds for the AIR calculation, which facilitates an information theoretic characterization of the AIR-PM detector.http://lup.lub.lu.se/record/4a1a9cd3-578e-44c8-b184-55bb1afc0caahttp://dx.doi.org/10.1109/TSP.2016.2641393scopus:85014949602wos:000395423500019engIEEE Transactions on Signal Processing; 65(6), pp 1622-1627 (2017)ISSN: 1053-587XSignalbehandlingA Soft-Output MIMO Detector with Achievable Information Rate based Partial Marginalizationcontributiontojournal/articleinfo:eu-repo/semantics/articletextComparison of two channel shortening approaches for MIMO-ISI channels
https://lup.lub.lu.se/search/publication/aad27d72-b2dd-45be-839f-902328cd4cfe
Hu, ShaRusek, FredrikAl-Dhahir, Naofal2016-09-12We consider a multiple-input multiple-output (MIMO) channel with inter-symbol interference (ISI) where signal detection is highly complex due to the large signal state-space dimensionality. A common strategy is to apply a front-end filter (FEF) to eliminate the ISI dimension (i.e. full ISI equalization). This FEF is then followed by a frequency non-selective MIMO detector. However, another, much less researched, strategy is to apply an FEF that eliminates the MIMO dimension and results in a set of parallel ISI channels followed by a bank of parallel single-input single-output (SISO) detectors. Which of the two strategies is better in a Shannon capacity sense? In this paper, we show that the answer to this question depends on system parameters such as SNR, number of antennas, ISI duration, and spatial correlation properties.http://lup.lub.lu.se/record/aad27d72-b2dd-45be-839f-902328cd4cfehttp://dx.doi.org/10.1109/WCNC.2016.7564884ISBN: 978-1-4673-9814-5ISBN: 978-1-4673-9815-2scopus:84989919673engTelekommunikationComparison of two channel shortening approaches for MIMO-ISI channelscontributiontobookanthology/conferenceinfo:eu-repo/semantics/conferencePapertextExploiting antenna correlation in measured massive MIMO channels
https://lup.lub.lu.se/search/publication/68f66d39-8a88-4351-a8be-8480a10c1365
Flordelis, JoseHu, ShaRusek, FredrikEdfors, OveDahman, GhassanGao, XiangTufvesson, Fredrik2016-12-21We investigate antenna correlation of an M-antenna massive multiple-input multiple-output (MIMO) setup with the purpose of obtaining a low-rank representation of the instantaneous massive MIMO channel. Low-rank representation bases using short-term and long-term antenna correlation statistics are defined, and their performance is evaluated with data sets obtained from channel measurements in both indoor and outdoor environments at 2.6 GHz. Our results indicate that the short-term bases can capture a larger amount of the channel energy compared to the long-term ones, but they have a limited timespan, one coherence time or less. On the other hand, the long-term bases are stable over time-spans of a few seconds. Hence, they can be obtained relatively easily. We also investigate a rank-p vector-scalar LMMSE channel estimator that exploits antenna correlation. Our results show that the investigated estimator can achieve a performance similar to that of full-rank LMMSE at a (2p + 1)/M times lower cost. The investigated estimator may be used in conjunction with estimators that exploit correlation in the frequency and time domains or, alternatively, in situations in which these estimators cannot be used, e.g., when pilot separation is larger than the channel coherence bandwidth or time.application/pdfhttp://lup.lub.lu.se/record/68f66d39-8a88-4351-a8be-8480a10c1365http://dx.doi.org/10.1109/PIMRC.2016.7794664ISBN: 9781509032549http://lup.lub.lu.se/search/ws/files/11792298/paper.pdfscopus:85010032081enginfo:eu-repo/semantics/openAccessTelekommunikationExploiting antenna correlation in measured massive MIMO channelscontributiontobookanthology/conferenceinfo:eu-repo/semantics/conferencePapertextOn the Design of Reduced State Demodulators with Interference Cancellation for Iterative Receivers
https://lup.lub.lu.se/search/publication/7b5c1a1f-79d4-4555-851a-100f5a3f2b0d
Hu, ShaRusek, Fredrik2015-09We consider the problem of designing demodulators for channels with memory that use reduced-size trellis descriptions for the received signal. We assume an overall iterative receiver, and for the parts of the signal not covered by the trellis description, we use interference cancellation based on the soft information provided by the outer decoder. In order to reach a trellis description, a linear filter is applied as front-end to compress the signal structure into a small trellis. This process requires three parameters to be designed: (i) the front-end filter, (ii) the feedback filter through which the interference cancellation is done, and (iii) a target response which specifies the trellis. While (i) and (ii) can be found in closed form, a numerical search is required for (iii). The numerical search is, however, very efficient and stable. Demodulators of this form have been studied before under the name channel shortening (CS), but the interplay between CS and interference cancellation has not been adequately addressed in the literature.http://lup.lub.lu.se/record/7448311http://dx.doi.org/10.1109/PIMRC.2015.7343440scopus:84958044523engElektroteknik och elektronikOn the Design of Reduced State Demodulators with Interference Cancellation for Iterative Receiverscontributiontobookanthology/conferenceinfo:eu-repo/semantics/conferencePapertextA Low-complexity Channel Shortening Receiver with Diversity Support for Evolved 2G Devices
https://lup.lub.lu.se/search/publication/6c675ec1-9da6-4896-b351-074cfcfe6cf8
Hu, ShaKröll, HaraldHuang, QiutingRusek, Fredrik2016-05The second generation (2G) cellular networks are the current workhorse for machine-to-machine (M2M) communications. Diversity in 2G devices can be present both in form of multiple receive branches and blind repetitions. In presence of diversity, intersymbol interference (ISI) equalization and co-channel interference (CCI) suppression are usually very complex. In this paper, we consider the improvements for 2G devices with receive diversity. We derive a low-complexity receiver based on a channel shortening filter, which allows to sum up all diversity branches to a single stream after filtering while keeping the full diversity gain. The summed up stream is subsequently processed by a single stream Max-log-MAP (MLM) equalizer. The channel shortening filter is designed to maximize the mutual information lower bound (MILB) with the Ungerboeck detection model. Its filter coefficients can be obtained mainly by means of discrete-Fourier transforms (DFTs). Compared with the state-of-art homomorphic (HOM) filtering based channel shortener which cooperates with a delayed-decision feedback MLM (DDF-MLM) equalizer, the proposed MILB channel shortener has superior performance. Moreover, the equalization complexity, in terms of real-valued multiplications, is decreased by a factor that equals the number of diversity branches.http://lup.lub.lu.se/record/6c675ec1-9da6-4896-b351-074cfcfe6cf8http://dx.doi.org/10.1109/ICC.2016.7511498scopus:84981335378engTelekommunikationA Low-complexity Channel Shortening Receiver with Diversity Support for Evolved 2G Devicescontributiontobookanthology/conferenceinfo:eu-repo/semantics/conferencePapertextReciprocity Calibration for Massive MIMO: Proposal, Modeling and Validation
https://lup.lub.lu.se/search/publication/7f9779c0-1634-4c58-928f-efe16178911a
Vieira, JoaoRusek, FredrikEdfors, OveMalkowsky, SteffenLiu, LiangTufvesson, Fredrik2017-03-17This paper presents a mutual coupling based calibration method for time-division-duplex massive MIMO systems, which enables downlink precoding based on uplink channel estimates. The entire calibration procedure is carried out solely at the base station (BS) side by sounding all BS antenna pairs. An Expectation-Maximization (EM) algorithm is derived, which processes the measured channels in order to estimate calibration coefficients. The EM algorithm outperforms current state-of-the-art narrow-band calibration schemes in a mean squared error (MSE) and sum-rate capacity sense. Like its predecessors, the EM algorithm is general in the sense that it is not only suitable to calibrate a co-located massive MIMO BS, but also very suitable for calibrating multiple BSs in distributed MIMO systems. The proposed method is validated with experimental evidence obtained from a massive MIMO testbed. In addition, we address the estimated narrow-band calibration coefficients as a stochastic process across frequency, and study the subspace of this process based on measurement data. With the insights of this study, we propose an estimator which exploits the structure of the process in order to reduce the calibration error across frequency. A model for the calibration error is also proposed based on the asymptotic properties of the estimator, and is validated with measurement results.application/pdfhttp://lup.lub.lu.se/record/7f9779c0-1634-4c58-928f-efe16178911ahttp://dx.doi.org/10.1109/TWC.2017.2674659http://lup.lub.lu.se/search/ws/files/25556784/FINAL_VERSION.pdfscopus:85027885273wos:000401083800024enginfo:eu-repo/semantics/openAccessIEEE Transactions on Wireless Communications; 16(5), pp 3042-3056 (2017)ISSN: 1536-1276TelekommunikationMassive MIMOreciprocity calibrationmutual couplingexpectation maximizationvalidationReciprocity Calibration for Massive MIMO: Proposal, Modeling and Validationcontributiontojournal/articleinfo:eu-repo/semantics/articletextAn information theoretic characterization of channel shortening receivers
https://lup.lub.lu.se/search/publication/82de250a-db85-430b-8b25-60366169a36a
Rusek, FredrikEdfors, Ove2016-04-01Optimal data detection of data transmitted over a linear channel can always be implemented through the Viterbi algorithm (VA). However, in many cases of interest the memory of the channel prohibits direct application of the VA. A popular and conceptually simple method in this case, studied since the early 1970s, is to first filter the received signal in order to shorten the memory of the channel, and then to apply a VA that operates with the shorter memory. We shall refer to this general concept as a channel shortening (CS) receiver. Although studied for almost four decades, an information theoretic understanding of what such a simple receiver solution is actually doing is not available. In this paper, we show that an optimized CS receiver has a direct correspondance to the chain rule of mutual information. Furthermore, we show that the tools for analyzing the ensuing achievable rates from an optimized CS receiver are precisely the same as those used for analyzing the achievable rates of a minimum mean-square-error (MMSE) receiver.http://lup.lub.lu.se/record/82de250a-db85-430b-8b25-60366169a36ahttp://dx.doi.org/10.1109/TCOMM.2016.2536664scopus:84964355109wos:000375032400011engIEEE Transactions on Communications; 64(4), pp 1490-1502 (2016)ISSN: 0090-6778Elektroteknik och elektronikChannel shortening detectionMismatched mutual informationMismatched receiversReceiver designReduced complexity detectionAn information theoretic characterization of channel shortening receiverscontributiontojournal/articleinfo:eu-repo/semantics/articletextLower bounds on the information rate of intersymbol interference channels based on the Ungerboeck observation model
https://lup.lub.lu.se/search/publication/e049b5be-b089-4e9a-9e8d-d0ec79e34d20
Rusek, FredrikFertonani, Dario2009We consider the problem of lower bounding the information rate of intersymbol interference channels via Monte Carlo algorithms.
We adopt a novel approach based on the Ungerboeck observation model, unlike the existing ones that are all based on the Forney model.
The two approaches, yet equivalent in the case of full-complexity detection, lead to different results in the case of reduced-complexity detection, which is the only viable option when the channel memory is large.
In particular, the proposed approach significantly outperforms the existing ones at low-to-medium values of the signal-to-noise ratio.http://lup.lub.lu.se/record/1277678wos:000280141401042scopus:70449476546engElektroteknik och elektronikIntersymbol InterferenceShannon CapacityLower bounds on the information rate of intersymbol interference channels based on the Ungerboeck observation modelcontributiontoconference/paperinfo:eu-repo/semantics/conferenceObjecttext