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1.	Aabel, Lise, et al. (författare) A TDD Distributed MIMO Testbed Using a 1-bit Radio-Over-Fiber Fronthaul Architecture 2024 Ingår i: IEEE Transactions on Microwave Theory and Techniques. - 0018-9480 .- 1557-9670. ; In Press Tidskriftsartikel (refereegranskat)abstract We present the uplink and downlink of a time-division duplex distributed multiple-input multiple-output (D-MIMO) testbed, based on a 1-bit radio-over-fiber architecture, which is low cost and scalable. The proposed architecture involves a central unit (CU) that is equipped with 1-bit digital-to-analog and analog-to-digital converters, operating at 10 GS/s. The CU is connected to multiple single-antenna remote radio heads (RRHs) via optical fibers, over which a binary radio frequency (RF) waveform is transmitted. In the uplink, a binary RF waveform is generated at the RRHs by a comparator, whose inputs are the received RF signal and a suitably designed dither signal. In the downlink, a binary RF waveform is generated at the CU via bandpass sigma-delta modulation. Our measurement results show that low error-vector magnitude (EVM) can be achieved in both the uplink and the downlink, despite 1-bit sampling at the CU. Specifically, for point-to-point over-cable transmission between a single user equipment (UE) and a CU equipped with a single RRH, we report, for a 10-MBd signal using single-carrier (SC) 16 quadratic-amplitude modulation (QAM) modulation, an EVM of 3.3% in the downlink, and of 4.5% in the uplink. We then consider a CU connected to three RRHs serving over the air two UEs, and show that, after over-the-air reciprocity calibration, a downlink zero-forcing precoder designed on the basis of uplink channel estimates at the CU achieves an EVM of 6.4% and 10.9% at UE 1 and UE 2, respectively. Finally, we investigate the ability of the proposed architecture to support orthogonal frequency-division multiplexing (OFDM) waveforms, and its robustness against both in-band and out-of-band interference.
2.	Aabel, Lise, 1992, et al. (författare) Distributed Massive MIMO via all-Digital Radio Over Fiber 2020 Ingår i: Conference Record - Asilomar Conference on Signals, Systems and Computers. - 1058-6393. ; 2020-November, s. 319-323 Konferensbidrag (refereegranskat)abstract A crucial challenge in the implementation of distributed massive multiple-input multiple-output (MIMO) architectures is to provide phase coherence while, at the same time, limit the complexity of the remote-radio heads (RRHs), which is important for cost-efficient scalability. To address this challenge, we present in this paper a phase-coherent distributed MIMO architecture, based on off-the-shelf, low-cost components. In the proposed architecture, up- and down-conversion are carried out at the central unit (CU). The RRHs are connected to the CU by means of optical fibers carrying oversampled radio-frequency (RF) 1-bit signals. In the downlink, the 1-bit signal is generated via sigma-delta modulation. At the RRH, the RF signal is recovered from the 1-bit signal through a bandpass filter and a power amplifier, and then fed to an antenna. In the uplink, the 1-bit signal is generated by a comparator whose inputs are the low-noise-amplified received RF signal and a suitably designed dither signal. The performance of the proposed architecture is evaluated with satisfactory results both via simulation and measurements from a testbed.
3.	Bencivenni, Carlo, 1986, et al. (författare) Aperiodic Switched Array for Line-of-Sight MIMO Backhauling 2018 Ingår i: IEEE Antennas and Wireless Propagation Letters. - 1548-5757 .- 1536-1225. ; 17:9, s. 1712-1716 Tidskriftsartikel (refereegranskat)abstract A switchable aperiodic array solution is proposed to increase the minimum channel capacity of a multiple-input-multiple-output (MIMO) backhauling system over an extended line-of-sight communication range. The objective is to propose a low-complexity design solution that is cost-effective in regard to the manufacturing, installation, and its design process. It is shown that by adding only one switch and one auxiliary antenna per base station, a 4 × 4 MIMO system can be realized that is capable of minimizing the capacity loss that both regular and aperiodic arrays suffer from as a function of the link distance.
4.	Bohlin, Patrik, 1974, et al. (författare) Optimized Data Aided Training in MIMO Systems 2004 Ingår i: IEEE Vehicular Technology Conference, VTC 2004 Spring, Milan, Italy. Konferensbidrag (refereegranskat)
5.	Bohlin, Patrik, 1974, et al. (författare) Performance Evaluation of MIMO Communication Systems based Superimposed Pilots 2004 Ingår i: IEEE International Conference on Acoustics, Speech, and Signal Processing; Montreal, Que; Canada; 17 May 2004 through 21 May 2004. - 1520-6149. ; 4, s. IV-425-IV-428- Konferensbidrag (refereegranskat)abstract In the recent past, a number of publications have suggested superimposed pilots (SIP) for channel estimation in MIMO systems. However, the performance gain achieved by SIP compared to conventional (time-multiplexed) training is still questionable. In this paper we introduce a framework for modeling a general SIP scheme, of which the conventional scheme is a special case. Utilizing this framework, we derive a lower bound on the channel capacity to compare the performance between the two systems. It is found that in certain scenarios (high SNR, many receive antennas and short coherence times), it is beneficial to also transmit data during the training mode (i.e., use SIP). The main conclusion though, is that for most cases where these kind of schemes would be realistic, i.e. for systems with coherence lengths of more than a few symbols, the general SIP-scheme reduces to the conventional scheme. Hence, rendering the same capacity.
6.	Castaneda, Oscar, et al. (författare) 1-bit Massive MU-MIMO Precoding in VLSI 2017 Ingår i: IEEE Journal on Emerging and Selected Topics in Circuits and Systems. - : Institute of Electrical and Electronics Engineers (IEEE). - 2156-3365 .- 2156-3357. ; 7:4, s. 508-522 Tidskriftsartikel (refereegranskat)abstract Massive multi-user (MU) multiple-input multiple-output (MIMO) will be a core technology in fifth-generation (5G) wireless systems as it offers significant improvements in spectral efficiency compared to existing multi-antenna technologies. The presence of hundreds of antenna elements at the base station (BS), however, results in excessively high hardware costs and power consumption, and requires high interconnect throughput between the baseband-processing unit and the radio unit. Massive MU-MIMO that uses low-resolution analog-to-digital and digital-toanalog converters (DACs) has the potential to address all these issues. In this paper, we focus on downlink precoding for massive MU-MIMO systems with 1-bit DACs at the BS. The objective is to design precoders that simultaneously mitigate MU interference and quantization artifacts. We propose two nonlinear 1-bit precoding algorithms and corresponding very large-scale integration (VLSI) designs. Our algorithms rely on biconvex relaxation, which enables the design of efficient 1-bit precoding algorithms that achieve superior error-rate performance compared with that of linear precoding algorithms followed by quantization. To showcase the efficacy of our algorithms, we design VLSI architectures that enable efficient 1-bit precoding for massive MU-MIMO systems, in which hundreds of antennas serve tens of user equipments. We present corresponding field-programmable gate array (FPGA) reference implementations to demonstrate that 1-bit precoding enables reliable and high-rate downlink data transmission in practical systems.
7.	Coldrey, Mikael, 1972, et al. (författare) A Capacity Comparison between Time-Multiplexed and Superimposed Pilots 2004 Ingår i: Proc. 38th Asilomar Conference on Signals, Systems and Computers, Monterey, USA. Konferensbidrag (refereegranskat)
8.	Coldrey, Mikael, 1972, et al. (författare) Analysis of spectral-based localization of spatially distributed sources 2003 Ingår i: 13th IFAC Symposium on System Identification, SYSID 2003, Rotterdam, Netherlands. Konferensbidrag (refereegranskat)
9.	Coldrey, Mikael, 1972 (författare) Beamforming-based Estimators for Spatially Distributed Signals 2002 Ingår i: PCC Workshop & RadioVetenskap och Kommunikation, RVK '02, Stockholm, Sweden. Konferensbidrag (refereegranskat)
10.	Coldrey, Mikael, 1972 (författare) Channel Estimation in Wireless Communication Systems Employing Multiple Antennas 2004 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract Antennas that are able to adaptively direct the transmitted (and received) energy are of great interest in future wireless communication systems. The directivity implies reduced transmit power and interference, and also a potential for increased capacity. The focus of this thesis is on channel estimation in wireless communication systems that employ multiple antennas. The thesis is divided into two parts, whereof the first part addresses the problem of parameter estimation of a distributed source. Due to, e.g., local scattering around the transmitter, the source, as seen from the receiver, appears spatially distributed. A characterization of the spatial channel, in particular mean direction of arrival and spatial spread, is of interest for system optimization and performance prediction. Low-complexity beamforming-based estimators are introduced for the estimation of direction and spatial spread of the distributed source. Despite not exploiting the model, the proposed non-parametric estimators are found to show very competitive performance compared to more complex parametric methods. Also provided is a statistical analysis of their performance. The second part of the thesis deals with Multiple-Input Multiple-Output (MIMO) channel estimation. A simple and straightforward way of estimating the unknown channel is to transmit known training/pilot sequences. In the recent past, a number of publications have suggested Superimposed Pilots (SIP) for channel estimation in MIMO systems. However, the performance gain achieved by SIP compared to conventional (time-multiplexed) training is still questionable. To evaluate the performance of the various training-based schemes, a lower bound on the ergodic channel capacity of a general training-based scheme applied to a block-wise flat-fading MIMO channel is derived and evaluated. It is found that in certain scenarios (many receive antennas and short channel coherence times), it is beneficial to also transmit data during the training mode (i.e. use SIP). The main conclusion though, is that the general SIP-scheme quite often reduces to the conventional time-multiplexed scheme, and, hence, renders the same capacity.
11.	Coldrey, Mikael, 1972 (författare) Direction and Spread Estimation of Spatially Distributed Signals via the Power Azimuth Spectrum 2002 Ingår i: IEEE International Conference on Acoustics Speech and Signal Processing, ICASSP '02, Orlando, USA. Konferensbidrag (refereegranskat)
12.	Coldrey, Mikael, 1972 (författare) Estimation and Performance Analysis of Wireless Multiple Antenna Communication Channels 2006 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract The focus of this thesis is on channel estimation in wireless communication systems that employ multiple antennas.The thesis is divided into two parts, whereof the first part addresses theproblem of parameter estimation of distributed sources. Due to, e.g., localscattering around the transmitter, the source (as seen from the receiver)appears spatially distributed. A characterization of the spatial channel, inparticular mean direction of arrival and spatial spread, is of interest for optimizationand performance prediction of future communication systems. Nonparametricbeamforming-based estimators are derived and analyzed for thelocalization of distributed sources, and it is found that they provide goodestimation performance. Parametric generalized beamformers are also presentedand analyzed. It is found that they also yield competitive performancecompared to existing algorithms.The second part of the thesis deals with Multiple-Input Multiple-Output(MIMO) and Single-Input Single-Output (SISO) channel estimation. A simpleand straightforward way of estimating the unknown channel is to transmitknown training/pilot sequences. In the recent past, a number of publicationshave suggested Superimposed Pilots (SIP) for channel estimation in communicationsystems. However, the performance gain achieved by SIP comparedto conventional (time-multiplexed) training is still questionable. To evaluatethe performance of the various training-based schemes, a lower bound on the mutual information of a general training-based scheme applied to block-wise fading MIMO and SISO channels is derived and maximized. It is found that in certain scenarios it is beneficial to also transmit data during the training mode (i.e., use SIP). The main conclusion though, is that the general SIP-scheme quite often reduces to the conventional time-multiplexed scheme, and, hence, renders the same performance. The theory is also extended to the case when detected data symbols are used as additional training symbols, which significantly improves the channel estimation performance. An improved channel estimate leads to an improvement in the effective Signa-lto-Noise Ratio (SNR), and, thus, a higher coding rate may successfully be applied. The extended scheme shows a performance close to the fundamental capacity of the noncoherent MIMO channel.
13.	Coldrey, Mikael, 1972, et al. (författare) Generalization and Analysis of the Conventional Beamformer for Localization of Spatially Distributed Sources 2006 Ingår i: In Proc. 14th European Signal Processing Conference, EUSIPCO 2006. Konferensbidrag (refereegranskat)
14.	Coldrey, Mikael, 1972, et al. (författare) Higher order MIMO outdoor-to-indoor measurements using repeaters 2010 Ingår i: IEEE Vehicular Technology Conference. - 1550-2252. - 9781424425198 Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract In this paper we present results from a outdoor- to-indoor MIMO measurement campaign where we study the effect of repeaters on singular values, spatial richness, capacity, and delay spread distributions. We present results from different repeater scenarios such as, e.g. different outdoor/indoor repeater deployments with single/dual polarized antennas. The measurements show that repeaters significantly enhance the received SNR while they increase the delay spread and decrease the spatial richness of the MIMO channel. However, the main conclusion is that a significant increase in channel capacity is attained when deploying repeaters mostly thanks to their ability to provide spatial multiplexing over the high-quality eigenmodes they may provide. The number of such eigenmodes is limited by the number of repeaters, deployment, and their antennas. © 2010 IEEE.
15.	Coldrey, Mikael, 1972, et al. (författare) Multiple-Antenna Channel Estimation Using Detected Symbol Information 2004 Ingår i: PCC Workshop & Nordic Radio Symposium NRS '04, Oulu, Finland. Konferensbidrag (refereegranskat)
16.	Coldrey, Mikael, 1972 (författare) On the use of beamforming for estimation of spatially distributed signals 2003 Ingår i: IEEE International Conference on Acoustics Speech and Signal Processing, ICASSP '03, Hong Kong, China, APR 06-10. ; 5, s. 369-372 Konferensbidrag (refereegranskat)abstract In wireless communications, spatially distributed signals originate from local scattering around the transmitter. Spatially distributed signals cause conventional high-resolution methods to show degradation in performance, since the underlying data model is of full rank theoretically. In this paper, a relationship between the distribution of the spatial spread and the Power Azimuth Spectrum is derived. Using this relationship, two simple and robust estimators based on the conventional beamformer and Capon's beamformer for estimation of the nominal direction and spatial spread are found. The proposed estimators show very good performance in numerical examples.
17.	Coldrey, Mikael, 1972, et al. (författare) Performance Evaluation of MIMO Wireless Communication Systems Employing Pilot Training for Channel Estimation 2003 Ingår i: PCC Workshop & Ericsson/TeliaSonera Research Days 2003, Stockholm, Sweden. Konferensbidrag (refereegranskat)
18.	Coldrey, Mikael, 1972, et al. (författare) Training-Based MIMO Systems: Part I - Performance Comparison 2005 Rapport (övrigt vetenskapligt/konstnärligt)abstract A simple and straightforward way of estimating the unknown communication channel is to transmit known training/pilot sequences. Conventionally, these non-information carrying pilots are time-multiplexed with the data and, therefore, consume valuable bandwidth. Hence in the recent past, a number of publications have instead suggested superimposing pilots to the data to perform channel estimation in, e.g., MIMO systems. However, the performance gain achieved by using superimposed pilots (SIP) compared to the more conventional time-multiplexedpilot scheme is not well studied and motivates further research. In this paper we attempt to make a fair comparison between the two schemes, their respective maximum information rate is computed and compared using lower bounds. The theory is applied to a block-wise flat-fading MIMO channel and it is found that in certain scenarios (such as many receive antennas and/or short channel coherence times), it is beneficial to use SIP. Finally, an abstract projection-based viewpoint of the training-based models using vector spaces is given.
19.	Coldrey, Mikael, 1972, et al. (författare) Training-Based MIMO Systems: Part II - Improvements using a Variable Coding Rate Scheme 2005 Rapport (övrigt vetenskapligt/konstnärligt)abstract In this paper the channel throughput performance is addressed for MIMO systems that employ transmission of known training sequences for channel estimation. In the companion paper, it was found that systems based on superimposed pilots (SIP) suffer from poor channel estimates, since there is interfering data power present during channel estimation. By allowing channel re-estimation, this data can be utilized as additional training power, which significantly improves the channel estimates. As the channel estimates improve, a higher coding rate can successfully be applied. These variable coding rate schemes can also be applied to conventional time-multiplexed pilot schemes. However, our numerical examples show a larger performance increase in channel throughput if applied to a SIP scheme.
20.	Ding, Liqin, 1988, et al. (författare) Shannon Capacity of LOS MIMO Channels with Uniform Circular Arrays 2022 Ingår i: IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC. ; 2022-September Konferensbidrag (refereegranskat)abstract The Shannon capacity for the line-of-sight (LOS) multiple-input multiple-output (MIMO) channel between two perfectly aligned uniform circular arrays (UCAs) is derived from the first principles in a tutorial fashion. It is well known that harmonically related complex exponentials (also known in the literature as orbital angular momentum (OAM) modes) are eigenmodes for the spatially continuous channel. We show that the corresponding eigenvalues can be expressed as Bessel functions of the first kind. We also show that the spatially discrete channel between two UCAs with the same finite number of Hertzian dipole antennas on both sides has eigenmodes that are spatially sampled continuous OAM modes, and discrete eigenvalues that are aliased versions of the continuous eigenvalues. Through numerical solution of Maxwell's equations, we verify that the discrete eigenvalues for UCAs with realistic dipole antennas are the same as with the Hertzian dipoles for the studied geometries (1~km hop distance, UCA radius 1 and 2 m, carrier frequency 70 GHz) as long as antenna spacing is not very dense.
21.	Ettefagh, Yasaman, 1989, et al. (författare) Performance of Quantized Massive MIMO with Fronthaul Rate Constraint over Quasi-Static Channels 2023 Ingår i: IEEE Access. - 2169-3536 .- 2169-3536. ; 11, s. 56935-56950 Tidskriftsartikel (refereegranskat)abstract We provide a rigorous framework for characterizing and numerically evaluating the error probability achievable in the uplink and downlink of a fully digital quantized multiuser multiple-input multiple-output (MIMO) system. We assume that the system operates over a quasi-static channel that does not change across the finite-length transmitted codewords, and only imperfect channel state information (CSI) is available at the base station (BS) and at the user equipments. The need for the novel framework developed in this paper stems from the fact that, for the quasi-static scenario, commonly used signal-to-interference-and-distortion-ratio expressions that depend on the variance of the channel estimation error are not relatable to any rigorous information-theoretic achievable-rate bound. We use our framework to investigate how the performance of a fully digital massive MIMO system subject to a fronthaul rate constraint, which imposes a limit on the number of samples per second produced by the analog-to-digital and digital-to-analog converters (ADCs and DACs), depends on the number of BS antennas and on the precision of the ADCs and DACs. In particular, we characterize, for a given fronthaul constraint, the trade-off between the number of antennas and the resolution of the data converters, and discuss how this trade-off is influenced by the accuracy of the available CSI. Our framework captures explicitly the cost, in terms of spectral efficiency, of pilot transmission—an overhead that the outage capacity, the classic asymptotic metric used in this scenario, cannot capture. We present extensive numerical results that validate the accuracy of the proposed framework and allow us to characterize, for a given fronthaul constraint, the optimal number of antennas and the optimal resolution of the converters as a function of the transmitted power and of the available CSI.
22.	Ingason, Tryggvi, 1983, et al. (författare) Impact of frequency selective channels on a line-of-sight MIMO microwave radio link 2010 Ingår i: IEEE Vehicular Technology Conference. - 1550-2252. - 9781424425198 Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract Two line-of-sight (LoS) multiple-input multipleoutput (MIMO) systems for point-to-point microwave radio links are simulated and analyzed. Both 2×2 spatially-separated, singlepolarized and 4×4 spatially-separated, dual-polarized systems are considered in the presence of a frequency selective and time varying channel. Time dependence is introduced by moving the antennas during the simulation and thereby introducing the need for an adaptive space-time-equalizer (STE). Our results indicate that a time-varying frequency selective LoS MIMO channel puts higher requirements on the STE and introduces a higher SNR penalty compared to a single-input single-output (SISO) system. © 2010 IEEE.
23.	Jacobsson, Sven, 1990, et al. (författare) Massive MU-MIMO-OFDM Downlink with One-Bit DACs and Linear Precoding 2018 Ingår i: Proceedings - IEEE Global Communications Conference, GLOBECOM. - 2334-0983 .- 2576-6813. ; , s. 1-6 Konferensbidrag (refereegranskat)abstract Massive multiuser (MU) multiple-input multiple- output (MIMO) is foreseen to be a key technology in future wireless communication systems. In this paper, we analyze the downlink performance of an orthogonal frequency division multiplexing (OFDM)-based massive MU-MIMO system in which the base station (BS) is equipped with 1-bit digital-to-analog converters (DACs). Using Bussgang’s theorem, we characterize the performance achievable with linear precoders (such as maximal-ratio transmission and zero forcing) in terms of bit error rate (BER). Our analysis accounts for the possibility of oversampling the time-domain transmit signal before the DACs. We further develop a lower bound on the information-theoretic sum-rate throughput achievable with Gaussian inputs.Our results suggest that the performance achievable with 1-bit DACs in a massive MU-MIMO-OFDM downlink are satisfactory provided that the number of BS antennas is sufficiently large.
24.	Jacobsson, Sven, 1990, et al. (författare) Massive MU-MIMO-OFDM uplink with direct RF-sampling and 1-Bit ADCs 2019 Ingår i: 2019 IEEE Globecom Workshops, GC Wkshps 2019 - Proceedings. Konferensbidrag (refereegranskat)abstract Advances in analog-to-digital converter (ADC) technology have opened up the possibility to directly digitize wideband radio frequency (RF) signals, avoiding the need for analog down- conversion. In this work, we consider an orthogonal frequency- division multiplexing (OFDM)-based massive multi-user (MU) multiple-input multiple-output (MIMO) uplink system that relies on direct RF-sampling at the base station and digitizes the received RF signals with 1-bit ADCs. Using Bussgang's theorem, we provide an analytical expression for the error-vector magnitude (EVM) achieved by digital down-conversion and zero-forcing combining. Our results demonstrate that direct RF-sampling 1-bit ADCs enables low EVM and supports high-order constellations in the massive MU-MIMO- OFDM uplink.
25.	Jacobsson, Sven, 1990, et al. (författare) Massive multiuser MIMO downlink with low- resolution converters 2018 Ingår i: International Zurich Seminar on Information and Communication (IZS 2018) Proceedings. Konferensbidrag (refereegranskat)abstract In this review paper, we analyze the downlink of a massive multiuser multiple-input multiple-output system in which the base station is equipped with low-resolution digital-to-analog converters (DACs). Using Bussgang’s theorem, we characterize the sum-rate achievable with a Gaussian codebook and scaled nearestneighbor decoding at the user equipments (UE). For the case of 1-bit DACs, we show how to evaluate the sum-rate using Van Vleck’s arcsine law. For the case of multi-bit DACs, for which the sum-rate cannot be expressed in closed-form, we present two approximations. The first one, which is obtained by ignoring the overload (or clipping) distortion caused by the DACs, turns out to be accurate provided that one can adapt the dynamic range of the quantizer to the received-signal strength so as to avoid clipping. The second approximation, which is obtained by modeling the distortion noise as a white process, both in time and space, is accurate whenever the resolution of the DACs is sufficiently high and when the oversampling ratio is small. We conclude the paper by discussing extensions to orthogonal frequency-division multiplexing systems; we also touch upon the problem of out-of-band emissions in lowprecision-DAC architectures.
26.	Jacobsson, Sven, 1990, et al. (författare) Nonlinear 1-bit precoding for massive MU-MIMO with higher-order modulation 2017 Ingår i: Conference Record - Asilomar Conference on Signals, Systems and Computers. - 1058-6393. ; , s. 763-767 Konferensbidrag (refereegranskat)abstract Massive multi-user (MU) multiple-input multiple-output (MIMO) is widely believed to be a core technology for the upcoming fifth-generation (5G) wireless communication standards. The use of low-precision digital-to-analog converters (DACs) in MU-MIMO base stations is of interest because it reduces the power consumption, system costs, and raw baseband data rates. In this paper, we develop novel algorithms for downlink precoding in massive MU-MIMO systems with 1-bit DACs that support higher-order modulation schemes such as 8-PSK or 16-QAM. Specifically, we present low-complexity nonlinear precoding algorithms that achieve low error rates when combined with blind or training-based channel-estimation algorithms at the user equipment. These results are in stark contrast to linear-quantized precoding algorithms, which suffer from a high error floor if used with high-order modulation schemes and 1-bit DACs
27.	Jacobsson, Sven, 1990, et al. (författare) On Out-of-Band Emissions of Quantized Precoding in Massive MU-MIMO-OFDM 2017 Ingår i: Conference Record of the Asilomar Conference on Signals Systems and Computers. - 1058-6393. - 9781538618233 ; , s. 21-26 Konferensbidrag (refereegranskat)abstract We analyze out-of-band (OOB) emissions in the massive multi-user (MU) multiple-input multiple-output (MIMO) downlink. We focus on systems in which the base station (BS) is equipped with low-resolution digital-to-analog converters (DACs) and orthogonal frequency-division multiplexing (OFDM) is used to communicate to the user equipments (UEs) over frequency- selective channels. We demonstrate that analog filtering in combination with simple frequency-domain digital predistortion (DPD) at the BS enables a significant reduction of OOB emissions, but degrades the signal-to-interference-noise-and-distortion ratio (SINDR) at the UEs and increases the peak-to-average power ratio (PAR) at the BS. We use Bussgang’s theorem to characterize the tradeoffs between OOB emissions, SINDR, and PAR, and to study the impact of analog filters and DPD on the error-rate perfor- mance of the massive MU-MIMO-OFDM downlink. Our results show that by carefully tuning the parameters of the analog filters, one can achieve a significant reduction in OOB emissions with only a moderate degradation of error-rate performance and PAR.
28.	Jacobsson, Sven, 1990, et al. (författare) One-Bit Massive MIMO: Channel Estimation and High-Order Modulations 2015 Ingår i: 2015 IEEE International Conference on Communication Workshop, ICCW 2015. - 9781467363051 ; , s. 1304-1309 Konferensbidrag (refereegranskat)abstract We investigate the information-theoretic throughout achievable on a fading communication link when the receiver is equipped with one-bit analog-to-digital converters (ADCs). The analysis is conducted for the setting where neither the transmitter nor the receiver have a priori information on the realization of the fading channels. This means that channel-state information needs to be acquired at the receiver on the basis of the one-bit quantized channel outputs. We show that least-squares (LS) channel estimation combined with joint pilot and data processing is capacity achieving in the single-user, single-receive-antenna case. We also investigate the achievable uplink throughput in a massive multiple-input multiple-output system where each element of the antenna array at the receiver base-station feeds a one-bit ADC. We show that LS channel estimation and maximum-ratio combining are sufficient to support both multiuser operation and the use of high-order constellations. This holds in spite of the severe nonlinearity introduced by the one-bit ADCs.
29.	Jacobsson, Sven, 1990, et al. (författare) Throughput Analysis of Massive MIMO Uplink With Low-Resolution ADCs 2017 Ingår i: IEEE Transactions on Wireless Communications. - 1558-2248 .- 1536-1276. ; 16:6, s. 4038-4051 Tidskriftsartikel (refereegranskat)abstract We investigate the uplink throughput achievable by a multiple-user (MU) massive multiple-input multiple-output (MIMO) system, in which the base station is equipped with a large number of low-resolution analog-to-digital converters (ADCs). Our focus is on the case where neither the transmitter nor the receiver have any a priori channel state information. This implies that the fading realizations have to be learned through pilot transmission followed by channel estimation at the receiver, based on coarsely quantized observations. We propose a novel channel estimator, based on Bussgang's decomposition, and a novel approximation to the rate achievable with finite-resolution ADCs, both for the case of finite-cardinality constellations and of Gaussian inputs, that is accurate for a broad range of system parameters. Through numerical results, we illustrate that, for the 1-bit quantized case, pilot-based channel estimation together with maximal-ratio combing, or zero-forcing detection enables reliable multi-user communication with high-order constellations, in spite of the severe nonlinearity introduced by the ADCs. Furthermore, we show that the rate achievable in the infinite-resolution (no quantization) case can be approached using ADCs with only a few bits of resolution. We finally investigate the robustness of low-ADC-resolution MU-MIMO uplink against receive power imbalances between the different users, caused for example by imperfect power control.
30.	Kolomvakis, Nikolaos, 1985, et al. (författare) Downlink Performance of Regularized ZF in Massive MIMO Systems Subject to IQ Imbalance 2017 Ingår i: IEEE International Conference on Communications. - 1550-3607. Konferensbidrag (refereegranskat)abstract This paper studies the impact of in-phase and quadrature imbalance (IQI) on the downlink of a single-cell multiuser multiple-input multiple-output (MU-MIMO) system with large antenna arrays. Moreover, we consider a time-division duplex (TDD) system where we assume uplink/downlink channel reciprocity in the downlink precoding design and study the impact of uplink/downlink channel mismatch on the downlink rate caused by different IQI on the base station (BS) and users equipment (UEs). A tractable analytical expression to predict the achievable downlink rates is derived for the regularized zero-forcing (RZF) and based on this expression, we prove that, as the number of BS antennas grow large, the achievable downlink rate of each UE is limited either by the RX IQI at the UEs or by the simultaneous presence of the transmit/receive IQI at the BS and TX IQI at the UEs, when there is uplink/downlink channel mismatch.
31.	Kolomvakis, Nikolaos, 1985, et al. (författare) IQ Imbalance in Multiuser Systems: Channel Estimation and Compensation 2016 Ingår i: IEEE Transactions on Communications. - 0090-6778 .- 1558-0857. ; 64:7, s. 3039-3051 Tidskriftsartikel (refereegranskat)abstract In this paper, we consider the uplink of a single-cell multi-user single-input multiple-output (MU-SIMO) system with in-phase and quadrature-phase imbalance (IQI). Particularly, we investigate the effect of receive (RX) IQI on the performance of MU-SIMO systems with large antenna arrays employing maximum-ratio combining (MRC) receivers. In order to study how IQI affects channel estimation, we derive a new channel estimator for the IQI-impaired model and show that the higher the value of signal-to-noise ratio (SNR) the higher the impact of IQI on the spectral efficiency (SE) of MRC receivers. Moreover, a novel pilot-based joint estimator of the augmented MIMO channel matrix and IQI coefficients is described and then, a low-complexity IQI compensation scheme is proposed which is based on the IQI coefficients' estimation and it is independent of the channel gain. The performance of the proposed compensation scheme is analytically evaluated by deriving a tractable approximation of the ergodic SE assuming transmission over Rayleigh fading channels with large-scale fading.Furthermore, we investigate how many MSs should be scheduled in massive MIMO systems with IQI and show that the highest SE loss occurs at the optimal operating point.Finally, by deriving asymptotic power scaling laws, and proving that the SE loss due to IQI is asymptotically independent of the number of BS antennas, we show that massive multiple-input multiple-output (MIMO) is resilient to the effect of RX IQI.
32.	Kolomvakis, Nikolaos, 1985, et al. (författare) Massive MIMO in Sparse Channels 2014 Ingår i: 2014 IEEE 15th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC). - 2325-3789. - 9781479949038 ; :October, s. 21-25 Konferensbidrag (refereegranskat)abstract Massive multi-user multiple-input multiple-output (MU-MIMO) systems are cellular networks where the base stations (BSs) are equipped with hundreds of antennas, N, and communicate with tens of mobile stations (MSs), K, such that, N >> K >> 1. Contrary to most prior works, in this paper, we consider the uplink of a single-cell massive MIMO system operating in sparse channels with limited scattering. This case is of particular importance in most propagation scenarios, where the prevalent Rayleigh fading assumption becomes idealistic. We derive analytical approximations for the achievable rates of maximum-ratio combining (MRC) and zero-forcing (ZF) receivers. Furthermore, we study the asymptotic behavior of the achievable rates for both MRC and ZF receivers, when N and K go to infinity under the condition that N/K -> c >= 1. Our results indicate that the achievable rate of MRC receivers reaches an asymptotic saturation limit, whereas the achievable rate of ZF receivers grows logarithmically with the number of MSs.
33.	Kolomvakis, Nikolaos, 1985, et al. (författare) Massive MIMO Systems with IQ Imbalance: Channel Estimation and Sum Rate Limits 2017 Ingår i: IEEE Transactions on Communications. - 0090-6778 .- 1558-0857. ; 65:6, s. 2382 - 2396 Tidskriftsartikel (refereegranskat)abstract This paper studies the impact of in-phase and quadrature imbalance (IQI) on single-cell multiuser multiple-input multiple-output (MU-MIMO) systems with large antenna arrays. Moreover, we consider a time-division duplex (TDD) system where we assume uplink/downlink channel reciprocity in the downlink precoding design. First, we investigate the effect of transceiver IQI on the uplink channel estimation by deriving the linear minimum-mean-square-error (LMMSE) estimator for the IQ-impaired model and prove that only the receiver IQI at the base station (BS) limits the estimation accuracy. Then, we study the impact of uplink/downlink channel mismatch on the downlink rate caused by different IQI on the BS and user equipments (UEs). We prove that the achievable downlink rate of each UE is limited either by the receiver IQI at the UEs or jointly by the transmit and receive IQI at the BS, when there is mismatch between the uplink and downlink channels.
34.	Kolomvakis, Nikolaos, 1985, et al. (författare) Massive MIMO with IQ Imbalance: Performance analysis and compensation 2015 Ingår i: IEEE International Conference on Communications. - 1550-3607. - 9781467364324 ; , s. 1703-1709 Konferensbidrag (refereegranskat)abstract In this paper, we consider the uplink of a single-cell massive multiple-input multiple-output (MIMO) system with inphase and quadrature-phase imbalance (IQI). This scenario is of particular importance in massive MIMO systems, where the deployment of lower-cost, lower-quality components is desirable to make massive MIMO a viable technology. Particularly, we investigate the effect of IQI on the performance of massive MIMO employing maximum-ratio combining (MRC) receivers. In order to study how IQI affects channel estimation, we derive a new channel estimator for the IQI-impaired model and show that IQI can substantially downgrade the performance of MRC receivers. Moreover, a low-complexity IQI compensation scheme, suitable for massive MIMO, is proposed which is based on the IQI coefficients' estimation and it is independent of the channel gain. The performance of the proposed compensation scheme is analytically evaluated by deriving a tractable approximation of the ergodic achievable rate and providing the asymptotic power scaling laws assuming transmission over Rayleigh fading channels with log-normal large-scale fading. Finally, we show that massive MIMO effectively suppresses the residual IQI effects, as long as, the compensation scheme is applied.
35.	Kolomvakis, Nikolaos, 1985, et al. (författare) Quantized Uplink Massive MIMO Systems with Linear Receivers 2020 Ingår i: IEEE International Conference on Communications. - : Institute of Electrical and Electronics Engineers Inc.. - 9781728150895 ; 2020-June Konferensbidrag (refereegranskat)abstract This paper considers the uplink of a single-cell multi-user massive multiple-input multiple-output (MIMO) system. Each receiver antenna of the base station is assumed to be equipped with a pair of analog-to-digital converters (ADCs) to quantize the real and imaginary part of the received signal. We propose a novel Bussgang-based weighted zero-forcing (B-WZF) receiver, which distinguishes the clipping and granular distortion. Numerical results demonstrate that for sufficiently high SNR and users that do not experience deep large-scale fading, the B-WZF brings significant performance gain over existing linear receivers in the literature, when the training sequence length is higher than the number of users. © 2020 IEEE.
36.	Kolomvakis, Nikolaos, 1985, et al. (författare) Reconstruction of clipped signals 2023 Patent (övrigt vetenskapligt/konstnärligt)abstract Using information contained in clipped samples from aualog-to-digital (ADC) conversion to improve receiver performance, by, for example, reducing the clipping distortion caused by ADCs due to its data resolution constraints. This provides an advantage over existing solutions, which perform suboptimally because the existing solution discard information in tire clipped samples.
37.	Lioliou, Panagiota, 1982, et al. (författare) Efficient Channel Estimation Techniques for Amplify and Forward Relaying Systems 2012 Ingår i: IEEE Transactions on Communications. - 0090-6778 .- 1558-0857. ; 60:11, s. 3150-3155 Tidskriftsartikel (refereegranskat)abstract In this paper, we present a channel estimation scheme for Amplify-and-Forward (AF) relaying systems, using measurements at the destination only. A Least-Squares (LS) based channel estimation algorithm is developed, that provides the destination with full knowledge of all channel responses involved in the transmission. To investigate the algorithm performance, the Cramer-Rao lower bound (CRB) is analytically computed and compared with the asymptotic covariance of the proposed estimator. Since the existing estimator does not reach the CRB, we also propose and analyze an improved algorithm by taking into account the noise characteristics via weighted LS (WLS). The improved algorithm is asymptotically efficient, since it attains the CRB as the SNR tends to infinity.
38.	Lioliou, Panagiota, 1982, et al. (författare) Performance Analysis of Relay Channel Estimation 2009 Ingår i: 43rd Asilomar Conference on Signals, Systems and Computers; Pacific Grove, CA; United States; 1 November 2009 through 4 November 2009. - 1058-6393. - 9781424458271 ; , s. 1533-1537 Konferensbidrag (refereegranskat)abstract Amplify-and-Forward (AF) relays can be used to enhance the channel in Multiple-Input-Multiple-Output (MIMO) wireless communication systems. However, optimizing the channel requires Channel State Information (CSI). This paper is concerned with the performance of relay channel estimation. A Least Squares (LS) based algorithm for the estimation of all relevant channel parameters was recently proposed by the authors. It is based on creating different compound channels by varying the gain factors at the relays. From this, the individual links from source to relay and from relay to destination, are revealed using LS. To investigate the algorithm performance, the Cramér-Rao lower Bound (CRB) is computed and compared with the asymptotic covariance of the proposed estimator. Furthermore, we propose and analyze an improved algorithm that is efficient for high Signal-to-Noise Ratio (SNR). Computer simulations demonstrate the performance of the algorithm. The results indicate that the asymptotic expressions are in good agreement with the simulations.
39.	Lioliou, Panagiota, 1982, et al. (författare) Self-Interference Suppression in Full-Duplex MIMO Relays 2010 Ingår i: 44th Asilomar Conference on Signals, Systems and Computers. - 1058-6393. - 9781424497218 ; , s. 658-662 Konferensbidrag (refereegranskat)abstract Full-duplex relays can provide cost-effective cover-age extension and throughput enhancement. However, the main limiting factor is the resulting self-interference signal which deteriorates the relay performance. In this paper, we propose a novel technique for self-interference suppression in full-duplex Multiple-Input Multiple-Output (MIMO) relays. The relay employs transmit and receive weight filters for suppressing the self-interference signal. Unlike existing techniques that are based on zero forcing of self-interference, we aim at maximizing the ratio between the power of the useful signal to the self-interference power at the relay reception and transmission. Our simulation results show that the proposed algorithm outperforms the existing schemes since it can suppress interference substantially with less impact on the useful signal.
40.	Makki, Behrooz, 1980, et al. (författare) Finite Block-length Analysis of Large-but-finite MIMO Systems 2019 Ingår i: IEEE Wireless Communications Letters. - 2162-2345 .- 2162-2337. ; 8:1, s. 113-116 Tidskriftsartikel (refereegranskat)abstract This letter studies the performance of point-to-point multiple-input-multiple-output (MIMO) systems in the presence of large but finite numbers of transmit and receive antennas. We use some recent results on the achievable rates of finite-length codes to analyze the system performance in the cases with short packets. The results are presented for different fading models. With codewords of finite length, we derive closed-form expressions for the error probability/throughput and determine the minimum number of antennas which are required to satisfy different error probability/throughput requirements. As we show, while the error-constrained required number of antennas is sensitive to the length of short codewords, its effect is negligible for codewords of moderate/long length.
41.	Persson, Patrik, et al. (författare) Design and evaluation of a 2 x 2 MIMO repeater 2009 Ingår i: European Conference on Antennas and Propagation, EuCAP 2009, Proceedings. - 9783000245732 ; , s. 1509-1512 Konferensbidrag (refereegranskat)abstract In this paper the design and measurement-based evaluation of an amplify-and-forward 2 x 2 MEMO repeater is presented. During the design phase, simulations were made to study different scenarios and repeater parameter settings. The evaluation of the repeater was made using field trials in Kista, Stockholm, Sweden. Three scenarios were considered; coverage, outdoor-to-indoor, and LOS MEMO rank enhancement. The overall results showed good improvement in received signal strength, ranging from only a few dB up to 25 dB depending on the particular measurement scenario. Moreover, the concept of repeater-aided rank enhancement in LOS also showed good results.
42.	Seifi, Nima, 1981, et al. (författare) Adaptive Multicell 3-D Beamforming in Multiantenna Cellular Networks 2016 Ingår i: IEEE Transactions on Vehicular Technology. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9545 .- 1939-9359. ; 65:8, s. 6217-6231 Tidskriftsartikel (refereegranskat)abstract We consider a cellular network with multiantenna base stations (BSs) and single-antenna users, multicell cooperation, imperfect channel state information (CSI), and directional antennas, each with a vertically adjustable beam. We investigate the impact of the elevation angle of the BS antenna pattern, which is denoted as tilt, on the performance of the considered network when employing either a traditional single-cell transmission or a fully cooperative multicell transmission. Using the results of this investigation, we propose a novel hybrid multicell cooperation technique in which the intercell interference is controlled via either cooperative beamforming in the horizontal plane or coordinated beamforming in the vertical plane of the wireless channel, which is denoted as adaptive multicell 3-D beamforming. The main idea is to divide the coverage area into two disjoint vertical regions and adapt the multicell cooperation strategy at the BSs when serving each region. A fair scheduler is used to share the time slots between the vertical regions. It is shown that the proposed technique can achieve performance comparable with that of a fully cooperative transmission but with significantly lower complexity and signaling requirements. To facilitate computationally efficient simulation and design space exploration, accurate approximations of the user ergodic rate are proposed for different transmission strategies under imperfect CSI.
43.	Seifi, Nima, 1981, et al. (författare) An Efficient Signaling for Multi-mode Transmission in Multi-user MIMO 2010 Ingår i: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings. - 1520-6149. - 9781424442966 ; , s. 3402-3405 Konferensbidrag (refereegranskat)abstract In this paper the downlink of a multi-user MIMO (MUMIMO)system with multi-mode transmission is considered.We propose a low-complexity algorithm for selecting usersand the corresponding number of data streams to each user,denoted as user transmission mode (UTM). The selectionis only based on the average received signal-to-noise ratio(SNR) from the base station (BS) for each user. This reducesthe overall amount of feedback for scheduling, as opposedto techniques that assume perfect instantaneous channelstate information (CSI) from all users. Analytical averagethroughput approximations are derived for each user at different UTMs. Simulation results demonstrate that the proposed algorithm provides performance close to dirty paper coding (DPC) with considerably reduced feedback.
44.	Seifi, Nima, 1981, et al. (författare) Coordinated 3D Beamforming for Interference Management in Cellular Networks 2014 Ingår i: IEEE Transactions on Wireless Communications. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-2248 .- 1536-1276. ; 13:10, s. 5396-5410 Tidskriftsartikel (refereegranskat)abstract We consider downlink transmission in a cellular network consisting of multi-antenna base stations (BSs), single-antenna mobile users, directional antennas each with a vertically adjustable beam, and control signaling delays in feedback and backhaul links. We propose a novel transmission technique in which intercell interference management is performed via coordinating the beamforming jointly in the horizontal and vertical planes of the wireless channel, denoted as coordinated 3D beamforming. In the horizontal plane, we focus on intercell interference cancelation (ICIC) and investigate its performance when the provided channel state information (CSI) is impaired due to delay/mobility. It is demonstrated that the superiority of ICIC over conventional intracell maximum ratio transmission is highly dependent on the CSI accuracy. In the vertical plane, we consider intercell interference control via coordinatively adapting the elevation angle of the BS antenna pattern, denoted as tilt, to the locations of the scheduled users. It is shown that with perfect CSI interference management should be performed in the horizontal plane using ICIC, while at high delay/mobility it should be done in the vertical plane via coordinated tilt adaptation. For intermediate values of delay/mobility, joint interference management in both planes is required.
45.	Seifi, Nima, 1981, et al. (författare) Coordinated Single-Cell vs Multi-Cell Transmission with Limited-Capacity Backhaul 2010 Ingår i: IEEE Asilomar Conf. on Signals, Systems, and Computers, Pacific Grove, CA, Nov. 2010. - 1058-6393. - 9781424497218 ; , s. 1217-1221 Konferensbidrag (refereegranskat)abstract Base station coordination is an efficient techniqueto transcend the limits on spectral efficiency imposed by intercellinterference. In this paper, we compare the performance ofdifferent coordination strategies with different amount of channelstate information (CSI) and data sharing among the coordinatingbase stations. We focus on the effect of limited backhaul capacityin a two-cell network. Contrary to the common belief, we showthat coordination strategies with no data and only limited CSIsharing is preferred to those with full data and CSI sharing whenthe backhaul capacity is relatively low and the edge SNR is high.
46.	Seifi, Nima, 1981, et al. (författare) Elevation plane spatial multicell interference mitigation with No CSI sharing 2015 Ingår i: IEEE Globecom Workshops, GC Wkshps 2015, San Diego, United States, 6-10 December 2015. - 9781467395267 Konferensbidrag (refereegranskat)abstract We study downlink transmission in a cellular network with multi-antenna base stations (BSs) and single-antenna users. We propose a novel coordinated transmission strategy that controls the intercell interference (ICI) via exploiting the elevation plane of the wireless channel. The key idea is to divide the area of each cell into two disjoint vertical regions and to serve the non-adjacent vertical regions in the neighboring cells according to a set of pre-determined coordinated transmission patterns. A scheduler is used to dynamically allocate the available time-slots among transmission patterns. By appropriately adapting the elevation angle of the antenna main beam, denoted as tilt, at the BSs in each pattern, ICI can be effectively suppressed. Unlike the state-of-the-art coordinated transmission schemes, the proposed technique requires no channel state information sharing among BSs and/or infeasible BS tilting capabilities, but achieves comparable performance.
47.	Seifi, Nima, 1981, et al. (författare) Impact of base station antenna tilt on the performance of network MIMO systems 2012 Ingår i: IEEE Vehicular Technology Conference. - 1550-2252. - 9781467309905 ; , s. Art. no. 6239994- Konferensbidrag (refereegranskat)abstract We study the downlink of a multicell MIMO system where clusters of multi-antenna base stations jointly serve multiple single-antenna users, commonly referred to as a network MIMO system. Most of the previous studies on network MIMO have only considered the azimuth pattern of the antenna, while ignoring the elevation pattern. In this paper, we consider both the azimuth and the elevation patterns and investigate the impact of the elevation angle tuning parameter, denoted as the antenna tilt, on the performance of such systems. Using system simulations, it is shown that the promised performance gains of network MIMO systems over conventional non-coordinated systems, crucially depend on the choice of the right tilt setting including the tilt type, i.e., mechanical or electrical, and the tilt angle. In particular, for tilt angles smaller than the optimum, network MIMO with intra-site coordination performs almost as well as the conventional system; while for tilt angles larger than the optimum, the performance of network MIMO with intra-site is similar to that of network MIMO with inter-site coordination.
48.	Seifi, Nima, 1981, et al. (författare) Joint transmission mode and tilt adaptation in coordinated small-cell networks 2014 Ingår i: 2014 IEEE International Conference on Communications Workshops, ICC 2014. - 9781479946402 ; , s. 598-603 Konferensbidrag (refereegranskat)abstract We study downlink transmission in a wireless network consisting of small-sized cells, denoted as a small-cell network (SCN). We consider multi-antenna base stations (BSs) and single-antenna users, BS coordination, 3D antenna patterns, and fair scheduling. We investigate the impact of the elevation angle of the BS antenna pattern, denoted as tilt, on the performance of the considered SCN when employing either a conventional uncoordinated transmission mode or a fully coordinated transmission mode. Using the results of this investigation, we propose a novel hybrid-mode transmission technique that can achieve a performance comparable to that of a fully coordinated transmission but with a significantly lower complexity and signaling requirement. The main idea is to divide the coverage area into two so-called vertical regions and jointly adapt the transmission mode and the tilt at the BSs when serving each region. A fair scheduler is used to share the time-slots between the vertical regions and among the users in each region.
49.	Seifi, Nima, 1981, et al. (författare) Multimode Transmission in Network MIMO Downlink with Incomplete CSI 2011 Ingår i: Eurasip Journal on Advances in Signal Processing. - : Springer Science and Business Media LLC. - 1687-6172 .- 1687-6180. ; 2011, s. Art. no. 743916- Tidskriftsartikel (refereegranskat)abstract We consider a cooperative multi-cell MIMO (a.k.a network MIMO) downlink system with multiantennabase stations (BSs), which are connected to a central unit and communicate with multi-antennausers. In such a network, obtaining perfect channel state information (CSI) of all users at the central unitto exploit opportunistic scheduling requires a substantial amount of feedback and backhaul signaling.In this paper, we propose a scheduling algorithm based only on the knowledge of the average receivedSNR at each user from all the cooperating BSs, denoted as incomplete CSI. Multi-mode transmission isapplied that is able to adaptively adjust the number of data streams transmitted to each user. Utilizingthe results of random matrix theory, an analytical framework is proposed to approximate the ergodicrate of each user with different number of data streams. Using these approximate ergodic rates, a jointuser and mode selection algorithm is proposed, where only the scheduled users need to feed backinstantaneous CSI. Simulation results demonstrate that the developed analytical framework providesa good approximation for a practical number of antennas. While substantially reducing the feedbackoverhead, it is shown that the proposed scheduling algorithm performs closely to the opportunisticscheduling algorithm that requires instantaneous CSI feedback from all users.
50.	Seifi, Nima, 1981, et al. (författare) Throughput Optimization for MISO Interference Channels via Coordinated User-Specific Tilting 2012 Ingår i: IEEE Communications Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-2558 .- 1089-7798. ; 16:8, s. 1248-1251 Tidskriftsartikel (refereegranskat)abstract In this letter, we propose a novel framework to enhance the throughput in multiple-input single-output (MISO) mutually interfering channels via selecting the tilt angles at all base stations (BSs) in a coordinated fashion. In the proposed framework, multiple BSs adjust their tilt angles jointly to maximize the sum throughput of the scheduled users, denoted as coordinated user-specific tilting. Assuming the availability of location information of the scheduled users at all BSs, accurate analytical expression for user ergodic rate is provided, which enables a decentralized deployment of the proposed framework at each BS. Simulation results show that the proposed coordinated user-specific tilting scheme outperforms the conventional schemes employing a fixed tilt angle at each BS.

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