This paper presents an overview of progress in the area of multiple input multiple output (MIMO) space-time coded wireless systems. After some background on the research leading to the discovery of the enormous potential of MIMO wireless links, we highlight the different classes of techniques and algorithms proposed which attempt to realize the various benefits of MIMO including spatial multiplexing and space-time coding schemes. These algorithms are often derived and analyzed under ideal independent fading conditions. We present the state of the art in channel modeling and measurements, leading to a better understanding of actual MIMO gains. Finally, the paper addresses current questions regarding the integration of MIMO links in practical wireless systems and standards.

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From theory to practice: an overview of MIMO space-time coded wireless systems

The escalating teletraffic growth imposed by the proliferation of smartphones and tablet computers outstrips the capacity increase of wireless communications networks. Furthermore, it results in substantially increased carbon dioxide emissions. As a powerful countermeasure, in the case of full-rank channel matrices, MIMO techniques are potentially capable of linearly increasing the capacity or decreasing the transmit power upon commensurately increasing the number of antennas. Hence, the recent concept of large-scale MIMO (LS-MIMO) systems has attracted substantial research attention and been regarded as a promising technique for next-generation wireless communications networks. Therefore, this paper surveys the state of the art of LS-MIMO systems. First, we discuss the measurement and modeling of LS-MIMO channels. Then, some typical application scenarios are classified and analyzed. Key techniques of both the physical and network layers are also detailed. Finally, we conclude with a range of challenges and future research topics.

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Survey of Large-Scale MIMO Systems

In this paper, we first verify a previously proposed Kronecker-structure-based narrow-band model for nonline-of-sight (NLoS) indoor multiple-input-multiple-output (MIMO) radio channels based on 5.2-GHz indoor MIMO channel measurements. It is observed that, for the narrow-band case, the measured channel coefficients are complex Gaussian distributed and, consequently, we focus on a statistical description using the first- and second-order moments of MIMO radio channels. It is shown that the MIMO channel covariance matrix can be well approximated by the Kronecker product of the covariance matrices, seen from the transmitter and receiver, respectively. A narrow-band model for NLoS indoor MIMO channels is thus verified by these results. As for the wide-band case, it is observed that the average power-delay profile of each element of the channel impulse response matrix fits the exponential decay curve and that the Kronecker structure of the second-order moments can be extended to each channel tap. A wide-band MIMO channel model is then proposed, combining a simple COST 259 single-input-single-output channel model and the Kronecker structure. Monte Carlo simulations are used to generate indoor MIMO channel realizations according to the models discussed. The results are compared with the measured data using the channel capacity and good agreement is found.

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Modeling of wide-band MIMO radio channels based on NLoS indoor measurements

In this article an overview of the scheduling algorithms proposed for fourth-generation multiuser wireless networks based on multiple-input multiple-output technology is presented. In MIMO systems a multi-user diversity gain can be extracted by tracking the channel fluctuations between each user and the base station, and scheduling transmission for the "best" user. Based on this idea, several opportunistic scheduling schemes that attempt to improve global capacity or satisfy users with different QoS requirements have been proposed. Transmit beamforming procedures aimed at increasing the channel fluctuations have been proposed. The simultaneous exploitation of both spatial and multi-user diversity is not straightforward; however, it may be achieved by a refined selection of the "best" user. In addition, a multiple access gain can be obtained from a simple SDMA/TOMA system. Finally, several resource allocation schemes are discussed for this hybrid multiple access system.

An overview of scheduling algorithms in MIMO-based fourth-generation wireless systems

This article gives an overview of the high speed downlink packet access (HSDPA) concept; a new feature which is coming to the Release 5 specifications of the 3GPP WCDMA/UTRA-FDD standard. To support an evolution towards more sophisticated network and multimedia services, the main target of HSDPA is to increase user peak data rates, quality of service, and to generally improve spectral efficiency for downlink asymmetrical and bursty packet data services. This is accomplished by introducing a fast and complex channel control mechanism based on a short and fixed packet transmission time interval (TTI), adaptive modulation and coding (AMC), and fast physical layer (L1) hybrid ARQ. To facilitate fast scheduling with a per-TTI resolution in coherence with the instantaneous air interface load, the HSDPA-related MAC functionality is moved to the Node-B. The HSDPA concept facilitates peak data rates exceeding 2 Mbps (theoretically up to and exceeding 10 Mbps), and the cell throughput gain over previous UTRA-FDD releases has been evaluated to be in the order of 50-100% or even more, highly dependent on factors such as the radio environment and the service provision strategy of the network operator.

High Speed Downlink Packet Access: WCDMA Evolution

In evolving 3G systems, the proportional fair packet scheduling algorithm promises an attractive trade-off between average cell capacity and user fairness by exploiting multi-user selection diversity in time-shared channels. However, in these systems the option of antenna transmit diversity already exist, whose performance benefit is dependent on the considered scheduling strategy. This study focuses on the interaction between proportional fair packet scheduling and open or closed-loop transmit diversity techniques. The basic interaction is studied by using an analytical model. Additionally, Monte Carlo simulations are conducted to study the effect of scheduling delays. In the low mobility Rayleigh fading case, the potential average cell capacity gain with proportional fair scheduling, compared to a simple round robin in time approach, is on the order of 60% for single-antenna transmission. It reduces, however, to around 35% when dual antenna transmit diversity is additionally deployed. Comparing the performance of the different transmit diversity schemes to single-antenna transmission, conditioned on proportional fair scheduling, it is found that the closed-loop schemes provide a benefit over a wide range of terminal speeds, whereas open-loop space-time block coding exhibits limited performance and in some cases even a loss.

Interaction of transmit diversity and proportional fair scheduling

This document gives an overview of recent achievements of various research teams, both academic and industrial, in the characterisation and the exploitation of the essential properties of multiple antenna systems. It is organised into two main parts. The first part looks at recent modelling activities of Multiple-Input Multiple-Output (MIMO) radio channels, while the second tackles advanced antenna processing suggested in the Multiple-Input Single-Output (MISO) and MIMO perspectives. In the second part, a strong emphasis is given to the standardisation of the evolution of the Third Generation (3G) cellular systems within the Third Generation Partnership Project (3GPP).

https://www.ursi.org/Proceedings/ProcGA02/papers/p0563.pdf

Recents Advances in Propagation Characterisation and Multiple Antenna Processing in the 3GPP Framework

There is a growing interest in the standardisation ofMultiple Input Multiple Output (MIMO) schemes forHigh Speed Downlink Packet Access (HSDPA) forfuture releases of UMTS. The I-METRA project,building on the legacy of METRA, has set up as one ofits priorities to contribute to this standardisation effortby developing and evaluating pertinent transmission andreception schemes for this type of systems. I. INTRODUCTION The concept of HSDPA has been recently standardisedin 3GPP for UMTS [1]. It considers enhancements thatcan be applied to UTRA to provide very high-speeddownlink packet access by means of a high-speeddownlink shared channel (HS-DSCH). Among theseenhancements, the I-METRA project is focused onMIMO antenna processing techniques which arenecessarily also related to the evolution of otherprocedures, such as Adaptive Modulation and Coding(AMC) or Hybrid Automatic Repeat on Request (H-ARQ), in order to include adaptive MIMO techniques inthe HS-DSCH structure.Within the HSDPA concept, it is expected thatmodulation and coding are adapted to channel, trafficand user requirements. In this procedure, the userequipment (UE) should be able to estimate the channelstate information and translate it into a metric, which istransmitted to its serving Node B within the DPCCH-HS channel (an uplink Dedicated Physical Controlchannel associated with HS-DSCH). It is required thatthis metric value can be mapped to the downlinkchannel FER. The Node B is continuously selecting theuser to be served and its data rate based on a schedulingalgorithm and on that feedback information receivedfrom all active users. The uplink DPCCH-HS alsocarries H-ARQ acknowledgements, which are used bythe Node B to update the user traffic queues. The use ofAMC and H-ARQ procedures along with Fast CellSelection and Stand-Alone DSCH [2] will provide up to10.7Mbps of data rate. It is envisioned that the use ofMIMO techniques on top of these other procedures willallow doubling this upper bound [3].In the first stage of I-METRA

Adaptive Modulation schemes for MIMO HSDPA

A downlink space-time block coding transmission scheme is assessed via Monte Carlo simulations in a direct sequence code division multiple access context. Raw symbol error rate performance of the space-time transmit diversity scheme defined in the UTRA/FDD standard serves as a benchmark. Exploiting space-time code properties, the new scheme facilitates radio channel equalization and space-time decoding prior to despreading. Assuming ideal equalization, the proposed diversity scheme is therefore own cell interference free and outperforms conventional, schemes in frequency selective fading environments. With increasing channel estimation error, however, the proposed scheme tends to suffer from multiple access interference.

Modified space-time transmission in DS-CDMA downlink facilitating MISO channel equalization

The paper studies different degrees of channel quality-based scheduling freedom for the downlink of a DS-CDMA system where the base station (BS) and each mobile station (MS) have 2 antennas. The system combines opportunistic user time slot scheduling, opportunistic spatial multiplexing, and adaptive modulation and coding (AMC) with limited and unlimited allowable constellation sizes. System performance is investigated in a time dispersive propagation environment, with varying degrees of antenna correlation. Three transmission schemes with increasing degrees of resource allocation freedom are identified. The full freedom scheme allows for time slots and transmit antennas to be independently allocated to different users and is compared to two schemes of more limited flexibility in antenna allocation. For a system with little multi user diversity gain, i.e., fewer than 5 users, the option of single antenna selection transmit diversity is favorable, while dual antenna transmission with independent antenna-to-user allocation becomes increasingly important when a higher degree of multi user diversity can he exploited.

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Lars Torsten Berger

Papers

Interaction of transmit diversity and proportional fair scheduling

Recents Advances in Propagation Characterisation and Multiple Antenna Processing in the 3GPP Framework

Adaptive Modulation schemes for MIMO HSDPA

Modified space-time transmission in DS-CDMA downlink facilitating MISO channel equalization

Effects of multi user MIMO scheduling freedom on cellular downlink system throughput