There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Wireless Communications

Mobile users' data rate and quality of service are limited by the fact that, within the duration of any given call, they experience severe variations in signal attenuation, thereby necessitating the use of some type of diversity. In this two-part paper, we propose a new form of spatial diversity, in which diversity gains are achieved via the cooperation of mobile users. Part I describes the user cooperation strategy, while Part II (see ibid., p.1939-48) focuses on implementation issues and performance analysis. Results show that, even though the interuser channel is noisy, cooperation leads not only to an increase in capacity for both users but also to a more robust system, where users' achievable rates are less susceptible to channel variations.

http://dns2.asia.edu.tw/~ysho/YSHO-English/2000%20Engineering/PDF/IEE%20Tra%20Com51,%201927.pdf

User cooperation diversity. Part I. System description

Multi-user MIMO offers big advantages over conventional point-to-point MIMO: it works with cheap single-antenna terminals, a rich scattering environment is not required, and resource allocation is simplified because every active terminal utilizes all of the time-frequency bins. However, multi-user MIMO, as originally envisioned, with roughly equal numbers of service antennas and terminals and frequency-division duplex operation, is not a scalable technology. Massive MIMO (also known as large-scale antenna systems, very large MIMO, hyper MIMO, full-dimension MIMO, and ARGOS) makes a clean break with current practice through the use of a large excess of service antennas over active terminals and time-division duplex operation. Extra antennas help by focusing energy into ever smaller regions of space to bring huge improvements in throughput and radiated energy efficiency. Other benefits of massive MIMO include extensive use of inexpensive low-power components, reduced latency, simplification of the MAC layer, and robustness against intentional jamming. The anticipated throughput depends on the propagation environment providing asymptotically orthogonal channels to the terminals, but so far experiments have not disclosed any limitations in this regard. While massive MIMO renders many traditional research problems irrelevant, it uncovers entirely new problems that urgently need attention: the challenge of making many low-cost low-precision components that work effectively together, acquisition and synchronization for newly joined terminals, the exploitation of extra degrees of freedom provided by the excess of service antennas, reducing internal power consumption to achieve total energy efficiency reductions, and finding new deployment scenarios. This article presents an overview of the massive MIMO concept and contemporary research on the topic.

/pdf/massive-mimo-for-next-generation-wireless-systems-1eydagg3js.pdf

Massive MIMO for next generation wireless systems

Multiuser diversity is a form of diversity inherent in a wireless network, provided by independent time-varying channels across the different users. The diversity benefit is exploited by tracking the channel fluctuations of the users and scheduling transmissions to users when their instantaneous channel quality is near the peak. The diversity gain increases with the dynamic range of the fluctuations and is thus limited in environments with little scattering and/or slow fading. In such environments, we propose the use of multiple transmit antennas to induce large and fast channel fluctuations so that multiuser diversity can still be exploited. The scheme can be interpreted as opportunistic beamforming and we show that true beamforming gains can be achieved when there are sufficient users, even though very limited channel feedback is needed. Furthermore, in a cellular system, the scheme plays an additional role of opportunistic nulling of the interference created on users of adjacent cells. We discuss the design implications of implementing. this scheme in a complete wireless system.

https://web.stanford.edu/~dntse/papers/oppbf_it.pdf

Opportunistic beamforming using dumb antennas

In this paper we look at the effect of discrete constellation alphabets on linear precoding for the downlink of multiuser (MU) MIMO in the context of LTE. We underline the fundamental difference in the approach of precoding if the alphabets are assumed to be discrete constellations rather than the idealized Gaussian assumption. We show that the problem of finding global optimal linear precoder taking into account discrete inputs is non-convex and we propose a method of finding a near optimal linear precoder. Underlining the viability of MU MIMO for future wireless communications as LTE, we further propose in this paper a precoding strategy based on low resolution LTE precoders which necessitate 2 bits feedback from the users. The proposed strategy encompasses geometrical interference alignment at eNodeB and the use of low complexity MU detectors at the users. On one hand, this strategy relegates the interference seen by each user by a geometric scheduling algorithm while on the other hand, users exploit the structure of this interference in the detection process. Simulation results validate improved performance of the proposed strategy over single user schemes.

/pdf/near-optimal-linear-precoder-for-multiuser-mimo-for-discrete-3n9lh8kx94.pdf

Near Optimal Linear Precoder for Multiuser MIMO for Discrete Alphabets

The integration of all the heterogeneous wireless networks deployed along European railway lines constitutes a key technical challenge to improve global efficiency of railway system. Emerging Cognitive Radio (CR) technologies can answer this challenge. The paper presents the first results obtained in the ANR project CORRIDOR (Cognitive Radio for High Speed Railway through Dynamic and Opportunistic spectrum Reuse) which is the first research project devoted to CR systems for Railway in France, taking into account very demanding railway applications: GSM-R which is the radio link of the ERTMS system, and other operational needs such as video surveillance of the inside of trains, data related to maintenance and Internet on board for passengers.

Cognitive Radio for High Speed Railway through Dynamic and Opportunistic spectrum Reuse

The focus of this study is the concept of multiuser diversity in the context of a single-cell multiuser channel. The primary result of this work is that using proper dynamic resource allocation strategies on Rayleigh fading channels can yield a non-negligible power boost which can be as high as 5 or 6 dB, for a moderate number of users employing a joint resource allocation policy. This gain grows logarithmically with the number of users. The main practical issue arising from these results is fairness. Users must wait (or the base station in the case of the broadcast channel) until their channel conditions are favourable enough to transmit. We show in this work that multiuser diversity can still be achieved in a multi-carrier system using simple orthogonal multiplexing even under the constraint that each user receives the same constant bandwidth resource at any time instant.

Achieving multiuser diversity under hard fairness constraints

Future wireless communication systems characterized by tight frequency reuse, adaptive modulation and coding schemes and diversified data services will be interference limited by interfering signals of diverse rates and strengths. We propose in this paper a low complexity algorithm for spatial interference cancellation in the presence of one strong interferer. This algorithm is based on an earlier proposed low complexity max log MAP detector. It encompasses two strategies for interference cancellation which have been termed as partial interference cancellation (PIC) and absolute interference cancellation (AIC). Their corresponding selection in the receiver is dictated by the relative strength and the rate of interfering stream. In the scenario of interfering stream being relatively weak or of higher rate as compared to the desired stream, the mobile station (MS) resorts to PIC while when the interfering stream is relatively stronger or is of lower rate, the MS switches to AIC. Finally we analyze the performance of proposed algorithm by simulations.

/pdf/spatial-interference-cancellation-algorithm-ykghsadzau.pdf

Spatial Interference Cancellation Algorithm

We consider the downlink of a multiuser multi-cell system. Each cell is equipped with multiple antennas transmitting over M parallel channels. We study the benefits of Distributed Antenna arrays on the performance of such systems and the gain on fairness between users offered by this technique,compared to co-located antenna systems. We show that the Macro-Diversity introduced by distributed antennas combined with the Max-Min allocation algorithm, not only considerably increases the system Spectral Efficiency but also offers a remarkable enhancement on the Minimum Allocated Rate and thus the fairness between users.

/pdf/wideband-channel-allocation-in-distributed-antenna-systems-1giidns9df.pdf

Raymond Knopp

Papers

Near Optimal Linear Precoder for Multiuser MIMO for Discrete Alphabets

Cognitive Radio for High Speed Railway through Dynamic and Opportunistic spectrum Reuse

Achieving multiuser diversity under hard fairness constraints

Spatial Interference Cancellation Algorithm

Wideband Channel Allocation in Distributed Antenna Systems