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 consider low-complexity detection of coded spatial data streams with uniform power and non-uniform rate distribution in a single-user MIMO system. The receiver decodes these different streams as if facing a multiple access channel (MAC). Conventional receiver solutions for such schemes are based on successive interference cancellation (SIC) by employing a linear minimum mean square error (MMSE) successive stripping detector, where the optimality is nonetheless constrained to Gaussian codebooks. As a remedy, this paper introduces a novel near-optimal low-complexity max-log-MAP demodulator for a $2\times n_{r}$ system ( $n_{r}$ is the number of receive antennas) which reduces the complexity of detection from ${\cal O}(\vert\chi\vert^{2})$ to ${\cal O}(\vert\chi\vert^{1})$ , where $\vert\chi\vert$ indicates the size of the signal set. In the sequel, we extend the proposed low-complexity demodulation scheme to higher-dimensional MIMO systems via a hybrid detector, where significant complexity saving is realized at the expense of slight performance degradation.

Low Complexity BICM MIMO OFDM Demodulator

This paper considers a communication network comprised of two nodes, which have no mutual direct communication links, communicating two-way with the aid of a common relay node (RN), also known as separated two-way relay (TWR) channel. We first recall a cut-set outer bound for the set of rates in the context of this network topology assuming full-duplex transmission capabilities. Then, we derive a new achievable rate region based on hash-and-forward (HF) relaying where the RN does not attempt to decode but instead hashes its received signal, and show that under certain channel conditions it coincides with Shannon's inner-bound for the two-way channel [1]. Moreover, for binary adder TWR channel with additive noise at the nodes and the RN we provide a detailed capacity achieving coding scheme based on structure codes.

/pdf/hash-and-forward-relaying-for-two-way-relay-channel-3hsxqmnwk4.pdf

Hash-and-forward relaying for two-way relay channel

https://www.eurecom.fr/en/publication/1872/download/cm-toufiss-060801.pdf

Channel allocation algorithms for multi-carrier multiple-antenna systems

Interference management between uncoordinated eNBs and UEs is a crucial aspect of Heterogeneous Networks in LTE, especially if the interference can be exploited at the receiver. In this paper, a novel preprocessing Block QR decomposition technique is proposed for a low-complexity maxlog-MAP receiver, for interference detection in a 4×4 HetNet scenario. The authors show that for various SNR, the proposed receiver detection scheme has minimal mutual information loss for Gaussian signals for the 4×4 and 8×8 case. The proposed preprocessing method is implemented in an LTE simulation test bench with practical codes and compared with a brute-force search max-log-MAP algorithm.

/pdf/detection-using-block-qr-decomposition-for-mimo-hetnets-7ocvhufh3y.pdf

Detection using block QR decomposition for MIMO HetNets

This paper presents a practical implementation of a novel three-message transmission strategy for the Gaussian half-duplex relay channel based on Turbo code superposition encoding and interference-aware successive interference cancellation. The impact of finite block-length and discrete input constellations on the Block Error Rate (BLER) performance is evaluated through extensive simulations on an LTE simulation test bench and compared to the theoretical performance of asymptotically large block-length Gaussian codes. For the practically relevant BLER value of 10−2 and by varying the direct source-destination link strength, the maximum spectral efficiency gap between theory and the presented implementation is found to be of 0.458 bits/dim when the strength of the source-destination and relay-destination links is the same and of 0.681 bits/dim when the relay-destination link is 5 dB stronger than the source-destination link. These values indicate that practical implementations of high-performing HD relay techniques for future Heterogeneous Network deployments are within reach. A comparison with a baseline strategy without direct source-destination transmission, as currently proposed in the LTE standard for relay scenarios, shows superior performances of the proposed scheme. In particular, the rate gain is of a factor of 2 when the strength of the source-destination and relay-destination links is the same and of a factor of 1.2 when the relay-destination link is 5 dB stronger than the source-destination link, thereby highlighting the critical importance of physical-layer cooperation in broadband wireless systems.

/pdf/an-lte-implementation-of-a-novel-strategy-for-the-gaussian-3t6n68octj.pdf

Raymond Knopp

Papers

Low Complexity BICM MIMO OFDM Demodulator

Hash-and-forward relaying for two-way relay channel

Channel allocation algorithms for multi-carrier multiple-antenna systems

Detection using block QR decomposition for MIMO HetNets

An LTE implementation of a novel strategy for the Gaussian half-duplex relay channel