Multiple Antenna Terminals Research Articles

Simulation of Multiple-Antenna Terminal Performance in Massive MIMO Systems Based on Indoor Measurements

In massive MIMO systems the uplink pilot signals transmitted by a terminal define the channel seen by the base station. This gives the terminal some degree of freedom selecting an uplink pilot transmission strategy. In this paper, we investigate the benefit of different pilot transmission strategies when increasing the number of antennas in the terminal. Building on previous work on a simulation framework for Multiple-antenna terminals in 5G massive MIMO systems, this paper presents simulated performance results for various transmission schemes. The results are calibrated to reflect a real communication situation in a large auditorium. Emulating the measurement set-up, we show that the framework can be tuned to generate channel distributions that match measured data. Under generalized conditions, we perform simulations for different terminal transmission-strategies, both related to single stream and multiple streams. All evaluations are based on terminals with four antennas integrated into real Sony Xperia smartphone-chassis, tuned to 3.7 GHz. The measurements are conducted by using the Lund University Massive MIMO testbed with its 100 antennas. The results clearly show the advantage of increasing the antenna-count also at the terminal side in massive MIMO systems.

Secure Communication in Relay-Assisted Massive MIMO Downlink With Active Pilot Attacks

In this paper, the achievable secrecy rate of a relay-assisted massive multiple-input multiple-output (MIMO) downlink is investigated in the presence of a multi-antenna active/passive eavesdropper. The excess degrees-of-freedom offered by a massive MIMO base-station (BS) are exploited for sending artificial noise (AN) via random and null-space precoders. An active eavesdropper contaminates the uplink channel estimates by sending pilot sequences identical to those of the legitimate users/relay. This active pilot contamination makes the massive MIMO BS implicitly beamform the confidential signals toward the active eavesdropper during two-hop downlink transmissions. The achievable secrecy rates are derived by taking the detrimental effects of actively contaminated channel state information with estimation errors and spatially correlated fading at the multiple-antenna terminals into account. The secrecy rate loss incurred by active pilot attacks over passive eavesdropping is investigated, and the secrecy rate gap between random and null-space-based AN is compared. A novel transmit power control policy is designed to efficiently allocate transmit power at the BS/relay for payload data and AN sequences for maximizing the achievable secrecy rate. Our results reveal that active pilot contamination attacks significantly degrade the achievable secrecy rate in dual-hop transmissions, and the corresponding detrimental effects cannot be asymptotically mitigated in the infinite BS antenna regime.

A Simulation Framework for Multiple-Antenna Terminals in 5G Massive MIMO Systems

The recent interest in massive multiple in multiple out (MIMO) has spurred intensive work on massive MIMO channel modeling in the 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 the 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 3rd Generation Partnership Project. The simulation results are compared with the measured terminal performance when communicating with the Lund University'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 the 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.

Optimal Cooperative Beamforming Design for MIMO Relay Channels

Motivated by the rapid growth of wireless applications with multiple antenna terminals, multiple input multiple output (MIMO) relaying has gained much attention now a days due to its improved data rate, coverage extension and improved signal to noise ratio (SNR). In this paper, a transmit beamforming design for MIMO decode-and-forward (DF), MIMO amplify-and-forward (AF) and MIMO compress-and-forward (CF) half-duplex two-hop relay channels with a direct source–destination link is considered. Network model consist of source, relay and destination. Design includes four different cases in terms of the number of antennas deployed at source, relay and destination node which contains 2:2:1 scenario, Ns:1:1 scenario, Ns:Nr:1 scenario and Ns:Nr:Nd scenario. Here, beamforming design is based on exact capacity formulation and low-complexity explicit expressions are used. Effect of parameter such as number of antennas on the performance of MIMO relay channels is also investigated. Optimal beamforming design for MIMO AF relaying scheme achieves high performance gain and higher information rates than MIMO DF and MIMO CF relaying scheme.

On the Impacts of Composite Fading on Large-Scale Multi-User MIMO Systems

This letter considers the uplink of a large-scale multi-user multiple-input-multiple-output (MIMO) system where multiple single antenna terminals transmit simultaneously to an array of hundreds of antennas. The base station (BS) is assumed to have perfect channel state information (CSI). Under Rayleigh fading and lognormal shadowing, the probability density function (PDF) of the received signal-to-interference-plus-noise-ratio (SINR) is approximated by a lognormal distribution for different receiver types and closed-form expressions for the outage probability are presented. The effects of shadowing on the performance of the system are quantified.

Low-Complexity End-to-End Performance Optimization in MIMO Full-Duplex Relay Systems

In this paper, we deal with the deployment of full-duplex relaying in amplify-and-forward (AF) cooperative networks with multiple-antenna terminals. In contrast to previous studies, which focus on the spatial mitigation of the loopback interference (LI) at the relay node, a joint precoding/decoding design that maximizes the end-to-end (e2e) performance is investigated. The proposed precoding incorporates rank-1 zero-forcing (ZF) LI suppression at the relay node and is derived in closed-form by solving appropriate optimization problems. In order to further reduce system complexity, the antenna selection (AS) problem for full-duplex AF cooperative systems is discussed. We investigate different AS schemes to select a single transmit antenna at both the source and the relay, as well as a single receive antenna at both the relay and the destination. To facilitate comparison, exact outage probability expressions and asymptotic approximations of the proposed AS schemes are provided. In order to overcome zero-diversity effects associated with the AS operation, a simple power allocation scheme at the relay node is also investigated and its optimal value is analytically derived. Numerical and simulation results show that the joint ZF-based precoding significantly improves e2e performance, while AS schemes are efficient solutions for scenarios with strict computational constraints.

Experimental Investigation of the Directional Outdoor-to-In-Car Propagation Channel

The demand for wireless channel models, including realistic user environments, is increasing. This motivates the work on more detailed models that are reasonably simple and tractable but with adequate statistical performance. In this paper, we present an investigation of the spatial outdoor-to-in-car radio channel at 2.6 GHz. Specifically, we investigate the performance of a multiple-antenna smartphone mock-up in the hand of a user. We evaluate and utilize a composite channel approach to combine measured antenna radiation patterns with an estimated spectral representation of the multipath channel outside and inside the car in two different scenarios. The performance of the method is investigated, and comparisons with direct channel measurements are performed. Statistical and directional properties of the outdoor-to-in-car channel are presented and analyzed. It is found that the composite method, despite near-field problems when estimating plane-wave channel parameters in a very narrow environment, constitutes a tool that provides reasonably viable statistical results. In addition, we have found that the introduction of the car in the propagation environment increases scattering and eigenvalue dispersion, whereas it decreases pairwise antenna signal correlation. These statistical properties are found to increase slightly the possible diversity and the spatial multiplexing gains of multiple-antenna terminals when located inside cars. This positive effect, however, is small compared with the negative effect of car penetration loss.

Application of Perfect Space Time Codes: PEP Bounds and Some Practical Insights

The design of perfect space time codes constructed from cyclic division algebra (CDA) on the quasi-static MIMO channel has received lots of attention in industry over the last few years. However, the recent standards that use multiple antennas terminals such as IEEE 802.11n, IEEE 802.16e or LTE are based on more realistic assumptions involving the use of outer codes and multi-taps channels. In this paper, the question regarding the utility of using a high dimensional modulation schemes such as space time codes in a standard context is addressed. For this, we evaluate the performance of the perfect space time codes in term of an upper-bound on the pairwise error probability, and we compare it to the one of simple spatial division multiplexing (SDM) schemes. Finally, simulation results considered in the context of IEEE 802.11n show that the 2 × 2 MIMO perfect code, i.e. the golden code performs relatively similarly as the SDM scheme in the presence of a good outer code.

Opportunistic Relaying for MIMO Amplify-and-Forward Cooperative Networks

We consider a wireless relay network with multiple antenna terminals over Rayleigh fading channels, and apply distributed space-time coding (DSTC) in amplify-and-forward (A&F) mode. The A&F scheme is used in a way that each relay transmits a scaled version of the linear combination of the received symbols. It turns out that, combined with power allocation in the relays, A&F DSTC results in an opportunistic relaying scheme, in which only the best relay is selected to retransmit the source's space-time coded signal. Furthermore, assuming the knowledge of source-relay CSI at the source node, we design an efficient power allocation which outperforms uniform power allocation across the source antennas. Next, assuming M-PSK or M-QAM modulations, we analyze the performance of the proposed cooperative diversity transmission schemes in a wireless relay networks with the multiple-antenna source and destination. We derive the probability density function (PDF) of the received SNR at the destination. Then, the PDF is used to determine the symbol error rate (SER) in Rayleigh fading channels. We derived closed-form approximations of the average SER in the high SNR scenario, from which we find the diversity order of system R min{N s , N d }, where R, N s , and N d are the number of the relays, source antennas, and destination antennas, respectively. Simulation results show that the proposed system obtain more than 6 dB gain in SNR over A&F MIMO DSTC for BER 10?4, when R = 2, N s = 2, and N d = 1.

Characteristic Mode Based Tradeoff Analysis of Antenna-Chassis Interactions for Multiple Antenna Terminals

The design of multiple antennas in compact mobile terminals is a significant challenge, due to both practical and fundamental design tradeoffs. In this paper, fundamental antenna design tradeoffs of multiple antenna terminals are presented in the framework of characteristic mode analysis. In particular, interactions between the antenna elements and the characteristic modes and their impact on design tradeoffs are investigated in both theory and simulations. The results reveal that the characteristic modes play an important role in determining the optimal placement of antennas for low mutual coupling. Moreover, the ability of antenna elements to localize the excitation currents on the chassis can significantly influence the final performance. To demonstrate the effectiveness of the proposed approach, a dual-band, dual-antenna terminal is designed to provide an isolation of over 10 dB for the 900 MHz band without additional matching or decoupling structures. A tradeoff analysis of bandwidth, efficiency, effective diversity gain and capacity is performed over different antenna locations. Finally, three fabricated prototypes verify the simulation results for representative cases.

Iterative Multiuser Detection for Cooperative MIMO Systems over Quasi-Static Fading Channels

In this paper, we propose a cooperative multiuser system for multiple-antenna terminals under quasi-static fading. The system employs a bit-interleaved code modulation scheme that combines space-time coding and cooperation based on the decode-and-forward protocol at the transmitter and iterative multiuser detection based on parallel interference cancellation and minimum mean square error filtering at the receiver. We investigate an approach that allows to provide diversity from different channels to each user up to full diversity. Frame error rate performance under Monte Carlo simulations is shown to confirm the effectiveness of the proposed schemes.

MIMO two-way relay channel with superposition coding and imperfect channel estimation

This paper deals with the superposition coding (SPC) scheme in multiple-input multiple-output two-way relay channels subject to imperfect channel estimation. In this scenario, two multiple antenna terminals, which are unable to communicate directly, exchange information with each other via a multiple antenna relay. We determine the impact of the channel estimation error degradation on the achievable rate region for two main SPC techniques: (a) SPC without channel state information (CSI) at the users, (b) SPC with an imperfect CSI at the users where a waterfilling power allocation is employed. We demonstrate that imperfect CSI significantly improves the achievable rate at low signal-to-noise ratios (SNRs) while it becomes less critical at high SNRs. In addition, a SPC power allocation technique that incorporates the average channel statistics and does not require any instantaneous CSI is also investigated. We show how the available power is split between the two bi-directional (superimposed) data flows in order to maximize the system performance and to support fairness as well as to maximize the achievable sum-rate.

Multi-hop MIMO relay networks: diversity-multiplexing trade-off analysis

A multi-hop relay network with multiple antenna terminals in a quasi-static slow fading environment is considered. The fundamental diversity-multiplexing gain tradeoff (DMT) is analyzed in the case of half-duplex relay terminals. While decode-and-forward (DF) relaying achieves the optimal DMT in the full-duplex relay scenario, it is shown that the dynamic decode-and-forward (DDF) protocol achieves the optimal DMT if the relay is constrained to half-duplex operation. For the latter case, static DF protocols are considered as well, and the corresponding DMT performance is shown to fall short of the optimal performance, which indicates that dynamic channel allocation is required for optimal DMT performance. The optimal DMT is expressed as the solution of a convex optimization problem and explicit DMT expressions are presented for some special cases. In the case of multiple relays, it is shown that the optimal diversity gain, which is achieved by exploiting the available "hop-diversity", is dominated by the neighboring two-hops with the minimum diversity gain.

Transmitter Layering for Multiuser MIMO Systems

A novel structure for multiple antenna transmissions utilizing space-time dispersion is proposed, where the original data stream is divided into substreams which are modulated onto all available transmit antennas using stream-specific transmit signature sequences. In order to achieve this, the transmit antennas are partitioned into groups of antennas, called partitions. The signals from the data streams are independently interleaved by partition over the entire transmission frame. The interleaved partitions are then added over all substreams prior to transmission over the MIMO channel. At the receiver, a low-complexity iterative detector adapted from recent CDMA multiuser detection research is used. It is shown that with careful substream power assignments this transmission methodology can efficiently utilize the capacity of rank-deficient channels as it can approach the capacity limits of the multiple antenna channel closely over the entire range of available signal-to-noise ratios and system sizes. This transmission methodology and receiver structure are then applied to multiuser MIMO systems where several multiple antenna terminals communicate concurrently to a joint receiver. It is shown that different received power levels from the different MIMO terminals can be beneficial and that higher spectral efficiencies can be achieved than in the single-terminal case.