MIMO Links Research Articles

Joint Channel Estimation for Three-Hop MIMO Relaying Systems

We propose a novel joint channel estimator for a relaying MIMO communication system. Considering a three-hop relaying protocol, our combined alternating least squares (Comb-ALS) algorithm obtains cooperative diversity by fully exploiting the tensor algebraic structures of the available cooperative MIMO links. This is achieved by coupling the tensor data for the different relay-assisted links to iteratively estimate the channel matrices. Simulation results corroborate the effectiveness of the proposed tensor-based joint channel estimator in comparison with a sequential tensor-based method and a sequential LS estimator.

Random Matrix Theory Based Resource Allocation in Correlated MIMO Systems with ARQ Feedback

We consider resource allocation under partial feedback in a spatially correlated MIMO link, when the ARQ protocol is implemented at the MAC layer. We propose a design framework, which makes use of results from random matrix theory (RMT), to find the rate as well as the input covariance matrix that maximize the long term goodput. We consider partial feedback in terms of positive/negative acknowledgment bits (ACK/NAK), which comes essentially for free since they are always present in the signaling of the upper layers. We provide explicit expressions of the long term goodput, which, in association with a RMT based approximation of the mutual information enable us to optimize the resource allocation problem. Interestingly, the simulations show that the asymptotic optimization analysis is still valid for MIMO sizes as small as 2x2.

Channel Multiplexing Technique Utilizing Electric and Magnetic Components of a Radio Wave

A wireless channel multiplexing method which makes use of the electric and magnetic field components of an electromagnetic wave as separate information carriers is presented. It is based on the observation that the rates of attenuation for electric and magnetic fields with respect to distance are different for electric and magnetic sources in the near-field region. Capacity enhancement is obtained in free space without any scattering. Measured indoor channels at 1 MHz confirm capacity enhancement of the proposed 2 × 2 MIMO link compared with the reference SISO link.

Design of a Practical and Compact mm-Wave MIMO System with Optimized Capacity and Phased Arrays

In this paper we evaluate the feasibility of short range outdoor mm-wave MIMO links in the 70 GHz portion of the E-band (71–76 GHz). We use phased arrays in order to strongly reduce the impact of the multipath components, thus making the channel mainly line-of-sight (LOS). We design the array using a simple patch as a single element and simulate the performances for a 200 m link and a MIMO system with equal element spacing at the transmitter and the receiver. Each node of the MIMO system consists of a uniform rectangular array (URA) where the single element is a patch antenna, in order to achieve higher gains and narrow beams. Such configuration is much more compact compared to the antennas currently employed for the same bandwidth. We optimize the interelement distances at the transmitter and the receiver and evaluate the capacity achievable with different array sizes. The results show that, for the proposed link budget, capacity up to 29 bit/s/Hz is achievable at a range of 200 m, with practical dimensions. We also show that the beamforming capabilities make the design much more flexible than the single reflector antenna systems. In the last part of the paper, we verify that our antenna can also operate in rainy conditions and longer ranges.

Link Scheduling for Exploiting Spatial Reuse in Multihop MIMO Networks

Multiple-Input-Multiple-Output (MIMO) has great potential for enhancing the throughput of multihop wireless networks via spatial multiplexing or spatial reuse. Spatial reuse with Stream Control (SC) provides a considerable improvement of the network throughput over spatial multiplexing. The gain of spatial reuse, however, is still not fully exploited. There exist large numbers of additional data streams, which could be transmitted concurrently with those data streams scheduled by stream control at certain time slots and vicinities. In this paper, we address the issue of MIMO link scheduling to maximize the gain of spatial reuse and thus network throughput. We propose a Receiver-Oriented Interference Suppression model (ROIS), based on which we design both centralized and distributed link scheduling algorithms to fully exploit the gain of spatial reuse in multihop MIMO networks. Further, we address the traffic-aware link scheduling problem by injecting nonuniform traffic load into the network. Through theoretical analysis and comprehensive performance evaluation, we achieve the following results: 1) link scheduling based on ROIS achieves significant higher network throughput than that based on stream control, with any interference range, number of antennas, and average hop length of data flows. 2) The traffic-aware scheduling is enticingly complementary to the link scheduling based on ROIS model. Accordingly, the two scheduling schemes can be combined to further enhance the network throughput.

Cross‐layer optimization for fair end‐to‐end rate allocation in WMNs with MIMO links

ABSTRACTMultiple‐input multiple‐output (MIMO) technology can bring substantial throughput improvement in wireless mesh networks (WMNs) via a combination of interference suppression and spatial multiplexing. In this paper, we investigate a cross‐layer design problem in MIMO‐based WMNs, which involves end‐to‐end rate allocation, routing, and stream control scheduling. Scheduling is addressed by using a set of transmission configurations (TCs), and a greedy heuristic algorithm is proposed to identify a good subset of all possible TCs. We define three optimization problems to find the maximum throughput, max–min fair and proportional fair rate allocation solutions. We propose a cross‐layer scheme to solve these problems. We also discuss how the proposed cross‐layer scheme can be implemented on the basis of the IEEE 802.16 mesh mode. Simulation results show that the performance of the proposed schemes with the heuristic algorithm is close to the optimal. The computational complexity of obtaining the solution using the proposed heuristic algorithm is substantially less than that of finding the optimal solution. Copyright © 2012 John Wiley & Sons, Ltd.

A Distributed Cooperative MAC Protocol for QoS Improvement and Mobility Support in WiMedia Networks

A Distributed Medium Access Control (D-MAC) protocol based on UWB for high-rate Wireless Personal Area Networks is specified by the WiMedia Alliance. D-MAC protocol is suitable for ubiquitous connection in home networks, military/medical applications due to its inexpensive cost, low power consumption, high data rate, and distributed approach. In contrast to IEEE 802.15.3, D-MAC makes all devices have the same functionality. And its networks are self-organized and provide devices with functions such as access to the medium, channel allocation to devices, data transmission, quality of service and synchronization in a distributed manner. D-MAC fundamentally removes the problems of the centralized MAC approach revealed at IEEE 802.15.3 MAC by adopting a distributed architecture. However, the current D-MAC can't prevent QoS degradations, occurred by mobile nodes with low data rate due to bad channel status, which cause critical problems in QoS provisioning to isochronous streams and mobile applications. Therefore, we propose a distributed cooperative MAC protocol for multi-hop WiMedia networks using virtual MIMO links. Based on instantaneous Channel State Information among WiMedia devices, our proposed protocol can intelligently select the transmission path with higher data rate to provide advanced QoS with minimum delay for real-time multimedia streaming services.

Characterization of Measured Indoor Off-Body MIMO Channels with Correlated Fading, Correlated Shadowing and Constant Path Loss

Indoor off-body wireless MIMO links between a mobile user equipped with wearable textile patch antennas and a fixed base station exhibit specific channel behavior due to the near presence and movements of the human body. Therefore, they require a dedicated channel model that captures the effects of correlated small-scale Rayleigh fading and correlated lognormal shadowing. A methodology is presented to construct such a model, allowing to predict the bit error characteristics and channel capacity curves based on the shadowing and fading correlation matrices that are extracted from channel measurements. It is shown that by separating shadowing, including effects caused by movement and reorientation of the human body, from small-scale fading, the main mechanisms of the off-body communication link are accurately captured by the model. A clear dependence of the shadowing correlation values on the physical layout of the antenna system is found. In our measurements, shadowing is not significantly decorrelated by polarization diversity or front-to-back diversity whereas the small-scale fading is clearly decorrelated. From the model, MIMO channel realizations with identical bit error rate and channel capacity characteristics as the measured channel can be quickly generated for link emulation purposes.

On the scheduling, multiplexing and diversity trade-off in MIMO ad hoc networks: A unified framework

In this paper we study the fundamental scheduling, multiplexing and diversity trade-off in MIMO ad hoc networks. In particular, we propose a unified framework for the scheduling-multiplexing and scheduling-diversity sub-problems that constitutes a major step towards solving the overall problem. The two sub-problems are motivated by a fundamental trade-off between scheduling full multiplexing (diversity) gain non-interfering links and scheduling interfering links using lower multiplexing (diversity) gain in conjunction with interference nulling. First, we cast each sub-problem as a cross-layer optimization problem that jointly decides the scheduling and MIMO stream allocation, subject to signal-to-interference-and-noise-ratio (SINR) constraints. Second, we characterize the problem as non-convex integer programming which is quite challenging to solve. Hence, we shift our focus to characterize the optimal for the simple case of two links. The main result of this paper is that the two fundamentally different sub-problems give rise to structurally similar SINR-based decision rules which constitute the basis for a resource allocation algorithm with linear complexity in the number of links, namely Iterative MIMO Link Scheduling (IMLS), that solves the two sub-problems and achieves significant gains for any number of links. Numerical results exhibit more than twofold/quadratic improvement over scheduling non-interfering links with full multiplexing/diversity gain, for plausible scenarios. IMLS and its variants reveal an important throughput-fairness trade-off which is an interesting topic for future research.

QoS-Aware Base-Station Selections for Distributed MIMO Links in Broadband Wireless Networks

The distributed multiple-input-multiple-output (MIMO) techniques across multiple cooperative base stations (BS) can significantly enhance the capability of the broadband wireless networks in terms of quality-of-service (QoS) provisioning for wireless data transmissions. However, the computational complexity and the interfering range of the distributed MIMO systems also increase rapidly as the number of cooperative BS's increases. In this paper, we propose the QoS-aware BS-selection schemes for the distributed wireless MIMO links, which aim at minimizing the BS usages and reducing the interfering range, while satisfying diverse statistical delay-QoS constraints characterized by the delay-bound violation probability and the effective capacity technique. In particular, based on the channel state information (CSI) and QoS requirements, a subset of BS with variable cardinality for the distributed MIMO transmission is dynamically selected, where the selections are controlled by a central server. For the single-user scenario, we develop two optimization frameworks, respectively, to derive the efficient BS-selection schemes and the corresponding resource allocation algorithms. One framework uses the incremental BS-selection and time-sharing (IBS-TS) strategies, and the other employs the ordered-gain based BS-selection and probabilistic transmissions (OGBS-PT). The IBS-TS framework can yield better performance, while the scheme developed under the OGBS-PT framework is easier to implement. For the multi-user scenario, we propose the optimization framework applying the priority BS-selection, block-diagonalization precoding, and probabilistic transmission (PBS-BD-PT) techniques. We also propose the optimization framework applying the priority BS-selection, time-division-multiple-access, and probabilistic transmission (PBS-TDMA-PT) techniques. We derive the optimal transmission schemes for all the aforementioned frameworks, respectively. Also conducted is a set of simulation evaluations which compare our proposed schemes with several baseline schemes and show the impact of the delay-QoS requirements, transmit power, and traffic loads on the performances of BS selections for distributed MIMO systems.

MIMO-Pipe Modeling and Scheduling for Efficient Interference Management in Multihop MIMO Networks

Multiple-input-multiple-output (MIMO) technology, which is a recent breakthrough in wireless communications, has been shown to significantly improve channel capacity in single-user systems. However, obtaining a rigorous understanding of the possible MIMO gains in multihop networks is still an open topic. One grand challenge is that multihop wireless networks are interference limited and that the interference introduces coupling across various layers of the protocol stack, including the physical (PHY), medium access control (MAC), network, and transport layers. The fundamental differences between multihop networks and point-to-point settings dictate that leveraging the MIMO gains in multihop networks requires a domain change from high-SNR regimes to interference-limited regimes. In this paper, we develop a cross-layer optimization framework for effective interference management toward understanding fundamental tradeoffs among possible MIMO gains in multihop networks. We first take a bottom-up approach to develop a MIMO-pipe model based on PHY interference and extract a set of {( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ri</i> , SINR <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i> ) }, where each pair ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ri</i> , SINR <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i> ) corresponds to a meaningful stream multiplexing configuration for individual MIMO links [with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ri</i> being the rate and SINR <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i> being the signal-to-interference-plus-noise ratio (SINR) requirement]. Using this link abstraction model, we study MIMO-pipe scheduling for throughput maximization. Based on continuous relaxation via randomization, we study the structural property of the optimal scheduling policy. Our findings reveal that, in an optimal strategy, it suffices for each MIMO link to use one stream configuration only (although each individual MIMO link can have multiple stream configurations). In light of this structural property, we then formulate MIMO-pipe scheduling as a combinatorial optimization problem, and by using a multidimensional 0-1 knapsack approach, we devise centralized approximation algorithms for both the dense network model and the extended network model, respectively. Next, we also develop a contention-based distributed algorithm, in which links update their contention probability based on local information only, and characterize the convergence and the performance of the distributed algorithm.

Adaptive MIMO transmission techniques for broadband wireless communication systems [Topics in Wireless Communications

Link adaptation is a way to increase data rates in wireless systems by adapting transmission parameters such as the modulation and coding rate. While link adaptation in single antenna systems is now mature, its application to multiple-input multiple-output communication links, presented in several emerging wireless standards, has been challenging. The main reason is that the space-time transmission strategy can also be adjusted in MIMO communication links, introducing a new dimension for adaptation. This means that practical MIMO link adaptation algorithms must also provide a dynamic adaptation between diversity and multiplexing modes of operation. This article reviews a recently proposed framework for adaptive MIMO architectures and shows how to use this framework to reduce adaptive control overhead. We also discuss practical implementation issues. Simulations in an IEEE 802.16e (mobile WiMAX) system illustrate the frame-work's potential improvements in data rates.

Cross-Layer Design for End-to-End Throughput Maximization and Fairness in MIMO Multihop Wireless Networks

MIMO links can significantly improve network throughput by supporting multiple concurrent data streams between a pair of nodes and suppressing wireless interference. In this paper, we study joint rate control, routing, and scheduling in MIMO-based multihop wireless networks, which are traditionally known as transport layer, network layer, and MAC layer issues, respectively. Our aim is to find a rate allocation along with a flow allocation and a transmission schedule for a set of end-to-end communication sessions so as to maximize the network throughput and also to achieve the proportional or weighted fairness among these sessions. To this end, we develop Transmission Mode Generating Algorithms (TMGAs), and Linear Programming- (LP-) and Convex Programming- (CP-) based optimization schemes for the MIMO networks. The performances of the proposed schemes are verified by simulation experiments, and the results show that the different schemes have different performance benefits when achieving a tradeoff between throughput and fairness.

Cooperative power scheduling for a network of MIMO links

A cooperative power scheduling algorithm developed by Wang, Krunz and Cui is extended for an ad hoc network of MIMO links. This algorithm, referred to as price-based iterative water filling (PIWF) algorithm, is a distributed algorithm by which each link computes its power scheduling through an iterative and cooperative process. The cooperation among all links is achieved by adaptive price factors applied by each link. Compared to a centralized power scheduling algorithm, the PIWF algorithm is much more efficient in computation although not as efficient in network throughput. Compared to a non-cooperative counter-part by Demirkol and Ingram where all price factors are zero, the PIWF algorithm requires additional in-network computation but is more efficient in network throughput.

Slot Antenna Performance and Signal Quality in a Smartphone Prototype

Antenna position and user grip on smartphone-like devices may lead to obstruction of radio signal paths and antenna detuning. A multiple-input-multiple-output (MIMO) outdoor propagation measurement campaign is presented, which collected data for slot antennas on a smartphone prototype. The antennas are in four positions, two polarizations, and one was obstructed due to operator grip. All MIMO links were measured within the channel's coherence time while standing, walking, and driving. We show how signal levels change due to obstruction, position, and motion and that signal fluctuations increase significantly, thus tending to impair service quality. We also examine how proximity of the operator's hand affects the antenna's radiation and input characteristics.

Measurement-Based Evaluation of Interlink Correlation for Indoor Multiuser MIMO Channels

The properties of the single-link multiple-input-multiple-output (MIMO) propagation channels have been analyzed in many studies based on extensive propagation measurements. In this letter, we extend the measurement-based analysis to dual-link MIMO propagation channels. Here, we examine interlink correlation by evaluating the correlation of eigenvectors obtained from multiple MIMO links. We also investigated how the physical propagation phenomena produced the interlink correlation. Results indicate that interlink correlation may exist. Sometimes it is found to be very high also for far separated receive units, depending on similarity of dominant propagation mechanisms between multiple MIMO links and also on antenna array properties.

Effects of channel dispersion and path correlations on system-wide throughput in MIMO-based cellular systems

We compute cell-wide mean throughputs for a wide range of multiple-input-multiple-output (MIMO) system design parameters, in both single- and multi-cell scenarios, using realistic models for channel dispersion, path correlations, and cochannel interference. We assume the use of single-carrier modulation, and quantify the degradation in downlink throughput as the channel dispersion increases. Our results highlight the potential harm even from mild dispersion in MIMO links. In studying path correlations, we assume linear arrays at each end of the link, and quantify the interplay among device size, number of antenna elements, path correlation, and carrier frequency. We show that, under practical size constraints, the use of additional antenna elements can be counterproductive.

Joint Linear Filter Design in Multiuser Cooperative Nonregenerative MIMO Relay Systems

This paper addresses the filter design issues for multiuser cooperative nonregenerative MIMO relay systems in both downlink and uplink scenarios. Based on the formulated signal model, the filter matrix optimization is first performed for direct path and relay path respectively, aiming to minimize the mean squared error (MSE). To be more specific, for the relay path, we derive the local optimal filter scheme at the base station and the relay station jointly in the downlink scenario along with a more practical suboptimal scheme, and then a closed-form joint local optimal solution in the uplink scenario is exploited. Furthermore, the optimal filter for the direct path is also presented by using the exiting results of conventional MIMO link. After that, several schemes are proposed for cooperative scenario to combine the signals from both paths. Numerical results show that the proposed schemes can reduce the bit error rate (BER) significantly.

On the multicell processing capacity of the cellular MIMO uplink channel in correlated rayleigh fading environment

In the context of cellular systems, it has been shown that multicell processing can eliminate inter-cell interference and provide high spectral efficiencies with respect to traditional interference-limited implementations. Moreover, it has been proved that the multiplexing sum-rate capacity gain of multicell processing systems is proportional to the number of base station (BS) antennas. These results have been also established for cellular systems, where BSs and user terminals (UTs) are equipped with multiple antennas. Nevertheless, a common simplifying assumption in the literature is the uncorrelated nature of the Rayleigh fading coefficients within the BSUT MIMO links. In this direction, this paper investigates the ergodic multicell-processing sum-rate capacity of the Gaussian MIMO cellular multiple-access channel in a correlated fading environment. More specifically, the multiple antennas of both BSs and UTs are assumed to be correlated according to the Kronecker product model. Furthermore, the current system model considers Rayleigh fading, uniformly distributed UTs over a planar coverage area and power-law path loss. Based on free probabilistic arguments, the empirical eigenvalue distribution of the channel covariance matrix is derived and it is used to calculate both optimal joint decoding and minimum mean square error (MMSE) filtering capacity. In addition, numerical results are presented, where the per-cell sum-rate capacity is evaluated while varying the cell density of the system, as well as the level of fading correlation. In this context, it is shown that the capacity performance is greatly compromised by BS-side correlation, whereas UT-side correlation has a negligible effect on the system's performance. Furthermore, MMSE performance is shown to be greatly suboptimal but more resilient to fading correlation in comparison to optimal decoding.

Delay-optimal power and precoder adaptation for multi-stream MIMO systems

In this paper, we consider delay-optimal MIMO precoder and power allocation design for a MIMO link in wireless fading channels. There are L data streams spatially multiplexed onto the MIMO link with heterogeneous packet arrivals and delay requirements. The transmitter is assumed to have knowledge of the channel state information (CSI) as well as the joint queue state information (QSI) of the L buffers. Using L-dimensional Markov decision problem (MDP), we obtain optimal precoding and power allocation policies for general delay regime, which consists of an online solution and an offline solution. The online solution has negligible complexity but the offline solution has worst case complexity part((N + 1)L) where N is the buffer size. Using static sorting of the L eigenchannels, we decompose the MDP into L independent 1-dimensional subproblems and obtained low complexity offline solution with linear complexity order part(NL) and close-to-optimal performance.