Multiple-input Multiple-output Links Research Articles

Scaling Up MIMO: Opportunities and Challenges with Very Large Arrays

Multiple-input multiple-output (MIMO) technology is maturing and is being incorporated into emerging wireless broadband standards like long-term evolution (LTE) [1]. For example, the LTE standard allows for up to eight antenna ports at the base station. Basically, the more antennas the transmitter/receiver is equipped with, and the more degrees of freedom that the propagation channel can provide, the better the performance in terms of data rate or link reliability. More precisely, on a quasi static channel where a code word spans across only one time and frequency coherence interval, the reliability of a point-to-point MIMO link scales according to Prob(link outage) ` SNR-ntnr where nt and nr are the numbers of transmit and receive antennas, respectively, and signal-to-noise ratio is denoted by SNR. On a channel that varies rapidly as a function of time and frequency, and where circumstances permit coding across many channel coherence intervals, the achievable rate scales as min(nt, nr) log(1 + SNR). The gains in multiuser systems are even more impressive, because such systems offer the possibility to transmit simultaneously to several users and the flexibility to select what users to schedule for reception at any given point in time [2].

ON SOME OPTIMAL MIMO ANTENNA COEFFICIENTS IN MULTIPATH CHANNELS

This paper derives some optimum transmit and receive antenna coe-cients in wireless multipath channels based on the spherical vector wave multimode expansion of the multiple-input multiple-output (MIMO) channel matrix. The derived antenna coe-cients satisfy the following speciflc optimization criteria: (i) maximum MIMO mean efiective link gain (link MEG) based on the multimode channel realizations or (ii) maximum MIMO link MEG based on the multimode correlation matrix or (iii) correlation minimization by diagonalization of the MIMO full-correlation matrix. It is shown that the proposed approach leads to matrix equations belonging to the nearest Kronecker product (NKP) problem family, which in general have no trivial solution. However, we show that exact solutions are provided to the posed NKP problems under the assumption of the Kronecker model for the MIMO full-correlation matrix. The results are illustrated by numerical examples. The proposed approach is a complement to existing antenna pattern analysis methods.

From Spectrum Pooling to Space Pooling: Opportunistic Interference Alignment in MIMO Cognitive Networks

We describe a noncooperative interference alignment (IA) technique which allows an opportunistic multiple input multiple output (MIMO) link (secondary) to harmlessly coexist with another MIMO link (primary) in the same frequency band. Assuming perfect channel knowledge at the primary receiver and transmitter, capacity is achieved by transmiting along the spatial directions (SD) associated with the singular values of its channel matrix using a water-filling power allocation (PA) scheme. Often, power limitations lead the primary transmitter to leave some of its SD unused. Here, it is shown that the opportunistic link can transmit its own data if it is possible to align the interference produced on the primary link with such unused SDs. We provide both a processing scheme to perform IA and a PA scheme which maximizes the transmission rate of the opportunistic link. The asymptotes of the achievable transmission rates of the opportunistic link are obtained in the regime of large numbers of antennas. Using this result, it is demonstrated that depending on the signal-to-noise ratio and the number of transmit and receive antennas of the primary and opportunistic links, both systems can achieve transmission rates of the same order.

Crossover Digital Predistorter for the Compensation of Crosstalk and Nonlinearity in MIMO Transmitters

This paper proposes a novel crossover digital predistorter (CO-DPD) model to compensate for crosstalk and nonlinearity in multiple-input multiple-output (MIMO) radio systems. Crosstalk can take place before or after the power amplifiers, designated herein as nonlinear and linear crosstalk, respectively. This paper demonstrates that, contrary to linear crosstalk, nonlinear crosstalk significantly affects the performance of the digital predistortion algorithm; and, it cannot be embedded and compensated for by the conventional channel matrix inversion algorithm at the receiver side of MIMO links. Based on a parametric study of system-level simulations and measurements, it was found that for a -20-dB nonlinear crosstalk level, the use of a memory multibranch polynomial predistorter, along with the channel matrix inversion algorithm, bounds the adjacent channel power ratio (ACPR) for a wideband code division multiple access (WCDMA) signal to -46 dBc and the error vector magnitude (EVM) for a world interoperability for microwave access (WiMAX) signal to -43 dB in MIMO links. The proposed CO-DPD was investigated and analyzed for the MIMO transmitter with N = 2, where N is the number of RF front-ends. Its performance was evaluated through measurements, the experimental results obtained show that, in the presence of -20-dB nonlinear crosstalk, the proposed CO-DPD improve the ACPR of the WCDMA signal by 13 dB to -56.81 dBc from those obtained using a conventional digital predistorter. The same improvement was observed in the EVM measurement of the WiMAX signal, where the EVM decreases from -21.22 dB for the conventional DPD to -49.71 dB for the proposed CO-DPD.

An Optimization Framework for Demand-based Fair Stream Allocation in MIMO Ad Hoc Networks

In this paper, we investigate the problem of scheduling flows for fair stream allocation (or, stream scheduling) in ad hoc networks in which the transmitter and receiver use multiple antennas called Multiple Input Multiple Output (MIMO) technology. Our main contributions include: i) the concept of stream allocation to flows based on their traffic demands or class, ii) stream allocation to flows in the network utilizing single user or multiuser MIMO communication, iii) achieving the proportional fairness of the stream allocation in the minimum possible schedule length, and iv) performance comparison of the stream scheduling in the network for single user and multiuser communication and the tradeoff involved therein. We first formulate demand-based fair stream allocation as an integer linear programming (ILP) problem whose solution is a schedule that is guaranteed to be contention-free. We then solve this ILP in conjunction with binary search to find a minimum length contention-free schedule that achieves the fairness goals. We show that an implementation of our algorithm for a number of sample topologies in fact yields minimum length schedules that achieve the fairness goals. We then give performance comparison results that show the benefit of multiuser MIMO links over single user links at higher traffic workloads in the network. Finally, we also give a greedy heuristic for stream scheduling and compare its performance with the ILP-based algorithm in terms of the fairness goals achieved in a given schedule length.

Constraint Design and Throughput Evaluation in Multi-Band Wireless Networks Using Multiple-Input Multiple-Output Links

Recent advances in radio technology enabled software-defined radios (SDRs), which can switch from one frequency band to another at minimum cost, are making dynamic multi-band access and sharing possible. Also, recent advances in signal processing combined with those in antenna technology, enabled MIMO (Multiple-Input Multiple-Output) capabilities, which created great opportunities for enhancing the throughput of wireless networks. Together SDRs and MIMO empower the next-generation wireless networks, such as mesh networks, with dynamic and adaptive bandwidth sharing and use along time, frequency, and space. In this paper, we design and model the radio and interference constraints on multi-hop wireless networks, when they are both SDR-capable and MIMO-equipped. The developed constraint models were then used to formulate and solve multi-hop routing problems in wireless networks with MIMO and multi-band capabilities. We evaluated and analyzed the throughput that multi-hop wireless networks can achieve, when they are both allowed to communicate on multiple bands and equipped with MIMO links.

MIMO ANTENNA MODELLING USING THE EFFECTIVE LENGTH MATRICES

A hybrid electromagnetic-network analysis of the antennas- channel multiple input multiple output (MIMO) communication sub- system is presented. The analysis is based on the antenna efiective and realized efiective length matrices, which relate in a compact math- ematical way the radiated and received electric fleld intensities to the network characteristics of actual and coupled transmitting (Tx) and receiving (Rx) multi-element antenna (MEA) systems. The efiective length matrices are calculated via the active power gain and phase antenna patterns obtained by means of any full wave computational electromagnetics (CEM) fleld solver. It is shown that the realized efiec- tive length matrix is suitable for the S-parameter analysis of a MIMO communication link, while the efiective length matrix is convenient for its Z-parameter analysis. The efiective length matrix framework is ap- plied to a free space 2£2 coupled dipoles MIMO system and its results are in excellent agreement to those obtained by a Method of Moments (MoM) based fleld solver.

Modulation-mode and power assignment in broadband MIMO systems

Since the capacity of multiple-input multiple-output (MIMO) systems increases linearly with the minimum number of antennas at both, the transmitter as well as the receiver side, MIMO schemes have attracted a lot of attention. However, non-frequency selective MIMO links have reached a state of maturity. By contrast, frequency selective MIMO links require substantial further research, leading in this contribution to a joint optimization of the number of activated MIMO layers and the number of bits per symbol along with the appropriate allocation of the transmit power under the constraint of a given fixed data throughput. Our results show that in order to achieve the best possible bit-error rate, not necessarily all MIMO layers have to be activated. .

Optimal Power Schedule for Distributed MIMO Links

We present an optimal power scheduling scheme to maximize the throughput of a set of distributed multiple-input multiple-output (MIMO) wireless links. This scheme exploits both spatial and temporal freedoms of the source covariance matrices of all MIMO links. In particular, the source covariance matrix of each MIMO link is allowed to vary within a block of time (and/or frequency) slots. This scheme, also referred to as space-time power scheduling, optimizes an integration of link scheduling and power control for MIMO links. The computational problem involved in this scheme is non-convex. However, a gradient-projection algorithm developed for this scheme consistently yields a higher capacity than all other existing schemes.

Routing in ad-hoc networks with MIMO links: Optimization considerations and protocols

Smart antennas include a broad variety of antenna technologies ranging from the simple switched beams to the sophisticated digital adaptive arrays. While beam-forming antennas are good candidates for operation in strong line of sight (LOS) environments, it is the multiple input multiple output (MIMO) technology that is best suited for multipath environments. In fact, the MIMO links exploit the multipath induced rich scattering to provide high spectral efficiencies. The focus of this work is to identify the various characteristics and tradeoffs of MIMO links that can be leveraged by routing layer protocols in rich multipath environments to improve their performance. To this end, we study the relative benefits of the different strategies of operation in MIMO links both analytically and through simulations. Using the insights gained from the study, we propose a routing protocol called MIR for ad-hoc networks with MIMO links, that optimally leverages the various characteristics of MIMO links in its mechanisms to improve the network performance. We show the effectiveness of the components of the proposed protocol by evaluating its performance through ns2 simulations for a variety of network conditions. Results indicate that MIR is able to provide significant performance gains of about 100% over baseline approaches.

Space-Time Power Scheduling of MIMO Links—Fairness and QoS Considerations

Power control, beamforming and link scheduling are all important operations to improve the power-and-spectral efficiency of networks of multiple-input multiple-output (MIMO) wireless links. We call a joint optimization of the above operations the space-time power scheduling (STPS) scheme. The STPS scheme is formulated as joint optimization of the transmitter covariance matrices of all active MIMO links over all dimensions of space and time, which includes the dimension of frequency as a dual form of time. In this paper, we address the proportional fair (PF) and quality-of-service (QoS) issues of the STPS scheme, which are important for networks with asymmetric topology and/or asymmetric traffic demands. Both slow fading channels and fast fading channels are considered. We demonstrate that the PF-STPS scheme provides a very attractive tradeoff between sum capacity and rate distribution for asymmetric links. We also demonstrate that the QoS-STPS scheme has a much higher power-and-spectral efficiency than the previously existing QoS based scheme that do not exploit the temporal freedom. Efficient optimization algorithms for both PF-STPS and QoS-STPS are provided. The STPS scheme is a centralized cooperative scheme which requires a scheduler. For ad hoc networks, this scheduler can be elected adaptively among eligible nodes in the network.

Joint bandwidth allocation, element assignment and scheduling for wireless mesh networks with MIMO links

With the unique features of spatial multiplexing and interference suppression, Multiple Input Multiple Output (MIMO) techniques have great potential in the improvement of network capacity over conventional antenna technologies. In order to exploit the benefit of simultaneous transmissions provided by MIMO, researchers have proposed a number of cross-layer optimizations and MAC layer designs to increase the throughput of wireless mesh or ad hoc networks, where the number of elements in the antenna arrays are pre-allocated or evenly assigned to the routers. In this paper, we argue that using the same number of elements in each antenna array in all routers is not a necessary condition for the improvement of system performance. This is because the requirement for the number of elements is quite different for each router. Especially at those critical routers that have huge aggregate traffic toward the gateway, more elements are needed not only for the traffic relay but also for the interference suppression. Based on this observation, we define the joint problem of bandwidth allocation, element assignment and scheduling to characterize the throughput benefits of cross-layer optimizations. We propose a Cost-Aware Element Assignment (CAEA) technique to minimize the total number of the antenna elements when still achieving the optimal bandwidth allocation. In addition, to verify the efficiency of the CAEA assignment, a heuristic Traffic-aware Stream-controlled Link Scheduling (TSLS) algorithm is proposed to provide a schedulable bandwidth allocation. We demonstrate through extensive simulations that our solutions (CAEA, TSLS) not only effectively save the total cost on antenna elements but also perform close to optimal on the average.

Space–Time Power Schedule for Distributed MIMO Links Without Instantaneous Channel State Information at the Transmitting Nodes

A space-time optimal power schedule for multiple distributed multiple-input multiple-output (MIMO) links without the knowledge of the instantaneous channel state information (CSI) at the transmitting nodes is proposed. A readily computable expression for the ergodic sum capacity of the MIMO links is derived. Based on this expression, which is a non-convex function of power allocation vectors, a projected gradient algorithm is developed to optimize the power allocation. For a symmetric set of MIMO links with independent identically distributed channels, it is observed that the space-time optimal power schedule reduces to a uniform isotropic power schedule when nominal interference is low, or to an orthogonal isotropic power schedule when nominal interference is high. Furthermore, the transition region between the latter two schedules is seen to be very sharp in terms of nominal interference-to-noise ratio (INR). For MIMO links with correlated channels, the corresponding space-time optimal power schedule is developed based on the knowledge of the channel correlation matrices. It is shown that the channel correlation has a great impact on the ergodic capacity and the optimality of different power scheduling approaches.

MIMO Link Layer Transmission Techniques Based on Cross Layer Design

In this paper, we propose and evaluate the performance of new link layer frame transmission techniques for MIMO (multiple input multiple output) wireless systems from a link layer perspective using frame error rate (FER), frame loss rate (FLR) and link layer throughput metrics. We first introduce a novel link layer transmission scheme for wireless networks when channel state information (CSI) is available at the transmitter (CSIT). This new scheme is based on an adaptive algorithm diagonal precoder which selects the proper orthogonal sub-channels of the MIMO link to use in order to increase the link layer throughput while maintaining an acceptable frame loss rate. Computer simulations show that the proposed MIMO-aware link layer algorithm provides a significant performance improvement. Second, we propose and evaluate the performance of different ARQ (automatic repeat request) retransmission strategies. Our performance evaluation shows that by decreasing the multiplexing gain and therefore increasing the diversity gain for retransmitted frames, the frame loss rate decreases dramatically while the link layer throughput is only marginally affected.

A Differential Modulation Scheme for MIMO Links with Distributed Transmit Antennas

This letter proposes a differential en/decoding scheme for multiple-input multiple-output (MIMO) communication links with distributed transmit antennas, in which neither the transmitter nor the receiver knows channel knowledge. The proposed scheme is based on the layered differential space-time codes we present. Simulation results show that the bit error rate (BER) performance is nonidentical in different channel propagation delay cases. The optimum delay time for two transmit antennas system, for example, is about 0.6Ts (where Ts denotes symbol period) when the links achieve better performance.

Optimized Power Allocation for Multiantenna Systems Impaired by Multiple Access Interference and Imperfect Channel Estimation

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper presents an optimized spatial signal shaping for multiple-input–multiple-output (MIMO) “<emphasis emphasistype="boldital">ad hoc</emphasis>”-like networks. It is adopted for maximizing the information throughput of pilot-based multiantenna systems that are affected by spatially colored <emphasis emphasistype="boldital">multiple access interference</emphasis> (MAI) and channel estimation <emphasis emphasistype="boldital">errors</emphasis>. After deriving the architecture of the minimum mean square error (MMSE) MIMO channel estimator, closed-form expressions for the maximum information throughput that is sustained by the MAI-affected MIMO links are provided. Then, we present a novel power allocation algorithm for achieving the resulting link capacity. Several numerical results are provided to compare the performance achieved by the proposed power-allocation algorithm with that of the corresponding MIMO system that works in <emphasis emphasistype="boldital">MAI-free</emphasis> environments and is equipped with <emphasis emphasistype="boldital">error free</emphasis> (e.g., <emphasis emphasistype="boldital">perfect</emphasis>) channel estimates. By so doing, we are able to give an insight on the ultimate performance loss that is induced in MIMO systems by spatially colored MAI and imperfect channel estimates. Finally, we point out some implications about space-division multiple access strategies arising from the proposed power allocation algorithm. </para>

Broadcast scheduling with MIMO links in multi-hop ad hoc networks

As the current medium access control protocols with Multiple Input Multiple Output (MIMO) links only bear point to point service, broadcast scheduling algorithm in ad hoc networks with MIMO links is proposed. The key to the proposed broadcast scheduling algorithm is the time slot scheduling algorithm which guarantees collision-free transmissions for every node and the minimum frame length. The proposed algorithm increases the simultaneous transmissions of MIMO links efficiently. Due to the interference null capacity of MIMO links, the interference node set of each node can decrease from two-hop neighbors to one-hop neighbors possibly. Simulation results show that our algorithm can greatly improve network capacity and decrease average packet delay.

Simultaneous 5 GHz co-channel multiple-input multiple-output links at microcellular boundaries: interference or cooperation?

Simultaneous real-time 4 x 4 multiple-input multiple-output measurements from three base stations to the same mobile are analysed. Such measurements can be applied to determine the real-time scale of interference from neighbouring cells due to lack of orthogonality between their respective eigenvectors. Also, new and existing schemes are compared that can be applied in equally valid ways to suppress or take advantage of the unwanted base station links at cellular boundaries; thus it opens up the possibility of either interference or cooperation on a microcellular boundary.

MMSE transmitter design for correlated MIMO systems with imperfect channel estimates: power allocation trade-offs

We investigate the transmit precoder design problem for a multiple input multiple output (MIMO) link with correlated receive antennas, considering the effect of channel estimation. We work with the total mean squared error (MSE) as the performance measure, and develop transceiver structures considering the effect of channel estimation and the correlation of the MIMO link. The proposed transceiver structures are optimum in the sense of minimizing the total MSE and distributing the total MSE equally among the parallel data streams. Motivated by the substantial effect the channel estimation process can have on the system performance, we next investigate the problem of how the correlated MIMO link should distribute its total available power between power expended for channel estimation versus data transmission. The optimum power allocation problem between the training sequences for channel estimation and data transmission for the correlated MIMO link is shown to have a unique solution, that is different than the uncorrelated case. It is observed that the proposed transceiver achieves near-minimum MSE values via a relatively wide range of power allocation parameters. This is in contrast to the transceiver that is oblivious to the estimation errors when a more precise power allocation strategy is needed to achieve the best performance. Our results demonstrate that the correlation structure of the MEMO link has a profound effect on the performance, and that the transceiver optimization should be done by taking both the correlation and the channel estimation process into account

Capacity scaling laws in MIMO relay networks

The use of multiple antennas at both ends of a wireless link, popularly known as multiple-input multiple-output (MIMO) wireless, has been shown to offer significant improvements in spectral efficiency and link reliability through spatial multiplexing and space-time coding, respectively. This paper demonstrates that similar performance gains can be obtained in wireless relay networks employing terminals with MIMO capability. We consider a setup where a designated source terminal communicates with a designated destination terminal, both equipped with M antennas, assisted by K single-antenna or multiple-antenna relay terminals using a half-duplex protocol. Assuming perfect channel state information (CSI) at the destination and the relay terminals and no CSI at the source, we show that the corresponding network capacity scales as C = (M/2) log(K) + O(1) for fixed M, arbitrary (but fixed) number of (transmit and receive) antennas N at each of the relay terminals, and K rarr infin. We propose a protocol that assigns each relay terminal to one of the multiplexed data streams forwarded in a "doubly coherent" fashion (through matched filtering) to the destination terminal. It is shown that this protocol achieves the cut-set upper bound on network capacity for fixed M and K rarr infin (up to an O(1)-term) by employing independent stream decoding at the destination terminal. Our protocol performs inter-stream interference cancellation in a completely decentralized fashion, thereby orthogonalizing the effective MIMO channel between source and destination terminals. Finally, we discuss the case where the relay terminals do not have CSI and show that simple amplify-and-forward relaying, asymptotically in K, for fixed M and fixed N ges 1, turns the relay network into a point-to-point MIMO link with high-SNR capacity C = (M/2) log(SNR) + O(1), demonstrating that the use of relays as active scatterers can recover spatial multiplexing gain in poor scattering environments