On the MIMO Interference Channel with Reconfigurable Antennas and Partial CSIT

Recently, a technique entitled “Blind Interference Alignment (BIA)” was proposed, which allows interference to be aligned within a reduced subspace without knowledge of the channel state information at the transmitter. The key to realizing the BIA scheme is the use of a receive antenna that is capable of switching among multiple channel-states (with channel diversity) in a pre-determined way. In this paper, we exploit the ESPAR antenna, which uses only a single radio frequency chain as the receiver to provide the necessary beampattern switching for the BIA technique. Moreover, in the proposed ESPAR based BIA scheme, beam-steering of each ESPAR receiver is designed according to its position relative to the transmitter to increase the received SNR. Furthermore, we study the proposed BIA scheme by applying the ESPAR antennas to a 2-D cellular setting in order to illustrate the ESPAR beam pattern design and associated improvements in the performance of the BIA scheme. In order to keep the number of coordinating BSs at a reasonable level, we study the case where only three adjacent BSs operate the BIA scheme coordinatively. This is found to be reasonable since the other inter-cell interference is not significant due to the ESPAR beam-steering and the longer propagation distance. The simulation shows that our scheme provides comparable sum rate to that of BIA scheme operated in all cells.

Visible light communications (VLC) have been proposed as an alternative to the radio-frequency (RF) communications because of the huge unlicensed bandwidth available and a low cost implementation. In this context, one of the most interesting applications of VLC is their use in vehicular communications using the headlights of trucks, cars or motorbikes. However, the vehicular communications are subject to blocking from other elements of the network or weather effects such as rain or fog. To solve these issues we propose the use of multiphotodiode receivers following an angle diversity distribution. In particular, in this work we focus on a configuration that considers a single signal processing chain connected to the set of photodiodes through a selector referred to as reconfigurable photodetector. First, we obtain the experimental evaluation of the optical channel considering high-power optical transmitters and several angle diversity configurations. After that, an offline evaluation of the blind interference alignment (BIA) scheme is carried out in comparison with a diversity scheme such as maximum ratio combining (MRC).

We derive the degrees of freedom (DoF) for K-user single-input single-output interference channels with a reconfigurable antenna at receivers, assuming linear coding strategies without channel state information at transmitters. Our DoF converse builds upon the inherent feature that channel state's changing patterns of the links towards the same receiver are always identical. We prove the linear sum DoF is upper-bounded by n* K/K+n*(n*-1), n* where n* is the optimal number of preset modes among N. Moreover, we propose a linear scheme that achieves the upper-bound if K/n* is an integer. For the other cases, we give a guideline for the achievable scheme.

In recent years, several experimental studies have come out to validate the theoretical findings of interference alignment (IA), but only a handful of studies have focused on blind interference alignment. Unlike IA and other interference mitigation techniques, blind IA does not require channel state information at the transmitter (CSIT). The key insight is that the transmitter uses the knowledge of channel coherence intervals and receivers utilize reconfigurable antennas to create channel fluctuations exploited by the transmitter. In this work, we present a novel experimental evaluation of a reconfigurable antenna system for achieving blind IA. We present a blind IA technique based on reconfigurable antennas for a 2-user multipleinput single-output (MISO) broadcast channel implemented on a software defined radio platform where each of the receivers is equipped with a reconfigurable antenna. We further compare this blind IA implementation with traditional TDMA scheme for benchmarking purposes. We show that the achievable rates for blind IA can be realized in practice using measured channels under practical channel conditions. Additionally, the average error vector magnitude and bit error rate (BER) performances are evaluated.

A novel interference alignment (IA) scheme based on the errors-in-variables (EIV) mathematic model has been proposed to overcome the channel state information (CSI) estimation error for the MIMO interference channels. By solving an equivalently unconstrained optimization problem, the proposed IA scheme employing a weighted total least squares (WTLS) algorithm can obtain the solution to a constrained optimization problem for transmit precoding (TPC) matrices and minimizes the distortion caused by imperfect CSI according to the EIV model. It is shown that the design of TPC matrices can be realized through an efficient iterative algorithm. The convergence of the proposed scheme is presented as well. Simulation results show that the proposed IA scheme is robust over MIMO interference channels with imperfect CSI, which yields significantly better sum rate performance than the existing IA schemes such as distributed iterative IA, maximum signal-to-interference-plus-noise ratio (Max SINR), and minimum mean square error (MMSE) schemes.

This paper studies the blind interference alignment (BIA) of a heterogeneous 3-user 2x1 broadcast channel (BC). In the BC considered, the transmitter equipped with two antennas broadcasts information symbols to three receivers with each possessing a unique reconfigurable antenna. By heterogeneous it means that the three 2x1 links connecting the transmitter and the three receivers experience independent block fading and different coherence times. To meet the requirement of BIA on the 3-user 2x1 BC, the heterogeneous channel structure could be simply transformed into a homogeneous 3-user broadcast channel by applying reconfigurable antennas to perform mode switching. However, the DoF gain achieved by BIA should compensate the expense on frequent mode switching. To this end, an utility measure is established, and an optimization problem maximizing the achievable utility value is modeled. The solution of the optimization problem is derived, and the impact of the three coherence times on the optimal solution is investigated.

We investigate the secure degrees of freedom (SDoF) for the two-way 2×2×2 MIMO interference channel (IC) under three wiretap models, i.e., the confidential messages (CM) model, the untrusted relays (UR) model, and the combined CM and UR (CM-UR) model. For the general case of arbitrary antenna configuration under each wiretap model, we derive the upper bound on SDoF with Markov chain and secrecy constraints, and obtain the achievability schemes with designed interference neutralization, cooperative jamming, and interference alignment schemes. To gain insight on these bounds, we further consider the special case where each user node has M antennas and each relay node has N antennas, and highlight the modification process when achieving the maximum SDoF. For such special case, we show that the optimum SDoF of the CM model is achieved in the regimes M ≥ N and M <; (N/2); and for the UR model and CM-UR model, the optimum SDoF is achieved in the regimes N ≤ (M/2), N > 2M, and N = M.

We consider the K-user MISO interference channel where each transmitter has multiple antennas and each receiver has a reconfigurable antenna capable of switching among several pre-set modes. The channels in different modes are assumed to be independent. We propose an interference alignment scheme without channel information at transmitters. We show that the achievable degree of freedom of the proposed scheme outperforms that in the traditional K-user MISO interference channel without the reconfigurable antenna.

Massive MIMO has been proposed as a means of achieving huge spectral efficiency in cellular networks. However, its implementation is usually based on linear precoding schemes using Channel State Information (CSI). For this reason, the use of the Time Division Duplex (TDD) mode is typically assumed since it removes the dependence on the number of antennas at the transmitter to acquire CSI. Unfortunately, most of the cellular network operate in the Frequency Division Duplex (FDD) mode in which the acquisition of CSI depends proportionally to the number of transmit antennas. For this reason, the implementation of Massive MIMO is handicapped to few transmit antennas for the current cellular networks. In this paper we study the use of reconfigurable antennas that can switch among a set of preset modes, i.e., radiation patterns, through Blind Interference Alignment schemes. We show that it is a solution to exploit a very large number of antennas at the transmit side, overcoming the bottleneck that today constitutes the acquisition of CSI in Massive MIMO systems operating in FDD mode.

We show that the three-user M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> × M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> Multiple-Input Multiple-Output (MIMO) interference channel where each transmitter is equipped with M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> antennas and each receiver is equipped with M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> antennas has (M) d(M, N) = <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Δ</sup> min (M/2-1/κ, N/2+1/κ) degrees of freedom (DoF) normalized by time, frequency, and space dimensions, where = <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Δ</sup> min(M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> , M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> ), N = <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Δ</sup> max(M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> , M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> ), κ = <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Δ</sup> ⌈M/N-M⌉. While the DoF outer bound of d(M, N) is established for every M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> , M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> value, the achievability of d(M, N) DoF is established in general subject to a normalization with respect to spatial extensions, i.e., the scaling of the number of antennas at all nodes. In particular, we show that qd(M, N) DoF are achievable for the three-user qMT × qMR MIMO interference channel, for some positive integer q, which may be seen as a spatial extension factor. q is the scaling factor needed to make the value qd(M, N) an integer. Given spatial extensions, the achievability relies only on linear beamforming based interference alignment schemes and requires neither channel extensions nor channel variations in time or frequency. In the absence of spatial extensions, it is shown through examples how essentially the same interference alignment scheme may be applied over time-extensions over either constant or time-varying channels. The central new insight to emerge from this paper is the notion of subspace alignment chains as the DoF bottlenecks. The subspace alignment chains are instrumental both in identifying the extra dimensions to be provided by a genie to a receiver for the DoF outer bound, as well as in the construction of the optimal interference alignment schemes. The DoF value d(M, N) is a piecewise linear function of M, N, with either M or N being the bottleneck within each linear segment, whereas the other value contains some redundancy, i.e., it can be reduced without reducing the DoF. The corner points of these piecewise linear segments correspond to two sets, A={1/2, 2/3, 3/4, ...}and B={1/3, 3/5, 5/7, ...}. The set A contains all those values of M/N and only those values of M/N for which there is redundancy in both M and N, i.e., either can be reduced without reducing the DoF. The set B contains all those values of M/N and only those values of M/N for which there is no redundancy in either M or N, i.e., neither can be reduced without reducing the DoF. Because A and B represent settings with maximum and minimum redundancy, essentially they are the basis for the DoF outer bounds and inner bounds, respectively. Our results settle the question of feasibility of linear interference alignment, introduced previously by Cenk et al., for the three-user M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> × M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> MIMO interference channel, completely for all values of M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> , M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> . In particular, we show that the linear interference alignment problem (M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> × M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> , d) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> (as defined in previous paper by Cenk et al.) is feasible if and only if d ≤ d(M, N). With the exception of the values M/N ∈ B, and only with that exception, we show that for every M/N value there are proper systems (as defined by Cenk et al.) that are not feasible. Evidently the redundancy contained in all other values of M/N manifests itself as superfluous variables that are not discounted in the definition of proper systems, thus creating a discrepancy between proper and feasible systems. Our results show that M/N ∈ A are the only values for which there is no DoF benefit of joint processing among co-located antennas at the transmitters or receivers. This may also be seen as a consequence of the maximum redundancy in the M/N ∈ A settings.

We consider a downlink heterogeneous network (HetNet), where several picocells coexist in the same spectrum as a macrocell. For a general framework of the multiple-input multiple-output (MIMO) network consisting of arbitrary number of picocells and macro users within the macrocell, designing a closed-form solution to achieve the degrees of freedom (DoF) of (C + K)d while guaranteeing the minimum overall antenna usage is an open problem, where C, K and d are the number of picocells, marco users within the macro cell and data streams per user, respectively. Therefore, by relaxing the minimum antenna utilization constraint, we present a zero-forcing based transceiver design at first, which outperforms the existing closed-form solution based on interference alignment (IA) in terms of antenna usage in most cases. That is to say, alignment processing is not necessary in most cases for the existing scheme because simple zero-forcing scheme with efficient antenna utilization can also attain the same DoF. Besides, we propose two IA schemes without any iterative computations, which have the same antenna configuration but different CSI feedback overhead. Also, our proposed IA schemes require lower usage of antennas compared to the zero-forcing scheme and existing IA scheme.

The explosive demand for high data rates and the need ubiquitous wireless connectivity has led to the phenomenon of network densification, the deployment of large number of base station within a geographic area. Since the radio spectrum is limited, wireless transmissions need to share a common resource which results in interference. Interference in turn creates a bottleneck on the communication rate. Recently three breakthroughs have been made to increase the capacity of wireless networks while mitigating interference : multiple input multiple output (MIMO) beamforming, interference alignment (IA) and pattern reconfigurable antennas (PRA). While a great deal of progress has been made on understanding each individually, relatively little is known about how to use these techniques in combination to increase system performance. In this dissertation, we analyze and experimentally evaluate how these new technologies can be leveraged to enhance the rate performance of communication networks. Specifically, we address three problems: i) enhancing multi-user MIMO performance with PRA, ii) improving IA performance with PRA-based antenna selection, and iii) enabling practical blind IA with PRA. To understand the impact PRAs on MU-MIMO and IA, it is necessary to evaluate their performance through measurements. Simulation-based studies often reiterate over set of simplistic channel models and unrealistic assumptions. This dissertation focuses on a measurement-based evaluation approach. For each of these problems, a hardware testbed implementation and measurement methodology is developed for evaluating the performance of the aforementioned technologies through efficient and repeatable experiments. The first two major contributions of this dissertation focus on multi-user MIMO (MU-MIMO) and IA. While the theoretical rate gains of MU-MIMO and IA are substantial, the performance gains in practical systems and measured channels have been limited. One of the key limiting factors are spatially correlated user channels. In this work, we propose MU-MIMO and IA transmission schemes that leverage the pattern diversity provided by PRAs to mitigate the effects of spatial correlation in wireless channels. PRAs are capable of dynamically adjusting their radiation pattern, providing an additional degree of freedom that can be exploited to improve MU-MIMO and IA performance by treating the array configuration and radiation characteristics as additional components in the joint optimization of adaptive system parameters. We quantify the benefits of pattern diversity on the rate performance of MU-MIMO and IA systems through extensive measurements. Furthermore, we develop efficient antenna mode selection algorithms to realize those benefits without increasing the system complexity or overhead, making these techniques suitable for practical implementation. The third contribution of this dissertation focuses on a transmission technique known as blind IA. In contrast to MU-MIMO and conventional IA, blind IA is signaling scheme that suppresses interference in multi-user systems, without the knowledge of channel state information at the transmitter (CSIT). The key to performing IA without CSIT is the use of PRAs that are capable of dynamically switching among a fixed number of radiation patterns to introduce artificial fluctuations in the channel. In this dissertation, we develop a novel implementation of a blind IA system on a software defined radio platform where each of the receivers is equipped with a PRA. Our work provides the first experimental evaluation of PRA-based blind IA schemes and demonstrates significant gains in rate and reliability over traditional orthogonal transmission schemes such as TDMA.

In this paper, we propose a novel iterative interference alignment (IA) scheme for the single-input single-output (SISO) interference channel system using minimum total mean square error criterion under the individual transmitter power constraints. We show that interference alignment under such criterion could be realized through an iterative algorithm. The convergence of the proposed algorithm is discussed. Simulation results, compared with several existing IA schemes, show that the proposed scheme can effectively improve the BER performance of the SISO interference channel system while maintaining the same degree of freedom as Cadambe-Jafar scheme.

This paper proposes an Interference Alignment (IA) scheme in two-user downlink frequency selective broadcasting channels, referred to as semi-blind interference alignment (semi-BIA). Based on the frequency selectivity structure, interference can be aligned at users without precise channel coefficients at the Base Station (BS) nor reconfigurable antennas at users. Compared with traditional interference control schemes, for example, Frequency Division Multiple Access (FDMA), the proposed scheme can obtain more degrees of freedom (DoF), which means the overall throughput is improved. Besides, this semi-BIA scheme can also apply to X channels with two single-antenna transmitters and two single-antenna receivers. Simulation results demonstrate that the proposed scheme performs well in terms of achievable data rate and bit error rate (BER) evaluation.

In this paper, we investigate the effect of feedback on two-user MIMO interference channels. At first, the capacity region of MIMO interference channels with feedback is characterized within a constant number of bits, where this constant is independent of the channel matrices. Further, the generalized degrees of freedom region for the MIMO interference channel with feedback is characterized.