Multiuser Millimeter Wave Beamforming Strategies with Quantized and Statistical CSIT

To alleviate the high cost of hardware in mmWave systems, hybrid analog/digital precoding is typically employed. In the conventional two-stage feedback scheme, the analog beamformer is determined by beam search and feedback to maximize the desired signal power of each user. The digital precoder is designed based on quantization and feedback of effective channel to mitigate multiuser interference. Alternatively, we propose a one-stage feedback scheme which effectively reduces the complexity of the signalling and feedback procedure. Specifically, the second-order channel statistics are leveraged to design digital precoder for interference mitigation while all feedback overhead is reserved for precise analog beamforming. Under a fixed total feedback constraint, we investigate the conditions under which the one-stage feedback scheme outperforms the conventional two-stage counterpart. Moreover, a rate splitting (RS) transmission strategy is introduced to further tackle the multiuser interference and enhance the rate performance. Consider (1) RS precoded by the one-stage feedback scheme and (2) conventional transmission strategy precoded by the two-stage scheme with the same first-stage feedback as (1) and also certain amount of extra second-stage feedback. We show that (1) can achieve a sum rate comparable to that of (2). Hence, RS enables remarkable saving in the second-stage training and feedback overhead.Comment: submitted to TW

Joint Wireless Information and Energy Transfer with Reduced Feedback in MIMO Interference Channels

To determine the transmission strategy for joint wireless information and energy transfer (JWIET) in the MIMO interference channel (IFC), the information access point (IAP) and energy access point (EAP) require the channel state information (CSI) of their associated links to both the information-decoding (ID) mobile stations (MSs) and energy-harvesting (EH) MSs (so-called local CSI). In this paper, to reduce th e feedback overhead of MSs for the JWIET in two-user MIMO IFC, we propose a Geodesic energy beamforming scheme that requires partial CSI at the EAP. Furthermore, in the two-user MIMO IFC, it is proved that the Geodesic energy beamforming is the optimal strategy. By adding a rank-one constraint on the transmit signal covariance of IAP, we can further reduce the feedback overhead to IAP by exploiting Geodesic information beamforming. Under the rank-one constraint of IAP's transmit signal, we prove that Geodesic information/energy beamforming approach is the optimal strategy for JWIET in the two-user MIMO IFC. We also discuss the extension of the proposed rank-one Geodesic information/energy beamforming strategies to general K-user MIMO IFC. Finally, by analyzing the achievable rate-energy performance statistically under imperfect partial CSIT, we propose an adaptive bit allocation strategy for both EH MS and ID MS.Comment: accepted to IEEE Journal of Selected Areas in Communications (IEEE JSAC), Special Issue on Wireless Communications Powered by Energy Harvesting and Wireless Energy Transfe

A virtual MIMO dual-hop architecture based on hybrid spatial modulation

International audienceIn this paper, we propose a novel Virtual Multiple-Input-Multiple-Output (VMIMO) architecture based on the concept of Spatial Modulation (SM). Using a dual-hop and Decode-and-Forward protocol, we form a distributed system, called Dual-Hop Hybrid SM (DH-HSM). DH-HSM conveys information from a Source Node (SN) to a Destination Node (DN) via multiple Relay Nodes (RNs). The spatial position of the RNs is exploited for transferring information in addition to, or even without, a conventional symbol. In order to increase the performance of our architecture, while keeping the complexity of the RNs and DN low, we employ linear precoding using Channel State Information (CSI) at the SN. In this way, we form a Receive-Spatial Modulation (R-SM) pattern from the SN to the RNs, which is able to employ a centralized coordinated or a distributed uncoordinated detection algorithm at the RNs. In addition, we focus on the SN and propose two regularized linear precoding methods that employ realistic Imperfect Channel State Information at the Transmitter. The power of each precoder is analyzed theoretically. Using the Bit Error Rate (BER) metric, we evaluate our architecture against the following benchmark systems: 1) single relay; 2) best relay selection; 3) distributed Space Time Block Coding (STBC) VMIMO scheme; and 4) the direct communication link. We show that DH-HSM is able to achieve significant Signal-to-Noise Ratio (SNR) gains, which can be as high as 10.5 dB for a very large scale system setup. In order to verify our simulation results, we provide an analytical framework for the evaluation of the Average Bit Error Probability (ABEP)

Space-Time Encoded MISO Broadcast Channel with Outdated CSIT: An Error Rate and Diversity Performance Analysis

Studies of the MISO Broadcast Channel (BC) with delayed Channel State Information at the Transmitter (CSIT) have so far focused on the sum-rate and Degrees-of-Freedom (DoF) region analysis. In this paper, we investigate for the first time the error rate performance at finite SNR and the diversity-multiplexing tradeoff (DMT) at infinite SNR of a space-time encoded transmission over a two-user MISO BC with delayed CSIT. We consider the so-called MAT protocol obtained by Maddah-Ali and Tse, which was shown to provide 33% DoF enhancement over TDMA. While the asymptotic DMT analysis shows that MAT is always preferable to TDMA, the Pairwise Error Probability analysis at finite SNR shows that MAT is in fact not always a better alternative to TDMA. Benefits can be obtained over TDMA only at very high rate or once concatenated with a full-rate full-diversity space-time code. The analysis is also extended to spatially correlated channels and the influence of transmit correlation matrices and user pairing strategies on the performance are discussed. Relying on statistical CSIT, signal constellations are further optimized to improve the error rate performance of MAT and make it insensitive to user orthogonality. Finally, other transmission strategies relying on delayed CSIT are discussed

The Impact of CSI and Power Allocation on Relay Channel Capacity and Cooperation Strategies

Capacity gains from transmitter and receiver cooperation are compared in a relay network where the cooperating nodes are close together. Under quasi-static phase fading, when all nodes have equal average transmit power along with full channel state information (CSI), it is shown that transmitter cooperation outperforms receiver cooperation, whereas the opposite is true when power is optimally allocated among the cooperating nodes but only CSI at the receiver (CSIR) is available. When the nodes have equal power with CSIR only, cooperative schemes are shown to offer no capacity improvement over non-cooperation under the same network power constraint. When the system is under optimal power allocation with full CSI, the decode-and-forward transmitter cooperation rate is close to its cut-set capacity upper bound, and outperforms compress-and-forward receiver cooperation. Under fast Rayleigh fading in the high SNR regime, similar conclusions follow. Cooperative systems provide resilience to fading in channel magnitudes; however, capacity becomes more sensitive to power allocation, and the cooperating nodes need to be closer together for the decode-and-forward scheme to be capacity-achieving. Moreover, to realize capacity improvement, full CSI is necessary in transmitter cooperation, while in receiver cooperation optimal power allocation is essential.Comment: Accepted for publication in IEEE Transactions on Wireless Communication

Secrecy Wireless Information and Power Transfer in Fading Wiretap Channel

Simultaneous wireless information and power transfer (SWIPT) has recently drawn significant interests for its dual use of radio signals to provide wireless data and energy access at the same time. However, a challenging secrecy communication issue arises as the messages sent to the information receivers (IRs) may be eavesdropped by the energy receivers (ERs), which are presumed to harvest energy only from the received signals. To tackle this problem, we propose in this paper an artificial noise (AN) aided transmission scheme to facilitate the secrecy information transmission to IRs and yet meet the energy harvesting requirement for ERs, under the assumption that the AN can be cancelled at IRs but not at ERs. Specifically, the proposed scheme splits the transmit power into two parts, to send the confidential message to the IR and an AN to interfere with the ER, respectively. Under a simplified three-node wiretap channel setup, the transmit power allocations and power splitting ratios over fading channels are jointly optimized to minimize the outage probability for delay-limited secrecy information transmission, or to maximize the average rate for no-delay-limited secrecy information transmission, subject to a combination of average and peak power constraints at the transmitter as well as an average energy harvesting constraint at the ER. Both the secrecy outage probability minimization and average rate maximization problems are shown to be non-convex, for each of which we propose the optimal solution based on the dual decomposition as well as suboptimal solution based on the alternating optimization. Furthermore, two benchmark schemes are introduced for comparison. Finally, the performances of proposed schemes are evaluated by simulations in terms of various trade-offs for wireless (secrecy) information versus energy transmissions.Comment: to appear in IEEE Transactions on Vehicular Technolog

Quality of service optimization in the Broadcast Channel with Imperfect transmit channel state information

[Resumen]Este trabajo considera un sistema Broadcast Channel (BC) que consiste en un transmisor equipado con múltiples antenas y varios usuarios con una o más antenas. Dependiendo del número de antenas en el lado receptor, tales sistemas son conocidos como Multiple-User Multiple-Input Single-Output (MU-MISO), para usuarios con una única antena, o Multiple-User Multiple-Input Multiple-Output (MU-MIMO), para usuarios con varias antenas. Este modelo es adecuado para sistemas actuales de comunicaciones inalámbricas. Respecto a la dirección del flujo de datos, diferenciamos entre el canal downlink o BC, y canal uplink o Multiple Access Channel (MAC). En el BC las señales se envían desde la estación base a los usuarios, mientras que la información perteneciente a los usuarios es transmitida a la estación base en el MAC. En este trabajo nos centramos en el BC donde la estación base aplica precodificación lineal aprovechando las múltiples antenas. La información sobre el estado del canal se asume perfecta en todos los usuarios. Sin embargo, los usuarios no cooperan, y la estación base solo tiene información de canal parcial obtenida a través de un canal de realimentación en los sistemas Frequency-Division Duplex (FDD), que tiene un ancho de banda limitado. Esta limitación fuerza a los usuarios a aplicar algunos métodos, como quantización, para reducir la cantidad de datos a enviar a la estación base. La combinación de la información proporcionada por los usuarios es interpretada en la estación base como información de canal estocástica, y constituye un factor crítico en el diseño de los precodificadores. En la literatura se han considerado varios métodos para evaluar el rendimiento del BC, a saber, Signal to Interference-plus-Noise Ratio (SINR), Minimum Mean Square Error (MMSE), y tasa. Algunos trabajos calculan las medidas correspondientes para cada usuario mientras que otros consideran la suma de todos ellos como la métrica de interés. En nuestro caso, nos centramos en la tasa como figura de mérito. En particular, estamos interesados en garantizar ciertas tasas por usuario. De esta manera, evitamos situaciones injustas que surgen de utilizar la tasa suma como criterio, en las que a los usuarios con canales pobres se les asignan tasas bajas, o incluso cero. Además, reducir la cantidad de potencia necesaria para satisfacer las restricciones de calidad de servicio mencionadas es una característica deseable en los sistemas de comunicaciones inalámbricas. Así, abordamos el problema de optimización consistente en minimizar la potencia total en el transmisor empleada para cumplir un conjunto de restricciones de calidad de servicio, expresadas como tasas por usuario. Durante los últimos años el problema de minimización de potencia ha sido estudiado ampliamente para información tanto perfecta como imperfecta de canal, en los escenarios BC. Asumir conocimiento de canal perfecto es poco realista y, por tanto, consideramos que los usuarios env´ıan la información de canal a la estación base por medio de un canal de realimentación, normalmente disponible en los est´andares de comunicación recientes. Aunque algunos autores han empleado modelos de incertidumbre limitada para el conocimiento de canal tales como rectangular, elipsoidal, o esférico, y han aprovechado esa asunción para resolver el problema de minimización de potencia, no asumimos una forma particular para esa incertidumbre sino un modelo de error estocástico. En el modelo de sistema considerado, MU-MIMO, el número de antenas en la estación base es mayor que el número de antenas en cada usuario, e.g. MU-MISO. Además, los usuarios no cooperan para separar las señales recibidas. Debido a ésto y a la falta de grados de libertad en los usuarios, es necesario el uso de filtros transmisores, también llamados precodificadores, para eliminar las interferencias entre usuarios. De este modo, en este trabajo diseñamos conjuntamente los precodificadores lineales y los filtros receptores minimizando la potencia total en el transmisor sujeta a restricciones de tasa por usuario. Esta formulación del problema no es convexa y, por tanto, es complicada de manejar. Por este motivo, aplicamos la desigualdad de Jensen a las restricciones de tasa para obtener otras basadas en el MMSE. Como consecuencia, nuestro objetivo es diseñar los precodificadores y filtros que minimizan el MMSE para todos los usuarios. Para ello, distintos tipos de dualidades basadas en SINR,Mean Square Error (MSE), o tasa, han sido empleadas para el diseño de los filtros como fórmulas para intercambiar entre el BC y el MAC por conveniencia. En particular, empleamos la dualidad de MSE con conocimiento de canal imperfecto. Además, para la distribución de potencias, explotamos el marco teórico de las standard Interference Function, planteado para resolver el algoritmo de control de potencia. De esta manera, proponemos un algoritmo para solucionar el problema de minimización de potencia en el BC. Para comprobar la factibilidad de las restricciones de calidad de servicio, proponemos un test que permite determinar si el algoritmo converge o no. Además, el algoritmo propuesto permite resolver el problema dual, ésto es, encontrar los objetivos de tasa balanceados correspondientes a una potencia total en el transmisor. Finalmente, algunas aplicaciones de la minimización de potencia surgen de diferentes escenarios y se resuelven por medio del algoritmo propuesto. Usando el lenguaje de programación MATLAB se simulan experimentos con el objetivo de mostrar el rendimiento de los métodos propuestos.[Resumo]Este traballo considera un sistema Broadcast Channel (BC) que consiste nun transmisor equipado con múltiples antenas e varios usuarios cunha ou máis antenas. Dependendo do número de antenas no lado receptor, tales sistemas son coñecidos como Multiple-User Multiple-Input Single-Output (MU-MISO), para usuarios cunha única antena, ou Multiple-User Multiple-Input Multiple-Output (MU-MIMO), para usuarios con varias antenas. Este modelo é adecuado para sistemas actuais de comunicacións sen fíos. Respecto á dirección do fluxo de datos, diferenciamos entre a canle downlink ou BC, e a canle uplink ou Multiple Access Channel (MAC). No BC os sinais env´ıanse dende a estación base aos usuarios, mentres que a información pertencente aos usuarios é transmitida á estación base no MAC. Neste traballo centrámonos no BC onde a estación base aplica precodificación lineal aproveitando as múltiples antenas. A información sobre o estado da canle asúmese perfecta en todos os usuarios. Por contra, os usuarios non cooperan e a estación base só ten información da canle parcial obtida a través dunha canle de realimentación nos sistemas Frequency-Division Duplex (FDD), que ten un ancho de banda limitado. Esta limitación forza aos usuarios a aplicar algúns métodos, como quantización, para reducir a cantidade de datos que se envían á estación base. A combinación da información proporcionada polos usuarios é interpretada na estación base como información da canle estocástica, e constitúe un factor crítico no deseño dos precodificadores. Na literatura consider´aronse varios métodos para avaliar o rendemento do BC, a saber, Signal to Interference-plus-Noise Ratio (SINR), Minimum Mean Square Error (MMSE), e taxa. Algúns traballos calculan as medidas correspondentes para cada usuario mentres que outros consideran a suma de todos eles como a métrica de interese. No noso caso, centrámonos na taxa como figura de mérito. En particular, estamos interesados en garantir certas taxas por usuario. Deste xeito evitamos situación inxustas que xurdan de utilizar a taxa suma como criterio, nas que aos usuarios con canles pobres se lles asignan tasas baixas, ou incluso cero. Ademais, reducir a cantidade de potencia necesaria para satisfacer as restricci ´ons de calidade de servizo mencionadas ´e unha característica desexable nos sistemas de comunicacións se fíos. Así, acometemos o problema de optimización consistente en minimizar a potencia total no transmisor empregada para cumprir un conxunto de restricións de calidade de servizo, expresadas como taxas por usuario. Durante os últimos anos o problema de minimización de potencia foi estudado amplamente para información tanto perfecta como imperfecta de canle, nos escenarios BC. Asumir coñecemento perfecto de canle é pouco realista e, por tanto, consideramos que os usuarios envían a información de canle á estación base por medio dunha canle de realimentación, normalmente dispoñible nos estándares de comunicación recentes. Aínda que algúns autores empregaron modelos de incerteza limitada para o coñecemento de canle tales como rectangular, elipsoidal, ou esférico, e aproveitaron esa asunción para solucionar o problema de minimización de potencia, non asumimos unha forma particular para esa incerteza sen´on un modelo de error estocástico. No modelo de sistema considerado, MU-MIMO, o número de antenas na estación base é maior que o número de antenas en cada usuario, e.g. MU-MISO. Ademais, os usuarios non cooperan para separar os sinais recibidos. Debido a isto e á falta de graos de liberdade nos usuarios, é preciso o uso de filtros transmisores, tamén chamados precodificadores, para eliminar as interferencias entre usuarios. Deste xeito, neste traballo deseñamos conxuntamente os precodificadores lineais e os filtros receptores minimizando a potencia total no transmisor suxeita a restriccións de taxa por usuario. Esta formulación do problema non é convexa e, por tanto, é complicada de manexar. Por este motivo, aplicamos a desigualdade de Jensen ´as restriccións de taxa para obter outras baseadas no MMSE. Como consecuencia, o noso obxectivo é deseñar os precodificadores e filtros que minimizan o MMSE para todos os usuarios. Para iso, distintos tipos de dualidades baseadas en SINR, Mean Square Error (MSE), ou taxa, foron empregadas para o deseño dos filtros coma fórmulas para intercambiar entre o BC e o MAC por conveniencia. En particular, empregamos a dualidade de MSE con coñecemento de canal imperfecto. Ademais, para a distribución de potencias, explotamos o marco teórico das standard Interference Function, formulado para resolver o algoritmo de control de potencia. Desta maneira, propomos un algoritmo para resolver o problema de minimización de potencia no BC. Para comprobar a factibilidade das restricci óns de calidade de servizo, propomos un test que permite determinar se o algoritmo converxe ou non. Ademais, o algoritmo proposto permite resolver o problema dual, ´ısto é, atopar os obxectivos de taxa balanceados correspondentes a unha potencia total no transmisor. Finalmente, algunhas aplicacións da minimización de potencia xorden de diferentes escenarios e resólvense por medio do algoritmo proposto. Usando a linguaxe de programación MATLAB simúlanse experimentos co obxectivo de mostrar o rendemento dos métodos propostos.[Abstract]This work considers a Broadcast Channel (BC) system, where the transmitter is equipped with multiple antennas and each user at the receiver side could have one or more antennas. Depending on the number of antennas at the receiver side, such a system is known as Multiple-User Multiple-Input Single-Output (MU-MISO), for single antenna users, orMultiple-UserMultiple-InputMultiple-Output (MU-MIMO), for several antenna users. This model is suitable for current wireless communication systems. Regarding the direction of the data flow, we differentiate between downlink channel or BC, and uplink channel or Multiple Access Channel (MAC). In the BC the signals are sent from the Base Station (BS) to the users, whereas the information from the users is sent to the BS in the MAC. In this work we focus on the BC where the BS applies linear precoding taking advantage of multiple antennas. The Channel State Information (CSI) is assumed to be perfectly known at each user. However, the users do not cooperate, and the BS only has partial CSI obtained via a feedback link in Frequency-Division Duplex (FDD) systems, which is bandwidth limited. This limitation forces the users to apply some methods, like quantization, to reduce the amount of data to be sent to the BS. The combination of the information provided by the users is interpreted as stochastic CSI at the BS, so that the partial CSI is critical for the design of the precoders. Several criteria have been considered to evaluate the BC performance in the literature, namely, Signal to Interference-plus-Noise Ratio (SINR), Minimum Mean Square Error (MMSE), and rate. While some works compute the corresponding metric for each of the users, others consider the sum of all of them as the value of interest. In our case, we concentrate on rate as figure of merit. In particular, we are interested in guarantying certain per-user rates. That way, we avoid unfair situations of the sum rate criterion arising when the channels for some of the users are poor with assigned low, even zero, rates. Moreover, reducing the amount of power required to fulfill the mentioned Qualityof- Service (QoS) restrictions is a desirable feature for a wireless communication system. Thus, we address the optimization problem consisting on minimizing the total transmit power employed at the BS to fulfill a set of given QoS constraints, expressed as per-user rates. The power minimization problem has been widely studied during the last years for both perfect and imperfect CSI at the BS scenarios. The assumption of perfect CSI is rather unrealistic so that, as we mentioned previously, we consider that the users send the channel information to the BS by means of the feedback channel, usually available in recent wireless communication standards. Although some authors have employed XIV bounded uncertainty models for the CSI such as rectangular, ellipsoidal, or spherical, and have taken advantage of that assumption to solve the power minimization problem, we do not assume a particular shape for that uncertainty, but is modeled as a stochastic error. In the considered MU-MIMO system model the number of antennas at the BS is larger than the number of antennas at each user, e.g. MU-MISO. Moreover, the users do not cooperate to separate the received signals. Due to that and to the lack of degrees of freedom at the users, it makes necessary the use of transmit filters, also denoted as precoders, to remove inter-user interference. Thus, in this work we jointly design the linear precoders and receive filters minimizing the total transmit power subject to per-user rate constraints. This problem formulation is non-convex. As a consequence, it is difficult to deal with. For such a reason, we apply the Jensen’s inequality to the rate constraints to obtain a MMSE based restrictions. Consequently, our aim is to find the precoders and the filters that minimize the MMSE for all the users. To that end, several types of dualities based on SINR, Mean Square Error (MSE), or rate have been employed for the design of the filters as conversion formulas that allow to switch between the BC and the MAC for convenience. We employ the MSE BC/MAC duality for imperfect Channel State Information at the Transmitter (CSIT). Furthermore, for the power allocation design, we take advantage of the standard Interference Function (IF) framework, proposed to solve the power control algorithm. In such a way, an algorithm is proposed to solve the power minimization problem in the BC. To check the feasibility of the QoS constraints, we propose a test that allows to determine the convergence of the algorithm. Additionally, the proposed algorithm can be employed to solve the dual problem, i.e., find the balanced targets for given total transmit power. Finally, some applications of the power minimization problem arising from different scenarios are studied and solved by means of the proposed algorithm. Simulation experiments are carried out using the technical programming language MATLAB in order to show the performance of the proposed methods