Walter C. Freitas

QoS based Radio Resource Allocation and Scheduling with Different User Data Rate Requirements for OFDMA Systems

In this paper a radio resource allocation (RRA) and scheduling algorithm to maximize the satisfaction of users served by an orthogonal frequency division multiple access (OFDMA) system is proposed. The satisfaction oriented resource allocation (SORA) algorithm, based on low complexity heuristics, divides the users into two sets: satisfied and unsatisfied. Then, it performs resource allocation taking advantage of channel frequency selectivity and the characteristics of each set. The main advantage of the proposed algorithm is the capacity of keeping a high number of users satisfied by resources from users that would be unsatisfied in any case.

Hybrid MIMO Transceiver Scheme with Antenna Allocation and Partial CSI at Transmitter Side

Hybrid multiple-input multiple-output (MIMO) transceiver scheme (HMTS) combines transmit diversity and spatial multiplexing, thus achieving at the same time the two possible spatial gains offered by MIMO channels. In the design of HMTS spatial diversity and spatial multiplexing branches are disposed in parallel in order to achieve diversity and multiplexing gains at the same time. Since the spatial multiplexing branches have no protection, they are more susceptible to the fading effect becoming the bottleneck in the performance of the whole transceiver. In this paper, we propose a solution to this bottleneck in the hybrid MIMO transceiver scheme using a partial channel state information at the transmitter side. The idea is to perform an antenna allocation. Thus, the most powerful subchannels are allocated to the most susceptible layers (spatial multiplexing branches). Through this solution we decrease the performance imbalance between the two layers of the HMTS G3+1, increasing the whole transceiver performance with low complexity feedback requirements

Switching between Hybrid MIMO Structures for Video Transmission Based on Distortion Model

In this paper, we propose a switching between hybrid Multiple-Input Multiple-Output (MIMO) structures based on a distortion model per Group of Pictures (GOP) to improve the transmission of video streaming over wireless systems. The proposed algorithm uses measurements of the application and physical layers. In the application layer, a distortion model for GOP is used based on the packet loss rate and media data units. In the physical layer, the switching between hybrid MIMO structures is realized by simultaneously taking advantage of multiplexing and diversity gains provided by hybrid MIMO structure. The simulation results presented in this article show that the switching between hybrid MIMO structures based in the distortion model per GOPs is capable of improving the objective and subjective video quality at the end user.

Performance of Joint Antenna and User Selection Schemes for Interference Alignment in MIMO Interference Channels

In multiuser MIMO systems, the transmitter can select a subset of antennas and/or users which have good channel conditions to maximize the system throughput using various selection criteria. Furthermore, the Interference Alignment (IA) method, which is based on the concept of precoding, can provide interference free dimensions. It offers different trade-offs between complexity and performance. The basic idea of Interference Alignment consists in precoding the transmitted signals such that they are aligned at the receiver where they constitute interference, while at the same time disjointed from the desired signal. We analyze the performance of antenna selection and multiuser diversity together in order to allow opportunistic IA using chordal and Fubini-Study distances. Analyses and simulations verify the behavior of the proposed scheme.

Clustering-based Assignment within CoMP systems

More and more Coordinated Multi-Point (CoMP) systems are attracting interest as a means to improve the performance of conventional cellular networks, particularly thanks to the joint processing approach. However, the large amount of signaling and computer operations required deserves special attention regarding both backhaul and radio resources constraints. Partitioning the set of transmission points into clusters is likely to reduce this coordination cost, but the search for the best formations of clusters may lead to expressive further computational efforts. Herein we introduce a novel approach, the Clustering-based Assignment Algorithm (CbAA), to cluster dynamically the transmission points on the downlink in order to transmit coherently to multiple users. When compared to static clustering, the proposed CbAA provided expressive gains in terms of system spectral efficiency. Compared to state-of-the-art dynamic clustering approaches, CbAA can waive the exhaustive search over all possible formations as well as the repetitive precoder calculations during the clustering procedure.

Blind Receiver for Multi-Layered Space-Frequency Coded MIMO Schemes Based on Temporally Extended Linear Constellation Precoding

This work formulates a new receiver for the blind decoding of multi-layered space-frequency codes (MLSFC) in multiple input multiple output (MIMO) systems based on orthogonal frequency division multiplexing (OFDM). We introduce a slight modification on the standard MLSFC transmit structure with linear constellation precoding, which consists in extending the constellation rotation across multiple OFDM symbols. Thanks to this added feature, we formulate the received signal as a three-way array following a parallel factor (PARAFAC) model exploiting the powerful uniqueness property of this tensor model, blind MLSFC decoding is guaranteed under mild conditions using an alternating least squares estimation algorithm. Our preliminary numerical results illustrate the bit error rate (BER) performance of the proposed receiver in comparison with the non-blind zero-forcing based MLSFC receiver that assumes perfect channel knowledge.

Overload Prediction Based on Delay in Wireless OFDMA Systems

In this work we deal with Congestion Control (CC) strategies to protect the Quality of Service (QoS) of Real Time (RT) services in Orthogonal Frequency Division Multiple Access (OFDMA)-based and packet-switched networks such as Long Term Evolution (LTE) and Worldwide Interoperability for Microwave Access (WiMAX). Specifically, the first contribution of this paper is the adaptation of a CC strategy previously proposed for High-Speed Downlink Packet Access (HSDPA) to these modern systems. This CC strategy comprises in a coordinated manner the functionalities scheduling, Admission Control (AC) and Load Control (LC). The second contribution is the proposal of a new feature to be added to this CC strategy in order to allow for an early detection of overload situations based on the packet delay of RT flows. In the results we show that our proposed overload prediction based on delay can efficiently prevent QoS degradation of RT flows.

Joint Opportunistic Beamforming and Subcarrier Assignment for Maximization of User Satisfaction in OFDMA Systems

In this paper, we consider a system based on Orthogonal Frequency Division Multiple Access (OFDMA) and investigate the benefits of combining the Satisfaction Oriented Resource Allocation (SORA) algorithm, which aims at maximizing the number of satisfied users in a system, and Orthogonal Random Beamforming (ORB), which provides spatial multiplexing gains and has low computacional complexity, low signaling demands, and high transparency to the User Equipment. Our investigations have shown that considerable gains in terms of throughput and user satisfaction can be achieved by combining SORA and ORB.

A Modified Successive Interference Cancellation Algorithm for Hybrid Diversity/Spatial Multiplexing MIMO Schemes

In this paper we propose a modified successive interference cancellation algorithm and its performance is evaluated considering hybrid diversity/spatial multiplexing MIMO schemes. As consequence of this modification, the diversity layer of hybrid schemes is now able to achieve the full diversity and thereby improves performance considering bit error rate, while offering greater efficiency in bits per channel use (pcu) when compared with traditional STBC using the same number of transmit antennas.

Block diagonalization with subspace based external interference mitigation

In this work we propose a variation to the Block Diagonalization (BD) algorithm described that mitigates the interference from an external interference source which does not participate in the joint transmission inherent to the BD algorithm. The idea consists of precoding the signal at the transmission side so that the received signal at each receiver lies in a subspace orthogonal to the subspace containing the external interference. It is similar to how interference alignment algorithms work, but here instead of limiting the caused interference at the unintended receivers to a specific subspace, we are limiting the desired signal at the intended receivers. Since this “limiting” aspect reduces the dimensions available for the useful signal, a trade-off is involved between how many useful streams can be transmitted and how much of the external interference energy can be canceled. The proposed algorithm is analyzed and we show different metrics that can be used to determine the “good point” in this trade-off.