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Cross Layer QoS Guarantees in Multiuser WLAN Systems

The use of real-time delay-sensitive applications in wireless systems has significantly increased during the last years. Consequently, the demand to guarantee certain Quality of Service (QoS) is a challenging issue for the system’s designers. A cross-layer based dynamically tuned queue length scheduler is presented in this paper, for the Downlink of multiuser WLAN systems with heterogeneous traffic requirements. An opportunistic scheduling algorithm is applied, while users from higher priority traffic classes are prioritized. A trade-off between the throughput maximization of the system and the guarantee of the users QoS requirements is obtained. Therefore the length of the queue is dynamically adjusted to select the appropriate conditions based on the operator requirements.

CAC for Multibeam Opportunistic Schemes in Heterogeneous WiMax Systems Under QoS Constraints

Spatial scheduling in a heterogeneous WiMax Downlink channel, based on partial channel state information at the transmitter (CSIT), is carried out through a multibeam opportunistic beamforming technique. In contrast to existing literature, this paper considers a more practical perspective and presents a strategy where a minimum rate per user is required, which can only be guaranteed under a certain system outage restriction. Within this scenario, the paper then formulates the minimum rate and maximum/mean delay expressions in closed form expressions. Furthermore, in opportunistic schemes a high number of users is required to improve the sum rate system performance, but for practical considerations, a large number of available users induces a large service delay. A dynamic call admission control (CAC) is then proposed to regulate the number of users, and to guarantee the service to all the users in the cell under rate and delay requirements. The derived expressions are then tested via simulations in several transmission scenarios, where users running delay-constrained (DC) and delay-tolerant (DT) applications coexist.

An improved partial CSIT random beamforming for multibeam satellite systems

A novel multiple input multiple output (MIMO) transmitting scheme is presented for multibeam satellite systems, where the objective is to increase the system sum rate, availability and variance performance.

Robust Power Allocation Schemes for Multibeam Opportunistic Transmission Strategies Under Quality of Service Constraints

Scheduling in a Broadcast (BC) channel based on partial Channel State Information at the Transmitter (CSIT) is carried out in an opportunistic way, where several orthogonal beams are randomly generated at the Base Station transmitter to simultaneously deliver several users with their intended data. The paper presents a power allocation over the transmitting beams, where a minimum rate per user restriction is required for each scheduled user, standing as a potential Quality of Service (QoS) indicator for the system behaviour. However, in practical wireless scenarios the CSIT is imperfect due to non-accurate estimation, so that robust schemes are required to meet the system demands. Based on the allowed system outage in the QoS achievement, different robust power allocation schemes are proposed, which are efficiently solved through convex optimization tools. The presented strategies are later compared via simulations for the different scenarios and the system specifications.

Multiuser MAC Protocols for 802.11n Wireless Networks

The emerging 802.11n standard establishes the integration of MIMO technology in WLANs with the goal of achieving high data rates. However there are still many open issues regarding MAC protocol design for MIMO based systems, especially in order to exploit the multiuser capabilities of the MIMO channel. In this paper we investigate practical solutions to implement multiuser downlink transmission in infrastructure 802.11n based WLANs. A low-complexity beamforming transmission technique is employed at the physical layer and four MAC schemes that vary in complexity and efficiency are presented and evaluated through computer simulations.

Dual polarization for MIMO processing in multibeam satellite systems

A multibeam satellite scenario is considered along this paper to test for the benefits of dual polarization. Several multiple input multiple output (MIMO) transmitting schemes are proposed to deal with the dual polarization, where the results show that dual polarization is better employed as an interference rejection tool.

Robust Multibeam Opportunistic Schemes Under Quality of Service Constraints

Scheduling in a broadcast (BC) channel based on partial channel state information at the transmitter (CSIT) is carried out in an opportunistic way, where several orthogonal beams are randomly generated at the base station transmitter to simultaneously deliver several users with their intended data. Within a more practical perspective of the opportunistic systems, this paper first presents a transmission scheme where a minimum rate per user restriction is required for each scheduled user. This minimum rate is demanded by each user to properly decode and manage its received signal, which stands as a possible Quality of Service (QoS) indicator for the system behaviour. Then, the paper considers an imperfect CSIT situation where robust schemes are required to meet the QoS restrictions. Two robust opportunistic transmission philosophies are presented through a power allocation over the transmitting beams, and they are efficiently solved via convex optimization tools.

Quality of Experience for Spatial Cognitive Systems within Multiple Antenna Scenarios

Multiple Input Multiple Output (MIMO) adds a new dimension, the spatial one, to be optimized in Cognitive Radio (CR) by offering service simultaneously to more than one user. In this paper, statistical optimization techniques are applied to assess the performance of the Quality of Experience (QoE) in CR systems, where each user has different demands. A Multiuser scenario is considered where the transmitter can accomplish either a random or an opportunistic scheduling approach. Closed form expressions are derived for different scenarios and obtained for three QoE indicators in the system. The performance of primary and secondary users in such scenarios are mathematically formulated and compared their results to computer simulations.

Energy Optimization for Bidirectional Multimedia Communication in Unsynchronized TDD Systems

Energy efficiency is one of the main system resources that has to be optimally allocated, due to its impact on the amount of generated interference and the battery life for multimedia communication. In this paper, we concentrate on the time-division duplexing (TDD) option and how to efficiently allocate energy on both uplink (UL) and downlink (DL) channels that share the same bandwidth but over different time intervals. One of the main advantages of TDD is the possibility for load compensation between UL and DL, by allocating a larger percentage of time for the operation in DL if the system is congested in DL, and vice versa. While this allocation can be done on each cell independently, the result is that adjacent cells would have a different compensation value, thus generating cross interference, which decreases the system performance, mainly on the cell-edge users. This paper presents an energy allocation in order to compensate the unsynchronization in the percentage allocation, where the objective is to guarantee minimum quality of service satisfaction at all receivers, while the least amount of energy is consumed.

Spatial diversity scheme to efficiently cancel ISI and ICI in OFDM-OQAM systems

This paper is based on an Offset Quadrature Amplitude Modulation (OQAM) Orthogonal Frequency Division Multiplexing (OFDM) transmission scheme that is operated without a Cyclic Prefix (CP), where the multiple transmitting antennas are employed to substantially reduce the inherent intersymbol and intercarrier interference. The proposed scheme avoids the use of the CDMA technology to get rid of the interference. The nonemployment of the CP increases the spectral efficiency in comparison with classical CP-OFDM systems, as it does not employ the CP for its correct performance. On the other hand, the nonemployment of the CP comes at cost of Intersymbol Interference (ISI). This paper presents a method which cancels the interference terms by employing a multiantenna precoding strategy based on spatial diversity OQAM-OFDM scheme, so that the overall system can get the advantage of the CP removal while no ISI is generated. Moreover, the proposed system benefits from the multiuser gain through an opportunistic scheduler at the transmitter side to select the user with the best channel characteristics at each instant. The resultant scheme OQAM-OFDM-MIMO data rate is obtained in a closed form expression and proved to be higher than the classical CP-OFDM systems.