Ismael Gutiérrez

High-Voltage Multicarrier Spread-Spectrum System Field Test

High-voltage (HV) power lines have been used as a communications medium since the 1920s. Those point-to-point links were typically based on single-sideband amplitude modulation. These days, the state of the art in HV power-line carrier (PLC) communications comprises the combination of analog systems, mainly for teleprotection tasks, and digital systems, used for voice and data transmission. Beside traditional core services (monitoring, operation management, and limitation and removal of failures), electrical utilities would like to satisfy the increasing need of new internal applications. In that way, quadrature amplitude modulation and, most recently, multicarrier modulation (MCM)-based modems are beginning to play an important role in HV PLC systems. Although the typical 4-kHz bandwidth has been recently increased up to 32 kHz, this paper proposes a low-power 256-kHz bandwidth multicarrier-spread-spectrum (MC-SS)-based physical layer. Based on channel measurements, the MC-SS symbol has been designed and tested in order to increase the user bit rate while delivering reduced power spectral density and bit-error rate.

Contiguous frequency-time resource allocation and scheduling for wireless OFDMA systems with QoS support

The orthogonal frequency division multiple access (OFDMA) scheme has been selected as a potential candidate for many emerging broadband wireless access standards. In this paper, a new joint scheduling and resource allocation scheme is proposed for the OFDMA systems using contiguous subcarrier permutation. The proposed resource allocation algorithm provides contiguous sets of frequency-time resource units following a rectangular shape yielding a reduction on the required burst signalling. The joint scheduling and resource allocation process is divided into two phases: the QoS requirements fulfilment and the input buffers emptying status. For each phase, a specific prioritization function is defined in order to obtain a trade-off between the fairness and the spectral efficiency maximization. The new prioritization scheme provides a reduction of 50% of the 99th percentile from the delivered packets delay in case of non real-time services, and 30% of the packet loss rate in case of real-time services compared to the proportional fair scheduling function. On the other hand, it is also demonstrated that using the rectangular data packing algorithm, the number of required bursts per frame can be reduced up to a few tenths without compromising the performance.

Adaptive Resource Management for a MC-CDMA System with Mixed QoS Classes Using a Cross Layer Strategy

Wireless communications are experiencing a great increasing demand, both user and bandwidth capacity. The efficiency in the resource management and usage will be a dramatically important issue since spectrum and power is a scarce resource, even more in the wireless environment. Under these circumstances, adaptive resource management will lead to high spectral efficiency rates and power saving algorithms. Previous works have focused on increasing the system capacity by means of complex power and rate adaptation techniques at the expense of unfair systems. The present proposal proposes a low complexity algorithm for adaptive resource allocation considering different quality of service classes in a multicarrier-code division multiple access (MC-CDMA) system, which is considered according to several researches, as a promising air interface scheme for the beyond 3G systems.

Prioritization function for packet data scheduling in OFDMA systems

In this paper, the authors developed a new prioritization function integrated within a new resource allocation process for an OFDMA system. In this paper a new prioritization function is proposed where the resource allocation problem is divided in two phases; the minimum requirements satisfaction, and; the spectral efficiency maximization of the unallocated resources. Compared with the Proportional Fair (PFS) and the Prioritization Function (PRF) algorithms whose have been designed based on the average bit rate requirements without considering neither the states of the buffers or the VBR nature of the traffic, the proposed scheme overcome these restrictions by the support of a designed Packet Data Scheduler (PDS) based on the input buffers status.

Peak Reduction of Multi-Carrier Systems by Controlled Spectral Outgrowth

One of the major drawbacks with multi-carrier (MC) modulations is the large peak-to-average power ratio (PAPR) of the transmitted signal which significantly reduces the power amplifier efficiency and the system performance. Several PAPR reducing methods which do not decrease the system performance have been proposed. In this paper, we present a new low complexity PAPR reduction technique based on the controlled spectral outgrowth (CSO) of the nearby subcarriers which neither requires the transmission of additional information nor reduces the system performance and is independent of the mapping scheme. The proposed method can be easily combined with most of the methods in the literature to provide a further peak reduction in any MC or MC spread spectrum (MC-SS) system. An analog PAPR reduction about 2.5 dB at 10-4 symbol clip probability and a power increase about 0.12 dB are obtained for a 256-subcarrier of OFDM transmission

New prioritization function for user scheduling in OFDMA Systems

In this paper a new prioritization function for Orthogonal Frequency Division Multiple Access (OFDMA) is proposed where the resource allocation problem is divided in two phases, minimum requirements satisfaction, and spectral efficiency maximization. For each phase a different prioritization function is considered. The performance of the proposal is evaluated, via computer simulation, by the packet delay statistics obtained for different scenarios and also compared to other prioritization functions from the literature.

Intercell Interference Investigation in a MC-CDMA System with Iterative Demapping

The increasing interest on multimedia applications and services in portable computing communication will create at same time a large demand for high speed wireless data on communication devices. The 4th generation (4G) mobile communication is expected to offer 100 Mbps or higher data rates in downlink. orthogonal frequency division multiplexing (OFDM) and multiple carrier code division multiple access (MC-CDMA) are potential candidates for next generation of mobile radio communications for achieving hight data rate transmissions. In cellular networks, the frequency reuse is one of the main parameters to consider since it is more suitable to optimize the spectrum reuse. Using a frequency reuse factor of one, it could be a possible system using the same frequency in all the network. In this proposal the sensitivity and the impact of an interfering base station based on a MC-CDMA system is investigated and analyzed using different channel coding and decoding processes.

Space-Time Adaptation and MIMO Standardization Status

The use of multiple antennas at the transmitter and/or receiver sides has been used for years in order to increase the signal to noise ratio at the receiver or for beam steering (a.k.a. beamforming) to reduce the amounts of interference at the receiver. However, one of the main benefits of using a Multiple Input Multiple Output (MIMO) channel is the increase of the channel capacity. Nowadays, by using different space-time-frequency coding techniques, orthogonal (or quasi orthogonal) virtual paths between transmitter and receiver can be obtained. These virtual paths can be used in order to increase the spectral efficiency or in order to increase the signal diversity (i.e. space diversity). In fact, a fundamental trade-off between diversity and multiplexing capabilities exists and must be considered when designing a multiple antenna system. In this chapter, the following issues are addressed:  We start by describing and reviewing the (ergodic) capacity of the MIMO channel in case of perfect channel state information at the transmitter (CSIT). Afterwards, the outage capacity is studied leading to the diversity-multiplexing trade-off.  Then, we analyze different spatial adaptation and precoding mechanisms that can be applied to increase the performance of the system (either in terms of throughput or robustness). A new spatial adaptation algorithm proposed by the authors and called Transmit Antenna and space-time Coding Selection (TACS) is described showing some performance results that illustrate the improve on performance and/or throughput.  Finally, the well-known Space-Time coding techniques are reviewed, and a summary of the MIMO techniques adopted in WiMAX2 (IEEE 802.16e/m) is provided.

Mixed TUSC and Band AMC Permutation Zone in OFDMA Systems with Limited Feedback

In this paper a new permutation zone which combines adjacent and distributed subcarrier permutation is proposed for Orthogonal Frequency Division Multiple Access (OFDMA) systems. For such purpose two types of burst are defined: localized and distributed. A new resource allocation and scheduling algorithm that exploits such structure is then proposed. Furthermore, the effects of limited feedback are also investigated where only the n-best subchannel quality metrics are feedback. As shown in the simulation results, a tradeoff between spectral efficiency, delay and load signaling requirements is obtained which can be adjusted by modifying the number of sub- channels signaled in the uplink. Additionally, the proposed scheme turns out to be a very efficient way to increase the system fairness and therefore improving the quality of service.

Transmit Antenna and Code Selection for Mimo Systems with Linear Receivers

In this paper the combination of Transmit Antenna Selection with Linear Dispersion Code Selection is studied. Two optimization criteria are proposed (bit error rate minimization and throughput maximization). The performance of the proposed spatial link adaptation scheme is evaluated under low mobility environments concluding that maximum spatial diversity is achieved as well as a smooth transition between codes with low spatial multiplexing rate, high spatial diversity (suitable for low SNR), and codes with high multiplexing rate but low diversity order (suitable for high SNR) in order to maximize the overall system throughput.