André Noll Barreto

A dynamic link adaptation algorithm for IEEE 802.11 a wireless LANs

We present a simple but powerful dynamic link adaptation mechanism for wireless LANs, which are based on the IEEE 802.11a standard. Using this method, a transmitter is able to detect whether the quality of the link is improving or deteriorating, and based on this information to switch to a higher or lower transmission rate, respectively. For determining the link quality, the transmitter employs only information that is available locally. Therefore, the proposed method can be implemented without changes or enhancements to the current IEEE 802.11 standard. The efficiency of our method is investigated and evaluated by means of simulations.

Adaptive bit loading for wireless OFDM systems

We investigate the use of adaptive bit loading in existing OFDM-based W-LAN standards. The performance gain is assessed through simulation and the use of higher-rate codes is considered to improve the performance. Some practical aspects, such as peak-to-average power ratio, channel estimation, signalling and spectral flatness are also addressed.

Joint signal precoding in the downlink of spread-spectrum systems

The downlink capacity tends to be the limiting factor in future communications systems. We propose two schemes for the downlink of code-division multiple-access systems that help reduce the multiuser interference by jointly precoding the transmitted signal based on knowledge of the downlink channel. Methods that operate on blocks of bits are first developed and a bitwise simplification of these for systems with large processing gains are derived. The use of multiple transmit antennas are also considered. These methods provide a substantial capacity increase without requiring complex multiuser detectors at the mobile terminals, and can be applied for instance to time-division duplex systems, which have reciprocal channels.

Optical Networking With Variable-Code-Rate Transceivers

We evaluate the impact of variable-code-rate transceivers on cost, capacity and survivability of wavelength-routed optical networks. The transmission rate and reach trade-off is quantified for two hypothetical coded modulation schemes (aggressive and conservative) in a wavelength routing network with 50-GHz-spaced channels. The aggressive scenario assumes the 64-QAM modulation format, a small gap to capacity, and a small excess bandwidth. The conservative scenario considers the 16-QAM modulation format, and a larger capacity gap and excess bandwidth. The performance of the conservative and aggressive technologies is evaluated in three representative networks. Transparent reaches are calculated by means of an existing analytical method which assumes the AWGN hypothesis for the nonlinear noise. It is shown that variable-code-rate transceivers enable the concept of soft protection, in which the protection lightpath operates at a data rate which is lower than the corresponding working lightpath, in a way to avoid regeneration. This is specially attractive in the transport of IP traffic, where capacity reduction (in average up to 25%) may be tolerable during a repair time. It is also shown that variable-code-rate transceivers have the potential to offer significant savings in terms of transceiver usage and wavelength occupation, when compared to current fixed-rate transceivers operating at 100, 200 or 400 Gb/s. Finally, practical variable-code-rate transceivers may achieve a discrete set of N code rates, yielding a quantized capacity-versus-reach curve. The system impact of N is evaluated for several network scenarios.

Capacity increase in the downlink of spread spectrum systems through joint signal precoding

In this paper we propose two precoding schemes for the downlink of CDMA systems which help reduce the multiuser interference by jointly precoding the transmitted signal based on knowledge of the downlink channel. These methods provide a substantial capacity increase without requiring complex multiuser detectors at the mobile terminals and can be applied for instance to TDD systems, which have symmetrical channels.

Power and rate adaptation in IEEE 802.11a wireless LANs

We present a simple but powerful dynamic link adaptation mechanism for wireless LANs that is compliant with the IEEE 802.11a standard. Using this method, a transmitter is able to estimate the current link quality and, based on this information, to adapt the transmission parameters, namely the transmission data rate or the transmission power level, dynamically. To determine the link quality, the transmitter employs only information that is available locally. Therefore, the proposed method can be implemented without changes or enhancements to the current IEEE 802.11 standard. The efficiency of our method is investigated and evaluated by means of simulations.

On the downlink capacity of TDD CDMA systems using a pre-RAKE

The pre-RAKE technique for the downlink transmission in TDD-based CDMA wireless systems has been shown to provide greater capacity and a simpler receiver design at the mobile receiver. Nevertheless this technique is very affected by fast changing channels, since it relies on uplink channel measurements for the downlink transmission. In this paper a semi-analytical approach to the performance, including channel coding is presented. Furthermore we analyse the pre-RAKE performance including its behaviour with changing channels. Finally we show that the pre-RAKE maximises the correlation between the received and the desired signal, and this demonstration provides us the means to obtain an instant power control at bit level that improves performance significantly.

Performance of Cooperative Eigenvalue Spectrum Sensing with a Realistic Receiver Model under Impulsive Noise

In this paper we present a unified comparison of the performance of four detection techniques for centralized data-fusion cooperative spectrum sensing in cognitive radio networks under impulsive noise, namely, the eigenvalue-based generalized likelihood ratio test (GLRT), the maximum-minimum eigenvalue detection (MMED), the maximum eigenvalue detection (MED), and the energy detection (ED). We consider two system models: an implementation-oriented model that includes the most relevant signal processing tasks realized by a real cognitive radio receiver, and the theoretical model conventionally adopted in the literature. We show that under the implementation-oriented model, GLRT and MMED are quite robust under impulsive noise, whereas the performance of MED and ED is drastically degraded. We also show that performance under the conventional model can be too pessimistic if impulsive noise is present, whereas it can be too optimistic in the absence of this impairment. We also discuss the fact that impulsive noise is not such a severe problem when we take into account the more realistic implementation-oriented model.

Antenna transmit diversity for wireless OFDM systems

We investigate several transmit-diversity techniques for wireless OFDM systems employing multiple antennas. We concentrate our investigation on methods that can be applied with few modifications to existing standards. Existing methods such as delay diversity, clustering and antenna interleaving are investigated and compared. A new approach that yields better results, consisting in applying different phase shifts on each subcarrier and on each antenna is also presented. Some practical implementation issues are also addressed.

Concept for universal access and connectivity in mobile radio networks

Mobile communications is characterized by a fast growing number of users and an increasingly heterogeneous data traffic due to emerging multimedia packet services. Likewise, users desire more mobility support, thus demanding permanent availability at any place and any speed. The conventional answer to this problem is building new wireless standards. But these standards require totally new hardware, users cannot use their old terminals, and no wireless communication among networks is possible which considerably constrains the availability demand. Instead, we propose a unified way for basic network access and connectivity maintenance. It is embodied by a perpetually available signalling channel-the network access and connectivity channel (NACCH) and a common functionality trunk, shared by several networks. With this concept, networks can communicate and, hence, form a mega-network where users have universal access and roaming support.