Miguel Luzio

On the design of frequency-domain equalizers for OQPSK modulations

This paper considers frequency-domain receiver design for OQPSK signals (Offset Quaternary Phase Shift Keying). It is shown that conventional QPSK receivers are not suitable to OQPSK schemes since they lead to significant interference between in-phase (I) and quadrature (Q) components. To overcome this problem we propose several linear receivers where there is no I-Q interference. Our receivers are suitable to OQPSK schemes, significantly outperforming frequency-domain receivers designed for QPSK modulations.1

On the Design of Linear Receivers for SC-FDE Schemes Employing OQPSK Modulation

This paper considers the design of linear frequency-domain receivers with oversampling for OQPSK (Offset Quaternary Phase Shift Keying) signals. Based on a receiver structure able to eliminate the residual interference between in-phase (I) and quadrature (Q) components at the sampling instants, a method that minimizes the overall residual ISI (Inter-Symbol Interference) plus IQI (I-Q interference) is proposed. Therefore, this receiver significantly outperforms previous frequency-domain OQPSK receivers.

On the design of iterative FDE receivers for OQAM modulations

This paper considers receiver designs for SC-FDE schemes (Single-Carrier with Frequency-Domain Equalization) employing M-ary OQAM signals (Offset Quadrature Amplitude Modulation). It is shown that the interference between the in-phase (I) and quadrature (Q) components at the sampling points can lead to severe performance degradation. Therefore, receivers without residual I-Q interference (IQI), as well as receivers with IQI cancellation were developed. Our performance results show that the new receivers are suitable to M-ary OQAM schemes.

Pragmatic Frequency Domain Equalization for Single Carrier with Offset Modulations

Offset signals can be designed to have reduced envelope fluctuations allowing high amplification efficiency. When combined with SC-FDE (Single-Carrier with Frequency-Domain Equalization) schemes, they become excellent candidates for broadband wireless systems with severe power constraints. However, the performance of these modulations with conventional FDE receivers is sub-par, even for receivers specifically designed for offset modulations. This is especially serious when large offset QAM (Quadrature Amplitude Modulation) constellations are used. In this paper we consider SC-FDE schemes combined with offset modulations and study the reason behind the poor performance of conventional FDE implementations. We also present pragmatic FDE receivers for offset modulations that have low complexity and excellent performance.

On the Use of Multiple Grossly Nonlinear Amplifiers for an Efficient Amplification of OQAM Signals with FDE Receivers

This paper considers the use of several grossly nonlinear amplifiers for an efficient amplification of Mary-OQAM signals (Offset Quadrature Amplitude Modulation) with FDE (Frequency Domain Equalization) and the impact of phase and gain unbalance between different amplifiers. It is shown that for some 16-QOQAM signals it is possible to employ two grossly nonlinear amplifiers to perform an amplification that is formally equivalent to a linear amplification. However, the phase and gain of these amplifiers need to be carefully selected, since we can only accept gain errors below 10% and phase errors below 0.1 rad (roughly 6 degrees).

Analytical characterization of nonlinearly distorted TC-OQAM signals

Based on nonlinear OQPSK (Offset Quadrature Phase Shift Keying) format it is possible to define a wide class of TC-OQAM signals (Trellis Coded-Offset Quadrature Amplitude Modulation) with good gains. This paper investigates nonlinear effects in TC-OQAM signals, taking advantage of the the nonlinear OQPSK format. This allows an analytical characterization of the transmitted signals when affected by nonlinear devices such as those inherent to a nonlinear power amplification, which can be used for obtaining the PSD (Power Spectral Density) of the transmitted signals, the design of the optimum receiver as the corresponding performance.

A Pragmatic Design of Frequency-Domain Equalizers for Offset Modulations

Offset modulations such as OQPSK (Off- set Quaternary Phase Shift Keying) and OQAM (Offset Quadrature Amplitude Modulation), combined with SC- FDE (Single-Carrier with Frequency-Domain Equalization) are an interesting choice for broadband wireless systems, that require low-complexity transmitters and efficient power amplification. However, the FDE perfor- mance can be rather poor due to interference between the in-phase and quadrature components at the sampling instants, especially when large constellations are employed. In this paper we propose pragmatic iterative and non- iterative FDE designs for offset modulations. Our performance results show that both iterative and non-iterative pragmatic FDE receivers have excellent performance, even for large OQAM constellations.

Frequency-domain parallel multiuser detection for quasi-constant envelope OQPSK schemes with high spectral efficiency

In this paper we consider a FDMA (Frequency Division Multiple Access) system employing OQPSK modulations (Offset Quaternary Phase Shift Keying) with quasi-constant envelope. To increase the overall spectral efficiency we consider spectral overlapping between different frequency channels. We present an iterative frequency-domain receiver with parallel multiuser detection. Our performance results show that our receiver is able to cope with the strong ACI levels (Adjacent Channel Interference) inherent to spectral overlapping, as well as with the interference between the in-phase (I) and quadrature (Q) components of the OQPSK signals, even for severely time-dispersive channels.

Efficient receivers for SC-FDE with offset modulations

Offset QAM signals (Quadrature Amplitude Modulation) allow good power-bandwidth trade-offs, making them good candidates for broadband wireless systems, especially when combined with SC-FDE schemes (Single-Carrier with Frequency-Domain Equalization). However, FDE receivers designed for nonoffset modulations have poor performance when employed with offset modulations and even FDE receivers specially designed for offset modulations have poor performance when large offset QAM constellations are employed. This is especially serious for non-uniform constellations. In this paper we study the reason behind the poor performance of offset modulations with conventional FDE schemes and present pragmatic FDE receivers for offset modulations that have low complexity but allow excellent performance, even for large and highly non-uniform offset QAM constellations.1

Power efficient coded 16-OQAM schemes over nonlinear transmitters

High power and spectral efficiencies are crucial on modern wireless communications. M-QAM (Quadrature Amplitude Modulation) modulations have been adopted on HSDPA (High Speed Downlink Packet Access) and satellite links, due his higher spectral efficiency. However, due to large envelope fluctuations levels, nonlinear operations such as an high power amplification are undesirable since can they lead to performance degradation and spectral widening. Nonlinearly coded M-OQAM (Offset Quadrature Amplitude Modulation) modulations can be viewed as a particular case of nonlinear OQPSK signals (Offset Quadrature Phase Shift Keying) when the pulse modulation is designed to take advantage from the nonlinear format. In this paper we present nonlinearly encoded 16-OQAM schemes, based on two nonlinear OQPSK signals, designed to allow higher amplification efficiencies due its robustness against nonlinear distortions. The proposed approach assures an higher robustness against nonlinear effects, namely spectral regrowth and performance degradation. The consequences are improved performances over several drive operations of nonlinear amplifiers and a better compromise between bandwidth and power efficiencies. As we can see further the posted simulation results support our assumptions.