Hisham A. Mahmoud

OFDM for cognitive radio: merits and challenges

Cognitive radio is a novel concept that enables wireless systems to sense the environment, adapt, and learn from previous experience to improve the quality of the communication. However, CR requires a flexible and adaptive physical layer in order to perform the required tasks efficiently. In this article, CR systems and their requirement of a physical layer are discussed, and the orthogonal frequency division multiplexing technique is investigated as a candidate transmission technology for CR. The challenges that arise from employing OFDM in CR systems are identified. The cognitive properties of some OFDM-based wireless standards also are discussed to indicate the trend toward a more cognitive radio.

Sidelobe suppression in OFDM-based spectrum sharing systems using adaptive symbol transition

In this letter, we introduce a new method for orthogonal frequency division multiplexing (OFDM) sidelobe suppression. An extension is added to OFDM symbols that is calculated using optimization methods to minimize adjacent channel interference (ACI) while keeping the extension power at an acceptable level. Using this technique, interference to adjacent signals is reduced significantly at the cost of a small decrease in the useful symbol energy. The proposed method can be used by cognitive radio (CR) systems to shape the spectrum of OFDM signals and to minimize interference to licensed users (LU), or to reduce the size of guard bands used in conventional OFDM systems.

Error vector magnitude to SNR conversion for nondata-aided receivers

Error vector magnitude (EVM) is one of the widely accepted figure of merits used to evaluate the quality of communication systems. In the literature, EVM has been related to signal-to-noise ratio (SNR) for data-aided receivers, where preamble sequences or pilots are used to measure the EVM, or under the assumption of high SNR values. In this paper, this relation is examined for nondata-aided receivers and is shown to perform poorly, especially for low SNR values or high modulation orders. The EVM for nondata-aided receivers is then evaluated and its value is related to the SNR for quadrature amplitude modulation (QAM) and pulse amplitude modulation (PAM) signals over additive white Gaussian noise (AWGN) channels and Rayleigh fading channels, and for systems with IQ imbalances. The results show that derived equations can be used to reliably estimate SNR values using EVM measurements that are made based on detected data symbols. Thus, presented work can be quite useful for measurement devices such as vector signal analyzers (VSA), where EVM measurements are readily available.

Initial Ranging for WiMAX (802.16e) OFDMA

Ranging is one of the most important processes in the new mobile WiMAX standard. Power adjustment, timing offset estimation, and synchronization between a base station (BS) and all users within a cell are done through the ranging process known as initial ranging. In this paper we discuss the details of initial ranging and some of the proposed algorithms as well as a novel algorithm to carry out a successful ranging process. Performance curves and computational complexity comparisons will be presented. It is observed that the proposed algorithm offers a better trade-off between computational complexity and performance

Spectrum shaping of OFDM-based cognitive radio signals

In this paper, Orthogonal Frequency Division Multiplexing (OFDM) spectrum shaping using various techniques to reduce Adjacent Channel Interference (ACI) is investigated. The trade-offs between interference level and spectral efficiency, power consumption, and computational complexity are discussed. Simulation results along with detailed analysis showing the advantages and disadvantages of each technique are presented.

IQ Imbalance Correction for OFDMA Uplink Systems

Direct conversion receivers are attractive for low cost systems as they avoid intermediate frequency (IF) filters. However, the direct conversion from radio frequencies (RF) to complex inphase (I) and quadrature (Q) baseband signals in one mixing step introduces additional front-end distortions. These IQ distortions lead to a degradation in the system performance. The problem becomes more significant in orthogonal frequency division multiple access (OFDMA) systems where multiple user signals with different IQ impairments are combined in the uplink (UL) signal. In this paper, detection methods for OFDMA-UL signals corrupted by IQ distortions are investigated. The received signal as a function of transmitted signals, IQ parameters, and communication channels is mathematically formulated. We designed a novel pilot pattern that is used by two proposed estimation and compensation methods of IQ impairments to the signal. Proposed methods were shown to significantly improve the system performance.

An efficient initial ranging algorithm for WiMAX (802.16e) OFDMA

Ranging is one of the most important processes in the mobile WiMAX standard. Power adjustment, timing offset estimation, and synchronization between a base station (BS) and all users within a cell are done through the ranging process referred to as initial ranging. In this paper, we discuss the details of initial ranging together with some of the proposed algorithms, as well as a novel algorithm to carry out a successful ranging process. Performance curves and computational complexity comparisons are presented. The system performance is evaluated for both additive white Gaussian noise (AWGN) channels and practical multipath fading channels with multiuser interference. It is shown that the proposed algorithm offers a reduced complexity ranging method that can be employed in practical WiMAX-based BSs.

Improved Channel Estimation in OFDM Systems with Synchronization Errors and Back-Off

In orthogonal frequency division multiplexing (OFDM) systems, synchronization errors in the receiver cause a linear phase rotation at the output of the discrete Fourier transform (DFT) block. The correlation between the channel coefficients at different subcarriers is weakened due to this phase rotation. As a result, the performance of minimum mean-square error (MMSE) channel estimation degrades significantly. In this paper, we propose two novel algorithms to improve MMSE channel estimation under synchronization errors. Theoretical analysis and simulation results are presented to compare the performance of the proposed algorithms with that of the conventional algorithm.