Alexandros-Apostolos A. Boulogeorgos

I/Q-Imbalance Self-Interference Coordination

In this paper, we present a novel low-complexity technique, which improves the performance of single-antenna multi-carrier communication systems, suffering from in-phase and quadrature (I/Q)-imbalance (IQI) at the receiver. We refer to the proposed scheme as I/Q-imbalance self-interference coordination (IQSC). This technique not only mitigates the detrimental effects of IQI, but, through appropriate signal processing, also coordinates the self-interference terms produced by IQI in order to achieve second-order frequency diversity. However, these benefits come at the expense of a reduction in transmission rate. More specifically, IQSC is a simple transmit diversity scheme that improves the signal quality at the receiver by elementary signal processing operations across symmetric (mirror) pairs of subcarriers. Thereby, the proposed transmission protocol has a similar complexity as Alamoutis space-time block coding scheme and does not require extra transmit power nor any feedback. To evaluate the performance of IQSC, we derive closed-form expressions for the resulting outage probability and symbol-error rate. Interestingly, IQSC outperforms not only existing IQI compensation schemes but also the ideal system without IQI for the same spectral efficiency and practical target error rates, while it achieves almost the same performance as ideal (i.e., IQI-free) equal-rate repetition coding. Our findings reveal that IQSC is a promising low-complexity technique for significantly increasing the reliability of low-cost devices that suffer from high levels of IQI.

Energy Detection Spectrum Sensing Under RF Imperfections

Direct-conversion radio (DCR) receivers can offer highly integrated low-cost hardware solutions for spectrum sensing in cognitive radio (CR) systems. However, the DCR receivers are susceptible to radio frequency (RF) impairments, such as in-phase and quadrature-phase imbalance, low-noise amplifier nonlinearities, and phase noise, which limit the spectrum sensing capabilities. In this paper, we investigate the joint effects of RF impairments on energy detection-based spectrum sensing for CR systems in multi-channel environments. In particular, we provide the novel closed-form expressions for the evaluation of the detection and false alarm probabilities, assuming Rayleigh fading. Furthermore, we extend the analysis to the case of CR networks with cooperative sensing, where the secondary users suffer from different levels of RF imperfections, considering both scenarios of error free and imperfect reporting channel. Numerical and simulation results demonstrate the accuracy of the analysis as well as the detrimental effects of RF imperfections on the spectrum sensing performance, which bring significant losses in the spectrum utilization.

Spectrum Sensing in Full-Duplex Cognitive Radio Networks Under Hardware Imperfections

Direct-conversion radio transceivers can offer reprogrammable and low-cost hardware solutions for full-duplex (FD) cognitive radio networks (CRNs). However, they are susceptible to radio-frequency (RF) impairments, such as in-phase (I) and quadrature (Q) imbalance (IQI), which can significantly limit spectrum sensing capabilities. This paper is devoted to quantifying and evaluating the effects of IQI in single- and multichannel energy detectors operating in FD mode under both cooperative and noncooperative spectrum sensing scenarios. In this context, closed-form expressions are derived for the false alarm and detection probabilities in the general case, where partial self-interference suppression (SIS) and joint transmitter (TX) and receiver (RX) IQI are considered. Furthermore, simplified closed-form expressions for the special cases, where either the RF front end is ideal or the SIS technique is perfect, are also presented. The presented analytical results have been verified through extensive simulations and indicate that the IQI and partial SIS can significantly affect spectrum sensing accuracy in FD-based CRNs. Specifically, if ideal RF front end is assumed, spectrum sensing error can significantly increase, leading to a reduction in the CRN performance and a negative effect on the performance of primary (PR) networks. Hence, when designing spectrum sharing algorithms for FD-based CRNs, the hardware impairments should be considered to improve the CRN performance while minimizing the negative effects on PR users.

Energy detection under RF impairments for cognitive radio

Direct-conversion radio receivers can offer highly integrated low-cost hardware solutions for spectrum sensing in cognitive radio systems. However, these receivers are susceptible to radio frequency (RF) impairments, such as in-phase and quadrature-phase imbalance, low-noise amplifier nonlinearities and phase noise, which limit the spectrum sensing capabilities. In this paper, we study the joint effects of RF impairments on energy detection based spectrum sensing in multi-channel environments. In particular, we provide an analytical framework for the evaluation of the probabilities of detection and false alarm, assuming Rayleigh fading and approximating the joint effects of RF impairments by a Gaussian distribution. Numerical results illustrate the detrimental effects of RF imperfections on the spectrum sensing performance, which bring significant losses in the spectrum utilization.

Spectrum Sensing With Multiple Primary Users Over Fading Channels

We investigate the impact of multiple primary users (PUs) and fading on the spectrum sensing of a classical energy detector (ED). Specifically, we present the novel closed-form expressions for the false-alarm and detection probabilities in a multiple PUs environment, assuming the Nakagami-m fading and complex Gaussian PUs transmitted signals. The results reveal the importance of taking into consideration the wireless environment, when evaluating the ED spectrum sensing performance and selecting the ED threshold.

How Much Does I/Q Imbalance Affect Secrecy Capacity?

Radio frequency front ends constitute a fundamental part of both conventional and emerging wireless systems. However, in spite of their importance, they are often assumed ideal, although they are practically subject to certain detrimental impairments, such as amplifier nonlinearities, phase noise, and in-phase and quadrature (I/Q) imbalance (IQI). This letter is devoted to the quantification and evaluation of the effects of IQI on the secrecy capacity of a wiretap channel. Novel closed-form expressions are derived for the secrecy outage probability for the case of ideal transmitter and I/Q imbalanced legitimate users and eavesdroppers receivers. Numerical results and simulations illustrate the detrimental effects of IQI on the physical layer security performance, and reveal that IQI should be seriously taken into consideration in the design of secure wireless systems.

The effects of RF impairments in vehicle-to-vehicle communications

Radio frequency (RF) front-ends constitute a fundamental part of both conventional and emerging wireless communication systems. However, in spite of their importance they are often assumed ideal, although they are practically subject to certain detrimental impairments, such as amplifier nonlinearities, phase noise and in phase and quadrature (I/Q) imbalance (IQI). The present work is devoted to the quantification and evaluation of the RF IQI effects in the context of realistic wireless vehicle-to-vehicle (V2V) communications over double-Nakagami-m fading channels. Novel closed form expressions are derived for the corresponding outage probability for the case of ideal transmitter (TX) and receiver (RX), ideal TX and I/Q imbalanced RX, I/Q imbalanced TX and ideal RX, and joint I/Q imbalanced TX/RX. The offered analytic results have a relatively convenient algebraic representation and their validity is extensively justified through comparisons with respective results from computer simulations. Based on these, it is shown that cascaded fading results to considerable degradations in the system performance and that assuming ideal RF front-ends at the TX and RX induces non-negligible errors in the outage probability evaluation that can exceed 20% in several V2V communication scenarios.

On the effects of I/Q imbalance on sensing performance in full-duplex cognitive radios

Direct-conversion radio transceivers can offer reprogrammable and low-cost hardware solutions for full-duplex (FD) cognitive radio (CR) devices. However, they are susceptible to radio frequency (RF) impairments, such as in-phase (I) and quadrature (Q) imbalance (IQI), which can significantly constrain the spectrum sensing capabilities. This paper is devoted to quantify and evaluate the effects of IQI in the context of spectrum sensing in FD CR systems, in which self-interference suppression (SIS) techniques are employed. Specifically, closed form expressions are derived for the false alarm and detection probabilities, under three different scenarios: imperfect SIS with joint transmitter (TX) and receiver (RX) IQI, imperfect SIS and ideal TX/RX RF front-end, and perfect SIS and ideal TX/RX RF front-end. The derived analytical results are validated through extensive simulations, which reveal that IQI has a detrimental impact on the spectrum sensing performance of the FD CR transceiver, which brings significant losses in the spectrum utilization.

Inter-band carrier aggregation in heterogeneous networks: Design and assessment

This paper deals with the performance assessment of the Long Term Evolution (LTE)-Advanced Release 12 physical downlink channel, emphasizing on the Carrier Aggregation (CA) technology and its recent advances, such as the challenging interband non-contiguous solution. By processing the LTE-Advanced waveforms in the time domain (instead of the more common baseband), we describe the underlying system model and the associated simulation setup in detail. The error performance of the system is evaluated under different physical layer parameters and CA scenarios, according to the latest updates of the Third Generation Partnership Project (3GPP) technical specifications. Our analysis reveals that the Heterogeneous Band (HetBand) non-contiguous CA technology can be efficiently applied to the design of next generation mobile broadband networks, given that the exploitation of both unlicensed and frequency dispersed bands might be a promising solution against the spectrum scarcity.

Comparison of CSMA/CA protocols applied in wireless body area network standards

This paper deals with the performance assessment of the wireless body area network (WBAN) standards proposed by the Institute of Electrical and Electronics Engineers (IEEE), namely IEEE 802.15.4 and IEEE 802.15.6, emphasizing on the different carrier sensing multiple access/collision avoidance (CSMA/CA) mechanisms. By comparing the different proposals, we provide an engineering insight, which elaborates the reasons of the performance differences and reveals important trade-offs in terms of successful packet transmission and energy consumption. These trade-offs should be seriously taken into consideration when designing a WBAN.