Secure communication in frequency selective channels with fade-avoiding subchannel usage

Digital Photography X -- 3 February 2014 through 5 February 2014 -- 105377The random nature of fading channels is an enabling factor for achieving secrecy against eavesdropping. By considering the channel frequency selectivity, we propose an adaptive transmission scheme in which the faded subchannels of the legitimate channel are not used for conveying information. Thus, capacity reduction in legitimate channel is minimized while causing a reduction of the eavesdropper channel capacity proportional to the unused subchannels. Besides improving communication secrecy with the intelligent subchannel usage, populating the unused subchannels with artificially-generated noise that further disturbs the eavesdropper's reception is proposed. Since each receiver has its own channel state information (CSI) but not other's, eavesdroppers cannot discard the distortion which is integrated into the transmitted signal as a function of the legitimate pair's CSI. Positive secrecy capacity and outage probabilities are provided as well as error performance example in a communication scenario. It is showed that while careful usage of the subchannels in frequency selective channels improves the secrecy, introducing the fade-filling noise creates an error floor for the malicious nodes.IEEEIEEE Communications SocietyIEEE New South Wales Section Histor

N-continuous OFDM with CP alignment

In this paper, we propose an out-of-band (OOB) reduction method which incorporates with multipath channel and N-continuous OFDM structure. By exploiting the inevitable guard periods between OFDM symbols, i.e., cyclic prefix (CP), the correction component which maintains symbol continuity is aligned with the CP duration after passing through the multipath channel. Thus, the proposed method enables disturbance-free data subcarriers at the receiver while maintaining the continuity of OFDM symbols. This is achieved by the generalization of conventional N-continuous with using more degrees-of-freedom stemming from the CP duration. As it eliminates the interference on the data symbols caused by the correction term, it yields no modification at OFDM receiver, which makes the proposed scheme backward-compatible to conventional OFDM receivers

A practical physical-layer security method for precoded OSTBC-based systems

In this work, we investigate the security performance obtained by employing a practical precoded orthogonal space time block coding method (POSTBC) in MISO wireless networks. In particular, space time codewords are precoded with an optimum matrix that minimizes the error rate at only the legitimate user (Bob). The acquired results depict that there exists a security gap region in the resulting BER performance as a consequence of using POSTBC. Moreover, we enhance the performance more by developing a new hybrid and green security method called precoding along with partial pre-equalizing (PCPPE). In this method, the transmitted symbols are precoded by a new precoder composed of both the original precoder and a new designed unitary matrix that maps Bob's channel amplitudes or phases estimated over the transmitting antennas into 2D orthonormal matrix. Additionally, three issues associated with the proposed security method have been tackled. Including: The slight increase in the transmit power, the appropriate selection process of the optimal precoding matrix, and the effect of imperfect channel estimation and reciprocity. The comparative simulation results prove that PCPPE method provides a secure link among the legitimate parties without sacrificing Bob's reliability although an eavesdropper is assumed to be fully aware of the used method and the original selected precoding matrix indicator (PMI)

Time-asymmetric and subcarrier-specific pulse shaping in ofdm-based waveforms

WOS: 000365016700014In orthogonal frequency-division multiplexing (OFDM) systems, suppressing the out-of-band (OOB) emission with pulse shaping comes with a price. The expense emerges in designing the transition between consecutive symbols. The price can be either the reduction of spectral efficiency (SE) with symbol extension for better transition or the introduction of intersymbol and intercarrier interference (ISI and ICI) due to reduced cyclic prefix (CP) size. In this paper, we propose a time-asymmetric and per-subcarrier (per-SC) pulse-shaping method to minimize the introduced interference without sacrificing SE and OOB performances. Although time-asymmetric pulse reduces the interference due to shortened CP by exploiting the asymmetry in channel delay profile, it causes spectral growth. Thus, the time asymmetry of the pulse-shaping function is gradually increased for the inner SCs that have wider spectral room to the OOB region. A generalized Kaiser window with adjustable time asymmetry is introduced to the provided framework. Theoretical and simulation results showed that the proposed technique achieves superior BER performance while achieving the same level of OOB suppression of pulse shaping and SE of plain OFDM.InterDigital CommunicationsThis work was supported by InterDigital Communications Inc. The review of this paper was coordinated by Prof. S.-H. Leung

Waveform design principles for 5G and beyond

Various emerging applications in future wireless networks require rethinking the access techniques not only from the capacity perspective but also satisfying a wide variety of requirements under the same framework. Traditional Orthogonal Frequency Division Multiplexing (OFDM)-based schemes still suffer from some limitations that stand in front of their existence in future technologies. The main goal of this study is to pave the way towards rethinking the waveform design by providing the main components of a wireless communication system. The concept of the multidimensional lattice is introduced, exploiting the angular dimension that opens a new horizon in research for 5G technology and beyond. The study proposes a new way of waveform design procedure that considers the multidimensional grid, which is essential for exploiting the opportunities in spatial domain effectively. In addition, the waveform design criteria for various applications and frequency bands are presented

A windowing technique for optimal time-frequency concentration and aci rejection in ofdm-based systems

WOS: 000366928600028In this paper, we introduce a windowing technique, which provides optimal time-frequency containment and maximal adjacent channel interference (ACI) rejection for orthogonal frequency-division multiplexing (OFDM)-based systems. Instead of using a single pulse shape function for all subcarriers, multiple functions are considered in order to maximize the time-frequency containment of the OFDM waveform. The main strategy is to concentrate the spectrum of windowing functions into a given bandwidth while achieving maximum suppression in the out-of-band region. This is achieved by employing prolate-based windowing functions which give optimal spectral concentration for time-limited pulse shapes. The windowing functions are designed per-subcarrier basis in order to exploit available concentration band for each subcarrier. In addition, the proposed concept is considered for the receive filtering in the presence of ACI. It is shown that the optimal spectral concentration property also maximizes ACI rejection for OFDM receivers.InterDigital CommunicationsThis work was supported by InterDigital Communications Inc. The associate editor coordinating the review of this paper and approving it for publication was R. Dinis