Hakan Alakoca

OFDMA-based network-coded cooperation: design and implementation using software-defined radio nodes

Benefits of network coding towards enhancing communication quality, both in terms of robustness or data transmission rates, make it a significant candidate as a future networking technology. Conventionally, network coding is mostly used in wired infrastructures, where transmission errors between nodes are negligible. Capturing the provided benefits of network coding via straightforward extension from wired networks to wireless networks is not trivial. In addition to the challenges introduced through the wireless channel impairments, we can also capture the spatial diversity gain provided by the broadcast nature of the wireless channels. In this work, we design and implement a network-coded cooperation (NCC) system that operates in real time through the use of software-defined radio (SDR) nodes for the first time in the literature. We specifically target wireless networks. Our system is based on orthogonal frequency division multiple access (OFDMA) that provides a practical means to enable high transmission rates through the use of narrowband subcarriers. The developed testbed is composed of three source nodes, a relay node and two destination nodes. The transmission of the proposed NCC-OFDMA system is completed in two phases; the broadcast and the relaying phases. Multiplexing of source nodes’ signals is achieved through OFDMA technique. In the broadcast phase, an OFDMA signal is transmitted to relay and destination nodes. In the relaying phase, the relay node first detects the OFDMA signal, generates network-coded symbols, and then transmits these symbols to destination nodes. At the end of these two phases, the destination nodes determine the source nodes’ signals by using network decoders. The destination nodes make use of both the uncoded and network-coded symbols, which are received in broadcast and relaying phases, respectively. Destination nodes then perform network decoding. Through real-time bit error rate and error vector magnitude measurements, we show that the NCC-OFDMA system can significantly improve the communication quality and robustness, while enabling data transmission between multiple users, as known from theoretical analyses. Some features of this implemented NCC-OFDMA system have the potential to be included in 5G standards, due to the improved radio resource usage efficiency.

CP and pilot jamming attacks on SC-FDMA: Performance tests with software defined radios

We designed and implemented single carrier-frequency domain multiple access (SC-FDMA) based system by using software defined radio (SDR) nodes. We aimed to investigate the performance of an LTE system against smart jamming attacks, which target synchronization and channel estimation processes. Cyclic prefix (CP) based jamming attacks and pilot jamming attacks are evaluated and observed for both uplink and downlink directions of an LTE-like system. Based on error vector magnitude measurements, we observed that an SC-FDMA system is more sensitive to CP and pilot jamming attacks when compared to OFDMA.

OFDMA system identification using cyclic autocorrelation function: A software defined radio testbed

In this paper, cyclic autocorrelation function (CAF) is used for signal type identification. Orthogonal frequency division multiple access (OFDMA) signals with different cyclic prefix lengths are generated by using software defined radio nodes. According to test results, by using the OFDMA frame structure, it is possible to distinguished between OFDMA signals, hence signal parameters can be blindly differentiated. Additionally, jamming signal and OFDMA signals can also be distinguished from each other. Real-time measurement results are given to demonstrate the performance of the investigated approach.

Performance of TCP over 802.11ac based WLANs via testbed measurements

This paper presents measurement results collected from an IEEE 802.11ac based wireless test-bed to experimentally demonstrate how 802.11ac affects the throughput performance of a WLAN with different Transmission Control Protocol (TCP) variants implemented in Linux kernel. We compare the performance of 802.11ac with 802.11n for both single and multi-hop transmissions in a wireless mesh network (WMN). Our first observation is that the single-hop performance of 802.11ac outperforms 802.11n, however, as the number of transmission hops increases 802.11ac does not have a significant advantage over 802.11n. In addition, our measurement results show that the best throughput performance with the minimum standard deviation is achieved by TCP CUBIC and TCP BIC for multi-hop transmission. Lastly, we present the results on the behavior of congestion window (CW) with each TCP variant, and observe that the convergence time of the CW is smaller with 802.11ac for both single hop and multi-hop scenarios.

Identification of Non-Conforming Cordless Phone Signals in Licensed Bands

Identification of interference signals is critical in telecommunication systems increasing the need for automated signal identification. As deep convolutional neural networks demonstrate significant achievements in pattern recognition problems, it can be inferred that deep learning methods will give successful results in the field of wireless communications, especially about identification of signal. This paper investigates a new approach of signal identification based on deep convolutional neural network with the convolution architecture for feature extraction (CAFFE) framework. Authors provide the identification for the types of interference signals based on non-conforming digital enhanced cordless telecommunications (DECT) devices. For training NVIDIA-DIGITS, the NVIDIA Deep Learning GPU Training System, is used. The classification accuracy of the system under additive white Gaussian noise and Rayleigh fading channel conditions is observed to be high despite low signal to noise ratio values.

Chirp subcarrier jamming attacks: an OFDM based smart jammer design

In this study a new jammer design for orthogonal frequency division multiplexing (OFDM) based systems is in- troduced. We present three new vulnerability attack scenarios based on chirp signals. These are the conventional chirp attack, the cyclic prefix chirp attack and the pilot tone chirp attack. Bit error rate (BER) performances are investigated in presence of chirp based attacks. It is shown that the performance degradation is increased when compared to Gaussian distributed jammers. Chirp signals are also preferable due to nominal peak-to-average power ratios (at least 7 dB lower than Gaussian counterparts).

Angle of arrival (AoA) estimation by using software defined radios

Source direction of arrival (DoA) or angle of arrival (AoA) estimation is one of the vital problems in many wireless communication applications such as radar, radio astronomy, sonar and navigation. The resolution of a source direction of arrival estimation can be improved by an uniform linear array (ULA) system with innovative signal processing techniques. Super resolution techniques take the advantage of linear array antenna structures to better process the incoming waves. These techniques also have the ability to describe the direction of multiple targets. In this paper, MUSIC algorithm is implemented on USRP nodes using LabVIEW platform. Four receiver antennas are used to detect the AoA of a single carrier and unmodulated RF signal at 2.45 GHz in an indoor environment. The experimental results are shown for a single source which is tested at two different arbitrary angles to confirm the real-time implementation of MUSIC algorithm with some errors which occur due to lack of some hardware imperfections. Moreover, these hardware imperfections are studied.

The impact of MAC layer parameters in system performance of 802.11ac multi-hop networks

The effect of two important MAC parameters on performance of IEEE 802.11ac and 802.11n based wireless local area networks (WLANs) have been investigated experimentally. In the first experiment set-up, effects of Request to Send/Clear to Send (RTS/CTS) packets on throughput performance are observed according to the number of hops. In the second set-up, the impact of A-MPDU technique on throughput performance is observed by using single and multi hop transmissions. The impact of the A-MPDU technique on throughput performance in 802.11ac based WLANs is more significant than 802.11n networks. Furthermore, the impact of AMPDU technique can be significant in a single hop transmission, but increasing number of hops transmission decrease A-MDPU effects. Considering the disadvantage of using A-MDPU (i.e. packet waiting delay), it may not be used in multihop transmission. Finally, using RTS thresholds, RTS/CTS packets do not affect the throughput significantly. However, as the RTS threshold increases, the RTS/CTS packet exchange will significantly effect the performance.

Design and implementation of a software defined radio based OFDMA network

OFDM technique has wide usage in modern communication systems, it can offer efficient use of frequency spectrum and high service quality for users. In literature, there are some software defined radio implementations of OFDM technique. OFDMA technique that can deliver advantages of OFDM technique to multi-user and dynamic based systems is more important for modern communications systems. In this study, for the first time in literature, multi-transmitter OFDMA, which is seen a generalized case of OFDM, is implemented using software defined radio technology. Handicaps of multi-user systems are tackled and solved with our implementation. This implementation is also an important step for understanding the basics and structure of modern and future communication technologies.

Implementation of pre-FFT beamforming in MIMO-OFDM

In this study, real-time software defined radio implementation of the point to point multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) system using pre-fast Fourier transform (pre-FFT) beamforming technique in both the transmitter and the receiver is done for the first time in the literature. By taking benefit of the MIMO systems spatial multiplexing property, the performance of the implemented pre-FFT beamforming approach is demonstrated in terms of bit error rate and error vector magnitude measurements. As a result of the test results, the effect of the beamforming methods on the performance of communication systems are clearly observed.