Novel pseudorandom phase generation in transform domain communication systems

Transform domain communication system (TDCS) using transform domain processing is demonstrated as having enhanced interference avoidance capability under adverse environmental conditions. Many transform methods can be used in the TDCS, a method is proposed in the paper which is based on random-ambiguity transform (RAT) plus fractional Fourier transform (FRFT). The proposed TDCS and its response to nonstationary interference scenarios are modeled and simulation results obtained using MATLAB. The proposed method has a good ability to anti any narrowband continuous nonstationary jammers. The system was evaluated using an E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> of -7.0 dB and interference energy-to-signal energy (J/E) ratios ranging from 0 dB to 16.0 dB. For BCSK (binary cyclic shift key) data modulation cases, the average TDCS bit error performance improvement is more than 14 dB relatives to a DSSS system according to different J/E. Collectively, these results indicate that the proposed TDCS using RAT-FRFT for interference mitigating is a viable option in nonstationary jammers scenarios

As a cognitive radio (CR) modulation technique, transform domain communication system (TDCS) has been proposed by utilizing white space spectrum for the network access and achieving low probability of interception (LPI). In the previously reported TDCS- based cognitive radio networks (CRN), however, the non-zero periodic correlation function between any pair of users results in multiuser interference (MUI). In this paper, a novel framework under the CR constraints is presented by employing a two- dimension spreading scheme (i.e., time and frequency domains). We first obtain a class of spreading sequences with almost perfect periodic correlation function for arbitrary spectrum utilization pattern. Then, an universal criteria is presented for synchronous MUI-free CRNs when employing TDCS. Simulation results demonstrate that the proposed TDCS-based CRN architecture is a preferable candidate for distribution wireless networks, such as CR Ad-hoc wireless sensor networks.

Addressing the control channel design problem: OFDM-based transform domain communication system in cognitive radio

As an overlay cognitive radio system, transform domain communication system (TDCS) has been proposed to support multiuser communication with low spectral density by using spectrum bin nulling and frequency domain spreading. It has been shown that TDCS has a low spectrum efficiency compared to other wireless communication systems. To solve this problem, this paper presents a cluster-based TDCS scheme to improve spectrum efficiency. Different from the utility of clusters in OFDM systems, the entire unoccupied spectrum bins are divided into L clusters, where each one is used for transmitting the data stream independently, to obtain L times spectrum efficiency of the traditional scheme. Simulation results demonstrate that the proposed TDCS using random allocation scheme is shown to provide the capability of improving spectrum efficiency without seriously degrading power efficiency.

The Internet of Things (IoT) leads the era of interconnection, where numerous sensors and devices are being introduced and interconnected. To support such an amount of data traffic, wireless communication technologies have to overcome available spectrum shortage and complex fading channels. The transform domain communication system (TDCS) is a cognitive anti-interference communication system with a low probability of detection and dynamic spectrum sensing and accessing. However, the non-continuous and asymmetric spectrum brings new challenges to the traditional TDCS block-type pilot, which uses a series of discrete symbols in the time domain as pilots. Low efficiency and poor adaptability in fast-varying channels are the main drawbacks for the block-type pilot in TDCS. In this study, a frequency domain non-uniform pilot design method was proposed with intersecting, skewing, and edging of three typical non-uniform pilots. Some numerical examples are also presented with multipath model COST207RAx4 to verify the proposed methods in the bit error ratio and the mean square error. Compared with traditional block-type pilot, the proposed method can adapt to the fast-varying channels, as well as the non-continuous and asymmetric spectrum conditions with much higher efficiency.

In this paper, we propose a novel fractional Fourier transform (FrFT) based multi-carrier order division multi-access communication system, in which each user is uniquely identified by an FrFT order. Transform domain communication system (TDCS) with FrFT scheme is also proposed to synthesize the wide-band baseband waveforms in all FrFT domains with different users' FrFT orders, which enhances the interference avoidance capability of this system under most of interference. Therefore, multiple independent data streams can be transmitted by using FrFT---OFDM in the same time and different FrFT domains. However, chirp bases, as the new kind of carriers with different modulated rates, are merely mutually approximately orthogonal. There is a problem of energy leakage between multiple chirp carriers, which possibly causes the multiple chirp carriers inter-shielding to influence the FrFT---OFDM demodulation performance. An efficient allocation algorithm of multiple chirp carriers by presetting the carrier parameters is proposed to solve this problem. Based on MC-CDMA, a variable bit rate system structure is proposed for TDCS with FrFT scheme under different channel environments. In order to simplify the process of modulation and demodulation of TDCS with FrFT scheme, a whole new cyclic shift key modulation mode in FrFT domain is also proposed. Both theories and simulations confirm strictly the validity of the proposed system.

In this paper, we investigate the impact of Doppler shift on bit error rate (BER) in OFDM-based transform domain communication system (TDCS) and propose a novel Doppler shift estimation algorithm to recover the system performance. The Doppler shift estimation algorithm in OFDM systems is robustly designed and can be applied in OFDM-based TDCS. However, the accuracy of these existing estimation algorithms will degrade seriously when the signal to noise ratio (SNR) is low. An attractive feature of the proposed algorithm is that it can utilize characteristics of OFDM-based TDCS to estimate Doppler shift accurately with SNR as low as -10dB. Simulation results demonstrate that the proposed algorithm can eliminate the effects of additive noise to estimation accuracy. Furthermore, we show that the system performance improved significantly after Doppler shift compensation.

Spectrum sensing is a key technology and prerequisite for Transform Domain Communication Systems (TDCS). The traditional approach typically involves selecting a working sub-band and maintaining it without further changes, with spectrum sensing being conducted periodically. However, this approach presents two main issues: on the one hand, if the selected working band has few idle channels, TDCS devices are unable to flexibly switch sub-bands, leading to reduced performance; on the other hand, periodic sensing consumes time and energy, limiting TDCS’s transmission efficiency. In contrast to previous studies that unrealistically modeled the problem as a Markov Decision Process (MDP), this study accounts for the fact that TDCS devices cannot fully observe the entire spectrum state and must rely on historical observations, along with the current state of sub-bands, to make informed decisions. We innovatively model this as a Partially Observable Markov Decision Process (POMDP). Moreover, we consider both the number of skipped time slots and the selection of idle sub-bands, establishing distinct termination conditions for each action. By assigning different weights to balance sensing overhead and spectrum utilization while reducing conflicts, the algorithm’s adaptability and performance are improved. To address the Q-value overestimation problem inherent in traditional Deep Recurrent Q-Network (DRQN) due to the use of a single network, we propose a DDRQN-BandShift strategy that combines Double Deep Q-Network (DDQN) and DRQN. Simulation results show that the proposed scheme significantly improves TDCS transmission efficiency while effectively reducing sensing costs.

MMP-DCD-CV based sparse channel estimation algorithm for underwater acoustic transform domain communication system

This paper addresses web object size which is one of important performance measures and affects to service time in multiple access environment. Since packets arrive according to Poission distribution and web service time has arbitrary distribution, M/G/1 model can be used to describe the behavior of the web server system. In the time division multiplexing (TDM), we can use M/D/1 with vacations model, because service time is constant and server may have a vacation. We derive the mean web object size satisfying the constraint such that mean waiting time by round-robin scheduling in multiple access environment is equal to the mean queueing delay of M/D/1 with vacations model in TDM and M/H2/1 model, respectively. Performance evaluation shows that the mean web object size increases as the link utilization increases at the given maximum segment size (MSS), but converges on the lower bound when the number of embedded objects included in a web page is beyond the threshold. Our results can be applied to the economic design and maintenance of web service.

The transmitter identification of the DTV systems becomes crucial nowadays. Transmitter identification (TxID, or transmitter fingerprinting) technique is used to detect, diagnose and classify the operating status of any radio transmitter of interest. A pseudo random sequence was proposed to be embedded into the DTV signal before transmission. Thus, the transmitter identification can be realized by invoking the cross-correlation functions between the received signal and the possible candidates of the pseudo random sequences. Gold sequences and Kasami sequences are two excellent candidates for the transmitter ID sequences as they provide a large family of nearly-orthogonal codes. In order to investigate the sensitivity of the transmitter identification in different topologies and Kasami sequences with different length, we present the analysis here for four different geometric layouts, namely circular distribution, doubly concentric and circular distribution, square array and hexagonal tessellation. The covered area and the lowest received signal-to-interference ratio are considered as two essential parameters for the multiple-transmitter identification. It turns out to be that the larger the Kasami sequence length, the larger the received signal-to-interference ratio. Our new analysis can be used to determine the required Kasami sequence length for a specific broadcasting coverage.

Direct sequence spread spectrum (DSSS) communication has been extensively adopted in military and civilian fields because of its high processing gain, anti-jamming capability and low probability of interception (LPI). In this paper, non-cooperative interception of DSSS signal in multiple signals environment is presented. The proposed algorithm is based on the fluctuations of autocorrelation, and also banks upon proper thresholding criteria. This methodology comes out to be a robust practical method for blind detection of DSSS signals in a dense multiple signals environment. In the proposed approach, firstly DSSS signals are blindly detected by leveraging the variations in autocorrelation of the received signal. Then, the presence of desired DSSS signal is confirmed when correlation peaks cross the threshold at multiple positions after a fixed period. In a real-time environment, signals other than spread spectrum with much better signal-to-noise ratio (SNR) can also co-exist. Among these multiple signals, the high SNR signals disturb the threshold selection criteria for detection. Consequently, this leads to erroneous or miss detection of the DSSS signal. The proposed scheme aims to reduce the effect of better SNR signals on the blind detection of DSSS signal. In order to validate the proposed algorithm, QPSK and 16-QAM signals are considered to co-exist with the desired DSSS signal. Analytical and numerical analysis signify the efficiency of the proposed algorithm with acceptable computational complexity, making it suitable for real-time practical systems.

We present an integrable solution for the detection of defective sensor elements using sigma-delta-(/spl Sigma//spl Delta/)-modulation and a matched filter. The sensor element is stimulated using a pseudo random binary sequence (PRBS). The sensor signal is read out and the analog output is digitized using a /spl Sigma//spl Delta/-modulator. The binary pulse density stream of the /spl Sigma//spl Delta/-modulator is the output of the sensor system and thus should ideally contain the PRBS. A matched filter has the task of detecting the pseudo random sequence in the pulse density stream and its sampled output is compared to a threshold thus making it possible to judge the functionality of the sensor element. By evaluating the magnitude of the matched filter output it is also possible to measure the sensor sensitivity. We present a discrete solution of this method, but an integrated chip using a standard 1.2 /spl mu/m CMOS process has been designed and is being fabricated.

Based on the analysis of the structures of isomorphic multiplicative groups of extended Galois fields, it is established that any cyclic shift of a pseudo-random Gold sequence can be transformed into a function belonging to the complete set of analogues of Rademacher functions of the corresponding dimension. This made it possible to develop a new algorithm for fast synchronization of Gold sequences based on the calculation of their discrete convolution using fast spectral transformation in a truncated basis of Walsh–Hadamard functions. The gain of the developed algorithm in terms of the number of arithmetic operations compared to the traditional method of calculating discrete convolution increases with increasing sequence length N and for N=511.1023 is approximately 3.4 times.

The effects of filtering in transmitters on the bit error performance of a binary asynchronous direct sequence spread spectrum communication system are examined in a multiple access environment. Computer simulations are carried out to evaluate the effects of non-linear phase Chebyshev and Butterworth filters and linear phase Bessel, Gauss and Nyquist roll-off filters. The performance degradation of the system is compared and discussed in terms of the BT product of the filter or the transmitted spectrum. >