Agile Detection of DRM Signal via GLRT

Orthogonal frequency division multiplexing (OFDM) has been widely adopted in dual-function radar-communication (DFRC) systems. However, with random communication symbols (CS) embedded in the DFRC waveform, the transmit signal has a random ambiguity function that affects the radar's delay-Doppler estimation performance, which has not been well explored. This paper addresses this gap by first characterizing the outlier probability (OP) -the probability of incorrectly estimating a target's (on-grid) delay-Doppler bin -in OFDM DFRC for any given CS realization. This subsequently motivates the OFDM DFRC waveform design problem of minimizing the OP w.r.t the CS probability distribution (i.e., the input distribution). Conditioned on the CSs, the OP only depends on the CS magnitudes. Hence, we consider the following two schemes for the above optimization: CSs with (1) constant magnitude input distribution (phase shift keying), and (2) variable magnitude input distribution (Gaussian). For (1), minimizing the OP reduces to the familiar power allocation design across OFDM's subcarriers and symbols, with uniform power allocation across OFDM subcarriers and a windowed power allocation across OFDM symbols being near-optimal. For (2), the mean and variance of the Gaussian distribution at each subcarrier is optimized, with an additional communication constraint to avoid the zero-variance solution where no CSs are carried. We observe that subcarriers with strong communication channels feature a large variance (favour communications) while the others are characterized by a large mean (favour radar). However, the overall power allocation (i.e., the sum of the squared mean and variance) across the OFDM subcarriers and symbols is similar to (1). Simulations for (2) show that while random CS magnitudes benefit communications, they degrade radar performance, but this can be mitigated using our optimized input distribution.

In this paper, designs for the New Radio (NR) physical uplink control channel (NR PUCCH) spanning 1 orthogonal frequency division multiplexing (OFDM) symbol duration (short-PUCCH) is investigated. Two short-PUCCH formats i.e., Format A, in which demodulation reference signal (DMRS) and uplink control information (UCI) are FDMed, and Format B, in which DMRS and UCI are TDMed, are proposed and evaluated by computer simulation. For Format A, various DMRS overheads can be realized by adjusting the numbers of DMRS subcarriers and that of UCI subcarriers for the given 1 OFDM symbol. For Format B, we assume no FDM between DMRS and UCI, so that discrete Fourier transform (DFT)-spread OFDM (DFT-s-OFDM) waveform is applicable; in order to adjust DMRS overhead, we locate more than one OFDM symbols within the given 1 OFDM symbol duration by using higher subcarrier spacing (SCS) for the short-PUCCH with Format B. The link-level evaluation results and link-budget analysis show that Format A offers better link-level performance than Format B, while better link-budget performance is achieved by Format B, with taking into account the peak-to-average power ratio (PAPR) power back-off margin.

In orthogonal frequency division multiplexing (OFDM) systems, a number of OFDM symbols form one subframe and consecutive subframes may be allocated to different users. Automatic gain control (AGC) is commonly employed in OFDM systems because the power of receiving signals can vary over a large dynamic range. After measuring the power of cyclic prefix (CP) of the first OFDM symbol in a subframe, AGC adjusts the gain before receiving the useful portion (UP). Otherwise, there is inter-carrier interference (ICI). CP length is usually chosen equal to the delay spread of the multipath channel. After passing through the channel, CP of the first OFDM symbol in a subframe contains the power of the last OFDM symbol in the prior subframe. When these two subframes are allocated to distinct users with different transmit powers, AGC gain is inaccurate and system performance is degraded. To solve this problem, we extend the CP of the first OFDM symbol in a subframe to include an AGC portion (AP). On the assumption that transmitted time domain signal is circularly symmetric complex Gaussian random variable, the required length of AP is also derived. Simulation results show that the proposed scheme has a gain of 0.0dB ~ 6.2dB at BLER of 0.1 over the prior scheme with 0.34% overhead.

In this work, a new technique for mitigating the impulsive noise impact on the orthogonal frequency division multiplexing (OFDM) system is proposed, and its performance is compared with a previously published technique (referred to as time-domain interleaving (TDI) technique). In the TDI technique, the samples contaminated by impulsive noise are spread in timedomain over N OFDM symbols by using an interleaver after the inverse fast Fourier transform (IFFT) process. Accordingly, the effect of the impulsive noise burst will be averaged over N OFDM symbols, which reduces the impact on the bit error rate (BER) considerably. However, to achieve the same goal, the proposed system is using an additional orthogonal transform in form of an IFFT at the output of the interleaver. In general, the two techniques have shown a superior improvement in BER performance compared to that of the standard OFDM system, which suffers from error floors at high values of signal to noise ratio (SNR). However, for quadrature phase shift keying (QPSK) modulation with low signal to impulsive noise ratio (SIR), the proposed technique outperforms the TDI technique for different impulsive noise distributions. For high modulation order 16 and 64 quadrature amplitude modulation (QAM) in severe impulsive noise channels, the proposed technique demonstrates more robustness than the TDI, which suffers from error floors for the considered values of SIR and impulsive noise distributions.

In high‐mobility scenarios, the time variation of mobile radio channels leads to a loss of orthogonality among subcarriers in orthogonal frequency division multiplexing (OFDM) systems, resulting in intercarrier interference (ICI) and performance deterioration. Conventional channel estimation schemes are usually based on pilot tones, which are distributed in each OFDM symbol to estimate the channel variation. Hence, the channel estimator itself suffers from ICI. In this study, a new estimation scheme, which does not suffer from ICI, is proposed to estimate the channel variation within OFDM symbols. The main idea is to zero‐pad (ZP) the OFDM symbol in the time domain. Then, in the middle of the ZP interval, an impulse signal is inserted as a pilot sample, which is used to estimate the channel at the pilot signal in the OFDM symbol. Finally, a linear model is used to estimate the channel variation over an OFDM symbol. Additionally, we derive the mean squared error (MSE) of the proposed estimation technique under the constraint that the channel varies linearly within OFDM symbols. Simulation results show that our scheme can achieve a substantial improvement in the bit error rate (BER) performance of OFDM, in spite of the OFDM symbol length being increased. Moreover, in many cases, the new scheme can achieve the same BER performance as the perfect knowledge of channel state information (CSI). Theoretical analysis and numerical simulations show that our scheme achieves excellent performance with much lower computational complexity.

A novel technique for Peak-to-Average Power Ratio (PAPR) reduction based on dithering is presented. The dithering method consists of adding a pre-defined random noise pattern to the transmitter time-domain Orthogonal Frequency Division Multiplexing (OFDM) symbol. The noise addition is performed to reduce the high peaks of the time-domain OFDM symbol. For each OFDM symbol, there is a process of selecting the most appropriate pre-defined noise pattern which achieves the lowest PAPR. We show that with merely 0.25 dB power increase to the time-domain transmitter OFDM signal, we attain the same Error-Vector Magnitude (EVM) in the receiver in different modulation schemes, with over 2 dB reduction in PAPR. The dithering information is transferred between the transmitter and the receiver using the pilot bits, thereby avoiding capacity reduction.

The inter-carrier interference (ICI) of the orthogonal frequency division multiplexing (OFDM) symbols caused by the transducers dynamic in underwater acoustic (UWA) communications has been presented in this paper. Both the practical experiment and simulation results show that the othorgonality property of OFDM symbols can be affected by the transducers dynamic during the transient period of these devices especially when the OFDM symbol length is comparatively short. A small cyclic prefix (CP) method is proposed to overcome this problem without much loss of the throughput and the bandwidth efficiency. The small CP, with the length just longer than the transient response of the transducer, can eliminate the ICI problem. The theoretical analysis and the simulation results show the efficiency of the method.

This paper deals with optimal training design and placement over multiple orthogonal frequency division multiplexing (OFDM) symbols for the least squares (LS) channel estimation in multiple-input multipleoutput (MIMO) OFDM systems. First, the optimal pilot sequences over multiple OFDM symbols are derived by co-cyclic Jacket matrices based on the minimum mean square error (MSE) of the LS channel estimation. Then, an enhanced channel estimation method using sliding window is proposed to improve further the performance for the optimal pilot sequences in fast-varying channels. Simulation results show that the enhanced method can efficiently improve the performances for the optimal pilot sequences over two and four OFDM symbols, especially in fast-varying channels.

A novel Peak to Average Power Ratio (PAPR) reduction method leveraging the rare occurrence of large magnitude Orthogonal Frequency Division Multiplexing (OFDM) symbols is proposed. A distinct innovation of this approach is the exploitation of the sparseness structure of a bit-plane binary number representation of time domain OFDM symbols. By encoding the sparsely populated non-zero entries in the binary bit plane matrix, the dynamic range of OFDM symbols to be transmitted can be significantly reduced without increasing bit error rate. Smaller dynamic range of OFDM symbols implies reduced PAPR, and hence better performance. This method incurs no data rate reduction penalty which may occur when side information must be transmitted via some data subcarriers. Simulation results reveal that this proposed new method out-performs current PAPR reduction methods by wide margins.

In this work, the performance of a novel linear interpolator scheme that uses K successive orthogonal frequency division multiplexing (OFDM) symbols is examined. The estimator collects K successive OFDM symbols to create a virtual super symbol (VSS) with K-P pilots using P pilots from each OFDM symbol. The (K - 1) middot P pilots are shifted in frequency in such way that in the VSS their distance is divided by K compared to standard symbols. Successive least squares (SLS) channel estimation through linear interpolation is performed by using more than one symbol in the LS estimator introducing the weight channel change (WCC) factor that represents the change of the wireless channel between the successive symbols used. A novel analysis deriving exact expressions for the mean square error (MSE) as a function of pilot spacing, channel statistics and WCC for an OFDM system where data subcarriers also exist before the first and after the last pilot is given. The MSE and the bit error rate (BER) of the estimator are characterized by the number and distance of the pilots, the Doppler spread and the WCC factor. Simulations show the effect of each factor on MSE and BER performance.

The statistical analysis of failure detection decisions in terms of the instantaneous probabilities of false alarm and correct detection for a specified failure magnitude at each check-time have previously been performed for several different failure detection techniques that utilize a Kalman filter. By performing a discrete-time specialization of a result of Gallager and Helstrom on a tightened upper bound for continuous-time level-crossing probabilities, upper bounds on the probabilities of false alarm and correct detection over a time interval have been obtained for the specific technique of CR2 tailnre detection (to allow an accounting for the effect of time correlations of the filter estimates). When these upper bounds are optimized to be as tight as possible to the desired probabilities, the resulting optimization problem for discrete-time is a collection of quadratic programming (QP) problems, which may easily be solved exactly without recourse to approximate solutions as were resorted to in the continuous-time formulation. This technique for evaluating tightened upper bounds on the false alarm and correct detection probabilities may be of general interest, since it can be applied to any failure detection technique or signal detection technique that can relate an exceeding of the deterministic decision threshold by the test statistic directly to a deterministic level being exceeded by a scalar Gaussian random process.

It is usually assumed that N samples of the time domain orthogonal frequency division multiplexing (OFDM) symbols have an identical Gaussian probability distribution (PD) in the real and imaginary parts. In this article, we analyze the exact PD of M-QAM/OFDM symbols with N subcarriers. We show the general expression of the characteristic function of the time domain samples of M-QAM/OFDM symbols. As an example, theoretical discrete PD for both QPSK and 16-QAM cases is derived. The discrete nature of these distributions is used to reconstruct the distorted OFDM symbols due to deliberate clipping or amplification close to saturation. Simulation results show that the data reconstruction process can effectively lower the error floor level.

In this paper, an algorithm that estimates the parameters of impulse noise (IN) is proposed in power line communication. The Peak to Average Power Ratio (PAPR) of the Orthogonal Frequency Division Multiplexing (OFDM) symbol sent by the transmitter is reduced by the selective mapping (SLM) technology. In addition, the peak value of each OFDM symbol is sent with a separate control channel. At receiver, the frequency characteristics of IN is calculated using the received peak value and the OFDM symbol. The noise used for simulation comes from actual measurement. Results from simulations show that the proposed algorithm can estimate the IN well under different IN environments, and the obtained parameters are very valuable for mitigating IN in power line communication.

Different guard interval techniques for the Orthogonal Frequency Division Multiplexing (OFDM) transmission are suggested to reduce the interference between successive symbols (i.e. Inter-Symbol Interference, ISI) and interference between different carriers within the same symbol (i.e. Inter-Carrier Interference, ICI). In this paper, the impact of replacing the Cyclic Prefix (CP) by zeros insertion (ZI) before the Inverse Fast Fourier Transform (IFFT) process on OFDM is studied. Associated with each OFDM symbol, the zeros will be added in the transmitter before the IFFT process. For symmetry of the OFDM symbol, the zeros will be inserted with the same length among the OFDM frames. The ZI acts as a buffer region where delayed information from the previous symbols can get stored. The receiver has to exclude samples from the ZI which got corrupted by the previous symbol when choosing the samples for an OFDM symbol. The motivation of using the ZI instead of the cyclic prefix is the reduction in the transmission rate and the high performance in reducing of the channel distortion. The performance comparison among the proposed ZI technique, the CP, Zero Padding (ZP), and Known Symbol Padding (KSP) is introduced in this paper. The results show that the KSP has a slightly worse performance; however, the proposed ZI approach provides better performance than the other techniques and achieves 20% reduction in the data redundancy relative to the redundancy in the CP-OFDM system.

Orthogonal Frequency Division Multiplexing (OFDM) is a very attractive technique for high data rate transmission with good bandwidth efficiency. OFDM system utilize the cyclic prefix (CP) to mitigate the effect of inter symbol interference (ISI) produced by the multipath effect of wireless channel. The CP insertion only mitigates the effect of inter block interference of OFDM symbols, whereas the intra block interference remain present in the OFDM symbol. The intra block OFDM symbol interference degrades the error performance of the OFDM system. In order to improve the error performance of the OFDM system forward error correction scheme can be utilized. Many FEC schemes for single carrier and multicarrier systems have been proposed in the literature. Convolution and LDPC codes are two popular coding schemes for FEC. In this paper we have considered Convolution and LDPC codes for improving the error performance of OFDM system. The BER performance of convolution ally coded a LDPC coded OFDM is evaluated for various code rates in AWGN and fading channel. We have also compared the performance of coded OFDM system with an OFDM system without FEC.