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A Robust Audio Watermark Representation Based on Linear Chirps

In this paper, we introduce a novel watermark representation for audio watermarking, where we embed linear chirps as watermark signals. Different chirp rates, i.e., slopes on the time-frequency (TF) plane, represent watermark messages such that each slope corresponds to a unique message. These watermark signals, i.e., linear chirps, are embedded and extracted using an existing watermarking algorithm. The extracted chirps are then postprocessed at the receiver using a line detection algorithm based on the Hough-Radon transform (HRT). The HRT is an optimal line-detection algorithm, which detects directional components that satisfy a parametric constraint equation in the image of a TF plane, even at discontinuities corresponding to bit errors. Simulation results show that HRT correctly detects the embedded watermark message after common signal processing operations for bit error rates up to 20%. The new watermark representation and the postprocessing stage based on HRT significantly improve the performance of the watermark detection process and can be combined with existing watermark embedding/extraction algorithms for increased robustness

Effects of Channel Models and Rake Receiving Process on UWB-IR System Performance

In ultra-wideband impulse radio (UWB-IR) systems, multipath-delayed received pulses may overlap if two consecutive multipaths arrive within less than the pulse duration. This condition makes it nontrivial for the Rake receiver to capture enough multipath energies for reliable communications. In this paper, we study in detail the effects of channel models and Rake receiving process on the performance of UWB-IR systems. Specifically, we consider Tc- and T-spaced channel models and different Rake receiver implementations to maximize the captured signal energy, where we discuss their validity for real system considerations. Also, we show that code shift keying (CSK) impulse modulation can achieve about 1 dB performance gain over conventional pulse position modulation (PPM) when there is pulse overlapping, as it randomizes pulse transmit locations as opposed to fixed pulse transmit locations used by PPM.

Combined M-ary code shift keying/binary pulse position modulation for ultra wideband communications

In this paper, a novel modulation format is proposed for time-hopping pulse position modulation (TH-PPM) ultra wideband (UWB) communications, where a near-orthogonal set of user-specific TH codes is employed for M-ary code shift keying (MCSK) to carry additional data. A particular TH code, selected by log/sub 2/ M-bit data, modulates the basic pulses that are time-shifted by binary PPM (BPPM), resulting in combined MCSK and BPPM (i.e., MCSK/BPPM). The combined modulation can achieve a high data rate without affecting pulse shaping and further improve the bit-error-rate (BER) performance subject to the same signal energy per bit.

Compressive sensing for ultra-wideband channel estimation: on the sparsity assumption of ultra-wideband channels

Due to the sparse structure of ultra-wideband UWB multipath channels, there has been a considerable amount of interest in applying the compressive sensing CS theory to UWB channel estimation. The main consideration of the related studies is to propose different implementations of the CS theory for the estimation of UWB channels, which are assumed to be sparse. In this study, we investigate the suitability of standardized UWB channel models to be used with the CS theory. In other words, we question the sparsity assumption of realistic UWB multipath channels. For that, we particularly investigate the effects of IEEE 802.15.4a UWB channel models and the selection of channel resolution both on channel estimation and system performances from a practical implementation point of view. In addition, we compare the channel estimation performance with the Cramer-Rao lower bound for various channel models and number of measurements. The study shows that although UWB channel models for residential environments e.g., channel models CM1 and CM2 exhibit a sparse structure yielding a reasonable channel estimation performance, channel models for industrial environments e.g., CM8 may not be treated as having a sparse structure due to multipaths arriving densely. Furthermore, it is shown that the sparsity increased by channel resolution can improve the channel estimation performance significantly at the expense of increased receiver processing. Copyright

Robust audio watermarking using a chirp based technique

In this study, we propose a new spread spectrum audio watermarking algorithm that embeds linear chirps as watermark messages. Different chirp rates, i.e., slopes on the time-frequency (TF) plane, represent watermark messages such that each slope corresponds to a different message. We extract the watermark message using a line detection algorithm based on the Hough-Radon transform (HRT). The HRT detects the directional elements that satisfy a parametric constraint in the image of a TF plane. The proposed method not only detects the presence of watermark, but also extracts the embedded watermark bits and ensures the message is received correctly. The results show that the HRT detects the embedded watermark message even after common signal processing operations such as MPEG audio coding, resampling, lowpass filtering and amplitude re-scaling.

The effect of channel models on compressed sensing based UWB channel estimation

Ultra-wideband (UWB) multipath channels are assumed to have a sparse structure as the received consecutive pulses arrive with a considerable time delay and can be resolved individually at the receiver. Due to this sparse structure, there has been a significant amount of interest in applying the compressive sensing (CS) theory to UWB channel estimation. There are various implementations of the CS theory for the UWB channel estimation based on the assumption that the UWB channels are sparse. However, the sparsity of a UWB channel mainly depends on the channel environment. Motivated by this, in this study we investigate the effect of UWB channel environments on the CS based UWB channel estimation. Particularly, we consider the standardized IEEE 802.15.4a UWB channel models and study the channel estimation performance from a practical implementation point of view. The study shows that while UWB channel models for residential environments (e.g., CM1 and CM2) exhibit a sparse structure yielding a reasonable channel estimation performance, channel models for industrial environments (e.g., CM8) may not be treated as having a sparse structure due to multipaths arriving densely. The results of this study are important as it determines the suitability of different channel models to be used with the CS theory.

Achievable performance of Bayesian compressive sensing based spectrum sensing

In wideband spectrum sensing, compressive sensing approaches have been used at the receiver side to decrease the sampling rate, if the wideband signal can be represented as sparse in a given domain. While most studies consider the reconstruction of primary users signal accurately, it is indeed more important to analyze the presence or absence of the signal correctly. Furthermore, these studies do not consider the achievable lower bounds of reconstruction error and how well the selected method performs correspondingly. Motivated by these issues, we investigate in detail the primary user detection performance of Bayesian compressive sensing (BCS) approach in this paper. Accordingly, we (i) determine the BCS signal reconstruction performance in terms of mean-square error (MSE), compression ratio and signal-to-noise ratio (SNR), and compare it with the conventionally used basis pursuit approach, (ii) determine how well BCS performs compared with the Bayesian Cramer-Rao lower bound (BCRLB) of the signal reconstruction error, and (iii) assess the probability of detection performance of BCS for various SNR and compression ratio values. The results of this study are important for determining the achievable performance of BCS based spectrum sensing.

Code Shift Keying Impulse Modulation for UWB Communications

In this paper, the system performance of M-ary code shift keying (MCSK) impulse modulation is studied in detail and compared to M-ary pulse position modulation (MPPM) under single- and multi-user scenarios. For that, bounds on the semi- analytic symbol-error rate (SER) expressions are derived and simulation studies are conducted. When practical implementations of MCSK and MPPM are considered, it is shown that MCSK can provide about 2 dB performance gain over MPPM as it reduces the effects of multipath delays on the decision variables by randomizing locations of the transmit pulse.

Spectral Characteristics of M-ary Code Shift Keying Based Impulse Radios: Effects of Code Design

This paper analyzes the power spectral density (PSD) characteristics of ultra wideband (UWB) signals modulated by M-ary code shift keying (MCSK) that can be combined with binary pulse position modulation (BPPM) and binary pulse amplitude modulation (BPAM), which we refer to as MCSK based impulse radios (IR). MCSK based IR are modified IR that were designed to increase the data rate of conventional IR by embedding the data on a time hopping (TH) code randomly selected from a set of M distinct TH codes per user. This random selection also results in increased effective TH code period, which- helps smoothing the continuous spectrum and suppressing the discrete spectral components. In combination with the random code selection, design of the TH code set for each user is very important for spectrum shaping and multiple access (MA) capability, and is addressed in detail in the paper.

Linear combination of pulses for coexistence in the IEEE 802.15.4a standard

The low data rate, accurate location-ranging Wireless Personal Area Network (WPAN) standard IEEE 802.15.4a, which is based on ultra-wideband (UWB) impulse radios, suggests using linear combination of pulses to reduce interference to coexisting primary systems. In this paper, we consider possible implementations of linear combination of pulses as suggested in the standard, and study the generated notches in the resulting code spectrum. Using the z-transform approach, we show that the number of these notches is limited and the notch locations are not flexible due to the limited number of pulses allowed by the standard. We also make some suggestions for increasing the number of notches at desired frequencies.