Inter-pulse Interference Research Articles

Improved low-complexity transmitted reference pulse cluster for ultra-wideband communications

Transmitted reference pulse cluster (TRPC) signalling was recently proposed for robust low-rate ultra-wideband (UWB) communications. TRPC provides significant performance gains over conventional transmitted reference (TR) UWB, and overcomes the critical long delay line problem associated with TR UWB. Thus, it is suited for practical implementation of a UWB communication system. The compact spacing of the pulses in TRPC leads to inter-pulse interference (IPI). In this study, the authors show that there exists an asymmetry between the two conditional distributions of the autocorrelation decision variable because of this IPI. This implies that the conventional zero decision threshold (ZDT) is not optimal for a TRPC autocorrelation receiver, and hence the authors derive the optimal maximum likelihood decision threshold. The authors propose two practical techniques for threshold determination. The first technique employs a training sequence whereas the other simply uses the decision variables of the received data signals to estimate the threshold. Simulation results show that in terms of the bit error rate (BER) performance, both the training-assisted adaptive decision threshold (TA-ADT) and the data-assisted ADT (DA-ADT) can achieve significant gains over the conventional ZDT-based detection for TRPC.

UltraWide Band Cognitive Pulse Shaping under Physical-Layer QoS Constraints

Ultra Wide Band (UWB) communication systems operate in the frequency range between 0 and 10.6 GHz so they induce the Scientific Community to solve the problem of coexistence with concurrent telecommunication services. This is the leading reason why both the Federal Communications Commission (FCC) and the European Telecommunications Standards Institute (ETSI) gave strict indications about the spectral limits to be respected and require the transmitter and receiver to be compliant with these spectral masks. To this end, it is mandatory to carefully shape the UWB pulse, for this can be accurately designed so as to avoid severe performance reduction while guarding inter-systems coexistence. The UWB technology and, more, the pulse shaping allow to apply the cognitive paradigm where the transmitter and receiver are the actors of this functionality since the performance are tied to channel features and interference presence. The widespread choice of Gaussian-like pulses has proven, however, largely suboptimal from a power emission point of view since they fail to optimize performance. Goal of this contribution is to show how to achieve a good compromise between spectral emission, rate and synchronization errors robustness, via a modified version of the Parks-McClellan method, considering channel impairments due to its frequency-selective nature and to the inter-pulse interference.

Spatial Capacity of UWB Networks with Space-Time Focusing Transmission

Space-time focusing transmission in impulse-radio ultra-wideband (IR-UWB) systems resorts to the large number of resolvable paths to reduce the interpulse interference as well as the multiuser interference and to simplify the receiver design. In this paper, we study the spatial capacity of IR-UWB systems with space-time focusing transmission where the users are randomly distributed. We will derive the power distribution of the aggregate interference and investigate the collision probability between the desired focusing peak signal and interference signals. The closed-form expressions of the upper and lower bound of the outage probability and the spatial capacity are obtained. Analysis results reveal the connections between the spatial capacity and various system parameters and channel conditions such as antenna number, frame length, path loss factor, and multipath delay spread, which provide design guidelines for IR-UWB networks.

From a discrete to a continuous model for interpulse interference with a frequency-comb laser

We have investigated correlation patterns generated by a frequency-comb laser in a dispersive unbalanced Michelson interferometer and apply the developed formalism to the case of distance metrology. Due to group velocity dispersion, the position of the brightest fringe of the correlation pattern, which is used for distance determination, cannot be derived by simply using the definition of group refractive index of the dispersive medium. It is shown that the discrete spectrum of the optical frequency comb gives rise to correlation functions which can be represented by a series, namely a discrete correlation series. We have developed a general formalism, valid for any pulse train, extending the discrete model to a continuous model of cross-correlation functions using the Poisson summation. Our model is relevant for any offset and repetition frequency of the frequency comb. From the continuous cross-correlation model we show that, even for a homogeneous dispersive medium the position of the brightest fringe varies nonlinearly for small delay distances and stabilizes for longer ones. We have compared the theoretical results to measurements of pulse propagation in air for path-lengths up to 200 m.

Transmit-reference UWB system based on m-sequence codes

In this article, a novel transmit-reference (TR) signaling scheme is proposed for ultra-wideband (UWB) system, where by invoking m-sequence codes the reference and data pulses can be transmitted side by side to increase the data rate. This structure enables demodulation with a simple and practical autocorrelation receiver despite existence of severe inter-pulse interference (IPI). To evaluate detection performance of the new designs, closed-form expression for bit error probability (BRP) is theoretically derived and simulation results confirm the analytical results. In addition, the proposed scheme can be extended to M-ary systems to further improve the data rate and power efficiency.

Multiuser performance of M-ary orthogonal coded/balanced UWB transmitted-reference systems

A tractable and compact closed-form expression on the channel-averaged signal-to-interference-plus-noise ratio (SINR) is derived for M-ary orthogonal coded/balanced transmitted-reference (BTR) systems, taking into consideration both inter-pulse interference (IPI) and multiple-access interference (MAI) in dense multipath ultra-wideband (UWB) channels. The UWB channel here is a realistic and standard one with lognormal channel gain distribution and double independent Poisson arrival distribution of cluster and ray. Hence, the analytical framework developed here can be applied to typical UWB channel models, especially considering the channel sparseness and cluster overlapping observed in realistic UWB channels. Based on the channel-averaged SINR, the effect of inter-pulse distance (between the reference and data pulses in BTR) on the multiuser performance is fully investigated. A proper selection of user-specific inter-pulse distances in multiuser scenario is then determined to maximize the user capacity for a given UWB channel.

Digital Receiver Design for Transmitted Reference Ultra-Wideband Systems

A complete detection, channel estimation, synchronization, and equalization scheme for a transmitted reference (TR) ultra-wideband (UWB) system is proposed in this paper. The scheme is based on a data model which admits a moderate data rate and takes both the interframe interference (IFI) and the intersymbol interference (ISI) into consideration. Moreover, the bias caused by the interpulse interference (IPI) in one frame is also taken into account. Based on the analysis of the stochastic properties of the received signals, several detectors are studied and evaluated. Furthermore, a data-aided two-stage synchronization strategy is proposed, which obtains sample-level timing in the range of one symbol at the first stage and then pursues symbol-level synchronization by looking for the header at the second stage. Three channel estimators are derived to achieve joint channel and timing estimates for the first stage, namely, the linear minimum mean square error (LMMSE) estimator, the least squares (LS) estimator, and the matched filter (MF). We check the performance of different combinations of channel estimation and equalization schemes and try to find the best combination, that is, the one providing a good tradeoff between complexity and performance.

Orthogonally-spread block transmissions for ultra-wideband impulse radios

Differential, transmitted reference (TR) and energy detection (ED) based ultra-wideband impulse radios (UWBIR) can collect the rich multipath energy offered by UWB channels with a low-complexity receiver. However, they perform satisfactorily only when the channel induced inter-pulse interference (IPI) is negligible. This can be achieved by appending a guard interval with duration greater than or equal to the channel's delay spread to each frame an operation limiting the maximum achievable data rate. As a remedy, this Letter advocates block transmissions in conjunction with orthogonal spreading sequences to remove the introduced IPI. The resultant scheme requires no channel knowledge besides timing offset and incurs slightly more complexity than non-block alternatives, while it increases the data rate at no cost in error performance. Given a fixed data rate of 25 Mbps, the novel block scheme exhibits about 1.8 dB gain relative to its non-block counterpart in single-user simulated tests.

Performance analysis of transmitted‐reference UWB systems with narrowband interference suppression

AbstractIn this paper, we propose a receiver structure for transmitted‐reference ultra‐wideband (TR‐‐UWB) systems with both narrowband interference (NBI) and inter‐pulse interference (IPI) mitigation capabilities. The effects of additive white Gaussian noise (AWGN) and the IEEE 802.15.4a fading channel are also taken into consideration. We adopt band‐stop filtering to suppress the NBI. For IPI, it is statistically removed by a sum‐and‐average process. Theoretical analysis is carried out to study the lower bound of the bit‐error rate (BER) performance of the proposed receiver. Numerical results show that the proposed receiver is able to provide satisfactory performance and is robust to variations in the system design parameters. It is also observed that the proposed receiver is able to deliver good performance even when there is zero delay between the reference and data pulses, which can effectively double the system throughput. Copyright © 2008 John Wiley & Sons, Ltd.

Effects of the pulse area and pulse number on the population dynamics of atoms interacting with a train of ultrashort pulses

We theoretically investigate the effects of the pulse area and the pulse number in a train of ultrashort pulses with an ultrahigh repetition rate in terms of the population dynamics. Because of the interpulse interference the upper state population exhibits an oscillatory behavior as a function of the laser detuning. The pulse area and the pulse number significantly affect the periodic structure of population oscillation.

Statistical Analysis of UWB Channel Correlation Functions

Various performance metrics of impulse-radio (IR) ultrawideband (UWB) receivers are closely connected to the correlation functions of the multipath channel responses to UWB pulses. Interpulse interference is related to the autocorrelation function (ACF) of the received pulse (RP), the RP energy and its fading correspond to the ACF at zero lag, and multiple access interference is connected to the cross-correlation function (CCF) between two channel pulse responses. Each realization of the multipath channel shows different correlation functions due to the ldquorandomnessrdquo of the UWB propagation environment. This paper derives the first- and second-order statistics of the ACF and CCF, capturing this randomness. Such results are useful for incorporating the multipath channel into the performance and design optimization studies of UWB systems. The analysis is based on a model of the received UWB pulse. The model describes the random channel response by two continuous functions of the excess delay time-one expresses the power, the other expresses power variations-and by a prototype pulse shape representing all linear system components including the band limitation of the RP and antenna effects. The analytical results are validated through the analysis of simulated and measured channel responses.

Multiple Access Performance of Balanced UWB Transmitted-Reference Systems in Multipath

Recently, a novel balanced transmitted-reference (TR) system has been proposed for UWB communications. This TR system is capable of properly eliminating the inter-pulse interference (IPI) between the reference and data pulses in a single user multipath environment. In this paper, we investigate its multiple access (MA) performance by evaluating the second- order moments of noise and interference terms. The analytical framework developed here can also be used to accurately evaluate the MA performance of conventional TR system. It is shown that the proposed balanced TR system can achieve comparable MA capacity as conventional TR, while operating at higher information rate. On the other hand, given a target information rate, the balanced TR system in conjunction with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary signaling offers higher MA capacity than conventional TR system. The performance results suggest there exists a trade-off between the inter-pulse distance, thus the achievable information rate, and the MA capacity.

M-ary orthogonal coded/balanced ultra-wideband transmitted-reference systems in multipath

A new M-ary orthogonal coded signaling is introduced to avoid the inter-frame interference that is especially detrimental to realizing high rate ultra-wideband (UWB) transmitted-reference (TR) systems. To further increase the information rate, the inter-pulse interference by an overlap of multipath-delayed pulses is controlled by integrating the signaling and a pair of balanced matched Alters in a joint manner, so as to permit a shorter time delay between the reference and data pulses in TR systems. To evaluate an achievable information rate increase relative to conventional TR, the symbol error probability (SEP) is theoretically derived for the proposed M-ary orthogonal coded/balanced TR system, considering the realistic IEEE standard UWB channel models. In addition, we consider the issue of receiver complexity and present two alternative low- complexity receiver implementations for the proposed TR system.

Optimal MIMO UWB-IR Transceiver for Nakagami-fading and Poisson-Arrivals

In this contribution, we develop a (novel) fam- ily of Multiple-Input Multiple-Output (MIMO) UWB Impulse-Radio (UWB-IR) transceivers for Orthogonal PPM- modulated (OPPM) coded transmissions over (baseband) multipath-faded MIMO channels. To by-pass expensive channel-estimation procedures, the MIMO channel path- gains are assumed to be fully unknown at the receiver. Thus, according to the UWB-IR statistical channel-models currently reported in the literature for both indoor/outdoor application scenarios, we develop and analyze three versions of the resulting noncoherent transceiver, that are optimal for Nakagami, Gaussian, and Log-normal distributed channel- gains, respectively. As dictated by the Saleh-Valenzuela (SV) UWB model, the resulting noncoherent Maximum- Likelihood (ML) Decoder explicitly accounts for the Poisson- distribution of the path-arrivals. Hence, after analytically evaluating the performance of the proposed noncoherent transceiver via suitable versions of the Union-Chernoff bound, we prove that the family of Space-Time OPPM (STOPPM) recently presented in the Literature is able to at- tain full-diversity in the considered multipath-affected appli- cation scenario. To corroborate the carried out performance analysis, we report several numerical results supporting both the medium/long coverage ranges attained by the proposed STOPPM-coded noncoherent transceiver, and its perfor- mance robustness against the degrading effects induced by Inter-Pulse-Interference (IPI), spatially-correlated multipath fading and mistiming.

Transmitter-Side Multipath Preprocessing for Pulsed UWB Systems Considering Pulse Overlapping and Narrow-Band Interference

We analyze a prerake diversity combining scheme for pulsed ultrawideband systems to shift signal processing needs from the receiver to the transmitter. We consider the realistic case that received pulses carrying the same transmitted symbol could overlap with one another in optimizing the prerake scheme based on zero-forcing and eigenanalysis techniques. We show that in the presence of interpulse interference caused by pulse overlapping, the optimum prerake combining scheme in the sense of maximizing the received signal-to-noise ratio derived by using the eigenanalysis technique performs the same as a conventional rake with maximal ratio combining. We also analyze the different behaviors of prerake and rake schemes in the presence of tone interferers and provide numerical results to validate the conclusions in the presence of in-band-modulated interferers. We find that when the number of narrow-band interference (NBI) signals is small, the prerake scheme outperforms the rake scheme; as the number of NBI sources increases, however, this performance gap reduces.

Hexagonal Multicarrier Modulation: A Robust Transmission Scheme for Time-Frequency Dispersive Channels

By regarding signal transmission as tiling of the time-frequency plane, we propose a multicarrier transmission scheme on hexagonal lattice, coined the term hexagonal multicarrier modulation. To optimally combat the impact of the propagation channels, the underlying lattice parameters and the pulse shape of modulation waveform are jointly optimized to adapt to the channel scattering function from a minimum energy perturbation point of view. It comes out that there exist two equivalent pulse and lattice matching criteria for each channel scattering function to achieve minimal interpulse interference (IPI). Robustness of the proposed system against the wireless channels is analyzed in terms of robustness factor. Numerical analysis and simulation results show that the hexagonal multicarrier transmission system outperforms conventional orthogonal-frequency-division multiplexing (OFDM) and lattice-OFDM (LOFDM) systems with regard to robustness against channel dispersion

Performance analysis of b-bit digital receivers for TR-UWB systems with inter-pulse interference

An ultra-wideband (UWB) transmitted reference (TR) system transmits an un-modulated pulse and a delayed modulated pulse pair. Then, a correlation receiver uses the former to demodulate the latter. Because of the long spread of a typical UWB channel, time delay between the two pulses is preferable to be larger than the channel delay spread for reduced noise at the receiver. However, for bandwidth efficiency, that delay should be made small, resulting in inter-pulse interference at the receiver. In this paper, digital receivers are constructed for TR-UWB systems including inter-pulse interference. A typical mean matching technique, appropriate for both PPM and PAM schemes, is implemented digitally to obtain a good template for symbol detection. Joint estimation and detection performance of this family of digital receivers, using finite number of bits in analog-to-digital conversion and finite noisy observations, is analyzed. Closed form results are derived and verified by computer simulations. In addition, the effect of time offset between the reference pulse and information carrying pulse is studied. Overlap of the two pulses does not incur noticeable performance degradation. The proposed analytical framework can be applied to study detection performance of other related digital receivers not covered in this paper

On the optimum linear receiver for impulse radio systems in the presence of pulse overlapping

In impulse radio ultra-wideband systems, multipath delay may cause received pulses to overlap with each other. Pulse overlapping causes inter-pulse interference (IPI) which may, especially in dense multipath environments, severely limit the system performance. In this letter, we build a mathematical model with pulse overlapping considered and derive an optimum minimum mean-square error (MMSE) receiver. A simpler RAKE receiver is to take samples for each received pulse and perform maximal ratio combining (MRC) by ignoring the IPI. We then show, by an analytical approach, that the optimum linear MMSE receiver performs exactly the same as the simpler MRC receiver.

Optical interference and pulse propagation in multimode fibers

Abstract The role played by optical interference in determining the propagation of pulses in multimode fibers is examined under conditions of mode independence and mode coupling. Single pulses and pulse-pairs generated by a source that has arbitrary temporal and spatial coherence are considered, and the role played by the spatial integration at the detector, which effectively is a form of spatial incoherence, is investigated. It is shown that three forms of optical interference may result under different conditions of source spatiotemporal coherence—intramodal, intermodal, and interpulse interference. These effects change the profile of received pulses in a, in general, nonlinear way.

A Stripline-Implemented Baseband Signal Processor for Data and Clock Regeneration of PQM

A baseband signal processor for 1 Gbit/s pulse quaternary modulation (PQM), a pulse modulation format, has been developed and tested. A coupled stripline network is implemented for signal formatting, along with a frequency/phase tracking pulse demultiplexer which reduces predetection bandwidth and interpulse interference. Measured BER performance is presented along with computer simulation predictions.