Impulse Radio Systems Research Articles

Searchback Algorithms for TOA Estimation in Non-coherent Low-rate IR-UWB Systems

Non-coherent ultra-wideband (UWB) receivers offer an attractive low-complexity solution to ranging with UWB radios. They use a simple receiver architecture that can operate at very low sampling rates compared to the Nyquist rate. In this paper, time-of-arrival (TOA) estimation performances of two different searchback algorithms for non-coherent impulse radio UWB systems are analyzed: jump back and search forward (JBSF) and serial backward search (SBS). Average ranging errors for both algorithms are formulated using noise-based ranging thresholds and at sub-Nyquist sampling rates. For JBSF, a lower bound on the average ranging error is obtained using an error-minimizing threshold. Also, a semi-analytic technique for the selection of the searchback window length is presented. For SBS, noise-based threshold selection for both single-cluster and multi-cluster channels are introduced. Computer simulations in IEEE 802.15.4a channels provide insights on the selection of some of the critical parameters for accurate ranging. The simulation results show that JBSF typically performs better than SBS if the inter-cluster delay parameter (K) is not selected appropriately for SBS. In both cases, it is shown that even at sampling rates much smaller than the Nyquist rate, ranging accuracies on the order of 33 cm can be obtained using a non-coherent UWB receiver.

On UWB Impulse Radio Receivers Derived by Modeling MAI as a Gaussian Mixture Process

One of the main problems in ultra wide band (UWB) impulse radio (IR) systems is the need to cope efficiently with multiple access interference (MAI). As outlined recently in the literature, this can be achieved by modeling MAI as a Gaussian mixture (GM) random process and deriving the receiver in accordance with a maximum likelihood paradigm. Depending on the chosen context, both rake and chip-matched solutions can be identified. In this paper we compare different solutions, generalize them to a QAM context, and investigate their implementation complexity and robustness to impulsive interference. We also investigate efficient methods to lower the computational demand.

Optimization of CDMA Systems with Respect to Transmission Probability, Part II: Signal to Noise Plus Interference Ratio Optimization

Code division multiple access (CDMA) systems commonly use power control mechanism to reduce the amount of interference between the users. In part I of this paper, we introduced the concept of probability control that optimizes system performance with respect to the users' transmission probabilities. The importance of probability control was demonstrated by proving that probability control alone optimizes the mutual information rates over a frequency-flat slow-fading multiple access channel. In this part we extend this result to the optimization of the average signal-to-noise plus interference ratio (SIR) in a CDMA system over a frequency-selective slow- fading channel model and any network topology. We prove that there is a group of CDMA receivers, in which the average SIR (ASIR) of all users is maximized using probability control (while all users transmit their maximal allowed power). This receivers group includes, among others, the common matched filter (MF) RAKE receiver, as well as the sophisticated minimal mean square error (MMSE) RAKE receiver. Simulations demonstrate for a 2- users scenario that both users achieve significantly higher ASIR using probability-control instead of power-control. Results are applicable both for CDMA and impulse radio (IR) systems.

Noncoherent Multiple-Symbol Detection in Coded Ultra-wideband Communications

Recently, ultra wideband (UWB) impulse radio systems based on noncoherent detection, such as transmitted reference and differential modulation systems, have drawn a lot of attention for their potential applications in low-rate and low-cost communications. In this paper, a unified approach is presented to develop and analyze a noncoherent multiple-symbol detection (MSD) algorithm for various UWB signaling formats in multi- path environments. Pair-wise error probabilities of coherent and noncoherent MSD algorithms are derived and compared. The intrinsic connection between the coherent and noncoherent MSD algorithms is revealed, tradeoff curves between their performance and complexity are obtained, and optimal transmission signal design rules for noncoherent detection are provided. The bit error rates are shown through simulations to verify the analysis and to evaluate the performance of the developed algorithms in realistic environments.

Differentially-Encoded Di-Symbol Time-Division Multiuser Impulse Radio in UWB Channel

Ultra-wideband (UWB) communication systems are expected to operate in a highly frequency-selective multipath fading environment. To exploit multipath diversity gains in a multiuser scenario, we developed a differentially-encoded, di-symbol time-division multiuser impulse radio (d2TD-IR) system with delay-sum autocorrelation receivers. In traditional time-division multiple access systems, each user transmits a single pulse during a symbol duration in a pre-assigned chip which is longer than maximum excess delay of the channel. However, due to the exponential decay property of UWB channel, we proposed the use of much shorter chip duration, which significantly increases the transmission rate. Because dense pulse transmission will induce multiuser interference, two time-hopping access sequences, which alternately encode the odd- and even-index symbols, are employed with delay-sum autocorrelation receivers to maximally suppress the interference. It was shown that when the chip duration is properly chosen, the proposed system outperforms the conventional time-hopping impulse radio system at high signal-to-noise ratio. This paper also proposed a method to estimate the optimal chip duration when only the average power decay profile of the UWB channel is known.

0.7-GHz-Bandwidth DS-UWB-IR System for Low-Power Wireless Communications

A direct-sequence ultra-wideband impulse radio (DSUWB-IR) system is developed for low-power wireless applications such as wireless sensor networks. This system adopts impulse radio characterized by a low duty cycle, and a direct-sequence 0.7-GHz bandwidth, which enables low-power operation and extremely precise positioning. Simulation results reveal that the system achieves a 250-kbps data rate for 30-mdistance wireless communications using realistic specifications. We also conduct an experiment that confirms the feasibility of our system.

Frequency domain detectors for ultra-wideband communications in short-range systems

In this paper, we propose an original detection scheme for high rate short-range impulse radio ultra-wideband systems. The proposed receiver relies on both the introduction of the cyclic prefix at the transmitter and the use of a frequency domain multiuser detector at the receiver. Zero forcing (ZF) and minimum mean square error (MMSE) detection strategies have been investigated and compared with the classical RAKE, considering a scenario where several mobile terminals communicate with a base station in an indoor environment characterized by severe multipath propagation. The results show that the MMSE receiver achieves the best performance, irrespective of the number of active terminals, both in the uplink and in the downlink communications. Hence, the proposed approach is well suited in indoor wireless environments where the multipath propagation tends to increase the effects of both the inter-path and the inter-user interference.

Direct Blind Adaptive MOE Receiver for PAM Modulated Time-Hopping UWB System Over Frequency-Selective Fading Channel

Ultra wideband impulse radio systems have attracted great attention for their promised applications in high-speed short-range indoor wireless communication systems. Among the various modulation and multiple access schemes, this paper deals with time-hopping (TH) antipodal pulse amplitude modulation operating in the presence of a multipath fading downlink channel. We first employ a constrained optimization technique to design a batch mode blind (without exploiting training sequences and undesired users’ time-hopping (TH) codes) mobile station receiver. To reduce the computational complexity, we propose a blind adaptive receiver that is based on the criterion of maximizing the receiver’s minimum possible output energy. The algorithm jointly and iteratively optimizes the weight vector and channel impulse response to improve system performance. Simulation results show that the proposed adaptive receiver converges to the optimum batch mode receiver. Moreover, the algorithms are shown to be robust to multi-user interference and near-far problems.

Two Blind Adaptive Mobile Receivers in Time-Hopping PAM UWB Impulse Radio System

This paper deals with the design of blind mobile station (MS) receiver for time-hopping (TH) ultrawide-band (UWB) impulse radio (IR) system employing antipodal pulse amplitude modulation (PAM). In the presence of multipath fading channel, batch-mode minimum output energy (MOE) receiver, as well as a blind channel estimator, is first developed. To reduce the computational complexity, we propose two blind adaptive algorithms to determine the weight vector of the MS receiver. The rational of the algorithms premises on iteratively maximizing the minimum possible receiver's output energy. The first (indirect) approach is derived by first developing an adaptive blind channel estimator, then the updated channel impulse response (IR) is used to calculate the MS receiver's weight vector. Meanwhile the second (direct) method jointly and iteratively optimizes the weight vector and channel IR to improve system performance. Simulation results demonstrate convergence of both algorithms.Moreover, the algorithms are shown to be robust to multiuser interference (MUI) and near-far problems.

Pulse position modulated space–time trellis coding for ultra-wideband impulse radio multiple-input multiple-output communication systems

The design of pulse position modulated (PPM) space-time trellis codes (STTC) for ultra-wideband impulse radio (UWB-IR) multiple-input multiple-output (MIMO) communication systems over slow and fast fading multipath channels is considered. First, A probability of error analysis is carried out to derive upper bounds on pairwise symbol error probability at high and low signal-to-noise ratios (SNRs). From the upper bounds, A new distance notion is introduced and novel design criteria for optimal (in error rate performance) M-ary PPM STTC are deduced for UWB. An optimal binary-PPM STTC is designed for two transmit antennas. Finally, simulation results of the UWB-IR MIMO system, using the optimal STTC, confirm significant improvement in bit-error-rate performance over the uncoded UWB-IR single-input single-output system and also over previously proposed space-time coding scheme for UWB, at higher SNR.

Effect of Group Delay in RF BPF on Impulse Radio Systems

This paper presents an analysis of the effects of RF filter characteristics on the system performance of an impulse radio. The impulse radio system transmits modulated pulses having very short time duration. Information can be extracted in the receiver side based on the cross-correlation between received and reference pulses. Accordingly, the pulse distortion due to in-band group delay variation can cause serious degradation in system performance. In general, RF band pass filters inevitably cause non-uniform group delays to the signal passing through the filter that are proportional to its skirt characteristic due to its resonance phenomenon. In this work, a small signal scattering parameter, S 21 , which is a frequency domain parameter, and its Fourier transform are utilized to characterize the output pulse waveform under the condition that the input and output ports are matched. The output pulse waveform of the filter is predicted based on the convolution integral between the input pulse and filter transfer function, and the analysis result is compared with previously reported experimental result. The resulting bit error rate performances in a bi-phase modulation and a pulse position modulation based impulse radio system are also calculated. Moreover, improvement of system performance by the pulse shaping method, a potential solution for pulse waveform distortion, is analyzed.

Performance Analysis of Single-User Ultra-Wide Band Impulse Radio (UWB-IR) with Super-Orthogonal Turbo Codes (SOTC)

Low-complexity low-rate super-orthogonal turbo codes (SOTC) are proposed in this work to replace the implicit repetition code (RC) in ultra-wide band impulse radio (UWB-IR) systems to improve the transmission range and system throughput. Various receivers, including the matched-filter and the RAKE receivers, are examined for data detection in the additive white Gaussian noise (AWGN) channel and the indoor IEEE 802.15.3a channels. The performance of SOTC-coded UWB with perfect and imperfect timing and channel information is analyzed and corroborated by computer simulation. It is demonstrated that the SOTC-based UWB-IR system can achieve significant performance improvement over the conventional direct-sequence UWB (DS-UWB) system encoded by RC over both ISI and ISI-free channels.

Cramer–Rao bounds for channel estimation in UWB impulse radio

This paper addresses the Cramer–Rao bound (CRB) calculation for channel parameter estimation in impulse radio ultra-wide band systems. We consider a time-hopping code scheme with binary pulse position and pulse amplitude modulation formats. We first derive in closed-form the (resp. modified) CRB for the multipath channel parameters in the data-aided (resp. non-data-aided) context. Unlike existing methods, the calculations are derived taking into account the overlapping between signal echoes due to multipaths. We illustrate the benefit of taking into account the overlapping assumption on realistic channel propagation environment. Simulations show that the CRB using the non-overlapping assumption clearly overestimate the performance.

Performance of UWB Impulse Radio With Planar Monopoles Over On-Human-Body Propagation Channel for Wireless Body Area Networks

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <?Pub Dtl=""?>Ultrawideband (UWB) is a promising technology for wireless body area networks (WBANs). This paper studied the impacts of 3.1–10.6 GHz on-human-body UWB channel on the impulse radio WBAN system. A performance evaluation method is presented for the realistic UWB WBAN systems, which observes the waveform distortion along the signal path. The measurement and characterization of the 3.1–10.6 GHz on-human-body UWB channel are devised to generate the radiographs of path loss and delay spread for the first time. The performance of the UWB impulse radio WBAN transceiver in terms of bit error rate (BER) is evaluated based on the waveform distortion and on-human-body channel measurement, which shows the human body effect is more significant than the environment effect, especially when the propagation channel contains no line-of-sight path. Various candidate pulse shapes and modulation schemes for UWB WBAN are studied and their performances with the measured WBAN channel are evaluated and compared. </para>

Two-Stage Acquisition in Time-Hopping Impulse Radio Systems for UWB Communications

Ultra wideband impulse radio is a promising radio technology for networks delivering extremely high data rates at short ranges. The use of extremely short duration pulses however makes synchronization task more difficult. In this paper a two-stage acquisition with serial search noncoherent correlator for time hopping impulse radio is proposed. The proposed two-stage acquisition scheme gets chip timing synchronization, and aligns phase of the local time-hopping code in two stages successively. With the aid of flow-graph approach, the analytical expressions are presented for the mean acquisition time and the probability of acquisition. Numerical results in a slow fading channel show that the proposed two-stage acquisition method can offer much shorter mean acquisition time or much higher probability of acquisition than that of the conventional acquisition.

The Tradeoff Between Processing Gains of an Impulse Radio UWB System in the Presence of Timing Jitter

In time hopping impulse radio, Nf pulses of duration Tc are transmitted for each information symbol. This gives rise to two types of processing gains: (i) pulse combining gain, which is a factor Nf, and (ii) pulse spreading gain, which is Nc = Tf/Tc, where Tf is the mean interval between two subsequent pulses. This paper investigates the tradeoff between these two types of processing gains in the presence of timing jitter. First, an additive white Gaussian noise (AWGN) channel is considered, and approximate closed-form expressions for bit error probability (BEP) are derived for impulse radio systems with and without pulse-based polarity randomization. Both symbol-synchronous and chip-synchronous scenarios are considered. The effects of multiple-access interference (MAI) and timing jitter on the selection of optimal system parameters are explained through theoretical analysis. Finally, a multipath scenario is considered, and the tradeoff between processing gains of a synchronous impulse radio system with pulse-based polarity randomization is analyzed. The effects of the timing jitter, MAI, and interframe interference (IFI) are investigated. Simulation studies support the theoretical results.

Characterization and Performance of High-Frequency Pulse Transmission for Human Body Area Communications

SUMMARY With the rapid progress of electronic and information technology, an expectation for the realization of body area network (BAN) has risen. However, on-body transmission characteristics are greatly dependent on the frequency, and a high-speed transmission is difficult due to the remarkable signal attenuation at higher frequencies. In this study, we proposed a pulse transmission system with the frequencies at dozens of mega-hertzes. The system was based on an impulse radio (IR) scheme with bi-phase modulation. By using the frequency-dependent finite difference time domain (FD 2 TD) method, we investigated the on-body transmission characteristics and derived a path loss expression. Based on the transmission characteristics, we also investigated the influences of white Gaussian noises and other narrow-band interferences on the communication link budget and bit error rate (BER) performance. The results have shown the feasibility of the proposed on-body IR communication system.

Maximum Free Distance Binary to $M$-ary Convolutional Codes for Pseudo Chaotic Type Time Hopping PPM Impulse Radio UWB

The topic of spectral line suppression is of major interest in the design of pulse position modulated time hopping impulse radio ultra wideband communications systems. Previously pseudo-chaotic time hopping (PCTH) has been introduced to eliminate spectral lines and improving bit error rates (BER). In this letter, new binary to M-ary maximum free distance convolutional codes are introduced showing similar power spectral density characteristics as those obtained with PCTH and better BER for hard and soft decision decoding

Rake Reception of UWB-IR Indoor Radio with Reflector

Ultra-wideband impulse-radio systems have the ability to resolve multiple paths of the transmit radio and to mitigate the fading. Rake reception is capable of combining these paths, thereby improving the signal-to-noise ratio. In LOS channel, however, the improvement may be comparatively small for the cost of increasing receiver system complexity. This is because the LOS path should be dominant for the total energy in all paths. In this paper the distribution of the energy captured by Rake receiver is first presented for 160 measured LOS channel cases and then discussed. Rake reception with reflector is next suggested in order to effectively increase the signal energy without increasing the complexity, that is, increased number of Rake fingers. The use of reflector is also suggested for non-LOS channel and experimentally discussed where the Rake gain is compared with conventional Rake without reflector. The measurement results show the usefulness.

Blind signal reception in downlink time‐hopping ultrawideband communication system

AbstractUltra wideband (UWB) impulse radio (IR) system is currently being considered for several applications due to its attractive features that include low‐power carrierless modulation and ample multipath diversity. Among the various modulation and multiple‐access schemes, this paper deals with time‐hopping (TH) antipodal pulse amplitude modulation (PAM), operating in the presence of multipath‐fading downlink channel. We develop two types of low‐complexity mobile station (MS) receivers. One is based on the RAKE reception scheme and the other is designed to meet the minimum‐output‐energy (MOE) criterion. However, since accurate channel information is crucial for reliable operation, thereby we propose a blind (non‐data aided) channel estimator. Simulation results show that the MOE receiver outperforms the RAKE receiver in multiuser environment. Moreover, both the channel estimation algorithm and the MOE reception schemes are shown to be reliable and near‐far resistant. Copyright © 2007 John Wiley &amp; Sons, Ltd.