Cross-Modulation Interference and Mitigation Technique for Ultrawideband PPM Signaling

The detection issues of ultrawideband (UWB) signals depend on the type of modulation scheme that is used during the transmission. Cross-modulation interference (CMI) is a problem that is specific to UWB pulse-position-modulation (PPM) signaling. In this paper, the effects of CMI on the performance of noncoherent UWB receivers are analyzed. The probabilities of error for transmitted-reference (TR) and energy detector (ED) receivers in the presence and absence of CMI are derived. Optimal and suboptimal CMI avoidance algorithms, which are based on novel acquisition techniques, are proposed for Rake receivers. The results show that the performance degradation in both receivers, which is due to the CMI effects, can be significant, depending on the modulation index. TR receivers still can be functional in the presence of CMI, and the target performance level determines the modulation index to be used. It is unlikely that effects of CMI on the performance of ED receivers in the presence of CMI are more severe relative to TR receivers, and the performance level is not acceptable. As a result, PPM signaling is not an appropriate modulation technique for ED receivers that are operating in the CMI region, unless CMI mitigation algorithms can be developed. Furthermore, the proposed optimal and suboptimal algorithms are two promising schemes for avoiding the CMI effects and, consequently, for improving the performance of Rake receivers operating in the CMI region.     

Non-Coherent Fourth-Order Detector for Impulse Radio Ultra Wideband Systems: Empirical Evaluation Using Channel Measurements

Low-complex and low-power non-coherent energy detectors (EDs) are interesting for low data rate impulse radio (IR) ultra wideband (UWB) systems, but suffer from a loss in performance compared to coherent receivers. The performance of an ED also strongly depends on the integration interval (window size) of the integrator and the window position. This paper presents a non-coherent fourth-order detector (FD) which can discriminate between Gaussian noise signals and non-Gaussian IR-UWB signals by directly estimating the fourth-order moment of the received signal. The performance of the detectors is evaluated using realistic channels measured in a corridor, an office and a laboratory environment. The results show that bit-error-rate (BER) performance of the proposed FD receiver is slightly better than the ED in low signal-to-noise ratio (SNR) region and its performance improves as the SNR increases. In addition, BER of the FD receiver is less sensitive to overestimation of the integration interval making it relatively robust to variations of the channel delay spread. Finally, a criteria for the selection of integration time of the proposed detector is suggested.     

Mitigation of channel estimation error in TR–UWB system based on a novel MMSE equalizer

The time reversal (TR) technique combined with the ultra-wideband (UWB) system offers a new potential for decreasing the cost and complexity of the UWB receivers. In spite of TR–UWB's good performance in perfect channel state information (CSI), it is very sensitive to the channel estimation error. The effect of channel imperfection on the TR–UWB system is considered in this paper. At first, based on a minimum mean square error (MMSE) equalizer receiver, a prefilter is calculated in closed form to improve the performance of the TR–UWB system in an imperfect CSI scenario. Furthermore, for comparison purposes, a similar calculation for prefilter is carried out based on a simple matched filter (MF) receiver. Then, in order to improve the MF receiver performance, a two-stage iteration-based algorithm is developed. The initial value for this iteration-based improved algorithm is considered to be a prefilter which is calculated in the TR–UWB system with MMSE equalizer. This optimized algorithm causes the channel estimation error in the TR–UWB system to become zero in some steps. Finally, exhaustive simulations are done to demonstrate the performance advantage attained by the improved algorithm.     

A Survey on Various Coherent and Non-coherent IR-UWB Receivers

The recent developments in radio technologies, paves its way to impulse radio (IR) ultra-wideband (UWB) communication, which is used for low power, short range and high bandwidth communication, thereby exploiting a large portion of radio spectrum. In this paper, a brief review of the work done by various researchers on coherent and non-coherent IR-UWB receivers has been analysed, based on their bit error rate (BER) performances, as well as pros and cons of using these receivers. An in depth study on the receivers concludes that, non-coherent IR-UWB receiver is preferred over its counterpart coherent IR-UWB receiver even though it comes at the expense of poor BER performance. The simulation results prove that, though the performances are same, the low complexity of energy detector (ED) receivers gives an edge over the autocorrelation receivers. Further, ED receiver suffers from noise, which paves way to using weighted ED (WED) receiver. The superiority of WED receivers over all the other non-coherent UWB receivers is further confirmed by the simulation performed in AWGN and IEEE 802.15.4a UWB channels. It can also be concluded from the review that, some special receivers such as generalized likelihood ratio test, multi-symbol differential detector and decision feedback differential transmitted reference, when clubbed with UWB systems, lead to further improvement in BER performance.     

Performance of ultra-wideband communications with suboptimal receivers in multipath channels

The performance of a single-user ultra-wideband (UWB) communication system employing binary block-coded pulse-position modulation (PPM) and suboptimal receivers in multipath channels is considered. The receivers examined include a RAKE receiver with various diversity combining schemes and an autocorrelation receiver, which is used in conjunction with transmitted reference (TR) signaling. A general framework is provided for deriving the performance of these receivers in multipath channels corrupted by additive white Gaussian noise (AWGN). By employing previous measurements of indoor UWB channels, we obtain numerical results for several cases which illustrate the tradeoff between performance and receiver complexity.     

Performance analysis of non-coherent UWB receivers at different synchronization levels

Non-coherent ultra-wideband (UWB) receivers require no channel state information for demodulating the received signal. The primary non-coherent receiver in the UWB literature is the autocorrelation receiver, which autocorrelates the received signal at specific time lags, circumventing problems of template signal design and multipath energy combining. A unique advantage of the UWB autocorrelation receiver is its robustness to synchronization errors, which has not been explored yet to date. This paper investigates two major UWB schemes employing autocorrelation receivers: the transmitted reference (TR) scheme (R. Hoctor and H. Tomlinson, 2002) and the differential (DF) scheme (M. Ho et al., 2002). Performance is analyzed for TR and DF receivers at different synchronization accuracy levels, their robustness to synchronization errors is shown, and the existence of a tradeoff between performance and synchronization complexity for non-coherent UWB receivers is revealed. As a result of our analysis, comparisons of TR and DF schemes are also made in the presence of synchronization errors, which have not been addressed before. Simulations corroborate our findings.     

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     

A Single Input-Multiple Output Time Reversal UWB Communication System

Time reversal is a promising technique for the improvement of UWB communication systems. Intersymbol interference (ISI) limits the system performance in such wireless systems. This paper presents a general ISI analysis for time reversal UWB communication systems. The time reversal UWB system gives good performance for rates below the coherence bandwidth but at higher data rates the performance of the system is limited by intersymbol interference and bit error rate saturates even for high signal-to-noise ratio. To mitigate the ISI effects, a single input/multiple output (SIMO) time reversal UWB system is used and its performance is analyzed. It is shown that by using a SIMO TR transceiver, ISI reduces and the system capacity increases. Transmitted signal power at SIMO time reversal decreases, therefore in low data rate SISO performance is better than SIMO, But in high rate scenario, SIMO TR suppresses the ISI better than the SISO TR and its performance is better than SISO TR. It is possible to compensate the reduced power by using a receiver with more sensitivity.     

Ultra-Wideband Communications using Hybrid Matched Filter Correlation Receivers

Transmitted-reference (TR) schemes for time-hopping impulse radio (TH-IR) ultra-wideband (UWB) communications allow the use of simple receiver structures that are able to combine energy from different multipath components without channel estimation. A conventional TR receiver consists of a simple delay-and-multiply operation combined with an integrator. On the downside, it shows a performance loss due to non-linear operations on noise terms (generation of noise-noise cross-terms) when forming the decision variable. This paper describes a hybrid receiver structure for UWB communications that reduces these noise-noise cross-terms by first performing a "matched filtering" operation matched to the time-hopping sequence of pulses. The receiver retains most of the simplicity of the conventional TR receiver, but requires an analog correlator for the time-hopping sequence of pulses. The performance the proposed receiver is analyzed in both AWGN and multipath channels. For the AWGN case, the exact expression for the bit error probability is obtained, which takes into account the nonGaussian nature of the noise-noise cross-terms arising in the correlators. For the multipath case, both inter-frame interference and multipath interference from the reference pulse to the data pulse are considered, and approximate closed-form expressions are derived based on the assumption of a large integration interval. Also approximate criteria for optimal integration interval are obtained for the best receiver performance. Simulation studies are presented to analyze the performance of the proposed receiver structure and to confirm the theoretical analysis     

Smart UWB System Based on STC and TR Techniques for BER Performance Enhancement and Interference Rejection

In this research, a novel smart UWB system is introduced. The proposed system is based on using an adaptive maximum ratio combining (MRC) Rake receiver. The proposed adaptive Rake receiver uses Genetic algorithm (GA) to adaptively select the delays of the fingers of the Rake receiver depending on the channel impulse response. It adaptively selects the delays that will allow the Rake receiver to capture most of the energy in the multipath components with minimum complexity. This adaptive Rake receiver is referred to as a GA Rake. The adaptive GA Rake is applied to a single-input single-output and space time coding (STC) multi-input single-output UWB systems. The performance of those systems using a GA Rake is compared to their performance when using a conventional MRC-Rake receiver and showed a great enhancement in performance with less receiver complexity. Also, in this paper, the smart UWB system using STC is modified by using the time reversal (TR) pre-coding technique. The modified system is referred to as a TR smart UWB system. This modification leads to more enhancements in performance and more reduction in receiver complexity over the smart UWB system. Moreover, this paper also shows the ability a TR smart UWB system in combating interference from other UWB systems.     

On the Transceiver Types of IR-UWB Systems at Sub-Nyquist Sampling Rates

In this paper, we present a unified performance analysis for different impulse radio (IR) ultra-wideband (UWB) transceiver types employing various modulation options and operating at sub-Nyquist sampling rates. Stored reference (SR), transmitted reference (TR), and energy detector (ED) receivers are considered employing one of the binary phase shift keying (BPSK), pulse position modulation (PPM), and on-off keying (OOK) modulation types. Realistic UWB channel models (the IEEE 802.15.4a channels) and practical pulse shapes (the root-raised cosine pulse) are used to characterize the statistics of the captured energies of different transceiver types at low sampling rates. The bit error rate (BER) expressions for different transceiver/modulation types are provided explicitly in additive white Gaussian noise channels. In multipath channels, the BER expressions are conditioned on the captured energies; then, the captured energy histograms at sub-Nyquist rates are used towards a semi-analytic evaluation of the BER for different transceiver/modulation combinations. The analyses are then verified via simulations using IEEE 802.15.4a channel models. The results show that in addition to their lower implementation complexities, the TR and ED receivers may be more favorable compared to SR receivers at low sampling rates in terms of their BER characteristics as well.

Performance of UWB Receivers with Partial CSI Using a Simple Body Area Network Channel Model

Ultra wideband (UWB) communication is a very promising candidate for the use in wireless body area networks (BAN). The high UWB peak data rate allows for medium average data rates in combination with a very low duty cycle, which is the key for a very low power consumption. Devices in a wireless BAN require low complexity. Hence, mainly non-coherent receivers such as energy detector and transmitted-reference receiver are suited. In this paper, the symbol-wise maximum-likelihood (ML) detectors for pulse position modulation (PPM) and transmitted reference pulse amplitude modulation (TR PAM) are derived assuming partial channel state information (CSI) at the receiver. Additionally, also the ML detectors for a combination of PPM and TR PAM are presented. The performance of the derived receiver structures is evaluated using a novel BAN channel model not distinguishing line-of-sight and non line-of-sight situations. This simple channel model is based on 1100 channel measurements in the frequency range between 2 and 8 GHz, which were measured in an anechoic chamber. Using the BAN channel model, performance of the derived receiver structures is evaluated showing that the knowledge of the average power delay profile (APDP) at the receiver improves performance substantially. Requiring only slightly more complexity such receivers are a well suited alternative to non-coherent receivers for the use in a BAN.     

Equivalent system model and equalization of differential impulse radio UWB systems

A discrete-time equivalent system model is derived for differential and transmitted reference (TR) ultra-wideband (UWB) impulse radio (IR) systems, operating under heavy intersymbol-interference (ISI) caused by multipath propagation. In the systems discussed, data is transmitted using differential modulation on a frame-level, i.e., among UWB pulses. Multiple pulses (frames) are used to convey a single bit. Time hopping and amplitude codes are applied for multi user communications, employing a receiver front-end that consists of a bank of pulse-pair correlators. It is shown that these UWB systems are accurately modeled by second-order discrete-time Volterra systems. This proposed nonlinear equivalent system model is the basis for developing optimal and suboptimal receivers for differential UWB communications systems under ISI. As an example, we describe a maximum likelihood sequence detector with decision feedback, to be applied at the output of the receiver front-end sampled at symbol rate, and an adaptive inverse modeling equalizer. Both methods significantly increase the robustness in presence of multipath interference at tractable complexity.     

Analysis of weighted non-coherent receiver for UWB-OOK signal in multipath channels

Non-coherent receivers are attractive for pulsed Ultra-WideBand （UWB） systems due to the implementation simplicity. However, they have to face the shortage of performance degradation. Several techniques were proposed to alleviate the noise effect and promote the receiver performance, among which is the weighted combining of multiple integration sub-intervals. In this paper, the performance of the weighted non-coherent receiver for UWB On-Off Keying （UWB-OOK） signal in multipath channels is analyzed, in terms of bit-error-rate. In addition, a closed-form expression of the approximately near-optimal weighting coefficient set is derived, and two simple weighting coefficient sets are proposed as well. Finally, the analytic results are verified via the computer simulations, which reveal obvious performance improvements to the conventional energy detector.     

Designing Time-Hopping Ultrawide Bandwidth Receivers for Multiuser Interference Environments

The multiple-user interference (MUI) in time-hopped impulse-radio ultrawide bandwidth (UWB) systems is impulse-like and poorly approximated by a Gaussian distribution. Therefore, conventional matched filter receiver designs, which are optimal for Gaussian noise, are not fully efficient for UWB applications. Several alternative distributions for approximating the MUI process and the MUI-plus-noise process in UWB systems are motivated and compared. These distributions have in common that they are more impulsive than the Gaussian approximation, with a greater area in the tails of the probability density function (pdf) compared to a Gaussian pdf. The improved MUI and MUI-plus-noise models are utilized to derive new receiver designs for UWB applications, which are shown to be superior to the conventional matched filter receiver. Multipath propagation is abundant in UWB channels and is exploited by a Rake receiver. A Rake receiver uses multiple fingers to comb the multipath rays with a conventional matched filter implemented in each finger. Rake structures utilizing the new receiver designs that are suitable for reception of UWB signals in multipath fading channels are provided. An optimal performance benchmark, based on an accurate theoretical model for the interference that fully explains the features of the MUI pdf, is also presented. Analysis and simulation results are shown for the novel receivers, which demonstrate that the new designs have superior performance compared to the conventional linear receiver when MUI is significant. Several adaptive receivers are shown to always match or exceed the performance of the conventional linear receiver in all MUI-plus-noise environments. Parameter estimation for the new receivers also is discussed.      

Analysis of UWB transmitted-reference communication systems in dense multipath channels

Transmitted-reference (TR) signaling, in conjunction with an autocorrelation receiver (AcR), offers a low-complexity alternative to Rake reception. Due to its simplicity, there is renewed interest in TR signaling for ultrawide bandwidth (UWB) systems. To assess the performance of these systems, we develop an analytical framework based on the sampling expansion approach. In particular, we derive closed-form expression for the bit-error probability (BEP) of TR signaling with AcR that can be used to exploit multipath diversity inherent in wideband channels. We further extend our analysis to the BEP derivation of modified AcR with noise averaging. Our methodology does not require the Gaussian approximation and is applicable for any fading scenario, provided that the correlator output signal-to-noise ratio (SNR) can be characterized in terms of a characteristic function. We show that the validity of the conventional Gaussian approximation depends on the time-bandwidth product and the number of transmitted pulses per symbol. Our results enable the derivation of a computationally simple lower bound on the BEP of TR signaling with AcR. This lower bound allows us to obtain the SNR penalty associated with an AcR, as compared with All-Rake and Partial-Rake receivers.     

Kasami Code-Shift-Keying Modulation for Ultra-Wideband Communication Systems

Ultra-wideband (UWB) communication systems are generally applied to short-range wireless communications. In order to achieve higher rates or to support multiple access capabilities, direct sequence spread spectrum (DSSS) techniques have been introduced to UWB systems, and multiple pulses corresponding to a certain pseudo-noise (PN) code are transmitted to represent a symbol. In addition, the concept of M-ary code shift keying (M-CSK) was introduced into DSSS systems to achieve higher rates. In this work, we propose an M-CSK modulation technique based on the large set of Kasami sequences since it possesses good code properties, including a large code set size and low cross correlations. The modulation and demodulation schemes are developed, and the system performance in additive white Gaussian noise (AWGN) and UWB channels exposed to multiple user interference is investigated thoroughly. It was found that the Kasami M-CSK modulation is superior to other M-CSK modulation schemes in the bandwidth efficiency, and therefore a higher data rate can be achieved. Furthermore, based on our proposed demodulation scheme, the hardware complexity of receivers can be greatly reduced to O(M^1/3), and the implementation of receivers for a very large M becomes feasible.     

Noncoherent Ultra-Wideband (De)Modulation

Ultra-wideband (UWB) radios have received increasing attention recently for their potential to overlay legacy systems, their low-power consumption and low-complexity implementation. Because of the pulsed or duty-cycled nature of the ultra-short transmitted waveforms, timing synchronization and channel estimation pose major, and often conflicting, challenges and requirements. In order to address (or in fact bypass) both tasks, we design and test noncoherent UWB (de)modulation schemes, which remain operational even without timing and channel information. Relying on integrate-and-dump operations of what we term "dirty templates," we first derive a maximum likelihood (ML) optimal noncoherent UWB demodulator. We further establish a conditional ML demodulator with lower complexity. Analysis and simulations show that both can also be applied after (possibly imperfect) timing acquisition. Under the assumption of perfect timing, our noncoherent UWB scheme reduces to a differential UWB system. Our approach can also be adapted to a transmitted reference (TR) UWB system. We show that the resultant robust-to-timing TR (RTTR) approach considerably improves performance of the original TR system in the presence of timing offsets or residual timing acquisition errors     

A novel self-pilot-based transmit-receive architecture for multipath-impaired UWB systems

In the last few years, ultra-wideband (UWB) systems became an appealing technology for wireless communication applications. Unfortunately, when the transmission channel is affected by intersymbol interference (ISI), system performance of UWB systems equipped with receivers based on conventional matched filters presents error-floor phenomena. Aimed by these considerations, in this letter, we present a novel transmit-receive scheme allowing blind channel estimation and minimum mean-square error linear channel equalization. Essentially, the proposed scheme exploits a very short duration of the UWB pulse for achieving reliable blind deconvolution of the received signal. A nice feature of the resulting system is that blind deconvolution of the received signal is achieved without power and throughput losses. Simulation results support the effectiveness of the proposed scheme, and show that it is able to gain about 8 dB over current UWB receivers based on matched filtering on several test channels impaired by ISI.     

Multipath energy combining for fast UWB acquisition without channel knowledge

In this paper, we study the effectiveness of the multipath energy combining for the coarse acquisition of UWB signals. The performances of different detectors, including single pulse correlator (SPC), energy detector (ED), and transmittedreference (TR), are derived and compared in terms of the mean acquisition time (MAT) and the false acquisition rate (FAR) using flow graph analysis in a two-step random search framework. The results show that both the ED and the TR can achieve a lower acquisition time than the SPC under the same FAR constraint due to their multipath energy combining capabilities. Simulation results also reveal that the ED scheme is more vulnerable to multiuser interference than the TR and the SPC schemes.