Transmitted Reference Systems Research Articles

A Solution to the Non-Ideal Delay Line Problem in Transmitted Reference Pulse Cluster Schemes for UWB Communications

The conventional transmitted reference (TR) technique in ultra wide-band (UWB) communications has provided a springboard for a lot of studies, and a TR pulse cluster (TRPC) structure was proved to have robust performance. Most of TR and TRPC related studies hitherto are based on the assumption that the delay lines (DLs) in their schemes are ideal. However, the group delay ripple (GDR) of nonideal DLs triggered by practical imperfect factors at implementation causes an unexpected distortion in both TR and TRPC UWB systems, and there are few studies so far. In this article, we first analyze the impact of the GDR of nonideal DLs on the performance of TR and TRPC UWB systems. Then a new design solution for both TR and TRPC transceivers is proposed, where the GDR information of the nonideal DL at the receiver side is introduced into transmitter side. The performance of the new design solution is analyzed and simulation results prove the robustness and efficiency of this new design method in reducing the impaction caused by nonideal DLs. Moreover, the proposed design solution can also greatly relieve the high design requirement of DLs in both the transmitter and receiver side and therefore offer valuable insight for the practical transceiver design in both TR and TRPC related UWB systems.

A Frequency Offset Transmit Reference System in Dense Multipath Environments: Propagation Effects and Design Considerations

Frequency offset transmit reference (FoTR)—a noncoherent spread spectrum technique—is considered in dense multipath fading environments, in the context of wideband communication. The interplay between system parameters and propagation effects is investigated. Analytical relations between key design parameters and the channel delay spread are derived, which provide a framework for determining reasonable system parameters that optimize the performance. An approximated closed-form expression for the outage probability is also obtained, which is shown to be fairly accurate for low outage probabilities. It is shown that FoTR suffers significantly from noise-enhancement, but, is nonetheless, robust against frequency-selective fading. A large value of frequency offset can significantly deteriorate the performance, particularly in environments with large channel delay spreads. This restricts the maximum data rate as well as the multiple-access capability of the scheme. Despite the limitations, FoTR can be a useful communication scheme for low data rate sensor networks deployed in dense multipath environments, particularly where the design demands a simple receiver and a low outage probability.

Method for Bandwidth Efficiency Increasing of M-ary PPM Transmitted-Reference UWB Communication Systems

One of the main drawbacks of conventional transmitted reference (TR) ultra-wideband systems is the fact that time delay between the reference and the data pulse has to be at least equal to channel delay spread, in order to avoid inter-pulse interference (IPI). This requirement drastically limits the achievable data rate of conventional TR systems based on M-ary pulse position modulation (PPM). In this paper, the new method for M-ary TR PPM scheme bandwidth efficiency increasing is proposed. In the proposed method the orthogonal codes are used such that only the pulses which can cause IPI are coded. Usage of such type of coding allows the reduction of time delay between the reference and the data pulse, whereas IPI is avoided. The proposed method achieves higher bandwidth efficiency than the conventional TR PPM method for the similar BEP performance level. Furthermore, the results show that an additional increase in bandwidth efficiency can be achieved by using higher modulation levels. Additionally, the analysis on tradeoffs between the BEP performance, bandwidth reduction and system costs is given in this paper.

Implementation of FM-DCSK modulation scheme on USRP platform based on complex envelope

FM-DCSK transmitted reference modulation scheme with autocorrelation receiver offers a very robust and simple transceiver configuration. However the noise performance of such a system is relatively poor. Although the theoretical noise performance of transmitted reference systems has been derived, its validation by real measurements has not yet been carried out. The paper shows the implementation of an FM-DCSK system on USRP platform based on the complex envelope approach. The theoretical noise performance of FM-DCSK modulation scheme with autocorrelation receiver is validated through real measure-

Novel signaling and detection schemes for ultra-wideband transmitted-reference systems

Ultra-Wideband (UWB) impulse radio systems apply sub-nanosecond pulses and are able to resolve multipath due to their fine time-resolution capability. To take full advantage of the available signal energy, a Rake receiver with high hardware complexity is required. To remedy this difficulty, an improved scheme called the transmitted reference (TR) systems is proposed. TR systems do not require the complex task of channel estimation and render a simple receiver structure. However, the low-complexity detection in the conventional TR scheme is achieved at the cost of 50% energy/bandwidth disadvantage. This disadvantage has become the popular research focus of UWB TR systems. In this paper a performance-enhanced signal and detection scheme, namely the dual averaging N-bit differential transmitted reference/balanced transmitted reference (A2NDTR/BTR) system, is proposed to increase the data rate while still maintain the overall system performance. Numerical results suggest that this proposed scheme is able to achieve superior data rate while maintain the same or better bit error performance as compared with others.

Performance of Non-Polarized Noise Modulated Communications System in the Presence of Interference

The noise modulated non-polarized communication system, developed by us recently, is an ultra-wideband transmitted-reference system designed for secure interferencefree communications. By using ultra-wideband random noise to spread the signal and by using polarization diversity for transmitting the spread message and the reference signal, we make the transmitted signal appear totally featureless and noise-like. However, the system may encounter intentional and non-intentional interference since its operating bandwidth is very large. This paper derives analytical models to characterize system performance in partial-band jamming channels and channels containing narrowband interference. Models compare very well with simulations and results validate successful system operation under low signal-to-interference ratio conditions. Research also reveals that the center frequency of transmitted signal must be suitably protected to prevent link outage due to the presence of cross-interference terms. Since multipath acts as self-interference for communication systems, a new method which can improve system performance in multipath environments is also introduced in this paper.

IR-UWB Transmitted-Reference Systems With Partial Channel Knowledge: A Receiver Design Based on the Statistical Invariance Principle

This paper deals with detection algorithms for impulse-radio ultrawideband transmitted-reference systems. For binary modulation schemes, we propose a novel detector that operates on the partial autocorrelations of the received signal as computed over time intervals (bins) of size less than the symbol period. Assuming that the receiver has knowledge of the partial autocorrelations of the channel response over those bins, we derive the minimum error probability detector (MEPD). In doing so, we exploit some invariance properties of the detection problem under consideration. It turns out that the receiver adopting such a strategy is superior to a conventional TR system, the more so the smaller the bin size is. Also, we address the problem of estimating the partial autocorrelations of the channel response and propose a simple solution by exploiting a training sequence. The impact of the estimation errors on the receiver performance is shown to be marginal.

New receivers for generalized UWB transmitted reference systems with improved performances

Generalized ultra-wide bandwidth (UWB) transmitted reference (TR) receivers proposed previously in the literature are improved with respect to the energy allocation in the data packet and the integration interval length in the correlator. The improvement refers to a best-effort technique to reduce the receiver bit error rate. Simulation results show that the improvement can provide a performance gain of up to 4.2 dB in signal-to-noise ratio. These results give useful guidance on the design of generalized UWB TR receivers.

On the performance of analog multiuser detection in ultra-wideband transmitted-reference system

The detection performance is evaluated for our proposed analog multiuser receiver in Ultra-WideBand (UWB) transmitted-reference system. In the presence of dense multipath and multi-access signals, the performance analysis is difficult due to the complicated waveform of received impulse. We develop an approach to analyze the steady-state Signal-to-Interference-plus-Noise (SINR) of the detector output. The multipath-spread impulse is fitted to an exponentially decaying profile in the analysis. A closed-form expression of steady-state SINR is further deduced for the proposed Least Minimum Square (LMS) detector. The analysis is validated by simulations in Line-Of-Sight (LOS) and Non-Line-Of-Sight (NLOS) channel respectively. Based on the theoretical results, the multipath delay spread is employed to determine the optimal width of the integration window of the detector.

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.

Weighted high data rate ultra wideband transmitted-reference system in dense multipath fading channels

A novel weighted high data rate single-user ultra-wideband transmitted-reference (UWB-TR) system is proposed and its performance in log-normal slow fading channels is investigated, which has not been addressed before. This system employs a novel signalling scheme, which provides a nearly inter-symbol interference (ISI) free-channel response at the receiver and eliminates the need for equalisation. It also enhances the reference pulse quality by employing optimum power allocation. The performance of the proposed system is compared with that of the differential transmitted-reference (DTR) system in IEEE 802.15.3a Channel Model 1 (CM1) both analytically and via simulations by taking into account noise, log-normal slow fading, inter-path/pulse interference (IPI) and ISI. Both analytical and simulation results show that our system significantly outperforms the DTR one. It is shown that the proposed method is robust against small changes in both weighting coefficients and integration interval, whereas the UWB-DTR scheme is too susceptible to such changes in the integration interval.

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.

Improved receivers for generalized UWB transmitted reference systems

Three new receivers for generalized transmitted-reference ultra-wide bandwidth systems are proposed. The receivers are derived by optimally combining the channel estimates using the reference symbols, the data decisions and the data symbols. The structures are very simple. Simulation results show that they outperform previous receivers.

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.

Analysis of Channel-Averaged SINR for Indoor UWB Rake and Transmitted Reference Systems

In this paper, we derive a closed-form expression for channel-averaged signal-to-interference-plus-noise ratio (SINR) for ultra-wideband (UWB) Rake receiving system in an indoor multiuser communication scenario, given that the interference level is fluctuating due to asynchronous transmissions among users. The indoor wireless channel model adopted here is a standard one, recently released by IEEE 802.15 study group 3a . We propose a theoretical framework to derive the channel-averaged SINR considering the lognormal channel gain distribution and the double independent Poisson arrival distribution of cluster and ray, and show that our analysis is well coincident with the simulation results. With this framework, we demonstrate that our analysis can be applied to theoretically determine the optimum integration interval for a UWB transmitted reference system, even in a multiuser scenario.

UWB Radio: Digital Communication with Chaotic and Impulse Wavelets

Radio communications via channels already occupied by traditional telecommunication systems can be achieved by using ultra-wideband (UWB) radio where extremely wideband wavelets are used in order to reduce the power spectral density (psd) of transmitted signal. Since the recovery of these UWB carriers is not feasible, noncoherent demodulation techniques have to be used. The letter evaluates and compares the noise performances of the feasible noncoherent UWB modulation schemes, namely, that of the noncoherent pulse polarity modulation and the transmitted reference system.

Analysis of Channel-Averaged SINR for Indoor UWB Rake and Transmitted Reference Systems

Analysis of Channel-Averaged SINR for Indoor UWB Rake and Transmitted Reference Systems

Quantized UWB Transmitted Reference Systems

Digital implementation of ultra-wideband receivers requires analog-to-digital conversion (ADC) at an extremely high speed, thereby limiting the available bit resolution. Herein, the effect of low bit resolution quantization on the performance of UWB transmitted reference receivers is investigated. It is verified that the gain of the automatic-gain-control (AGC) has a significant effect on the achievable performance. Because of the considerable performance loss of conventional transmitted reference receivers in the presence of a low resolution ADC a new family of receiver structures optimized and tailored to quantized observations is presented. In particular, the generalized- likelihood ratio test (GLRT) based on the quantized samples is derived and shown to provide modest performance gains relative to the infinite resolution GLRT rule employed on the quantized received signal suggesting that conventional receiver structures can also be employed in the presence of a low resolution ADC. Results reveal that four bits of resolution in combination with an optimal choice for the AGC gain are sufficient to closely approach the performance of an infinite resolution receiver.

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

Analysis of the Channel Energy Capture in Ultra-Wideband Transmitted Reference Systems

In this letter, we analyze the expected value of the channel energy capture as a function of the integration interval of the correlator in a transmitted reference system. A modified Saleh-Valenzuela channel model is employed. We observe that for the system specifications (such as channel model, data rate, and signal-to-noise ratio) considered here, an integration interval that captures approximately 84%-89% of the expected value of the channel energy provides a bit-error probability close to the minimum one.