Huilin Xu

Ultra-wideband technology: Yesterday, today, and tomorrow

In recent years, ultra-wideband communications have attracted increasing interests, since the FCC certification of the UWB spectral mask. UWB research and development have made unprecedented progress in a wide range of applications, including WPAN for high- data-rate multimedia networking, wireless connection and localization among low-power distributed sensors, and body area networking (BAN) for medical as well as entertainment purposes. This paper will provide an overview of UWB technology fundamentals, recent developments, together with envisioned applications and open problems.

High-Resolution TOA Estimation with Multi-Band OFDM UWB Signals

In this paper, we propose a simple model-based time-of-arrival (TOA) estimation technique for multi-band orthogonal frequency division multiplexing (MB-OFDM) signals based on the ECMA-368 standard [1]. The proposed technique does not need oversampling at the receiver. The key idea is to minimize the energy leakage from the first channel path due to mis-sampling. We use realistic channel models and assume that no channel statistical information is known at the receiver. In our approach, the multi-band signals are coherently combined in order to improve the resolution of the TOA estimation. We compare the performance of the proposed scheme with the well-known maximum-likelihood based algorithm, namely, space-alternating generalized expectation-maximization (SAGE). Simulations based on the ECMA-368 standard parameters in both IEEE 802.15.3a and IEEE 802.15.4a standard channel models show that our approach outperforms the SAGE algorithm by avoiding the local maxima. Furthermore, we have also shown that our approach can achieve a root mean square error of TOA estimation much smaller than the receiver sampling interval and it is robust to the narrowband interference.

Interference avoidance in 3GPP femtocell networks using resource partitioning and sensing

Interference problems in cochannel femtocell networks may cause significant performance degradation for certain femtocell/macrocell users. These problems and potential solutions are recently being investigated within the 3GPP. The goal of the present paper is to review the simulation framework for femtocell networks in 3GPP and briefly summarize some of the important inter-cell interference coordination (ICIC) techniques that have been discussed. Then, a new intercell interference avoidance (ICIA) method is proposed for femtocell networks which is based on resource partitioning and sensing. Different than prior art techniques, the proposed approach does not require the use of X2 interface or over-the air signaling, both of which may be infeasible/costly for femtocells with the available releases of the 3GPP.

Differential UWB Communications With Digital Multicarrier Modulation

As a high-rate alternative of impulse radio (IR) ultrawideband (UWB) communications, analog and digital multicarrier UWB radios with coherent detection have been introduced. In this paper, we investigate digital multicarrier differential (MCD) modulation and demodulation schemes for UWB communications. In our approach, the differential encoding and decoding are carried out across multiple digital carriers in the frequency domain. Hence, channel estimation is bypassed by using neighboring carriers as reference carriers. Compared with the transmitted reference TR-UWB which relies on delaying and correlating the received signal segments in time domain, our MCD-UWB avoids the need for analog delay elements. In addition, our digital-carrier based approach does not incur any spectrum expansion. We also show that our approach enables variable data rates, and remains operational even in the presence of severe interframe interference (IFI). In addition, the small spacing between differentially encoded carriers ensures that the data rate is not limited by the channel coherence bandwidth. Furthermore, our schemes are flexible in providing variable-rate transmissions. Simulations are also carried out to corroborate our theoretical analysis.

A New Modulation Scheme for Rapid Blind Timing Acquisition using Dirty Template Approach for UWB Systems

Timing acquisition is one of the major challenges in ultra-wideband (UWB) communications. The timing with dirty template (TDT) approach is an attractive technique for UWB systems, which is characterized by its low complexity and fast acquisition in the data-aided (DA) mode. However, in the non-data-aided (NDA) mode, the performance of this approach degrades due to the random symbol effect. In this paper, we propose to overcome this issue by adopting orthogonal pulse-shape modulation (PSM) proposed in L.B. Michael et al. (2002), (M. Pinchas and B.Z. Bobrovsky, 2003). Our algorithm uses a train of alternating orthogonal Hermite pulses to modulate the transmitted symbols. The application of the TDT approach to the proposed scheme shows an enhancement of synchronization speed in the NDA mode. Simulations confirm performance improvement of TDT with the proposed modulation relative to the original TDT in terms of the mean square error (MSE) and the acquisition probability.

Mistiming performance analysis of the energy detection based toa estimator for MB-OFDM

In this letter, we apply energy detection based time- of-arrival (ToA) estimation to multi-band orthogonal frequency-division multiplexing signals. We analyze the mistiming performance of the ToA estimator in the Nakagami-m channel. Analysis shows that the slope of the probability of mistiming curve increases with the number of subbands and the Nakagami-m parameter. This is known in communications as diversity. Simulations are carried out in various channels to corroborate our theoretical analysis.

Full length article: Orthogonal bi-pulse UWB: Timing and (de)modulation

In this paper, we propose a novel orthogonal bi-pulse ultra-wideband (UWB) system, which uses an even pulse and an odd pulse to convey information symbols in an alternating manner. Due to the orthogonality of these pulses, their corresponding received waveforms remain orthogonal after propagating through multipath channels. Then we consider two major challenges in the realization of our proposed UWB system: timing synchronization and symbol demodulation. In particular, the idea of timing with dirty template (TDT) in [L. Yang, G.B. Giannakis, Timing Ultra-Wideband signals with dirty templates, IEEE Trans. on Commun. 53 (11) (2005) 1952-1963] is employed for timing synchronization and the noncoherent scheme in [L. Yang, G.B. Giannakis, A. Swami, Noncoherent Ultra-Wideband (de)modulation, IEEE Trans. Commun. 55 (4) (2007) 810-819] is used to bypass channel estimation. Both of these techniques are characterized by correlating adjacent waveform segments. In the implementation of these techniques, we will gradually reveal the advantages of our proposed system. The correlation of adjacent waveform segments only contains the information of a single symbol. This enables a significant enhancement of the synchronization speed of TDT when no training sequence is transmitted. For the same reason, our demodulation approach completely mitigates the inter-symbol interference (ISI) in the second paper referred to, above, and entails a simple demodulator even in the presence of unknown timing errors. Simulations are also carried out to corroborate our analysis.

ToA estimation for MB-OFDM UWB by suppressing energy leakage

In this paper, we estimate the time-of-arrival (ToA) of wireless channels by using the widely adopted equally-spaced channel estimator for the multi-band orthogonal frequency-division multiplexing (MB-OFDM) ultrawide-band (UWB) system. By suppressing the energy leakage due to the imperfect sampling of the leading channel paths, a novel ToA estimator is proposed for the MB-OFDM UWB system. This ToA estimator is evaluated and compared with the space alternating generalized expectation maximization (SAGE) algorithm in IEEE 802.15.4a UWB channels. Simulation results show that the proposed ToA estimator performs well in all channel models and outperforms SAGE by directly locating the first channel path.

Timing with Dirty Templates for Low-Resolution Digital UWB Receivers

Timing synchronization is a major challenge in ultra-wideband (UWB) wireless communication systems. Recently, a timing technique using dirty templates (TDT) has been proposed for rapid synchronization. The TDT technique efficiently collects the multipath energy even when the propagation channel and the spreading codes remain unknown. However, these TDT algorithms rely on delaying and correlating the received analog waveforms and entail an analog delay line, which is not easy to fabricate at the IC level. In this paper, we investigate and evaluate TDT algorithms at a digital UWB receiver with low-resolution analog-to-digital converters (ADC) in both data-aided and non-data-aided modes. The analysis and simulations show that the TDT algorithms remain operational for digital UWB receivers with negligible performance degradation

Differential UWB Communications with Digital Multi-Carrier Modulation

As a high-rate alternative of impulse radio (IR) ultra-wideband (UWB) communications, analog and digital multi-carrier UWB radios with coherent detection have been introduced. In this paper, we investigate digital multi-carrier differential (MCD) modulation and demodulation schemes for UWB. In our approach, the differential encoding and decoding are carried out across multiple digital carriers in the frequency domain. Hence, channel estimation is bypassed by using neighboring carriers as reference carriers. Compared with the transmitted reference (TR) UWB which relies on delaying and correlating received segments in time domain, our MCD-UWB avoids the need for the analog delay element. In addition, our digital carrier based approach does not incur any spectrum expansion. We also show that our approach enables variable data rates, and remains operational even in the presence of severe inter-frame interference (IFI). In addition, the small spacing between differentially encoded carriers guarantees that the data rate is not limited by the channel coherence bandwidth. Simulations are also carried out to support our theoretical analysis