Ultra-wideband impulse radio systems with multiple pulse types

Ultra wideband-impulse radio (UWB-IR) systems transmit data by ultra short pulses. The UWB-IR systems can achieve a time diversity gain by transmitting some pulses for each information symbol. In L. Yang et al. (2002) the space time (ST)-UWB-IR system was proposed to achieve a space-time diversity gain by transmitting space-time block coded symbols with some antennas. In the ST-UWB-IR system, the transmit antennas transmit the same symbol over an ST code block simultaneously. If all the transmit antennas can transmit different symbols independently, that is, if it separates the symbols from each antenna, we can expect a higher transmission rate than the ST-UWB-IR system. Moreover, at the same transmission rate, we can expect a larger diversity gain by sending more pulses for each symbol. This paper proposes a space hopping (SH)-UWB-IR system. In the SH-UWB-IR system, to allow each user to transmit different symbols from different antennas, a unique time hopping sequence is assigned to each antenna, which is known to the receiver. In addition, to get the larger diversity gain in the SH-UWB-IR system, the transmitted symbols are interleaved over the transmit antennas. From the results of our computer simulation, we show that when the number of users is small, the bit error rate (BER) of the SH-UWB-IR system is superior to that of the ST-UWB-IR system at the same data rate.

Multiuser detection of time-hopping PPM UWB system in the presence of multipath fading

The possibility to accurately localize tags by using wireless techniques is of great importance for several emerging applications in the Internet of Things. Precise ranging can be obtained with ultra wideband (UWB) impulse radio (IR) systems, where short impulses are transmitted, and their time-of-arrival (ToA) is estimated at the receiver. Due to the presence of noise and multipath, the estimator has the difficult task of discriminating the time intervals where the received waveform is due to noise only, by those where there are also signal components. Common low-complexity methods use an energy detector (ED), whose output is compared with a threshold, to discriminate the time intervals containing noise only from those containing signal plus noise. Optimal threshold design for these methods requires knowl- edge of the channel impulse response and of the receiver noise power. We propose a different approach, where ToA estimation is based on model selection by information theoretic criteria (ITC). The resulting ToA algorithms do not use thresholds, and do not require any information about the channel or the noise power level. These blind, universal ToA estimators show, for completely unknown multipath channels and in the presence of noise with unknown power, excellent performance when compared with ideal genie-aided schemes.

In-car wireless data communications systems require a short-range unlicensed radio communications technology that causes a very low level of interference in the other, already deployed radio links and networks, offers low and medium data rate, can reuse the already occupied radio-frequency (RF) bands, and assures low probability of message collisions. Ultrawide band (UWB) impulse radio employs RF pulses with very short duration to carry the information; consequently, it is an optimal candidate for the in-car wireless communications and intravehicular wireless sensor networks. Data rate and shape of RF carrier pulse determine the performance of a UWB radio link. To limit the interference caused, the maximum power radiated by an UWB device is restricted by the Federal Communications Commission (FCC) in the U.S. Introducing a new mathematical model and starting from the FCC regulations, analytical expressions for the calculation of FCC power limits are derived here. It is shown that the low- and high-rate UWB impulse radio systems are peak and average power limited, respectively. The relationship between the mathematical model and the parameters of an UWB carrier pulse used in a built UWB radio is established. The performances of RF carrier pulses known from the literature are evaluated and compared. All expressions derived are verified by measurements.

The link budget of the transmission gain is usually evaluated by using the Friis' transmission formula. However, it is not directly applicable to ultra wideband impulse radio (UWB-IR) transmission systems. This paper presents the link budget evaluation formula in the term of transmission gain for the UWB-IR systems that takes into account the transmitted waveform, its distortion due to the antennas, the channel and the correlation receiver. Since the antennas are significant pulse-shaping filters in UWB-IR, the various kinds of the antennas are experimentally examined, especially focused on the effect of the received signal and the isotropic template waveforms.

Low-complexity low-rate super-orthogonal turbo codes (SOTC) are proposed in this work to replace the implicit repetition code (RC) in an ultra-wide band impulse radio (UWB-IR) system to improve BER, transmission range and system throughput. Various receivers including matched-filter and RAKE receivers are adopted for data detection in the additive white Gaussian noise (AWGN) channel and the indoor IEEE 802.15.3a channel. The performance of SOTC-encoded UWB is analyzed and validated by computer simulation. It is shown that the SOTC-based UWB-IR system can achieve significant performance improvement over the conventional DS- UWB system encoded by RC.

1.1 Impulse Radio (IR) Ultra Wideband (UWB) Communications Ultra wideband (UWB) communication is a fast emerging technology that offers new opportunities such as high data rates, low equipment cost, low power, precise positioning capability and extremely low interference Ghawami (2005). A wide range of possible UWB communication system applications includes radars due to UWB ultra high precision ranging at the centimeter level, wireless personal area networks (WPAN), sensor networks, imaging systems, UWB positioning systems, etc. Yang (2004), Kshetrimayum (2009). From a commercial point of view, the high data rates are the most attractive feature of UWB systems for which speeds of over 100Mb/s have been demonstrated Ghawami (2005). There exist three main types of UWB technologies: impulse radio (IR UWB), direct sequence (DS UWB), and multi-band orthogonal frequency division multiplexing (MB OFDM) Ran (2009). IR UWB communication technique is essentially different from all other communication techniques because it is carrier free and uses for communication between transmitters and receivers very narrow radio frequency (RF) pulses generated from the UWB pulse generator, while traditional transmission systems transmit information by varying the power, frequency, and/or phase of a sinusoidal wave in a modulation process Kshetrimayum (2009), Yao (2007). Consequently, complicated frequency mixers and local oscillators for carrier frequency up and down conversion are not necessary Yao (2007). IR UWB modulation techniques are especially important because the UWB spectrum has been made available by the Federal Communication Commission (FCC) and it can be used with IRs developed to date Lin (2005). Waveforms for IR UWB are designed to obtain a flat frequency response over the bandwidth of the pulse and to avoid a DC component. The large spectrum of a UWB signal may interfere with existing users. In order to keep this interference to the minimum, FCC specifies spectral masks for different applications, i.e. the allowed power output for specific frequencies Ghawami (2005). A contiguous bandwidth of 7.5GHz is available in the frequency interval of (3.1− 10.6)GHz at a maximum power output of −41.3dBm/MHz which is considered as extremely low Ghawami (2005). The power spectral density (PSD) of UWB systems defined as a ratio of the transmitted power P in watts and a signal bandwidth B in hertz is extremely low as compared to other communication systems due to the very wide bandwidth B of short pulses that are typically of nanosecond or picosecond order Ghawami (2005). The selection of the impulse signal type for IR UWB communication system is essential since it determines

The primary characteristic of the ultra-wideband (UWB) indoor radio channel is the large frequency diversity, which translates into a large number of resolvable paths and, consequently, in a low signal energy-per-path. Code acquisition is particularly difficult in such a hostile environment if the available diversity is not exploited. This paper deals with the initial code acquisition for an UWB impulse radio (IR) system in a dense multipath environment. In particular, we propose an approach based upon a non-coherent Rake detector (NCRD), which combines the energy collected from different paths in a non-coherent fashion. System performance of the NCRD is analyzed in multipath fading channels, showing that satisfactory performance is achievable with relatively low complexity.

Power efficient ultra-wideband based wireless communication for wireless body area network applications

Pulse-based communication systems are a promising candidate in the deployment of future wireless networks, thanks to the potential robustness and capacity guaranteed by the transmission of pulses of very short duration. In the framework of pulse-based communication systems, the Impulse Radio-Ultra Wide Band (IR-UWB) technique recently received particular attention. In the past, most of the UWB research focused on hardware and physical layer aspects in order to solve the technological challenges posed by IR-UWB. UWB peculiar characteristics may, however, also stimulate innovative higher layers' design. This work addresses Medium Access Control (MAC) issues for IR-UWB communication systems and extends the discussion to wireless optical communication systems. Typical characteristics of an IR-UWB system are presented, and solutions proposed for such systems at the MAC layer are reviewed. The applicability of such solutions to an optical wireless communications system is then discussed, highlighting analogies and differences between IR-UWB and wireless optical systems.

In this paper, a joint blind synchronization and demodulation scheme is developed for ultra-wideband (UWB) impulse radio systems. Based on the prior knowledge of the direct-sequence (DS) spread codes, the proposed approach can achieve frame-level synchronization with the help of frame-rate samples. Taking advantage of the periodicity of the DS spread codes, the frame-level synchronization can be carried out even in one symbol interval. On the other hand, after timing acquisition, these frame-rate samples can be re-utilized also for demodulation. Thus the acquisition time and the implementation complexity are reduced considerably. The performance improvement can be justified by both theoretical analysis and simulation results, in terms of acquisition probability and bit error rate (BER).

Non-coherent ultra-wideband (UWB) impulse radio systems are easier to implement than their coherent counterparts, because the strict synchronization requirements are relaxed. The trade-off for simplicity is a loss in performance. The pulse-repetition interval (PRI) for non-coherent UWB schemes is limited by the maximum channel delay, to avoid inter-symbol interference (ISI). This affects the maximum data rate achievable. For a coherent system, ISI can be mitigated by equalization, which is generally not applicable for non-coherent methods. In a non-coherent energy detector for instance, the square-law device at the receiver destroys the multipath phase information, making complete ISI mitigation impossible. This paper proposes the use of zero-forcing equalizer based on the partial channel information derived from the energy received over an integration time for on-off keying (OOK) transmission. The proposed scheme does not suffer from noise enhancement because the multipaths do not fade simultaneously. The performance of the proposed system is extensively simulated and the results are discussed in detail.

We investigate timing tolerances of ultra-wideband (UWB) impulse radios. We quantify the bit-error rate (BER) sensitivity to epoch timing offset under different operating conditions, including frequency-flat fading channels, dense multipath fading channels, multiple access with time hopping, and various receiver types including sliding correlators and RAKE combiners. For a general correlation-based detector, we derived in Part I unifying expressions for the decision statistics as well as BER formulas under mistiming, given fixed channel realizations. In Part II, we provide a systematic approach to BER analysis under mistiming for fading channels. The BER is expressed in terms of the receiver's energy capture capability, which we quantify under various radio operating conditions. We also develop the optimal demodulator in the presence of timing offset and show a proper design of the correlation-template that is robust to mistiming. Through analyses and simulations, we illustrate that the reception quality of a UWB impulse radio is highly sensitive to both timing acquisition and tracking errors. © 2005 IEEE.

Ultrawideband (UWB) signals are extremely power limited due to the severe power spectral density constraints obligated by the Federal Communications Committee. Therefore, the design of radiation efficient antennas for UWB signaling is a challenging step in the design and development of impulse radio UWB (IR-UWB) communication systems, especially when these signals coexist with previously standardized narrowband (NB) wireless services. In this paper, a dual-band-notched quad-element UWB multiple-input multiple-output (MIMO) antenna design is presented. The proposed design is capable of suppressing NB interference (NBI). The NBI suppression capability of the proposed design is tested both experimentally and through simulations. Moreover, the experimentally measured spectral characteristics of the proposed design are employed in simulating the bit error rate (BER) performance of IR-UWB systems, considering the biphase modulation (BPM) scheme. Both experimental and simulation results show that the proposed design is capable of improving the BER performance of BPM-modulated IR-UWB systems in an NBI environment by suppressing the NBI signals located at six different frequencies within the bandwidth of the two band notches of the proposed design, while introducing a negligible distortion to the IR-UWB signaling waveforms.

In order to boost the data-rate in Impulse Radio UWB systems (which is necessary in some applications such as video stream), we propose to allocate more than one time-hopping code to the same user. In the case of a rake receiver, a new kind of interference associated with extra codes allocated to the user of interest is induced. Consequently the impact of these extra codes on the data rate is not straightforward. In order to evaluate the benefit of our proposition, we derive a closed-form for the multicode interference variance in the context of a time-hopping ultra wideband impulse radio based system. This enables us to theoretically analyze the influence of the number of additional codes on the real achievable data rate.