This paper updates the advanced progress in ultra-wideband (UWB) research and development (R&D) in Institute for Infocomm Research (I2R), Singapore. The background and history of the UWB R&D in I2R is first reviewed. The activities in I2R, SERC of A*STAR (Science and Engineering Research Council of Agency of Science, Technology and Research, Singapore), and iDA (Infocomm Development Agency, Singapore) are reviewed. In particular, the past and ongoing programs at a national level are introduced. Then, the R&D projects related to UWB technology especially in the past 4 years in I2R are briefly introduced. In particular, the projects related to UWB high-data rate hardware and firmware based on WiMedia UWB standard PHY/MAC for multimedia streaming, as well as UWB antenna designs are addressed. In the project entitled "UWB for multimedia streaming", a dedicated FPGA-based PHY validation / evaluation platform is developed and available for IP customers to test UWB based receivers as shown in Figure 3. The platform consists of one analog board containing PLL and high speed ADC / DACs and one FPGA board containing 2x Xilinix's Spartan 3 (XC3S5000) FPGAs. The main application of the platform is to evaluate the performance of the receiver under real world environment. As an example, one platform can be programmed as a transmitter, transmitting deterministic data at a fixed data rate and another platform can be used as the receiver with packet error rate calculator. The platforms can then be connected to RF TX/RX, after which real world performance over the air (i.e. packet error rate v.s. distance) can be evaluated. In the UWB antenna and Arrays project, we involve issues related to UWB antenna and arrays, from fundamentals, design as well as applications. Many designs have finished for fundamental research and industry applications. In particular, the technology to reduced ground plane effects of small UWB antenna has been patented and applied to UWB portable devices. Last, the new R&D focuses in I2R will be mentioned. Since this year, we have been more focusing on a variety of industry UWB applications although we are still researching into many technology challenges. Our current projects include

This paper proposes a novel concept to aim at low-profile broadband antennas for UWB applications. The performance of the proposed UWB antenna is achieved by slotting the vertical ground plane under the radiating element of the antenna to reduce the required height for the UWB band and introducing an additional feed point in the horizontal direction with an extended vertical ground plane as shown in Figure 1. The notches on the vertical ground plane are normal to radiating element of the antenna. The additional feed point horizontally introduced is to decrease the antenna impedance to that of the RF cable to achieve an impedance match. Its performance in the UWB band features acceptable gain, omni-directional radiation in horizontal planes within �}30o elevation angles with a single feed point and fits within the small space available in most laptop computers. It also covers the 5 GHz WLAN band well.

As an example, though the design may be implemented in a number of other materials, the antenna was etched onto a 10 mm �~50 mm �~20 mil 25N Arlon PCB slab with dielectric constant ƒÃr=3.38 and 0.0025 loss tangent at 10 GHz. In this implementation the ground plane surface is in good contact with the metal cover and RF cable outer conductor. The antenna is installed at the top of the cover as shown in Figure 2. The bent rim of the cover has a height of 12 mm (inside) and a slant of ~10o. A 12 cm long RF feed cable is installed along the rim of the display cover. The minimum distance between the rim of the LCD panel and the bottom of the antenna is about 3 mm. The thickness of the LCD panel is about 5 mm. The top of the antenna is 2 mm below the edge of the cover rim.

There has been a considerable surge of research interest in the Ultra Wideband (UWB) radio technology worldwide. The planar types of monopoles have been demonstrated to provide UWB impedance bandwidth with satisfactory radiation patterns. Recently, studies on different shapes of planar UWB antennas have increased drastically. However, there isn't much explanation on how the planar UWB monopoles can achieve such a wide bandwidth and retain a seemingly omnidirectional pattern across the entire bandwidth. In this paper, we will provide further insight into the operation of the planar UWB monopoles. It is realised that the overlapping of the closely distributed resonance modes in the planar UWB monopoles is responsible for an ultra wide -10 dB bandwidth. At the low frequency end, the UWB monopole operates mostly in the standing wave modes. With the increase of the frequency, the antenna operates in a hybrid mode of standing and travelling waves. At the high frequency limit, travelling wave becomes dominated in the antenna operation. The time domain performance of the UWB monopoles has also been investigated in this paper. Different type of input signals are studied and compared. The fidelity of each type of the input signals is quantified. It is noted that for the same antenna, different type of input signals will have different ringing effects. Low fidelity happens in the cases of the mismatch between the signal spectrum and the transfer function. Among the different types of input signals, the 4th order Rayleigh pulse can completely conform to the FCC emission mask therefore is well suited for DS-UWB systems. Using modulated Gaussian pulse as input signals, a very high fidelity can be achieved and the received pulse has the least distortion. This narrow band pulse with carrier is suitable for MB-OFDM UWB systems.

Since the declaration of Ultra-Wideband (UWB) frequencies (from 3.1 GHz to 10.6 GHz) by the Federal Communications Commission (FCC) in 2002 [1], antenna design for this new communications standard has attracted increasing interest. Commercial UWB systems require small low-cost antennas with omnidirecctional radiation patterns, large bandwidth and non-dispersive behaviour [2].

It is a well-known fact that planar monopoles of different geometry, such as circular, elliptical or square, are compact broadband omnidirecctional antennas [3]. Additionally, planar monopoles are also non-dispersive [4]. Due to all these appealing features, planar monopoles are very suitable for UWB applications, and growing research activity is being focused on them. As described in [5], the shape and size of planar monopoles can be optimized for different purposes, using the Theory of Characteristic Modes [6]. This theory leads to modal solutions for arbitrary shaped antennas, and provides physical interpretation of radiation phenomena taking place on them.

Recently, a double feed square monopole has been proposed for UWB application [7]. This monopole presents a novel feeding configuration that consists of a splitting network connected to two symmetrical ports on its base. Using the Theory of Characteristic modes it has been demonstrated that the insertion of two symmetric feed ports prevents the excitation of horizontal currents and assures that only the dominant vertical current mode is present in the structure. As a result, an improvement in the polarization properties and impedance bandwidth of the square monopole is achieved. However, the design in [7] needs of an additional feeding network below the ground plane, that complicates the antenna and increases its cost. This inconvenient can be solved by fabricating the antenna in microstrip technology. In this way, the feeding network and the antenna lay in the same plane, and both can be constructed over a single Printed Circuit Board (PCB). Moreover, the use of a dielectric substrate leads to a more compact antenna.

This paper proposes an adapted version of the planar monopole in [7] to microstrip technology. The new antenna accomplishes UWB specifications by combining several design strategies.

[1] FCC 1st Report and Order on Ultra-Wideband Technology, Feb. 2002.
[2] H. Schantz, "The Art and Science of Ultrawideband antennas", Artech House 2005.
[3] N.P. Agrawall, G. Kumar, and K.P. Ray, "Wideband planar monopole antennas," IEEE Trans. Antennas Propag., vol. 46, pp.294-295, Feb. 1998.
[4] Z. N. Low, J. H. Cheong, and C. L. Law, "Low-Cost PCB Antenna for UWB Applications", IEEE Antennas and Wireless Propagation Letters, Vol. 4, pp. 237-239, 2005.
[5] M. Cabedo, E. Antonino, D. Sánchez, M. Ferrando, "On the Influence of the Shape of Planar Monopole Antennas in the Impedance Bandwidth Performance," Spanish URSI 2005 Symposium, Sept. 2005, Gandia, Spain.
[6] R. F. Harrington and J. R. Mautz, "Theory of characteristic modes for conducting bodies," IEEE Trans. Antennas Propagat., vol. AP-19, pp. 622-628, Sept. 1971.
[7] E. Antonino-Daviu, M. Cabedo-Fabrés, M. Ferrando-Bataller, and A.Valero-Nogueira, "Wideband double-fed planar monopole antennas," Electronics Letters, vol. 39, no.23, , pp. 1635-1636, November 2003.

* This work has been supported by Spanish Ministerio de Ciencia y Tecnología (TEC2004-04866-C04-01).

In Ultra-Wideband (UWB) system development, antennas have to be assessed with respect to their impedance matching and radiation performance. Impedance matching is easily expressed as return loss but the quantification of radiation performance is not so obvious. Nevertheless it is crucial to ensure that the antenna has a sufficiently stable pattern in the directions of interest, over the operating bandwidth, or in other words the antenna transfer function does not vary significantly over the range of operating directions. It has been proposed that this assessment can be conducted with the help of the Pattern Stability Factor (PSF). PSF enables one to compare two or more antennas for pattern stability, define pattern-stable bandwidth of an antenna as well as the overall bandwidth of an antenna, and assess the effect of antenna-device integration on radiation performance. Calculation of PSF can be done easily with either the impulse response in the time domain or directional transfer functions in the frequency domain. Transfer functions are acquired through time-domain electromagnetic simulation followed by Fourier transform or wideband sweeps in a frequency-domain simulation. Experimental determination of PSF can be done with conventional antenna range measurements or with two-port scattering parameter measurements using a vector network analyzer. In this paper we will present preliminary results of the latter.

In the proposed paper ther will be a comparison of the principles and concepts regarding the evaluation of different antenna concepts. Especially the qualification and optimization of bow-tie antennas versus slot antennas regarding the correlation properties of the antenna impulse response will be investigated. Coming from the foundations of antenna impulse response characterization the degradation effects in realistic impulse systems will be analyzed. Very important means for this purpose are the directional correlation properties of the different antennas.

With these tools a bowtie antenna concept and a diamond shaped slot concept are compared and optimization strategies for these concepts are shown.

Pattern difference representation for slot antennas.

UWB technology holds potential for a wide variety of new short range devices for communications, measurement, location tracking, imaging, surveillance and medical systems. In this context, a multiband programmable platform has been developed. The goal of the platform is to provide an open architecture with on the shelf components in order to rapidly prove the feasibility of algorithms for a low-cost UWB solution. One of the challenges of the implementation of UWB systems is the development of a suitable antenna that would cover a large bandwidth, which would include some forbidden frequency bandwidth.

For our application, a wide slot antenna with fork-shaped microstrip line fed structure [1] has been optimized for UWB applications, this prototype can cover whole UWB frequency range (3.1-10.6GHz). This antenna has been already presented in different papers [1,2] and has good performances for UWB applications [3]. The wide slot antenna is fed by 50 ohms microstrip feed line with a fork-shaped tuning stub, which is printed on the opposite side of the substrate and placed symmetrically with respect to the center line of the rectangular wide slot. The antenna is placed above a limited ground at a given distance, this position and its impact on radiation characteristics will be comment.

This paper presents different solutions to realize one or more filtering functions, to avoid interfering with existing 5.2/5.8 GHz wireless local area network (WLAN) systems for example. Two types of solutions have been studied. The first one is to modify the input of the antenna by a adding some stub or inserting some spurline [4,5]. The second one is to modify the radiating part with notches or stripes around the wide slot on the patch and also to insert some spurline at the right place. Details of all these solutions will be explained and experimental results will be presented and discussed.

References:

[1] X. Qing, M.Y. W. Chia, X. Wu " Wide-slot Antenna for UWB Applications", Antennas and Propagation Society International Symposium, 2003. IEEE, Volume 1, 22-27 June 2003 Page(s):834 - 837.
[2] M. Klemm, I.Z. Kovcs, G.F. Pedersen, G. Troster 'Novel small-size directional antenna for UWB WBAN/WPAN applications' Antennas and Propagation, IEEE Transactions onVolume 53, Issue 12, Dec. 2005 pp. 3884 - 3896.
[3] C. Marchais, G. Le Ray, A. Sharaiha "UWB Antennas Time Domain Characterization", 11th International Symposium on Antenna Technology and Applied electromagnetics (ANTEM 2005) ;Saint-Malo (France) ; pp. 136-137 ; 15-17 Juin 2005.
[4] W.H. Tu; K. Chang; Compact microstrip bandstop filter using open stub and spurline, Microwave and Wireless Components Letters, IEEE [see also IEEE Microwave and Guided Wave Letters, Volume 15, Issue 4, April 2005 pp. 268 - 270.
[5] C. Nguyen and K. Chang, "On the analysis and design of spurline bandstop filters," IEEE Trans. Microw. Theory Tech., vol. 33, no. 12, pp. 1416-1421, Dec. 1985.

The description of antennas, UWB or not, is usually placed at two extreme levels of precision, and consequently of complexity. Either the antennas are fully characterized (both numerically and experimentally e.g. in an anechoic chamber) or they are represented by a single scalar parameter prosaically referred as its "gain", and taking, e.g. a value of about 0 dBi for an omni-directional antenna. Typically, the former is adopted by the antenna specialists and the later by "the rest of the world", i.e., for example, the specialists of the propagation channel, of the "link level", coexistence or system analyses. The later approach is clearly not satisfactory because of its obvious lack of realism. But the former is neither satisfactory for reverse reasons: excessive volume of data, pointless precision (for numerous simulators stochastic by nature) or excessive complexity (particularly for UWB antennas); or for a practical one: the unavailability of the full data set.

We propose a drastic compression of the "antenna representation data set" based on a double parametric modelling. Starting from the measurement (or e.m. simulation) of the antenna Directional Transfer Function (DTF) in the Frequency Domain (FD) [1], a poles/residues extraction of the computed Impulse Response (DIR) is performed (SEM), followed by an expansion over a spherical modes basis. Both projections being linear, the reverse process is also possible. The adopted algorithm for the SEM is the "Full" Matrix Pencil method [2], whereas the spherical modes expansion is based on the TM and TE to r modes [3, 4].

Following this procedure, it is shown that the DIR may be expanded as:

where the coefficients R_nmp are the modal residues (for the pole p and the TM or TE (n,m) mode), the ψ_n,m are the normalized vectors of the spherical basis, and the s_p = α_p ± jΩ_p are the poles.

The set of the descriptive parameters is hence reduced to a few Rnmp complex coefficients leading to data compression factors as high as 200 (99.5 %).

As an example, the magnitude of the modal residues for a [2.5–10.5] GHz planar dipole (supposed omni-directional for simplicity) and the comparison of the residues and the reconstructed ones after spherical modes expansion as a function of the elevation angle are shown fig. 1 and 2.

References

[1] Ch. Roblin, S. Bories and A. Sibille: Characterization tools in the Time Domain, IWUWBS, Oulu, June 2003.

[2] T. P. Sarkar, S. Park, J. Koh and S. Rao: Application of the Matrix Pencil method for estimating the SEM poles of source-free transient responses from multiple look directions, IEEE Trans. on Antennas and Propagation, Vol. 48, n° 4, april 2000.
[3] R. F. Harrington: Time-Harmonic Electromagnetic Fields, McGraw-Hill, 1961.
[4] Ch. Roblin: A Preliminary Approach of the Statistical Modelling of Antennas for the UWB Communications, ICEAA, Turin, Sept. 2005.

The large spectral occupancy of ultrawideband (UWB) wireless signals significantly complicates the design of multi-antenna arrays and the characterization of propagation channels determining the performance of such arrays. This survey paper develops generic techniques for double-directional UWB channel characterization, and surveys methods for the extraction of the parameters of multipath components. The key channel attributes affecting multi-antenna performance, such as the angular spread and spatial correlation, are discussed in the light of measurement results. Guidelines for UWB array design are provided. Throughout the discussion, the differences from narrowband systems are highlighted.

An FIR-filter controlled linear antenna array for ultra wideband short pulses will be presented in this contribution. Electronic array steering with FIR-filters, where one filter is placed behind each antenna element of the array (see Fig.), can be considered as an extension of true-time delays with one more degree of freedom. This method allows ultra wideband radiation pattern synthesis only by adjusting the filter coefficients, whereas the incremental time delays of the filter stay constant. In microwave systems those coefficients are broadband amplifiers with bi-phase adjustable gain. The incremental time delays can be realized by transmission lines of certain length. The concept has been presented in our previous publications [GeMiC 2005].

In realistic environments, the signal, received by the antenna array will be distorted due to the antenna characteristic and will be seriously affected by multipath propagation. Hence, for the determination of the filter coefficients we take into account the antenna transfer characteristic, which is mainly characterized by the radiation pattern and input impedance. For wide bandwidth those parameters can become a strong function of frequency. If this dependence is moderately flat, we can use FIR-filters to equalize the frequency dependence of the antenna.

In order to partially compensate for the multipath propagation we correlate the received signal with a certain reference signal. Receiving the signal under the main beam angle, the FIR-filters work similar to true-time delays and signals are superimposed constructively from each antenna. With an appropriate channel model and with knowing the antenna characteristics this signal can be simulated without considering the antenna array and becomes a reference signal for the correlator. Using this concept, we can determine a radiation pattern in time-domain, which correlates well with the desired radiation pattern, even under realistic assumptions.

We will demonstrate our concept on various simulations, where we design a linear array radiation pattern for null steering in a desired direction. The ultra wideband radiation pattern will then be investigated in time-domain. Finally, methods will be discussed in order to keep the number of antenna elements and the filter order within reasonable limits.