1. Introduction

The ultra-wideband (UWB) communication technology with a long history is developed rapidly in the past few decades. Since 1989, the UWB was first employed by the Defense Advanced Research Projects Agency (DAPRA) as a term, and the DAPRA also proposed the bandwidth definition of the UWB. In fact, the UWB technology was only authorized to be applied in military communications. Since February 2002, the development of UWB has undergone a great change. The Federal Communications Commission (FCC) finally released the UWB spectrum globally for data communication or radar and security field for civilian application and redefined the bandwidth of UWB, which specifies that the UWB radiofrequency signal has a fractional bandwidth (FBW) greater than 20% or 10 dB absolute bandwidth greater than 500 MHz. According to the definition of FCC part 15 [1], the authorized band allocated to the UWB communication systems is ranging from 3.1 to 10.6 GHz. Unprecedented 7.5 GHz of bandwidth is the largest bandwidth of any commercial terrestrial system has ever allocated. The 3 dB FBW of the UWB can reach 109%, and FCC emission mask specified that the transmission power does not exceed 41.3 dBm/MHz (75 nW/MHz). The way of sharing the spectrum with extremely low-power spectral densities (PSD) is of paramount significance in present intense crowned spectrum circumstance. The major merits of the UWB are as follows:

Firstly, high date rate: according to the Shannon formula for channel capacity [2], the maximum infallibility information date rate of the system in the additive white Gaussian noise (AWGN) channel can be expressed as

$$C = B \log\_2 \left( 1 + \frac{S}{N} \right) \tag{1}$$

The stub-loaded quintuple-mode resonator is employed to design UWB bandpass filter with two transmission zeros near the lower and the upper cutoff frequencies in [30]. To address the issue of harmonic effect to obtain wide stopband, the stepimpedance resonator (SIR) is utilized to design UWB bandpass filters with remoted harmonic [33–37]. A UWB bandpass filter with more than 30 GHz out-of-band attenuation is approached by using SIR in [34]. The novel ring resonators are considered as an effective way to design UWB bandpass filters attributed to its miniature size and multiple resonance behavior [38–46]. In [39], a design of UWB

by using quintuple ring resonator is proposed. In [42], UWB with switchable bandwidth is also investigated by implementing a ring resonator, and tunable passband ratio of 1.22:1.13:1 is obtained. Another major category of UWB bandpass with desired UWB passband performance is based on the parallel-coupled lines [47, 48]. In [48], by using parallel-coupled microstrip lines, a UWB bandpass filter with a passband from 3.1 to 10.6 GHz of less than 1 dB insertion loss is accomplished; meanwhile, the attenuation level can reach 40 dB in stopband. In order to cater for the urge demand for miniaturization, UWB bandpass filters with multilayer structures have been extensively investigated and reported [49–58]. In [57], design of an eight-pole UWB filter is demonstrated; meanwhile the proposed UWB filter not only has merits of miniature circuit size but also processes a 38.1 dB out-of-band suppression by utilizing the multilayer structure. In addition to the aforementioned techniques, there were also other routines to obtain the UWB bandpass filter, such as semi-lumped UWB bandpass filter [58, 59] and UWB bandpass filter designed with right-/left-banded structure [60–62]. Furthermore, for the purpose of achieving the UWB communication while eliminating other inferences of current communication systems, notch band UWB bandpass filter is presented [60, 63–77] and

, without feedlines)

bandpass filter with extremely compact circuit size (0.46 cm<sup>2</sup>

Review on UWB Bandpass Filters

DOI: http://dx.doi.org/10.5772/intechopen.87204

will be demonstrated in detail in Section 6 of this chapter.

of this chapter.

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2. Regulation and application

This chapter mainly focuses on the various approaches to achieve UWB bandpass filter and the discussion of several conventional methods for highperformance UWB filter with wide stopband, high out-of-band attenuation, sharp selectivity, and miniaturization. Therefore, the organization of this chapter is as follows: in Section 2, application scenarios of UWB, development history of UWB, and the UWB regulations established by the FCC are briefly demonstrated. In Section 3, the major specifications of the UWB filters as well as the foundation of design methodology are illustrated. Section 4, the key section, focuses on varied approaches to realize the UWB filter design. Common ways for accomplishing the design of UWB filters can be classified into the following categories: one of the general methods of designing UWB bandpass filter is using multimode resonator (MMR) (Section 4.1), and similar to the method in Section 4.1, UWB bandpass filters are also realized by using a stub-loaded multimode resonator (SLMMR) (Section 4.2). The methods of implementing the UWB bandpass filter with multilayer structure, parallel-coupled line, and step-impedance resonator design methodology are, respectively, reviewed in Sections 4.3–4.5. In order to fulfill the requirement to eliminate the RF interference within the UWB band, the UWB bandpass filter with notch band has been designed and reported extensively, which is reviewed in Section 5. Section 6, the Conclusion section, will be given at the end

The UWB wireless communication has been only authorized to the military

communication for 42 years. Since 2002, the FCC released the unlicensed

where B stands for channel width and S/N denotes the signal-to-noise ratio. Hence, it can be concluded from Eq. (1) that even if the signal-to-noise ratio values are as low as 0.1 (�10 dB), the system's data rate still can reach as high as 1 Gbps. It fully demonstrates the extremely high date rate of the UWB system. Secondly, strong anti-interference ability: UWB resorts carrierless communication with nanosecond pulses. With Fourier transform, it can be derived that the power spectral density is dramatically wide with low-energy density, which reveals that the UWB system is of excellent concealment. Thirdly, high resolution ratio of time and space: the UWB is operating at high frequency with a nanosecond resolution of time domain, and the short wavelength at the RF enables spatial resolution of 0.1 m approximately. The rapid development of 5G [3] and the Internet of Things (IoT) [4] has an urgent demand for high response speed and high positioning accuracy, and the UWB can perfectly meet this requirement. The emergence of key reports and research process, whether from an academic or engineering perspective, has greatly advanced the development of UWB over the past few decades.

The UWB bandpass filters served as key building block in UWB wireless communication systems to regulate the FCC UWB masks have aroused much research interest in this century. And various attempts to design UWB have been reported continuously. The UWB bandpass with a FBW of more than 20% have been reported with simple design methodology and excellent passband performance since 2012 [5, 6]. However, for the FCC authorized specification, 109% of the FBW is actually an unprecedented challenge in approaching UWB bandpass filters design. Despite the well-established comprehensive design theory for narrowband bandpass filters with varied specification [7–10], the synthesis design methods for UWB bandpass filters are not suitable to employ existing powerful design theory foundations.

Various techniques have been presented to develop the UWB bandpass filters. One of the straightforward methods is cascading a low-pass filter and a high-pass filter to accomplish UWB bandpass filter [11–13]. Though considerable wideband is realized in [11], the occupied circuit size needs to be further reduced. To achieve UWB bandpass filters with compact size and simple design process, multimode resonator (MMR) has been presented [14–25]. In [16], the UWB bandpass filter is achieved with wide stopband, and 40 dB attenuation can be realized within frequency ranging from 12.0 to 16.0 GHz. In [19], quintuple-mode resonator is introduced to design UWB bandpass filter and sharp shirt, and wide upper stopband is achieved simultaneously. A UWB bandpass filter with 20 dB out-of-band suppression up to 25.1 GHz is proposed in [21].In [25], a novel MMR with interdigitalcoupled-microstrip line sections is implemented, which can excite seven transmission poles to design UWB bandpass filter with high roll-off rate. In summary, design of UWB bandpass filters by using MMR is of compact size and with multitransmission poles, whereas the range of out-of-band rejection is still insufficient since harmonic effects. Similar to the MMR, the stub-loaded multimode resonator (SLMMR) is another ideal structure to design UWB bandpass filters owing to its simple structure and easy design procedure [26–33]. In [26], a highly selective UWB bandpass filter is achieved by short-circuit stub-loaded structure, which can excite 11 resonant modes to fulfill the requirement of UWB with miniature circuit size.
