Nowadays, rail traffic is expected to evolve into a new era of ‘smart rail mobility’, where trains, infrastructure, travellers and goods will be increasingly interconnected. Railway communications are required to support various high-data-rate applications, the communication system should be carefully designed, which makes railway scenario becomes a more and more important communication scenario. In this study, the channel characteristics are analysed for rural railway at 28 GHz. Four different deployments are considered via calibrated ray-tracing simulations in the target environment. The large-scale parameters of the mmWave channel, including the path loss, root-mean-square delay spread, Rician K-factor, angular spreads and cross-polarisation ratio are explored. The statistical properties, decorrelation distance and cross-correlations are analysed as well. The studied channel characteristics can be used to support the link level and system level design of the communication system in the similar environment.

The rising demand of high-capacity wireless services on-board has driven the research interest on radio communications in public transportation systems. In this experimental study, the authors focus on the intra-car communication links in a high-speed train (HST), which have been rarely reported in the literatures. Specifically, the authors have carefully measured and modelled the radio propagation characteristics for intra-train link. Measurements at both 2.4 and 5.8 GHz have shown that propagation losses inside the train car are much lower than free space, where path-loss exponent increases as the receiver moves away from the transmitter. Otherwise, the shielding loss and frequency dependent penetration losses caused by the train car body shell are parameterised. In addition, the small-scale fading is also evaluated to formulate a complete propagation model for intra-car communications. This work aims to support the Internet access for passengers,for onboard sensor networks and broadband moving-relay solutions.

The performance of communication-based train control (CBTC) systems, in terms of frequency of handoffs [changing association from one access point (AP) to another] and duration of service interruptions, strongly depends on the employed handoff algorithm. In this study, the received signal strength used for the calculation of these metrics is generated by a custom vector parabolic equation (VPE) solver explicitly developed to achieve high accuracy in complex tunnel environments. The authors demonstrate that VPE is as effective and accurate as measurements in estimating the performance of CBTC systems. Additionally, they present a methodology for selecting a handoff algorithm that most enhances the performance of a CBTC system, where APs have already been deployed. The impact of handoff algorithms on system performance is investigated by considering multiple system configurations in a 5 km, curved, rectangular tunnel. A multiple-attribute decision making approach is presented to rank handoff algorithms based on their performance. To validate this proposed simulation-based framework, a CBTC system deployed in the London Underground is presented, where measurement-based ranking is compared against the one derived using VPE simulations.

Machine-to-machine communication can realise considerably better information cognition of fixed and mobile equipment during rail transit. To assess the network access performance of the machine-type communication (MTC) of railway signal equipment for next-generation intelligent transportation system, the forecasting model of the machine communication traffic of railway signal equipment was divided into station indoor model, station outdoor model, and section movement model on the basis of cell units, and the traffic quantity and signaling overhead of the three models were calculated respectively. Based on Poisson's distribution and the Markov update process, an improved Markov-modulated Poisson Process (IMMPP) of the source traffic model was designed. Compared with the 3GPP traffic model, the IMMPP model maintains a good balance between the complexity and the relatively high accuracy of the signal equipment in the station outdoor and section scenarios. The simulation experiments showed that the IMMPP consumed 0.44 s more than 3GPP model 2 in the section scenario and 1.99 s in the station outdoor scenario, and the simulation equipment was in accordance with the actual railway signal equipment. The research result can provide a reference for the description of MTC traffic of other specialised equipment in the railway environment.

An ensemble learning-based approach for context detection for high-speed railway (HSR) is proposed, evaluated, and compared against various machine learning algorithms. The context information is collected and formatted from the realistic physical data, which are measured by the field test in a commercial 4G cellular network on-board high-speed train in an open area, HSR station, and typical urban area. The radio propagation knowledge implies that the channel model is implemented in feature extractions. The independent out-of-bag errors show ensemble learning approaches; especially, random forests-based algorithm achieves very accurate context detection (up to 93.5%), which is much higher than single tree (66%). Other ensembles with sub-spaces of discriminant (67%) and K-nearest neighbour (69%), as well as linear discriminant analysis (55.7%), and support vector machine (SVM) (27.7%). Furthermore, the features are selected based on the feature importance evaluation in the first round training. The selected predominant features and the reasonable number of individual trees construct the refined random forests, which obtain a high accuracy (92.6%) and 70% time reducing. This experimental study benefits from physical radio knowledge to support advanced long-term evolution (LTE) network for railway and future smart rail applications.

Device-to-device (D2D) communications help in improving the performance of wireless services in cellular networks via enabling cooperation among mobile users. Further, compared to traditional pairwise cooperation, cluster-wise cooperation can achieve even higher spectral and power efficiency. D2D clustering certainly can be a great supporting technology for the future of intelligent transportation. In this study, the authors investigate the D2D–relay clustering network in the vehicular scenario and propose a new clustering scheme named iterative greedy user clustering (IGUC). By exploiting those vehicles with better channel state, system resources can be saved. Here, they mainly focus on the question which is by applying D2D–relay clustering among vehicles, how much bandwidth could be saved in the uplink. This problem is mathematically formulated into a non-linear binary programming problem with the goal being minimum spectrum usage. Due to the NP-hardness, the low-complexity suboptimal algorithm IGUC is introduced to tackle it. Simulation results show that compared to the non-cooperative uplink system, IGUC saves a considerable proportion of the spectrum. Furthermore, clusters in denser and further areas tend to produce a higher gain. Influence of the fast time-varying characteristic of vehicular wireless channels is also investigated, upon which an adaptive switch mechanism is proposed to further facilitate IGUC.

In vehicular relay networks, the quality of communication link has a significant impact on the stable and reliable communication requirements. Most earlier studies focused on the influence of relay probability on link connectivity while little attention has been paid to the impact of packet collision. In complex mobile scenarios, vehicle density is proportional to relay probability, while proportional to data packet collision probability. Meanwhile, vehicle density and packet collision affect the quality of communication link in the relay network. This condition inspires them to study the link connectivity in vehicular relay networks based on carrier sense multiple access with collision avoidance. First, the relationships among vehicle density, vehicle speed, packet collision probability and relay probability are studied in relay communication. Second, due to the high-speed movement of vehicles, an analysis model, considering the mobility of vehicle nodes, is conducted to analyse the downlink performance in relay communication networks. Finally, a platoon scheme is proposed to reduce packet collision probability, considering the vehicle density and backoff window size on vehicle-to-roadside unit communication. Extensive numerical results indicate that the platoon model has a great advantage in terms of downlink performance, especially in the case of dense traffic flow.

Unmanned aerial vehicles (UAVs) with multi-input–multi-output technologies are considered as a promising platform to provide future high-speed wireless communication services. In this study, considering three-dimensional (3D) antenna arrays and 3D arbitrary trajectories of the UAV and mobile terminal, the authors propose a new 3D non-stationary geometry-based stochastic channel model for UAV-ground communication systems. Under 3D non-isotropic scattering scenarios, the computation and optimisation methods of time-variant channel parameters are investigated. In addition, the theoretical statistical properties such as the autocorrelation function, cross-correlation function, Doppler power spectrum density, level-crossing rate, and average fading duration are analysed and derived. Finally, the usefulness of the proposed model as well as theoretical derivations are verified by the comparison between theoretical, simulated, and measured results.

The unmanned aerial vehicles (UAVs) have been widely applied in various fields due to their advantages like high mobility and low cost. Reliable communication is the premise to ensure the connectivity between UAV nodes. To provide reasonable references for the design, deployment, and operation of UAV communication systems, the precise prediction of radio channel parameters are required. In this study, the authors propose prediction methods for path loss and delay spread in air-to-ground millimetre-wave channels based on machine learning. Random forest and K-nearest-neighbours are the algorithms employed in the methods. Then, a feature selection scheme is proposed to further improve the prediction accuracy and generalisation performance of the machine-learning-based methods. Generally, machine learning algorithms require massive data for training purpose. However, measuring data is time-consuming and costly, especially when the scenario or frequency changes. Therefore, transfer learning methods are introduced to predict path loss with limited data. The proposed methods for path loss prediction are compared to Okumura–Hata and COST-231 Hata models. The lognormal distribution is the contrast model in delay spread prediction. Based on the data generated by ray-tracing software, the new methods have a smaller root mean square errors than contrast models.

This paper proposes the design of a microstrip patch antenna with an asymmetric polariser for adaptive polarisation adjustment. The asymmetric polariser is placed in the outer perimeter of a circular radiating patch and is mechanically rotated from 0° to 360° to adjust the axial ratio (AR) and the tilt angle (TA). To verify the operating principle of the proposed antenna, its radiating properties are theoretically formulated based on the cavity model, and three sample antennas with different shapes of the polariser are fabricated to measure their antenna characteristics. Then, a field test is conducted to observe variations of the received signal strength for incoming signals with time-varying polarisation properties. The results demonstrate that the mechanical rotation of the polariser is suitable to minimise the polarisation mismatch with improvement of the received signal strength from −44.0 to −30.3 dBm in a harsh multipath environment.

This paper presents empirical results of an innovative beam-scanning leaky-wave antenna (LWA), which enables scanning over a wide angle from − 35° to + 34.5° between 57 and 62 GHz, with broadside radiation centred at 60 GHz. The proposed LWA design is based on composite right/left-handed transmission-line (CRLH-TL) concept. The single-layer antenna structure includes a matrix of 3 × 9 square slots that is printed on top of the dielectric substrate; and printed on the bottom ground-plane are Π- and T-shaped slots that enhance the impedance bandwidth and radiation properties of the antenna. The proposed antenna structure exhibits metamaterial property. The slot matrix provides beam scanning as a function of frequency. Physical and electrical size of the antenna is 18.7 × 6 × 1.6 mm³ and 3.43  × 1.1  × 0.29, respectively, where is free space wavelength at 55 GHz. The antenna has a measured impedance bandwidth of 10 GHz (55–65 GHz) or fractional bandwidth of 16.7%. Its optimum gain and efficiency are 7.8 dBi and 84.2% at 62 GHz.

Here, a new algorithm, based on compressed sensing (CS), is presented for generating broad nulls in linear and planar adaptive antenna arrays through the control of only a small number of elements. In particular, sparse recovery theorem and convex optimisation are used to generate the broad nulls by perturbing the complex weights of a minimum number of elements. The problem is first formulated as a sparse recovery problem and then relaxed to the form of a convex optimisation problem. In addition to the nulling constraints, another constraint is added to ensure that the perturbed elements do not cause a pointing error in the synthesised array pattern. Also, one more constraint is used to set a predefined peak value for the array response in the sidelobe region. The optimisation problem is then solved iteratively using the iterative re-weighted -norm minimisation technique to reduce the number of perturbed array elements to the lowest possible number while satisfying the constraints on the radiation pattern. Simulations were conducted for linear and planar arrays. The results show that the proposed algorithm is capable of forming the required nulls by changing the complex weights of only a small percentage of the array elements.

This study presents a low-profile broadband microstrip patch antenna with filtering response. The proposed antenna consists of a rectangular patch and four parasitic gap-coupled elements, two L- and two rectangular-shaped patches. A broadband quasi-elliptic boresight gain response is obtained without using any extra filtering circuits. The input impedance of each radiating element, i.e., driven patch and parasitic elements, is matched to its radiating quality factor and the couplings between patches are optimised for broadband impedance bandwidth with filtering response. Prototype hardware is designed and fabricated on Kappa 438 substrate with a relative permittivity of 4.4 and thickness of 3.2 mm. The antenna exhibits a total size of 25 × 23 × 3.2 mm³ with relative impedance bandwidth (voltage standing wave ratio<2) of 60% ranging from 4.4 to 7.8 GHz. The experimental results demonstrate good performance with nearly flat gain and good filtering response. The proposed filtering antenna exhibits low pulse distortion in time domain which makes it a good candidate for location-aware Internet-of-things applications employing the IEEE 802.15.4 ultra-wideband standard. Switchable sector base-station antenna system is studied to demonstrate the capability of this design to enhance the localisation and communication performance of the wireless network.

Here, the authors apply Lorenz–Lorentz and Maxwell Garnett homogenisation theory in order to extract the effective constitutive parameters of a metamaterial. The authors demonstrate the significance of extracting the polarisability coefficients of a metamaterial taking in consideration the interaction fields of a unit cell neighbours in an infinite periodic structure. The validation process is done using full wave analysis by comparing reflection and transmission coefficients of the periodic structure to the results from a bulk characterised by the extracted electrical effective parameters. The presented method helped us to evaluate the effective parameters of non-dispersive metamaterials with highly interaction fields. In addition, several new geometries of metamaterials that are characterised by unique non-dispersive diagonal and are presented.

A planar electronically pattern-reconfigurable antenna based on Alford-loop-type hexagon shaped radiator is presented. The antenna consists of three half-wavelength dipole radiators arranged on both sides of an FR-4 substrate. It has an overall profile of 0.47λ₀ × 0.47λ₀ × 0.012λ₀ (λ₀ free space wavelength at 2.35 GHz). The bottom ground layer is tapered to allow impedance matching. Three PIN diodes are used to control the direction of radiation pattern by switching them in a controlled manner. Thus the antenna provides reconfigurable radiation patterns in six spatial directions with by switching three PIN diodes in ON and OFF states. The antenna has a measured impedance bandwidth of 2.35–to 2.45 GHz (4.16% fractional Bandwidth) and steers the beam in six directions 0°, 60°, 120°, 180°, 240°, and 300° covering the entire azimuthal plane. The antenna offers a maximum measured gain of 2.66 dB with simulated efficiency over 75%. Further, an investigation of non-linear effects due to PIN diodes shows that the antenna operates in the linear region up to an input power level of 27 dBm where the strength of received spurious harmonic and intermodulation signals is negligible. It makes the antenna suitable for indoor and short-range communication applications in the 2.4 GHz band.

Here, the authors present a class of lumped-element low-pass and bandpass absorptive filter circuits. To begin with, the authors discuss a new type of absorptive low-pass filter (LPF) structure that has an excellent impedance matching performance in both passband and stopband. This work provides low-pass prototype values for the proposed absorptive LPFs in terms of those for the conventional reflective filters, namely g-parameters, so that the new type of absorptive LPF can be designed in a straightforward manner. In addition, it is shown that traditional low-pass-to-bandpass frequency transformations can be applied to the absorptive low-pass prototype cases in the design of absorptive bandpass filters (BPFs). Second- and third-order LPF and BPFs are designed and measured to verify the new type of absorptive filter and its design method.

At millimetre-wave frequencies, the discrepancy between simulation and measurement comes from the relative permittivity tolerance and fabrication tolerance. This discrepancy can mainly cause resonant frequency shift. A tunable waveguide-to-microstrip transition for the W-band antenna is proposed to overcome this discrepancy. A typical transition consists of a microstrip probe on a substrate and a waveguide back short in a fixed structure. The tunability has been designed by making a movable back short through WR-10 waveguide. This design can change the back-short distance mechanically to tune resonant frequency. The tunability of this design has been verified with a W-band substrate integrated waveguide (SIW) slot array radar antenna.

The evolution of a wireless communication system to the fifth generation is accompanied by a huge improvement in the performance of the communication system through providing high data rate with a better coverage area. Hence, 5G technology requires access to millimetre wavebands where more spectra are available for high-speed applications. This technological progress motivates the research community to develop the essential microwave components, specially couplers, through the use of state of the art guiding structure such as printed ridge gap waveguide (PRGW). In this study, the design of ultra-wideband 3-dB planar quadrature hybrid coupler based on PRGW technology is proposed. A systematic design procedure for the proposed coupler is presented and illustrated. The proposed directional coupler is fabricated and measured, where the measured results show good agreement. The presented coupler covers over 26% bandwidth centred at 30 GHz. The presented coupler has a measured wide impedance bandwidth of a 26.5% centred at 30 GHz based on −10 dB threshold. The phase difference between output ports is over 23% relative bandwidth. On the other hand, the total bandwidth is limited by the amplitude imbalance, which has a bandwidth of 13% for ±  0.75 dB variation.

A planar DC-blocker suitable for differential mode signalling applications is designed and fabricated. The theory of this component is explained in a new form which utilises the wave scattering transfer matrix. The proposed interpretation of the transfer matrix is most suitable for series (cascade) elements like DC-blockers. In addition to the theoretical enhancement, design of a compressed balanced DC-blocker inserted through a shielded broadside coupled stripline (SBCSL) transmission line (TL) is presented. The return loss of better than 10 dB is obtained at 50-ohm differential-mode input ports of the fabricated DC-blocker in the entire frequency range of 5.6–8.4 GHz. The other achievement of this paper is design and fabrication of a wideband substrate-integrated waveguide (SIW)-mediated balun structure for single-ended measurement of a balanced SBCSL component. The fabricated balun exhibits a nearly perfect coaxial-mode to coupled-stripline differential-mode conversion in the full range of 5–9 GHz. The presented balun is successfully utilised to derive the scattering parameters (S-parameters) of the fabricated balanced SBCSL DC-blocker.

The authors introduce a miniature and gain enhancement method of dielectric resonator antenna (DRA) using a metallic cap. The structure of the proposed antenna consists of a stacked cylindrical dielectric resonator, a metallic cap that is located on the resonator, and a ground plane. By adjusting the size of the metallic cap while retaining the size of the antenna, the proposed antenna operates in the lower frequency band and shows improved realised gain. As the radius of metallic cap increases, wavelength at z-axis increases and the resonance frequency gets lower. In addition, tangential field at side wall surface increases, which leads to the enhancement of realised gain at resonance frequency. The authors have fabricated a prototype of the antenna for the experimental verification. The simulation results are in a close agreement with the experimental finding.

The proposed study contains the design and analysis of TM ₁₁ to HE ₁₁ (hybrid) mode converter at 42±0.2 GHz which is an integral component of the transmission line that transmits 200 kW power from gyrotron to the plasma in SST-1 Aditya tokamak. HE ₁₁ mode is preferred, as the transmission loss in corrugated waveguide is less in comparison with other modes. For generating HE ₁₁ mode from TM ₁₁ mode, the depth of the annular slots in the mode converter is varied gradually from zero to one-quarter wavelength. The conversion efficiency and radiation pattern of the proposed mode converter depends on the corrugation profile and the length of the mode converter. Both the parameters are optimised numerically to achieve the highest conversion efficiency. Keeping in mind mechanical fabrication of mode converters optimisation has been carried out. It converts 95.6% power of TM ₁₁ mode into HE ₁₁ (82.2% TE ₁₁+13.4% TM11) mode respectively. The 1 dB compression bandwidth of 19.04% (8 GHz) at 42 GHz is obtained. The length of mode converter is 40.83λ mm. The reflection coefficient and VSWR are −25 dB and 1.1 respectively. The power in side lobes and isolation from cross polar component is less than −20 dB and −25 dB respectively.

This article proposes CORPS beamforming network (BFNs) with both beam and null steering capability. All simulation results are presented for scannable mono-beam configuration but it can be extended to the multi-beam system. Complex inputs are optimised by means of the PSO algorithm to get the array factor with the desired characteristics that are side lobe level (SLL), directivity (D), and null depth. All sub-blocks of the BFN are designed in the centre frequency of 2.4 GHz.

A wideband dual-band dual-polarised patch antenna is proposed here. It consists of a ground that has a cross slot and a rectangle hole, a feeding structure that consists of two π-shape strips, and a radiating patch that consists of a square loop strip and a small square patch with a cross slot. Two feeding ports are used to generate two orthogonal polarisations. A prototype has been fabricated and measured. The measured −10 dB impedance operating bands of the two ports can cover the bands of 3.59–4.21 GHz and 5.42–6.34 GHz, the relative bandwidth are 15.9% and 15.6%. The measured isolations between the two ports are more than 20 and 15 dB at the lower and higher bands, respectively. The antenna has a good radiation performance at the lower and higher bands.

Here, a novel design of a planar monopole antenna is presented for wireless body area network (WBAN) wearable applications. The design is fabricated on flexible liquid crystalline polymer (LCP) substrate. The antenna is fed by a uni-planar coplanar waveguide transmission line and has total size of 51 × 22 mm². In comparison with its recent published peers, the antenna has the thinnest thickness of 0.1 mm. The antenna is simulated and tested while physically bent. Proposed design along with simulation and experimental results are discussed in this work. Bottom rectangular slabs were added as a modification to the design in order to restore gain at higher frequencies. For the entire operational bandwidth, numerical far field simulations show the suitability of the proposed design to human body loading. A full study targeting specific absorption rate (SAR) has been performed to calculate the maximum input power to the proposed design. The objective has been to prevent any harm to human body as specified by the relevant international non-ionising radiation exposure standards.

The conventional Doherty output combiner limits the bandwidth of operation in the asymmetric Doherty power amplifiers (DPAs). In this study, an output combiner that is designed using a harmonic-tuned continuum mode active load modulation technique is proposed with the main target of extending the bandwidth of operation in the asymmetric DPAs. Through this methodology, design equations are derived and multiple harmonic impedance solutions are provided. This offers greater freedom for designing the output combiner of the asymmetric DPA which consists of the impedance inverting network and the impedance transforming network. Moreover, enhancements in the efficiencies at back-off and saturation power levels are made possible. The efficacy of the design strategy is demonstrated by the realised broadband asymmetric DPA prototype working within the band of 1.4–2.45 GHz. The experimental results have shown 54.55% bandwidth of operation, accounting for about 0.95–12.55% increment in bandwidth as compared with some recently published DPAs in the literature. Substantially, drain efficiencies within 35.5–52% at 6 dB output power back-off level and 47.5–64.2% at peaking power level are also recorded from the experiments. The maximum output power and gain are recorded at 43.52 dBm and 13.03 dB, respectively.

This paper presents a step-by-step procedure for the design of a slot array antenna in groove gap waveguide technology. The design procedure is based on defining an effective width for groove gap waveguides. To accomplish this, a direct equivalent correspondence is assumed between the groove gap waveguide and a rectangular waveguide. It is demonstrated that once the effective width of the groove gap waveguide is found, conventional equations for the design of slot-array rectangular waveguide antenna can be utilised to accurately design a slot array antenna in gap waveguide technology. Thus, unlike other methods available in literature, the proposed method only requires the knowledge of propagation constant in the dominant mode of the groove gap waveguide. Since analytic equations are utilised, the design of a slot array groove gap waveguide antenna with very good impedance matching and desired side-lobe level can be accomplished in a fast optimisation process. Using the proposed method, slot array antennas with uniform and Chebyshev distribution of slots are designed. Finally, a prototype of a designed antenna is fabricated and its reflection coefficient and radiation pattern are measured to provide an experimental verification.

The variations of parameters in magnetically coupled resonant wireless power transfer (MCR-WPT) system leads to system detuning problem. When the operation frequency is shifted from the resonant frequency of the system, output power of load (PL) will reduce greatly. The mathematical formulation of PL is similar to a multimodal function of the operation frequency. All the maximums of PL are needed to confirm the operation frequency. This study describes a dynamic niche artificial bee colony (DNABC) algorithm for locating all the optimums of multimodal function. By judging the increment of individual fitness, DNABC algorithm divides the solution space into peaky niche and candidate niche, and updates the boundary of niches dynamically. It transforms searching multiple maximum values in the whole solution space into searching one global maximum in each niche. Taking a 3-coil MCR-WPT system as an example, the simulation and experiment results show that DNABC algorithm is affected little by the number of population and has high convergence speed and high optimisation accuracy.

In this paper, a high gain, aperture efficient, low profile compact, near zero index (NZI) metasurface (MS) lens antenna with enhanced radiation characteristic is proposed. A high-performance transmissive MS is designed by using modified double circular ring resonator as unit cell with measured 3 dB transmission band from 8.80 to 11.90 GHz. It is observed that a triple layer stacked MS lens has the maximum ability to converge the electromagnetic radiation. The focusing mechanism of multiple stacked MS is studied by using measured E- and H-plane radiation pattern plot and H-Field distribution. A high gain low profile antenna is designed by placing the proposed small single surface NZI MS lens over a microstrip patch antenna acting as a microwave source. The proposed NZI MS lens is found to increase the measured gain of E- and H-plane by 10.70 dB in boresight direction at operating frequency 10.15 GHz. The front to back ratio is improved by 8 and 3 dB in E- and H-plane, respectively. The proposed MS lens antenna is able to achieve a broadside gain of 97.50% of the maximum gain achieved due to ideal effective radiation surface.

Recently, considerable attention has arisen over the frequency-coded chipless radiofrequency identification (RFID), but there is no general consensus on the system specification scheme including encoding techniques, resonators with narrowband and harmonic-rejected features, and a frequency spectrum used. The purpose of this study is to develop the design rules that improve the capacity and reliability of the chipless RFID. The proposed system development is cast into three phases. Phase one is to determine the encoding scheme that enhances robustness. The limitation of the conventional one-bit-per-resonator approach is identified, and frequency shift encoding is designed to be the kernel of the chipless RFID. Phase two is to determine the resolution of the frequency spectrum by analysing the radar cross-section for 37 resonators. Four resonators exhibit the narrowband and harmonic-rejected features, and thus they are recommended to be implemented in the chipless tag. Phase three is to determine the specification on the frequency spectrum. For a 21-bit example system, the frequency spectrum that meets the trade-off between the broadband complexity and the size of the tag is 2.0–5.0 GHz. The proposed design rules are validated by conducting simulation and measurement.