# Half-Elliptical Resonator Lowpass Filter with a Wide Stopband for Low Band 5G Communication Systems
## 1 Introduction  
Lowpass filters (LPFs) are one of the highly demanded devices in the modern communication circuit. In [1], a compact LPF is proposed with distinguishing features, such as sharp roll-off and low insertion loss. Butterworth and Chebyshev microstrip filters are designed in [2] using stepped-impedance structures. A microstrip LPF with a sharp roll-off using the analytical method is presented in [3]. A compact LPF is presented in [4] based on defected ground structure; however, this method has a complex fabrication process, which could be restrictive in many applications [5]. In [6], an ultra-wide stopband LPF is designed with a sharp response, but this filter has a complicated structure. Stepped-impedance structures, open tubs and radial-shaped resonators have widely been used in designing microstrip filters, particularly lowpass filters with a large stopband [7, 8, 9]. Square split-ring resonators are used to shape a small microstrip LPF with a new structure in [10], but this filter suffers from high insertion loss in pass-band. An electromagnetic band-gap structure has improved the efficiency of the applied lowpass filter in [11]. Unfortunately, this method has a complex fabrication process. Other resonators, such as U- and T-shaped resonators, and a complementary square split ring resonator were used in [12, 13, 14, 15]. T-shaped and circular-shaped resonators are used in [13] to design a lowpass filter to achieve a sharp roll-off and extended stopband, where work, equivalent circuits and equations of transmission zeros (TZs) were presented; however, the size of the paper is undesirably large. The T-shaped resonators were utilized in [16, 17] to present compact lowpass filters, which were relatively large. In [18], a modified U-shaped resonator is applied to create a compact LPF, but the figure of merit (FOM) parameter for this LPF is lower than the state of the art. In [19], a compact LPF was introduced using compact microstrip resonator cells. These cells present a wide suppression level, but insertion loss in passband for this work is poor.  
In [20], lumped components (L and C) were used to reduce the circuit size and provide a wide stopband; however, the application of lumped components increased the complexity of the filter, and lumped elements cannot be used for high frequencies above 24 GHz.  
Furthermore, neural networks, which are useful tools in solving real-life engineering problems [21; 22; 23; 24; 25; 26; 27; 28], have been recently used to model the passive circuit, like LPF. However, designing microwave circuits based on neural networks is not a straightforward approach, which is more suitable for device modelling. In [29], open stubs with bent structures were used to provide a wide stopband, where several stubs were needed. The relative stopband of the filter is 1.72. Moreover, the proposed LPF is a suitable device for low-band 5G applications.  
The 5G spectrum provides remarkably faster mobile speed and lower delay compared with previous generations. The 5G spectrum has low-band, mid-band and high-band frequency bands. Low-band, which is also called sub 1 GHz, is a frequency band below 1 GHz. The proposed LPF correctly passes the sub 1 GHz signals and suppresses unwanted harmonics at higher frequency bands.