The fifth generation (5G) wireless communication is the next major phase of mobile communications standards beyond the current LTE-Advanced. In order to meet 5G faster speed (1-10 Gbps), millimeter wave frequency band has been recommended for 5G wireless communication. The unlicensed 60-GHz band has been targeted for short-range, high data rate communication. This band is attractive for a number of reasons, including a large amount of spectrum available (7 GHz), suitability for frequency reuse, and high FCC EIRP limit (40 dBm) . One major challenge in the implementation of 60-GHz wireless is high path loss, implying a need for high-gain beamforming antennas.
Due to the 5G demand for high data rate communication and treasure frequency bandwidth, phased array systems are found significant in wireless communications. Depending on the operating environment, phased arrays can adapt their radiation patterns as well as cancel out information in unwanted directions. This significantly contributes to the reception quality and data capacity in communication systems. Mobile communications networks use MIMO technology to achieve high data rates. With MIMO beamforming, it is possible to reduce transmit power and still get better channel characteristics.
The beam-forming network (BFN) is main component of multiple beams antennas. Beam-forming networks are complex networks used to precisely control the phase and amplitude of radio frequency (RF) energy passing through them, which is conveyed to the radiating elements of an antenna. In 5G system, the devices will use smart array to beam-shape its own receiver sensitivity and transmit energy. The Butler matrix has been used extensively over the years in the beam-forming system. The Butler matrix consists of 90 degree hybrid couplers and fixed phase shifters. It has N input ports and N output ports. As a beam-forming network, it is used to drive an array of N antenna elements. It can produce N orthogonally space beams overlapping at the –3.9 dB level, and having the full gain of the array.
Design of Butler Matrix
As seen in Fig.1, an 8-way Butler matrix is composed of twelve 3-dB quadrature couplers, eight phase shifters, and sixteen crossovers. The microstrip line based 3-dB coupler and phase shifter are designed individually before integrating to a Butler matrix.
Fig. 1. Block diagram of the proposed 8×8 switched beam
Butler matrix is realized on a Liquid Cristal Polymer (LCP) substrate material of 4 mil thick and dielectric constant of 3.2. LCP substrate can be used as a low-cost dielectric material for high-frequency antenna design that provides very reliable performance at high frequencies. LCP has very reliable electrical properties up to millimeter waves like low dielectric constant of 3.2 and low loss tangent of 0.002-0.004 at 60 GHz. Its permeability is (moisture absorption ~0.02 %) comparable to that of glass and very close to that of ceramic and it has low Coefficient of Thermal Expansion (CTE) as low as 8·10-6/K. These material properties make LCP an ideal candidate for Multi-Chip-Module (MCM) technology looking at the growing millimeter wave applications market.
Fig. 2. Layout design of beamforming network (a) 8×8 Butler matrix layout Design (b) Design parameters of Hybrid Couple and Crossover
The layout design is shown in Fig. 2. The Butler matrix consists of eight RF inputs that independently feed eight RF outputs with varying phase delays. The purpose of this network is to uniformly feed the eight antenna elements with progressive phase delays of 22.5 degree which is determined by the respective input port selected. This creates eight fixed radiating beams accessed by eight independent inputs. The use of hybrid couplers provides high isolation between each input port, which allows a switch network to toggle between these ports without affecting the antenna performance. The individual components, as well as the entire Butler matrix structure, were designed using Keysight Advanced Design System (ADS). Using the principle of reciprocity, this structure can be utilized in the same manner for transmitting or receiving antenna applications. The overall size is about 25×18 mm2.
Hybrid coupler is used to generate signals 90° out of phase at its outputs. The port closer to the input port (port-2) is leading in phase by 90 degrees. The port located on the same side as the input port is isolated since there is no power reaching it. The crossover also known as 0 dB couplers are effective means of crossing two RF transmission line signals with a minimal coupling between them. By cascading two 90° hybrids we can implement a crossover. The phase shifter is implemented using microstrip transmission line.
Fig. 3. Simulation result of 8×8 Butler matrix (a) Simulated reflection coefficient for each port of the Butler matrix (b) Phase plot at each output ports
The simulated S-parameter of the Butler matrix is shown in Fig. 3(a). These results correspond to the assigned ports illustrated in Fig. 2(a) shows the reflection coefficients when looking into each port. It maintains a return loss greater than 10 dB across the frequency band of 58.3–61.5 GHz. Fig. 3(b) shows the simulated phase shift at adjacent antenna elements for excitations.
- Microstrip Antenna Array Design
The design started with the accurate determination of the single antenna element impedance characteristics allowing tuning the elementary antenna element over the desired frequency band. The dimensions of the single element are optimized by means of a full-wave analysis based on the Method of Moment (MOM).Table I provides an overview of the array specifications. The excitation of the patch is done by the center feeding along the microstrip feed line.
TABLE I : Design Parameter of Antenna Array
|Operating Frequency||60 GHz|
|Substrate Thickness||h = 4 mil|
|Substrate Permittivity||εr = 3.2|
|Loss Tangent||tan δ = 0.004|
|Bandwidth of -10dB return loss||1 GHz|
|Maximum Gain ( Array)||22 dBi|
To have the possibility of dual linear polarization, a square geometry of the patch has been chosen. The array is designed to operate at f=60 GHz with a bandwidth of -10 dB return loss of 2.2 GHz and peak gain of 22 dBi. The substrate has a thickness of h=4 mil with dielectric constant of 3.2. The design parameters of 8 elements linear array with a single transformer are shown in Fig. 4. Length and width of patch element are L=W=54 mil and the spacing between array elements is 0.75 λ0. The width of the transformer has a significant effect on the return losses due to changes in its characteristic impedance.
Fig. 4. Antenna Design (a) Linear Array of 8 elements (b) 8×8 planar arrays fed with eight output port of Beamforming Network
The return loss subsequently is optimized by adjusting the patch dimensions and impedance transformer at the feeding point. As shown in Fig. 5, a bandwidth of 1.1 GHz for -10 dB return losses has been achieved. The half power beamwidth (HPBW) of the array is 8°◦and it undergoes negligible fluctuations over the frequency range from 59.5 to 60.5 GHz. In this frequency range, the side lobe levels and maximum gain fluctuate ±3.5dB and ±0.7dB respectively.
|Fig. 5. Simulated result of 1×8 linear Antenna (1) Returns Loss plot (b) Antenna Radiation Pattern at 60 GHz|
2. Phase array system design
Antenna array is integrated with 8×8 Butler matrix in a single chip. As shown in Fig. 6(a), the antenna is placed on a top layer of chip stack up while Butler matrix network is at the bottom layer. Both antenna and BFN shares a common ground plane. The output of Butler matrix is connecting through via to linear microstrip array element. The complete system is simulated and antenna beam scanning is plotted in Fig.6 (b) with RF signal to eight input ports of Butler matrix. The gain of 8×8 element array antenna is 22 dBi. The designed antenna would support the future 5G mobile communication proposed to operate at the mmWave frequency and beam scanning capability.
Array antenna and beamforming network are placed in a single 35x35mm2 chip as shown in Fig. 5(c). This chip can be easy integrated with other circuits thereby making fabrication more cost-effective and suitable for 5G. For MIMO, the antenna array can be seamlessly integrated with RF.
A beam steerable phased array antenna with 22 dBi peak gain at 60 GHz for 5G MIMO system has been designed and modeled. The designed phased array system has 8×8 Butler matrix as its feed to achieve a steerable beam ranging from -30 to +30 degrees. Both the feed and the array antenna are designed in a single two layer 35x35mm2 chip thereby making fabrication more cost-effective and suitable for 5G applications.