Moving Up in Frequency – Why D-band is the Next Frontier for XHaul

This article describes an Innovate UK-funded collaborative project on next-generation mmWave technology projects up to 175 GHz, to enable emerging 5G wireless XHaul requirements up to 100Gbps.

E-band is well established as an attractive and cost effective, high capacity solution for mmWave XHaul applications, capable of supporting radio links upto 20Gbps. The spectrum allocations available in E-band are 71 – 76 GHz and 81 – 86 GHz. However, the ever-increasing demand for data means that still higher capacity is going to be required in 5G XHaul networks, and the need for links up to 100Gbps has already been identified. Moving up the frequency spectrum to W-band and D-band systems, where huge amounts of further bandwidth become available, will likely prove to be part of the solution.

Higher order modulation techniques can also increase data rates, but these demand higher signal-to-noise ratios and very linear components in order to keep error rate to a minimum. Multi-channel systems like XPIC (cross-polarization interference cancelling) and line-of-sight MIMO have also been demonstrated to provide enhanced data rates, but require more expensive radio equipment.

Higher Bands

W-band refers to the range of the radio frequency from 92 GHz to 114.5 GHz, and D-band normally refers to frequencies in the range 130 – 175 GHz, as shown in Figure 1. The atmospheric attenuation in W-band is very similar to that in E-band, while the rain attenuation in D-band is only around 2 dB higher and is almost flat across the frequency range.

Figure 1 - Radio Frequency Bands from 71 to 174.8 GHz

An antenna of the same size will also give a higher gain in D-band compared to one at a lower frequency. All these features make the D-band system a good candidate for the next generation ultra-high capacity wireless links. System simulations carried out by the ETSI mWT ISG (GR mWT 008) suggest that link distances of several hundred metres are practical at D-Band frequencies with antenna sizes comparable to those at E-Band.

In 2018, the European CEPT Electronic Communications Committee (ECC) issued recommendation 18(01) identifying sub-bands totaling more than 30 GHz over the following frequencies, for potential use at D-band for fixed service backhaul and fronthaul: 130 – 134 GHz, 141 – 148.5 GHz, 151.5 – 164 GHz and 167 – 174.8 GHz. These bands, and the bandwidth each makes available, are summarized in Figure 2. This CEPT recommendation generated a significant level of interest in developing the D-band component technologies that will be required to produce such links in the future.

In terms of the semiconductor technologies for use in D-band, the proposed bands can be supported with III-V MMIC technology. 100nm InGaAs mHEMT devices can be used for power amplifiers (PA) up to 155 GHz, and InP pHEMT devices on a 100nm process can be used for PAs up to 170 GHz. InP DHBT processes with fMAX greater than 650 GHz are emerging which are showing promising performance across the whole of D Band. For low-noise amplifiers (LNA), devices on both 35nm InP pHEMT and 50nm InGaAs mHEMT processes can perform up to 185 GHz and beyond.

Figure 2 - Available Bandwidth in the D-Band

Silicon-based technologies have demonstrated PAs and LNAs operating up to 160 GHz in the case of SiGe and 140 GHz in the case of SOI-CMOS. SiGe technology continues to develop and specialist foundries are offering processes with Fmax > 400 GHz. All of these devices would be custom, however, as no standard commercial devices are currently available in these frequency bands.

Collaborative Project

An Innovate UK-funded project was carried out to build upon previous research work on D-band components, and to develop methods for integrating them into a transmit-receive module for use in high speed links. In particular, it concentrated on developing a robust method for making low loss connections between the active circuits in the module and to the external interface, which is generally a waveguide port connected to an external antenna. Filtronic and the UK’s NPL (National Physical Laboratory) collaborated on the project, which concluded successfully in January 2019. Filtronic explored designs and assembly techniques to provide low loss D-band transitions between mmWave integrated circuits (MMIC) and various external circuits – these designs were successfully fabricated and demonstrated. New methods of on-wafer calibration and measurement were developed collaboratively by NPL and Filtronic, and these were described in detail in a presentation by Xiaobang Shang of NPL at the ARMMS RF and Microwave Society meeting in November 2019. Figure 3 shows the test setup used by NPL for these measurements.

Figure 3 - Test Setup used by NPL for Measurements

The work included trials of hot via technology. This is where via holes that connect between the top surface of the IC and pads on the backside of the ICare bonded directly to the tracks on the circuit board. This technique eliminates excess bond wire inductance, and allows surface mounting of MMICs.

Ongoing Work

Filtronic continues to cooperate with NPL on D-band measurements, and NPL was recently awarded a grant from EURAMET for “Knowledge Transfer of Planar Calibration and Measurement Techniques at Millimetre-wave Frequencies”. Filtronic is the primary industry supporter for this project. Filtronic is also an Industrial Partner within the UK EPSRC “DLINK” project, led by Lancaster University and the University of Glasgow, established to explore use cases within D Band.