Ultra Low Latency 71~76/81~86GHz E-band Radiolink Design

Abstract

For the special purpose of a time-sensitive system, such as a financial network, banking network, or medical network for real-time access in hospital, a very low latency of several tenths of a nano-second with a 1.25 Gbps transceiver is required. However, most commercial radiolinks are too slow at 5 ~ 350 s of latency through the use of Layer-2 topology with higher-level digital modulation. In this work, a unique topology of an ultra low latency transceiver was implemented by using high-speed ASK modulation with the scheme of physical Layer-1 transmission. In order to achieve a range of tenths of a nano-second of ultra-low latency with simple low cost, the direct conversion architecture was tried first. It was simple but still needed to be revised to address several problems. First, it was too difficult to build an ASK modulator directly at a 70/80 GHz millimeter-wave as it was too sensitive at its physical dimension. Secondly, the flatness was important, but it was difficult to secure the required bandwidth. Most of all, the conversion loss was significantly increased when the received power is low. In order to solve these problems, a heterodyne transceiver was considered. The ASK modulation was conducted at the IF stage. By adopting the heterodyne topology, the burdens of building an ultra wideband ASK modulator in 70/80 GHz millimeter-wave range was alleviated. However, building a 1.25 Gbps ASK modulator in the IF stage presented another new challenge. Several wide band design techniques were proposed as well as trial and error. According to the measurement results, the sensitivity was -45 dBm for 1.25 Gbps under BER 10-12, or error free, and one-way latency was measured by 19.1 ns, which is a superior achievement compared to existing commercial radiolinks worldwide. It was field tested at 4.1 km and showed a good match with its link budget. As a field proven solution, this research result has been partially adapted to a financial network in service between Chicago and New York.