Polymer Microwave Fiber Communication Links

Since the last decade, opportunities have been risen to integrate entire radio systems – operating at mm-Wave frequencies – in one single transceiver chip. This is merely made possible through the continuous scaling of nanometer CMOS technology. Indeed, as predicted by Moore’s Law, transistors became so small that they are able to operate at extremely high frequencies, even above 100 GHz. The high carrier frequency increases the absolute bandwidth, resulting in high-speed communication links. Of course, CMOS transistors are made small to reduce the cost, but they are by far not optimized to operate at mm-Wave frequencies. Wireless communication at these frequencies is therefore only possible over very short distances that are not industrially viable. However, electromagnetic (EM) waves do not only propagate in open air. A polymer microwave fiber (PMF) serves as a directive channel and appears to be an excellent transmission channel. The fiber guides the EM wave and confines the energy. This idea was already suggested in the late sixties, but back then integrated circuits were simply not fast enough. Therefore, the concept of transferring EM waves through polymer fiber was only possible at lower frequencies – below 10 GHz – leading to fibers that have a diameter of several centimeters thick and make use of expensive and bulky transmitters and receivers. Today, almost 60 years later, it is possible to communicate between low-cost silicon transceivers through a polymer fiber at extremely high frequencies – above 120 GHz. Based on the combination of three low-cost elements – (1) standard CMOS transmitters and receivers, (2) on-chip integrated antennas and (3) low-loss polymer fibers – a novel physical layer for multi-gigabit communication systems is discussed. The result is a communication technique that is competitive with copper wireline and fiber-optics. The PMF waveguides that we tend to use have low insertion losses at mm-Wave frequencies and are therefore suitable for PMF communication over several meters distance. Furthermore, the high carrier frequency enables to integrate the antenna on the chip or in the package and enables high bandwidth resulting in high data rates. This doctoral dissertation discusses the opportunities, the design issues and the performance of multi-gigabit PMF communication links. First, a 160-GHz three-stage transformer-coupled fully-differential power amplifier with single-ended input and output is presented in a 40-nm bulk CMOS technology. The mm-Wave design and layout challenges are discussed. A combination of several design techniques are proposed and discussed in detail. A tapered gate-connection network is optimized to increase the gain per stage. Slow-wave transmission lines and on-chip transformers act as matching networks. Capacitive neutralization in the differential pairs and a series resistor in the bias lines result in a complete stability. We propose to package the mm-Wave transceiver chips using flip-chip packaging instead of wire-bonding. Compared to this conventional technique, flip-chip packaging is a superior packaging technique for high-frequency applications. The results are evaluated with in-house measurements and experiments. Two 120-GHz simplex PMF communication links are implemented in a 40-nm bulk CMOS technology. The directive channel, the coupler and the CMOS circuitry are described in detail. Complementary measurement results are profoundly analyzed. As a proof of concept, four different simplex demonstrators are presented. Every demonstrator setup is built with a specific purpose to showcase the abilities and opportunities and to valorize the PMF communication. The demonstrators explore various types of couplers, the properties of the channel and its coating, bending and radiation losses, connector misalignments and its tolerances, maximum achievable data rates, etc. Next, bidirectional communication is discussed. An orthogonal behaviour between up and downlink has to be exploited to achieve two-way communication. Three bidirectional PMF examples at 120 GHz are presented and developed: (1) an electrical balance duplexer (EBD) design, exploiting common and differential mode orthogonality, (2) a substrate integrated waveguide-based directional coupler on PCB, exploiting orthogonal propagation directions, and (3) an in- band full-duplex (IBFD) micromachined ortho-mode transducer (OMT) design, exploiting orthogonal polarities. The 120-GHz IBFD PMF transceiver chip with integrated antenna and tunable EBD is proposed and implemented in a 40-nm bulk CMOS technology. Simulation and measurement results are provided. This thesis has advanced the field of PMF communication links, facilitating a clear understanding and insightful results of a mm-Wave transceiver design, the polymer waveguide and the low-loss couplers. Nevertheless, several remaining steps towards a first PMF product have to be taken and many unanswered questions have to be addressed.status: publishe