Millimeter (mm) wave communication holds the key to the next phase of the ongoing wireless revolution, in which wireless catches up with wires in speed. The several gigahertz (GHz) of unlicensed spectrum in the 60 GHz “oxygen absorption” band, which is a part of the broader mm-wave band, is well-suited to short-range multiGigabit indoor (e.g., 10 meters range) and outdoor (e.g., 100-500 meters range) wireless links. Figure 1 shows the spectrum allocation around the 60 GHz band for some regions.
While the mm-wave spectrum has been used for military communications and radar applications for decades (with RFICs based on expensive packaging techniques and compound semiconductor processes) its potential for commercial use is only emerging now, driven by recent successes in building mm-wave transceivers in inexpensive silicon processes using low-cost packaging techniques. Moreover, the exponentially increasing demand for wireless multimedia has fueled intense efforts in both research and standardization for indoor mm wave communication. Millimeter wave communication can lead to a quantum leap in communication networks at several levels: Figure 2 shows some examples.
What are the critical challenges?
Realizing the potential for revolutionary networking advances using mm-wave spectrum poses a host of fundamental questions in communication hardware and signal-processing co-design, as well as network architectures and protocols that are intimately linked to the unique characteristics of mm-wave propagation, captured via realistic 60 GHz channel modeling.
The multiGigabit speeds at 60 GHz put severe power and processing requirements when conventional transceiver designs for say 2.4 and 5 GHz bands are used. For example, high precision analog-to-digital converters (ADCs) become a major bottleneck at multiGigabit speeds. Therefore 60 GHz requires rethinking radio transceiver hardware and signal processing designs.
Millimeter wave links are fundamentally different from those at lower carrier frequencies such as the 2.4 and 5 GHz WiFi bands. Propagation loss for omnidirectional transmission is significantly worse at higher carrier frequencies, while directional antennas are much easier to implement at these frequencies. As a result, mm wave links are inherently directional. Given the small wavelength, mm-wave links have a limited ability to diffract around obstacles and are therefore susceptible to blockage. These peculiar traits of mm-wave links have a profound impact on mm-wave network protocol design.