The explosive growth of mobile communications, and the overly crowded and expensive spectrum have pushed both system engineers and operators to make their systems as spectrally efficient as possible in order to accommodate the increasing traffic demand. This article is a tutorial introduction to the orthogonal subchannel (OSC) technique. OSC was adopted to improve the capacity of the GSM/EDGE radio access network GERAN, and it is a new concept in which two users can simultaneously share the same GSM radio resource (time slot and frequency) in both the downlink and in uplink directions. Potentially, OSC could not only provide increased network capacity, but also reduce network-associated costs through more efficient usage of hardware and spectrum resources. In addition, this article presents some challenges related to this method, as well as solutions and their respective impact. The results provided herein may contribute to guidelines for network dimensioning and optimization, as well as list potential enhancements to the OSC radio resource management mechanisms needed to further exploit the benefits of OSC. Currently, in real OSC network deployments a capacity gain of 50 percent has been achieved at the cell level. As an indication of the importance of OSC, GSMA awarded it (called Quad Rate) the Best Technology Breakthrough award at Mobile World Congress 2012 [1].