A user??™s
speech was directly converted by an FM modulator to be sent over the wireless medium. This system had
voice quality issues, in particular at the cell fringe, and suffered from low user capacity [1??“3].
Here comes Digital! The second generation (2G) cellular systems used digital modulation schemes
such as /4-Shifted Differential Quaternary Phase Shift Keying (/4-DQPSK) and Gaussian Filtered
Minimum Shift Keying (GMSK). The speech was digitized, error protected, and transmitted in a specific
time, frequency, or code domain. These advances helped improve voice quality and user capacity. These
2G systems are North American TDMA System (IS-136), Group Special Mobile (GSM), North
American Spread Spectrum System (IS-95), and Japan Digital Cellular (JDC), to name a few, with GSM
being the de facto Global Standard, in other words, the more widely used system throughout the world.
These 2G cellular systems suffered not only from low user capacity, but also from low user data
rates, since applications were being created that really wanted higher data rates in order to fully
exploit their benefits. With advances in signal processing and advancements in technology used in
communication devices, the 2G systems had their lifetime extended into what has been commonly
accepted as 2.
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