Overview

When designing a wireless communication system, there is always a tradeoff between bandwidth efficiency and power efficiency when it comes to selecting the appropriate modulation scheme. GSM, for example, uses GMSK which, as will be discussed later on, is a constant envelope modulation scheme that can operate in conjunction with highly efficient power amplifiers. This is true despite the variations introduced to the modulated signal due to filtering. GSM occupies 200 kHz of bandwidth and offers a raw data rate of 22.8Kbps. Enhanced Data-rate for GSM Evolution (EDGE), on the other hand, occupies the same bandwidth as GSM but offers a raw data rate of 300 Kbps. The reason for the increased data rate is the use of the more bandwidth-efficient 8-PSK modulation. This spectral efficiency, however, comes at the cost of increased power due to the linear power amplifier required for differential 8-PSK signaling.

The factors presented above offer by themselves a challenge for the designer who aims to design a modem that uses a single modulation scheme, such as a plain GSM phone, for example. The design becomes much more complicated, and meeting the various requirements much more difficult when designing an SDR modem that supports multiple waveforms that often operate simultaneously. Therefore, a deep understanding of the nuances of the various modulation schemes is essential.

The advent of VLSI and DSP has enabled the communication engineer to apply more cost-effective and robust digital modulation techniques to support modern data and multimedia applications such as voice...