High Speed Communication Circuits and Systems

This project will explore issues associated with implementing a GMSK (Gaussian Minimum Shift Keying) transmitter that is intended for use in GSM cell phones. This communication standard encodes information using phase modulation of a constant envelope sine wave signal. By constraining the transmitter output to have constant envelope, a nonlinear power amplifier can be used that broadcasts the signal with high efficiency (usually around 50% efficiency, as compared to about 10% efficiency encountered with linear power amps). A proposed transmitter for this task is shown in Figure 1, which employs a PLL that contains an I/Q modulator in its feedback loop. As you proceed through the project, you will (hopefully) come to understand the value of placing the I/Q modulator in the PLL feedback loop rather than directly using its output as the transmitter output, and develop a good understanding of RF transceivers in general. Figure 2 displays the proposed receiver used to select and demodulate the desired GMSK RF signal, which is implemented using a direct conversion architecture. For simplicity, we will ignore the many difficulties encountered with this structure, such as high sensitivity to DC offsets, antennae impedance variations, and local oscillator feedthrough. The noise source in the figure represents the overall input-referred noise of the receiver (it is not, of course, purposefully added!).