Gaussian Minimum Shift KeyingGMSKSoftware Defined Radio EE 194SDRSpring 2006Brad Gaynor

GMSKGaussian Minimum Shift KeyingModulation scheme used in GSMBandwidth-time productDescribes the amount the symbols overlapBT = 0.3 for GSM networksGood spectral efficiencyAt the expense of some inter-symbol interference (ISI)Data rate: 270.8 kbps

MSKGMSK is based on MSKMinimum Shift KeyingLinear phase changesSpectrally efficientAt baseband, bit transitions are represented by cycle sinusoid

Generating a GMSK Waveform

The figures in the following slides are from the University of Hull:http://www.emc.york.ac.uk/reports/linkpcp/appD.pdfGMSK as implemented by quadrature signal processing at baseband followed by a quadrature modulator

Pulse ShapingInput: Binary pulse train (+1/-1)Each binary pulse goes through a LPF with a Gaussian impulse responseThe filter smoothes the binary pulsesThe filter output is truncated and scaledThis process results in a train of Gaussian shaped pulses

Summing and IntegrationThe pulses are summed together (left)The signal is integrated over time to obtain a continuous waveform which captures the bit transition information (right)

I&Q SignalsThe resulting waveform is divided into In-Phase and Quadrature componentsIn-phase: LeftQuadrature: RightThe two signal components are then up-converted to the carrier frequency

GMSK PropertiesImproved spectral efficiencyPower Spectral DensityReduced main lobe over MSKRequires more power to transmit data than many comparable modulation schemes