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Gaussian Minimum Shift Keying GMSK

Gaussian Minimum Shift Keying GMSK. Software Defined Radio EE 194SDR Spring 2006 Brad Gaynor. GMSK. Gaussian Minimum Shift Keying Modulation scheme used in GSM Bandwidth-time product Describes the amount the symbols overlap BT = 0.3 for GSM networks Good spectral efficiency

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Gaussian Minimum Shift Keying GMSK

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  1. Gaussian Minimum Shift KeyingGMSK Software Defined RadioEE 194SDR Spring 2006 Brad Gaynor

  2. GMSK • Gaussian Minimum Shift Keying • Modulation scheme used in GSM • Bandwidth-time product • Describes the amount the symbols overlap • BT = 0.3 for GSM networks • Good spectral efficiency • At the expense of some inter-symbol interference (ISI) • Data rate: 270.8 kbps

  3. MSK • GMSK is based on MSK • Minimum Shift Keying • Linear phase changes • Spectrally efficient • At baseband, bit transitions are represented by ½ cycle sinusoid

  4. 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.pdf • GMSK as implemented by quadrature signal processing at baseband followed by a quadrature modulator

  5. Pulse Shaping • Input: Binary pulse train (+1/-1) • Each binary pulse goes through a LPF with a Gaussian impulse response • The filter smoothes the binary pulses • The filter output is truncated and scaled • This process results in a train of Gaussian shaped pulses

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

  7. I&Q Signals • The resulting waveform is divided into In-Phase and Quadrature components • In-phase: Left • Quadrature: Right • The two signal components are then up-converted to the carrier frequency

  8. GMSK Properties • Improved spectral efficiency • Power Spectral Density • Reduced main lobe over MSK • Requires more power to transmit data than many comparable modulation schemes

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