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Published byKarlee Blackler Modified over 2 years ago

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Typical Digital Modulation Methods M-PSK DPSK M-QAM (Linear amplifiers required “AM”) FSK OQPSK/MSK – I and Q phase transitions are offset in time so that 180 o phase shifts are never encountered XX XX I Q X X X I Q X OQPSK – Offset Square Pulses MSK – Offset Rounded Pulses

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X X X I Q X X X X X 01 10 00 11 /4-DQPSK Encoder Memory Data 1 of 8 Phase Select Delay X X X X

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Pulse Spectra Contributes to Adjacent Channel ISI Rounding the pulses makes the spectrum more square and reduces ISI potential.

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1.0 0.5 fNfN fNfN Low pass filtering of the data pulses improves spectral behavior f b /2 NRZ f N = f b RZ f b AMI BTBT BTBT Trapezoidal Raised Cosine

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Discussion: Nyquist chose to use filters with these special characteristics because they yielded to tractable mathematical analysis and provided practical and useful results. However, trapezoidal and raised cosine filter characteristics cannot be realized with discrete components, but can be very closely approximated with non-real-time digital filters. By incorporating a time delay, the filter can operate on previous as well as “future” data pulses to realize the desired characteristics.

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CDMA IS-95 PCS Voice Signal Vo-Coder 8-13 kb/sec PCM Convolutional Encoding Block Interleaving Walsh Code Spreading 1.23 Mchips/sec Modulation: QPSK Uplink OQPSK Downlink Tx 800 Mhz Cell site demodulates and reconstructs digital uplink stream, then re-modulates for downlink. Each user has unique orthogonal Walsh code

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Modulator/Demodulator Performance Carrier Oscillator cos( o t) Splitter Unbalance Error /2 -e-e I Data: I(kT) Q Data: Q(kT) (leakage) S(t)

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Ideal Demodulator Carrier Recovery cos( o t) Splitter /2) LPF Recovered Q Data: Q R (kT) Recovered I Data: I R (kT) Splitter S(t)

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