【正文】 ? In Offset QPSK, the phase transitions are limited to 900, the transitions on the I and Q channels are staggered. ? In ?/4 QPSK the set of constellation points are toggled each symbol, so transitions through zero cannot occur. This scheme produces the lowest envelope variations. ? All QPSK schemes require linear power amplifiers I Q I Q I Q Conventional QPSK ?/4 QPSK Offset QPSK 29/01/2020 Property of R. Struzak 30 Multilevel (Mary) Phase and Amplitude Modulation ? Amplitude and phase shift keying can be bined to transmit several bits per symbol. – Often referred to as linear as they require linear amplification. – More bandwidthefficient, but more susceptible to noise. ? For M=4, 16QAM has the largest distance between points, but requires very linear amplification. 16PSK has less stringent linearity requirements, but has less spacing between constellation points, and is therefore more affected by noise. 16 QAM 16 APSK 16 PSK 29/01/2020 Property of R. Struzak 31 Distortions Perfect channel White noise Phase jitter 29/01/2020 Property of R. Struzak 32 GMSK ? Gaussian Minimum Shift Keying (GMSK) is a form of continuousphase FSK in which the phase change is changed between symbols to provide a constant envelope. Consequently it is a popular alternative to QPSK ? The RF bandwidth is controlled by the Gaussian lowpass filter bandwidth. The degree of filtering is expressed by multiplying the filter 3dB bandwidth (B) by the bit period of the transmission (T), . by BT ? GMSK allows efficient class C nonlinear amplifiers to be used 29/01/2020 Property of R. Struzak 33 Modulation Spectra ? The Nyquist bandwidth is the minimum bandwidth that can represent a signal (within an acceptable error) ? The spectrum occupied by a signal should be as close as practicable to that minimum, otherwise adjacent channel interference occur ? The spectrum occupied by a signal can be reduced by application of filters Nyquist Minimum Bandwidth Frequency Relative Magnitude (dB) Adjacent Channel 29/01/2020 Property of R. Struzak 34 Bandwidth Efficiency 2l og 1 c a pa c i t y ( bi t s pe r s e c ond )ba ndwi dt h of t he m odu l a t i ng b a seba nd s i g na l ( Hz ) e ne r gy pe r bi t no i se powe r de ns i t y ( wa t t s/ Hz )T hus t ot a l s i gna l powe r t ot a l noi se b b bbbbbf E fWWfWEEfW????????????????? powe r ba ndwi dt h us e e f f i c i e nc y= bi t s pe r s e c ond pe r HzbfW?29/01/2020 Property of R. Struzak 35 Comparison of Modulation Types Modulation Format Bandwidth efficiency C/B Log2(C/B) Errorfree Eb/No 16 PSK 4 2 18dB 16 QAM 4 2 15dB 8 PSK 3 4 PSK 2 1 10dB 4 QAM 2 1 10dB BFSK 1 0 13dB BPSK 1 0 29/01/2020 Property of R. Struzak