Digital Modulation-FSK,PSK,QFSK

1、1Principles of Digital ModulationDr Mike Fitton,mike.fittontoshiba-Telecommunications Research LabToshiba Research Europe Limited2Principles of Digital Modulation: Outline of Lecturesl Introduction to digital modulationl Relevant Modulation Schemes (QPSK, GMSK, M-Ary Schemes)l Coherent and Differential Receptionl The impact of the mobile channel on digital modulation noise and interference random FM (narrowband fading) intersymbol interference (wideband fading)3Digital Modulation Basics4Digital

2、Modulation Basicsl The bit rate defines the rate at which information is passed. l The baud (or signalling) rate defines the number of symbols per second. Each symbol represents n bits, and has M signal states, where M = 2n. This is called M-ary signalling.l The maximum rate of information transfer through a baseband channel is given by: Capacity fb = 2 W log2M bits per second where W = bandwidth of modulating baseband signal5Amplitude Shift Keying (ASK)l Pulse shaping can be employed to remove

3、spectral spreading.l ASK demonstrates poor performance, as it is heavily affected by noise and interference.BasebandDataASK modulatedsignalA cos ct A cos ct0 06Frequency Shift Keying (FSK)l Bandwidth occupancy of FSK is dependant on the spacing of the two symbols. A frequency spacing of 0.5 times the symbol period is typically used.l FSK can be expanded to a M-ary scheme, employing multiple frequencies as different states.BasebandDataFSK modulatedsignalf1 f1f0 f0where f0 = A cos(c-)t and f1 = A

4、cos(c+)t 7Phase Shift Keying (PSK)l Binary Phase Shift Keying (BPSK) demonstrates better performance than ASK and FSK.l PSK can be expanded to a M-ary scheme, employing multiple phases and amplitudes as different states.l Filtering can be employed to avoid spectral spreading.BasebandDataBinary PSK modulatedsignals1 s1s0 s0where s0 = -A cos ct and s1 = A cos ct 8Nyquist & Root-Raised Cosine Filtersl The Nyquist bandwidth is the minimum bandwidth than can be used to represent a signal.l It is impo

5、rtant to limit the spectral occupancy of a signal, to improve bandwidth efficiency and remove adjacent channel interference. l Root raised cosine filters allow an approximation to this minimum bandwidth.+10-60-50-40-30-20-100-4 -2 -1 0 1Frequency Offset from Carrier(f-fc)Ts (Hz)2 3-3 4Magnitude (dB)Nyquist MinimumBandwidthNyquist bandwidth on the QPSK spectrum9Modulation - QPSKlQuadrature Phase Shift Keying is effectively two independent BPSK systems (I and Q), and therefore exhibits the same pe

6、rformance but twice the bandwidth efficiency.lQuadrature Phase Shift Keying can be filtered using raised cosine filters to achieve excellent out of band suppression.lLarge envelope variations occur during phase transitions, thus requiring linear amplification.Cos Wc t900Odd DataEven Data(NRZ)(NRZ)QPSKQ-ChannelI-ChannelQ(-1,-1)(-1,1)I(1,1)(1,-1)Wc = Carrier Frequency, I = In phase channel, Q = Quadrature channel10Types of QPSKlConventional QPSK has transitions through zero (ie. 180o phase transit

7、ion). Highly linear amplifier required.lIn Offset QPSK, the transitions on the I and Q channels are staggered. Phase transitions are therefore limited to 90o.lIn pi/4-QPSK the set of constellation points are toggled each symbol, sotransitions through zero cannot occur. This scheme produces the lowest envelope variations.lAll QPSK schemes require linear power amplifiers.11GMSK - Gaussian Minimum Shift Keyingl GMSK is a form of continuous-phase FSK, in which the phase is changed between symbols to

8、 provide a constant envelope. Consequently, it is a popular alternative to QPSK.l The RF bandwidth is controlled by the Gaussian low-pass filter bandwidth.l The degree of filtering is expressed by multiplying the filter 3dB bandwidth by the bit period of the transmission, ie. by BT.l As BT is lowered the amount of intersymbol-interference introduced increases and this results in either a fixed power penalty or anirreducible error floor.l GMSK allows efficient class C non-linear amplifiers to be

9、used, however even with a low BT value its bandwidth efficiency is less than filtered QPSK.12Minimum Shift Keying (MSK)2T 4T 6T 8T0Time-2pi-pi0pib bb bPhase2pi0Time-2pi-pi0piData -1+12pi4Tb2Tb 8Tb6TbPhaseTimel In MSK phase ramps up through 90 degrees for a binary one, and down 90 degrees for a binary zero.l For GMSK transmission, a Gaussian pre-modulation baseband filter is used to suppress the high frequency components in the data. The degree of out-of-band suppression is controlled by the BT product.MSK possible phase transitions MSK phase transitions for data:(00111000.)13GMSK SignalsTxGLPFVCONRZDATAl In MSK , the BT is infinity and this allows the square bit transients to directly modulate the VCO.l In GMSK, low values of BT create significant intersymbolinterference (ISI). In the diagram, the portion of the symbol energy acts as ISI for adjacent symbols.l If BT is less than 0.3,

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