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Minimum-shift keying

From Wikipedia, the free encyclopedia

In digital modulation, minimum-shift keying (MSK) is atype of continuous-phase frequency-shift keying that was

developed in the late 1960s. Similar to OQPSK, MSK isencoded with bits alternating between quarternarycomponents, with the Q component delayed by half thesymbol period. However, instead of square pulses asOQPSK uses, MSK encodes each bit as a half sinusoid. Thisresults in a constant-modulus signal, which reducesproblems caused by non-linear distortion. In addition tobeing viewed as related to OQPSK, MSK can also beviewed as a continuous phase frequency shift keyed(CPFSK) signal with a frequency separation of one-half thebit rate.

Mathematical representation

The resulting signal is represented by the formula

where aI(t) and aQ(t) encode the even and odd information respectively with a sequence of

square pulses of duration 2T. Using the trigonometric identity, this can be rewritten in a form where

the phase and frequency modulation are more obvious,

where bk(t) is +1 when aI(t) = aQ(t) and -1 if they are of opposite signs, and kis 0 ifaI(t) is

1, and otherwise. Therefore, the signal is modulated in frequency and phase, and the phasecontinuously and linearly changes.

Gaussian minimum-shift keyingIn digital communication, Gaussian minimum shift keying or GMSK is a continuous-phasefrequency-shift keying modulation scheme. It is similar to standard minimum-shift keying (MSK);

Modulation techniques

Analog modulation

AM SSB QAM FM PM SM

Digital modulation

FSK ASK OOK PSK QAMMSK CPM PPM TCM OFDM

Spread spectrum

CSS DSSS FHSS THSS

See also: Demodulation, modem

Mapping changes in

continuous phase

Contents

1 Mathematical representation 2 Gaussian minimum-shift keying

2.1 Applications 2.2 Spectral efficiency

3 See also 4 References

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however the digital data stream is first shaped with a Gaussian filter before being applied to afrequency modulator. This has the advantage of reducing sideband power, which in turn reduces out-of-band interference between signal carriers in adjacent frequency channels. However, the Gaussianfilter increases the modulation memory in the system and causes intersymbol interference, making itmore difficult to discriminate between different transmitted data values and requiring more complexchannel equalization algorithms such as an adaptive equalizer at the receiver.

Applications

GMSK is most notably used in the Global System for Mobile Communications (GSM).

Spectral efficiency

GMSK has high spectral efficiency, but it needs a higher power level than QPSK, for instance, inorder to transmit the same amount of data reliably.

See also

Constellation diagram used to examine the modulation in signal space (not time).

References

Subbarayan Pasupathy,Minimum Shift Keying: A Spectrally Efficient Modulation, IEEECommunications Magazine, 1979

Document from the University of Hull giving a thorough description of GMSK. Another good discussion (University of Toronto) PSK/GMSK comparison & illustrations (Aerospace)

Retrieved from "http://en.wikipedia.org/wiki/Minimum-shift_keying"Categories: Radio modulation modes

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