FM modulation index
Minimum shift keying is a special type of continuous phase-frequency shift keying (CPFSK) wherein the peak frequency deviation is equal to ¼ the bit rare. In other words, MSK is continuous phase FSK with a modulation index of 0.5 .The modulation index of an FSK signal is similar to the FM modulation index, and is defined as KFSK = (2∆F)/ Rb , where ∆F is the peak RF frequency deviation and Rb is the bit rate. A modulation index of 0.5 corresponds to the minimum frequency spacing that allows two FSK signals to be coherently orthogonal, and the name minimum frequency implies the minimum frequency separation that allows orthogonal detection. Two FSK signals VH (t)and VL (t) are said to be orthogonal if
MSK is sometimes referred to as fast FSK, as the frequency spacing used is only half as much as that used in conventional noncoherent FSK.MSK is a spectrally efficient modulation scheme and is particularly attractive for use in mobile radio communication systems. It possesses properties such as:
- Constant envelope
- Spectral efficiency
- Good BER performance
- Self-synchronizing capability.
The block diagram of an MSK transmitter is shown in figure-1. Multiplying a carrier signal with cos (Î t/2T) produces two phase-coherent signal at fc +1/4T and fc +1/4T.these two FSK signals are separated using two narrow bandpass filters and appropriately combined to form the in-phase and quadrate carrier components x(t) and y(t) respectively. These carriers are multiplied with the odd and even bit streams mI (t) and mQ (t), to produces the MSK modulated signal sMSK (t).
Fig 1 : Block diagram of MSK transmitter.
The block diagram of an MSK receiver as shown in figure-2.The received signal SMSK (t) is multiplied by the respective in-phase and quadrature carriers x (t) and y (t).The output of the multipliers are integrated over two bit periods and dumped to a decision circuit at the end of each two bit periods. Based on the level of the signal at the output of the integrator, the threshold detector decides whether the signal is a 0 or a 1.The output data streams correspond to mI (t) and mQ (t), which are offset, combined to obtain the demodulated signal.
Fig 2 :Â Block diagram of an MSK receiver.
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