MSK, GMSK and advantages of spread spectrum

MSK

Minimum Shift Keying (MSK) is binary continuous phase FSK with modulation index h = 0.5, so the tone spacing is R_b/2, the minimum for orthogonal coherent detection.

• The phase changes linearly by ±π/2 in each bit, with no discontinuity.
• Equivalent to OQPSK with half-sinusoidal pulse shaping: s(t) = a_I cos(πt/2T_b) cos ω_ct + a_Q sin(πt/2T_b) sin ω_ct.
• Constant envelope; main lobe 1.5R_b wide with side lobes falling as 1/f⁴; BER same as BPSK.

GMSK

Gaussian MSK passes the NRZ data through a Gaussian low-pass filter before MSK modulation.

data->[Gaussian LPF, BT]->[FM, h = 0.5]-> GMSK

• The filter smooths phase changes, reducing side lobes and adjacent channel interference.
• BT (3 dB bandwidth × bit time) sets the trade-off: GSM uses BT = 0.3. Smaller BT means narrower spectrum but more ISI.
• Keeps the constant envelope, so efficient class-C amplifiers can be used. Used in GSM, DECT.

Advantages of spread spectrum in wireless communication

1. Interference and jamming rejection – narrowband interference is spread at the receiver and reduced by the processing gain.
2. Multipath resistance – delayed copies more than one chip late are uncorrelated with the code; a RAKE receiver can even combine them for diversity gain.
3. Multiple access (CDMA) – many users share the same band at the same time using different codes; universal frequency reuse.
4. Low probability of interception – the signal is below the noise floor and looks like noise without the code.
5. Privacy/security – the PN code acts as a key.
6. Graceful degradation – capacity is soft-limited; extra users only raise the noise level slightly.
7. Accurate ranging – fast chips give good timing resolution (used in GPS).