An amplitude-based signaling scheme such as amplitude shift keying (ASK) or quadrature amplitude modulation (QAM) is inherently vulnerable to performance degradation in a fading environment. Thus, for fading channels, the preferred choice for a signaling scheme is a frequency or phase-based modulation type.

In considering orthogonal FSK modulation for fading channels, the use of MFSK with M = 8 or larger is useful because its error performance is better than binary signaling. In slow Rayleigh fading channels, binary DPSK and 8-FSK perform within 0.1 dB of each other.

In considering PSK modulation for fading channels, higher-order modulation alphabets perform poorly. MPSK with M = 8 or larger should be avoided.

Example: Phase Variations in a Mobile Communication System

The Doppler spread fd=V/λfd=V/λ size 12{f rSub { size 8{d} } = {V} slash {λ} } {} shows that the fading rate is a direct function of velocity. Table 1 shows the Doppler spread versus vehicle speed at carrier frequencies of 900 MHz and 1800 MHz. Calculate the phase variation per symbol for the case of signaling with QPSK modulation at the rate of 24.3 kilosymbols/s.

Assume that the carrier frequency is 1800 MHz and that the velocity of the vehicle is 50 miles/hr (80 km/hr). Repeat for a vehicle speed of 100 miles/hr.

Table 1

Solution

At a velocity of 100 miles/hr:

Δθ / symbol = f d R t × 360 o Δθ / symbol = f d R t × 360 o size 12{ {Δθ} slash {"symbol"} = { {f rSub { size 8{d} } } over {R rSub { size 8{t} } } } times "360" rSup { size 8{o} } } {}

= 132 Hz 24 . 3 × 10 3 symbols/s × 360 o = 132 Hz 24 . 3 × 10 3 symbols/s × 360 o size 12{ {}= { {"132"" Hz"} over {"24" "." 3 times "10" rSup { size 8{3} } " symbols/s"} } times "360" rSup { size 8{o} } } {}

= 2 o / symbol = 2 o / symbol size 12{ {}= {2 rSup { size 8{o} } } slash {"symbol"} } {}

At a velocity of 100 miles/hr: Δθ/symbol=4o/symbolΔθ/symbol=4o/symbol size 12{ {Δθ} slash {"symbol"} = {4 rSup { size 8{o} } } slash {"symbol"} } {}

Thus, it should be clear why MPSK with a value of M > 4 is not generally used to transmit information in a multipath environment.

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This work is licensed by Sinh Nguyen-Le and Tuan Do-Hong under a Creative Commons Attribution License (CC-BY 2.0), and is an Open Educational Resource.

Last edited by Tuan Do-Hong on Nov 15, 2007 7:01 am -0600.