The Effects of the Amplitude and Delay Slope Components of Frequency Selective Fading on QPSK, Offset QPSK and 8 PSK Systems

Amplitude and delay slope are significant components of inband distortion resulting from multipath fading. In this paper, we present and compare computer simulated results of signal to noise (
S/N
) degradation for probability of error (P
e
) of 10
^-4
, due to these distortion components on Quadrature Phase Shift Keying (QPSK), Offset Quadrature Phase Shift Keying (Offset QPSK) and 8 Phase Shift Keying(8 PSK)systems. For all systems the transmitter and receiver filtering is modeled so as to provide a 50% excess bandwidth raised cosine spectrum at the receiver detector input. The QPSK and Offset QPSK systems are studied with bit rates of 44.7 Mbits/s, the so-called T3 rate, and the 8 PSK system is studied with bit rates of 44.7 Mbits/s and 67 Mbits/s, a 67 Mbit/s 8 PSK system having the same modulated frequency spectrum as 44.7 Mbit/s QPSK and Offset QPSK systems. When the frequency spectrum is the same for all systems, then for a given amplitude slope, results show that QPSK is the least degraded, followed by Offset QPSK, then 8 PSK, and for a given delay slope, Offset QPSK is the least degraded, followed by QPSK, than 8 PSK. When the bit rate is the same for all systems, then for a given amplitude slope, QPSK is the least degraded. 8 PSK is less degraded than Offset QPSK for values of amplitude slope less than 0.47 dB/MHz, but more degraded for higher values. For equal bit rates and a given delay slope, Offset QPSK is the least degraded, followed by 8 PSK, then QPSK. By relating degradation results to published information on the occurrence of amplitude and delay slope distortion components during multipath fading, estimates of
S/N
degradation for a P
e
of 10
^-4
due to these distortion components as a function of frequency selective fade depth are made. Finally, measured
S/N
degradations for a P
e
of 10
^-4
due to amplitude and delay slope on a 44.7 Mbit/s Offset QPSK system with filtering similar to the computer model are presented and compared to the computer simulation results.