This paper describes the design and performance of the 16 QAM carrier recovery PLL for service-channel transmission using FSK additional modulation. When FSK additional modulation is applied, tracking phase error occurs and corrupts the 16 QAM signal. To reduce the phase error without widening the loop noise bandwidth is an important issue in the 16 QAM-FSK double modulation scheme. An analytical expression to estimate the peak phase error due to class 4 partial response FSK is established, By using the approximation formula, an optimization of the carrier recovery PLL is presented. Moreover, experiments and computer simulations are carried out for the 200 Mbit/s 16 QAM and 64 kbit/s SC double modulation system. As a result, it is demonstrated that a 0.1 dB 16 QAM equivalent power loss and an 8 dB frequency deviation margin of SC transmission performance can be obtained.

A novel long haul 5 GHz 16 QAM digital radio system, which has 200 Mbit/s transmission capacity within the 40 MHz interleaved channel allocation, is proposed and described. It is designed to be overbuilt on existing FDM-FM routes with an approximately 50 km repeater spacing. To achieve the 5 bit/s/Hz RF spectral efficiency, the 16 QAM modulation and Nyquist cosine roll-off spectral shaping techniques (\alpha = 0.5) are investigated. Then a new signal shaping filter, differential encoding and carrier recovery techniques are presented. Finally, the effects of TWT amplifier nonlinearity on a 16 QAM signal are experimentally investigated.

Multipath fading effects on a 200 Mbit/s 16 QAM digital radio system have been experimentally investigated. It was found that the outage probability can be accurately estimated using only the in-band frequency response without knowledge of the delay difference between two incident waves. New methods to evaluate the improvement factors for both space diversity reception and dynamic equalizing, which are required to combat multipath fading effects, were also proposed. In particular, the significance of noticing the delay dispersion sign was pointed out when attaining sufficient equalization effect. The validity of these evaluations was confirmed by simulation experiments and a field trial.