This paper presents a new Frequency Shift Keying (FSK) demodulation method using the Short Time-Discrete Fourier Transform (ST-DFT) analysis to combat large frequency offset with time variation in low earth orbit (LEO) satellite communications systems. This demodulation method can demodulate the received signal only by searching for the instantaneous spectrum energy peaks without complicated carrier recovery. In addition, it is insensitive to the signal-to-noise ratio (SNR) degradation caused by the excessively wide bandwidth of the receiver front-end band pass filter. Furthermore, the ST-DFT analysis combined with a differential encoding scheme gives FSK demodulation method a potential robustness against large and fast time-varying frequency offset.

A direct-conversion receiver with a full digital logic FSK demodulator is presented. It is developed from the quadricorrelator which is known as a frequency detector. We show that the performance of the receiver converges to that of the analog quadricorrelator receiver as the number of mixing axes increases, and obtain the optimum filter bandwidth by computer simulation.

In this paper we propose a two-stage address coding scheme to transmit two data symbols at once within a frame in a MFSK/FH-CDMA system. We compare it with the conventional system using single-stage address coding. Assumed that the address codes of all users are known in the receiver. A multiuser detection scheme is applied and the performance is evaluated by computer simulations to show the improvement in bit error rate (BER) compairing to the conventional system. We also investigate the performance of error-correcting coding and decoding in the two-stage address coded MFSK/FH-CDMA system. An erasure decoding scheme is modified for the two-stage address coded system and is utilized to improve spectral efficiency or to increase user capacity in the MFSK/FH-CDMA system. Finally, we investigate a hybrid scheme of combining the multi-user detection scheme and the error-correcting decoding scheme for the two-stage address coded MFSK/FH-CDMA system. The performance is evaluated by computer simulations.

Cross-phase modulation (XPM) induced by residual intensity modulation in coherent optical frequency-shift-keying (FSK) frequency division multiplexing (FDM) transmission systems that use dispersion-shifted fibers is evaluated theoretically and experimentally in terms of spectral profile deformation. The bit-error rate is measured in a 2.5-Gbit/s 4-channel 40-km dispersion-shifted fiber transmission experiment, and we confirm experimentally and theoretically that the power penalty in the presence of residual intensity modulation of over 4 mWp-p exceeds 1dB. Experimental results show that the penalty due to XPM is large even when the power of the newly generated lights caused by four-wave mixing is 20-dB less than that of signals. This confirms that residual intensity modulation must be reduced in continuous-phase (CP)-FSK-FDM systems even though they are designed to avoid generating four-wave mixing.