Theoretical and experimental results on QPSK spectrum used for data transmission

The demand for a high bit rate data transmission in space applications has been rapidly increasing especially where one deals with Earth observing missions. In a multi-carrier operation, the problem of interference between carrier spectra appears. Several modulation techniques for digital transmission have recently been investigated for the purpose of achieving narrow signal spectra with power concentrated within a given bandwidth. While designing a communication system, particularly a satellite communication system, special attention is needed to use the primary resources namely, the transmitted power and channel bandwidth. Space communication links are both bandwidth- and power-limited. The quadrature phase shift keying (QPSK) modulation scheme is best suited for satellite communication since bandwidth and power required is optimum. In this paper, some observations on QPSK spectral spikes and the methods to eliminate them as well as pulse shaping circuits to improve spectrum bandwidth are presented.