A High Bit Rate Miniaturized QPSK Modulator for Satellite Data Transmission

Introduction Space communication systems are designed based on the prime requirements of size, weight, on-board power requirements and reliability. Various factors limit the transmission of data from satellites and the design of the system becomes more complex with an increase in data rates. Indian Remote Sensing (IRS) satellites provide data in a variety of spatial, spectral and temporal resolutions. The data transmission requirement for IRS satellites has increased from 20Mbps to 640Mbps. There is a need to improve the resolution of images and hence there is increase in demand for higher data rate transmitters. This calls for developing new data transmitting systems—the modulator in particular. Data transmission from space to ground needs to be carried out in the frequency band allotted by International Telecommunication Union (ITU). The S-band is overcrowded and moreover higher data rates cannot be accommodated in the allowable 20MHz bandwidth. The other bands allotted for data transmission from space to ground are X-band and Ka-band with 375MHz and 1500MHz bandwidth, respectively. At present, data transmission from remote sensing satellites is confined to X-band, with data rates as high as 210Mbps on two different carriers using QPSK modulation. The next generation satellite will transmit data rates as high as 640Mbps on a single carrier using dual polarization. If the data rates are increased further, transmission at Ka-band is the only option. In a digital communication system, especially in satellite links, there is a trade-off between bandwidth efficiency and power efficiency when selecting a suitable modulation scheme. The QPSK modulation scheme, which is optimum in terms of power and bandwidth efficiency, is widely used for high bit rate data transmission for terrestrial as well as satellite communications. A miniaturized QPSK modulator was designed at X-band using micro-strip line techniques. It was tested for its performance up to a data rate of 800 Mbps. The high data rate ensures the transmission of images acquired by high resolution cameras to ground. Realization of the unit and its test results are explained in this paper.