Modulation & Modulator

                     

         In the televisions systems, signals can be carried between a limited frequency spectrum, which a concrete lower and upper frequencies. A modulator is a device charged of transporting a signal inside another signal to be transmitted. Is able to transform a low-frequency signal into a other frequency signal. As a result of, the frequency can be controlled and the problem above can be solved. Mainly, the aim of modulate a signal consist in changing a parameter of the wave according to the variations of the modulation signal (information to be transmitted).

                         

                        There are four modulation modes available, with QPSK and 8PSK intended for broadcast applications in non-linear satellite transponders driven close to saturation. 16APSK and 32APSK, requiring a higher level of C/N, are mainly targeted at professional applications such as news gathering and interactive services.

                        Adaptive Coding and Modulation (ACM) allows the transmission parameters to be changed on a frame by frame basis depending on the particular conditions of the delivery path for each individual user. It is mainly targeted to unicasting interactive services and to point-to-point professional applications.

                         

                        DVB-S2 offers optional backwards compatible modes that use hierarchical modulation to allow legacy DVB-S receivers to continue to operate, whilst providing additional capacity and services to newer receiver.

Digital modulation is often a means of transmitting payload data from orbiting satellites to ground stations.  In one such approach, quadrature phase-shift-keying (QPSK) modulation provides both spectral and power efficiency. In a QPSK modulator, two data streams simultaneously modulate a carrier signal. For optimum use of available satellite power, an unbalanced QPSK (UQPSK) modulator is often used. The design and simulation for such a modulator will be shown here, along with methods for demodulating such signals.

Communications channels in satellite-communications (satcom) systems, as well as communications systems in general, can be categorized as being power- or bandwidth-limited. For power-limited communications channels, coding schemes are typically applied to save power at the expense of bandwidth. In bandwidth-limited channels, spectrally efficient modulation schemes are often used to conserve bandwidth. The prime goal of spectrally efficient modulation is to optimize bandwidth efficiency, which is defined as the ratio of the data rate to the channel bandwidth (in b/s/Hz). A secondary goal of such a modulation scheme is to achieve high bandwidth efficiency with minimum signal power.

                        QPSK modulation results in optimum use of both spectrum and power when the data rates for the two channels—or the in-phase (I) and quadrature (Q) signal components—are the same. But when two data streams at different bit rates from two independent payloads must be transmitted, the normal procedure of formatting the data for modulation onto the carrier becomes very complicated. The easiest way is to transmit the two different data streams on the two QPSK channels by direct modulation, which results in the two channels of the modulator having different data rates. The higher of the data rates determines the bandwidth required for the modulated carrier and the transmitter output power will be equally shared by the two different data streams.

                        To optimize available transmit power from a satcom system, an UQPSK modulator is proposed here, where the power level of the low-data-rate channel can be reduced to achieve the same carrier-to-noise (E_b/N_o) ratio for both channels. This can be achieved by unbalancing the amplitudes of the carrier’s signal components in the I and Q channels of a conventional QPSK modulator. The amplifier power following the modulator is shared between the I and Q channels in proportion to the amplitudes of the I and Q signal components of the modulated carrier.

                        Figure 1 shows a block diagram of the proposed UQPSK modulator. In a conventional QPSK modulator, the carrier source signal is divided equally by a 3-dB/90-deg. branch line hybrid coupler to yield two equal-amplitude carriers in quadrature. Both carriers are modulated with I and Q data streams using binary-phase-shift-keying (BPSK)modulation. The BPSK-modulated carriers are combined in an in-phase Wilkinson power combiner to produce a QPSK-modulated carrier. When data of different bit rates are to be modulated, an UQPSK modulator can be used.

                       

                        1. This block diagram represents the proposed UQPSK modulator.

                        In the case of an UQPSK modulator, for optimum utilization of power, an attenuator is added to the low-data-rate channel to reduce the power. A phase shifter is added tothe high-data-rate channel to compensate for the phase shift introduced by the attenuator in the low-data-rate channel. The resulting output isa UQPSK-modulated signal. The mainlobe bandwidth will be determined by the high-data-rate channel. Figure 2 shows a plot of the I and Q data, along with the simulated UQPSK spectrum.