Modulation is the process of converting information that can be sent successfully through a medium. The various modulation schemes have contributed towards success of wireless standards, providing efficient transmission with minimal bandwidth and power requirements.

Sampling and Quantization digitize the analog information, and differ from Digital Modulation, where the digital symbols are transmitted across the analog channel.

The Quadrature Form of Modulator using In-phase (I) and Quadrature-phase (Q) channel method acts as the predominant method used in digital modulation. Basically the information bits are given to band-pass signal processing which splits the given bits into two streams as I and Q. This is a conventional way of doing modulation in which channel I and Q are not transmitted but only the real signal is transmitted. The channel I and Q are independent to each other and is used to create the symbol. Hence QPSK is said to be a two dimensional signal.

Quadrature Phase Shift Keying (QPSK) is one of the most popular digital modulations. The term "Quadrature" implies, the carrier can have one of the four possible phases, at a given time slot. The phase shift of 90º in each time period, gives four possible states in the complete cycle of 360º. In this type of digital modulation, each phase shift represents 2-bits of information, called ' Symbol '.

Hence it has four symbols in total and each stands for two bits. The means of conveying information through phase variations i.e. the cosine carrier is varied in phase by keeping the amplitude and frequency as constant is called Quadrature Phase Shift Keying (QPSK). Although, this process may seem to be insignificant at initial stage, but this modulation scheme enables the carrier to transmit the 2-bits of information instead of 1-bit. This doubling makes the bandwidth of the carrier, more effective

The QPSK modulators are mainly applicable in the communication world

The most popular digital wireless LAN standard uses a variety of PSKs depending on the data-rate required

It is also used in the Code Division Multiple Access (CDMA) system

Satellite Communication for sending data over cable networks

Higher order modulation scheme such as QPSK modulator is preferred due to its improved spectral efficiency

Robust against noise

Bandwidth efficiency of the carrier gets doubled due to QPSK modulation

VHDL

Simulation: modelsim5.8c

Synthesis: Xilinx 9.1

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