Simulink-based Simulation of Quadrature Amplitude Modulation ( QAM ) System

Adaptive modulation system is one of the key techniques in building a broadband mobile communication network because of increasing shortage of wireless communication channels. Quadrature amplitude modulation (QAM) has been widely used in adaptive modulation because of its efficiency in power and bandwidth. To better understand the QAM system, a Simulink-based simulation system is designed. In the paper, the theory of M-ary QAM and the details of the simulation model are provided. In the simulation model, the parameter settings for random generator, QAM modulation and demodulation, AWGN wireless channel are provided. Error rates of QAM system versus the signal-to-noise ratio (SNR) are used to evaluate the QAM system for adaptive modulation. The model can be used not only for the criteria for adaptive modulation but also for a platform to design other modulation systems. Introduction With the fast development of modern communication techniques, the demand for reliable high date rate transmission is increased significantly, which stimulate much interest in modulation techniques. Different modulation techniques allow you to send different bits per symbol and thus achieve different throughputs or efficiencies. QAM is one of widely used modulation techniques because of its efficiency in power and bandwidth. In QAM system, two amplitude-modulated (AM) signals are combined into a single channel, thereby doubling the effective bandwidth. However, it must also be noted that when using a modulation technique such as 64-QAM, better signal-to-noise ratios (SNRs) are needed to overcome any interference and maintain a certain bit error ratio (BER). The use of adaptive modulation can increase the transmission rate considerable by matching modulation schemes to time varying channel conditions, which justifies its popularity for future high-rate wireless applications. Crucial to adaptive modulation is the requirement of channel state information at the transmitter. In figure 1, a general estimate of the channel state information for different modulation techniques is provided. As you increase your range, you step down to lower modulations (in other words, QPSK), but as you are closer you can utilize higher order modulations like QAM for increased throughput. In addition, adaptive modulation allows the system to overcome fading and other interference. Both QAM and QPSK are modulation techniques used in IEEE 802.11 (Wi-Fi), IEEE 802.16 Proceedings of The 2008 IAJC-IJME International Conference ISBN 978-1-60643-379-9 (WiMAX), and 3G (WCDMA/HSDPA) wireless technologies. The modulated signals are then demodulated at the receiver where the original digital message can be recovered. The use of adaptive modulation allows wireless technologies to optimize throughput, yielding higher throughputs while also covering long distances. Figure 1: Adaptive Modulation and Coding [1] To better understand the QAM system, a MATLAB/Simulink-based simulation system is designed in this paper. In the simulation model, the parameter settings for random generator, QAM modulation and demodulation, AWGN wireless channel are provided. Error rates of QAM systems versus the SNR are used to evaluate the QAM system for adaptive modulation. The model can be used not only for the criteria of adaptive modulation but also for a platform to simulate other modulation techniques. M-ary QAM Modern modulation techniques exploit the fact that digital baseband data may be sent by varying both envelope and phase/frequency of a carrier wave. Because the envelope and phase offer two degrees of freedom, such modulation techniques map baseband data into four or more possible carrier signals. Such modulation techniques are called M-ary modulation, since they can represent more signals than if just the amplitude or phase were varied alone. In an M-ary signaling scheme, two or more bits are grouped together to form symbols and one of M possible signals is transmitted during each symbol period. Usually, the number of possible signals is M =2, where n is an integer. Depending on whether the amplitude, phase, or frequency is varied, the modulation technique is called M-ary ASK, M-ary PSK, or M-ary FSK. Modulation which alters both amplitude and phase is M-ary QAM. As with many digital modulation techniques, the constellation diagram is a useful representation. It provides a graphical representation of the complex envelop of each possible symbol state. The constellation diagram of 16-QAM is shown in Figure 2. The constellation