ABSTRACT
In this project we are going to implement M-array QAM modulator, here M being 16. This paper is a demonstration of complete system generator based design of 16-QAM transmitter which can be easily implemented. Each of the possible symbols is a combination of a real and a complex value and can be plotted as a point in the complex plane. Performance in the sense we are going to analyze about eye diagram of our proposed modulation technique and also going to estimate BER. Each point represents one of the M possible symbols. Simulation can be done in MATLAB software. QAM (Quadrature Amplitude Modulation)
QAM (quadrature amplitude modulation) is a method of combining two amplitude-modulated (AM) signals into a single channel, thereby doubling the effective bandwidth. QAM is used with pulse amplitude modulation (PAM) in digital systems, especially in wireless applications.
Quadrature amplitude modulation (QAM) is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme. The two carrier waves of the same frequency, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers or quadrature components — hence the name of the scheme. The modulated waves are summed, and the final waveform is a combination of both phase-shift keying (PSK) and amplitude-shift keying (ASK), or, in the analog case, of phase modulation (PM) and amplitude modulation. In the digital QAM case, a finite number of at least two phases and at least two amplitudes are used. PSK modulators are often designed using the QAM principle, but are not considered as QAM since the amplitude of the modulated carrier signal is constant. QAM is used extensively as a modulation scheme for digital telecommunication systems. Arbitrarily high spectral efficiencies can be achieved with QAM by setting a suitable constellation size, limited only by the noise level and linearity of the communications channel.
The QAM modulator is of the type shown in Figure 1 below. The two paths to the adder are typically referred to as the ‘I’ (inphase), and ‘Q’ (quadrature), arms.
Not shown in Figure 1 is any bandlimiting. In a practical situation this would be implemented either at message level – at the input to each multiplier – and/or at the output of the adder. Probably both ! The motivation for QAM comes from the fact that a DSBSC signal occupies twice the bandwidth of the message from which it is derived. This is considered wasteful of resources.
QAM restores the balance by placing two independent DSBSC, derived from message #1 and message #2, in the same spectrum space as one DSBSC. The bandwidth imbalance is removed. In digital communications this arrangement is popular. It is used because of its bandwidth conserving (and other) properties. It is not used for multiplexing two independent messages. Given an input binary sequence (message) at the rate of n bit/s, two sequences may be obtained by splitting the bit stream into two paths, each of n/2 bit/s. This is akin to a serial-to-parallel conversion.
Because of the halved rate the bits in the I and Q paths are stretched to twice the input sequence bit clock period. The two messages are recombined at the receiver, which uses a QAM-type demodulator. The two bit streams would typically be band limited and/or pulse shaped before reaching the modulator. A block diagram of such a system is shown in Figure 2 below.
The QAM modulator is so named because, in analog applications, the messages do in fact vary the amplitude of each of the DSBSC signals. In QPSK the same modulator is used, but with binary messages in both the I and Q channels, as describe above. Each message has only two levels, ±V volt. For a non-bandlimited message this does not vary the amplitude of the output DSBSC. As the message changes polarity this is interpreted as a 1800 phase shift, given to the DSBSC. Thus the signal in each arm is said to be undergoing a 1800 phase shift, or phase shift keying – or PSK. Because there are two PSK signals combined, in quadrature, the twochannel modulator gives rise to a quadrature phase shift keyed – QPSK – signal.
The QAM receiver follows the similar principles to those at the transmitter, and is illustrated in idealised from in the block diagram of Figure 3. It is idealised because it assumes the incoming signal has its two DSBSC precisely in phase quadrature. Thus only one phase adjustment is required.
The parallel-to-serial converter block performs the following operations: 1. regenerates the bit clock from the incoming data. 2. regenerates a digital waveform from both the analog outputs of the I and Q arms. 3. re-combines the I and Q signals, and outputs a serial data stream. Not shown is the method of carrier acquisition. This ensures that the oscillator, which supplies the local carrier signal, is synchronized to the received (input) signal in both frequency and phase. In this experiment we will use a stole carrier to ensure that carrier signal in the transmitter and receiver are in synchronism with each other. (Please read about Costas Receiver to understand more about carrier acquisition).
In this experiment, two independent data sequences will be used at the input to the modulator, rather than having digital circuitry to split one data stream into two (the serialto-parallel converter). Two such independent data sequences, sharing a common bit clock (2.083 kHz), are available from a single SEQUENCE GENERATOR module. The data stream from which these two channels are considered to have been derived would have been at a rate of twice this – 4.167 kHz. Low pass filter band limiting and pulse shaping is not a subject of enquiry in this experiment. So a single band pass filter at the ADDER (summer) output will suffice, providing it is of adequate bandwidth. A 100 kHz CHANNEL FILTERS module is acceptable.