![]() ![]() The minus sign means the I wave is inverted (or phase shifted by 180°) relative to the I wave in Figure 1.7(a).Īs the number of symbols increases, more data bits are transmitted per symbol. This represents a symbol where the Q wave is at an amplitude of 3 and the I wave is at an amplitude of –3. To understand what each dot in the diagram represents, take the top left one. Figure 1.8 Constellation diagram for 16-QAM. Thus the dots are spaced at regular intervals, the same distance apart. There is a dot at the intersection of each of the points marked on the axes. There are 16 dots positioned between the axes in a square formation, four rows of four. The Y axis is labelled Q and likewise has points minus 3, minus 1, 1 and 3 marked on the axis. The X axis is labelled I and has points minus 3, minus 1, 1 and 3 marked on the axis. As there are 16 symbols, this version of QAM is called 16-QAM. So, in each symbol period, only one of the ‘dots’ is transmitted. Each dot in Figure 1.8 is a symbol, as it represents a unique combination of amplitude and phase of the I and Q waves. This is a plot of the I and Q amplitudes with I on the horizontal axis and Q on the vertical axis. The set of symbols in QAM can be conveniently represented on a signal constellation diagram (Figure 1.8). Negative amplitudes just mean that the wave is inverted. The I and Q waves remain orthogonal if either or both of them are inverted (multiplied by –1, or flipped vertically). If two signals are orthogonal, when they are transmitted simultaneously one can be completely recovered at the receiver without any interference from the other. These functions are said to be orthogonal to each other. ![]() You may recognise the I wave in Figure 1.7 as a sine function and the Q wave as a cosine function. Figure 1.7 (a) I (in-phase or sine) wave and (b) Q (quadrature or cosine) wave Then it falls again to the next trough, which is the same depth as the previous trough. The line then rises smoothly to a rounded peak the same distance above the X axis as the trough is below. Graph B shows the waveform starting above the origin, at maximum voltage. Then it rises again to the next peak, which is the same height as the previous peak, and then drops again to a trough of the same depth. The line then drops smoothly down to a rounded trough the same distance below the X axis as the peak is above. Graph A shows a sinusoidal waveform starting at the origin. The Y or vertical axes are labelled as voltage. The X or horizontal axes are labelled as time. This figure consists of two line graphs, labelled A and B, showing sinusoids out of phase with each other. ![]()
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