3/19/2014 Technical Notes : OFDM - Magna Design Net, Inc.

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Technical Notes > OFDM > Digital Modulation

II. Digital Modulation

1. Review of Dibital Modulation

Digital modulation modulates three parameters of sinusoidal signal. The three parameters are Amplitude

A, phase θ, and frequency f.

s(t) = A·cos( 2π·fc·t + θk )

There are three type digital modulation.

Amplitude Shift Keying

Phase Shift Keying

Frequency Shift Keying

In OFDM the combination of ASK and PSK is used such as QAM in Figure 1.

Figure 2 shows multi bit modulation.

Figure 1 : 3 types of digital modulation

Figure 2 : Multi bit modulation(QPSK)

2. Mathematics of Digital Modulation

Transmission signal s(t) is expressed as below.

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e j2πfc·t

( ak + jbk )

s(t) = cos( 2π·fc·t + θk ) = cosθk·cos( 2π·fc·t ) - sinθk·sin( 2π·fc·t )

Define ak = cosθk and bk = sinθk

s(t) = Re [ ( ak + jbk ) e j2πfc·t ]

Then, the transmission signal could be characterized as ( ak + jbk ) e j2πfc·t

Here,

Carrier Wave

Digital Modulation

Important fact is that digital modulation could be characterized in complex number.

3. QAM(Quadrature Amplitude Modulation)

The characteristics of digital modulation such as (ak+jbk) can be plot like figure 3. Figure 4 shows that

of 16 QAM and Figure 5 shows that of 64QAM.

Figure 3 : Constellation map of QPSK

Figure 4 : 16QAM

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Figure 5 : 64QAM

4.FDMA(Frequency Division Multiple Access)

The old analog TV broadcasting uses the FDMA. The each channel frequency is separated by

guardband.

Figure 6 : FDMA

5. Multi-Carrier Modulation

The multi-carrier modulation is used plural of FDMA channels as shown in upper one of Figure 7. Figure

7 shows the occupied channel frequency for both multi-carrier and single carrier modulation. The total

occupied bandwidth is same. In multi-carrier modulation, guardband is inevitable. In OFDM, each

spectrum of multi-carrier modulation can be overlapped.

Figure 7 : Frequency Spectrum

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