Different types of Digital Modulation

1.Digital Modulations

As for all recent communication systems, WiMAX/802.16 uses digital modulation. The now well-known principle of a digital modulation is to modulate an analogue signal with a digital sequence in order to transport this digital sequence over a given medium: fibre, radio link, etc. (see Figure 1). This has great advantages with regard to classical analogue modulation: better resistance to noise, use of high-performance digital communication and coding algorithms, etc







Figure 1: Digital modulation principle

2.Binary Phase Shift Keying (BPSK)

The BPSK is a binary digital modulation; i.e. one modulation symbol is one bit. This gives high immunity against noise and interference and a very robust modulation. A digital phase modulation, which is the case for BPSK modulation, uses phase variation to encode bits: each modulation symbol is equivalent to one phase. The phase of the BPSK modulated signal is π or -⁻π according to the value of the data bit. An often used illustration for digital modulation is the constellation. Figure 5.2 shows the BPSK constellation; the values that the signal phase can take are 0 or π.

Figure 2: The BPSK constellation
3. Quadrature Phase Shift Keying (QPSK)

When a higher spectral efficiency modulation is needed, i.e. more b/s/Hz, greater modulation symbols can be used. For example, QPSK considers two-bit modulation symbols.

Table 1 shows the possible phase values as a function of the modulation symbol. Many variants of QPSK can be used but QPSK always has a four-point constellation (see Figure 3). The decision at the receiver, e.g. between symbol ‘00’ and symbol ‘01’, is less easy than a decision between ‘0’ and ‘1’. The QPSK modulation is therefore less noiseresistant than BPSK as it has a smaller immunity against interference. A well-known digital communication principle must be kept in mind: ‘A greater data symbol modulation is more spectrum efficient but also less robust.’

Figure 3: Example of a QPSK constellation

Table 1: Possible phase values for QPSK modulation

Open table as spreadsheet

Even bits	Odd bits	Modulation symbol	ϕk
0	0	00	π/4
1	0	01	3π/4
1	1	11	5π/4
0	1	10	7π/4

4. Quadrature Amplitude Modulation (QAM): 16-QAM and 64-QAM

The QAM changes the amplitudes of two sinusoidal carriers depending on the digital sequence that must be transmitted; the two carriers being out of phase of +π/2, this amplitude modulation is called quadrature. It should be mentioned that according to digital communication theory, QAM-4 and QPSK are the same modulation (considering complex data symbols). Both 16-QAM (4 bits/modulation symbol) and 64-QAM (6 bits/modulation symbol) modulations are included in the IEEE 802.16 standard. The 64-QAM is the most efficient modulation of 802.16 (see Figure 4). Indeed, 6 bits are transmitted with each modulation symbol.

Figure 4: A 64-QAM constellation

The 64-QAM modulation is optional in some cases:

• license-exempt bands, when the OFDM PHYsical Layer is used

• for OFDMA PHY, yet the Mobile WiMAX profiles indicates that 64-QAM is mandatory in the downlink.