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 5.1). This has great advantages with regard to classical analogue modulation: better resistance to noise, use of high-performance digital communication and coding algorithms, etc.
Many digital modulations can be used in a telecommunication system. The variants are obtained by adjusting the physical characteristics of a sinusoidal carrier, either the frequency, phase or amplitude, or a combination of some of these. Four modulations are supported by the IEEE 802.16 standard: BPSK, QPSK, 16-QAM and 64-QAM. In this section the modulations used in the OFDM and OFDMA PHYsical layers are introduced with a short explanation for each of these modulations.
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.
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 5.1 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 5.3). 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.’
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 |
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 5.4). Indeed, 6 bits are transmitted with each modulation symbol.
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.
Having more than one modulation has a great advantage: link adaptation can be used (this process is also used in almost all other recent communication systems such as GSM/EDGE, UMTS, WiFi, etc.). The principle is rather simple: when the radio link is good, use a high-level modulation; when the radio link is bad, use a low-level, but also robust, modulation. Figure 5.5 shows this principle, illustrating the fact that the radio channel is better when an SS is close to the BS. Another dimension is added to this figure when the coding rate is also changed (see below).