In wireless systems, information transfer takes place through the process of modulationthat is, changes introduced onto the radio carrier wave. The data is combined with the carrier wave, creating the radio signal; the receiving end extracts the data, discards the carrier signal, and reconstructs the information. There are various modulation techniques for both analog and digital systems.
In order to represent the digital bitstream in analog format, one of three main characteristics of the carrier signal is altered:
- Amplitude With amplitude modulation, different amplitudes or power levels are used to represent the 1 or the 0.
- Frequency With frequency modulation (FM), also referred to as frequency-shift keying (FSK), different frequencies are used to differentiate between 1s and 0s.
- Phase of the wave With phase-shift keying (PSK), a shift from the expected direction of the signal is used to encode the 1 or the 0.
These signal characteristics are illustrated in Figure 5.3 in Chapter 5, "Data Communications Basics." Some modulation schemes use a combination of these characteristics to alter the carrier signal, most commonly combining amplitude and phase. Modulation techniques vary in several ways, including speed, immunity to noise, and complexity. Not surprisingly, many incompatible schemes exist.
The duration of a single cycle of a waveform is called the symbol time. Modulation schemes vary in their spectral efficiency, which is a measure of the number of digital bits that can be encoded in a single cycle of a waveform, or symbol. To get more bits per Hertz, many modulation techniques define more amplitude levels. To encode k bits in the same symbol time, 2k voltage levels are required. However, it becomes more difficult for the receiver to discriminate among many voltage levels with consistent precision as the speed increases. As discussed in Chapter 5, there are two categories of modulation schemes: single-carrier and multicarrier modulation. The following sections describe the modulation schemes that relate to wireless communication.
Single-Carrier Modulation Techniques
In single-carrier modulation schemes, a single channel occupies all the bandwidth. The following single-carrier techniques are commonly used in wireless systems:
- Gaussian Minimum-Shift Keying (GMSK) GMSK is a kind of continuous-phase frequency-shift keying modulation. GMSK produces one bit per symbol time. Starting with a bitstream of 1s and 0s, a bit-clock assigns each bit a timeslice. The baseband signal is generated by first transforming the 0/1 encoded bits into 1/+1 encoded bits. These encoded bits are then filtered, transforming the square-shaped pulses into Gaussian-shaped (or sinusoidal) pulses. The baseband signal is then modulated, using frequency modulation. Frequency shifts can be detected by sampling the phase at each symbol period. Transmitting a 0 or 1 bit is therefore represented by changing the phase. Each shift in the phase represents a bit. This technique is used in the GSM cellular system.
- Binary Phase-Shift Keying (BPSK) BPSK is the simplest form of PSK. It introduces a 180-degree phase shift (i.e., it uses two phases, separated by 180 degrees). It is able to modulate at only one bit per symbol time and is therefore unsuitable for high-data-rate applications. However, it is the most robust of all the PSK techniques because it takes a tremendous amount of distortion for the demodulator to reach an incorrect decision. For this reason, it is the best solution under noisy conditions. It is commonly used with the DSSS version of the original 802.11 radio link. (DSSS is discussed later in this chapter.)
- Quadrature Phase-Shift Keying (QPSK) QPSK introduces four different phase shifts0 degrees, 90 degrees, 180 degrees, and 270 degreesproviding two bits per symbol time. QPSK can operate in harsh environments, such as with over-the-air transmission. Because of its robustness and relatively low complexity, QPSK is widely used in situations such as with Direct Broadcast Satellite (DBS). It is also used with 802.11x WLANs. In 802.11b, it is used when operating at 5.5Mbps and 11Mbps, and in 802.11a and 802.11g, it is used when operating at 12Mbps or 18Mbps.
- Differential Phase-Shift Keying (DPSK) DPSK is a form of phase-shift keying used for digital transmission in which the phase of the carrier is discretely varied in relation to the phase of the immediately preceding signal element and in accordance with the data being transmitted. The demodulator determines the changes in the phase of the received signal rather than the phase itself. DPSK can be significantly simpler to implement than ordinary PSK because there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal. However, it produces more erroneous demodulations as a result. Differential Binary PSK (DBPSK) is used in low-speed 802.11 WLANs (operating at 1Mbps), and Differential Quadrature PSK (DQPSK) is used with the extended-rate 2Mbps 802.11 WLANs.
- Quadrature Amplitude Modulation (QAM) QAM modulates multiple levels of amplitude or uses both amplitude and phase to yield a higher spectral efficiency. As discussed in Chapter 5, various levels of QAM are referred to as nn-QAM, where nn indicates the number of states per Hertz. The number of bits per symbol time is k, where 2k = nn. Therefore, 4 bits per Hertz = 24 = 16-QAM, 6 bits per Hertz = 26 = 64-QAM, and 8 bits per Hertz = 28 = 256-QAM. QAM techniques are used in many applications, including digital cable television, cable modems, DSL modems, digital satellite systems, 802.11 (Wi-Fi), 802.16 (WiMax), and 3G W-CDMA/HSDPA systems.
Multicarrier Modulation Techniques
Multicarrier modulation techniques use an aggregate amount of bandwidth and divide it into subbands. Each subband is then encoded by using a single-carrier technique, and the bitstreams from the subbands are bonded together at the receiver. OFDM, discussed later in this chapter, is an increasingly popular multicarrier technique. It is used in European digital over-the-air broadcast and in many new and emerging wireless broadband solutions, including 802.11a, 802.11g, 802.16x, 802.20x, and Super 3G; it is the basis of the 4G and 5G visions.