gly, many incompatible schemes exist. The duration of a single cycle of a waveform is called the symbol time. Modula- tion 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, 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 modula- tion. The following sections describe the modulation schemes that relate to wire- less 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 continu- ous-phase frequency-shift keying modulation. GMSK produces one bit per symbol time. Starting with a bitstream of 1s and Os, 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 fil- tered, 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 chang- ing 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 Chapter 13 Wireless Communications Basics therefore unsuitable 