ors causing performance degradation in the particular application (interference, multipath fading, etc.). Power efficiency - particularly important in mobile applications where battery life is an important user acceptance factor. Modulation schemes with higher spectral efficiency (in terms of data bits per Hz of bandwidth) require higher signal strength for error-free detection. Implementation complexity - which translates directly into the cost of hardware to apply a particular technique. Some aspects of modulation complexity can be implemented in software, which has less impact on end-user costs. Table 4-9: Spectral Efficiency of Typical Modulation Techniques Modulation technique Spectral efficiency (Bits/Hz) 16-QAM 128-QAM 256-QAM Radio Communication Basics Simple Modulation Techniques On/Off keying (OOK) is perhaps the simplest modulation technique, where the carrier is turned off during a 0-bit and turned on during a 1-bit. OOK is a special case of amplitude shift keying (ASK) in which two amplitude levels represent 0- and 1-bits. The magnitude of the amplitude shift between these two levels is called the modulation index. Return to zero inverted (RZI) is the modulation technique used in IrDA for data rates up to 1.152 Mbps. It is a derivation of the non-return to zero (NRZ) modulation used in UART data transmission (Figure 4-21), in which a 1-bit is represented by a high state, a 0-bit by a low state, and the transition from high state to low state only occurs when a 1-bit is followed by a 0-bit. In contrast, a return to zero (RZ) transmission has a low-high-low pulse during the bit time for each 1-bit, while the RZI scheme inverts this to give a pulse for each zero bit or symbol. When the RZI modulated signal is received, the bit stream is recovered by triggering a high to low transition for each received pulse, as shown in Figure 4-22. The low state of the decoded signal returns to a high state and a high state remains high at the end of each bit period, unless another pulse is received. Data bits Non Ret