urn to zero (NRZ) Return to zero (RZ) Return to zero inverted (RZI) period Figure 4-21: NRZ, RZ and RZI Modulation Techniques Chapter Four Start RZI modulated signal Decoded signal Data bits Figure 4-22: RZI Bit Stream Decoding The advantage of the RZI scheme for IrDA is that it allows the transmitting LED to be off for most of the bit time, in order to conserve battery power. Phase Shift Keying Phase shift keying is a modulation technique in which the phase of the carrier is determined by the input bit or chip stream. There are several types of phase shift key (PSK) modulation including binary (BPSK) and quadrature (QPSK). Binary Phase Shift Keying BPSK is the simplest technique in this class, with the carrier phase taking one of two states, as shown in Table 4-10. A 0 input symbol (whether it is a bit or a chip) corresponds to a zero phase carrier while a 1-symbol corresponds to a 180° phase shifted carrier, resulting in the output waveform shown in Figure 4-23. Table 4-10: Binary Phase Shift Keying Symbol Carrier phase 0 degrees 180 degrees Radio Communication Basics Bit stream Zero phase carrier modulated carrier Figure 4-23: Binary Phase Shift Keying Modulation (BPSK) BPSK modulation is used by IEEE 802.11b at a data rate of 1 Mbps, and by IEEE 802.11a, in combination with OFDM, to achieve data rates of 6 and 9 Mbps. Quadrature Phase Shift Keying Instead of the two phase states used in BPSK, QPSK uses four distinct carrier phases, each of which is used to encode a symbol comprised of two input bits or chips. These four carrier phases are illustrated in Figure 4-24, which represents the phase of the carrier signal in the IQ plane (I = In phase, Q = Quadrature or 90 degrees out of phase). The angle of a given point amplitude circle QPSK constellation II/4-QPSK constellation Figure 4-24: QPSK Phase Constellation Chapter Four Table 4-11: Quadrature Phase Shift Keying Symbol Carrier phase 0 degrees 90 degrees 180 degrees 270 degrees on the plane from the I axis represents the phase angle and the distance of a poi