ot detect a signal encoded using another orthogonal code, since orthogonal codes by definition do not correlate with each other. Examples of an orthogonal code set are the Walsh codes, which can be easily generated from the procedure called the Hadamard transform. With each step to the right in Figure 4-20, the three light matrices are the same as the full matrix to the left, while the darker shaded matrix is the inverse of the one to the left. The Walsh codes can be read off as the lines in each matrix, SO the Walsh codes of length 4 are; 0000, 0101, 0011 and 0110. The property of orthogonality is the basis of CDMA and is used in 3G mobile telephony to ensure that many users, each assigned a unique orthogonal access code, can transmit and receive without interference within a single network cell. Digital Modulation Technique Introduction Modulation is the step in the digital signal processing sequence that transforms and encodes the data stream onto the transmitted RF or infrared signal. The spectrum spreading and multiple access techniques will result in a bit-stream transformed into a chip-stream which must now be modulated onto either a single or multiple carrier frequencies, or used to modulate the position or shape of a transmitted RF or Ir pulse. Chapter Four A wide variety of modulation techniques are used in wireless networking. These range from the simple return to zero inverted (RZI) used in IrDA at low data rates, through a variety of phase shift and code keying methods of increasing complexity, such as BPSK and CCK, used for example in IEEE 802.11b at intermediate data rates, to more complex methods, such as the HHH (1,13) code used in IrDA at high data rates. The selection of the best digital modulation technique for a specific application is driven by a number of criteria, the most important being: Spectral efficiency - achieving the desired data rate within the available spectral bandwidth (see Table 4.9). Bit error rate (BER) performance - achieving the required error rate given the specific fact