## Example of direct sequence CDMA (cont.)

The spread baseband sequence in IS-95 must then be modulated onto the carrier frequency for transmission. In both the forward and reverse link channels, two further second short code PN sequences are superimposed onto identical versions of the spread baseband stream, which are then filtered to restrict the signal bandwidth. The specification calls for a stop band rejection of better than 40 dB at 740 kHz. These two filtered spread baseband data streams form inputs to a QPSK modulator. In the forward path this is a standard QPSK approach; however in the reverse link, offset QPSK is used, making full use of the non-zero envelope properties of this technique to alleviate the design of the handset RF power amplifiers. |

In addition to the main traffic channel characteristics described, there are further pilot, synchronization and paging channels in the IS-95 standard for system control in this highly complex digital communications link.