for simultaneous reception of the same MS signal by several BSs during a soft handover. Thus, all MSs use zero offset of the short code. Second, there is a half-chip delay inserted into a quadrature branch after a multiplier by PN-Q. This is done to convert QPSK into its version called offset QPSK (O-QPSK). The latter is believed to be preferable for battery lifetime. The conventional QPSK modulator in Figure 11.4 may be thought of as two BPSK modulators, operating independently with cosine and sine CW carriers. When binary symbols in both branches change into the opposite simultaneously, the phase of the QPSK signal hops by 180 , and the transmitter power amplifier should have linear dynamic range around the doubled signal amplitude. In O-QPSK, due to half-chip time-offset of symbol bounds, when one of two modulating binary streams changes, the other remains constant. Therefore, the maximal hop of the resulting signal phase is only 90 , softening the demand on the amplifier linear dynamic range and helping to make the battery lifetime longer.

11.3.5 Evolution of air interface cdmaOne to cdma2000

One of the main stimuli for promoting 3G standards was the extension of functionality of a mobile handset from just a telephone device to a terminal capable of high-speed data exchange with the network, receiving and outputting multimedia information, access to Internet etc. All of these novelties demand much higher transmission rates as compared to 2G systems, and the ultimate rate associated with the 3G philosophy is 2 Mbps. Such a dramatic rate increase with no compromise of quality of service and number of users served calls for a broader system bandwidth. It is characteristic of cdma2000 projects that, unlike the UMTS concept (see Section 11.4), they consider IS-95 as the starting point and allow for backward compatibility with it. Of all the technologies united by the common name cdma2000, the one based on the MC-DS- CDMA principle (see Section 10.2.1) of the forward link looks most likely to be finally adopted. We cannot touch upon the details of this proposal here (whose extensive description is the subject of [69]), due to their complexity and possibly non-final state, so we give only a very short survey of the main ideas below.

With three carriers (in future this number may increase) the cdma2000 forward link just repeats three times the spectrum of IS-95 occupying total bandwidth of about 3.75 MHz and using in each of three 1.25 MHz subbands spreading and modulation techniques somewhat different from those of IS-95. Unlike IS-95, where BPSK data modulates the QPSK spreading sequence, in cdma2000 both data and spreading code are of QPSK type. This allows doubling the duration of the codestream symbols, given the data rate, thereby increasing by two times (up to 128) the spreading factor per subband within the same chip rate. Certainly, greater spreading factor means doubled number of forward physical channels, and consequently potentially greater number of users served. But even more importantly, the number of parallel channels granted by the network to the same user may permit transmission of data at the number-of-channel times higher total rate. This multicode mode inherited from the IS-95B specification is one of the main resources for approaching the rates pre-assigned by the 3G concept.

Another advancement of the cdma2000 forward link is employing transmit diversity, in particular a technique close to that discussed in Section 10.3.5. Since signals from different transmit antennas propagate along different paths, some extra pilot channels