Operating Manual Vol 1 - ES Documentation

Digital Standard 3GPP W-CDMA (FDD)SMIQ2.14.3.5.6 Effect of Scrambling CodeThe scrambling code permits to distinguish between base stations (downlink) and mobile stations (uplink).If different scrambling codes are used within a signal (in this example, the scenario under section 2.14.3.5.3 iscoded twice with different scrambling codes), the orthogonality between the channels will be lost.The following example shows the level conditions measured after despreading:Fig. 2-158 Effect of different scrambling codes on the power distributionThese conditions still allow error-free reconstruction of the bit streams. A certain guard interval shouldhowever always be observed between the transmitted channel power and the measured power of theother (non-orthogonal) channels.In this example a channel power of -10 dB is sufficient for error-free decoding, whereas with a power of-15 dB the bit error rate is about 10 % already. (The given values were evaluated at the physical layerusing a CDPA. Even under worse conditions error-free reconstruction of the bit stream can be madethrough channel coding!)2.14.3.5.7 Effect of Symbol Rates and Channelization Code NumbersThe channelization code generation method has the effect that with certain symbol rate/channelizationcode combinations the identical spreading sequences Si/Sq are generated. This is always the case withchannelization code number 0 (all bits 0), but also with the combination 960 ksps/channelization code 1(0011) 480 ksps/channelization code 2 (00110011).As a rule, the spreading sequences are identical if the products of symbol rate andchannelization code are identical.The despreader can therefore no longer differentiate between the two channels. Depending on the datacontents, the channel powers are either added up or cancel each other. Restoring of all source data isnot possible.The following signal contains• a DPCH with 960 ksps, channelization code number 1, 0 dB, data all 0• a DPCH with 480 ksps, channelization code number 2, 0 dB, data all 1An analysis at 960 ksps gives the following result:Fig. 2-159Cancellation possible in case of several channels with identical spreading sequences1125.5555.03 2.232E-9