Overview
WCDMA is a 3G technology after GSM. This technology is designed based on the CDMA system. CDMA stands for code division multiple access. This means that the available frequency channel is broken down by different code sequences that are multiplied by the user signals of the individual subscribers. All subscribers transmit on the same frequency and at the same time.

For WCDMA different base stations are distinguished by a different scrambling code, which makes cell planning a lot easier, since neighboring cells can re-use the same frequency! (However, the occupied "SNR" – or Signal to Noise Ratio is the limiting factor and characteristic for CDMA)

The data rate used by a terminal depends on spreading factor assigned to this particular terminal. If several terminals use the same spreading factor, the signals are distinguished through different code channels. At present the maximum data rate is 384 kbps. In the future it will be possible to combine several code channels to a multi-code link, allowing data rates up to 2 Mbps. However, when this is used the capacity of this frequency channel is used up, i.e. no other terminal can operate on this frequency channel. The reason for this is that there is no more "SNR" left for additional connections. This is the capacity issue indicated above.

In order to address higher data rates high speed downlink packet access (HSDPA) has been introduced into Release 5 of the WCDMA (3GPP) standard. HSDPA allows data rates of up to 10 Mbps (and 20 Mbps for Multiple Input Multiple Output (MIMO) systems) and is based on 16-QAM modulation. As the name suggests HSDPA is only available in the downlink direction, i.e. ideal for loading large Emails, surf the web or download videos.