Universal Mobile Telecommunication System (UMTS)
WCDMA

Since the TDMA and FDMA access at the air interface used in former 2G networks has not been sufficient for 3G networks, the UMTS adopted more sophisticated access method based on CDMA, explained in chapter 7. To be more specific, the WCDMA technique is utilized in UMTS networks. In WCDMA, user information bits are spread over a much wider bandwidth than in conventional CDMA case. This way, the capacity and data bit rates achieved by individual users in the system is increased. Similarly as in case of CDMA, user data are multiplied with quasi-random bits, called chips, derived from CDMA spreading codes. The UMTS uses the chip rate of 3.84 Mcps (Mega chip per second) and carrier bandwidth is equal to 5 MHz. The other distinguished characteristic of WCDMA based system is the frequency reuse factor 1. It means, the frequency used in all cells is the same (contrary to GSM, where each cell uses individual frequencies to minimize interference). Thus, radio resources are better utilized.

The utilization of WCDMA brings several important features into UMTS standard:

- Rake receiver – Radio channel propagation in mobile networks is characterized by multiple reflections, diffractions, and attenuation of the signal energy caused by obstacles such as buildings, hills, vegetation, etc. This results in so called multipath propagation and fading (strong signal attenuation at certain frequencies). WCDMA system combats these negative effects by rake receiver, which is able to effectively receive and combine individual signal’s component
- Power control – One of the most important aspects regarding WCDMA is fast closed loop power control, on the uplink in particular. Otherwise, single user could block a whole cell if transmitting with high power. Consequently, in WCDMA based systems, the power received by all users should be the same at the base station. Note that in case of HSPA this is only partly true as explained later.
- Soft handover – Since the adjacent cells use the same frequency, the users can be simultaneously connected to two base stations (two different codes have to be used) at the same time. This result in handover between two cells without any interruption, also known as soft handover as already described earlier.