1、附录 3：英文资料 Basic Concepts of WCDMA Radio Access Network 1. Background There has been a tremendous growth in wireless communication technology over the past decade. The significant increase in subscribers and traffic, new bandwidth consuming applications such as gaming, music down loading and video st
2、reaming will place new demands on capacity. The answer to the capacity demand is the provision of new spectrum and the development of a new technology - Wideband CDMA or hereinafter referred to as WCDMA. WCDMA was developed in order to create a global standard for real time multimedia services that
3、ensured international roaming. With the support of ITU (International Telecommunication Union) a specific spectrum was allocated - 20Hz for 3G telecom systems. The work was later taken over by the 3GPP (3rd Generation Partnership Project), which is now the WCDMA specification body with delegates fro
4、m all over the world. 2. WCDMA a development from GSM and CDMA Naturally there are a lot of differences between WCDMA and GSM systems, hut there are many similarities as well. The GSM Base Station Subsystem (BSS) and the WCDMA Radio Access Network (RAN) are both connected to the GSM core network for
5、 providing a radio connection to the handset. Hence, the technologies can share the same core network. Furthermore, both GSM BSS and WCDMA RAN systems are based on the principles of a cellular radio system. The GSM Base Station Controller (ESC) corresponds to the WCDMA Radio Network Controller (RNC)
6、. The GSM Radio Base Station (RBS) corresponds to the WCDMA RES, and the A -interface of GSM was the basis of the development of the Iu-interface of WCDMA, which mainly differs in the inclusion of the new services offered by WCDMA. The significant differences, apart from the lack of interface betwee
7、n the GSM BSCs and GSM Abis-interface to provide multi-vendor operability, are more of a systemic matter. The GSM system uses TDMA (Time Division Multiple Access) technology with a lot of radio functionality based on managing the timeslots. The WCDMA system on the other hand uses CDMA, which means t
8、hat both the hardware and the control functions are different. Examples of WCDMA-specific functions are fast power control and soft handover. Code Division Multiple Access and WCDMA Code Division Multiple Access (CDMA) is a multiple access technology where the users are separated by unique codes, wh
9、ich means that all users can use the same frequency and transmit at the same time. With the fast development in signal processing, it has become feasible to use the technology for wireless communication, also referred to as WCDMA and CDMA2000. In CDMA One and CDMA2000, a 1.25MHz wide radio signal is
10、 multiplied by a spreading signal (which is a pseudo-noise code sequence) with a higher rate than the data rate of the message. The resultant signal appears as seemingly random, but if the intended recipient has the right code, this process is reversed and the original signal is extracted. Using uni
11、que codes means that the same frequency is repeated in all cells. which is commonly referred to as a frequency re-use of 1. WCDMA is a step further in the CDMA technology. It uses a 5MHz wide radio signal and a chip rate of 3.84Mcps, which is about three times higher than the chip rate of CDMA2000.
12、The main benefits of a wideband carrier with a higher chip rate are: Support for higher bit rates Higher spectrum efficiency thanks to improved trunking efficiency (i.e. a better statistical averaging) Higher QoS Further, experience from second-generation systems like GSM and CDMA One has enabled im
13、provements to be incorporated in WCDMA. Focus has also been put on ensuring that as much as possible of WCDMA operators investments in GSM equipment can be reused. Examples are the re-use and evolution of the core network, the focus on co-siting and the support of GSM handover. In order to use GSM h
14、andover, the subscribers need dual mode handsets. 3. Radio Network Functionality For optimal operation of a complete wireless system i.e. from handset to radio access network (RAN) several functions are needed to control the radio network and the many handsets using it. All functions described in th
15、is section, except for Handover to GSM, are essential and therefore necessary for a WCDMA system. 3.1 Power control The power control regulates the transmit power of the terminal and base station, which results in less interference and allows more users on the same carrier. Transmit power regulation
16、 thus provides more capacity in the network. With a frequency re-use of 1, it is very important to have efficient power control in order to keep the interference at a minimum. For each subscriber service the aim is that the base station shall receive the same power level from all handsets in the cel
17、l regardless of distance from the base station. If the power level from one handset is higher than needed, the quality will be excessive, taking a disproportionate share of the resources and generating unnecessary interference with the other subscribers in the network. On the other hand, if power le
18、vels are too low this will result in poor quality. In order to keep the received power at a suitable level, WCDMA has a fast power control that updates power levels 1500 times every second. By doing that the rapid change in the radio channel is handled. To ensure good performance, power control is i
19、mplemented in both the up-link and the down-link, which means that both the output powers of the hanpset and the base station are frequently updated. Power control also gives rise to a phenomenon called cell breathing. This is the trade-off between coverage and capacity, which means that the size of
20、 the cell varies depending on the traffic load. When the number of subscribers in the cell is low (low load), good quality can be achieved even at a long distance from the base station. On the other hand, when the number of users in the cell is high, the large number of subscribers generates a high interference level and subscribers have to get closer to the base station to achieve good quality. 3.2 Soft and softer handover
