Key elements in the early stage of WCDMA System Construction Planning (1)

1. Introduction

At present, 3G is getting closer and closer to us, and the conditions for building a new 3G mobile communication network are basically mature. we can predict the near future, 3G will provide users with better network services in a more complex wireless environment.

Compared with 2G mobile communication networks, 3G mobile communication networks must provide more complete network coverage capabilities and support data services with higher speed requirements, it also integrates voice, data, and other real-time multimedia services. Due to the complexity of 3G technology, it poses new challenges and higher requirements for network planning and design. Among the 3G standard, the WCDMA standard is dominant in the global market because it can smoothly transition through the GSM network.

This article summarizes the years of IS95CDMA/1x network planning and construction experience, puts forward the key elements to be considered in the early stage of WCDMA system construction planning, and carries out in-depth analysis. Finally, we provide suggestions on the WCDMA system planning process.

2. Analysis of Common Problems in IS95CDMA/1x network construction and Its Reference

2.1 soft switch Problem Analysis

The main advantage of soft switching is the path diversity of forward and reverse business channels. Because only a small amount of power is required on the forward and backward links to obtain the diversity gain, this means that the total system interference is reduced and the average capacity of the system is increased. At the same time, the reduction of the transmit power of the mobile station prolongs the battery time, that is, the call time. Although soft switching brings unparalleled advantages to the system, in the CDMA downlink, the base station sends additional signals to the mobile station, and soft switching also produces more interference to the system. Because of the limited number of RAKE pointers on the receiver, the mobile platform may not be able to collect the energy transmitted by all base stations, therefore, the gain of the downstream channel depends on the macro diversity gain and the performance loss caused by the interference. At the same time, soft switching occupies multiple channel resources, increasing the complexity of equipment investment and system backup board, mainly manifested in the need to add additional CE units to the base station; an additional transmission link is required for the Abis interface, an additional RAKE modem is required for the mobile station, and additional links need to be added for different fan ranges in the base station. Therefore, too many soft switching areas have a negative impact on the network.

Based on actual engineering experience, when the actual network capacity reaches the expected load target, it is more appropriate to set the soft switch ratio to 35%. When the actual network capacity is not large, based on the operational experience of SPRINT in the United States, it is appropriate to set the soft switch ratio to no more than 50%.

At the initial stage of network construction, user growth takes some time. When the number of users is much lower than the design load of the network, the coverage of the Community exceeds the design coverage, resulting in too many overlapping areas of the Community, this leads to an excessively high soft switching ratio. Therefore, we should pay attention to the analysis of this problem and propose corresponding solutions.

To overcome the negative impact of too many soft switching areas on the network, we need to take appropriate measures to control too many soft switching areas in the WCDMA engineering design phase. The following methods can be used:

(1) Reasonable site selection according to network design requirements: when considering the NodeB base station with a common site, the original base station, such as the GSM base station, has an unsatisfactory location and a low height (too high or too low) when the antenna installation location is limited, and the surrounding buildings are blocked, avoid using the site as much as possible. Although the project investment is saved in the early stage, if the scope of the soft switchover is improperly controlled, the CE Resources in the later stages of the system are virtually wasted. In urban base stations, buildings with a base station higher than the average building height but lower than the highest building should be selected as the site. Avoid selecting the site where a new building may affect the coverage area in the future.

The confidentiality arrangement of the base station should be based on the traffic density distribution, and the future 1 ~ Within two years, you only need to adjust the channel configuration to meet the capacity requirements without making a large adjustment to the number of base stations, so as to avoid adverse consequences for network coverage and control of switching areas; the effective coverage of base stations should also be fully considered so that the system can meet the requirements for coverage.

(2) Reasonably set the neighbor list: the neighbor list setting should be adjusted regularly. In the initial stage, the system carries a small number of users. To improve the service quality of the system, more neighbors should be set in the list, as the number of online users increases, the coverage area of the base station is reduced, and the neighbor list should be shortened accordingly. On the one hand, the system search time is shortened, and on the other hand, the soft switchover area of the system is also reduced.

(3) reasonable adjustment of soft switching parameters: due to environmental changes, wireless business changes, human errors, and other reasons, the parameter settings are often unreasonable, reasonable adjustment of soft switching parameters is an important way to improve the control of soft switching areas.

2.2 Analysis of pilot pollution

Pilot pollution refers to signals with high intensity in the same region (more than the peak of the Rake receiver, that is, the number of coherent receivers. Sometimes, due to the unreasonable site layout or the influence of topography, too many wireless signals overlay the adjacent area, resulting in frequency pollution. The direct impact of pilot pollution is frequent calls. Once the pilot pollution problem occurs, it is difficult to eliminate it by means of optimization and adjustment. Therefore, in the WCDMA design stage, efforts should be made to overcome the pilot pollution problem.

In the network design phase, it is recommended to consider the following aspects based on years of engineering construction experience: (1) using reliable software simulation design, correct the model to accurately determine the coverage of the residential area and control the coverage beyond the district; (2) Select the appropriate site; (3) Select the appropriate NodeB transmit power; (4) fully estimate the impact of the expected number of users on network coverage.

2.3 interference problem analysis

In a CDMA network, the indicator for determining whether a mobile phone can access the network is Ec/Io instead of the receipt level. This is often the case during network testing. When the interference signal strength in a region is strong, the mobile phone receives a high level, but cannot access the network normally. Through testing and analysis, we can find out the causes of interference, including interference from other residential areas in the band, interference from direct release stations and indoor distribution systems, and interference from other systems.

In order to avoid these interference, the following aspects must be controlled during the WCDMA design and planning phase: (1) reasonably plan the site, antenna, and transmit power, and control the over-area coverage area; (2) when using the network signal of the direct station extension in low traffic areas, strict pre-design surveys, reasonable design, strict construction and review should be carried out, avoid the impact on network quality that is hard to solve in the future due to hasty construction. (3) Indoor Coverage mainly aims to absorb traffic and solve the indoor signal blind zone, indoor traffic is the main source of network traffic, the first problem to consider is to prioritize the use of existing indoor coverage systems and take appropriate measures to reduce possible interference.