Optimization of China Unicom Shanghai Branch´s GSM Network

China Unicom Shanghai Branch has successfully offered special optimization solutions to problems generally existing in metropolises such as Shanghai. The quality of its GSM network has been greatly improved.

The Branch has clear optimization guidelines:

• Systematical optimization
• Risk minimization of network optimization
• Optimization from the whole to the partial
• Optimization from the city center to the outside
• Orderly optimization

1 Purpose of Network Optimization
The purpose of network optimization is to find the major factors that affect the quality of network, and ultimately achieve better network performance. It can improve network quality, bringing a higher degree of customers´ satisfaction and enhancing an operator´s competence.
Network optimization and routine maintenance are tightly correlated with each other. Network optimization is implemented along with daily maintenance. Network construction is based on the optimization. Network planning and construction provide a foundation and platform for network optimization.

  The major problems that can be resolved through network optimization include low-level signal coverage, overreach coverage, traffic unbalance between uplink and downlink, island effect, radio interference, and other network problems caused by the above reasons, such as call loss, call failure and handover failure. In addition, the optimization can cut down regional traffic congestion or idleness caused by unbalanced traffic loads.

2 Optimization Guidelines
The optimization of China Unicom Shanghai Branch´s radio network complies with the following principles.

  (1) Systematical Optimization
  A network is an integral system. The possible effects of any partial optimization on the whole network should be carefully considered, otherwise new problems may be brought out. In the mean time, tests and statistical analysis are necessary once such partial network optimization is over.

  (2) Risk Minimization of Network Optimization
  Since network optimization is a kind of system engineering implemented on a running network, any adjustment and change to the network would cause negative effects on the whole network. It is necessary to take any risks into consideration while carrying out optimization. Frequent network adjustment should be avoided. It is encouraged to use network planning and optimization software and equipment to simulate the result of network adjustment. All data of the running network should be backed up. It is better to implement optimization during low traffic periods. An optimization trial can first goes in a chosen area.
 

  (3) From Whole to Partial
  An optimization plan should first focus on problems of the whole network, such as the unreasonable network structure and network congestion, and on some generally existed problems such as overreach coverage and signal interference.

  (4) From Center to Outside
  When a metropolitan network needs general optimization, it is important to settle down the radio parameters of central metropolitan areas first, and then conduct the optimization work in outer areas. The reason to do so is that the central area has the heaviest traffic and most limited frequency resources while there are more frequency resources in the outer area.

  (5) Orderly Optimization
  Performances of all the network functions are correlated with each other, so when there appear many problems in a network, they should be solved in a proper order. Network optimization should be implemented step by step. Primary problems should be settled first, then the secondary.

3 Network Features and Optimization Strategies
Solutions are made for China Unicom Shanghai Branch´s wireless network in view of specific network features.

3.1 Feature 1 and Its Countermeasures

  (1) Feature
  At present, there are more than 2 million subscribers on the GSM network of China Unicom Shanghai Branch. In order to meet the market demand, network construction is always in progress, which makes the construction period and the stable period alternate. In general, there are two times in a year to implement large-scale network expansion and frequency reallocation, which makes it difficult to maintain a stable network. Actually, once a whole network optimization comes to an end, a new phase of network construction starts.

  (2) Countermeasures

• Setup a steady-going network optimization team, and focus on the enhancement of technical strength. In 1998, Shanghai Branch established a special network optimization team, and since then the network optimization has become an important part of the routine maintenance work, changing the situation that equipment providers were responsible for network optimization while the branch only did the routine maintenance.
• Combine network optimization with network planning and network construction, and alternately carry out fixed period optimization and continuous optimization. Shanghai Branch merged its network optimization and planning departments, and let its optimization team fully participate into the network planning and design. Consequently the combination and complementation of network optimization and network designing have made the network development more objective and effective.
• Setup the radio parameter management system, statistical analysis system and optimization database to strengthen the optimization management and improve the accuracy and efficiency of optimization. With the self-developed network parameter management system, Shanghai Branch has successfully discovered and controlled network changes in time, and rectified abnormal network parameters. This has ensured the orderly network management throughout all phases of both routine operation and network construction.
• Statistical analysis is an effective method to find out problems and hidden troubles of the network. The statistical analysis software supports easy and quick data handling and inquiry. It can warn of cells in abnormal working conditions, such as the overflow or low traffic and the abnormal handover, number of calls and number of channels. The traffic statistics is analyzed daily, weekly and monthly.
• Routine network optimization includes frequent modification of network configuration and parameters. Shanghai Branch has developed a Network Optimization Database System to enhance the daily optimization management. All optimization team members use this integrated system to record all the optimization work, including who, when, what, why and how to do optimization and optimization results and status. Besides, the optimization plans are approved by the optimization supervisor and the optimization results are made by this system. Therefore, the system can not only record any changes of the network during optimization, but also offer a collection of optimization cases. It is a good tool for optimization experience sharing and optimization training.

3.2 Feature 2 and Its Countermeasures

  (1) Feature
  The network expands very fast and the frequency resources are quite limited, which leads to the demand for cutting-edge network technologies.

  (2) Countermeasures

• Keep studying new technologies and actively carry out application trials and make secured and reliable technology application plans based on actual network situations. Network development demands for applications of new technologies, but such applications take risks to the existing network. It should be very careful to plan and carry out any applications. Fortunately, we have made great successes through our elaborate plans and execution.
• Dynamically adjust network configuration, networking strategy and functional applications according to network developments. The mobile network is ever changing, so special solutions would be required at its different development phases. For instance, at the beginning of the launch of our dual-band network, there were few dual-band handset users. So we limited the coverage and capacity of dual-band base stations. But now dual-band handsets are dominant on the existing network. The 1 800 MHz band base stations have almost covered the whole city and the network capacity keeps expanding. Therefore, the adjustment of dual-band network priority, handover threshold and other parameters has been conducted for those changes.

3.3 Feature 3 and Its Countermeasures
  (1) Feature
The complex metropolis environment results in terrible uplink interference. The prosperous downtown, skyscrapers and dense buildings in Shanghai lead to a complex radio environment. Uplink interference has become one of the main factors impairing the network quality.

  (2) Countermeasures

• Enhance the network monitoring in order to discover interference in time. Generally, uplink interference influences several cells in an area. The statistical analysis and drive test are the main tools to find uplink interference. According to subscribers´ complaints, we use the statistics tool to preliminarily position the interference. The field strength test equipment is then used to locate the exact position of interference sources.
• Adjust antennas and network parameters to minimize the effect of interference. Repeaters are always installed in places difficult to find. Adjustment of antennas and cell parameters is necessary to minimize their impact on base stations. Most interfered base stations are generally located in the downtown, where the distance between base stations is short, and the upper layer is covered with the DCS (Digital Cross-Connection System) network. We adjust handover parameters to make GSM1800 cells of a dual-band base station load more traffic, and meanwhile lower down those antennas suffering uplink interference to narrow their service extent. In this way, the number of lost calls decreased a lot, although the call loss rate of the interfered cell keeps unchanged. Quality of the whole network is accordingly improved.
• Strengthen the work of searching for interference sources. Uplink interference raises the call loss rate and reduces the success rate of handover, greatly impairing the network quality. So it is extremely important for us to find and shut down the uplink interference sources, or to adjust antennas or network parameters to minimize the effect of interference when the source cannot be found out. What we have done to search for the interference source is as follows:
  Firstly, we use statistical analysis on traffic to locate base stations suffering uplink interference.
  Secondly, we use the directional antenna at the top of the base station to scan the uplink frequency band to find out the direction suffering the strongest uplink interference.
  Thirdly, we use the field strength analyzer to measure the uplink receiving level in areas where interference most possibly exists. Generally speaking, the background noise of the uplink frequency band should keep less than -100 dB, though it is allowed to reach -90 dB-100 dB occasionally. If the noise determined by the field strength analyzer is higher than -80 dB, we can conclude that there is an uplink interference source nearby.
  

  Lastly, we pay special attention to places where repeaters areinstalled, such as commercial or recreational public places. Generally, abnormally high receiving electrical level means the existence of repeaters. We shut down the repeaters once we find them.

3.4 Feature 4 and Its Countermeasures
  (1) Feature
  There are more than 3 200 buildings of 8 or more storeys. Elevated highways and dense high buildings make greater demands for better network coverage.

  (2) Countermeasures

• Strengthen field tests and technical analysis to ensure good network coverage over elevated highways. The drive test is one of the effective approaches to find out network problems. We have deployed an automatic drive test system with 8 testing terminals. The terminals are installed in cabs and automatically collect and transfer data to the monitoring center where the testing data are analyzed, so the troubles can be found out in time and optimization plans can be made later. In addition, to ensure good network quality, our optimization team frequently does tests on important roads. We also launch periodical network-comparing tests. With horizontal and vertical comparison between networks, we get an overall picture of network quality and can determine the focus of further network optimization.
• Actively conduct CQTs (Call Quality Tests) tests and pay great attention to the network coverage over high buildings. With such a test going in hot-spot areas, communication guaranteed important areas and main buildings, we evaluate the quality of voice, coverage and other services subjectively, and experience users´ real feelings.
• Reinforce the construction of indoor coverage systems. The outdoor coverage and communication quality of our GSM network have come up to an advanced level after a few years of construction and development. In order to improve the network competence and attract more high-end users, large-scale construction for key indoor coverage has become our priority projects since last year. A special indoor coverage construction team has been set up. Its sales and marketing division is responsible for the investigation of indoor coverage demands. The indoor coverage plan is mapped out according to the investigation results. The network optimization team participates in examining and approving indoor coverage solutions. In the light of actual situations, different solutions were adopted to make the indoor coverage projects more objective, better planed and economically effective.

Manuscript received: 2004-03-02