| Recently, with the development of computer and communication technology, mobile telecommunication grows rapidly, China holds the largest mobile communications network in the world, and has become the potential largest mobile communications market. As the constant increase of users'demands, the rapidly rise of voice traffic and the continuous increase of users'speed, the third generation mobile communication system develops rapidly. WCDMA system has been broadly adopted as one of the important air interface standard in 3G., Comparing with CDMA technology, WCDMA supports higher code rate, wider carrier bandwidth. It is much important that WCDMA has the new radio resource management method which can assure the quality of communication, improve the network coverage and the system capacity. Handoff is one of the essential functionality of mobile communications system, which can make an on-going call to a different sector or cell and maintain a call in progress across cell boundaries in mobile communications. As one of the important radio resource management methods, soft handoff is the most essential technology for seamless communication, it can decrease the transmitting power of mobile station, reduce the drop probability, so enhancing the network coverage, system capacity and the quality of communication consumedly.Because of coherent detection is employed in the uplink using pilot symbols in the WCDMA system, the uplink capacity and coverage are improved greatly. It has been proved that communication quality can be improved as the introduced of macro diversity gain which needs occupy additional network resource, so the downlink capacity can be the limiting factor for the system capacity in certain situations especially with those systems that support asymmetric services, the system performance of soft handoff in the downlink needs to be further investigated. Additionally, as the improve of system performance depends on soft handoff algorithm and parameter optimization, and the soft handoff algorithm usually decides system service quality and the efficiency of resource utilization, as a result, soft handoff algorithm needs to be further optimized.To study the soft handoff technology is the main content of this paper, including the influence of voice traffic and uplink, downlink capacity, as well as soft handoff algorithm optimization. Besides, using the MATLAB software to simulate based on Monte-Carlo method which generates random users who distribute uniformly in a cell. It can get the simulation results from several calculation using simulation model.As follows is the main content of this paper.Chapter one is the exordium which introduces the modern development of the third generation mobile communications and the basic structure of WCDMA system, as well as background and significance of investigation.In chapter two, the basic theory of soft handoff is analyzed. It is compared between hard handoff and soft handoff, and the basic process of soft handoff including measure, judgement and implementation, especially the process of measure is introduced. 1A~1F matters of intra-frequency measure which is used in soft handoff is discussed that is used in FDD model.The chapter three studies the performance of soft handoff. The influence of soft handoff to voice traffic is discussed, and the conclusion is that handoff can decrease the drop probability. Then it is discussed about uplink capacity of WCDMA system. The results of simulation shows: Capacity is decreased with the increase of ratio between intre-cell interference and intra-cell interference, however, as the combination of the selectivity incorporate in RNC and reverse power control technology. Soft handoff can reduce the MS's transmitting power, and decrease the interference of system from MS which can improve the uplink capacity and coverage.In the downlink, the system assigns another channel to users who are in soft handoff state which not only occupy additional resource but also increase the downlink interference and decrease the downlink capacity. The paper analyzes the downlink interference firstly, discusses the pilot strength that MS has received who are in two or three ways soft handoff state, also compares the powers which are assigned to downlink traffic channel by the base stations which are in soft handoff or not soft handoff state by simulation. As a result, it is concluded that soft handoff can decrease this power, assure the quality of communication and restrain QoS deterioration. At the same time, It is investigated that the connection between soft handoff thresholds and soft handoff ratio as well as the downlink capacity. It can be concluded by simulation that the value of handoff added thresholds T_ADD can be 3dB~5dB when the soft handoff ratio is controlled between 30%~40%; Another conclusion is also obtained, the downlink capacity decreases gradually with the increase of T_ADD, when the power of base station is specific(e.g. P=20w), a cell can add about 7 users if T_ADD decreases from 6dB to 0dB. So the value of T_ADD can be increased that more users can stay in soft handoff state if the system is in small load; but when the system is in the state of high load, the value of T_ADD should be decreased properly, and the cell can hold more users.In chapter four, the soft handoff algorithm is optimized and simulated. Firstly, the basic soft handoff algorithms are introduced including the soft handoff based on distance, IS-95, cdma2000 and UTRA soft handoff algorithm. Secondly, the optimization methods of soft handoff which were studied by former academician, and the methods are based on cell's traffic load or the MS's position messages. The soft handoff thresholds can be adjusted dynamically based on the load and position messages. This paper studies a new position assisted soft handoff algorithm based on handoff forecast which forecasts the soft handoff base on the MS's position messages. When MS is far away from the base station, the direction of MS is off the currently base station and the speed of MS is much fast, the probability of soft handoff is higher, so the added threshold T_ADD can be increased and the dropped threshold T_DROP can be decreased to make the neighboring base station add to active set faster, also the worst base station in the active set can be deleted much faster. By compared with UTRA soft handoff algorithm with simulation, it can be concluded that the new algorithm hold lower value of MASN and ASUR, also the variety range of them is small with the increase of T_ADD. When T_ADD is 3dB, the MASN of the new algorithm can be dropped by 12%, and ASUR can be decreased by 3%.As a result, the new algorithm can improve the recourse utilization efficiency. Additionally, the new soft handoff algorithm can decrease the outage probability, so the quality of service can be increased.Chapter five is the summarization and analysis of disadvantage. The conclusions of this paper own the value for the layout plan and optimization of the future mobile networks. By optimizing the soft handoff algorithm, the future mobile network can hold better utilization of recourse and quality of service. They also establish theoretical basic for the investigation of soft handoff in WCDMA system, especially the optimization of soft handoff algorithm. And they have reference significance for wireless resource layout in the WCDMA system. |
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