PHS network optimization analysis

PHS network optimization analysis

Abstract: Network optimization is the key to PHS operation. This article discusses the issues of PHS network optimization including: coverage, capacity, handover, paging area settings, interference, synchronization and other issues.

[Keywords] PHS network optimization coverage capacity switching interference synchronization

I. Introduction

As of the end of 2004, China's PHS users have reached 65 million, which has nearly doubled from the 35 million at the end of 2003. Such a rapid increase has posed a huge challenge to the quality of PHS network operations. The increase in the number of users requires operators to continuously expand the network. The network model of the original PHS network design has undergone tremendous changes. The traffic volume in some areas may have reached saturation or exceeded the original design capacity. City planning With the urban construction, the coverage of the original PHS network has also changed. This requires the PHS network to be optimized in time with changes in the environment and changes in business volume, so as to ensure the communication quality of the network and provide users with satisfactory services. PHS will continue to maintain the momentum of development. At present, PHS operators China Telecom and China Netcom have taken PHS network optimization as the focus of PHS work.

Second, PHS network optimized content

PHS network optimization is the key to ensuring the quality of PHS networks. Network optimization work has been running through the construction and operation of the network. The content of PHS network optimization involves the analysis of the coverage area of ​​the base station, the consideration of the cell capacity, the consideration of the frequency of handover and the success rate of handover, the setting of the paging area, the resolution of frequency interference, and the realization of accurate synchronization. Network optimization should achieve the best operation quality with the least resources.

The current PHS network has reached a certain scale, and its network optimization is mainly based on measured data, statistical analysis of traffic in hot spots, and analysis of CS signal coverage including signal strength distribution and CS coverage layers. For the measurement of the relative time difference of each Slave base station relative to the Master base station, the actual call road test is also required to obtain statistics of wireless events that occur during the entire call establishment to call release process, including: number of call failures (rate), normal release The number of times, the number of dropped calls (rate), the number of handover (Handover and TCH Switching) attempts, the number of handover failures (rate), the number of location update attempts, the number of location update failures (rate), and analyze the causes of various event failures through signaling Finally, summarize the current network problems, such as insufficient channel resources, excessive interference, unsatisfactory synchronization of base stations, and excessive handovers caused by unreasonable coverage, etc., thus deriving a network optimization plan. The collection of the above data requires the support of corresponding powerful testing tools and software, so that the optimization of the PHS network can be completed more quickly, scientifically and accurately. At present, PHS network-optimized drive test equipment and post-analysis software are still relatively rich, and the functions are also very powerful. The PHS drive test system can complete the geographic information annotation and analysis of drive test data and traffic statistics data. The graphics and tables can graphically depict the network quality status, providing a reliable and intuitive basis for adjusting and optimizing the network.

1. Coverage analysis in PHS network optimization

In the early days of network construction, the first step is to predict the coverage of the base station through the link budget. L [dB] = Ptx [dBm] + Gtx [dBi] −Prx [dBm] + Grx [dBi] −M [dB], where Ptx is the base station transmit power, Prx is the receiver sensitivity, M is the fading margin, Gtx And Grx are antenna gains. In the PHS system, the maximum base station CS transmission power is 500mw (actually used are 500mw, 300mw, 20mw, 10mw), the terminal PS transmission power is maximum 10mw, and the reception sensitivity is 16dBμV (-91dBm). For the propagation model of PHS, the COST231-WI model can be selected. This model is more suitable for the environment with many buildings under the microcellular structure. Its loss is composed of three parts: L0 + Lrts + Lmsd, where L0 is the free space loss, and Lrts is The loss of street diffraction, Lmsd is the loss of diffraction of multiple obstructions. Through the above parameters, the coverage of the PHS base station can be calculated. This calculation is only a theoretical value. Since the actual propagation environment is very complicated, the model must also be corrected by actual measurement. In the actual measurement process, an electronic map and corresponding drive test software can be used to obtain a distribution map of CCH channel signal strength within the PHS coverage area through drive test. Use terrain analysis tools to make terrain profiles for the 3D geographic and topographical features of the designated area. Using representative drive test data, the propagation model can be calibrated based on the analysis results. Usually, the 10mw CS coverage range is 50-150 meters; the 20mw CS coverage range is 100-300 meters; the 300mw base station coverage range is 300-1000 meters; the 500mw base station coverage range is 500-1500 meters. In addition, by conducting the roadside in the call state, the transmission power and BER of the upstream and downstream TCH channels can be recorded on the CS side and the PS side. According to the transmission power curve of CS and PS (who reaches the maximum transmission power first) and the change curve of BER, it can be judged whether the coverage is limited by uplink or downlink. Afterwards, the appropriate cell edge can be determined by adjusting the transmission power of the base station according to the test results to achieve the balance of uplink and downlink within the coverage area.

2. Capacity considerations in PHS network optimization

The standard PHS base station supports 1 control channel corresponding to 3 service channels, called 1C3T. With the increase in the number of base station channels required by dense traffic areas, 1C7T and 1C15T base stations have gradually emerged using group control technology. In the process of capacity optimization, the amount of traffic in the coverage area of ​​the base station must be considered. The distribution of traffic volume is related to the time and place. The busy traffic situation should be considered in the design. 1C3T base station can carry 0.899erl traffic at 5% blocking rate, 1C7T base station can carry 3.738erl traffic at 5% blocking rate, 1C15T base station can carry 10.633erl traffic at 5% blocking rate . The total traffic in the coverage area can be calculated by the formula: total traffic (erl) = effective area × number of people per square meter × PS ownership rate × average traffic (usually 0.02erl). It can be seen that the larger the coverage area of ​​the base station, the larger the total traffic in the coverage area and the smaller the capacity per unit area. Therefore, the size of the coverage area of ​​the base station should be designed based on the actual traffic volume in the area. In the capacity design, the switching factors must also be considered, and a certain margin should be reserved for the PS switching to ensure the success rate of the PS switching. When optimizing capacity, in order to take into account the communication quality of the network and the cost of network construction, it is generally reasonable to control the capacity utilization rate in the coverage area (total busy traffic divided by the capacity of the wireless network) within 60% to 75%. If the capacity utilization of the coverage area exceeds 75%, the network needs to be expanded. Capacity expansion can be achieved by cell splitting, or by group control technology to increase the number of base station traffic channels, such as the 1C7T and 1C15T mentioned above.

3. Solution to the handover problem in PHS network optimization

PHS switching is hard switching, that is, after the coverage edge signal is weak to a certain degree, the PS will first disconnect the original link and search for a stronger signal. This process takes about 1 to 2 seconds, which will cause voice Inconsistent, users can feel a short pause. PHS switching is divided into intra-CS switching and inter-CS switching. TCH switching mainly considers the bit error rate (threshold value 1 × 10E-6) and channel busy and idle; the switching between CS mainly considers the switching threshold level (the reference value of the broken link threshold: 26dBμV, the link establishment threshold Reference value: 32dBμV), while also considering the bit error rate and the busyness of the channel. Handover should ensure that mobile users can continuously communicate with acceptable communication quality during the movement. At present, the main problem encountered in PHS handover optimization is frequent handover. The reason for this problem is that PHS base stations are low-power base stations themselves and have a small coverage area. In the past, network construction used 10mw and 20mw low-power base stations. It adopts high-power base stations (500mw, 300mw) and low-power base stations to expand coverage and balance capacity, so as to appropriately reduce the frequency of handovers. Second, the signal levels of different base stations received in certain multi-base station coverage areas are all lower than the switching threshold level, resulting in frequent switching. Generally, the number of handovers within 15s does not exceed 3, and the time for each switch does not exceed 1.2s, which can ensure that the voice is clear during the handover, and the subjective feeling of the handover is not obvious; the number of handovers is 3 to 5 times within 15s, and there are handover requests exceeding 1.2s , Then subjectively feel the handover, but it has little effect on the continuity of the call; if the number of handovers exceeds 15 times within 15s and there are more than 1.2 seconds of handover requests, frequent handovers will affect the continuity of the call, and the user feels that the call quality is not good it is good. The solutions to this problem mainly include: adding a base station to increase the signal level in the area, or using a directional antenna to cover the area at a closer base station, or adjusting the switching threshold of each base station according to the distribution of the base station Level. At present, PHS has introduced seamless switching technology to improve the switching speed, but this requires PS support, and a large number of users still use the old PS, so we must continue to do the traditional PHS network switching optimization work, to a large number of old customers Provide quality service.

4. Principles for setting paging area in PHS network optimization

The paging area is composed of an area covered by a group of base stations, and the base stations in the area have the same paging area number. The network side will register the paging area where the PS is located. When the PS is called, the network side will cause all CSs in the paging area where the PS is located to send paging messages. Considering the processing capacity and network resource utilization rate of the PHS network, the maximum number of paging per hour of the PHS network is designed from 5000 (ZTE and Lucent ’s PHS network) to 8000 (UT Starcom ’s PHS network), so the paging area The design must consider the size of the traffic, which cannot be designed too large; and the paging area is too small will cause PS to frequently perform cross-region location registration, location registration will occupy CCH and TCH, which relatively reduces wireless resources and reduces The possibility of other PS switching success. Therefore, it is necessary to comprehensively consider these two factors to rationally design the size of the paging area. When optimizing the paging area, you can start from the following aspects: First, the border of the paging area should be selected in the area with low traffic density as far as possible, avoiding the intersection of intersections, trunk roads and other users with high density and frequent user flow Place; secondly, the paging area should not be long and narrow, preferably close to a regular hexagon, so as to reduce the probability of over-location registration; Finally, because the border area of ​​the paging area is poor for the called PS, the quality of service is required. Avoid falling into units that require high service quality, such as government agencies and management departments.

5. Resolution of interference problems in PHS network optimization

PHS interference is divided into intra-system interference and inter-system interference. The interference within the system is divided into adjacent frequency interference and co-frequency interference. Adjacent frequency interference is mainly solved by ensuring that the equipment has good adjacent channel selectivity (≥50dB). Co-channel interference is mainly solved through network optimization. Co-channel interference includes CCH co-channel interference and TCH co-channel interference. The base station density is too large and the antenna downtilt angle is too small is the main cause of co-channel interference. To solve this problem, it is necessary to appropriately reduce the transmit power of the antenna and reduce the coverage of the base station; or appropriately reduce the height of the base station antenna and adjust the downtilt angle of the antenna. This method can reduce the co-channel interference of CCH and TCH at the same time. The co-channel interference of TCH mainly depends on the interference avoidance mechanism. The interference avoidance mechanism reduces the TCH co-frequency interference by reallocating a frequency point or time slot with less interference for users. Interference in the system also includes the interference of high-power base stations to low-power base stations in the PHS network system. This can be solved by using low-power base stations for indoor coverage (1.9GHz wireless signal penetration is poor). Inter-system interference includes PHS and 900MHz / 1800MHz GSM system edge frequency signal interference, as well as frequency interference with future 3G systems. What needs to be solved at present is the interference with the GSM system. According to experience, when the PHS base station is facing the radiation direction of the main lobe of the GSM base station, the distance between the PHS base station and the GSM1800MHz base station is preferably more than 50 meters, and the distance from the GSM900MHz base station is preferably more than 20 meters; when it is not in the radiation direction of the main lobe of the GSM base station, It is best to be more than 10 meters apart.

6. Base station synchronization in PHS network optimization

Base station synchronization is a key issue in the PHS system. Because PHS uses TDD duplex mode, if the base stations are not synchronized, it will cause misalignment of the CCH time slots between the base stations and co-channel interference of the control channel, which seriously affects the normal operation of the base station. . The synchronization accuracy between base stations requires that the time difference between the base stations does not exceed 31.2 μs. If this value is exceeded, the base station is considered to be out of synchronization, and the base station will cause frequency interference to other neighboring base stations. PHS synchronization includes GPS synchronization and PSC synchronization. The purpose of GPS synchronization is to correct the phase of the wireless frame. The synchronization of the PSC is to control and maintain the phase of the wireless frame that has obtained the unified synchronization starting point through the line clock. In order to ensure the GPS synchronization effect, it is recommended that each Master base station has about 40 Slave base stations under normal circumstances, which can be increased to 50-60 Slave base stations in high-density areas. Only one Master base station can be installed in the low-density area. To improve the system's ability to resist catastrophe in the high-density area, it is recommended to add a Master base station as a backup.

Third, the PHS typical coverage area optimization method

PHS network optimization can be divided into outdoor dense traffic area optimization, outdoor sparse traffic area optimization, and indoor coverage optimization according to the type of coverage area. The following is an analysis of the optimization of these three typical types of coverage area.

1. Optimization of outdoor dense traffic area

Outdoor dense traffic area bears a large proportion of traffic, so the optimization of outdoor dense traffic area is a key point of PHS network optimization. Pay attention to the following aspects in optimization:

1) 500mw high-power base station coverage should be used, and the overlapping area of ​​adjacent base station coverage should not be too large to avoid excessive handover.

2) The power of the CCH channel and TCH channel of the base station can be adjusted so that PCCH ≤ PTCH, so that the CCH signal at the edge of the coverage area of ​​the base station is weak, and the PS is not easy to access. The PS in the call can ensure the quality of the call due to the strong TCH signal, and it is not easy to drop the call.

3) The boundary of the paging area should avoid falling into the outdoor dense traffic area, so as to avoid excessive channel resources for location update.

4) Adjust the size of the paging area in time, and split the cells with too high paging times.

5) The outdoor dense traffic area should adopt group control and layered coverage, which can reduce the probability of control channel interference.

6) The solution of the synchronization problem is the key. If the synchronization is not done well, the system will cause serious interference.

2. Optimization of outdoor sparse traffic area

For sparse traffic areas such as non-prosperous areas in cities, urban-rural integration areas, townships, and roads between urban and rural areas. You can try to increase the coverage area of ​​the base station to effectively reduce the cost of network construction. Specifically, a PHS base station amplifier can be used. For example, the CHOSEN-PHS series base station power amplifier can enhance the base station uplink signal by 16-18dB. When the base station antenna tilt is 5 degrees, the coverage radius can be expanded to about 2 kilometers. This improves the coverage radius of a single base station, reduces the number of cross-zone handovers, and improves the network's calling effect. For narrow and long areas such as highways, 500mW directional antennas can be used for coverage to reduce the switching frequency.

3. Optimization of indoor coverage

When PHS is used for indoor coverage, low-power base stations are mainly used, so that both coverage can be achieved and channel problems can be solved; proper use of repeaters can introduce outdoor signals into the indoor and can also cover indoor blind areas, but direct Stations cannot increase the channel; for the ground floor buildings, outdoor base stations can be built to cover the interior through signal penetration; for elevators, narrow corridors, etc., directional antennas can be used for coverage; for crowded shopping malls, supermarkets, etc. Ceiling antenna for coverage; for small restaurants and private rooms, 10m and 20m long feeders can be used to avoid the barrier of the wall to introduce signals into each room; for each floor of the building, power dividers, trunk amplifiers, combiners, Couplers, antennas and other devices separate and amplify signals, and achieve ideal coverage by building indoor distribution systems.

4. Summary

An important reason for the current PHS to attract customers is low tariffs, but the quality of the network is not satisfactory. To consolidate the customer base of PHS, it is necessary to strengthen the improvement of the communication quality of the PHS network. This requires constant attention to the PHS network. For optimization, adjust the network in time according to changes in environment, number of users, and business volume, effectively improve the coverage effect and user satisfaction of the PHS network, and enhance the market competitiveness of PHS.

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