This article analyzes the causes of network congestion in detail, and discusses some methods to solve the congestion from the base station hardware and network parameters. And by optimizing the description of the example, it can effectively reduce network congestion and increase traffic. The main lobe points to the high traffic area, which can balance the traffic distribution; strengthen the signal strength of the coverage area, enhance the carrier-to-interference ratio of the useful signal; deviate from the same frequency cell, effectively control interference; adjust the direction of the three cells of the directional station to avoid the cell The "crossover" phenomenon of the signal avoids the problem that the difference between the idle signal of the mobile phone and the signal at the time of connection is too large due to the imbalance of the upstream and downstream signals. D-sub Connector Contacts D-SUB coaxial contact,D-Sub Connectors Contacts,D-Sub Plug Connectors Contact,D-Sub Receptacle Connectors Contact ShenZhen Antenk Electronics Co,Ltd , https://www.antenkwire.com
<Keywords> Network optimization; wireless parameters; hardware adjustment; traffic balance; GSM;
A preface
With the development of the mobile communication industry and the proximity of China's accession to the WTO, the introduction of competition systems for operating companies and the entry of foreign capital, the result of network optimization is to directly address users and the market. Therefore, network optimization is particularly important for the competition of operating companies in the mobile market important. The quality of the network quality can be reflected in the following indicators: wireless call drop rate, channel blocking rate, call quality, etc., where network blocking will bring great inconvenience to the normal communication of mobile users and become the focus of user complaints. At the same time, as an important indicator for assessing the operation of the network, excessive network congestion will also cause the cracking of other network indicators. For example, excessive congestion will lead to high call drop, low handover success rate, reduced network connection rate, and users' inability Network failures such as Internet access, and excessive network congestion will prevent the normal absorption of traffic, resulting in operators unable to make full use of existing network resources to obtain higher economic benefits. Therefore, how to reduce congestion and improve network quality has become the focus of network optimization.
This paper combines the high congestion situation of Qingdao Unicom's actual network and the experience in network optimization at ordinary times, analyzes the causes of network congestion failures and proposes corresponding solutions.
Two: Causes of network congestion
2.1 Classification of wireless network congestion
In general, wireless network congestion can be attributed to the following two situations: when there is no signaling channel available in the immediate assignment, that is, SDCCH congestion, and the other case is that when the voice channel is assigned, the network does not have a traffic channel available, that is, TCH congestion .
2.2 Causes of wireless network congestion
2.2.1 Network equipment failure
There are two common network equipment failures: base station equipment failure and transmission failure.
2.2.1.1 Base station equipment failure
Partial TCH and MBCCH are blocked due to unstable PA or TPU operation or device damage. Damaged antenna feeder or poor internal and external connector contact causes the equipment to send and receive abnormally, or the base station combiner DUCOM / FICOM standing wave ratio is too high, high and low power amplifier Mixed use of PA equipment (HPA and MPA, PA25), etc.
2.2.1.2 Transmission failure
When the transmission of the ABIS port (PCMB) is momentarily interrupted or there is a high bit error rate, at this time, because the fault has to be transmitted to the BSC in the future, resulting in the BSC channel activation, due to the unavailability of the ground circuit resources, the The event counts towards blocking. At the same time, due to the failure and the future upload to the MSC, the BSC will also send a hand-in request, resulting in too many hand-in requests failing.
2.2.2 Unreasonable parameter setting of BSC database
2.2.2.1 Unreasonable parameter setting
The setting problem of BSC TIMER: BSCT7 and BSCT8 are set too long, so that the original channel cannot be released after switching or the time interval between the two switches is too long, causing unnecessary waste of system resources and causing waste of system resources; unreasonable setting of queuing parameters; unreasonable setting of switching threshold homargin ; The minimum cell access level rxlevelacessmin is unreasonable; the BTS transmit power setting is unreasonable; the relevant parameters of the C1 and C2 algorithms are improperly set; the ratio of the signaling channel and the traffic channel is unreasonably configured; it is located at the border of the LAC, BSC, and MSC CELLRESHYS parameter setting is unreasonable.
2.2.2.2 Problems with neighboring cells
It is mainly manifested in the hardware failure of the neighboring cell or the interference of the neighboring cell, which makes it impossible to cut in or cut out; the neighbor relationship of the missing serving cell makes the call unable to cut out until the call is dropped.
2.2.3 Too many authentication times, frequent registration of illegal users, and unreasonable location area boundaries
If the switch authentication times are set too much or users who do not have roaming authority (mainly some local users frequently go to the Internet frequently), if they remain powered on in a restricted area, they will continue to register on the network, but always Failure to authenticate will greatly increase the signaling load.
At the same time, the cells located at the borders of LAC, MSC and BSC will increase the signaling load due to frequent location updates or roaming parameters not set.
2.2.4 The design of traffic density exceeds expectations
Due to changes in the user base, congestion has occurred in some hotspots where the traffic exceeded the expected network plan.
2.2.5 Island island phenomenon
Due to the early stage of network construction, operators consider the base station construction height to be higher for expanding network coverage with less investment. As the network continues to expand, this situation will lead to more serious coverage of the base station, and It will inevitably cause the mobile station to remain in the calling service cell all the time. No matter how the surrounding signals change, due to the complexity of the neighbor relationship, it cannot be switched normally until the call is dropped.
2.2.6 Reasons for network coverage
Excessive coverage of the network will often cause network congestion: mainly due to unreasonable network settings on the hardware or excessively high base station antennas and unreasonable pitch angles; software may be due to the minimum network access level rexlevelacessmin setting is too small, BTS power setting Too large; in the wireless propagation environment, it may be caused by the spread of the propagation environment of certain cells of some base stations and the large number of users.
With the change of network capacity and wireless propagation environment, the situation of network congestion will continue to change, so the adjustment of the network must be continuously adjusted according to the wireless situation.
Three: Strategies and methods of traffic balance
For the adjustment method of network traffic, the SIEMENS base station system provides a rich set of methods to balance the traffic. This article mainly discusses the balance of traffic from two aspects of network hardware and parameter adjustment.
3.1 Hardware adjustment
Hardware adjustment is an extremely effective method to solve TCH and SDCCH blocking and balance cell traffic. Mainly through network expansion, adding microcells, station type adjustment, adjusting base station height, antenna direction and pitch angle to achieve balanced traffic 1. The purpose of absorbing traffic.
3.1.1 Network expansion
Network capacity expansion is the most effective way to solve network congestion. Adding base stations in busy traffic areas or expanding capacity in busy cells can quickly achieve the goal of reducing TCH congestion. However, with the increase of network capacity and the limitation of Unicom frequency resources, And the difficulty of project implementation will be limited.
For the expansion of the network, in addition to adding hardware, the use of new frequency reuse methods or the use of integrated frequency hopping is an effective method for solving the limitation of China Unicom's frequency resources and increasing network capacity.
3.1.2 Base station type adjustment
Due to the uneven distribution of users, the distribution of cell traffic is often severely unbalanced. In different cells of the same base station or different base stations in the same location, busy cells have tight resource resources and serious congestion, while idle cells have idle TCH and SDCCH and excess resources . In order to alleviate this contradiction to a certain extent, the station type of the planned base station can be adjusted. During our optimization process, due to the serious blockage of the base station, large-scale capacity expansion was carried out in areas with relatively loose frequency resources. A total of 25 TRXs were expanded to effectively improve the blocking situation. The following table shows the base stations before and after the expansion of Qilu cell 10051 Performance comparison.
Table 1 Comparison of wireless indicators before and after base station capacity expansion 
It can be seen from the above table that not only the blocking rate is reduced after the configuration adjustment, but also the wireless call drop rate and the handover success rate are improved to varying degrees.
3.1.3 Base station antenna adjustment
The adjustment of the base station antenna involves multiple aspects, and by adjusting the antenna to change the coverage of the base station, the effect of improving network blocking is also obvious. The antenna adjustment is mainly modified from the following aspects: the effective height of the antenna, the direction of the main lobe of the antenna, Changes in antenna pitch angle, etc.
3.1.3.1 Base station antenna height
Changing the height of the antenna is more obvious in network optimization for controlling coverage and absorbing traffic. For base stations with an early height that is too high, the height can be adjusted by reducing the height of the antenna layer and the actual effective height of the antenna. Theoretical analysis: h, h1, h2 represent the actual height of the base station cell antenna, the actual effective height after adjustment, and the antenna drop height. The antenna height gains before and after adjustment are:
â–³ b = 20log [(h1 + h2) / h] â–³ a = 20log (h1 / h), then the attenuation of the adjusted signal is 20log [h1 / (h1 + h2)] dB, this value has only theoretical reference It does not consider the obstruction of the actual building. It can be seen from the above: reducing the antenna height can significantly reduce the signal strength in the coverage area.
3.1.3.2 Principle of antenna main lobe direction adjustment
Table 2 Comparison of the adjusted effects of 2/2/2 Haier base stations in urban areas 
It can be seen from the table that the change of the antenna direction can effectively balance the traffic, but pay attention to the change of the neighboring relationship with the surrounding base stations and the degree of frequency interference during the adjustment process.
3.1.3.3 Antenna pitch adjustment
During the adjustment of the antenna pitch angle, it should be noted that the difference between the electronic tilt angle and the mechanical tilt angle: when the electrical tilt angle is used, the directional pattern can still maintain its original shape as the tilt angle increases; but the mechanical tilt angle is tilted down The signal strength of the area corresponding to the main beam of the antenna decreases rapidly. When the downtilt angle increases to a certain value, the front-to-back radiation ratio of the antenna should be considered. At this time, the coverage area corresponding to the main beam gradually sinks, and the side lobe gain increases. We usually discuss The fission of the pattern is not caused by excessive mechanical tilt angle depression, but by the increase of antenna side lobe gain. Theoretically, if the horizontal antenna is tilted down by 1 degree, the signal attenuation is 3.5dB, and if the antenna is tilted down by 10 Degree, the signal attenuation is 4dB, that is, the attenuation in the horizontal direction is not significant, but in the actual cell actually covering the ground, the received signal increases by about 8 ~ 12dB.
3.1.4 Add Micro Cell
The increase in microcells is mainly used to attract the traffic of macrocells, which can be considered as cells in the cell. SIEMENS provides a variety of microcellular equipment: from BS11 for 2 TRX to EMICO for 4 TRX. In places with large crowds, such as hotels, shopping malls, and stations, the transmission power of microcells is relatively small. Considering the closedness of the wireless environment in these places, microcells can be easily added to solve the traffic.
3.2 Adjustment of software parameters
In practical applications, the adjustment of the hardware is often restricted, and there are certain difficulties in the implementation of the project, or it is difficult to achieve the desired effect from the hardware adjustment alone. At this time, if you make fine adjustments in conjunction with the BSC database parameters, you can achieve Ideally, familiarity with BSC parameters is also a key task in network optimization. This article mainly discusses from the following aspects.
3.2.1 Changing the channel configuration of a cell
By changing the channel configuration of the cell, the blocking can be significantly improved, just like the expansion of the base station hardware. For example: 2 TRXs, according to 5% call loss, the traffic load that can be handled in a network configured with 13 TCH channels is 8.9erl, and when configured according to 14 TCH channels, it can handle 9.7erl.
According to the SIEMENS BSC CHANNELCONFIG and ETSI GSM specifications, the control channel configuration is as follows:
â‘ MAINBCCH = FCCH + SCH + BCCH + CCCH
â‘¡ MBCCHC = FCH + SCH + BCCH + CCCH + SDCCH / C4 (0..3) + SACCH / C4 (0..3)
③ SDCCH = SDCCH / C8 (0… 8) + SACCH (0… 7)
3.2.1.1 For cells with 2 TRXs, the configuration modification principles are as follows:
Table 3 2 TRX cell channel configuration principle table 
It can be seen from Table 3 that TCH blocking can be significantly reduced through channel adjustment, but during the adjustment process, it is necessary to pay attention to the MS call situation in the cell. In the case of severe network blocking, there are certain adjustments to this method. Risk, because the substantial decline in the number of RACH SLOT will cause users to be unable to make calls during peak traffic.
3.2.1.2 For three TRX cells, because two SDCCHs (16SDCCH) are configured, generally SDCCH will not block, and usually one SDCCH can be omitted to add a TCH, and the MAINBCCH is adjusted to MBCCHC. At this time, there is a total of SDCCH (12SDCCH).
3.2.2 Adjustment of C1 and C2 cell selection parameters
In the SIEMENS parameter design, the parameter for cell access is: rxlevmin. For high traffic areas, this value can be appropriately increased to logically reduce the coverage area to prevent blocking, but be careful not to set it too high, otherwise it will be in the cell A blind spot was created at the junction, causing dropped calls.
Currently, the MS selects the serving cell by calculating the size of the C1 value, and the MS will select the C1 value size area as the serving cell. The calculation formula of C1 is not repeated here. In it, we can see that by adjusting the BTS transmit power and the size of rxlevmin, the C1 value is reduced to reduce the user's selection opportunities, thereby reducing traffic.
In the SIEMENS system, based on C1, the C2 value is calculated to select the cell. The calculation formula for C2 is:
C2 = C1 + cell_reselecTIon_offset-temp_offset * A (penTIme is 0-31)
* [A = 0 (penTIme-T <0); A = 1 (pentime-T> 0)]; After activating the C2 algorithm, you can optimize the traffic load of the cell by adjusting the parameters PENTIME and CRO, and set CRO to negative Offset. You can also set the cell's first access level CRQ parameter to adjust. The normal level is generally set to 0, allowing MS access, but in some special cases can be set to 1 that does not allow MS access to restrict traffic.
The access level of each user is restricted in the GSM specification, which is divided into 10 types from 0 to 9 and stored in the SIM card. This parameter is provided in the system. Setting this parameter reasonably in real time can alleviate traffic jams. But in order to avoid that some users are often restricted, this parameter should be changed frequently.
3.2.3 Adjustment of switching parameters
Since handover can transfer traffic from one cell to another, six types of handover judgment algorithms are defined in the SIEMENS system, and reasonable setting of handover tolerance and handover threshold can balance traffic well.
According to the simple conditions of switching (QUALITY, RXLEVEL, DISTANCE, INTERFERENCE, POWERBUDGET), we can adjust the switching threshold to adjust the switching tolerance to balance the traffic. In the switching adjustment, the HOMARGIN parameter is to adjust the traffic comparison Effective parameters. In the ADJC setting, we can lower the HOMARGIN parameter threshold of idle cells, so that the call can be easily switched from the busy cell to the super idle cell and can prevent the occurrence of ping-pong handover, because ping-pong handover will not only increase the number of dropped calls Possibilities, and because the TCH resources in both the serving cell and the target cell will be occupied at the same time during the T3103 handover.
3.2.4 BSC timer (TIMER) adjustment and other parameter adjustment
Adjusting T3107 and T3103 can reduce the waste of TCH resources; the improvement of T3212 can reduce the load of periodic location updates on signaling, but the setting of this value must be less than the implicit shutdown time of MSC and note that this parameter must be set consistently across the network BSC ; Increasing T3122 can prevent users from frequently sending channel request information when the system has no resources to unnecessarily increase the load on the network RACH and CCCH.
At the same time, according to the quality of the wireless conditions, MAXRETR (maximum number of retransmissions) can be appropriately adjusted to adjust the size of the signaling load and the blocking situation. On the boundary problem of LAC area, the size of CELLRESH (reselection lag) can be appropriately increased to minimize the blocking of control signaling due to cell selection. Activating the queuing function has a significant effect on congestion caused by short-term traffic spikes.
The adjustment of the network parameters must be combined with the current wireless propagation environment and the actual network operating state. After the parameter adjustments, on-site testing and further fine-tuning must be performed until the optimal operating state is reached.
Four: conclusion
Network optimization is a long-term and meticulous work, and it has the importance of directly facing users and the market. Network optimization not only requires the attention of leaders, but also requires network optimization technicians to actively and proactively work to strive for every wireless The parameters are carefully studied to understand its effects and side effects. At the same time, the antenna parameters are carefully adjusted according to the actual situation. Reasonably adopting the methods introduced in this article can improve the network performance to a large extent and solve the network congestion. The congested and lost traffic is re-absorbed to achieve the purpose of increasing traffic. More importantly, reducing congestion will increase users' confidence in the network, improve the economic efficiency of the operating company, and enhance the company's competitive strength.
references
â‘ Han Binjie. GSM principle and network optimization. Beijing: Mechanical Industry Press, 2001 edition
â‘¡ Liao Ming. Methods to improve network congestion and increase traffic. Mobile communications. Number 4, 2000
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