In recent years, the digital mobile communication GSM network has developed rapidly in China. As of August 1999, the national GSM users have exceeded 20 million. According to conservative estimates, the number of mobile users will be close to fixed users in 2005, reaching 100 million users, of which GSM users will exceed 80 million, and users will be more concentrated. However, the user's rapid increase has also put tremendous pressure on the development of the network. Due to the limitation of frequency resources, the maximum number of users that can be carried by GSM900 should be around 40 million, which can not meet the development needs of GSM network. Therefore, the GSM1800 system is widely used. Application has become an inevitable way of GSM network development. At present, construction and application of the GSM1800 system have been started in Beijing, Shanghai, Guangzhou, Fujian, and Jiangsu.
The GSM900 and GSM1800 dual-band networks are one of the most effective methods for integrating network capacity and network coverage. The implementation of its main functions depends greatly on the setting of GSM900/1800 dual-band wireless network parameters. These parameters mainly include Adjustment of control parameters such as cell selection/reselection and dual-frequency traffic switching. Based on the 900MHz and 1800MHz wireless propagation characteristics and the dual-frequency networking principle, this paper focuses on some of the dual-band wireless network parameters and their adjustment methods.
2 GSM900/1800 dual frequency network2.1 900MHz and 800MHz wireless propagation characteristics
The radio wave propagation paths of different frequency bands are different. For the wireless communication system of the 900 MHz frequency band, the radio wave propagation mode is mainly the space, that is, the composite wave of the direct wave and the reflected wave. Since mobile phones are constantly subject to fluctuations in buildings, trees, and other obstacles or terrain that are geometrically and sized differently around (a few meters to tens of meters), the variation in the received field strength is quite complicated. This multipath combination of scattering, reflection, diffraction, or shadows on a straight path formed by obstacles caused by the propagation path causes the amplitude, phase, and angle of the signal arriving at the cell phone to be random, and thus the cell phone sometimes only When moving a few meters, the receiving field may have large fluctuations. The rate of change of this fluctuation is related to the moving speed of the mobile phone and the wavelength of the electric wave. The 1800MHz radio wave wavelength is half of 900MHz, so the multipath fading and shadowing effects it suffers will be more obvious.
The 900MHz can be estimated using the typical okumura-Hata propagation model (applicable frequency: 150-100MHz), while the 1800MHz is suitable for the improved Oklumura-Hata propagation model (applicable frequency: 1500-2000MHz).
L(dB)=A-13.82logHb+(44.9-6.55logHb).logb-a(Hm)
Where Hb: base station antenna height [30-200m];
d: propagation distance [1-20km];
Hm: mobile phone antenna height [1-10m];
a(Hm)=3.2(1og11.755Hm)-4.9
According to the specification, the GSM1800 mobile phone transmission power is generally 30dBm, which is 3dB lower than the GSM900 mobile phone. For the same material day debt line system, the GSM1800's day debt line loss is 0-2dB higher than the GSM900, and the 1800MHz radio wave is blocked by the building. Severe, the penetration loss is greater than the 900MHz wave. Therefore, the coverage area of ​​the GSM1800 system is 40%-58% smaller than the GSM900. In rural areas, the base station coverage radius of GSM1800 is up to 10km; in urban areas, it is generally around 0.6-1.1km. It can be seen that the networking mode of GSM1800 is inevitably different from GSM900.
2.2 GSM900/1800 Dual Frequency Networking
In a dual-band network, how to control and distribute traffic in a network with two frequency bands is the basis for establishing the network construction principle and network operation. The introduction of the GSM1800 system can be performed by separate networking and hybrid networking of the GSM1800 and the existing GSM900 system. Separate networking requires separate GSM 1800 devices, such as MSC, BSC, and BTS, while hybrid networking shares MSC or BSC with GSM 900. Although the individual networking has better advantages in network structure, wireless planning and operation and maintenance, the 1800MHz radio wave propagation characteristics determine the area covering the same area. The GSM1800 base station is more than the GSM900. Therefore, from the perspective of investment cost and equipment utilization, some provinces with relatively tight frequency resources have adopted the hybrid networking method to develop GSM 1800. The GSM1800 solves the capacity problem of the high traffic area, and promotes the use of the GSM 900/1800 dual-band mobile phone, and uses the GSM 900 network to achieve seamless coverage. In this sense, the so-called dual-band wireless network is essentially a two-layer network structure based on the GSM 900 network and GSM1800 network coverage area. In order to fully utilize the role of the GSM1800 system and improve the traffic carrying capacity of the network, the following principles are generally observed in dual-frequency networking:
(1) Establish a call on a preferred frequency band. In the early stage of the establishment of the GSM1800 system, due to the small number of dual-band mobile phone users, more idle channels and higher load ratio, the dual-band mobile phone should enter the GSM1800 system as much as possible through reasonable equipment, and reduce the GSM900 system. load.
(2) In general, both switch to the preferred frequency band. For dual-frequency users whose mobile speed is not fast, they should be cut into the GSM1800/cell as much as possible during the handover process. When the GSM 800 mobile phone moves at high speed, it should be switched to the GSM900 overlay to reduce the switching frequency of the mobile phone. Improve communication quality.
(3) Try to keep the call on the preferred frequency band.
(4) After the cell becomes congested, the call should be switched to the preferred frequency band.
3 dual-band wireless network parametersThe phone has two working states, namely the Idle state and the AcTIve state. It is in the Idle state before the phone establishes a connection with the network, and is in the AcTIve state after the connection is established. The mobile phone in the Idle state performs two actions of "cell selection and reselection". The purpose is to enable the mobile phone to access an ideal cell by selecting and reselecting, thereby reducing the number of cell handovers during a call; the mobile phone in the AcTIve state It is possible to perform a "cell switching" action. Therefore, the wireless network parameters related to the dual-frequency network mainly include: cell selection, reselection parameters, and cell handover parameters.
3.1 Multi-band indication (MulTIhand Reportng)
The parameter “Multi-band indication†is used to inform the mobile phone to report the neighbor content of the two frequency bands. In the single-band GSM system, when the mobile phone reports the neighboring area measurement results to the network, it only needs to report the contents of the six neighboring areas with the strongest signal in one frequency band. In the dual-band network, since the mobile phone is required to enter the GSM1800 preferentially, it is required that the mobile phone not only according to the strength of the signal but also the frequency band of the signal when reporting the measurement result. Since the traffic of the GSM1800 system is small in the initial stage of networking, it is generally expected that the mobile station can work as much as possible in this frequency band. Therefore, the handover priority of the GSM1800 cell should be set to be higher, and the corresponding multi-band indication should be selected as "3". In a dual-band network, it is recommended to set this parameter to 2, that is, the mobile phone needs to report the measurement results of the three neighboring cells in the neighboring cell table that contain the strongest signal strength in GSM 900 and GSM 1800, and the NCC is known and allowed. The neighboring area of ​​the currently used frequency band is reported in the remaining position, and if there are remaining positions, the situation of the remaining neighboring areas is reported, regardless of the neighboring area being in that frequency band. The Multi-Band Indication (MBCR) is included in the information element "Neighbor Cell Description" and is transmitted in the system messages 2ter and 5ter broadcasted by each cell.
3.2 Cell selection
Cell selection refers to the process of selecting a suitable cell registration after the mobile phone is turned on. According to the GSM specification, the cell selection process uses the C1 criterion, from which a set of BCCH frequencies (C1 > 0) arranged in order of field strength is obtained.
C1=(RxLev-RxLevAccMin)-MAX(MsTx PwrMaxCCCH-P,0)
Among them, RxLev: average receiving level of mobile phones;
RxLevAccMin: minimum access level of the mobile phone;
MsTxPwrMaxCCCH: The maximum allowable transmit power of the hand;
P: The actual maximum transmit power of the mobile phone.
3.3 Cell level definition
The parameter cell restriction prohibition (CBQ) is used to define the level of the cell, and the parameter is included in the information unit "cell selection parameter", and is periodically sent to the mobile phone in the system message broadcasted by each cell. Phase1 mobile phones do not understand CBQ information, and mobile phones are directly registered in the largest cell of C1 (and most Phase1 mobile phones use C1 algorithm for cell reselection);
The Phase 2 mobile phone first selects the cell with the largest C1 in the high level (CBQ=0). When the cell with the high level is not found, the cell registration with the largest C1 is selected in the cell with the lower level. However, for the Phase 2 GSM900 mobile phone, each cell selection, before getting the message of the cell with no higher priority, must scan all the BCCH channels for judgment, and there are other methods in the cell reselection process. To control the choice of cell in the idle state of the mobile phone.
In the initial stage of network construction, GSM1800 is the preferred network, and CBQ can be set to 0 (normal level), and CBQ of GSM900 is set to 1 (low level). However, when a dual-band mobile phone enters the network, as long as there is a GSM1800 cell with a receiving level greater than the incoming network level, the mobile phone will select the GSM1800 cell to register for the network.
3.4 cell reselection
After monitoring the surrounding cells, the mobile phone network will continuously read the BCCH of the six strongest neighboring cells from its BA (BCCH Allocation) list for at least 5 minutes, and read the BSIC codes of the six neighboring cells for at least 30 seconds. Looking for a more appropriate cell registration, there is a process of cell reselection. The cell reselection is judged according to the C2 and C1 parameters. If the mobile phone calculates that the C2 value of a neighboring cell (located in the same location area as the current cell) exceeds the C2 value of the current cell of the mobile phone and maintains for more than 5 seconds, the mobile phone will start the cell. Re-select to enter the cell. If the mobile phone measures a cell that is not in the same location area as the current cell, and the calculated C2 value exceeds the sum of the current cell C2 value and the cell reselection hysteresis parameter, and maintains for more than 5 seconds, the mobile phone will initiate cell reselection. And enter the cell.
In idle mode, system message 2 contains the handover neighbor table information, system messages 2, 2bis and 2ter are broadcast through BCCH, parameters CRO, To and PT are transmitted in the system message broadcasted by each cell, and the cell serving the mobile phone is heavy. selected.
3.5 cell switching
The handover process in the dual-frequency network is basically the same as that in the original GSM900 network, that is, the steps of channel measurement, pre-processing, handover threshold decision, and post-selection cell table generation are required. The purpose of cell handover is to reduce dropped calls. Therefore, all emergency handover types are reserved in GSM900 and GSM1800. There are two types of handover between the same frequency bands: Emergency handover and Better Cell handever. . The former includes signal strength of the uplink and downlink, poor voice quality or large interference; the latter aims to optimize the network so that the call can be better maintained by another cell, which is a normal handover. In the case of GSM1800 and GSM900, an emergency handover mode is adopted, but in order to prevent the phenomenon of "ping-pong switching" between dual-frequency networks, the PBGT (Power Budget for Neithour Cell, GSM specification 05.08) switching of different frequency bands can be turned off, and Appropriate time and field strength penalty value.
The inter-network handover between GSM900 and GSM1800 is the main means to achieve dual-frequency traffic control. Under normal circumstances, the work of GSM1800 network to absorb dual-frequency traffic has been basically completed through cell selection and reselection, and only a small part of the work needs to be completed by inter-network handover. Different systems can have different switching implementation methods. The Alcatel system uses priority band switching. The Ericsson system uses HCS (cell hierarchical structure) as the main means of dual-frequency traffic control.
The switching conditions of the Alcatel system are as follows:
If the traffic of the current serving cell reaches the set threshold and the field strength threshold of the handover candidate cell reaches the set threshold, the dual band handset will switch to the priority band. Another important switching factor in dual-band networks is the switching caused by speed. Since the GSM1800 generally adopts a microcellular structure, it is sensitive to the moving speed of the mobile phone. The GSM1800 system should have speed measurement capability. When the speed of the mobile phone reaches a certain threshold in the connected state, it is forced to switch into the GSM900 cell, thereby reducing the number of handovers and improving the communication quality. The handover neighbor table is included in the system message 5, and the system message 5ter contains the neighbor table of the non-current service band, and the mobile phone obtains the neighbor table of the cell handover through the SACCH.
3.6 Cell Hierarchy (HSC)
The purpose of the HCS is to make the signal strength received by the mobile phone reach the threshold of the cell in the priority band specified by the system, and the cell in the priority band is used as the serving cell, regardless of whether the signal of the other band is stronger. The HCS algorithm sets three parameters: LEVEL, LEVTHR, and LEVHYST. The LEVEL parameter is used to distinguish the level, GSM1800 is 1 and GSM900 is 2; LEVTHR is an important parameter. When the GSM1800 signal is higher than LEVTHR+LEVHYST, no matter how high the GSM900 signal is, no switching occurs; when the GSM1800 signal is lower than LEVTHR-LEVHYST Only to switch to GSM900. Since there is no frequency interference between GSM1800 and GSM900, LEVTHR can take a lower value.
4 ConclusionThe types and number of wireless interface parameters of the GSM dual-band network are numerous, and the adjustment methods are also various. Mastering the meaning and adjustment range of these parameters is the basis for the development of dual-band wireless network optimization, and will play a positive role in the processing of border roaming problems and traffic balancing. With the further development and application of GSM1800 and dual-band networks, the parameters of dual-band wireless networks must be adaptively adjusted according to the actual situation. This paper only introduces some parameter settings of the dual-band wireless network based on the specification, and hopes to develop dual-band networks. The optimization work provides useful help, and it is inappropriate to get criticism and correction from your peers.
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