DCS1800（Digital Cellular System at 1800MHz），1800MHzDigital cellular system。our countrymobile communicationThe development is extremely rapid. On the one hand, due to the limitation of 900MHz frequency resources, some regions have experienced the phenomena of tight frequency resources, insufficient channels, and excessive traffic density;On the other hand, multipleFrequency reuseThere are also many limitations in increasing capacity by means of, multi-layer coverage, frequency hopping and other technologies.The above problems can be completely solved only through the traffic sharing of the DCS1800 network.
Cell selection parameters refer to the parameters for selecting the best cell to register after the mobile phone is powered on.according toGSMSpecification: C1 criterion is used for cell selection. C1 calculation formula: C1=(RxLev － RxLev_Access_Min) － MAX (ms_txpwr_max_cch － P, 0) where RxLev: average reception level of mobile phone;RxLev_Access_Min: cell parameter, the minimum access level of the cell;Ms_txpwr_max_cch: cell parameter, the maximum allowable transmission power of the mobile phone;P: The actual maximum transmission power of the mobile phone.
After the mobile phone is powered on, it scans the GSM frequency points one by one to get a group of BCCH frequency points arranged according to the field strength, and C1 is greater than zero. If C1 is less than zero, the mobile phone cannot access the cell.For the Phase1 mobile phone, the phone will select the largest cell C1 as its service cell.
Cell level parameter CBQ
The cell level parameter (CBQ) is used to define the cell level, which is used to control the cell selection of the mobile phone.This parameter is included in the "Cell Selection Parameters" message. It is periodically sent to the mobile phone in the system information broadcast by BCCH in each cell.CBQ=0 represents a Normal priority cell;CBQ=1 represents a low priority cell.
Phase1The mobile phone is not controlled by the CBQ parameters. The mobile phone is directly registered in the largest cell C1, and generally uses the C1 algorithm to reselect cells.
The Phase2 mobile phone is controlled by the CBQ parameters. The mobile phone first selects the largest cell C1 in the high level (CBQ=0), and then selects the largest cell C1 in the low level cell for registration when the high level cell cannot be found.For the GSM 900 mobile phone in Phase 2, each cell selection must scan all channels for judgment before the message of the existence of a high priority cell is received. In the process of cell reselection, other methods are used to control the cell selection of the mobile phone in the idle state.
In major citiesDCS1800The network has developed maturelyGSM900The cell level parameters can be set according to the density, specific channel configuration and corresponding traffic load, and the traffic distribution of the dual frequency network can be adjusted to reduce unnecessary congestion and improve the call quality.
According to the actual network installation, there are many cases where the DCS1800 and the GSM900 share the same site, but the number of carrier frequencies installed in the DCS1800 is less than that in the GSM900.We should not only let the DCS1800 absorb a certain amount of traffic, but also not reduce the call quality and generate congestion. Therefore, we recommend the following settings before adjustment and optimization: RxLev_Access_Min (900M)=12, RxLev_Access_Min (1800M)=18, CBQ (900)=1 (low level), CBQ (1800)=0 (normal level), that is, the cell level (CBQ) of the GSM 900 is set to low level,dual-band handset When accessing the network, as long as there is a DCS1800 cell with a reception level greater than the access level (RxLev_Access_Min), the phone will select the GSM1800 cell to register for access.For the single frequency GSM 900 mobile phone in Phase 2, select the largest C1 cell in the 900M cell to register for network access.
Cell reselection parameters
aboutPhase2Cell reselection of mobile phones should be judged according to C2 and C1 parameters.
In idle mode, system information 2 contains the information of the handover neighbor cell table. System information 2, 2bis, and 2ter are broadcast through BCCH. Parameters CRO, temporary_offset, and pentalty_time are sent in the system information broadcast by each cell.
C2=C1+CRO-TO * H, where CRO: cell_reselect_offset, cell reselection adjustment value;TO：temporary_offset，Cell reselection temporary adjustment value, one of cell reselection parameters;H: H=0, pentalty_time – T<0; H=1, pentalty_time – T>0; T: the time maintained by the strongest neighbor cell, pentalty_time: the penalty time, when the value is 31, C2=C1-CRO. When the time maintained by the strongest neighbor cell is longer than the manually set penalty time (pentalty_time), it is easier for the mobile phone to reselect the cell as a service cell.CRO setting: for DCS1800, CRO=3-5;At penalty_time (penalty time), DCS1800, penalty_time=1-2 (40-60 seconds);For a GSM900 cell with small coverage, you can set penalty_time=1 (40 seconds).
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Preferred frequency band parameters
Preferred frequency band parameters（band_preference）
The band_preference parameter is used to specify the frequency band to be switched first.If the handover between multiple layers of networks is allowed (i.e. mb_preference=1), it can be used to control the handover between dual frequencies by setting the frequency band for priority handover.band_preference=1(PGSM)；band_preference=2(EGSM)；band_preference=4(DCS1800)；band_preference=8(PCS1900)。
The multi band neighbor cell reporting parameter (multiband_reporting) is used to set the method and priority of the mobile phone reporting the multi band neighbor cell measurement report to the system.During the call, in order to make each frequency band receive fair treatment, we reserved certain seats for different frequency bands in the six reported adjacent cells. By controlling the measurement report distribution of multi band adjacent cells, we can control network handover and adjust the traffic distribution.
multiband_reporting=0，The default mode is to report in the general way. Only six adjacent cells are reported in the order of the strongest reception level, regardless of multi band factors.
multiband_reporting=1，At least one strongest adjacent cell is reported in each frequency band, and the rest is reported in the general way.
multiband_reporting=2，At least two strongest adjacent cells shall be reported in each frequency band, and the rest shall be reported in the general way.
By setting the multi band adjacent cell report parameters and the frequency band of priority handover, it can be used to control the handover between dual frequencies at the same time.
Preferred frequency band mode parameters
The band_preference_mode parameter is used to specify the way to switch to the priority band. It needs to be matched with the band_preference_mode parameter to achieve the control of traffic between multi-layer networks.
band_preference_mode=0，Handover to the handover candidate cell with the strongest reception level;
band_preference_mode=1，It is mainly used for dual band separate BCCH and cell traffic transfer when busy. In the case of SDCCH to TCH connection, the TCH in the preferred frequency band (band_preference) is occupied first, and the rest is switched to the adjacent cell with the strongest reception level;
band_preference_mode=2，During handover, the BSS gives priority to the adjacent cells with the strongest reception level in the preferred frequency band.
band_preference_mode=3，It is equivalent to 1+2.
band_preference_mode=4，After the mobile phone occupies the TCH, the BSS gives priority to the adjacent cells with the strongest reception level in the preferred frequency band.
In practical application, the optimal frequency band mode parameters should be set according to different situations.Modes 1, 2 and 4 are commonly used.
For example, set band_preference to 4, and optimize the switchthreshold(handover_margin). You can transfer part of the GSM900 traffic to the DCS1800.Because the receiving quality of the DCS1800 network is good, the quality switching threshold can also be adjusted. When the receiving quality of the mobile phone is poor, the channel switched to the DCS1800 can be improvedCall quality。
