In the multiband networking, one of the most important purposes is to try to let GSM1800 network absorb or share traffic so as to satisfy the increasing requirement of network capacity and quality. The following principles should be followed.
z In the early stage of multiband network construction, try to let GSM1800 cells absorb multiband subscribers.
z Realize the continuous coverage of GSM1800 network in hot spot areas.
z When the number of multiband subscribers reaches a certain level, use different bands to share traffic thus to reduce handover and provide better service.
The carrier can realize different traffic control strategy through real-time adjustment of related parameters.
Different traffic control methods are used for different MS states. GSM1800 cell can have higher priority or better adjacent cell measurement comparison value through the configuration of system parameters. So when the subscriber turns on the mobile to select cell in idle mode or reselects cell in standby state, GSM1800 cell can be more likely to be the serving cell for multiband subscribers. In this way, the subscriber is more likely to wait at GSM1800 before a call connection; during the connection of MS call, the traffic distribution can be adjusted by directed retry. In connected state, try to connect as much as possible traffic to high level GSM1800 cells in lower layers through cell hierarchy and specifying different hierarchical cell structures (HCS); the multiband traffic handover can be used to make traffic load more rational.
The following describes in detail the cell selection, cell reselection, directed retry, cell hierarchy and specifying HCS, and multiband handover.
1) Cell Selection and Cell Reselection
In idle mode, the system guides the traffic absorption by controlling the process of MS cell selection and cell reselection.
When MS turns on, it first needs to select cell so that to confirm its serving cell. Principle of cell selection: cells allowing to be accessed and cells with high priority are first selected; for the cells with the same priorities, the cell with maximum C1 value is first selected. The C1 value of selected cell should be greater than zero. C1 value is calculated as follows:
( )
(
_ _ _ ,0)
_ _
1 RxLEV RxLEV Access MIN MAX MS TxPWR MAX CCH P
C = − − −
RxLEV Access MIN range: 0~63, 0 is corresponding with -110dBm, 63 is corresponding with -47dBm.
MS TxPWR MAX CCH value range:
z GSM900: 0~19 available, 0 is corresponding to 43dBm, 1 is corresponding to 41dBm. The value of higher level is 2dB greater than that of lower level.
z GSM1800: 0~15 available, 0 is corresponding to 30dBm, 1 is corresponding to 28dBm. Step is 2dB.
In the multiband network, owing to the strong fading of signals in GSM1800 frequency band, signals in GSM900 frequency band is stronger. In order to enable MS can be accessed to GSM1800 system, the cell selection priority can be controlled by setting value of cell bar qualify (CBQ) and cell bar access (CBA). The signals in GSM1800 cell are generally weaker than that in GSM900 cell. To enable the multiband MS to select GSM1800 cell preferentially, GSM1800 cell can be set as Normal and GSM900 cell as Low.
Table 2-15 Cell selection/reselection hierarchy
Case CBQ CBA Cell selection Cell reselection
1 0 0 Normal Normal
2 0 1 Barred Barred
3 1 0 Low Normal
4 1 1 Low Normal
When selecting cell, GSM900 cell is set as CBQ=1, CBA=0 and GSM1800 is set as CBQ=0, CBA=0. This enables GSM1800 cell to have a higher priority.
After MS completes cell selection, it should reselect cell in standby state in order to select a better serving cell. The parameter that decides cell reselection is C2. MS reselection principle is to select the cell with maximum C2 value as the serving cell. C2 depends on the following factors:
z C2=C1+CRO-TO×H(PT-T) (PT<31)
z C2=C1-CRO (PT=31)
Where, the value Cell Reselection Offset (CRO) decides the difficulty of cell reselection and Temporary Offset (TO) functions within penalty time (PT).
CRO value can be 0, 1, ↑ 63 with grade as unit, which are corresponding to 0=0dB;
1=2dB; 63=126dB respectively.
TO value can be 0, 1, and 7, which are corresponding to 0=0dB; 1=10dB; 6=60dB;
7=infinite respectively.
PT value can be 0, 1, and. 31, which are corresponding to 0=20s, 1=40s, and 30=620s respectively.
H ( ) = 0 if PT-T<0
C1 indicates the quality of radio channel. The greater C1 value, the better quality of channel. C2 is corrected manually. C2 value of each cell can be adjusted through CRO value. So the C2 value can be calculated according to CRO, TO, and PT so as to confirm the cell reselected for MS. That is to say, C2 value of GSM1800 cell can be greater than that of GSM900 cell by setting parameters that can affect C2 value, such as CRO. Therefore, though signals in GSM1800 cell are weaker that of GSM900 cell, GSM1800 cell can still be reselected for MS by setting parameters.
Parameters of cell selection and reselection can be flexibly used to control MS to select GSM1800 network as required in network planning; under the precondition that network quality is guaranteed, these parameters can be used to make MS establish calls in GSM1800 network so as to share the load of GSM900 network.
2) Directed retry
Provided that the process to initiate a call by an MS has completed switching, connection, control of some signaling and it is time to for SDCCH to assign TCH so as to connect the speech channel of both parties. However, it is found that the TCH of this cell is full. In this case, directed retry can be used to assign TCH of adjacent cells for MS from SDCCH thus to guarantee the successful connection. At the same time, the traffic is shared.
3) Layers and levels of network
Under the connected state, traffic between frequency bands can be distributed rationally through abundant Huawei multiband handover This is the core of multiband traffic guide and control strategy.
Huawei hierarchy handover algorithm divides a cell into 4 layers each layer with 16 levels. This meets the need of complicated networking circumstances. The design concept of this hierarchy has fully considered the collaboration with the current network equipment and the requirement of future network development. The cell layers and levels are as shown in Figure 2-37.
GSM 900
GSM 900 GSM 900 GSM 900
GSM900
Figure 2-37 Cell layers and levels
The GSM system covering the same area is divided into 4 layers. The high layer is the fourth layer, i. e. umbrella-like cellular, which is generally is a GSM900 cell with wide coverage. It has two functions: covering and quick connection of MS. The middle layer consists of GSM900 macro cells. These are the main cells of the system and most of subscribers gather in this layer. The followed layer consists of GSM1800 micro cells with small coverage. This layer is the main target for capacity expansion so as to solve the problem of short resource of frequencies. The bottom layer consists of GSM1800 Pico cells, which is to meet the requirements of hot spot and blind spot areas. For the priority, the cell in lower layer has a higher priority.
Considering the future network development, to make network planning and optimization more detailed and more flexible, the layer should combine with level division, that is to say, each layer should be divided into several levels. Each layer of these four layers is divided into 16 priorities.
For the description of handover, please refer to 2.3.6 . IV. Features of GSM1800
1) Propagation characteristics of GSM1800
The working frequency of GSM1800 is two times as that of GSM900. According to COST-231 model and practical experience, the propagation loss of GSM1800 inside stadia is 6dB greater than that of GSM900 and the propagation loss of GSM1800 outside stadia is 10dB greater than that of GSM900. The propagation loss inside buildings is 5~17dB higher (it varies from material to material). The fast fading of GSM1800 is a disadvantage to realize the fine coverage of GSM1800 and the condition of GSM1800 coverage is directly related with the performance of network. Moreover,
a) Outdoor coverage
The outdoor coverage can be easily realized if the distance between sites is not too far.
If necessary, besides the installation of GSM1800 equipment on the site of original GSM900 site, the new sites should be installed in proper places.
b) Indoor coverage
In order to guarantee the fine indoor coverage of GSM1800, the distance between BTSs in the city should not exceed 1000m. In the city with buildings in reinforced concrete structure, which penetration loss is very great, so it is recommended that the distance between BTSs should be 500~800m.
3) GSM1800 coverage mode
There are three coverage modes for GSM1800 network in multiband network: Fine continuous coverage, continuous coverage of hot spot areas, scattered coverage of hot spot areas.
a) Fine continuous coverage
This coverage mode has the following advantages: GSM1800 is easy to absorb traffic and has less handovers and high quality of operation; the frequency planning and network optimization is easy to be realized and the traffic distribution is easy to be controlled; after sites are constructed, if capacity expansion is needed, it is only needs to configure carrier instead of constructing new sites; and it is convenient to be constructed and maintained. The disadvantage is that the investment is large and it is hard to select sites in one time.
b) Continuous coverage of hot spot areas
This coverage mode has the following disadvantages: the traffic absorption of GSM1800 is limited and there are frequent multiband handovers; strict requirement for locating traffic hot spot; it is hard to plan frequencies and optimize network due to the irregular distribution of GSM1800 BTSs. The construction and maintenance is complicated. The advantage is that the site in highly intense areas can be gradual constructed so as to save the investment.
c) Scattered coverage of hot spot areas
This coverage mode has the following disadvantages: the traffic absorption of GSM1800 is low and there are frequent multiband handovers; strict requirement for locating traffic hot spot; it is hard to plan frequencies and optimize network due to the irregular distribution of GSM1800 BTSs. The construction and maintenance is complicated. The advantage is that the initial investment is small.