Soft capacity and soft cover planning for WCDMA

The second generation mobile communication system GSM mainly uses the time Division/Frequency Division multiple access wireless accessing way, its wireless network plan generally can divide into two steps. The first step is to forecast the loss of wireless path in order to ensure the wireless coverage of the required business area. The second step is to determine the capacity of the system by frequency planning based on estimates of the fields business volume. Usually in the wireless planning of GSM system, the main consideration is voice service and a small amount of data service (such as SMS service), so the capacity of the cell is a constant for the given channel quantity. Therefore, in GSM wireless planning, coverage and capacity planning can be carried out independently.

In WCDMA system, the concept of multimedia service and different QoS for each kind of business is introduced. The characteristics of the multi-service environment and WCDMA system make it possible to plan WCDMA systems in many different places than GSM systems. In particular, it should be noted that when planning WCDMA systems, the coverage of the community and the load should be considered together. By limiting the maximum transmitting power of the mobile station, this restricts the coverage of the cell in the uplink, while the downlink link restricts the capacity of the cell due to interference. In the WCDMA system, the Power Control (TPC), the gain from the soft switch and the softer switch, the power budget of the upper and lower link are considered in planning.

In a word, the wireless network planning of GSM system is the best point between the capacity and coverage of the cell, while WCDMA wireless network planning should seek the best point between capacity, coverage and different service quality, which increases the complexity of WCDMA wireless network planning correspondingly. (Computer science)

The capacity and coverage relationship in the wireless network planning of WCDMA system

WCDMA system itself is a disturbance-constrained system, in the network planning, capacity and coverage planning to be combined to consider, unlike GSM system network planning capacity and coverage is separate to consider. This is mainly due to the community's load will be allowed to the maximum spread loss of the community impact, that is, the impact on coverage, while the community load is the determining factor in the capacity of the community. When the load comparison hour WCDMA system is uplink coverage limited, when the load is relatively large, the row link capacity is limited. Therefore, the planning of coverage and capacity in network planning is different for upper and downstream links. In particular, 3G systems provide multimedia services, the quality of Service (QoS) of each business is different, so accurately planning coverage and capacity is more important.

1. Uplink coverage

In the initial layout stage of WCDMA wireless network planning, the uplink power budget is generally based on the link quality equation.

When the uplink load factor tends to 1 o'clock, the cell reaches its pole capacity or the incremental cell capacity. When the user's data rate is not very high (at this time the processing gain W/ri is relatively high), the uplink cell load factor and the coverage of the community is almost linear relationship.

2. Downlink coverage

WCDMA downlink and uplink have obvious differences, mainly in:

(1) The transmitting power of the base station is shared by all the mobile stations it serves;

(2) whether different mobile stations will be affected by the interference of neighboring communities, depending on the location of Mobile station in the community;

(3) The load factor of downlink Link has a great relationship with the orthogonal coefficient.

As with uplink, when the downlink load factor tends to 1 o'clock, the downlink capacity reaches its maximum value, that is, the pole capacity, at which time the required base station power reaches infinity. So in doing network planning, the load of the community can not be too large, load factor should be kept in a smaller than 1 value, so that the entire network to achieve a stable state.

3. Comparison between upper and lower links

From the above discussion, we can get the relationship between the upper and the downlink link coverage and the capacity in the WCDMA system wireless network planning, as shown in Figure 1. From the graph, we can see that the coverage of downlink link decreases rapidly with the increase of the load factor of the cell, and the uplink only when the load factor of the neighborhood is close to 1 o'clock, the coverage area of the cell is reduced obviously, so the coverage of downlink is more dependent on the load than that of uplink. The reason for this is that, regardless of the number of users in the downlink, the maximum transmit power is also 40dBm, it is shared by all users downlink, and in the uplink each additional user has its own amplifier. Therefore, in the downlink, even if the downlink load is low, the coverage as a user function is still decreasing with the increase in load.

From this we can know that when the load is low, the overlay is obviously limited to the uplink, and the capacity is limited to the downlink link. So when we do network planning, we focus on the uplink in the coverage planning, the use of mature technology to increase the coverage of the uplink, such as by increasing the antenna gain, reduce the base station RF part of the base station noise coefficient, reduce the antenna and base station Low-noise amplifier cable loss (using the Tower top amplifier MHA) and other ways to achieve. While the capacity planning is mainly for downlink, this can be achieved by using more carrier, transmitter diversity and base station sector.

Second, soft capacity and soft capacity planning

In the capacity and coverage planning of WCDMA systems, two scenarios may occur:

Capacity constraints this is generally due to the fact that the system load factor determined by the user's business characteristics of the planning area is greater than the maximum load factor that the system actually allows. There are two types of capacity constraints, one situation is that the uplink capacity is limited, for example, in rural areas, the scope of the planned community service is larger and the load factor is small; in the city, because the user is more, the community's load general plan is comparatively big, the base station's transmitting power limits the community service maximum user number, So often downlink capacity is limited.

The system load factor determined by the user's business characteristics of the planning area is less than the maximum load factor that the system actually allows. As with capacity constraints, there is a limit on upper and downlink coverage.

Because of the above two situations, and the soft capacity and soft cover must be considered when the WCDMA system is planned, we first assume that the planned community is limited to cover, to show how to determine the soft capacity and soft cover of the cell.

First, according to the maximum allowable load factor of the cell, the link budget equation and the field strength Prediction model calculate the coverage area of the cell, then calculate the load factor of the cell according to the business characteristics of the user in the coverage area (such as the user's moving speed, the type of service used and the coverage probability to be met). For real-time business, business types are often represented by erlangs, whereas for Non-real-time transactions, business types are expressed in Kbps. After calculating the load factor of the cell, comparing with the maximum allowable load factor of the system, there may be two kinds of conditions under the capacity and coverage limit.

(1) The maximum allowable load of the coverage system is less than the community load when calculating the coverage of the cell. This means that the plot load factor is too large when calculating the area coverage. In order for the two to be equal, a smaller load factor must be taken to recalculate the area coverage. Reducing the load factor means that the interference of the system decreases and the coverage area of the cell increases. The increase in coverage can absorb more users, which also increases the disturbance and load of the community. This process has been repeated until the last cell load factor size and according to the community scope of the user business characteristics of the load factor is equal to the size, then the community scope and load factor for the final planning of the community scope and capacity.

(2) Capacity constraints this situation is the opposite of the previous one, the maximum allowable load of the system is greater than the community load when calculating the coverage of the cell. This means reducing the coverage of the community, which reduces the number of users of the community service. As the coverage area of the cell is reduced, the load factor of the cell will be recalculated, the whole process is an iterative process, until the community load factor and the actual load factor of the system are equal according to the user's business characteristics in the community, then the coverage range of the community can be determined.

Third, concluding remarks

Because of the introduction of new business requirements in WCDMA system and the different from the second generation GSM system in wireless interface, the design of wireless network is different from GSM system. In this paper, the soft capacity and soft cover problems in WCDMA system are discussed in detail, which can be seen that the capacity and coverage should be considered at the same time in the network planning, and both of them have different influence on the upper and lower links. In addition, in this paper, how to determine the scope and capacity of the community in the initial stage of planning is discussed in more detail. All these conclusions have a good guiding significance for WCDMA wireless network planning engineers.