7. Radio Network Planning for WCDMA 7.1 Overview

With the emergence of 3G mobile communication technology, the construction of the UMTS network will bring a profound evolution, which makes higher requirements for network planning. At present, the public is greatly interested in this new technology. The construction of 3G mobile communication network is in the ascendant. This new mobile communication technology is different from the traditional GSM network planning in terms of essence. Worldwide people are developing new planning tool and algorithm and designing new work flow.

Comparison between UMTS network planning and GSM network planning:

1. GSM network planning

The GSM network planning is based on radio wave propagation analysis. According to the transmitting power and antenna configuration of BTS, its coverage area is calculated. Normally, only the downlink coverage area is calculated, so the GSM technology does not take uplink coverage area into consideration. The next step is performed by network planning engineers to analyze the required cell capacity. According to the calculated cell area, the traffic of cells can be estimated with the help of electronic map, and then the required channel numbers are calculated through traffic models (such as Erlang-B or Erlang-C). Next, it is the frequency distribution for BTSs, and the same frequency can only be reused among enough-distanced cells, to avoid interference.

To expand the network in the future, the network planning engineer just needs to distribute new channels to the corresponding cells. As long as there is appropriate frequency in the overall frequency planning and the expansion does not exceed the maximum capacity of the BTS, the network does not require other adjustments. Otherwise, new BTSs or sectors should be added, and new frequency calculation and channel distribution are required.

2. UTMS network planning

Compared with the GSM network planning, the UMTS network planning features the following differences:

Cell breathing:

The CDMA network is totally different from the GSM network. Since channels and users are not separated for consideration, there is tight relation between coverage and capacity. The cell with more traffic has less coverage area. In the CDMA network, more traffic means more interferences. This kind of effect of dynamic change of cell area is called as “cell breathing”. This can be illustrated through the following visual example. In a birthday party of a friend, many guests come. More people taking will cause it is harder to hear the voice of opposite party clearly. If in the beginning, you can talk with a friend at another end of the room, you can not hear what he is saying at all when the room is very noisy. It indicates that the “cell radius” of talking area is shortened. The UMTS network planning engineer faces a network changing dynamically.

In UMTS network planning, the network expandability should be taken into consideration first of all. The network planning engineer can not simply add frequency to the related cell as planning GSM network. In the beginning of network planning, a determined traffic signal redundancy should be taken into consideration, and this redundancy will be used as “compensation” to the interference caused by increased traffic. This shows that, from the very beginning, it is required to construct the network with smaller cells or more NodeBs, resulting in higher investment cost. If the traffic signal surplus is too small, there is only one way: adding NodeBs when expanding.

The network planning engineer should notice the above issues, because enhancing transmitting power simply can not reduce the receiving signal deterioration caused by traffic incensement.  Enhancing transmitting power can only improve the receiving signal of a cell but will add interference to the adjacent cells. As a result, the whole network communication quality will be influenced.

To enhance transmitting power, the valid range or capacity (for the CDMA network, they are homonymous) of the CDMA cell is limited. When the UMTS network transmitting power is doubled, the cell capacity is increased by 10%. The enhanced transmitting power raises the valid range of cell, but to satisfy the requirements of remote mobile subscribers, it is necessary to enhance transmitting power by multiple times, which will influence the talking quality of other mobile subscribers. Let’s return to the above party example. You can enhance your voice to continue the conversation with the friend at  another end of the room, but at the same time, other guests also raise voice to talk with others. As a result, the whole room is submerged in noise.

The corresponding relationship between transmitting power and cell capacity is gradual. Since the UMTS cell load is subject to saturate, the UMTS network planning engineer must reduce the full-load ratio. The detailed parameters depend on different services and how much the network carrier would like to risk. Usually, the full-load factor is preset as 60% in network design. Here, the “cell breathing” effect is used. The adjacent cells can mutually compensate load, called as soft load. Due to cost, the network capacity can not be increased on a large scale. The mathematical demonstration on the UMTS service with large scale data transmission shows that, as data transmission volume increases, it is more possible for a service cell to borrow capacity from the adjacent cells. This result is satisfied.

Near-far effect:

Another typical issue of the CDMA network is near-far effect. Since all the subscribers in the same cell share the same frequency, it is important that each subscriber in the whole system transmits signal at the minimum power. Let’s return to the above party example. If someone is shouting in the room, the conversation among all the other guests will be influenced. In the CDMA network, this problem can be solved through power control. For example, the UMTS network uses closed loop power control at frequency 1500 Hz. For GSM network, however, the power control works at 2 Hz and is for uplink only.

This kind of fast power control mechanism has been implemented in the UMTS hardware. But the network planning engineer will face the other case of this problem. When a subscriber is far away from the NodeB, he needs a majority of transmitting power, resulting in power shortage for other subscribers. This means that the cell capacity is related to the actual subscriber geographical distribution. When subscriber density is rather large, statistics average value can be used to solve this problem. When subscriber quantity is small, it is necessary to perform dynamic analysis to the network through simulations.

Uplink and downlink:

The UMTS network traffic is asymmetrical, that is, the data transmission quantity in the network uplink differs from that in downlink. The network planning engineer should calculate values in two directions and then combine them together properly. In this way, the network planning work will be very complicated. Uplink is a typical limit factor for the valid coverage range of UMTS cell, or we can say that uplink is coverage-limited and downlink is capacity-limited. The transmitting power in uplink is provided by UE and the one in downlink is provided by NodeB.

The above problems also occur in the existing CDMA network. For the UMTS network, these problems are more complicated. The UMTS network can satisfy different services with different requirements in communication quality and traffic at the same time, including simple voice service and packet service up to 2Mbps.

Integrated services:

In fact, the UMTS network should satisfy requirements of different services at the same time. So, the network planning engineer should take different services into consideration. For the service with low communication quality requirement, the UMTS cell has rather large coverage. For the service with high communication quality requirement, the cell has very small coverage. So, in the actual work, the network planning engineer should not just consider UMTS cell radius, because different services correspond to different cell radiuses. If the minimum cell radius, that is, the service with highest communication quality requirement, is used as the standard for network planning, the network establishment cost will be very high and it is not realistic. The UMTS network planning engineer should start from the cell radius of middle-class service. As a result, the actual valid range of the cell can only satisfy the requirement of high-class service partially. At present, many network planning software companies have started to develop valid algorithms for this kind of new UMTS network integrated service.

Other differences:

Compared with GSM network, the UMTS network features other differences. The GSM network solves capacity problem with sector partition method. The cell with too much traffic is divided into multiple sectors, and antennas are added correspondingly. This method is also used for the UMTS network, but its effect is not enough. The change of cell coverage will cause the near-far effect mentioned above, and overlapped sectors will interfere mutually because they use the same frequency.

The declination angle (mechanical or electronic) of antenna plays an important role in the UMTS network. It can reduce the interference among adjacent cells and raise cell capacity implicitly. In the actual application, large declination angle can be chosen to solve this problem.

In the WCDMA system, multi-path propagation is not a negative factor but an ideal result, because receiver can combine the signal with delay of at least 1 Chip (the UMTS network chip transmission rate is 3.84 Mcps, that is, 1 Chip=0.26 microsecond, equal to 78 meters) into valid signal.

In addition, the UMTS network also uses the soft handover. In this case, a mobile subscriber can connect to several cells. This method solves network signal fluctuation, but raises network traffic. The traditional Erlang model is not applicable any more.

Compared with 2G traditional GSM network, the UMTS network features many differences. Especially, the UMTS network can run asynchronously, which causes “non-orthogonality” of transmission channel. Let’s return to the party example again. Even if the perfect planning can be made theoretically, that is, planning the person to talk in the certain time, it is impossible to reach that ideal goal factually, because the watches of all the guests can not be synchronous exactly.

Through the above analysis, we can clearly see that the UMTS network planning needs more cost, compared with the current mobile communication network planning. The UMTS network planning is rather complicated, because many system parameters are closely related to each other and should be calculated at the same time. In the current mobile communication network, however, these parameters are calculated separately.