Soft Site Swap and Cutover in WCDMA Network

Lei Zengping Shi Hanjun Sun Yi

（ZTE Corporation, Shanghai 201203, China）

Abstrac t:Traditionally,when a radio network site is swapped,it is necessary to interrupt existing services and perform a step-by-step serial cutover,which involves complex arrangements and huge resource investments.Therefore,there is an urgent need to discuss a fast and low-risk solution for site moving and cutover.ZTE Base Band Unit and Remote Radio Unit（BBU+RRU）Node B adopts distributed structure and combines full frequency reuse of the WCDMA system to conveniently implement soft site moving and cutover.The solution is to build an overlay network and make a one-time cutover without any impact on the operation of the existing network.Moreover,it supports timely fallback,and can considerably simplify the site moving and cutover operation and lower the network risk.

T here are many site moving and cutover solutions to the Wideband Code Division Multiple Access（WCDMA）mobile communications system from traditional＂hard＂schemes to up-to-date＂soft＂ones.The profound analysis of these solutions in terms of application scenarios,technological principles,feasibility and implementation processes may offer theoreticalbases for site engineering and operations.

1 Disadvantages of Traditional Site Moving and Cutover Solutions

Traditionally,to begin a site moving and cutover engineering,the network is first divided into severalregions based on the coverage condition.Before going on to another region,the site operations,including moving,cutover and optimization,in one region have to be done.The entire engineering is performed in a gradual and step-by-step way until all sites in the network are replaced and optimized.Besides,the engineering is often scheduled at night when traffic is very low.

Figure 1 shows a traditional site moving and cutover flow.

The traditional site moving and cutover solution is also called＂hard site moving and cutover＂because it interrupts the services.It has several disadvantages,including:

·The services at the involved sites willbe interrupted at the night of the cutover.At the site with small equipment room,the interruption may take longer because the cutover is a serial process.

·It requires costly investments on resources such as engineering teams,test/optimization personnel and vehicles.

·The resource arrangement willbe very complicated due to huge resources investment.

·As the available resources are limited,only a small part of sites can be swapped and get cutover at a time,so the entire engineering will take a long time.

·The long engineering period will result in a considerable workload for the operator.

·The network's Quality of Service（QoS）decreases during the engineering,which may cause the drain of subscribers.

·Frequent coordination from the operator is needed during the engineering to complete data configuration,thus demanding more human resources and a higher technical level from the operator.

2 Soft Site Moving and Cutover

The primary intention of the operator for implementing site moving and cutover is to improve the network's QoSby replacing the old equipment with the new one.

▲Figure 1. Traditional site moving and cutover flow.

As there are often lots of subscribers in a current network,no operators expect to receive many complaints from the users,see many users to lose,or even see their brand being damaged due to improper site moving and cutover engineering.This means the impact of the engineering on the network performance should be minimized,ensuring the network's QoS will not be decreased considerably.Therefore,there is an urgent need to discuss the soft site moving and cutover solution.

▲Figure 2. Soft site moving and cutover flow.

2.1 Implementation

The basic idea of the soft site moving and cutover solution is to construct a new WCDMA network that runs parallel to the existing network,and then optimize it before cutover.This new network,which is in＂reserved＂state before cutover,is invisible to the users of the existing network except those who are invited for test.After all of its indexes are tested to meet the requirements,the new network will then be activated to take over the work of the existing network,and the old one will quit services.

If any serious quality problem is found in the new network,the old network can be quickly activated to resume its work.In this way,the network's QoSis guaranteed.

Figure 2 is a flowchart of the soft site moving and cutover using ZTEBase Band Unit and Remote Radio Unit（BBU+RRU）Node B.

From Figure 2,we can see that the soft site moving and cutover engineering can be divided into three stages:

（1）New Network Construction In this stage,the new network is constructed parallel to the existing network,that is to say,their sites share the same address.

（2）New Network Optimization

In this stage,the newly constructed network will be optimized.Once all of its Key Performance Indicators（KPI）meet the requirements,the cutover of all sites will be done simultaneously.

（3）Old Network Moving

If the new network performs very well through the observation period when it starts operation,the base station equipment of the old network will be swapped one by one.

2.2 Prerequisites

Obviously,the soft site moving and cutover solution using ZTE Base Band Unit and Remote Radio Unit（BBU+RRU）Node B presents an approach quite different from the traditional one:it swaps the sites after,rather than before,the cutover.

However,there are some prerequisites for implementing a soft site moving and cutover scheme.They are the following:

（1）Space

As the new Node B will share the equipment room of the old one during its construction stage,the space of the room should be enough to place both the old and new Node B equipment.With small size and flexible installation,ZTEBase Band Unit and Remote Radio Unit（BBU+RRU）can be easily installed in small space areas.

（2）Frequency

The operator should have idle frequencies so that the services of the current network will not be interfered during the construction and optimization of the new network.The full frequency reuse feature of a WCDMAsystem often enables the operator to have idle frequencies.

（3）Transmission

Separate transmission path should be provided by the operator at each site to construct the new network.

（4）Power Supply

Enough electric capacity should be reserved for the new network.The power consumption of ZTEBBU+RRU is small and supports AC power supply;so,its power supply requirement is quite low.

（5）Antenna-Feeder System

The antenna-feeder system at each site must be shared by the base stations of new and old networks.The sharing of the antenna-feeder system is a critical technology in the soft site moving and cutover solution.

2.3 Advantages

The advantages of soft site moving and cutover include:

（1）Independent Optimization

The new network will be optimized without affecting the services of the existing network.During the optimization,the cell attributes of the new network are set to＂reserved＂,and only the invited test users can access the new network,so that the existing network will not be affected.

（2）One-time Cutover

No site operation is required for the cutover from the old network to the new one,and the cutover is a one-step operation:just input a batch command at the Operation and Maintenance Center（OMC）to change the attributes of all cells in the new network from＂reserved＂to＂active＂,and change those of the old network from＂active＂to＂reserve＂.In this way,the states of the two networks are quickly converted,and a smooth cutover between the old and new networks are achieved.

（3）Quick Fallback in Case of Any Serious Problems

If any serious problems are found during the cutover,the system can quickly fall back to the old network;or if an unsolvable problem is found in the early running stage of the new network,causing the network quality to decrease dramatically,cutover operations may be done again,from the new network to the old one,to activate the old network and quit the new one.

（4）Minimum Operator Coordination

The soft site moving and cutover solution reduces the operator's coordination to a great extent.During the engineering,only the following operations need to be done by the operator:to provide current network data before the moving;to grant a permit for the engineering personnel to enter the site during the engineering;and to change the cell attribute settings during the cutover（use a batch command in the OMC）.As a result,fewer persons and a lower skill levelare required from the operator,which,in return,can quicken up the engineering progress,improve the engineering quality and control the risks.

（5）Less Neighboring Cell Adjustment

With little work involved in the cell adjustment,the network re-planning becomes simple and easy.This is especially useful in case new sites have to be constructed during the cutover.In this case,a comprehensive planning of the old and new sites can be made.Moreover,the neighboring cells of 2G and 3G networks can be adjusted respectively.

3 Soft Site Moving and Cutover Solutions and Their Applications

Currently,three soft site moving and cutover schemes are recommended:signal-antenna scheme,RRU combining scheme and combiner scheme.They all have their own advantages and disadvantages.

3.1 Single-antenna Scheme

（1）Schematic Diagram

Figure 3 is the schematic diagram of the single-antenna soft site moving and cutover scheme.

（2）Technical Principle

The single-antenna scheme is the simplest in terms of implementation:at a night during the installation of the new Node B,just take one receiving diversity antenna of the old Node B and connect it to the transmitting/receiving antenna of ZTERRU.In the connection,the old Node B is not required to be powered off;therefore,services will not be interrupted.

After ZTERRUis connected,the receiving diversity gains of both ZTEBBU+RRU and the old Node B will decrease by 3 d B.Therefore,this scheme requires the capacity and coverage of current network to reach a certain level.

（3）Application Scenarios

Being very simple and economical,the single-antenna scheme is often a preferred scheme for soft site moving and cutover engineering.But as it will lose 3 d Bof uplink capacity,the network capacity and coverage are required to reach a certain level.This scheme is best suitable in the following scenarios:

·The uplink load does not exceed 50%of the design capacity.That is to say,the load of the old Node B should not exceed 25%of the design capacity.

·The uplink coverage should be unlimited.In network planning,the coverage distance X is often determined by the uplink coverage,and the distance Y is calculated from the capacity and traffic density of the network.The planned coverage distance Z will then be expressed as Z=Min（X,Y）.If Z is smaller than X×85%,the uplink coverage is regarded as unlimited.

3.2 RRU Combining Scheme

（1）Schematic Diagram

Figure 4 is the schematic diagram of the RRU combining scheme for soft site moving and cutover.

（2）Technical Principle

The RRU combining scheme is also very simple in implementation:at a night during the installation of the new Node B,take one receiving diversity antenna of the old Node B and connect it to the parallel port of ZTERRU;and then,use a feeder to connect the parallel port with the receiving port of the old Node B.Besides,an attenuator（e.g.,30 d B）will be used to maintain the signal strength of the receiving diversity antenna at the same level as before.In the connection,the old Node Bis not required to be powered off;thus,services can be offered as usual.

After ZTERRU is connected,the downlink and uplink of the old Node Bwillnot be affected due to the active path function of ZTERRU.But at ZTEBBU+RRU end,the uplink coverage and capacity will be decreased by a certain degree as only one antenna is used.The 3 d Bloss in uplink capacity can be compensated with a dual-carrier configuration;however,there is still no solution for the uplink coverage loss.

（3）Application Scenarios

▲Figure 3. Single-antenna scheme.

▲Figure 4. RRU combining scheme.

The RRU combining scheme is simple and relatively economical;therefore,it is a secondary option for soft site moving and cutover.But because of the 3 d B loss in uplink capacity,it is applicable to the case where the uplink coverage is unlimited.

▲Figure 5. Combiner scheme.

▼Table 1. Advantage/disadvantage comparison of the three schemes

3.3 Combiner Scheme

（1）Schematic Diagram

Figure 5 is the schematic diagram of the combiner scheme for soft site moving and cutover.

（2）Technical Principle

The implementation of combiner scheme is as follows:at a night during the installation of the new Node B,connect the two antennas of the old Node B to the output ends of two combiners,and then connect the two feeder-antenna ports of the old Node B and ZTERRU with the input ends of the two combiners respectively.In the connection,the old Node B will be powered off for about 30 minutes（to avoid radio frequency interference of the transmitting antenna）.

After ZTERRU is connected,the downlink and uplink of the old Node B will not be affected due to the active path function of the combiner.Also,the use of two antennas in ZTEBBU+RRU ensures the capacity and coverage of the new network to be the same as the old one.

（3）Application Scenarios

Without any capacity or coverage requirement,the combiner scheme is applicable to all scenarios.Moreover,it is theoretically the best scheme as the integration of both the old and the new networks is kept before and after the cutover.However,the combiner is expensive and has to be customized;therefore,this scheme is not recommended in common scenarios.

4 Conclusions

Table 1 compares the advantages and disadvantages of the three soft site moving and cutover schemes discussed in Chapter 3.

In practice,different schemes should be selected for different applications.Abest scheme should be the one that can achieve the optimal network quality and be the most cost-effective,with actual network situations taken into account,including coverage,capacity and cost［1-4］.