Author Luo Shun Huawei ID 106922Department NTS, SSAR Product

FamilyWireless performance & RNP & RNO

Date 2009/11/25 Product Version

BSC6810V200R010C01B061

Reviewer

Title: Trouble shooting of an external interference problem in Operator C, Country B

Phenomenon Description

The customer complained that the RTWP was very high in lots of new UMTS2100 sites, and requested to solve it as soon as possible.

Alarm Information

NA

Cause Analysis

The reason of high RTWP could be: 1. The self-interference of WCDMA system, that is, with the increase of online user number, the RTWP increases somehow; 2. Interference, which includes external and internal interference, with different performance.

Handling Process

The network in the Region N is in UMTS850 originally, and with a rollout of some new UMTS2100 sites, it became a “mixed” network. At present two frequency points of 9639 and 9662 are available.However, after the commercial launch of some UMTS2100 sites, the customer complained the RTWP of these new sites were much more higher than existing UMTS850 sites, which can be displayed in the drive test logs, too:

1. According to the standard procedure of interference trouble shooting, we used the RNC performance data getting a statistic analysis of RTWP. It is proved that the UMTS2100 cells were in high RTWP, even in free hours, the RTWP was still over -90dBm, and 14 of them the RTWP was over -80dBm.

2. We started a backstage RTWP monitoring in LMT, and imported the result files into Nastar getting the following plot. It is clear that the RTWP was really very high, with a high noise floor and some peaks.

But we couldn’t collect RTWP monitoring plot because of some limitation.

3. We scanned the frequency band with YBT250 on one interfered site, but the result was not clear from the waterfall plot that there was no obvious interference around the old frequency point of 9639, because of some limitation on the field:

However, the downlink emission near 1877M was on a very high lever, which may cause a serious spurious, block or inter-modulation interference and a rising of noise floor:

Meanwhile, the installation of other telecom system could be the source of the interference:

We tried to collect more information but failed again.

4. Above all, it was still very hard to judge the reason of the interference, even with some useful information. At the same time we were checking the installation problem, we asked the R & D and knew that since RAN10, a new feature of frequency scanning was available on NodeB. The RRU works only as a receiver with the board blocked (the transmitter channel is closed), and changing frequency points from time to time to measure the total power within the band. In this process, the RRU only receives but doesn’t

transmit, so the RTWP will not be raised by NodeB itself.We tried to collect some scanning measurements for UL frequency points in four sites, in which it is very clear there was external interference:

The most important of all, the interference did exist around the frequency point of 9639. Let’s put WPEJBO04 as the example, we can see that the RTWP is very high from all three RRUs, although the first is higher than the other two:

But around other frequency points, like 9670, there was no interference, with a good noise floor:

Here we use one plot to explain the above result better, using one result of a RRU. The RTWP was high in two frequency bands, marking A and B, with a bandwidth of 10M and 6M respectively, and around the highest frequency, the RTWP became invalid (-5dBm, the largest value from the LMT measurement) because of some DL interference. We can see around 9670 the RTWP was nearly -106dBm, which is the base noise floor when the RRU is free with interference, so what we collected was valid. Even lots of UEs cannot raise the RTWP to a so high level because it is almost impossible for UEs to raise it to -85dBm during non-busy hours.Above all, our conclusion is, there was external interference around frequency point of 9639.

5. After a discussion with the customer, we changed the frequency point from 9639 with lots of interference to 9662 which is free of interference. After the same scanning operation after that, the RTWP decreased.Before the change of the frequency point:

After:

Suggestions and summary:

1. To the interference analysis, if we can collect more information as much as possible according to the “interference trouble shooting guide book” at the beginning of the whole process, we can work more effective.

2. The cost is controlled very strictly these days, and because of other factors like the access to sites, the on-field test in commercial sites is always very difficult. Therefore such kind of method of frequency point scanning will be more and more important in the future. It is suggested to add a chapter about that in the “interference trouble shooting guide book” and the “maintenance operation guide book”.

Attachment:

NA

Reference:

W-Interference Trouble Shooting Reference Book