RSCP and Ec/No

The CPICH-Common pilot channel is continuously transmitted from the NodeB.It carries no information and can be thought of as a beacon constantly transmitting the Scrambling Code of the cell.WCDMA cells are identified by their SC just like BCCH in the GSM.

It is this beacon that is used by the phone for its cell measurements for network acquisition and handover purposes (Ec, Ec/Io).In a WCDMA network the User Equipment (UE) receives signals from many cells

So Io is the Sum total of all of these signals. (dbm)

Note :Sometimes Io is referred to as No, RSSI

Now RSCP is the Received Signal code power of particular CPICH in dbm. Ec=Energy per Chip

From the previous two measures we can calculate a signal quality for each CPICH (SC) received.

Ec/Io = (Energy per chip / Noise spectral density) = RSCP/RSSI

Now lets understand relation between Ec/Io and Eb/NocNo, RSCP, RSSI, CPICH

RSSI: recieved signal strength indicator.RSSI= RSCP-Ec/Io

EcNo: energy per chip over the noise, a measure of the quality of the signal

RSCP: Energy after processing ie with gain from coding etcRSCP = RSSI + EcNo

RSSI: A dB measure of the signal arriving at the antenna before gain from processingCPICH: Pilot Channel of the cell.You have to measure two parameter on pilot ch (Ec/No & RSCP) to verify P-CPICH detection to minimise coverage holes, Ex: (RSCP > -100dBm (sufficient) & Ec/No > -14 (sufficient))& by correlation between low Ec/No & high RSCP you can find the interference & pilot pollution.

If you work with UMTS,'ve probably heard someone talk about RTWP. Its definition can be found in a dictionary of acronyms, such as http://acronyms.thefreedictionary.com/RTWP: Received Total Wideband Power.

Represents a measure of UMTS technology: the total level of noise within the UMTS frequency band of any cell.

RTWP is related to uplink interference, and its monitoring helps control the call drops - mainly CS. It also has importance in the capacity management, as it provides information for the Congestion Control regarding Uplink Interference.

In UMTS, the uplink interference may vary due to several factors, such as the number of users in the cell, the Service, Connection Types and Conditions of Radio, etc..

As our goal is to always be as simple as possible, we will not delve in terms of formulas or concepts involved. We will then know the typical values, and know what must be done in case of problems.

Typical Values

Ok, we know that RTWP can help us in checking the uplink interference, then we need to know its typical values.

In a network is not loaded, normal, acceptable RTWP Average value is generally around -104.5 and -105.5 dBm.

Values around -95 dBm indicate that the cell has some uplink interferers.

If the value is around -85 dBm, the situation is ugly, with strong uplink interferers.

Usually we have High, Low and Medium measures of RTWP. However, the maximum and minimum values are recommended only as auxiliary or reference, since they may have been caused by a peak of access, or even been forced to have a momentary value due to some algorithm i.e..

Thus, the value that helps us, and has the most accurate information is the same Mean RTWP!

For cases in which cell has two carriers, the difference between them RTWP should not exceed 6 dB.

Based on these typical values, most vendors have an alarm: RTWP "Very High. "

What to do in case of problems?

We have seen that RTWP can cause performance degradation, mainly CS Call Drops. Note: Actually, it's not RTWP that causes performance degradation. What happens is that when its value is 'bad', it's actually indicating the presence of interference - the latter being responsible for degradation.

But what can we do when we find bad values?

If RTWP is not at acceptable levels, some actions should be taken.

The first thing to do is check if there is a configuration issue with the RNC or NodeB. This is the most common case, especially in cases of new activations.

Once verified the parameter settings, the next step is the physical examination, especially jumpers and cables, often partially reversed. It also should be checked if there is faulty transmitters, or any other problem that could generate intermodulation between the NodeB and the antenna.

If the parameter settings and hardware are ok, the chance is very high that we have external interference, such as a Interferer Repeater.

In cases where there may be external interference, we must begin to act after such a prioritization based on how much this is affecting the cell KPI's across the network, if it carry high traffic, major subscribers, etc..

Note: There are many forms of interference in the uplink, both internal and external. Only a few are listed above. The deepening of all possibilities is beyond the goal of being simple to teach the concepts, but this is a suggestion for whoever wants to deepen the study, identification and elimination of interference.

In practice

to find - and eliminate - problems of interference is one of the biggest challenges in our area. For being such a complex problem, we recommend that be collected enough data for each investigation. Insufficient data collected can lead to erroneous conclusions, further worsening the problem.

The uplink interference may appear only in specific periods. Thus, it is recommended that data be collected from at least one week (7 days) for every 24 hours. Usually this amount of data is sufficient. In the figure below, we see different days and times - colorful - a fictional example where the interference occurred.

Data should be collected for the suspicious cell, but also for its adjacent cells, allowing it to make a triangulation increasing the chances of locating the source of interference.

Another way to locate the source of interference is to do a test in field. An antenna guy must gradually change the azimuth of the antenna, while another professional do RTWP measurements. That is, through the information directing the antenna and the respective values of RTWP, you can draw conclusions very good.

It is obvious that changing the online system may not be a good practice, and tests can be made with a Yagi antenna and a Spectrum Analyzer.

Vendors offer several ways to measure RTWP, using the OSS, performance counters and logs.

Conclusion

In this brief tutorial, we learn what is RTWP, and that the ideal typical value is about -104.5 dBm and -105.5 dBm.

As the RTWP is directly related to Uplink Interference - and we know that interference is the main cause of performance degradation - have concluded that improving RTWP, ie making is as close as possible to -105 dBm, improving the Call Drop Rate!IMPORTANT : Seizing the opportunity, see what was stated at the start of this tutorial - dictionary - by describing RTWP. Remember that this site has been the subject of a very interesting tutorial in the Tips Section. If you have not visited this section of the portal yet , I strongly recommend, because it has many issues that help in our growth in telecom and IT area.