Paging

The Paging Channel (PCH) is a downlink transport channel. The PCH is always transmitted over the entire cell. The transmission of the PCH is associated with the transmission of physical-layer generated Paging Indicators, to support efficient sleep-mode procedures.

Paging Channel selection

System information block type 5 (SIB 5) defines common channels to be employed in Idle mode. In a cell, a single or several PCHs may be established. Each Secondary Common Control Physical Channel (SCCPCH) indicated to the UE in system information may carry up to one PCH. Thus, for each defined PCH there is one uniquely associated PICH also indicated.

In case that more than a single PCH and associated PICH are defined in SIB 5, the UE shall perform a selection according to the following rule: The UE shall select a SCCPCH from the ones listed in SIB 5 based on IMSI as follows:

               "Index of selected SCCPCH" = IMSI mod K,

where K is equal to the number of listed SCCPCHs which carry a PCH (i.e. SCCPCHs carrying FACH only shall not be counted). These SCCPCHs shall be indexed in the order of their occurrence in SIB 5 from 0 to K-1.

"Index of selected SCCPCH" identifies the selected SCCPCH with the PCH and the uniquely associated PICH to be used by the UE. If the UE has no IMSI, for instance when making an emergency call without USIM, the UE shall use as default number IMSI = 0.

The UE may use Discontinuous Reception (DRX) in idle mode in order to reduce power consumption. When DRX is used the UE needs only to monitor one Page Indicator, PI, in one Paging Occasion per DRX cycle.

The Paging Indicator Channel (PICH) is a fixed rate (SF=256) physical channel used to carry the paging indicators. The PICH is always associated with an S-CCPCH to which a PCH transport channel is mapped. Picture below illustrates the frame structure of the PICH. One PICH radio frame of length 10 ms consists of 300 bits. Of these, 288 bits are used to carry paging indicators. The remaining 12 bits are not

formally part of the PICH and shall not be transmitted (DTX). The part of the frame with no transmission is reserved for possible future use.

Structure of Paging Indicator Channel (PICH)

Two Paging Procedures:

Paging procedure is used to transmit paging information to selected UEs in idle mode, CELL_PCH or URA_PCH state using the paging control channel (PCCH). Upper layers in the network may request paging, to e.g. establish a signalling connection. UTRAN may initiate paging for UEs in CELL_PCH or URA_PCH state to trigger a cell update procedure. In addition, UTRAN may initiate paging for UEs in idle mode, CELL_PCH and URA_PCH state to trigger reading of updated system information. UTRAN initiates the paging procedure by transmitting a PAGING TYPE 1 message on an appropriate paging occasion on the PCCH.

UE dedicated paging procedure is used to transmit dedicated paging information to one UE in connected mode in CELL_DCH or CELL_FACH state. Upper layers in the network may request initiation of paging. For a UE in CELL_DCH or CELL_FACH state, UTRAN initiates the procedure by transmitting a PAGING TYPE 2 message on the DCCH using AM RLC.

Two Paging Message Types:

PAGING TYPE 1 message is used to send information on the paging channel. One or several UEs, in idle or connected mode, can be paged in one message, which also can contain other information

PAGING TYPE 2 message is used to page an UE in connected mode (CELL_DCH or CELL_FACH state), when using the DCCH for CN originated paging.

PICH / S-CCPCH timing relation

Picture below illustrates the timing between a PICH frame and its associated single S-CCPCH frame, i.e. the S-CCPCH frame that carries the paging information related to the paging indicators in the PICH frame. A paging indicator set in a PICH frame means that the paging message is transmitted on the PCH in the S-CCPCH frame starting tPICH chips after the transmitted PICH frame.

Timing relation between PICH frame and associated S-CCPCH frame

tPICH = 7680 chips (3 slots)

Paging Block Periodicity (PBP): Period of the occurrence of Paging Blocks. (For FDD, PBP = 1).

Paging occasion: (FDD) The SFN of the PICH frame where the UE monitors its paging indicator (i.e.

the SFN of the PCCPCH frame in which the PICH frame begins).

What is RTWP?

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.