Frequently Asked Question

This section explains some of the most common questions that arise when using the ViCom BCH demodulator. They don’t have any particular order, so skip through them when you encounter a problem – maybe it’s one of the problems other people already had.

? Are there any other representation layouts other than the string format? Which ones are ASN.1 compli- ant?

! The ViCom API returns a string formatted version of the data according to the formatting rules defined in ASN.1 specification. Additionally, some prefixes are used to distinguish different versions of the UMTS RRC message standard (r3 for Release 99, r5 or r6), when necessary. There is no other mean- ingful form of the data returned from the ViCom library, except the raw byte-stream.

? Is the LAC parameter in the SBchCellIdentification structure filled at any time? ! ! It is only provided, if SIB 1 has been demodulated, which contains the LAC.

? I don’t get any results although I configured some PDUs to be demodulated. Even the MIB is not repor- ted. What’s wrong?

! Please check that your configuration allows the reporting of SIB 3 for that channel. You can verify this by checking if a PDU 17 is set either to mode 0 or 2 in the demo application. If not, configure the SIB 3 to be decoded in your start measurement setting.

? SIB10 and 16 are not enumerated in the ViCom etPDU. How can we select them (their corresponding PDUs) for decoding in ViCom, particularly SIB16?

! SIB 10 is not demodulated because it is only used in FDD and has been omitted completely as of UMTS TS25.331 Rel. 5. Currently SIBs which carry information in multiple occurrences are not demodulated in general. This is the reason, why SIBs 15.2, 15.3 and 16 have no related PDUs in the interface..

? BCH Ec/I0 threshold (lEcToIoThresholdInDB100) is the Ec/I0 threshold for which physical channel (P- CCPCH or CPICH)?

! It is the Ec/I0 of the P-CPICH. The data needed for demodulation are requested when the Ec/I0 of the P-CPICH is exceeding the given threshold. The CPICH is used as the scanner monitors all Node Bs by means of C-PICH Ec/I0 measurements and there is no extra measurement of the P-CCPCH Ec/I0 dur- ing the monitoring phase. But both values should be in a fix ratio per node B anyway. And actually both the P-CPICH and the P-CCPCH signals will be used for demodulation, as the channel estimation as a part of demodulation is done by the P-CPICH.

? What is the minimum configuration of decoded PDUs that have to be set to make the decoder work? ! You must request at least SIB3. No SIB PDU result will be provided until SIB3 has been decoded,

because the BCH Demodulator provides each SIB PDU together with CI (Cell Identity from SIB 3). E.g. if you request SIB 3 (= PDU 17) you will receive MIB, SIB3 and, if applicable, SB1 and SB2, after SIB 3 has been decoded by the BCH Demodulator.

? Once a SIB has been decoded for a Node B, is it ever decoded again at a later point in time? ! As long as the system is sure that the same signal is received, the first decoding result is considered

valid and returned. If there is an uncertainty about the signal source (for the same SC on the same channel, different frame timing or a longer pause between the measurement points occurred), the decoding process is repeated. To notify about this event, the scrambling code indicator is incremented. To be sure to have the same Node B in the case that two results have been found with two different indicator values, the CellID must be decoded. This behaviour can be changed when configuring the measurement. For each Node B, it is possible to define another strategy how often the PDUs shall be decoded. It is also possible to reset the stored results cache.

? Is there any effect in changing load for BCH demodulation in R&S ROMES with the R&S TSM Instru- ments in HiSpeed or HiDynamic mode?

! No, the load is fixed to 10%. This was a quite reasonable value that was found out with the R&S ROMES. With the NRE for the BCH demodulator the demodulations with much higher priority may be requested. The scheduling is influenced in a way that the R&S TSMx gets a maximum penalty in mea- surement rates on 10% between SIB decoding and not decoding.

? Is there a way to adjust R&S TSM Instruments Sync. Rate in ViCom as it is possible in R&S ROMES? If yes, how does this affect scheduling?

! If the R&S TSM Instruments are used with the R&S ViCom, new Node Bs are searched on each mea- surement. As this procedure (WCDMA synchronisation) is done with the same RF signal section as all the other measurements (RSCP, CIR etc.) the synchronisation does not affect the load of the R&S TSM Instruments or causes additional receiver usage. It affects only the processor load at the connec- ted PC. The difference in processing one RF section with and without synchronisation was up to 30% in the phase. At present, with more CPU cache, this difference is less than 10%. So we decided to syn- chronise on each RF sample with R&S ViCom and simplify R&S ViCom usage by elimination of another parameter. By the way, we are able to synchronise to a Node B in the high speed mode when it is visible for at least 1 ms.

7 R&S ViCom GSM Technology

The R&S ViCom GSM technology measurements offer a whole bunch of measurement methods to monitor and analyze GSM networks. Therefore, high sophisticated algo- rithms have been integrated in the R&S TSM Instruments software. Nevertheless, the API was designed to be as simple as possible. A basic understanding of the things happening behind the scene is still necessary to make full use of the power the device offers to you.

This chapter explains that functionality. In order to make you familiar with the different measurement modes the network scanner provides, the basic principles of how the network scanner actually finds the data are explained in the first section. The next sec- tion explains the usage of the sample application provided with the API. A step-by-step example helps you to do the first measurements.

● Measuring of GSM Signals... 80 ● GSM Measurements Demo Application...94 ● GSM BCH Demodulation... 99