INTERFERENCE in GSM

In GSM   systems, there can be interference to cells from their neighbor cells that use the same frequency or adjacent frequencies or both. There can even be interference from cells in other systems. Interference can be classified as Co-Channel Interference (CCI) and Adjacent Channel Interference (ACI) from the same system (intrasystem interference) or between different systems (intersystem interference).

Co-channel Interference

Because of frequency reuse in GSM  systems, the reused frequencies in other cells can interfere with the serving cell. CCI depends on the reuse plan. For example, the 7/21 reuse plan has a CCI greater than the 9/27 reuse and less than the 4/12 reuse plan. Therefore, to reduce the CCI it is recommended to increase the reuse pattern or, equivalently, increase the reuse distance between cells of the same frequency groups. The impact of increased CCI is a degradation in voice quality. The CCI is measured in terms of Carrier-to-Interference Ratio ( C/I) . For analog systems, C/I of 17 dB is considered appropriate for good voice quality.The BER for a digital system with and without diversity for different C/I levels.

Relationship between C/I and BER

Adjacent Channel Interference

The avoidance of using adjacent channels in neighbor cells is a good practice and is possible in a theoretical 7/21 reuse pattern. In practical systems, adjacent channel assignments in neighbor cells can occur quite often. When the carrier power is increased in a cell, it can cause interference to an adjacent channel in the neighbor cell. The IS-136 standard specifies acceptable levels for Carrier-to-Adjacent Interference Ratio (C/A) that produces acceptable voice quality in terms of BER for uplink interference as shown in figure

Overlay and Underlay Cells

In the overlay/underlay cell design, two cells (micro or macro) are defined at the same Radio Base Station (RBS) to transmit in the same direction, omnidirectional or sectored. The overlay cell has a lower output power, smaller coverage and provides service to Mobile Stations (MSs) close to the RBS. The underlay cell has higher power, wider coverage and provides service to an MS farther away from the RBS.

The underlay cell contains voice channels (analog or digital), control channels and a locating device. The overlay cell contains only a group of voice channels (digital or analog) and depends on the underlay cell to set up calls and perform handover. The reverse-link RSS during call origination or page response determines whether the overlay or the underlay cell is used.

Relationship between an overlay and underlay cell.

The main advantage of using overlay cells is to increase the traffic handling capacity without building new sites. Overlay cells can be used to reduce problems in the transition areas between small and large radius cells.

One of the disadvantages of the overlay cell is based on the assumption that its originating traffic is very close to the RBS. When the traffic is far away from the RBS, the overlay cell will not be used.

The ideal places to position overlay cells are on RBS sites, either in the center of or very close to major roads and highways, especially where the traffic is forced to move slowly during busy periods. They are most suitable at the edge of a suburban area. The overlay can handle the suburban traffic and the underlay can be used primarily for the rural area.

PDCH: Packet Data Channel in GPRS

PDCHs may also be used by TBFs in Dual Transfer Mode (DTM). The DTM TBFs may either use PDCHs already allocated for normal GPRS/EGPRS traffic or trigger allocation of additional PDCHs that then can be used for normal GPRS/EGPRS traffic too. Some PDCHs can later be returned to the CSD when they are no longer needed, or when CS traffic or Abis demand requires it.

There are different types of PDCHs; dedicated PDCHs, semi-dedicated PDCHs and on-demand PDCHs.

Dedicated PDCHs:

The number of dedicated PDCHs in a cell is set by the operator. These PDCHs are allocated at configuration from the CSD to the PSD for GPRS/EGPRS traffic and can then not be pre-empted by CS traffic. A GSL resource and an Abis resource is allocated at the time of allocation. The number of dedicated PDCHs can only be decreased by the operator.

Semi-dedicated PDCHs:

The number of semi-dedicated PDCHs in a cell is also set by the operator. These PDCHs are allocated at configuration from the CSD to the PSD for GPRS/EGPRS traffic and cannot be pre-empted by CS traffic. The number of semi-dedicated PDCHs can only be decreased by the operator. Semi-dedicated PDCHs will be activated first when there is need for more PDCHs for PS traffic, that is the GSL resource is not allocated until needed. The Abis resource is not allocated until needed either if the channel has a flexible Abis resource, When there is no longer any traffic need for a semi-dedicated PDCH (no TBF is reserved on it), it will be put in the PSD idle list (PIL). Eventually the GSL resource and the Abis resource (if a TCH with flexible Abis is used) will be returned to the respective pool.

On-demand PDCHs:

On-demand PDCHs are allocated from the CSD only when there is need for more PDCHs for PS traffic. At on-demand PDCH allocation also the GSL resource is allocated. The Abis resource is also allocated if the channel has a flexible Abis resource,  When there is no longer any traffic need for an on-demand PDCH (no TBF is reserved on it), it will be put in the PIL. Eventually the GSL resource and the Abis resource (if a TCH with flexible Abis is used) will be returned to the respective pool and the PDCH will be deallocated from the PSD and returned to the CSD. On-demand PDCHs can also be pre-empted by CS traffic.

Active Queue Management (AQM):

Active Queue Management (AQM) is a queue management feature for the downlink. For applications using TCP/IP as transmission (e.g. FTP, web browsing, email), it is important with a rapid feedback of the radio link data rate to the TCP/IP protocol. Feedback of the radio link data rate is reported to TCP/IP by discarding IP-packets. In this way the TCP/IP protocol will faster adjust its send rate according to the radio link capacity and TCP slow-starts are avoided. If the feature “QoS and Scheduling” in BSC is active (R97 QoS and/or R99 QoS), the queue management feature is applied to Interactive class and Background class data. The queue management feature is not applied to signalling messages, SMS, LLC acknowledged mode data, EIT data or Streaming class data when QoS is active. For SMS, signalling messages, LLC acknowledged mode data, EIT data and Streaming class data a maximum buffer is set and when the buffer is overflowed packets are discarded starting from front of the buffer.

If the feature “QoS and Scheduling” in BSC is passive, the queue management feature is applied to all payload data but not to signalling messages, SMS or LLC acknowledged mode data. For LLC acknowledged mode data and data not handled by AQM a maximum buffer is set and when the buffer is overflowed packets are discarded starting from front of the buffer.

Radio Network Optimization (RNO) Troubleshooting Tools

TEMS Investigation TEMS Investigation is a drive test tool within the TEMS product family. It consists of a TEMS mobile station, a PC with the TEMS Investigation software and a GPS receiver. The uplink and downlink information on the air-interface is monitored and recorded together with the positioning data from the GPS. TEMS Investigation, here referred to as TEMS, is a very powerful tool for field measurements during troubleshooting in specific areas of the network.

MTR Mobile Traffic Recording (MTR) records the events and measurements on both the uplink and downlink connected to a certain subscription, which can be useful when a subscriber complains and the cause is to be investigated. MTR is also very useful together with TEMS. From TEMS geographical information can be retrieved but not from MTR.

CER Channel Event Recording (CER) measures interference on the frequencies defined in the cell and is used when the performance of the channel allocation strategy is investigated. Idle Channel Measurement (ICM) or Differential Channel Allocation (DCA) is required for this recording,

CTR Cell Traffic Recording (CTR) collects data about connections in specific cells. Certain events can be used as triggers and all communication on up- and downlink is recorded. CTR could be used if there are specific problems found in any cell, such as an abnormal number of Traffic Channel (TCH) drops.

GMLOG The GPRS/EGPRS Mobile Logging (GMLOG) function logs protocol information in the RLC/MAC and the BSSGP protocols for one whole cell or four individual mobiles. All relevant information is logged together with related internal events. The Mobile Logging function is

a valuable function for GPRS/EGPRS verification and detailed troubleshooting purposes. The function can be used by the operator staff to verify network functions and characteristics. The function gives the possibility to correlate A-bis and Gb-data. It displays BSC internal events, which is not possible with external protocol analyzers. The logging capability is high, due to the fact that the logged data is directly transferred from the RPP to the logging client via Ethernet.

Read Also:

Measurement Tools for Radio Network Optimization (RNO)

Measurement Tools for Radio Network Optimization (RNO)

STS: Statistics and Traffic Measurement Subsystem (STS) is implemented in the BSC (and MSC). It gives statistics about events in different parts of the system such as cells and equipment. By continuously supervising the results from STS the operator can obtain a very good overview of the radio network performance which can help to detect problems early. For further information,

MRR Measurement Result Recording (MRR) collects information from the measurement results sent by the BTSs to the BSC. Information such as RXLEV, RXQUAL etc. is included. The tool is for instance used for routine supervision or for checking specific cells. MRR is a part of the Radio Network Optimization (RNO) package in OSS

TEMS Automatic Tems Automatic is a tool within the TEMS product portfolio, where several special mobile stations are placed in for instance taxis and buses. The set of mobiles are supervised centrally and the measurements are sent directly to this center. TEMS automatic provides the operator with information about subscriber perceived quality from many parts of the network.

R-PMO The real-time performance monitor provides real-time statistics in order to receive instant feedback of performance from sudden changes of the network, either by the network itself (e.g. hardware faults) or by operator initiated changes (i.e. parameter, feature or frequency changes). For operator initiated changes, faster tuning can be achieved. R-PMO also provides a high degree of detailed information, such as timestamps on events, and flexibility, such as user defined reports.

NOX Neighbouring Cell List Optimization Expert (NOX) is a tool meant as a support for the operator for optimization of the neighbouring cell relations. This is done by collecting and handling data from measurement reports, handover statistics and general network configurations. The outcome is suggestions to remove superfluous or add new neighbouring cell relations. The user can set whether the changes should be implemented automatically or require an approval by the user.

FOX Frequency Optimization Expert (FOX) measures for possible interferers in order to find suitable frequencies to define in cells. FOX supplies the operator with suggestions about frequencies at e.g. network/hardware expansions or frequency reallocations.

SYROX Synchronized Radio Network Optimization Expert (SYROX) is a tool intended to support the operator with planning of parameters that control the frequency hopping for a group of Synchronized Cells in order to minimize the interference in the network. Apart from the fact that a group of mutually synchronized cells is required, SYROX also requires that the optional BSS features FAS (FOX recording mode), and Flexible MAIO Management are available.

Read Also Topics:  Radio Network Optimization (RNO) Troubleshooting Tools

Tight BCCH Frequency Reuse

It means use Fewer BCCH frequencies in the network. Thus more frequencies can be used in other channel groups in order to increase the Network traffic capacity,

The capability of this feature is to make it possible to use fewer BCCH frequencies. This is achieved by applying separate pathloss and DTCB criteria to the BCCH CHGR. That makes it possible to use a tighter reuse than without the feature, or to enhance the speech quality and keeping the existing reuse.

Tighter BCCH frequency reuse is possible regardless of if subcells are used or not.

Statics Counters are used in Tight BCCH Frequency Reuse:

BCDTCBCOM is stepped by intra-cell handover attempt out of BCCH CHGR, BCCHDTCB criteria.

BCLOSSCOM is stepped by intra-cell handover attempt out of BCCH CHGR, BCCHLOSS critera.

BCDTCBSUC is stepped by successful intra-cell handover out of BCCH CHGR, BCCHDTCB criteria.

BCLOSSSUC is stepped by successful intra-cell handover out of BCCH CHGR, BCCHLOSS criteria.

In a tight BCCH re-use plan, careful planning is required in order to avoid BCCH confusion during handover, which can have a negative impact in the performance, particularly for fast moving MS (e.g. in highways); in this case, the mobile initially decode the correct BSIC of the neighbor cell and will continue measuring that frequency but won’t decode the BSIC information for every measurement report  the MS shall attempt to demodulate the SCH on the BCCH carrier of as many surrounding cells as possible, and decode the BSIC as often as possible, minimum at least once every 10 seconds, to confirm that it is monitoring the same cell), if both co-BCCH cells are closely located, after

a few seconds a fast moving mobile could be measuring a different cell (the co-channel BCCH) but will report it incorrectly with the old BSIC (the BSIC originally decoded from the neighbor cell) to the system. Since sometimes it’s impossible to avoid close co-BCCH cells in a tight re-use plan, then in order to minimize the effect of BSIC confusion, the closest co-channel BCCHs should be allocated away from highways, preferably towards areas with low mobility MS (e.g. residential areas).

Main Controlling Parameters

BCCHREUSE is a parameter activating/de-activating the feature Tight BCCH Frequency Reuse, set per cell.

BCCHLOSS is a parameter indicating maximum allowed path loss of the BCCH carrier, set per cell.

BCCHDTCB is a parameter expressing the limit of the calculated difference in signal strength between the BCCH of the serving cell and the strongest non-cosited neighbor cell within the same frequency band, set per cell.

BCCHLOSSHYST is a parameter for the hysteresis of the BCCHLOSS parameter, set per cell.

BCCHDTCBHYST is a parameter for the hysteresis of the parameter BCCHDTCB, set per cell.

Value Ranges and Default Values

Parameter name	Default value	Recommended value	Value range	Unit
BCCHREUSE	0 (NORMAL)		0,1 (0=NORM AL, 1=TIGHT)
BCCHLOSS	200		0 to 200	dB
BCCHDTCB	-63		-63 to +63	dB
BCCHLOSSHYST	3		3 0 to 63	dB
BCCHDTCBHYST	2		3 0 to 63	dB