GSM KPI Troubleshooting-Random Access Failure Rate(RASR)

1                                       Random Access Success Rate:-

1.1                                  Definition:-

It is the ratio between the Total no of failed random access (CS & PS) & the total no of RACH requests (CS & PS).

1.2                                  Formula:-

 Random Access Success Rate: = (RAACCFA/ (CNROCNT+PDRAC+RAACCFA))*100

 Counter Details:-

RAACCFA: =Total No of failed random access both CS & PS

CNROCNT: =Total No of accepted Random Access

PDRAC: = Number of accepted PS random accesses

MS requests for the allocation of SDCCH through RACH. This request from the MS on the uplink could either be as a page response (MS being paged by the BSS in response to an incoming call) or due to user trying to access the network to establish a call. Availability of SDCCH at the RBS will not have any impact on the Random Access Success.

1.3                                  Reason for degradation & Solution:-

*  Poor BCCH & BSIC Plan (Especially CO BCCH-BSIC).

*  Poor Coverage/Spillage

*Cells with High TA (Open Area Cells) & overshooting cells.

* Incorrect parameter settings (ACCMIN, MAXRET, CRO)

*  Hardware Issue (Faulty Antenna & Cable, Faulty TRX)

*  High Path Imbalance

*  Phantom RACH.

*  Incorrect Timer settings

1.3.1                            Poor BCCH & BSIC Plan (Especially CO BCCH-BSIC):--

If in a BSC the no of CO BCCH & BSIC usages are very high then it leads to high RACH Failures in that BSC & also if the cells in neighboring BSCs are also having CO BCCH BSIC then that will also impact on RACH Success. Change the Frequency & remove CO BCCH BSIC. 

1.1.1                            Poor Coverage/Spillage:--

Cells with Poor Signal Strength can cause High RACH failures. This happens for the cells located at cell boundary with Weak Coverage. Percentage of Call samples can be checked from MRR Recordings and can plan for Tilts, Azimuth Antenna Height Changes, and Power Changes. 

1.1.2                            Cells with High TA (Open Area Cells) & overshooting cells: ---

Cells which are facing towards Open area and can cause high RACH Failure due to Poor Coverage.

From MRR samples Tilt can Planned for the Cells & Azimuth change also if possible can be recommended.

1.1.3                            Incorrect parameter settings: ---

A.ACCMIN: -

It is the cell parameter that indicates the minimum received signal level at the MS required for accessing the system.

Range: 47 to 110

Default: 110

Incorrect ACCMIN values may lead to attempts from poor coverage regions. Optimized values for ACCMIN would ensure that MS camps on a cell with good downlink signal strength reducing the chances of failures. Normally it is defined as -110 due to which MS will attempt from poor coverage regions. Increase the Value to -102,-100 as MS will camp on a better cell as compared to ACCMIN -110.

B.CRO (Cell Reselection Offset (CRO) :-

Defines an offset to encourage or discourage MSs to select the cell while it is camping on another cell, i.e. perform a cell reselection.

Range: 0 to 63

Unit: dB

Default: 0

CRO is a value from 0 to 63. Each step represents a 2 dB step (0 to 126 dBm). This value is added to C1. A higher CRO value will make the cell tower more attractive to the MS. The higher the CRO, the more attractive the cell will be. The network might assign a CRO value to a cell if the network wanted to encourage mobile stations to utilize that cell. The network might want to do this in order to reduce the load on other cells during times of high traffic volume or to force MS's to a certain band.

If the neighbouring cells to a cell which is suspected to have poor RA Success Rate due to excess traffic from poor signal strength cell boundary , is kept higher than the cell in question , it will make the neighbouring cells more appealing for idle mode MS reselection across the cell boundaries. This approach can improve the RA Success Rate for cells with excess traffic on weak cell boundary (with good neighbours).

C.MAXRET:-

Defines maximum number of retransmissions an MS may do when accessing the system on RACH.

Type: Numeral

Range: 1, 2, 4, 7

Default: 4

Recommendation:--

·         In the areas where the radius of the cell is smaller than 3 km and the traffic volume is normal, such as urban non-busy areas, the MS MAX Retransmission should be set to 4.

·         For micro cells, the MS MAX Retransmission should be set to 2.

·         For micro cells where the traffic volume is heavy and cells where congestion occurs, the MS MAX Retransmission should be set to 1.

1.1.1                            Hardware Issue: ---

• Check Hardware Faults (TRX, Cell, Neighbour Cell blocked or down) and Follow the Hardware Failure Identification Procedure and Rectification.

·         Check for faulty antenna, loose /damaged jumper/feeder cables, VSWR alarms etc.

1.1.2                            High Path Imbalance:--

From MMR Recording we can find out the RX level average in DL & UL then we can calculate the Path loss simply by subtraction & if the output is high then we can say the path loss is high.

1.1.3                            Phantom RACH:--

·         The phantom RACH phenomenon is a general phenomenon in digital radio technology. It is therefore common to all GSM systems, not only Ericsson.

·         In the transceiver, the timeslot handler in charge of the RACH channel is listening for access bursts from the mobiles. These bursts contain a check sequence (8 bits) that is used to determine if the message is valid.

·         When the traffic is low in the cell, there is not many access bursts coming to the BTS. Most of the received signal is noise. If the receiver is sensitive, some of this noise might be interpreted as an access burst by the BTS: a certain bit pattern in the noise might match the valid check sequence.

·         When the traffic is high, the problem is not so disturbing as most of the noise will be covered by genuine access bursts. So there will not be so many phantom RACHs.

·         The more traffic you have, the less noise will be received in the RACH channel handler, and thus, the less phantom RACHs will be reported by the BTS.

·         This RACH phenomenon is unavoidable unless you diminish the receiver sensitivity or introduce a powerful filtering function in the signaling processor. But then you might lose some genuine access bursts.

·         When there is low traffic in a cell, it is possible to receive up to about 120 phantom RACHs per hour (one every 30 sec).

 

1.1.1                            Incorrect Timer Settings (T3122 Timer change):--

·         The timer is used to control how long the MS should wait with sending a new channel request after having received immediate assign reject. The timer value corresponds to the waiting indication value sent in the Immediate Assignment Reject message

·         Start Event:-When the MS receive Immediate Assignment Reject Message

·         Stop Event:-When the MS Sends RACH Request for the second time as in the 1st attempt the request gets rejected.

·         Default Value=10 Sec

·         Increase the value to 15 sec so that the RACH request attempt will decrease & will improve the RACH Success Rate