High Handover Failure Ratehttp://goengineer.wordpress.com/2009/09/16/high-handover-failure-rate/

High handover failures rate will probably be due to one or more of the following reason.

1. High Neighbour Interference

2. No Dominant Server

3. Database Parameters

Optimization for tips :

1. High Neighbour Interference

While handling off to the best neighbor the interference on the target cell frequency may result in the hand off failure.

TIPS:

When designing the cell frequencies care should be taken that there is proper frequency spacing between the cells to avoid neighbour interference. In most of the cases Ping Pong Handover starts i.e. the mobile hand off to a cell for better level and due to interference (Quality issues) hand off again to original cell. A through drive test can determine the “interfering frequencies” which should be eliminated from the frequencies plan.

2. No Dominant Server

If cell sites are designed poorly there might be areas where neighbour being received at the same level and some neighbour randomly look good for hand off for a certain amount of time. Such situation is disastrous because handoff decision will be hard and mostly it will end up unsuccessful handovers.

TIPS :

Antenna tilts provide the good way to reduce the footprint of the sites. Efforts should be made that a single dominant server should serve the specific area. Timing advance limitation is applied to cell areas where there are multiple servers.

3. Database Parameters

Received level, receive quality and power budget algorithm are set in the system information to define the criteria for handover. Improper values for these criteria may result in poor handoff.

TIPS:

Enable the “per neighbor” feature which displays the successful and unsuccessful handovers on a per cell basis. In optimization, monitor the following stats, which come under “cell statistic category”.

All those cells can be identified which are problematic in terms of hand off so one can focus only specific cell causing the major contribution towards poor HSSR. Ensure that handover margins are optimized. Rule of thumb is a 4 dB for adjacent frequencies and 6 dB per cell without adjacent frequencies. The following parameters can be played for defining the threshold for imperative and non-imperative handovers.

a. Defines the lower threshold for uplink quality handovers.

Ex: Range 0 to 1800

        Step size = 0.01

a value of 500 defines the lower threshold value of 5 (BER) for a quality handover to be triggered for uplink. The optimum for this threshold is 500.

b. Defines the lower threshold for downlink quality handovers.

c. Defines the lower threshold for received level uplink handovers.

Ex:   A value of 20 defines the threshold value of -90 dBm for a level handover to be triggered for uplink.

Range 0 to 63

Where 0 = -110 dBm

1 = -109 dBm

63 = -47 dBm

The optimum value for this threshold is 15 ( – 95 dBm). If the signal level goes below that, a level handover is initiated.

d. Defines the lower threshold for received level downlink handovers.

e. Defines the upper threshold for uplink interference handovers

f. Defines the upper threshold for downlink interference handovers

High Call Drop Rate (HCR) Guidelines

For healthy network the drop call rate should be less than 1%. There is again number of reasons, which could contribute towards higher dropped call rate is:

1. Drop on Handover

2. Low signal Level

3. Adjacent channel Interference

4. Co-channel Interference

5. Extraneous Interference

6. Link Imbalance

Optimization for Tips :

1. Drop on Handover

The call may drop on handover. It’s mostly high neighbor interference on the target cell, which causes the main problem. Sometime the mobile is on the wrong source cell (not planed for that area but serves due to the antenna overshoot) which may result in the drop call.

TIPS:

Within optima, monitor the following statistic. These statistics are defined under the category of BSC level statistics.

a. total and successful handover  on UL/DL quality

b. total and successful handover on UL/DL signal strength.

c. total and successful power budget handovers.

From the above statistics, quality or level must be estimated. 

2. Low signal Level

Signal level below -95 dBm is considered to be poor. If the mobile is unable to handoff to a better cell on level basis, the call will possibly be dropped. Topology or Morphology issues may

also be there like if Mobile enters into a tunnel or a building , higher RF losses will be developed.

TIPS:

First of all path balances should be checked. If path balances are deviating from the standard value then check the BTS transmitted power with the help of wattmeter. BTS may transmit low power because of the malfunctioning of radio or higher combiner losses. Also check the feeder losses, antenna connectors. Enable Downlink power control. Power control is being directional. The lower and upper receive level downlink power control values should be properly defined.

a. I_RxLev_DL_p

Defined the lower value for receive level for the power control to be triggered.

Range           0 to 63

Where           0 = -110 dBm

                       1 = -109 dBm

                       63 = -47 dBm

Example: If the value of 20 is set it means that the BTS will start transmitting more if it senses that downlink receive level is below -90 dBm.

b. U_RxLev_DL_p

Defines the upper threshold value for receive level for the power control to be triggered (Range is same as described above).

Example: On setting the value of 50 (equivalent to -60 dBm) BTS will lower down the power.

3. Adjacent and Co-channel Interference

Frequency planning plays a major role to combat adjacent channel and Co-channel Interference. Co channel is observed mostly when mobile is elevated and receives signals from cell far away but using the same frequencies.

TIPS:

An Optimization tools like Neptune could be helpful in identifying the interference on the particular area. Such frequencies can be cleaned from existing frequency plan. The following statistic can also be monitored to confirm that there is interferences issues in the cell. These stats are defined in optima under the category of BSC stats.

a. TCH Interference level 1

b TCH Interference level 2

c. TCH Interference level 3

d. TCH Interference level 4

When a TCH timeslot is idle it is constantly monitored for an uplink ambient noise. During a SACCH Multiframe an idle timeslot is monitored 104 times. These samples are the processed to procedure a noise level average per 480 ms. An interference band is allocated to an idle slot depending upon the interference level. The threshold for these levels can be set in the system parameters. Interference level 1 being the least ambient and interference level 4 being the most ambient. While planning the Network care should be taken that the cells do have the proper frequency spacing.

4. Extraneous Interference

Extraneous Interference might be from:

a. Others mobile network

b. Military communication

c. Cordless Telephones

d. Illegal radio communication equipment.

TIPS:

External interference is always measured through spectrum analyzer which can scan the whole band. Some spectrum analyzer can even decode voice from AMPS circuits or Cordless Phones.

5. Link Imbalance

Sometime the multifunctionality of vendor hardware becomes responsible for high Call Drop Rate. One of the possible scenarios could be :

a. Transmited and receiving antenna facing different direction

b. Transmited and receiving antennas with different tilts.

c. Antenna feeder damage, crossion or water ingress.

d. Physical obstruction.

SDCCH BlockingSDCCH Blocking is probably due to one or more reasons

1. No Access to SDCCH

2. Failure before Assignment of TCH.

3. High Paging Load

4. Incorrect or Inapproriate timer values

Optimization for tips :

1. No Access to SDCCH

This cases before has already been discussed

2. Failure before Assignment of TCH

This cases before has already been discussed

3. High Paging Load

Irreguler paging distribution in location areas results in SDCCH Blocking. Higher paging load in certain location area mean higher location updates on SDCCH resulting in SDCCH Blocking.

TIPS :

 A location area with a high paging load needs to be reduced in size to relieve SDCCH Blocking. A location area with low paging load need to be enlarged in size to reduced the overal number of location ares.

4. Incorrect or Inapproriate timer values

Timer rr_t3 111 sets the amount of time allowed to delay the deactivation of a traffic channel (TCH) after the disconnection of the main signaling link.

TIPS :

The suitable value for this timer is 1200 ms (max being 1500 ms). The timer will cause the BSS to wait before the channel in question is allocated another connection. A lower value of timer will result in higher capacity since the channel is held for less time before being released.

TCH BlockingTCH Blocking maybe due to the following reasons

1. Handover and Power budget margins

2. Cells too large

3. Capacity Limitations (Congestion)

4. Incorrect or Inappropriate Timer

Optimization for tips:

1. Handover and Power budget Margins

Handover Margins should be properly optimized to move the traffic to neighboring cell. Strict handover margins can results in lower handovers and ultimately congestion in cell.

TIPS:

6 dB handover margins are considered to be an appropriate margin for handover. A strict handover margin results in the strict criteria for Power Budget Handovers also. Setting a lower value of handover margin will initiate ping pongs handovers, which are not considered good for network health. (Handover margin have already been discussed).

2. Cells too Large

If cell are too large meaning antenna too high or antenna too shallow, it will pull in out of area traffic again causing congestion in the cell.

TIPS:

Consider reducing antenna height to reduce the footprint of the site. Also increase the antenna tilt (the max tilt is 12)

SDCCH Success Rate (SD-SUC)SDCCH Success Rate is percentage of MS Call Setup success due to Timeslots at the first call.

Where:

 SDCCH success rate = [(Call Setup Success - Call Setup Failure)/ Call setup success] x 100%

The call setup rate should be above 90% for a healthy network. However a CSSR of 85% – 90% is satisfactory.

There could be so many reasons for a poor CSSR. Some are described as follows:

1. No access to SDCCH

2. CM Service Reject

3. TCH Failure Assignment

4. Hardware Problem

Optimization Tips for:

 1. No Access to SDCCH.

BSS detects channel request (in the form of RACH) from a source, requesting resources for networks transactions. After validation of the RACH, BSS will attempt to allocate a dedicated channel (SDCCH) for the source. Once the availability of SDCCH channel is confirmed, the BSS will send immediate assignment to MS indicating the dedicated SDCCH sub-channel (via AGCH), whereby subsequent message exchange will be performed over the dedicated SDCCH.

Case:

a. Valid RACH (SDCCH Congestion)

Due to unavailability of SDCCH, BSS will response to MS with immediate assignment reject, terminating the transactions. In which case, call setup is termed as unsuccessful due to SDCCH congestion.

Invalid RACH (Invalid established cause detected in the received RACH)

b. Phantom RACHs

The received RACH is in fact generated from an “unknown source”, whereby it fails to continue the transaction after SDCCH has been allocated by the BSS. For instances, case of channel request detected by overshooting cells, handover access burst from distanced MS,

hardware deficiency, UL/DL imbalance path, MS moving out of range would carry the Phantom RACHs symptoms.

  TIPS:  

 Within the optima there are certain stats which can be monitored before coming to conclusion that there is SDDCH problem : 

a. SDCCH Blocking

b. SDDCH Congestion (Valid RACH)

If the SDCCH blocking greater than 1% or SDCCH Congestion greater than 2% then that mean that it is a capacity related issue and more slots should be assigned for SDCCH.

A TCH can be allocated by passing SDCCH. A parameter namely Immediate Assign Mode when enabled allocates TCH by passing SDCCH.

2. CM Service Reject

 CM Service Request (MOC) or Paging Response (MTC) to BSC/MSC. Inside the CM service request message (MS initiated service request), MS informs the network the types of service it requires (i.e. Mobile Originationl, Emergency Call, Short Message transfer or Suplementary services activity), wherby Paging response is specific to MTC. Subsequently, BSS embraces the information with its own initiated connection request BSSMAP message, send to MSC to approval. MSC will response with either connection confirmed, confirming the success in link establishment between MS-BSC-MSC, or connection Refused, Indicating the termination of the specific network transaction.

3. TCH Failure Assignment

Upon completion of MS/BSC/MSC link established, MS issues Assignment Request to BSC, Requesting TCH Assignment to the dedicated MS. Subsequently, BSS will attempt to allocated free TCH for MS voice messaging. Once Assignment Command is received by MS, stating the availability of TCH for the MS, it will move to the dedicated TCH and responds with Assignment Complete. In turns, BSS will submit Assignment Complete to MSC as to complete the signal activity.

Case :

TCH Congestion

TIPS :

For TCH Congestion certain features can be enabled like TCH queuing , Directed Retry and Congestion Relief.  In case of the TCH queuing feature is enable, MS will queue in the Original SDCCH , awaiting for the next available TCH. It is to be remainded that once Queuing timer

expires. BSS will also terminates transactions, in which case, call setup is termed as unsuccessfull due to TCH Congestion. The same situation also applies in situation where Congestion Relief feature is enabled. In the case of Directed Retry feature is enabled, MS will perform Handover to TCH of another cell if a valid handover neighbor is detected. The best thing to do is to add more radios in the cell to remove congestion.

Interference analysis on a particular carier can be done through an optimization tools like Neptune. Once interfering frequencies are determind, the frequency plan can be cleaned from such frequencies.

4. Hardware Problem 

Hardware failures also play major role for poor CSSR. Improper functinality of any BTS hardware can affect the overall performance of sites.

TIPS :

If there are no capacity or RF issues then equipment needs to be checked. Before starting the drivetest make sure that the cell site are free for any hardware alarms. The important parameter to check is the path balance. If path balances are not fine then start checking the power from radio to connected antennas. If we take the examples of GSM 900 scenario, the link budget defines that the radio should transmit 40 watts power  and at the top of the cabinet, 20 watts are recaived ( Considering the 3 dB loss of combiner). While checking the power, if any components seems to procedure more losses than expected, change that component. Similarly check the power at antenna feeder ports. Some time due to the water ingress, connectors get rusty and needs to be replaced.

TCH Call   drop

causes of TCH Call Drop

Usually, the typical causes for sending the Clear_Request mssg are as follows ;

1. Radio Inteface Failure

2. O & M Intervention

3. Equipment Failure

4. Protocol error

5. Preemption

Measurement Point Of successful TCH seizuresMeasurement Point of  successfull TCH seizures

1. When CH_ACT_ACK message is received in the immediate assignment process. Since there is no SDCCH available, TCH channel is directly assigned.

2. When CH_ACT_ACK message is received, the calling status is CS_WAIT_RR_EST(Waiting RR setup status) and the present channel is TCH.

 

3. When assignment complete message is sent in process of assignment.

 

4. When the MSG_ABIS_HO_DETECT message is received during incoming inter BSC inter cell handover, and the handover type is non-SDCCH handover.

1. When the MSG_ABIS_HO_DETECT message is received  during internal inter cell handover, and the handover type is non-SDCCH handover.

2. When the CLEAR-CMD message with the cause of HO_SUCC or CALL_CTRL is received from the MSC during inter-BSC handover , and the handover cause is directed retry.

Direct retry process:

When the CLEAR-CMD message, whose cause values is HO_SUCC or CALL_CTRL, is received from the MSC during inter BSC handover , and at the same time the handover cause is direct retry, one measurement point will be counted and added to successful TCH seizures times