CAUSES OF QoS AND POSSIBLE SOLUTIONS

Although the most common QoS shortfalls and suggested possible higher level solutions are discussed, a detailed description of the functionality to be fine-tuned and parameter settings is beyond the scope of this paper.
Because coverage, spectrum utilization, and traffic load differ from one area to another and from one network to another, engineers must determine optimized parameter values for a specific area of a network.


Accessibility Optimization:

SDCCH Congestion
• Causes
SDCCH availability, high number of location updates, high level of short message service (SMS) traffic, high number of call set-up bids
• Action
– Check historical statistics of SDCCH availability. In some systems, time slots may go into sleep mode. Historical data can show if certain time slots are constantly idle. If this occurs over a long period of time and especially during the busy hour (BH), a base transceiver station (BTS) restart and retest validation may be required.
– Check for high number of location updates, call set-ups, and SMS traffic. Increasing the cell reselect hysteresis (CRH) will delay GPRS reselection. It might be wise to expand SDCCH resources, if possible. This can be done at the expense of one TCH, which can be converted to eight SDCCHs. It is advisable to aim for no SDCCH congestion at all times.


TCH Congestion
• Causes
TCH availability, missing neighbors, missing assignments in neighbor list, traffic distribution
• Action
– Check TCH availability. TCH time slots may go into sleep mode. Real-time data can show if certain time slots are constantly idle. If this occurs over a long period of time and especially during the BH, a BTS restart and retest validation may be required.

– Check for cell mean holding time (MHT) and compare it with that of the surrounding cells in the area. Greater MHT may be due to missing or incorrect neighbor cell definitions. Check the radio plan for missing neighbor cell assignments.
– Use traffic management (load shedding) techniques that force traffic originating near the cell border to the surrounding cells. This can be achieved with optimum use of capacity-efficient features such as directed retry, cell load-sharing (traffic reason handover or changing the handover hysteresis parameters), and handover
offset between two neighbor cells.
– In a hierarchical cell structure, distribute traffic to lower or higher cell levels as required, using layer threshold and layer threshold hysteresis.
– Redistribute traffic among cells within the same layer, using early handover from a congested cell to another cell. This can be accomplished by adjusting handover hysteresis and handover offset.
Note: The traffic distribution actions mentioned above will improve GPRS performance. They will reduce TBF multiplexing and the number of PS immediate assignment rejections and will also increase GPRS throughput.


Retainability and Quality Optimization
Deterioration of Performance with Sudden Increase in the Number of TCH Drops
• Causes
Hardware problem, handover problem
• Action
– Check historical statistics of TCH availability. Check if there are any alarms on the cell or the transceiver or any of the TCH time slots.
– Check historical handover performance for the cell. If some external neighbor cells (belonging to a different BSC or mobile switching center MSC) show no successful hand-overs, but only attempts, missing or incorrect handover definitions on the parent BSC or MSC could be the reason.
– Check whether any neighbor cells have been deleted or whether any are not on the air. If any neighbor cells are not on the air, the serving cell may suffer TCH congestion and show increased MHT. There will be an increase of immediate PS assignment rejections, TBF multiplexing, and reduction of GPRS throughput.


TCH Drops due to Downlink Signal Quality
• Causes
Downlink interference, coverage
• Action
– Identify cell pairs that have a high number of handover attempts with reasonable downlink (DL) quality. This will help to identify the approximate area where mobiles experience DL interference. Check how and
where the serving cell frequencies are reused to identify the interfering frequencies and plan a frequency change. This is valid for base-band frequency hopping systems. For synthesizer hopping systems, change the
hopping sequence number (HSN). If the GPRS user is in a high interference area, there will be high value for block error rate (BLR) and poor throughput.
– When statistics show that drops are due to downlink quality, the drops may be due to poor coverage. This is more common in hierarchical cell structures where traffic is forced down to lower layers using aggressive layer thresholds of –90 dBm or lower. Change the layer threshold to initiate earlier handovers to higher layers. Also modify the imperative (urgent) handover parameters to initiate earlier urgent handovers to higher layers due to bad quality. For cells on the same layer, use hysteresis and hysteresis offset to initiate early handover and modify the imperative handover parameters to also initiate earlier handover due to bad quality.


TCH Drops due to Uplink Signal Quality
• Causes
Uplink interference, antenna feeder system, coverage
• Action
– Use cell traffic recording (CTR) and check the uplink quality for certain timing advance (TA) values. Check the frequency plan to see what frequencies are used in these areas and schedule a frequency retune.
– If the cell serves with a high TA value, make the cell less attractive in idle mode, using cell reselect offset (CRO).

– There could be a problem in the antenna or feeder systems. Investigate for any alarms on the site. Initiate damage assessment on coaxial and antenna systems.
– Consider increasing antenna downtilt to reduce the service area of the cell. This can be done if there is coverage overlap so that a coverage hole is not created.


TCH Drops due to Both Links (BL) Signal Strength
and due to Sudden Loss
• Causes
Coverage, hardware faults
• Action
– This type of problem occurs in areas where a cell serves a tube or tunnel. To confirm this, run CTR for this cell. Check the CTR file for both uplink and downlink signal strength. If any cell is a better server than this cell, then initiate early handover using hysteresis and hysteresis offset.
– In hierarchical cell structures, if the affected cell is in a lower layer and if a cell from a higher layer is stronger in CTR, make early handover to the higher layer using layer threshold.
– In a duplexed transmit/receive situation, a problem could exist in the antenna or feeder systems. Investigate for any alarms on the site. Check the antenna feeder system.


TCH Drops due to Uplink Signal Strength
• Causes
Coverage, hardware faults
• Action
– Check for any missing neighbor cell relations or to see if any defined neighbors are out of service. Mobiles traveling in certain directions will run out of coverage and drop out.
– Run CTR for the affected cell and check TA values. If TA values are high, restrict the coverage by making the cell less attractive in dedicated mode with CRO and in idle mode by initiating early handover with hysteresis and hysteresis offset.
– Consider installing a tower-mounted amplifier (TMA) to boost the uplink and see if there is room for a TMA installation in the tower.
– Check downtilt and calculate if the existing downtilt is correct for the intended coverage area. Increase downtilt if necessary.

– There could be a problem in the antenna or feeder systems. Investigate for any alarms on the site. Check the feeder and antenna systems for proper operation.


Handover Performance Optimization
Handover due to Degraded Signal Quality
• Causes
Downlink interference, uplink interference, coverage, antenna feeder system
• Action
– Identify cell pairs that have a high number of handover attempts due to degraded signal quality. Check to see how and where the serving cell frequencies are reused to identify the interfering frequencies and plan
a frequency change. This is valid for baseband frequency hopping systems. For synthesizer hopping systems, change the HSN.
– When statistics show that drops are due to downlink quality, the drops may be due to poor coverage. In such cases, check the layer and layer threshold for the cell. Changing layer threshold will help when the cells are on different hierarchical layers. If the cells are on the same layer, change the value of hysteresis and hysteresis offset to initiate earlier handover.
– Run CTR for the affected cell and check TA values. If TA values are high, restrict the coverage by making the cell less attractive in dedicated mode with CRO and in idle mode by initiating early handover with hysteresis and hysteresis offset.
– There could be a problem in the antenna or feeder systems. Investigate for any alarms on the site. Check the feeder system.


Handover Attempts but no Successful Handover Assignments
• Causes
Co-base station identity code/broadcast control channel (co-BSIC/BCCH) planning error, missing neighbor definition on the BSC and/or MSC
• Action
– Co-BSIC/BCCH planning errors occur when a cell has two neighbors with the same BSIC and the same BCCH. Mobiles report measurements of the surrounding cells with their BSICs and BCCHs; the BSC uses this combination to identify the cell identification (CI) of these cells and might direct the handover to the wrong cell. This can result in many dropped calls in the area. This can be identified from many handover attempts with no successful assignments. Change the BSIC of one of the neighbor cells.
– Check handover performance if there are attempts but no successful assignments for some external neighbor definitions (neighbors on a different BSC and/or MSC). This is due to incorrectly defined external
cells, i.e., the external neighbor cell has been incorrectly defined as a neighbor to the serving cell’s BSC with either wrong location area code (LAC) or BSIC or BCCH.


CONCLUSIONS
Operator competency in managing performance and optimizing QoS is not easily taught; it is developed, rather, mainly through trial and error. There are three main mechanisms for evaluating and optimizing
QoS—customer complaints, drive test analysis, and statistical analysis. These mechanisms have
advantages and disadvantages and can be utilized in parallel in large optimization projects.
Customer complaints can be objective but are also misleading, and this mechanism is reactive.
Drive tests are good for bench-marking and more ideal for verifying applied optimization solutions. Statistical analysis can identify trends but does not provide solutions. However, it can be a powerful tool for an experienced engineer with good analytically skills to use to identify problems and apply optimization solutions. The plethora of statistics generated in the network switches data must be organized before analysis.
For effective network performance and evaluation, the monitoring process and statistical analysis must take place at different levels: network-wide, by geographical area or region, by city, at the BSC level, and at the cell level. Optimization solutions vary in different areas and networks but, as discussed in this paper, a
generic approach can be developed to monitor and optimize the QoS as networks continuously change in response to changes in offered traffic and business priorities.

Source: Michael Pipikakis is
a network planning and wireless
technology manager for Bechtel’s
Europe, Africa, Middle East,
and Southwest Asia Region.