RSL Link.
TCH Blocking maybe due to the following reasons 1. Handover and Power budget margins 2. Cells too large 3. Capacity Limitations (Congestion) 4. Incorrect or Inapproriate Timer Optimization for tips : 1. Handover and Power budget Margins Handov Handover er Margin Margins s should should be be proper properly ly opti optimise mised d to move move the traffi traffic c to neigh neighbou bourin ring g cell. cell. Strict Strict hand hand TIPS : 6 dB hando handover ver margin margin is considered considered to be an an approriate approriate margin for for handover. handover. A strict strict handover handover mar 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 TIPS : Consider Consider reducing reducing antenna antenna height height to reduce reduce the footprin footprintt of the site. Also Also increase increase the antenna antenna tilt tilt (t
ver margins can results in lower handovers and ultimatelly congestion in cell.
gin results in the strict criteria for Power Budget Handovers also. Setting a lower value of handover mar
again causing congestion in the cell.
he max tilt is 12)
in will initiate ping pongs handovers, which are not considered good for network health. (handover mar
in have already been discussed).
SDCCH Blocking
SDCCH Blocking is probably due to one or m ore 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 cert TIPS : A location area with a high paging load needs to be reduced in size to relieve SDCCH Blocking. A l 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) TIPS : The suitable value for this timer is 1200 ms (max being 1500 ms). The timer will cause the BSS to
in location area mean higher location updates on SDCCH resulting in SDCCH Blocking.
cation area with low paging load need to be enlarged in size to reduced the overal number of location a
after the disconnection of the main signaling link.
ait before the channel in question is allocated another connection. A lower value of timer will result in hi
res.
gher capacity since the channel is held for less time before being released.
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 neighbour the interference on the target cell frequency may result in t TIPS: When designing the cell frequencies care should be taken that there is proper frequency spacing b 2. No Dominant Server If cell sites are designed poorly there might be areas where neighbour being received at the same l TIPS : Antenna tilts provide the good way to reduce the footprint of the sites. Efforts should be made that a 3. Database Parameters Received level, receive quality and power budget algorithm are set in the system information to defi TIPS: Enable the ―per neighbour‖ feature which dispalys the successful and unsuccessful handovers on a All those cells can be identified which are problematics in terms of hand off so one can focus only s 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 trigge b. Defines the lower threshold for downlink quality handovers. c. Defines the lower thr eshold for received level uplink handovers.
Ex : A value of 20 defines the threshold value of -90 dBm for a level handover to be triggered 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, d. Defines the lower threshold f or received level downlink handovers. e. Defines the upper threshold for uplink interference handovers f. Defines the upper threshold for downlink interference handovers
e hand off failure.
tween the cells to avoid neighbour interference. In most of the cases Ping Pong Handover starts i.e the
vel and some neighbour randomly look good for hand off for a certain amount of time. Such situation is
single dominant server should serve the specific area. Timing advance limitation is applied to cell area
e the criteria for handover. Improper values for these criteria may result in poor handoff.
per cell basis. In optimization, monitor the following stats, which comes under ―cell statistic category‖. ecific cell causing the major contribution towards poor HSSR. Ensure that handover margins are optimi
ed for uplink. The optimum for this threshold is 500.
for uplink.
a level handover is initiated.
mobile hand off to a cell for better level and due to interference (Quality issues) hand off again to origin
disastrous because handoff decision will be hard and m ostly it will end up unsuccessful handovers.
where there is multiple servers.
sed. Rule of thumb is a 4 dB for adjecent frequencies and 6 dB per cell without adjecent frequencies. T
l cell. A through drive test can determine the ―interfering frequencies‖ which should be eliminated from t
e following parameters can be played for defining the threshold for imperative and non-imperative han
he frequencies plan.
overs.
High Call Drop Rate (HCR) Guidelines
For healthy healthy network network the drop drop call rate rate should should be less less than 1%. There There are again again number of reasons, reasons, whi whi 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 handov handover. er. It’s mostly mostly high high neighbor neighbor interference interference on on the target target cell, which which caus caus TIPS : Within optima, monitor monitor the following following statistic. statistic. Theses Theses statistics statistics are defined defined under the category category of of BS a. total and successful handover on UL/DL quality 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 Signal level below below -95 -95 dBm is considere considered d to be poor. poor. If the mobile is unable unable to to handoff handoff to a better c TIPS: First of of all path balances balances should should be checked. checked. If path balances balances are are deviati deviating ng fro the standa standard rd value value t a. I_Rx Lev_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 value of 20 is set it means that the BTS will will start transmitting transmitting more if it sense sense b. U_RxLev_DL_p Defines the upper threshold value for receive receive level for the power control to be triggered (Rang 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 planning plays plays a major role to combat adjecent channel and Co-channel Interference. C TIPS : An Optimization tools like Neptune could be helpful in identifying the interference on the particular a 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 4. Extraneous Interference Extraneous Interference might be from : a. Others mobile network b. Military communication c. Cordless Telephones d. Illegal radio communition equipment. TIPS : External External interference interference is always always measured measured through through spectrum spectrum analyser analyser which can can scan the whole whole ban 5. Link Imbalance Sometime the multifunctionality of vendor hardware becomes responsible for high Call Drop Rate. a. Transmited and receiving antenna facing different direction b. Transmited and receiving antennas with different tilts.
c. Antenna feeder demage, crossion or water ingress. d. Physical obstruction.
h could contribute towards higher dropped call rate is :
s the main problem. Sometime the mobile is on the wrong source cell (not planed for that area but ser
C level statistics.
ll on level basis, the call will possibly be dropped. Topology or Morpology issues may also be there like i
en check the BTS transmited power with the help of wattmeter. BTS may transmit low power because
that downlink receive level is below -90 dBm.
is same as described above).
channel is observed mostly when mobile is elevated and receives signals from cell far away but using
ea. Such frequencies can be cleaned fr om existing frequency plan. The following statistic can also be
Multiframe an idle timeslot is monitered 104 times. These samples are the processed to procedure a n
d. Some spectrum analyser can even decode voice from AMPS circuits or Cordless Phones.
ne of the possible scanarios could be :
es due to the antenna overshoot) which may the result in the drop call.
f Mobile enters into a tunnel or a building , higher RF losses will be develoved.
f the malfunctioning of radio or higher combiner losses. Also check the feeder losses, antenna connect
he same frequencies.
onitored to confirm that there interferences issues in the cell. These stats are defined in optima under t
ise level average per 480 ms. An interference band is allocated to an idle slot depending upon the inter
rs. Enable Downlink power control. Power control is be directional. The lower and Upper recieve level d
e category of BSC stats.
ference level. The threshold for these levels can be set in the system parameters. Interference level 1 b
ownlink power control values should be properly defined.
eing the least ambient and interference level 4 being the most ambient. While planning the Network car
should be taken that the cell do have the proper frequency spacing.
SDCCH Success Rate is percentace of M S Call Setup success due to TimeSlot 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 satisf 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 detacts channel request (in the form of RACH) from a source, requesting resources for networ Case : a. Valid RACH ( SDCCH Congestion) Due to unavailability of SDCCH, BSS will response to MS with immediate assignment reject, t Invalid RACH (Invalid established cause detected in the received RACH) b. Phantom RACHs The received RACH is in fact genereted from an ―unknown source‖, wherby it fails to continue TIPS: Within the optima there are certain stats which can be monitored before coming to conclusion that t
a. SDCCH Blocking b. SDDCH Congestion (Valid RACH) If the SDCCH blocking greater than 1% or SDCCH Congestion greater than 2% than that mean tha A TCH can be allocated by passing SDCCH. A parameter namely Immediate Assign Mode when en 2. CM Service Reject CM Service Request (MOC) or Paging Response (MTC) to BSC/MSC. Inside the CM service requ
3. TCH Failure Assignment Upon completion of MS/BSC/MSC link established, MS issues Assignment Request to BSC, Reque Case : TCH Congestion TIPS : For TCH Congestion certain features can be enabled like TCH queuing , Directed Retry and Conge Interference analysis on a particular carier can be done through an optimization tools like Neptune. 4. Hardware Problem Hardware failures also play major role for poor CSSR. Improper functinality of any BTS hardware ca TIPS : If there are no capacity or RF issues then equipment needs to be checked. Before starting the drive
ctory.
s transactions. After validation of the RACH, BSS will attempt to allocate a dedicated channel (SDCCH
rminating the transactions. In which case, call setup is termed as unsuccessful due to SDCCH congesti
the transaction after SDCCH has been allocated by the BSS. For instances, case of channel request de
here is SDDCH problem :
it is a capacity related issue and mor e slots should be assigned for SDCCH. abled allocates TCH by passing SDCCH.
st message (MS initiated service request), MS informs the network the types of service it requires (i.e.
sting TCH Assignment to the dedicated MS. Subsequently, BSS will attempt to allocated free TCH for M
tion Relief. In case of the TCH queuing feature is enable, MS will queue in the Original SDCCH , awaiti nce interfering frequencies are determind, the frequency plan can be cleaned from such frequencies.
n affect the overall performance of sites.
est make sure that the cell site are free for any hardware alarms. The important parameter to check is t
for the source source.. One the avail availabi abili lity ty of SDCCH SDCCH channe channell is confir confirmed, med, the BSS will will send send immediat immediate e assig assig
on.
tect tected ed by over oversh shoo ooti ting ng cell cells, s, hand handov over er acce access ss burs burstt from from dist distan ance ced d MS, hard hardwa ware re defi defici cien ency cy,, UL/DL UL/DL im
obile Originationl, Emergency Call, Short Message transfer or Suplementary services activity), wherby
S voice messaging. messaging. Once Assignment Assignment Command is received received by MS, stating stating the availabili availability ty of TCH for t
ing for the next next avail availabl able e TCH. TCH. It It is to be be remain remainded ded that that once once Queu Queuing ing timer timer expire expires. s. BSS BSS will will also also term term
he path path balan balance. ce. If path path bala balance nces s are are not not fine fine then then start start checki checking ng the power power from from radi radio o to to conne connecte cted d ant ant
nment nment to MS indi indica cati ting ng the the dedi dedica cate ted d SDCCH SDCCH subsub-ch chan anne nell (via (via AGCH), AGCH), wher wherby by subs subseq eque uent nt messa message ge e
balance path, MS moving out of range would carry the Phantom Phantom RACH s symptoms.
Paging response is specific to MTC. Subsequently, BSS embraces the information with its own initiated
e MS, it will move to the dedicated TCH and responds with Assignment Complete. In turns, BSS will su
inates transactions, in which case, call setup is termed as unsuccessfull due to TCH Congestion. The s
ennas. If we take the examples of GSM 900 scenario, the link budget defines that the radio should trans
xchange will be preformed over the dedicated SDCCH.
connection request BSSMAP message, send to MSC to approval. MSC will response with either conne
bmit Assignment Complete to MSC as to complete the signal activity.
me situation also applies in situation where Congestion Relief feature is enabled. In the case of Directe
mit 40 watts power and at the top of the cabinet, 20 watts are recaived ( Considering the 3 dB loss of c
tion confirmed, confirming the success in link establishment between MS-BSC-MSC, or connection Re
d Retry feature is enabled, MS will perform Handover to TCH of another cell if a valid handover neighbo
mbiner). While checking the power, if any components seems to procedure more losses than expected
used, Indicating the termination of the specific network transaction.
r is detected. The best thing to do is to add more radios in the cell to remove congestion.
, change that component. Similarly check the power at antenna feeder ports. Some time due to the wate
r ingress, connectors get rusty and needs to be replaced.
