Basic RF ion

8/6/2019 Basic RF ion

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Definition of Optimization

Cellular system Optimization is the continuation of the design process that occurs aftersite construction, to ensure that a cell site is incorporated into the overall RF plan in away that maximizes its potential for coverage and capacity while at the same timeminimizes its detrimental effects on co-channel areas.

Elements of Good Optimization

1. An Optimization engineer must be aware of all aspects of RF engineering from thedesign process and the implementation stage before he/she can make well informeddecisions about optimization.

2. An Optimization engineer must consider the entire cellular system as whole and notjust piece by piece, that is practicing think globally, act locally.

3. An Optimization engineer must be proficient in his knowledge of important databaseparameters in order that he/she may know what to change and how the changes affectthe behavior of serving and neighboring sites.

4. An Optimization engineer must have an eye for detail for the entire network. Forexample, in matters relating to the maintenance of the neighbor list and databaseparameters.

Who is responsible for Optimization

Optimization Engineer Responsible for setting database parameters so that mobile station (MS) hand in and

hand out of each site at the appropriate signal levels and locations. The mainobjective for this is so that subscribers are able to experience continual calls withequal or better quality than the previous call whilst minimizing dropped handovers or

drop calls. He/she is also responsible for minimization of interference to co-channelcells by ensuring that the serving cell is not dragged beyond its intended servingarea.

RF Engineer Responsible for building sites that significantly improve coverage and capacity

without contributing to excessive interference to co-channel sites.

Frequency Planner Responsible for assigning channels that have the best chance of successful

Optimization.

Optimization Duties

1. Call quality maintenance


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Each optimization engineer in MNPP is specifically in charge of a givenarea. That engineers primary responsibility is to ensure that the call qualityand congestion of the area is at an acceptable level area via drop call andblocking statistics respectively as an indicator. Regular drive tests on the fieldwill also be necessary in order that real problems maybe discovered andattended to.

2. Neighbor list maintenance A correct and accurate neighbor list the heart of correctly definedhandovers. Therefore, an Optimization engineer should have strict controlover this list.

3. Reparenting of sites & LAC change Reparenting takes place when there is a need to transfer the site to anotherBSC as determined by capacity requirements specified by BSS networkdesign. An Optimization engineer must provide the correct neighbor list toBSS and carry out the drive test to verify handover performance wheneverthis takes place.

4. Frequency & BSIC retunes

This happens when new frequencies specified by the frequency plannerneed to be implemented in order that new sites can come up. Alternatively,retunes take places for interference control reasons. In any case, a neighborlist has to be provided to the frequency planner for this to take place.

5. Bringing into service (BIS) of new sites The BIS of new sites happens as and when RF engineering rolls out theirgrowth plans to meet capacity and coverage requirements. An Optimizationengineer must follow the procedure as in New Site Verification andFunctionality form that is to ensure that the hardware is in proper workingorder, check and verify for call origination and termination, ms powercontrol, site information is as in work order, familiarize with the coverage

area of the new site and ensure that this new site is integrated without causingany problems to the existing network.

Basic Optimization Theory

1. What is handover?


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Consider cell A and cell B. The MS travel from cell A to cell B. At the edge, weobserved that the rxlev perceived by the MS of cell B is stronger as the MS goes furtheraway from cell A. To sustain the call, the MS therefore should hand into cell B.

Handover is required to maintain a call in progress as a MS passes from one cellcoverage area to another and may also be employed to meet network managementrequirements e.g. relief of congestion.

In general, there are two types of handover, namely intracell and intercell. Intracellhandover occurs within a same cell from one carrier to another. It would only be requiredif an MS is on the radio channel that is subjected to co-channel interference. Poor signalstrength measurements taken on the original radio channel would not normally beimproved by performing intra-cell handovers as the signal strength of all radio channelswithin that cell would be similar.Intercell handover occurs between two different cells. It can be categorize into two,internal or external. Internal handover occurs between two cells within the same BSC.While external handover occurs between two cells of different BSC.Handover Criteria

i. Criteria 1

Rxlev_ncell (n) > Rxlev_min (n) + Max (0, Pa) (1)


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Where Pa = ms_txpwr_max (n) - PP = max power of ms = 2Watts or 33dBm

and ms_txpwr_max is the parameter max_tx_ms set in the database as 33dBm

Criteria 1 states that the neighbors receive level otherwise know as Rxlev_ncell must begreater that a minimum level for that neighbor. Currently the default value of theRxlev_min is -105dBm. The value is so low that the first criteria has no effects onhandover but in a correctly optimized system this value would be set depending on themeasure signal strength of the neighbor at the desired point of the handoff. Setting ahigher value of Rxlev_min will bias the traffic to stay on the serving cell longer. Bysetting a correct value of Rxlev_min, you can force MS to stay on a cell throughout itsintended coverage area even if a neighbor cell become stronger.Any neighbor failing Criteria 1 is not further considered in any handover decisionprocess.

ii. Criteria 2

PBGT (n) - Ho_margin > 0 (2)

Criteria 2 states that the PBGT(n) value must be greater than the ho_margin value whichis specified in the add neighbor parameter for a handoff to be requested.

iii. Power Budget Assessment ProcessHandover are generally triggered by a mathematical process based on Power Budgetcalculation. The power budget formula is given as:

PBGT = [ min (ms_txpwr_max, P) - Rxlev_DL - Pwr_C_D]serving

- [ min (ms_txpwr_max, P) - Rxlev_DL] (3)

neighborwhere ms_txpwr_max is the parameter max_tx_ms set in the database as 33dBm

Pwr_C_D is max_tx_bts - Actual BTS output power = power correction factorP is max power of the MS = 2 Watt or 33dBmRxlev_DL is the downlink receive level as measured by the MS

Substituting the values given and simplifying Equation 2 yields

PBGT = [ Rxlev_DL ] - [ Rxlev_DL + Pwr_C_D) (4)neighbor server

2. Interference

There are basically three types of interference that an Optimization engineer has to dealwith, namely co-channel interference, adjacent channel interference and externalinterference.


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Co-channel interference - comes from sites using the same frequency as its carrier somedistance away, typically 4 cells away (for n = 4 reuse pattern)

Adjacent interference - is a result of frequencies transmitting in the adjacent frequencyband next to the carrier in question. Every channel has therefore 2 adjacent channels.

External interference - can classified as interference that is external to our own MaxisNetwork. The most common source is Closed Circuit Television (CCTV) cameras thatare found in a wide area of retail and commercial shopping complexes. Other sourcesinclude interference from other operators namely, Celcom. This interference causes thedesensitization to our MS receiver causing the phone to be unable to receive our ownMaxis signals. Another source is Mobikom. This will cause high interference on idle toour sites and increasing the drop call.

Maintaining Carrier to Interferer ratio (C/I)

The most common specification in Optimization is what is know as Carrier to Inteferer

(C/I) ratio. This figure specifies the minimum requirement that the system can toleratebefore suffering from bad quality calls. The C/I has a direct reflection on the perceivedquality of the call. For co-channel interference, the interferer must be at least 12dB lowerthan the carrier. For adjacent channel interference, the interferer must not be allowed tobe stronger than the carrier by 9dB.

The goal of any good Optimization is to maximize the capacity of the cell withoutcomprising the quality of the call experienced by the subscriber. In order to achieve this,the C/I of every serving cell must be closely maintained. Should the C/I degrade, thesystem must be able to handover the call to its best neighbor in order that the subscribercan still experience a goof quality call.

3. Controlling Interference

An Optimization engineer must make decisions about the handover boundaries in orderto deal with interference that is inherent in the system. Below is a summary of some ofthe methods used for controlling interference.

3.1 Defining correct handover boundaries

This task involves knowing where the serving cell covers both for the BCCH and theTCH DRCUs. The Optimization engineer needs to deal with both power budget definedboundaries as well as quality defined boundaries. For power budget defined boundaries,these boundaries can be different due to irregularity of DRCU power levels. For qualitydefined boundaries, these boundaries can be different because the BCCH frequencies


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does not employ frequency hopping whilst the TCH has aggressive frequency over 9frequencies.

By knowing where your serving cell boundaries, you can ensure that your serving cellcovers the intended area of service before experiencing interference from co-channelsites. Should the server experiences interference, a handover to the best target shouldoccur. This will in turn ensure that the subscribers the best quality calls wherever he is.

3.2 Downtilting antennae

This option is done in conjunction with the RF engineer and is only used when you arecertain that the main beam and far field of an interference site is causing a problem atanother co-channel site. By downtilting the antenna, we can reduce the interfereranywhere from 1dB to 20dB. Note that the power of the side lobes of an antenna is notreduced by downtilts. This means that there is no benefits from downtilting if the sidelobes is causing interference.

3.3 Power down of sites

The powering down of a sites will enable you to meet the C/I at the areas you areexperiencing bad quality calls. This option, while being very effective in a localized area,has been known to cause other problems in other areas that are further away. Forinstance, the powering down of an interfering site can cause problem to in-building siteswhere it was originally serving. Therefore, it should be used only when necessary andwith great care.

3.4 Lowering down sites/antennae

This option is also done in conjunction with the RF engineer This method is only usedwhen you are certain that you do not need the site to serve so far, and that no areasespecially far field will suffer from low rf or bad quality after the lowering down.

Lowering down a site/antennae will let you have a better control over the site boundariesand thus, reducing interference.

3.5 Employing Power Control - Long term interference controlPower control is a feature available to GSM and can be described as follows. When theBTS/MS is receiving the MS/BTS transmission clearly, the BTS/MS will dynamicallyreduce the power until a level that will not compromise the quality of the call. This ideais based on the fact that the human ear is unable to differentiate between the voice qualityof Rxqual 0 and 1, and no subscriber should enjoy the rxqual of 0 while other subscribersin the network suffer from a bad quality call due to co-channel interference. Byemploying BTS/MS power control carefully, the average BTS/MS transmit power willreduce significantly whilst the C/I ratio is another co-channel site will increase therebyimproving the quality of the call.

4. Optimization Tools (TEMS) -Test Mobile System


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The basic tools Optimization engineers use is TEMS. Below is a summary of how itlooks like.

5. Handover Algorithms

There are a number of reasons that can cause a MS to handover from one cell to another.The Radio Sub-System (RSS) software decides on the appropriate handovers causevalues according to the following priority order:

i. uplink receive qualityii. uplink interferenceiii. downlink receive qualityiv. downlink interferencev. uplink receive levelvi. downlink receive levelvii. mobile station distanceviii. power budget


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If a handover is triggered through one decision process in the priority list, the othersfurther down the list are ignored. The handover trigger will then lead to furtherprocessing for Criteria 1 and Criteria 2.

5.1 UL/DL rxqualThe uplink and downlink quality handover procedures are identical. Rxqual

handover has the highest priority so as to reduce the possibility of a drop call. If ul/dlrxqual handover are enabled, the RSS will have to determine when the quality isconsidered bad, so that the MS will hand out of that serving cell. This threshold is thenset by the optimization engineer. The threshold is set at l_rxqual_ul/dl_h = 350 which isbetween a quality of 4 to 5 (rxqual 0 is best, rxqual 7 is worst). If hop_qual_enabled isturn on, then, the hopping carrier will then use l_rxqual_ul/dl_h_hopping. To enableul/dl rxqual handover, set ul/dl_rxqual_ho_allowed= 1.

5.2 UL/DL interferenceInterference handover igs a intracell handover, namely handover between radios in thesame cell. The concept behind this is most applicable for non-hopping cells, e.g.

concentric cells, where a particular channel (having been assigned to given radio) isinterfered greater than other channels in the same cell. Thusintra_cell_handover_allowed= 1 and interfer_ho_allowed= 1.

For an interference handover to occur, the MS must first qualify for an ul/dl rxqualhandover, at which point the BTS will determine whether to handover due to quality orinterference. This discriminating parameter is u_rxlev_ul/dl_ih. The default value is 40.As such, for a signal strength (rxlev) of above 40 with a quality (rxqual) of above 350,the BTS will initiate an interference handover and the MS will handover to anotherchannel within the same cell. If the rxlev is 40 and below with rxqual above 350, theBTS will initiate a quality handover and will handover to another cell.

5.3 UL/DL rxlevRxlev handover algorithm is basically a comparison of receive signal strength. It isinitiated if the signal strength of the MS or BTS decreases below a given threshold. Toenable, ul/dl_rxlev_ho_allowed= 1 and the threshold set is l_rxlev_ul/dl_h = 10.However, if power budget handover is also enabled, it is likely to qualify for pbgthandover before ever qualifying for rxlev handover. So, ordinarily, pbgt handover isdisabled when turning on rxlev handover.

A common usage for rxlev handover is when a cell is significantly powered down ordowntilted in order to combat interference. Since the powered down cell has shrunk, byenabling rxlev handover, the optimization engineer is able to extend the cells coverage.However, since power budget handover is disabled, a great amount of care must be takenwhen enabling rxlev handover.

5.4 Ms_distanceHere is a handover is initiated because the MS is being served by a cell to faraway, whichis determined y its timing advance value. The Motorola recommended value is 1.5times


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the cells serving area. The parameters used are ms_max_range = ,timing_advance_period= and ms_distance_allowed= 1.

Example: The MS hands out from a cell A to cell B, which is 12km away due to quality.Cell B has ms_distance handover turned on with a ms_max_range = 20 (approx 10km).Will the MS in cell B handout? ___________

5.5 Power budget handoverBy far, the most important handover and is enable by setting pwr_handover_allowed= 1.The default handover margin = 4, which simply means that whenever the power of aneighboring cell is greater than the server by 4dB, the MS will handover to this neighbor.As such, the cell boundary (of say Cell A) is defined by the relative signal strength of itsneighboring cells (say Cell B, C, D, etc.).

6. Microcell Handover

There are a total of 7 types of handovers and are popularly know as Algorithms and are

determined on a per neighbor basis. All this means is that the MS in a serving cell mayhandover to a different neighbor based on different handover conditions. Type 1 to 7 arevarious forms of Power Budget Handovers. The parameter used to set the handover typeis pbgt_alg_type = . The 3 main mechanisms for microcell handovers arehandover margins, delay timer and threshold. The delays are based on number ofSACCH multiframes (approximately in steps of 0.5s) and the threshold are based onsignal strength (rxlev dB).

6.1 Type 1 Algorithm (PBGT algorithm)This algorithm simply allows all the handover reasons to function is they are enabled.Type 1 handover is the used for macrocell to macrocell handover.

6.2 Type 2 Algorithm (Emergency handover)This algorithm is known as emergency handover since the MS will handover to anothercell with any reasons except power budget. Type 2 handover is typically used tohandover from a microcell to macrocell.

6.3 Type 3 Algorithm

Commonly known as around-the-corner handover and is defined by a handover margin,where [PBGT - ho_margin > 0]and as well as uplink and downlink rxlev thresholdwhere [ul_rxlev(s) < ul_rxlev_serv_l & dl_rxlev(s) < dl_rxlev_serv_l]. When all 3conditions are met, only then the MS will qualify for a handover.

6.4 Type 4 AlgorithmThe handover is determined by the handover margin and a delay timer which startedupon a hand in into that cell. Upon a hand in, a timer is started and no handovers areallowed until the timer expires. It may be used to keep fast moving mobiles on the macroand reduce handovers in cell that are extremely strong for only a short period of time.

6.5 Type 5 Algorithm


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The handover is determined by the handover margin, delay and a threshold. When thesignal strength of the target cell is above the specified threshold, the timer will start.Upon expiry of the timer, if PBGT (n) > ho_margin, the handover will be triggered. Thisalgorithm is commonly used to handover from macrocell to microcell.

6.6 Type 6 AlgorithmThis is a handover where the neighbor is made extremely unattractive when it is firstdetected, but becomes preferred neighbor after the expiry of a timer. The timer startswhen PBGT(n) > ho_margin, upon which a static offset is added to the ho_margin tofurther discourage handover to it. After the expiry of the timer, a dynamic offset issubtracted from previous calculation, thus encouraging handovers into the particular cell.The Type 6 algorithm may be viewed as a more sophisticated type 5 algorithm.

6.7 Type 7 AlgorithmThe algorithm is used to avoid handovers to neighboring cell with adjacent interferenceproblems. If the signal strength difference between the adjacent channels is greater thanthe parameteradj_chan_rxlev_diff, the neighbor will be taken off the candidate list.

7. Location Area Code

The location area code indicates the location of the cell. It is part of the GSM cellidentity (5 0 2 1 2 2021 27721) and is used to page mobiles within a group of cellsdefined by having the same Location Area Code (LAC). When a mobile moves from onecell to another cell having a different LAC, the MS will perform a Location Update(LAC update). This is to inform the network that the MS has now moved to a newlocation area. The area where 2 or more cells have different location areas, are known asLocation Area border.

8. Some extra points

8.1 Standalone Dedicated Control Channel (SDCCH)The SDCCH is used for call setup, validation, attaching/detaching of an IMSI, ShortMessage Service (SMS) and Location Update. An important factor in determiningSDCCH utilization is whether the cell is a Location Area border or not. SDCCH usage ofa cell in a LAC border is approx. 3 times that of a non-LAC border. Dynamic channelreconfiguration is also used to balance out the need for SDCCH blocks and the TCHtimeslots according to the traffic needs at a given time and reduce the blocking withoutthe permanent loss of TCHs.

8.2 Phantom RACHsPhantom RACHs occur primarily as the result of sporadic noise and interference.Channel requests from distant mobiles can be affected by such noise. Having co-channelco-bsic sites close to one another, increases Phantom RACHs. These in turn, clog up analready limited SDCCH timeslots resource and influence SDCCH congestion.

8.3 SDCCH Blocking


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SDCCH Blocking is influenced mainly by the Phantom RACHs and whether a cell is aLAC border cell. If a cell has abnormally high SDCCH blocking, then various otherproblems such as unable to make calls, and unable to send SMS may occur.

8.4 Common Control Channel (CCCH)Where SDCCH is crucial in making calls, Common Control Channels (CCCH) areessential for both making as well as receiving calls. On the uplink, it is used to sendRACH messages, while Paging Channel (PCH) and Access Grant Channel (AGCH)operate in the downlink direction. The AGCH is used to assign resources to the MS, suchas SDCCH, whereas the PCH is used to call a mobile. It should be noted however, thePCH and the AGCH are never used at the same time4.

The number of paging blocks required will be determined by the paging rate andthe number of subscribers within a given Location Area. The number of AGCH blocksare determined by the Call Rate, the Location Update Rate and the SMS Call Rate.

8.5 Traffic Channel (TCH) BlockingAlso commonly known as congestion and is probably the second most common

subscriber complain after drop calls. It is also influenced by the number of SDCCHblocks configured for a given site. When using dynamic SDCCH configuration 1 TCH(or more) is reconfigured to give 8 SDCCH sub-slots. Congestion relief is one method tocircumvent blocking. TCH block normally occur when the traffic for the particular cellsite increases or there is a site down event and may also due to hardware failure.

8.6 Congestion Relief

Congestion relief is designed to handover candidates meeting the criteria, when there iscongestion in a given cell, instead of the MS that is attempting to make the call. Thereare 2 types of CR. Type 1 only handover the required number of mobiles so that those

mobiles which are trying to originate within that cell will be able to access it. Type 2 willhandover all the mobiles which qualify for the handover due to congestion relief. Thefirst type of CR is the preferred choice.

Reference1. Basic RF Optimization2. Introduction to Digital Cellular CP023. BSS Database SYS024. BSS Database Application SYS035. Microcellular Principles SYS086. BSS Sub-system Planning SYS047. BSS Sub Stem Performance SYS05


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Basic RF ion

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