GSM Paging Issues & Analysis

Wireless Network Planning

Table of contents

Table of Contents Chapter 1 Special Topic...............................................................................................................1 1.1 The Problem of Coverage.................................................................................................1 1.1.1 Equipment Configuration........................................................................................1 1.1.2 Base Station Site Selection....................................................................................1 1.1.3 Antenna Engineering Design and Installation.........................................................3 1.1.4 Antenna feeder, combiner (divider), CDU connections...........................................6 1.1.5 Parameters settings and others..............................................................................6 1.2 TCH Congestion................................................................................................................9 1.2.1 The Causes of High TCH Congestion Ratio...........................................................9 1.2.2 Positioning Methods of High TCH Congestion Ratio............................................10 1.3 Voice Prompt...................................................................................................................13 1.3.1 Paging Strategy....................................................................................................13 1.3.2 Paging procedure.................................................................................................14 1.3.3 Analysis on the Problem ”Subscriber Out of the Service Area”............................15 1.3.4 Supplementary Notes...........................................................................................17 1.4 The Problem of Signal Fluctuation..................................................................................18 1.4.1 Examine the Stableness of the Base Station’s Transmission Power....................18 1.4.2 Cell Reselection (in idle state) or Handover (in conversation state)......................19 1.4.3 Power Control and DTX........................................................................................19 1.4.4 TRX Down............................................................................................................21 1.4.5 Interferences.........................................................................................................21 1.4.6 Cell Congestion....................................................................................................21 1.4.7 Multipath Effect.....................................................................................................22 1.4.8 Other causes........................................................................................................22 1.5 Other Main Problems......................................................................................................24 1.5.1 Poor Voice Quality................................................................................................24 1.5.2 Failure to attach the network................................................................................24 1.5.3 Slow to attach the network....................................................................................24 1.5.4 Access slow..........................................................................................................24 1.5.5

unilateral connection............................................................................................24

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Wireless network Planning

Chapter 10 Special Topic

Chapter 1 Special Topic 1.1 The Problem of Coverage At present there are more or less problem of coverage in the network. The problem of coverage concerns a large number of aspects. This section analyses the problem of coverage that is available on-line from the perspective of base station selection, carrier configuration, antenna feeder organization, antenna installation, installation quality and parameter setting, etc.

1.1.1 Equipment Configuration I. Combiner Configuration Solution recommendations under normal circumstances: For cells with less than, or equal to 2 TRXs, use EDU. For cells with less than, or equal to 4 TRXs, use CDU combiner. For cells with more than, or equal to 5 TRXs, consider using cavity combiner or fourin-one (SCU) combiner.

II. Tower Amplification For some areas or bands whose uplink signal quality needs to be strengthened, consider designing tower amplification.

III. Antenna Using high gain antenna can improve the base station’s coverage capacity to a certain extent. When doing network planning, first consider the network quality from the perspective of network coverage and select appropriate antenna. Under normal circumstances the 900M directional antenna gain shall be in the range of 15-17dBi; the 1800M directional antenna gain shall be in the range of 15-18dBi. For city areas the directional antenna with horizontal half power angle of 65 degrees is preferable. When high gain antenna is used, the problem of “Shadow right under the tower” should be taken into account. Preferably zero-point filling antenna should be used. If the omni-antenna is built over high mountains, the problem of “Shadow right under the tower” should be taken into account too. Preferably omni-antenna with built-in low incidence should be used.

1.1.2 Base Station Site Selection I. Bad base station location and antenna installation will lead to the problem of coverage. When there is a possibility that bad station location and antenna installation may lead 1



to the problem of coverage, the on-site surveying personnel shall insist their viewpoints from the perspective of network coverage. The issue shall be explained thoroughly if it cannot be solved ultimately. What shall be explained is also the base station’s coverage areas and objectives (in particular government offices, hotels, airports, residential areas, main roads, shopping centers and former cell’s coverage margins), as well as the cell’s carrier frequency configurations. Usually for intensively populated urban areas the height of the base station’s antenna is 25-30 meters; the height of the base station’s antenna in the suburbs (or the ones that point to the suburbs) is 40-50 meters. The planning personnel shall assist the network carriers to decide the coverage areas and objectives after a thorough study of the geographical features, the buildings’ absorption and loss features, and multipath effect to ensure that the base station will be constructed as expected. Furthermore, the base station, in particular the omni antenna base station shall not be too high. This is because the omni antenna’s vertical beamwidth is narrow. If the base station is too high, it will have bad coverage on the nearby area and good coverage on the faraway area where there aren’t many subscribers. Usually the height of the omni antenna base station shall not be 50-60 meters more than the covered area. For base stations that are too high, the built-in down-tilt omni antennas or antennas with broad vertical beam are desirable.

II. The antenna direction difference between the old and newly built network For directional cells, if the antenna direction of the newly built network is different from that of the old network, such as: the old antenna’s direction was 0/120/240, while the newly built antenna’s direction is 60/180/300. For omni cells, if the antenna direction of the newly built network is different from that of the old network, such as: the old antenna’s direction (transmit and diversity antenna) is 0/180, while the newly built antenna’s direction is 90/270. This will cause the change of the diversity surface and transmit antenna position. This may make the previous good coverage a bad one, and previous bad coverage a good one. But the mobile phone subscribers are only concerned with the change from good to bad, which will give rise to complaints. Solution recommendations: During the engineering design, the antenna shall be so designed as to make the direction of it as identical as that of the old one. In cases where the direction change is desired, the main beam of the directional antenna or the direction of the omni antenna shall point to the places where heavy coverage is emphasized. Due explanations are also desirable.

III. The difference of the antenna height between the old and newly built network The engineering design shall be so made as to ensure that the height of the new antenna is not lower than that of the old antenna. If the tower platform has not enough space to install the antenna, the explanations shall be given for decreasing the antenna height.

IV. Concerning the directional antenna’s down-tilt of the newly built network Preferably the down-tilt shall not change. In cases where such change is desired due to the increase of the base station in the urban area for purpose of controlling the coverage area, such change shall be duly recorded.

V. Change omni cell to directional cell As the half power angle of the directional antenna’s beamwidth is 65 or 90 degrees, and the beamwidth of the omni antenna is 360 degrees, after the change, the marginal coverage of the cell within the directional base station can be worse than that of the previous omni station. During the planning, make sure that the main beam 2



of the directional antenna points to the area where emphatic coverage is desired and describe the coverage differences between the omni-antenna and directional antenna.

1.1.3 Antenna Engineering Design and Installation I. The problem of the directional antenna installation： Generally, the base station’s uplink and downlink signals are balanced. Bad antenna installation may leave the mobile phone subscribers such impression that the base station’s uplink coverage capacity is weak. (1) The transmitting antenna is installed inverse. Transmitting feeder and the cell’s antenna were mistakenly connected or BTS set top jumper was mistakenly connected. Possible phenomenon clue for fault discovery: 

The mobile phone is somewhere in the cell but cannot call out (the receiving antenna is in the other direction, the uplink is bad);

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Unilateral call connection;

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From a distance of the cell, drop-off happens frequently (infrequent near the base station);

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Drop-off after frequent handovers (the handovers are found to be mostly caused by weak uplink signal strength or bad uplink quality);

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The field intensity distribution of the BCCH frequency band between the adjacent sectors were found to be displaced during the drive test;

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Unexpected severe interferences with the adjacent frequencies.

This type of error is easy to discover in the network. It can be found by checking installation or by analyzing the field intensity distribution of the BCCH frequency band during the drive test. (2) Wrong receiving antenna installation: This problem, which cannot be found by analyzing the field intensity distribution of the main BCCH frequency band in the downlink, can cause bad uplink in the current cell. After excluding the possibility that it is caused by transmitting antenna, check if the jumper or feeder of the receiving antenna are wrongly installed, also review the measurement report to see if the uplink level is too low, or if there are any call-out or handover problems, power control triggered by the uplink signal, or irrational handover ratio, etc.

II. The problem of shadow adjacent to the coverage area in the directional cell During the installation of the directional antenna, caution must be taken to avoid the creation of large area of shadow in its coverage area. Shadow usually comes into being as a result of huge hindrance near the base station, such as skyscrapers, or mountains. Caution must be taken to circumvent such hindrance during the installation. When the directional antenna is installed at the surface of the top of the building, caution must be taken so that the wave beam is not hindered by the edge of the building, and that the antenna is not installed near the edge, thus reducing or extirpating the shadow. Due to the complexity of the antenna surface, when the antenna must be installed away from the edge of the building, it must be installed high 3



above the antenna surface. For this reason, the bearing capacity of the building surface and the antenna’s against-wind force must be taken into account during the engineering.

III. The problem of the directional antenna diversity interval The effective horizontal interval of the 900M directional antenna space diversity should be more than 4m (at least greater than 3m); The effective horizontal interval of the 1800M directional antenna space diversity should be more than 2m.

IV. The problems during the omni-antenna installation Omni base stations are mostly located along the roads from villages to counties, or from counties to other cities. These base stations are mainly to serve the mobile subscribers along the road or subscribers in the villages. The common mistake that is made during the installation of the antenna feeder is that the distribution of the above subscribers are not fully taken into account, leading to the incomplete utilization of the performance of the base station, leaving the subscribers such impression that the coverage is bad. There are mainly 3 points concerning the design and installation of the omni antenna: (1) The interval between the omni antennas and the interval between the antenna and the tower (2) Diversity direction: the diversity surface should be directed to the area where subscribers are heavily populated (3) The position of the host transmitting antenna (TX) (the host transmitting antenna refers to that of the BCCH transmitting channel). The main transmitting antenna should be directed to the area where the subscribers are heavily populated. The following figure is on the direction of the base station omni antenna that covers the roads:

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M S _ T X P W R _ M A X (n ) R X L E V _ N C E L L (n )

M S _ T X P W R (n ) M s O p tL e v e l( n ) B T S 1H O

B T S 2

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I. Static Power Setting To reduce the interference with the adjacent frequencies, lower the height of the antenna, increase the down-tilt, or decreasing the base station’s transmitting power. Usually, decreasing the transmitting power can also worsen the indoor coverage. So what are commonly adopted arefalling down the height of the antenna and increasing the down-tilt.

II. RACH Parameters Setting To control the uplink access (call, paging response), and to balance between the coverage and call drop ratio, Huawei adds the parameter “RACH access threshold” in the BTS3.X base station. The parameter can go upward from –110dBm, and effectively controls the mobile phone’s uplink access.

III. Cell Selection and Reselection Parameters C1 and C2 decide the cell selection and reselection. RXLEV_ACCESS_MIN is commonly set between -100dBm to –105dBm. Setting the parameter too great will make it difficult for the mobile phone to select the cell, which, when viewed from the subscriber’s perspective, is the mobile phone’s call drop and no coverage; Setting the parameter too small will lead to the situation that the mobile phone can receive the signal but cannot call out. During the parameter setting, also note the differences of sensitivity and maximum transmitting power between the 900M and 1800M networks, ignoring which may cause the unbalance between the uplink and downlink.

IV. Other Parameters and Phenomena The setting of the random access error threshold can also restrict the mobile phone’s access. Its value, which is usually greater than 180, is set subject to the radio environment’s bottom noise and statistical result. The other parameters settings that have effect on the mobile phone attached, access, handover, and connection are: BSIC, NCC_PERMITTED, CELL_BAR_ACCESS, CELL_BAR_QUALIFY, radio link failure timer, the number of mobile phone’s maximum retransfer, the number of send distribution time slots (the number of extended distribution time slots), the number of SACCH multi-frames, the number of maximum physical retransfer, radio link connection timer, location update and paging-related parameters (including LAC distribution), handover-related parameters, power control-related parameters, flow control-related parameters, roaming permit-related parameters, TRX power settings of different CDU channels, etc. When the base station’s upper clock is bad, MCK (TMU) is failure, propagation is unstable, or the carrier frequency board is failure (no warning, sometimes stable interferences), the mobile phone’s accessing, call and handover (usually asynchronous handover) will be affected. When the system information is wrong or is not delivered correctly, the mobile phone’s accessing, call or handover will be affected. When the frequency interference is severe, the mobile phone’s accessing, call and handover will be affected. When the channel is congested the mobile phone’s handover and call access will be affected. Multipath effect can lead to signal fluctuation

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During the cell reselection and frequent location border update, the signal of the mobile phone will fluctuate. During the connection, direct re-try and load handover caused by congestion can lead to strong fluctuation of the signal; Priority handover (for layered network) and edge handover (the edge handover threshold is set too low and there is no PBGT handover) can also lead to strong fluctuation of the signal. If the power control and handover parameters are not set properly during the signal fluctuation, the fluctuation will be strengthened until call drop happens. The cross-area coverage or coverage in the border areas for different services can all cause problems, leading to the subscriber’s huge increase of roaming bill. To solve the problem, the coverage area should be controlled or enlarged during the planning and optimization phase to avoid mutual cell reselection or only single-direction cell reselection. Solve the problem by adjusting the height, down-tilt, and direction of the antenna, CRH and MS minimum access threshold, and BAI table. The mobile phone subscriber having not sufficient understanding on the terrain, construction material’s absorption loss, or multipath effect will tend to rise questions on coverage.

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1.2 TCH Congestion In 2001, the mobile phone companies in all the provinces presented five indexes as network quality criteria. They are: long distance call success ratio, traffic call drop ratio, traffic channel availability ratio, radio successful connection ratio, and worst cells ratio, the last two of which are directly influenced by TCH congestion ratio.

1.2.1 The Causes of High TCH Congestion Ratio The causes for high TCH congestion ratio are many, among others, data configuration problems, hardware problems and external interference. But in light of solution steps and sequence, it is desirable to first check the software and hardware problems of the equipment per se, and then check other equipment-unrelated factors such as external interference and constraints due to complex terrain (1) Interface A trunk circuit data configuration errors. (2) Carrier frequency board faults or unstable performance, leading to high congestion ratio. (3) Bad base station hardware installation causes unbalanced uplink and downlink level, leading to TCH congestion. (a) Uplink branch: antennatower amplifierfeeder linedischarge arresterset top connectordivider or CDUTRX board 

Divider cascaded half-rigid cable connection error, leading to uplink level discrepancy;

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Half-rigid cable distortion or loose connector, leading to the problem;

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Data bus problem.

(b) Downlink branch: TRXHPAcombiner or CDUset top connectordischarge arresterfeeder linetower amplifierantenna 

The transmit branch has antenna feeder – standing wave ratio warning;

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Half-rigid cable distortion or loose connector;

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Cell antenna connection error or TRX natural transmit channel discrepancy cause the discrepancy in the coverage direction and area of main BCCH carrier and expansion carrier, leading to TCH congestion.

(4) The repeater station is subordinated to the host cell. The repeater station doesn’t expand as the cell does. (5) Interferences leading to congestion (6) Isolated station or complex terrain causes TCH assignment failure, leading to congestion. (7) Huge real traffic leading to congestion.

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1.2.2 Positioning Methods of High TCH Congestion Ratio I. Remote Analysis of Congestion Ratio Cause (1) By traffic statistics analysis 

By conducting traffic statistics analysis on the cell’s TCH performance, check if TCH congestion is due to all-busy congestion. If it is, the problem can be solved by network optimization which delegates the cell’s traffic to other cells, or by advising the operator to expand.

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If the problem is not due to all-busy congestion, check if the congestion is related to interference, namely, check interference 1 to 5. If interfered, the cell’s call drop ratio can be a little high.

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Register the receiving performance measurement traffic statistics tasks. Query the traffic statistics result by object and see if the uplink/downlink measurement report in the same TRX is balanced to decide if the uplink/downlink hardware branch is balanced. Query the traffic statistics result by time and see if there is any exceptions regarding the measurement report number in the same cell to decide if the congestion is related to that board.

(2) View the warning Check the station warning to which the high-congestion-ratio cell belongs to see if there are any abnormal warning, such as VSWR warning, PCM synchronization lost warning, uplink data bus warning, etc, and decide if the congestion is related to that warning, taking into account the traffic statistics. (3) Remote maintenance terminal of the base station Check if the software of the boards in the base station is uniform. The version upgrade shall be subject to the notice from SUPPORT website. Use the maintenance terminal of the base station to congest in turn the cell carrier board’s TCH channels that have high congestion ratio to see if the high congestion ratio is related to that cell’s carrier board. Solution principles: 

If the fluctuation of the congestion ratio is related to the block of the carrier board’s channel, then very possibly that board is at fault. Check if there is any co-channel interference. Check the hardware performance of the uplink, downlink, and the board.

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If the congestion is not related to the carrier board, then the whole cell may be interfered or affected by the terrain.

(4) Use the Signaling Analyzer to diagnose the ABIS interface message. According to the call flow and TCH assignment failure statistics, use the Signaling Analyzer to trace the ABIS interface message at every high-congestion-ratio cell. The figure shown here uses MA10 Signaling Analyzer, the detailed analysis of which is as follows: Analyze the assignment command Assignment CMD delivered in SDCCH, as the TEI value in figure 10-2, to determine the carrier board that SDCCH is in. The carrier board that TCH is in can be determined by referring to the ARFCN radio frequency band. Determine if the assignment failures are all in a TRX. Also analyze the causes of the assignment failures by focusing on the mobile phone’s TA value, uplink/downlink level value, and uplink/downlink signal quality in the measurement

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report. Local calling is required. If assignment failure points to a certain TRX board, the cause can be one of the following: 

TRX board down or unstable performance;

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Caused by bad uplink/downlink level. The uplink branch/downlink branch hardware is at fault;

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Bad uplink/downlink signal quality. Decide which branch is interfered, taking into account the mobile phone’s TA value.

Figure 10-2 ABIS signaling If the assignment failures are randomly distributed over the carrier boards of the whole cell, the analysis on the measurement report may point to the following causes: (1) Complex terrain within the coverage area of the base station; (2)Interference of the frequency band within the whole the cell, such as the one from the repeater station

II. Examination on the local base station (1) Local maintenance: See if there are any abnormal warnings and solve them timely. (2) Check if there are any such hardware problems in the uplink and downlink antenna feeder branch as loose connector, inverse antenna, half-rigid cable connection error, and backplane loose wiring, etc. (3) Use mobile phones for calling in the same place; 

Calling by every carrier or by every channel to see if there are any time slots or boards that cannot be assigned.

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Check weather all the carrier’s downlink levels are approached. For 10



carrier board whose levels are uneven, replace the board or uplink/downlink antenna feeder system to look for the causes. Note: for frequency hopping cells, use command line parameters to change that cell to non-frequency-hopping cell for the convenience of local calling. (4) Do the drive test by the network optimization software ANT-PLOT to see if there is any handover exceptions, downlink interference for any clues on the problem of congestion. (5) Use the spectrum analyzer to look for the interference source. (6) See if the terrain of the station’s coverage area is complex.

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1.3 Voice Prompt The voice prompt in GSM is controlled by the MSC of GSM network and broadcast to the subscribers. MSC plays the pre-recorded voice prompt in the channel corresponding to the voice card according to the different cause values and data configuration of the message. Typical voice prompt problems can take the following forms: (1) When the called is in idle state, “The subscriber you’re calling is busy” or “The mobile phone you’re calling has turned off” is delivered. (2) When the called is in idle state, “The subscriber you’re calling is out of the service area” is delivered. (3) When the called is in idle state, “For the time being the subscriber you’re calling cannot be accessed” is delivered. The reasons that cause the above phenomena can be some of the following: Subscriber status management exception on the NSS side, roaming number fetch failure, or no paging response. In the following section we emphasize on the analysis of the problem “Subscriber out of the service area”.

1.3.1 Paging Strategy The primary causes for the problem of “subscriber out of the service area” are twofold: one is paging response time over; the other is no paging response. First let’s look at how the paging is done. The paging strategy can be divided into the following three parts as per MSC, BSC, and BTS.

I. MSC Paging Retransfer and Paging Mode Selection MSC can page the same message for a maximum of four times. If the latest paging is not responded, it will retransfer the paging message. The resend time interval is 3 seconds, 3 seconds, 2 seconds, and 2 seconds respectively. Two seconds after the last resend, i.e., twelve seconds after the first retransfer, if the paging is not responded, MSC will regard it as time-over and deliver prompt tone of “Subscriber out of the service area” to the subscriber. The paging methods MSC can choose are TMSI and IMSI.

II. BSC Paging Group Calculation and Paging Message Transfer between the Modules After BSC receives the Paging Request delivered by MSC, it will calculate the paging group that the paging belongs to by the last three digits of IMSI, the cell’s CCCH channel configuration, and the paging block configuration, and then deliver Paging Command to that cell. Under multiple modules conditions, the Paging Command needs to be transferred between the modules.

III. BTS Paging Queue and Paging Combination After BTS receives the Paging Command from BSC, it will put the paging in the Paging Group Queue that the Paging Command designates, and send the paging

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content of the paging group at intervals of the same paging frame period. At present, every queue length of the paging group in BTS is 9.In one paging reserve block, 2 IMSI paging or 4 TMSI paging can be delivered. So every time the paging is delivered, BTS must combine the paging according to the paging message type in the queue.

IV. MS Paging Detection and Paging Response When in idle state the mobile phone can not only receive the system information from the broadcast channel, but also detect paging in its Paging Sub-channel. Therefore when the mobile phone receives the paging that is directed to it, it will send Channel Request to the Network Side and finish an immidiate assignment process. If the assignment is successful, it will report the Paging Response in the assigned SDCCH channel and complete a call connection process. Faults in any one of the above-mentioned four steps will lead to the problem of “subscriber out of the service area”.

1.3.2 Paging procedure When in idle state, the mobile phone will stay in a cell that belongs to a location area. The mobile phone’s location area information is stored in VLR. When a mobile phone is being called, MSC will read the mobile phone’s location area and status information from VLR. If the mobile phone is in “Attached, Idle” state, it will send Paging Request to the BSC to which the location area belongs. The BSC will calculate the mobile phone’s paging subgroup, create Paging Command, and deliver the Paging Command to all the cells in the location area. When in idle state the mobile phone can not only receive the system information from the broadcast channel, but also detect any paging in the Paging Subchannel to which it belongs. Therefore after the mobile phone receives the paging that is directed to it, it will send Channel Request to the Network Side and finish an immidiate assignment process. If this immidiate assignment is successful, it will report Paging Response to the assigned SDCCH channel and complete a call connection process. Note: The Paging Response is reported in Establishment Indication. For example: all the cells in a region belong to one location area, the common control channel parameter configuration for the cells is as follows: One non-combined CCCH The number of frames between the same paging: 6 The number of access reserve block:1 Thus, there are 6x(9-1)=48 paging groups in every cell. The mobile phone will decide the paging group it belongs to according to the last three digits of its IMSI and the number of the paging groups, then detect in the relevant sub-channel. For a mobile phone whose number is 13013362000, and the IMSI is 460013361000037, the last three digits of its IMSI is 037, its paging sub group is 37.When in idle state, the mobile phone will always listen to the paging sub channel related to the No. 37 paging group. If someone calls 13013362000, the paging will be sent to that subscriber from the cells in that area. After the mobile phone receives the paging directed at 460013361000037, it will send a channel request, and finish an instant assignment according to the instant assignment from the network side. If there is idle SDCCH channel in the cell, a SDCCH channel will be assigned to that mobile phone, the mobile phone establishes the connection in the channel, and reports the paging response.

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1.3.3 Analysis on the Problem ”Subscriber Out of the Service Area” The reasons that cause “Subscriber out of the service area” are the time-out of the mobile phone’s response to the paging or no response. From the flow of the paging response report and analysis on the real cases, there are the following reasons that cause this problem:

I. MSC subscriber status management If MSC subscriber status management is down and the request cannot be delivered to BSC, then MS will fail to receive the paging and to respond, leading to time-out. In real situations, there are cases when MSC didn’t deliver the paging. There were also such abnormalities as faulted called being regarded as caller and 3Tick release abnormalities. Such abnormalities are non-recurrent. And can be identified by being traced by the GSM subscriber interface of MSC Maintenance Station.

II. Receiver in the base station If the receiver of the base station, including DSP, TRX, and antenna feeder, is down, it will fail to detect the mobile phone’s access pulse or to establish connection with the mobile phone within the time limit, the paging response (in essence the instruction to set up connection) cannot be reported to BSC, leading to paging response time out or no paging response. For example, we found that T2688 in somewhere is slow in getting online. After the amplifier and the antenna feeder are discarded, the access speed can be 10 seconds faster. As this situation involves air interface and sometimes can be caused by the problems of MS, the diagnosis of it is difficult and dictates experts equipped with special devices.

III. The differences of the mobile phone’s fake accessing and accessing mode According to the protocol, whenever the MS is performing open/close, it must update its position and do the IMSI detachment according to the system’s requirement. Some mobile phones, mostly Ericsson, don’t perform the above operations. If the mobile phone fails to update its position when attempting open, it will display the message that it has already been attached. But what happens is that there is no message reported and the subscriber status doesn’t change. Dialing that subscriber will receive the message that it is close. Another example is that IMSI is not detached when the mobile phone switches off, so the subscriber status is still “attached”. Paging is still delivered to that subscriber when it is called. “Out of service area” message will come out when the time is out. Different mobile phones have different accessing mode and speed after the drop-off, causing some to have long time in accessing, leading to the problem of “Subscriber out of the service area”. Under the protocols of GSM 02 and 03, after the call drop the mobile phone follows the principles of ASAP (as soon as possible) and energy saving to access the network. The mobile phone will attempt to access the network in receiving signal’s strength order. 900M mobile phones search 30 frequency bands; 1800M mobile phones search 40 BCCH frequency bands; Double frequency mobile phones search 70 frequency bands. After the failed attempt, the mobile phone will decide when to start next time according to its own algorithm. Mobile phones of different models have different algorithms. For example, some models of Motorola mobile phones can set accessing network frequency. Under the low speed accessing mode, it takes 50 minutes to access the network after the mobile phone drops off the network and re-enter the coverage area. In addition mobile phones of different manufacturers have different accessing mode. Some mobile phones, after several failures in finding the network, will stop doing so for a long time to come. So, this problem is related to the mobile phone itself and can be solved by the switch on and

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switch off of the mobile phone.

IV. Uplink / Downlink Unbalance of Radio Link The radio signal can be classified into uplink and downlink according to the propagation direction. Ideally the uplink-downlink are at balance, that is to say, at any area the base station and the mobile phone can receive (or not receive) the other side’s signal simultaneously. The uncertainty of the radio signal propagation and the discrepancy of the actual circumstances dictate that complete balance within the whole network is impossible. So in the network there must be some areas where he downlink signal can cover but the uplink signal cannot. In these areas, the subscriber can receive message sent by the network, while the network cannot receive the message reported by the subscriber, including paging response. Therefore in these areas it is very common for the “Subscriber out of the service area” message to occur. To solve such problem of “Subscriber out of the service area”, adjust the radio parameters such as “RACH access threshold”, “random access error threshold”, “MS minimum access level”, and “RSSI check” to optimize the balance. In particular, note that the displacements of the measured level value in the measurement report for different versions of base stations are different.

V. SDCCH Congestion After the mobile phone receives the paging command, it will send channel request to the network side. If there is no SDCCH channel available, or if the procedure to set up SDCCH channel fails, the paging response cannot be delivered to the network side, thus the problem of “Subscriber out of the service area” occurs. The causes for the SDCCH congestion are SDCCH all busy, random radio link failure, etc. For the SDCCH all busy situation, adjust that cell’s coverage area to reduce the SDCCH congestion. For congestion caused by other reasons, such as random radio link failure or surface link failure, the solution is subject to real situation.

VI. PCH Overload The paging message in the network is random. Due to the restrictions on the structure of the radio channels and the limitations on the capability to send the paging command, it is possible that some of the paging groups are overloaded, leading to failure to send the paging messages out timely, and the resend paging cannot be responded within the valid time, thus causing the problem of “Subscriber out of the service area”. This situation can be improved by modifying CCCH configuration parameters “the number of frames in the same paging”, “the number of access grant reserved block”, and “CCCH channel allocation”. Reducing “the number of access grant reserved block” to a proper extent will increase the number of PCH sub channels, thus increasing the capacity of the paging channels; Reducing “the number of frames in the same paging” will improve the frequency of sending the paging message; Increasing the number of CCCH channels in the cell can significantly improve the system’s paging capacity, but at the same time reducing the number of TCH configurations. Thus this method is seldom used. If PCH is severely overloaded, the size of the location area needs to be reduced, thus lowering the flow of the paging message.

VII. Mobile Phone Quality When the mobile phone’s radio frequency module is down, the antenna loosens, or the battery is down, its receiving sensitivity goes down, the uplink signal quality worsens, and the capability to receive the paging command and access the network lowers. Mobile phones with such problems are prone to have the phenomenon of 15



“Subscriber out of the service area”. The evaluation on the quality of the mobile phone sometimes requires special devices. The problems of the mobile phone can take the following forms: (1) Bad power supply leads to insufficient uplink transmitting power and uplink access failure. (2) Faulted mobile phone software leads to the mobile phone’s exceptional dead-end and failure to respond to the paging message. (3) Problems in the radio frequency part of the mobile phone will lead to failure to receive, unstable transmission, or high frequency error within some frequency band.

VIII. Non-comprehensive Coverage In areas where the radio signal coverage is not good, usually indoors, the signal quality is likely to be bad, and the call drop is not uncommon .As the subscriber status in the VLR has not changed, when that subscriber is being called, the network side can deliver paging message normally, but the subscriber cannot receive the paging message and cannot respond to the paging message. Or because of the low quality of the signal, the paging response cannot reach the network side. It is normal under this circumstance that “Subscriber out of the service area” occurs. For such “Subscriber out of the service area” phenomenon, the quality can be improved by increasing the number of the base stations and perfecting the coverage. There is a significant percentage of complaints concerning the problem of “Subscriber out of the service area” that is due to bad coverage.

1.3.4 Supplementary Notes In the wireless network, because of the radio channel congestion or radio signal coverage, it is certain that the problem of “No paging response” occurs. In some areas where the coverage is bad, bad signal quality will lead to the failure to deliver the paging or paging response to the peer side, causing the phenomenon “Subscriber out of the service area”. For example, due to the temporary congestion of the SDCCH channel, the MS cannot be assigned an SDCCH channel, and the paging response cannot be reported, in which case the first dial may receive the prompt tone of “Subscriber out of the service area” and the second dial may succeed. As the paging sub channel of the cell where the subscriber is in is fixed, so the chance for that sub channel overload increases significantly. Calling that subscriber many times will be very likely to encounter the phenomenon of “Subscriber out of the service area”. These phenomena are all normal no-paging-response. It is an exception only when the attempt to call an idle state subscriber with good signal continuously for a long time receives “Subscriber out of the service area”.

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1.4 The Problem of Signal Fluctuation The causes that can lead to the mobile phone’s signal fluctuation are: (1) Fluctuation in the base station’s transmission power (2) Cell reselection (in idle state) or handover (in conversation state) (3) Power control, DTX (4) TRX faults (5) Interference (6) Huge traffic, leading to channel congestion (7) Multipath effect

1.4.1 Examine the Stableness of the Base Station’s Transmission Power (1) Directly measure the base station’s output power (a) Use Spectrum Analyzer 8594, connect to the HPA’s amplifier interface that sends BCCH through 30dB attenuator. Spectrum analyzer 8594

30dB attenuator

HPA

Figure 10-3 Directly measure the base station’s output power (b) Connect the base 13M clock and CMD57 reference clock input by the clock line and synchronize the equipment to the frequency band to test every timeslot’s power, frequency error, and phase error. If the test result shows that every timeslot’s frequency error and phase error conform to the standards, and that base station’s 13M clock works stably, then the possibility that signal fluctuation is due to the 13M clock’s fluctuation can be excluded. After ensuring the output power stability, let’s check the installation quality of the antenna feeder to see if there is any instability in the standing waves. If the installation quality is good, then the possibility that the signal fluctuation is due to the equipment’s output power instability can be excluded. (2) Within the visual distance of the antenna (about 1km away. The purpose is to test the influence of the multipath propagation), test the base station’s transmitting level to see if it is stable. (a) Use the spectrum analyzer 8594 and do the test by feeding the signal received through omni-mini antenna to the spectrum analyzer directly, as shown in the following figure:

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spectrum analyzer 8594

Omni-mini antenna

Figure 10-4 Use the mini-antenna to test the base station’s transmitting level (b) Use the mobile phone SAGEM and the test software MobileShow to do the test at the place that is within visual distance and is 1 km away. Preserve and test the signal wave by the laptop computer to see weather the transmitting level is stable. By the above test, we can determine if the output power at the amplifier interface is stable. The shortcoming of it is that it cannot obtain the concrete value of the output power in the amplifier interface. It seldom happens that the output power of the base station is unstable.

1.4.2 Cell Reselection (in idle state) or Handover (in conversation state) Both cell reselection and handover can lead to the fluctuation of the signal. To find the causes, use the following testing methods: (1) Watch and test the mobile phone to see if the cell reselection or handover occur as the signal level changes; (2) Use the test mobile phone SAGEM and software MobileShow to see if the cell reselection and handover occur as the signal fluctuates. By the above tests we can decide if the signal level fluctuation is caused by the cell reselection or handover. This phenomenon normally happens at the edge of the cell. But if the network has serious trans-cell coverage,, frequent cell reselection and handover can also arise, leading to the fluctuation of the signal.

1.4.3 Power Control and DTX When the power control or DTX is down, the receiving level in the mobile phone fluctuates. If the threshold value for the uplink power control is set too low, the uplink signal from the mobile phone to the base station will be weak, This, together with the normal fluctuation in the radio space, will give rise to handover. During handover, if the adjacent signal’s quality is not strong to maintain the conversation, call drop will arise. Naturally the Level Indicator in the mobile phone will go down to one scale or zero scale from the previous full scale. Phenomenon: The Level Indicator points to full scale, suddenly the peer’s voice at the other end of the line cannot be heard in the middle of the conversation; the Level Indicator in the mobile phone points to one scale or zero scale. Call drop happens. After several seconds the conversation is on again. Procedure and Analysis: The Level Indicator pointing to full scale means that mobile phone is in good receiving quality. The level value at this time is about -75dBm. If the downlink level exceeds this value, the level fluctuation cannot be displayed in the mobile phone. So such fluctuation is not easy to be seen when the signal is strong. So when the network’s 18



downlink power control is on, it is desirable to set the downlink level control to -75dBm.One further note: the level indicated in the mobile phone is the mean value of its radio frequency level. So the signal fluctuation of the narrow pulse duration will be partly smoothed away. So the scale change in the Level Indicator is not the real-time performance of the receiving signal’s strength. It is delayed. During diagnosis, we did the following monitoring and testing: Test the downlink power at the combiner’s test interface, no fluctuation was found; the downlink power at the drop-off point was measured to be -85dBm by the spectrum analyzer. Though there was a fluctuation of 10-15dB, its minimum level was not enough to drop off the mobile phone; Next we monitored and tested the downlink level of the mobile phone by connecting it to mobile show, we found that starting from a certain time (subject to small differences every time), the level went down from –85dBm along the 60 degree slope, when it went to the middle of the slope, there happened a handover, but the strength of the signal continued to go down, until at last the level was almost the same with that of the other areas (about –105dBm), and maintained at this value for about 10 seconds and then it dropped off. If after 10 seconds it didn’t drop off, it went upward along the 60 degree slope after another 10 seconds, and there happened a handover in the middle of the slope. Then it continued to go upward, and maintained some time for conversation at about –85dBm.Then it repeated the above procedure, making its path like a sloped square wave, until it dropped off at some trough. By watching the Um interface from the background, we found that before or after the drop-off and handover, the downlink receiving level was strong and stable, and the uplink level was mostly at –105dBm.So it can be determined that the handover and drop-off were caused by the bad uplink quality, that is to say, because of the effect from the uplink power control, the uplink signal has almost approached the uplink level’s minimum threshold value when it reached the base station. That, along with the normal fluctuation under the radio environment, will cause the base station to think that the conversation quality in that cell is poor, so it orders the mobile phone to hand over to the adjacent cell. If the quality in the adjacent cell is good, the conversation goes on, otherwise, there will be a drop-off. In light of the call drop, the level indicated in the mobile phone after the handover is the downlink level of the adjacent cell. And the downlink level of the adjacent cell is quite small, so naturally the mobile phone’s level goes down to 1 scale or 0 scale. If the signal in the adjacent cell is not sufficient to sustain the conversation or the drop-off, the base station will order the mobile phone to hand over back to the previous cell. If this time it is successful, the conversation can go on, the level in the mobile phone will return to full scale from 0 scale. This is the reason why the conversation suddenly cuts off but doesn’t call drop, and after a while, the conversation recovers. When the subscriber is having long time conversation, due to the periodicity of the above square waves, along with the randomness of the environment’s fluctuation in the radio space, the subscriber will experience intermittent cutoff and recovery, (not necessarily cutoff at every trough, but certainly weakening of the conversation quality), until call drop happens. As the uplink power control makes any level from the mobile phone to the base station very weak, giving rise to drop-off regardless of the distance. Certainly as the radio fluctuation for the mobile phones that are near to the base station is small, so is its chance of call drop. As the indoor radio fluctuation is greater than outdoor radio fluctuation, so is its call drop ratio. In addition, the antenna is usually placed at high tower, , the radiation lobe is in fusiform, so the area right under the tower is blinded. The coverage only reaches to the edge of the lobe. The carrier’s office is usually located at that area, and they keep performing trial dialing, so the chance for the fluctuation is greater. Solution: First make some brief judgments according to the above analysis. If the situations are likewise, call the uplink power control parameters from the background and set the reasonable threshold value (preferably –80dBm.In addition the parameters concerning the handover need to be set as per the real situation, the reasonableness of which will affect the quality of the network and the fluctuation of the signal. Please do the modifications only after a thorough understanding of the meaning of each 19



parameter, and test the modifications.

1.4.4 TRX Down Phenomenon: The mobile phone’s Receiving Signal Indicator points to the full scale. When the subscriber dials the number and press , the mobile phone drops out within a few seconds, at the same time the Indicator lowers to one scale or zero scale. This phenomenon happens intermittently. Cause and Analysis: After key is pressed, BCCH assigns a TCH to the mobile phone through SDCCH. When the TRX where the TCH is located is down, such as no power output or output too small, then the TCH will fail to set up the communication, leading to drop-off, and the Indicator lowering from full scale to 1 scale or 0 scale. After a few seconds it will return to the Wait Status, and the Indicator returns to full scale. As the TCH’s assignment is dynamic, that is to say, the first subscriber occupies the first carrier frequency, the first time slot; the second subscriber occupies the first carrier frequency, the second time slot, and so on. After eight subscribers’ calls, (or one subscriber’s eight consecutive calls), TCH will switch to the second carrier frequency, thus leading to normal conversation after several calls. As the number of callers is large, the interval for the examiner to encounter such fault may vary. And the more the number of carrier frequencies in the cell, the more difficult for it to discover the cause. On the other hand, there is no fault warning when the carrier frequency is not being used, but the warning of Power Too Small after the assignment. Solution: See if there is any warning in the TRX at the background when the mobile phone is having call test. If yes, replace that TRX with the adjacent carrier frequency amplifier or TRX, decide where the fault is and substitute the standby board for the at-fault board. Furthermore, this method can be one of the effective ways to determine if there are any faults in the carrier frequency channels at the cell. At the opening phase, use two mobile phones and finish continuous testing on all the carrier frequencies at the cell within several minutes.

1.4.5 Interferences Interferences can lead to signal fluctuations. Interferences can cause the timeout for the DCS counter (90/the multiframes between the same paging) in the mobile phones at the current service area, thus leading to cell reselection, which when reflected on the mobile phone side, is the fluctuation of signals.

1.4.6 Cell Congestion First let us explain the problem by analyzing the signals: After a call is initiated by the mobile phone, first SDCCH channel is assigned, after the corresponding signaling is finished, a TCH will be assigned to the mobile phone for its use by the network. What is the normal procedure is that BSC activates a TCH channel by sending to BTS in the current cell a signal activation command, and then the channel is assigned to the mobile phone. But what actually happens is that (in times when the current cell is congested) the network gives the mobile phone a command to switch to other cells, whose signal is weaker than the current cell. As a result, the level goes down immediately, leading to the fluctuations of the signal. It is possible that after the

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penalty time the mobile phone switches to other cell again, whose signal is stronger, in which case the level indicated in the mobile phone goes up. Or after hooking the mobile phone is in IDLE state and receives the BCCH signal from the cell where the mobile phone is in, which reflected in the Indicator is the going up of the signal strength. Therefore the essential cause of this problem is too much traffic, causing congestion in the cell.

1.4.7 Multipath Effect Multipath effect is one of the main causes that lead to the signal fluctuation in the network. The performance of the radio communications system is largely constrained by the radio channel. The propagation path between the transmitter and receiver is very complex, ranging from the simple visual distance propagation to encountering such terrain as buildings, hills and trees. The form the electromagnetic wave takes to propagate is diverse. In the main, it can take the forms of reflection, diffraction, and scattering. Most cellular radio systems operate in the city. There is no visual path between the transmitter and receiver, and what’s more, the skyscrapers produce strong scattering loss. Furthermore, as different objects have different multipath reflections, the electro-magnetic waves, after passing paths of different length, interfere to produce multi-path loss. At the same time the strength of the electromagnetic waves attenuates as the distance between the transmitter and receiver increases. The attenuation in the radio environment can be classified into fast attenuation and slow attenuation according to the estimation on the field intensity. In skyscraper cities, as the height of the mobile antenna (mobile phone) is much lower than that of the surrounding buildings, there is in most cases no visual propagation between the mobile station to the base station, thus causing attenuation. Even if there is such a path for visual propagation, multipath effect may still arise due to the reflection of the surface and the surrounding buildings. The incident waves arrive in different directions and have different propagation delay. The signal received by the mobile station at any position in the space consists of many signal waves, which have randomly distributed amplitude, phase, and incident angle. These multipath components are combined in vectors by the receiver, thus giving rise to the attenuation and infidelity of the signal being received. Even when the mobile receiver is static, attenuation can still arise as the signal being received is affected by the propagation medium (the air flow change), or by the moving objects where the radio channel is in. The method the base station uses to solve the fast-attenuation phenomenon is space diversity (polarized diversity), i.e. host diversity antenna. The effectiveness of this host diversity receipt is guaranteed by the irrelevancy of the host diversity antenna receipt. By irrelevancy, it means the signal received by the host antenna doesn’t have the simultaneous attenuation feature with the signal received by the diversity antenna. This requires that the gap of the host diversity antenna is 10 times greater than the radio signal wavelength (for GSM 900M the antenna gap must be greater than 4 meters), or that the polarized diversity is used to guarantee the different attenuation features of the signals received by the host diversity antenna. Whereas the single antenna mobile station (mobile phone) is helpless before the fast-attenuation feature under the wireless environment. So multipath will lead to the fluctuation of the mobile phone’s receiving level.

1.4.8 Other causes (1) Poor antenna feeder connection will lead to ups and downs of the standing wave, which when reflected in the mobile phone, is the fluctuation of the signals (2) Serious propagation flickers will lead to the amplification of the on/off in the base 21



station. (3) Some mobile phones will also have fluctuation of the level when doing location area updating. This is especially so when in the location area border of the city. (4) The signal level after the assignment will fluctuate deeply due to the difference of the carrier combination method or the disaccord of transmission channel gain within the cell.

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1.5 Other Main Problems 1.5.1 Poor Voice Quality (1) Hardware faults, poor propagation and bad grounding will lead to poor voice quality. (2) Worse interference leads to poor voice quality, especially when the frequency is density reuse. (3) Poor coverage, leading to poor receiving level and receiving quality.

1.5.2 Failure to attach the network Cannot update the location area （The CGI table of MSC fails to keep record of the cell, leading to location area update failure and network access failure）

1.5.3 Slow to attach the network Possibly related to the mobile phone’s access mode. Generally when the mobile phone is opened, it will try to access the network according to the information stored when the mobile phone was closed last time. In cases when the information is invalid or there is no stored information, the mobile phone will try to access the network by scanning for the strongest signal. As the scanning method may differ, so does the access speed. Furthermore as the mobile phone needs to decode the scanned frequency band according to the level, when the cell has a low access priority, it can only access the network after ensuring that there is no higher priority. This will also cause the slow attach speed. As the network access procedure includes authority verification on the mobile phone, the speed is also related to the performance, propagation and system processing of the cell. Notwithstanding the foregoing, many factors shall be considered in an integrated way.

1.5.4 Access slow Possible causes: (1) Effect on the same number of paging multi-frames. The paging may be delayed for 0-0.235N seconds. Certainly its setting is related to many a factor such as location area. Its parameter change shall be considered in an integrated way. (2) Effect on the number of time slots during propagation extension. In areas where coverage is poor, when the first access request is not decoded successfully, resend may be delayed for 55x0.477 to 1 second. (Subject to the parameters) (3) Related to the system processing performance.

1.5.5 unilateral connection The common cause may come from the equipment or transmission. This problem will 23



be introducted in detail in other documents.

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GSM Paging Issues & Analysis

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