most likely interview questions

8/3/2019 Most likely Interview Questions

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Interview Questions

1. What are the optimization tools you use?

Drive test, analysis, others?

2. Are System Information Blocks (SIB) transmitted all the time?

No, system information block is multiplexed with synchronization channel. Synchronization

channel occupies the first time slot (TS) and SIB occupies the other 9 time slots.

3. How does UE camp (synchronize) to a NodeB?1. UE uses the primary synchronization channel (P-SCH) for slot alignment (TSsynchronization).

2. After aligning to NodeB time slot, UE then uses secondary synchronization channel (S-SCH)to obtain frame synchronization and scrambling code group identification.

3. UE then uses scrambling code ID to obtain CPICH, thus camping to a NodeB.

4. What could be the cause of soft handover failure?

UE issue. Resource unavailable at target NodeB. Inadequate SHO threshold defined. Etc.

5. What are the three sets in handover?

The 3 sets in handover are:

Active set – the list of cells which are in soft handover with UE. Monitored set – the list of cells not in active set but RNC has told UE to monitor. Detected set – list of cells detected by the UE but not configured in the neighbor list.

6. What are the major differences between GSM and UMTS handover decision?GSM:

Time-based: mobile measures of RxLev and RxQual – mobile sends measurement report

every SACH period (480ms).BSC instructs mobile to handover based on these reports.

UMTS:

Event-triggered reporting – UE sends a measurement report only on certain event “triggers”. o UE plays more part in the handover decision.



Interview Questions

7. What are the events 1a, 1b, 1c, etc.? Event 1A – a Primary CPICH enters the reporting range, i.e. add a cell to active set. Event 1B – a primary CPICH leaves the reporting range, i.e. removed a cell from active set. Event1C – a non-active primary CPICH becomes better than an active primary CPICH, i.e.

replace a cell.

Event 1D - change of best cell. Event 1E - a Primary CPICH becomes better than an absolute threshold. Event 1F – a Primary CPICH becomes worse than an absolute threshold.

8. What are event 2a-2d and 3a-3d?

Events 2A-2D are for inter-frequency handover measurements and events 3A-3D are for IRAT

handover measurements.

Event 3A: the UMTS cell quality has moved below a threshold and a GSM cell quality hadmoved above a threshold.

Event 3B: the GSM cell quality has moved below a threshold. Event 3C: the GSM cell quality has moved above a threshold. Event 3D: there was a change in the order of best GSM cell list.

9. What may happen when there’s a missing neighbor or an incorrect neighbor? Access failure and handover failure: may attempt to access to a wrong scrambling code. Dropped call: UE not aware of a strong scrambling code, strong interference. Poor data throughput. Poor voice quality. Etc.

10. What can we try to improve when access failure is high?When access failure is high we can try the following to improve RACH performance:

Increase maximum UE transmit power allowed: Max_allowed_UL_TX_Power. Increase power quickly: power_Offset_P0. Increase number of preambles sent in a given preamble cycle: preamble_Retrans_Max. Increase the number of preamble cycles: max_Preamble_Cycle. Increase number of RRC Connection Request retries: N300.

11. What are the conditions you typically set to trigger IRAT handover?

RSCP and Ec/Io are used to trigger IRAT handover:

RSCP ≤ -100dBm. Ec/Io ≤ -16dBm.

12. What are the typical KPIs you use to measure a network and what criteria? Access failure rate (≤ 2%). Call setup time (CS: over 95% of the time < 6-second for mobile-to-PSTN, 9-second for mobile-

mobile. PS: over 95% of the time < 5-second).



Interview Questions

Dropped call rate (≤ 2%). BLER: over 95% of the blocks ≤ 2%. Average DL/UL throughput for PSD: 210kbps for loaded, 240kbps for unloaded.

13. What is the typical UE transmit power?

Varies – most of the time below 0dBm.

14. Have your used Ericsson TEMS? If so: Do you know how to create command sequence? What are the call sequences you typically have? CS long call, CS short call, PSD call, etc. What are the typical commands you have for CS and PS call? Do you regularly stop and restart a new log file? Why and when to stop and start a new file? How do you stop a log file? Stop command sequence first, wait and make sure all equipment

are in idle mode before stop logging.

15. Did you work on neighbor prioritization?

Prioritization is based on geographical location of the cell defined in the neighbor list.

16. What is the typical event sequence of IRAT Handover from 3G to 2G

Event 2D – entering into compressed mode – measurement of 2G candidates – Event 3A – Verification of 2G resources – Handover from UTRAN Command from 3G RNC to UE.

17. What are the possible causes for an IRAT Failure? Missing 2G relations Non availability of 2G Resources Poor 2G Coverage Missing 3G Relations

18. What is Paging Success Ratio? What is the typical PSR that you have seen in a UMTS

network? PSR – Paging Responses to the Paging Attempts About 90%

19. What are the possible causes for a lower PSR?

Non-continuous RF Coverage – UE going in and out of coverage area frequently Very High „Periodic Location Update Timer‟ – Keeping UEs in VLR long time after it moved out

of coverage Lower Paging Channel Power

Access Channel Parameter Issues Delayed Location Update when crossing the LA / CN Boundaries



Interview Questions

20. What are the possible causes for a Drop Call on a UMTS network? Poor Coverage (DL / UL) Pilot Pollution / Pilot Spillover Missing Neighbor SC Collisions

Delayed Handovers No resource availability (Congestion) for Hand in Loss of Synchronization Fast Fading Delayed IRAT Triggers Hardware Issues External Interference

21. A UE is served by 2 or 3 SC in AS. It is identifying a SC from 3 rd tier, Stronger and

meets the criteria for Event1a or Event1c. But SHO did not happen because of missing

neighbor relations? How do you optimize this issue?

Study the Pilot spillover from the 3rd

Tier SC and control its coverage Even after controlling the coverage, if the spillover is there, Add the neighbor.

22. A UE is served by 2 SC in AS, a SC is coming in to Monitored Set and Event1a is

triggered. But UE is not receiving Active Set Update from NodeB and the call drops.

What could be possible causes for this drop? Delayed Handover Loss of Synchronization Fast Fading Pilot Pollution / Spillover issues

23. What is Hard Handover in UMTS? When will it happen? Hard Handover in UMTS is a break before make type Handover It can happen in the inter RNC boundaries where there is no Iur link.

24. What is the typical Call Setup Time for a 3G UE to 3G UE Call? What are the possible

RF related causes for a delayed CST in this type of call? 6 to 9 seconds Multiple RRC Attempts (UE is on poor coverage – need more than Access Attempt) Delayed Page Responses High Load on Paging and/or Access Channel

Paging / Access Parameters

25. What is Soft Handover Overhead? What is the typical value in UMTS network? Soft Handover Overhead is calculated in two ways. 1) Average Active Set Size – Total Traffic /

Primary Traffic. 2) Secondary / Total TrafficTypical Values are like 1.7 (Avg Active Set Size) or 35% (Secondary / Total )



Interview Questions

26. What will happen to the Soft Handover Overhead when you apply OCNS on the

network? And Why?

With OCNS(Orthogonal Channel Noise Simulator), the interference (load) increases. This leadsto reduction in Ec/Io of a Pilot, which reduces the pilot spillovers. Reduction in Pilot Spillover will

reduce the Soft Handover Overhead.

27. What are the possible causes for an Access Failure in UMTS? Missing Neighbors Poor Coverage Pilot Pollution / Spillover Poor Cell Reselection Core Network Issues Non – availability of resources. Admission Control denies Hardware Issues Improper RACH Parameters

External Interference

28. (FOR ERICSSON EXPERIENCED) What is RTWP? What is the significance of it?Received Total Wide-band Power

It gives the Total Uplink Power (Interference) level received at NodeB

29. (FOR ERICSSON EXPERIENCED) What is the System Reference Point at which all the

Power Levels are measured in Ericsson NodeB?

System Ref Point for E/// NodeB is at the output of TMA (Between TMA and Antenna)

30. What are the typical values for 'reportingrange1a' and 'reportingrange1b'?

3 dB and 5 dB respectively.

31. What will be the impact when you change 'reportingrange1a' from 3 to 4 dB and

'timetotrigger1a' 100 to 320 ms, without changing any other parameters?

Reduction in number of Event1a

Delayed Event1a trigger

Reduction in Average Active Set Size

Delay in Event1a could increase DL interference, which could lead to a drop call or increase in

Average Power Per User (reduction in cell capacity)



Interview Questions

32. What is Admission Control?

Admission Control is an algorithm which controls the Resource Allocation for a new call and

additional resource allocation for an existing call. Incase, if a cell is heavily a loaded and enough

resources in terms of power, codes or CEs are not available, admission control denies

permission for the additional resource requirement.

33. What is Congestion Control?

Congestion Control monitors the dynamic utilization of specific cell resources and insures that

overload conditions do not occur. If overload conditions do occur, Congestion Control will

immediately restrict Admission Control from granting additional resources. In addition,

Congestion Control will attempt to resolve the congestion by either down switching, or

terminating existing users. Once the congestion is corrected, the congestion resolution actions

will cease, and Admission Control will be enabled.

34. What is processing Gain?

Ans:- Processing gain is the ratio of chip rate over data bit rate,

usually represented in decibel (dB) scale. For example, with 3.84MHz chip rate and 12.2k data

rate, the processinggain is:

PG12.2k = 10 * log (3,840,000 / 12,200) = 25dB

35. How to Calculate Max Numbers of Users in Cell?

Ans:- W: chip rate (for UMTS 3,840,000 chips per second)

EbNo: Eb/No requirement (assuming 3dB for CS-12.2k)

i: other-cell to in-cell interference ratio (assuming 60%)R: user data rate (assuming 12,200 kbps for CS-12.2k)

η: loading factor (assuming 50%)

Take 12.2kbps as example:

M = W / (EnNo * (1 + i) * R) * η = 3,840,000 (3 * (1 + 0.6) * 12,200) * 0.5 = 32.8

The number of users could also be hard-limited by OVSF code space. Take CS12.2k for

example:

• A CS-12.2k bearer needs 1 SF128 code.

• Total available codes for CS-12.2k = 128 – 2 (1 SF64) – 2 (4 SF256) = 124.

• Consider soft-handover factor of 1.8 and loading factor of 50%: 124 / 1.8 *.05 = 34 uers/cell.

Eb/No

Eb/N0 (the energy per bit to noise power spectral density ratio) is an important parameter in

digital communication or data transmission. It is a normalized signal-to-noise ratio (SNR)

measure, also known as the "SNR per bit". It is especially useful when comparing the bit error

rate (BER) performance of different digital modulation schemes without taking bandwidth into

account.

http://telecomfunda.com/forum/viewtopic.php?f=42&t=1064






Interview Questions

36. What is Typical Gain of TMA?

Ans:- TMA typically has a 12 dB gain; however, the effective gain comes from noise

figure reduction and the gain is close or equivalent to the feeder loss

37. Definition RSCP,RSSI and Ec/Io

RSCP: The “Received Signal Code Power” (RSCP) is the collected RF energy after thecorrelation / descrambling process, usually given in dBm. Only this code power is of interest for the following receiver stages when judging on the quality of the reception.

Ec/Io: This is the ratio of the received energy per chip (= code bit) and the interference level,usually given in dB. In case no true interference is present, the interference level is equal to thenoise level.

Because of the system gain (see 1), the interference level can be higher than the wanted signallevel. Therefore, at the coverage border, the value of Ec/I0 is usually negative

RSSI: The Received Signal Strength Indicator (RSSI) is a value that takes into account bothRSCP and Ec/I0. It is usually given in dBm and can be calculated as follows: RSSI [dBm] =

RSCP [dBm] - Ec/I0 [dB]

38. What is SIR?

Ans:- SIR is the Signal-to-Interference Ratio – the ratio of the energy in dedicated physical

control channel bits to the power density of interference and noise after dispreading.

39. Definition of pole capacity?

Ans:- The uplink noise increases with the loading exponentially. When the uplink noiseapproaches infinity then no more users can be added to a cell – and the cell loading is close to

100% and has reached its “pole capacity”.

Mathematically, to calculate the uplink pole capacity we need to know:

W: chip rate (for UMTS 3,840,000 chips per second)

R: user data rate (assuming 12,200 kbps for CS-12.2k)

f: other-cell to in-cell interference ratio (assuming 65%)

EbNo: Eb/No requirement (assuming 5dB)

AF: Activity factor (assuming 50%)

Pole Capacity = (W/R) / ((1+f) * AF * 10^(EbNo/10)) = 120.6

To calculate the downlink pole capacity we also need to know:

α: downlink channels orthogonality factor (assuming 55%)

Pole Capacity = (W/R) / ((1- α +f) * 10^(EbNo/10)) = 64.06


http://telefunda.blogspot.com/2011/01/definition-rscprssi-and-ecio.html

http://www.telecomfunda.com/index.php/2010/12/whats-difference-between-rscprssi-and-ecno/




http://www.telecomfunda.com/forum/viewtopic.php?f=42&t=1323&start=0






http://telefunda.blogspot.com/2011/01/definition-rscprssi-and-ecio.html




Interview Questions

40. Why open loop power control is needed and how it works?

Ans:- Open Loop power control is used when no feedback mechanism is possible. An estimateof the required power is made from measurements and system information. This is used for initial network access and finding initial power settings during dedicated mode.

41. How does soft/softer handover work? Ans:- Soft/softer handover downlink: UE rake receiver performs maximum ratio combining, i.e.

UE combines multi-path signals and form a stronger signal.

•Soft handover uplink: RNC performs selection combining, i.e. RNC selects the better signal

coming from multiple NodeB.

•Softer handover uplink: NodeB performs maximum ratio combining, i.e. NodeB rake receiver

combines signals from different paths and forms a stronger signal.

42. Why is there “soft handover gain”?

Ans:- Soft handover gain comes from the following:

• Macro diversity gain over slow fading.• Micro diversity gain over fast fading.

• Downlink load sharing over multiple RF links. By maintaining multiple links each link could

transmit at a lower power, resulting in lower interference therefore a gain.

Advantages:

• Overcome fading through macro diversity.

• Reduced Node B power which in turn decreases interference and increases capacity.

• Reduced UE power (up 4dB), decreasing interference and increasing battery life.

Disadvantages:

• UE using several radio links requires more channelization codes, and more resources on the

Iub and Iur interfaces.

43. What are Fast Fading and Slow fading? Ans:- Fast fading is also called multi-path fading, as a result of multi-path propagation. Whenmulti-path signals arriving at a UE, the constructive and destructive phases create a variation insignal strength.

Slow fading is also called shadowing. When a UE moves away from a cell the signal strengthdrops down slowly.

44. What are fast fading margin and slow fading margin?

Ans:- To factor in the fast fading and slow fading, we need to have a margin in the link budget

and they are called fast fading margin and slow fading margin.

In link budget, the fast fading margin is usually set to 2-3; slow fading margin is set to 7-10.





http://telefunda.blogspot.com/2011/02/what-are-fast-fading-and-slow-fading.html

http://telefunda.blogspot.com/2011/02/what-are-fast-fading-and-slow-fading.html





Interview Questions

45. 3G KPI

Ans:--3G KPI Introduction

KPI Category

Accessibility AnalysisThe ability of a user to obtain a requested Accessibility service from the system.

RetainabilityThe ability of user to retain its requested service once connected for the desired duration.

IntegrityThe ability of a user to receive the requested service at desired quality.

MobilityThe ability to provide the requested service to the user with mobility.

AvailabilityThe percentage of time that a cell is considered available.

46. WCDMA KPI-ACCESSIBILITY ANALYSISACCESSIBILITY ANALYSIS:The ability of a user to obtain a requested Accessibility service from the system.These are the five steps to check Accessibility Analysis.We will see each of them in detail with parameter.

RAN Capacity utilization Analysis and monitoring

Transport and RNC Load Capacity utilization analysis and monitoring Audit Node B Hardware and License Capacity

Consistency check RBS and RNC parameter

Admission parameter adjustment

47. WCDMA KPI-RETAINABILITY ANALYSIS Ans:- RETAINABILITY ANALYSIS

The ability of user to retain its requested service once connected for the desired duration.Steps to Check Retainability Analysis

Drop speech due to soft handover actions

Drop speech due to missing neighbor

Drop speech due to UL Synchronization lost

http://telefunda.blogspot.com/2011/03/wcdma-kpi-accessibility-analysis.html

http://telefunda.blogspot.com/2011/04/wcdma-kpi-retainability-analysis.html










Interview Questions

48. Why Ec/Io is negative? Ans:- Pilot channel power is always less than the total cell power, the ratio of the Pilot power (Ec) to the total cell power (Io) is always less than one. Thus, when measured in decibels, thevalue of Ec/Io is always negative.

Here is the calculation.

Ec = Effect energyIo = Total energy and noise.

Ec/Io = 10.log[Effect energy / (total energy+noise)] < 10.log(1) = 0Ec/Io always negative

49. How many slots are there in a WCDMA Frame? How big is a frame in ms. how manychips are there in a slot?

WCDMA Frame is 15 slots wide. It is 10ms in length. There are 2560 chips in one slot. Chip rate

is 3840 Kc/sLength of frame = 10 msNumber of chips in a frame = 3840 *10=38400 chips.Number of chips in a slot = 38400/15= 2560 chips

50. What is a typical CPICH power?CPICH power typically takes about 8~10% of the total NodeB power. For a 20W (43dBm)NodeB, CPICH is around 2W (35.1 ~ 33dBm).In urban areas where in-building coverage is taken care of by in-building installations, theCPICH may sometimes go as low as 5% because:1) The coverage area is small since users are close to the site, and

2) More power can be allocated to traffic channels.

51. What is PICH and PCH? Ans:- PICH-Paging Indicator Channel

PCH-Paging Channel

PICH is used to indicate UE to when it should read to S-CCPCH (Carries PCH) whereas PCH isused to carry RRC Message “Paging type 1” which contains actual Paging information

52. Different Idle mode tasks UE performs Ans:- In Idle mode, the UE has no connection to the radio network Keeping UEs in Idle modeminimizes the use of resources both for the UEs and in the network. However, the UEs must stillbe able to access the system and be reached by the system with acceptable delays. For this,the following procedures need to be performed:

PLMN selection and reselection

Cell selection and reselection

http://telefunda.blogspot.com/2011/04/why-ecio-is-negative.html

http://telefunda.blogspot.com/2011/05/what-is-pich-and-pch.html

http://telefunda.blogspot.com/2011/05/different-idle-mode-tasks-ue-performs.html

http://telefunda.blogspot.com/2011/05/different-idle-mode-tasks-ue-performs.html

http://telefunda.blogspot.com/2011/05/what-is-pich-and-pch.html

http://telefunda.blogspot.com/2011/04/why-ecio-is-negative.html



Interview Questions

Location Area (LA) and Routing Area (RA) updating

Paging

System information broadcast

53. Explain the functionality of TPC Ans:- During Power Control, Transmit Power control(TPC) commands are used to power up or power down based on SIR target in the step of 0.5 dB ( 1 dB if the connection is made over Iur).

54. How is Initial RACH Power is calculated? Ans:- The initial power on the PRACH - the power of the first preamble - is determinedaccording to equation.

P_PRACH = L_PCPICH + RTWP + constantValueCprach

Where L_PCPICH is the path loss estimated by UE since it knows transmit & receive CPICHpower RTWP is received Total Wideband Power(uplink interference) measured by RBS .Constant Value Cprach is used by the UE to calculate the initial power on the PRACH . Thisparameter is configurable and decides at which level below RTWP preamble ramping will start.

55. How much power usually a Node B is allocated to control channels?

The power allocated to control channels may depend on equipment vendor recommendation.Typically no more than 20% of the total NodeB power is allocated to control channels, includingCPICH. However, if HSDPA is deployed on the same carrier then the total power allocated tocontrol channel may go up to 25 to 30% because of the additional HSDPA control channelsrequired.

56. How many time Inner Loop Power Control happens and what type of fading itcompensates?

Ans: 1500 Hz and compensates Fast Fading

57. Power Control in WCDMA?

Open Loop

Inner Loop

Outer Loop

Open Loop Power control- The Open loop power control technique requires that thetransmitting entity measures the channel interference and adjusts its transmission power accordingly. This can be done quickly, but the problem is that the interference estimation isdone on the received signal, and the transmitted signal probably uses a differentfrequency, which differs from the received frequency by the system‟s duplex offset. As uplinkand downlink fast fading (on different frequency carriers) do not correlate, this method gives theright power values only on average.

http://rfnetworkoptimization.blogspot.com/2011/05/explain-functionality-of-tpc.html

http://rfnetworkoptimization.blogspot.com/2011/05/how-is-initial-rach-power-is-calculated.html

http://rfnetworkoptimization.blogspot.com/2011/05/how-is-initial-rach-power-is-calculated.html

http://rfnetworkoptimization.blogspot.com/2011/05/explain-functionality-of-tpc.html



Interview Questions

Inner Loop Power Control- In this method the received signal-to interference ratio (SIR) ismeasured over a 667- microsecond period (i.e., one time slot), and based on that value, adecision is made about whether to increase or decrease the transmission power in the other end of the connection. Note that the delay inherent in this closed-loop method iscompensated for by making the measurements over a very short period of time. Thetransmit power control (TPC) bits are sent in every time slot within the uplink and the downlink.

There is not a neutral signal; all power control signals contain either an increase or decreasecommand.Outer Loop Power Control-The outer loop power control functions within the base stationsystem, and adjusts the required SIR value (SIRtarget), which is then used in the inner loopcontrol. Different channel types, which can be characterized by, for example, different codingand interleaving methods, constitute a channel‟s parameters. Different channel parameters mayrequire different SIRtarget values. The final result of the transmission process can only beknown after the decoding process, and the resulting quality parameter is then used to adjust therequired SIR value. If the used SIR value still gives a low quality bit stream, then the outer looppower control must increase the SIRtarget value. This change in the outer loop will trigger theinner loop power control to increase the mobile station transmission power accordingly.

Power control is necessary to keep the transmitted signal power level under control so as to

minimize the interference and keep the quality of signal to a desired level. The main functions

are:

1. Closed-loop power control

(a) Outer-loop power control

- Uplink outer-loop power control

- Downlink outer-loop power control

(b) Inner-loop power control

- Uplink inner-loop power control

- Downlink inner-loop power control

2. Open-loop power control

(a) Uplink open-loop power control

(b) Downlink open-loop power control

Closed-loop power control is the power control mechanism used in UMTS to solve near-far

problem, minimize interference and to keep the signal quality to optimum level. Closed-loop

power control is used in uplink (UL) as well as downlink (DL). However, the motive in both the

cases are different. In uplink, signals from different UEs reach NodeB with different power

strength, thus causing the stronger signal blocking the weaker one, resulting in near-far effect.

In downlink, there is no near-far effect, but the UEs near the cell-edge or in high interference

region may need extra power to overcome the increased other cell interference and weak signaldue to Rayleigh fading. 9633747429

Closed-loop power control can be divided into outer-loop and inner-loop power control. In case

of uplink, the RNC manages the outer-loop and Node B manages the inner-loop and for

downlink, UE manages the outer-loop and Node B manages the inner-loop.



Interview Questions

Inner-loop power control (also called fast closed-loop power control), operates at 1500 times per

sec (1.5 kHz) [From where did this value of 1.5 kHz come from?

Answer: A UMTS 10 ms frame consists of 15 TPC commands. This results in a power control

frequency of 1500 Hz (15/10ms)] and relies on the feedback information from the opposite end

of the link (or channel) to maintain the signal to interference (noise) ratio to a target level set bythe outer-loop power control. The transmission power is increased or decreased by a certain

fixed step size depending on whether the received SIR is below or above the target SIR.

Precise power control can lead to optimum use of bandwidth resulting in increase cell capacity.

The UL inner-loop power control lets the UE adjust its output power in accordance with one or

more TPC commands received in the downlink direction. Remember the increase and decrease

in power is limited by upper and lower bounds as defined in 3GPP TS 25.101. The upper bound,

i.e. UE maximum output power, is set depending on the Power class of UE. This can also be set

below the maximum capability of the UE through signaling when the link is established. The

lower bound, i.e. UE minimum output power defined as the mean power in one timeslot (TS),

and shall be less than -50 dB.

The DL inner-loop power control is used to control the transmission power of downlink channelsat Node B as per the TPC commands received from UE. However, in some situations Node B

may ignore the increase/decrease these TPC commands. For example, in case of congestion

(high load scenario), the Node B can ignore the TPC commands from UE.

Outer-loop power control is used to set the target quality value for inner-loop power control, i.e it

adjusts the target SIR in Node B which is used during inner-loop power control. Now the

question is why do we need to adjust the target SIR? Outer-loop power control tries to keep the

quality of a connection to desired value. Too high quality will waste the resources.

Open-loop power control is used to set the initial power of UE (in random access) and downlinkchannels. The TPC commands used in inner-loop power control are relative, so it needs a

starting point and this is defined by open-loop power control. Also, this is useful in setting the

power level in case of common shared channels, where it is difficult to send each UE the

necessary TPC commands. In case of uplink, UE and broadcasted cell/system parameters are

used to set initial access power on RACH. And in case of downlink, the measurement report of

UE is used to set the initial power of downlink channels.

The open loop power control tolerance is ±9dB under normal conditions and ±12dB under

extreme conditions.

57. What is the family of codes used for Chanelization in WCDMA?

Ans:- Family of codes used for Chanelization in WCDMA used is Orthogonal Variable SpreadingFactor.(OVSF)

58. What is Significance of Eb_No? Ans:- Eb_No is related to QOS of a service which in terms related to bit error rate.

Technically it is the minimum signal to noise needed by infrastructure



Interview Questions

equipment after despreading it signal. This is a value used to compare different

infrastructure vendors. Eb_No changes with the service type.

For AMR 12.2 is ~ 4db for Node B

For AMR 12.2 is ~ 8db for MS

59. Explain Timer T3212? Ans:- Periodic LA and RA updating is used to notify the network of the UEs availability, and toavoid unnecessary paging attempts for a UE that has lost coverage and is not able to inform theCN that it is inactive.

The periodic LA update procedure is controlled by a timer, called t3212, which gives the timeinterval between two consecutive periodic location updates. The value is sent by the WCDMARAN to UEs on the BCCH.

60:- What is the processing gain for 384 Kb/s service? Ans:- Processing Gain= 10 log (Chip rate /Bit rate)

=10*Log(3840/384)=10*log (10)=10*1=10

61. What effect is there on signal by spreading and dispreading?

Ans:- Spreading will increase the bandwidth of a as signal. A signal of 10 Kb/s will become 40Kb/s after spreading and will become 10 Kb/s after despreading. The processing gain termexpresses the gain achieved by spreading a narrow band signal over a wideband spectrum.This gain is the ratio between the spreading chip rate and the actual service bit rate measuredat the RLC level.

61. What is typical pole capacity for CS-12.2, PS-64, PS-128 and PS-384?

Ans:- With same assumptions as above:CS-12.2k: 120.6 (UL), 64.1 (DL).PS-64k: 34.8 (UL), 12.8(DL).PS-128k: 16.2 (UL), 8.4 (DL).PS-384k: 16.2 (UL), 2.8 (DL).PS-384k has only 128k on the uplink, therefore the uplink capacity is the same for both.

62. What do you understand by DRX cycle?

Ans:- The UE listens to the PICH only at certain predefined times, reducing power consumption.The periodicity of these searches is set by the system and the time interval is calledDiscontinuous Reception (DRX) cycle.Different DRX cycles are used for circuit switched and packet switched services in Idle mode. Aseparate DRX cycle is also used to page Connected mode UEs in state URA_PCH.



Interview Questions

63. What is cell selection criterion?

Ans:- Cell selection is based on: Qmean: the average SIR of the target cell.

Qmin: minimum required SIR.

P compensation: a correction value for difference UE classes.

S = Qmean - Qmin - Pcompensation

If S>0 then the cell is a valid candidate.

A UE will camp on the cell with the highest S.

64. Difference between PICH and PCH?

Ans:- PICH - Paging Indicator ChannelPCH-Paging Channel PICH is used to indicate UE to when it should read to S-CCPCH (CarriesPCH) whereas PCH is used to carry RRC Message “Paging type 1” which contains actualPaging information.

65. When is System information sent to UE?

Ans:- The system information is regularly broadcast to the UE on the BCCH. When a parameter in the system information is changed, all UE in a cell are notified by a paging message or by asystem information change indication message.

66). Explain Soft and Softer handover? Give some advantage and disadvantage for soft

handover

Ans:- In Soft Handover, the UE connection consists of at least two radio links established withcells belonging to different RBSs. In Softer handover, the UE connection consists of at least tworadio links established with cells belonging to the same RBS.It acts as macro diversity since UE is connected to more than one radio link at any given point,adds redundancy and reduces interference. However there is a tradeoff between soft/softer handover & system capacity.

A UE involved in Soft/Softer Handover uses several radio links, more DL channelization codes,and more DL power than a single-link connection.Consequently, if all the UEs connected to a particular RNC are considered, more resources areneeded in the RBSs, more resources over the Iub and Iur interfaces, and more resources in the

RNC. For this reason, the number of radio links involved in the Soft/Softer handover must belimited.



Interview Questions

67. How many types of handovers are there in UMTS?

Soft/Softer Handover

Inter Frequency Handover

Inter RAT Handover

Core Network Hard Handover Service based handover to GSM

HSDPA Mobility

68. What is Pilot Pollution?Simply speaking, when the number of strong cells exceeds the active set size, there is “pilotpollution” in the area. Typically the active set size is 3, so if there are more than 3 strong cellsthen there is pilot pollution.

Definition of “strong cell”: pilots within the handover window size from the s trongest cell. Typicalhandover window size is between 4 to 6dB. For example, if there are more than 2 cells (besides

the strongest cell) within 4dB of the strongest cell then there is pilot pollution.

69. Define Processing Gain for WCDMA?

Processing Gain is ratio between rate of spreaded signal and rate of non spreaded signal.Processing Gain = 10 log (Chip rate /Bit rate)

70. Define Active Set? Pros and Cons of having a small or longer Active Set

Active Set consists of group of cells that takes part in soft/softer handover & measure by UE.Typical size of Active set is 3 or 4 & generally a standard practice in all WCDMA networks.

A small active set size may provide more resources available due to less soft/softer handover

but at the expense of handover gain thereby reducing the capacity & link redundancy.

71. WCDMA Handover Fundamentals GSM to UMTS Ans:- Learn about the differences between the GSM handover and the WCDMA handover -WCDMA handover decision algorithm-WCDMA handover procedure-WCDMA handover parameters-WCDMA blind handover and directed retry

72. What is UE maximum transmit power in your link budget?

Ans:- UE Transmit Power limit is 21dBm.

73. What is a typical maximum path loss?-UMTS

Ans:- The maximum path loss is dependent on the service and vendor recommendations;typically it is in between 135 to 140dB for urban areas and between 150 to 160dB for rural areas





Interview Questions

74. What is the difference between dB and dBm?

Ans:- dBm is a unit of power level, measured in milli-watts in logarithm scale, that is,

dBm = 10 * log(W*1000) where W is the power in Watts

dB is not a unit, it is the difference in dBm

75. What is Packet Loss?

Ans:- Packet loss is the failure of one or more transmitted packets to arrive at their destination.This event can cause noticeable effects in all types of digital communications.

The effects of packet loss:

In data, packet loss produces errors. In videoconference environments it can create jitter . In pure audio

LEARN MORE

Network Performance Management Networking Resources communications, such as VoIP, it can cause jitter and frequent gaps in received speech. In the worst cases, packet loss can cause severe mutilation of received data, broken-up images,

unintelligible speech or even the complete absence of a received signal.The causes of packet loss include inadequate signal strength at the destination, natural or human-made interference, excessive system noise, hardwaref ailure, software corruption or overburdened network nodes. Often more than one of these factors is involved.In a case where the cause can not be remedied, packet loss concealment may be used tominimize the effects of lost packets.

76. What is Difference between ARQ and HARQ? Ans:- ARQ (Automatic Repeat Request), which is used in UMTS, a package received in error

will be discarded and a retransmission will be requested. If the retransmission is also erroneous,

another retransmission will be requested.

Using HARQ, an erroneous package will be stored at the receiver and a retransmission will be

requested. Even if the retransmission is faulty, the receiver attempts to combine the two

erroneous packages to reproduce the original package.

77. What is a typical UE sensitivity level?

Ans:- The service and load determines the UE sensitivity; in general, in no-load condition, the

sensitivity is between -105dBm and -120dBm. For Ericsson, the UE sensitivity level is

calculated at around:

CS12.2: -119 dBm

PS-64: -112 dBm

PS-128: -110 dBm

PS-384: -105 dBm

HSDPA: -95 dBm


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Interview Questions

78. What is a typical NodeB sensitivity level?

The service and load determines the NodeB sensitivity; in general, in a no-load condition, the

sensitivity is between -115dBm to -125dBm. For Ericsson, the NodeB

sensitivitylevel is calculated at around:

CS12.2: -124 dBmPS-64: -119 dBm

PS-128: -115 dBm

PS-384: -115 dBm

79. What is a typical NodeB maximum output power?-UMTS

Ans:- The maximum NodeB output power is usually 20W or 40W, that is, 43dBm or 46dBm.

Also upto 100W

80. Why We have Different AMR Source rates: 12.2

(GSM-EFR), 10.2, 7.95, 7.40 (IS-641), 6.70 (PDC-EFR), 5.90, 5.15 and 4.75 kbps? Is there

any benefit to keep 7.95 and 7.40?

Ans:- We have different Source rates to compitable with current technology.

For example:7.4 kbps is equal to the US-TDMA speech codec.

6.7 kbps is equal to the Japanese PDC codec.

81.Facts about HSDPA:-

HSDPA represents an evolution of the WCDMA radio interface, which uses very similar methods to those employed by EDGE (Enhanced Data Rates for GSM Evolution) technology for the GSM radio interface. The fundamental characteristics which enable the increase in the data

throughput and capacity with reduced latency are summarised below: Time and code multiplexing of the users Multi-Code transmission Fixed Spreading Factor (SF = 16) Shorter TTI = 2ms No DTX (Discontinuous transmission)for the data channel Adaptive modulation and coding (AMC) supporting higher order modulation Node B scheduling and link adaptation Node B retransmissions (H-ARQ – Hybrid Auotmatic Repeat-Request) No power control No soft handover

82. Explain HSPA(High Speed Packet Acess)?

Ans. High Speed Packet Access is a combination of two mobile telephony High Speed

Download Packet Access (HSDPA) & High Speed Upload Packet Access (HSUPA) that extends

& improves the performance of the WCDMA protocols.

http://www.telecomfunda.com/index.php/2010/10/some-facts-about-hsdpa/






Interview Questions

83. What is benefit of shorter TTI in HSDPA?

Ans:- 1) After every TTI the resources can be redistributed among the users. Therefore, the

resource usage is more efficient.

2) Each UE reports about the channel quality after every TTI by sending the CQI.

3) CQI is sent after the very short period of time of 2 ms, it is possible to effectively perform linkadaptation even in rapidly changing conditions.

84. What are the main technologies used for HSDPA

Ans:- Main Technology

1) Link adaptation (AMC)

2) Fast Hybrid Automatic Repeat Request (HARQ), and

3) Shorter Transmission Time Interval (TTI)

85. WHAT IS LATENCY IN HSDPA?

Ans:- Latency is the time a packet needs to travel from sender to receiver. While UMTS

typically features an end-to-end latency of approximately 200ms, HSDPA manages to lower the

delay times in transmission to around 100ms.

86. WHAT IS LINK ADAPTION?

Ans:- HSDPA uses link adaptation, which means the way of transmission is changed according

to the quality of the channel conditions.

If a user is in favorable conditions, for example close to the nearest antenna tower, this user will

be assigned a high data rate.

When the user moves into worse channel conditions, for example far away from

the antenna tower, the transmission parameters will be changed accordingly and thus the data

rate will be decreased.

87. Brief describe the advantages and disadvantages of soft Handover?

Ans:- Advantages:

• Overcome fading through macro diversity.

• Reduced Node B power which in turn decreases interference and increases capacity.

• Reduced UE power (up 4dB), decreasing interference and increasing battery life.

Disadvantages:

• UE using several radio links requires more channelization codes, and more resources on the

Iub and Iur interfaces.









Interview Questions

88) What are the RRC states?

a. Cell DCH

b. Cell FACH

c. Cell PCH

d. URA PCH

89) Describe different RRC States?

a. Cell DCH:

When UE is switched ON, it assumes the CELL_DCH State by default.

It exists when UE really requires UL/DL dedicated resource.

UE is characterized at cell level according to its current Active set.

Logical channels DCCH+DTCH are mapped to transport channel DCH.

E.g. Voice call(CS), Video telephony(CS+PS)

b.Cell FACH:

This happens when,

(1) UE doesn‟t require continuous circuit connection with n/w, but DL packets will work.

(2) UE is forced to release the n/w resources to decrease the congestion problems (flip-flop

between DCH and FACH).

(3) Eventual location updates to support CELL_PCH and URA_PCH states (flip-flop between

FACH and PCH).

E.g. Call on hold(CS), File downloading(PS)

No dedicated UL/DL is provided. However, UE will keep monitoring to DL common transport

channel FACH „continuously‟ and receives packet data meant for UE itself.

When UL transmission is required, it uses RACH for 1-2 frames or CPCH for few more frames,depending on common access procedures; still UL transmission will be „discontinuous‟.

UE is known on cell level, but this time it‟s based on the last cell update it made.

c. Cell PCH:

This happens when UE doesn‟t require an active connection(common/dedicated). But still needs

to be connected, as it‟s expecting a n/w initiated or terminated call(CS or PS) in future.

So, no UL transmission is allowed. Selects a PCH(based on algo.) and keep monitoring

(receiving) it in „discontinuous‟ mode(DRX).

Can commute only into CELL_FACH state.

E.g. No outgoing call or data receive for long time

UE is still identified on cell level, but now in the cell where UE made its last update, when it was

in CELL_FACH state.

At regular time interval, UE would have to move in CELL_FACH state to update the location

area. If UE is traveling, this update will become frequent because of changing cells.

In some cases UE can have the transition from CELL_DCH to CELL_PCH, but not vice versa; it

has to go via CELL_FACH.

d.URA PCH:



Interview Questions

This can be considered as a general case of CELL_PCH state. Because all the characteristics

are same except one.

That is, UE is known on URA(UTRAN Registration Area) level rather than cell level. It listens to

PCH(and updates URA when required).

If UE is traveling continuously from one cell to another. It would have to transit multiple time to

CELL_FACH state for location update. By assigning URA_PCH state, this condition is avoided,since 1 URA contains one or more cells and vice versa.

90) In which conditions UE will be in Cell FACH state?

This happens when,

UE doesn‟t require continuous circuit connection with n/w, but DL packets will work.

UE is forced to release the n/w resources to decrease the congestion problems (flip-flop

between DCH and FACH).

Eventual location updates to support CELL_PCH and URA_PCH states (flip-flop between FACH

and PCH).

E.g. Call on hold(CS), File downloading(PS)

91) What is the difference between Cell PCH and URA PCH state?

UE is known on URA(UTRAN Registration Area) level rather than cell level

If UE is traveling continuously from one cell to another. It would have to transit multiple time to

CELL_FACH state for location update. By assigning URA_PCH state, this condition is avoided,

since 1 URA contains one or more cells and vice versa

92) What is U-RNTI?

URNTI is a 32bit identity.

SRNC identifier + SRNTI makes URNTI

93) Describe the CELL search procedure?

The Cell Search procedure can be summarised into the following steps:

a.Cell Selection: Determine the cell whose received carrier power is the strongest

b.Slot Synchronisation: the UE reads the P-SCH of the strongest carrier and feeds the received

signal through a matched filter (that is matched to the Primary Synchronisation Code). The slot

timing is detected via detection of peaks in the output of the matched filter.

c.Frame Synchronisation and Scrambling Code Group Identification: the UE reads the S-SCH of

the strongest carrier, and correlates the received signal with all the possible 64 Secondary

Synchronisation Codes. The output that gives maximum correlation gives the group of the

Primary Scrambling Code used in the cell. This process also gives the frame timing since the

Secondary Synchronisation Code is different for each slot and the group tells the mapping

between slots and SSCs.

d.Scrambling Code Identification: The UE attempts then to read the P-CPICH so as to extract



Interview Questions

the Primary Scrambling Code used in the cell. The P-CPICH is always scrambled by the

Primary Scrambling Code of the cell and is spread a factor 256 and an OVSF whose index is

zero. Since the Primary Scrambling Codes are grouped into 64 groups of 8 Scrambling Codes

and the S-SCH has

given the Scrambling Code group only 8 possibilities remain. So the P-CPICH is unscrambled

separately withevery possible Scrambling Code (8) in the group and only one output willproduce correct results and the Primary Scrambling Code of the cell is determined.

e.Read Broadcast Information: Since the P-CCPCH is always scrambled by the Primary

Scrambling Code of the cell (which has now been determined) and is always spread using

SF=256 with OVSF index=1, The UE can read the P-CCPCH without any problem. The P-

CCPCH carries the BCH where system information blocks are broadcast throughout the entire

cell. The UE can acquire at this stage various information (e.g. Spreading Factors, OVSF

indexes and Scrambling Codes used in other downlink channels, important

parameters for PRACH transmission, Measurement Parameters, Transport Formats etc.) and

can then decodeother downlink channels, initiate a PRACH procedure, performs some specific

measurements etc.

94 ) Radio bearer configuration mappings?

Signalling radio bearer RB0 shall be used for all messages sent on the CCCH (UL: RLC-TM,

DL: RLC-UM).

Signalling radio bearer RB1 shall be used for all messages sent on the DCCH, when using RLC

unacknowledged mode (RLC-UM).

Signalling radio bearer RB2 shall be used for all messages sent on the DCCH, when using RLC

acknowledged mode (RLC-AM), except for the RRC messages carrying higher layer (NAS)

signalling.

Signalling radio bearer RB3 and optionally Signalling radio bearer RB4 shall be used for theRRC messages carrying higher layer (NAS) signalling and sent on the DCCH in RLC

acknowledged mode (RLC-AM).

Additionally, RBs whose identities shall be set between 5 and 32 may be used as signalling

radio bearer for the RRC messages on the DCCH sent in RLC transparent mode (RLC-TM).

95) What are the types of hand over?

Here are following categories of handover (also referred to as handoff):

Hard Handover

Hard handover means that all the old radio links in the UE are removed before the new radiolinks are established. Hard handover can be seamless or non-seamless. Seamless hard

handover means that the handover is not perceptible to the user. In practice a handover that

requires a change of the carrier frequency (inter-frequency handover) is always performed as

hard handover.

Soft Handover

Soft handover means that the radio links are added and removed in a way that the UE always



Interview Questions

keeps at least one radio link to the UTRAN. Soft handover is performed by means of macro

diversity, which refers to the condition that several radio links are active at the same time.

Normally soft handover can be used when cells operated on the same frequency are changed.

Softer handover

Softer handover is a special case of soft handover where the radio links that are added and

removed belong to the same Node B (i.e. the site of co-located base stations from which severalsector-cells are served. In softer handover, macro diversity with maximum ratio combining can

be performed in the Node B, whereas generally in soft handover on the downlink, macro

diversity with selection combining is applied.

Generally we can distinguish between intra-cell handover and inter-cell handover. For UMTS

the following types of handover are specified:

Handover 3G -3G (i.e. between UMTS and other 3G systems)

FDD soft/softer handover

FDD inter-frequency hard handover

FDD/TDD handover (change of cell)

TDD/FDD handover (change of cell)

TDD/TDD handover

Handover 3G - 2G (e.g. handover to GSM)

Handover 2G - 3G (e.g. handover from GSM)

The most obvious cause for performing a handover is that due to its movement a user can be

served in another cell more efficiently (like less power emission, less interference). It may

however also be performed for other reasons such as system load control.

96) What are types of measurements?

Intra-frequency measurements: measurements on downlink physical channels at the same

frequency as the active set. A measurement object corresponds to one cell. Detailed description

is found in subclause 14.1.

Inter-frequency measurements: measurements on downlink physical channels at frequencies

that differ from the frequency of the active set and on downlink physical channels in the active

set. A measurement object corresponds to one cell. Detailed description is found in subclause

14.2.

Inter-RAT measurements: measurements on downlink physical channels belonging to another

radio access technology than UTRAN, e.g. GSM. A measurement object corresponds to one

cell. Detailed description is found in subclause 14.3.

Traffic volume measurements: measurements on uplink traffic volume. A measurement object

corresponds to one cell. Detailed description is found in subclause 14.4.

Quality measurements: Measurements of downlink quality parameters, e.g. downlink transport

block error rate. A measurement object corresponds to one transport channel in case of BLER.

A measurement object corresponds to one timeslot in case of SIR (TDD only). Detailed

description is found in subclause 14.5.

UE-internal measurements: Measurements of UE transmission power and UE received signal



Interview Questions

level. Detailed description is found in subclause 14.6.

UE positioning measurements: Measurements of UE position. Detailed description is found in

subclause 14.7.

97) What is paging? Why paging is required?

Paging: This procedure is used to transmit paging information to selected UEs in idle mode,

CELL_PCH or URA_PCH state using the paging control channel (PCCH).

Paging can be initiated :

Upper layers in the network may request paging, to e.g. establish a signaling connection.

UTRAN may initiate paging for UEs in CELL_PCH or URA_PCH state to trigger a cell update

procedure.

UTRAN may initiate paging for UEs in idle mode, CELL_PCH and URA_PCH state to trigger

reading of updated system information.

UTRAN may also initiate paging for UEs in CELL_PCH and URA_PCH state to release the RRC

connection.

98) Type of Paging?

Paging Type 1

Paging Type 2(UE dedicated paging)

12)When dedicated paging is used?

a.This procedure is used to transmit dedicated paging information to one UE in connected mode

in CELL_DCH , CELL_FACH or CELL_PCH (FDD only) state. Upper layers in the network may

request initiation of paging.b.For a UE in CELL_DCH, CELL_FACH or CELL_PCH (FDD only) state, UTRAN initiates the

procedure by transmitting a PAGING TYPE 2 message on the DCCH using AM RLC. When not

stated otherwise elsewhere, the UTRAN may initiate the UE dedicated paging procedure also

when another RRC procedure is ongoing, and in that case the state of the latter procedure shall

not be affected.

c.UTRAN should set the IE "Paging cause" to the cause for paging received from upper layers.

If no cause for paging is received from upper layers, UTRAN should set the value "Terminating

– cause unknown".

99) In what situation UE goes to CELL PCH and URA PCH states?

a.Cell PCH:

This happens when UE doesn‟t require an active Connection(common/dedicated). But still

needs to be connected, as it‟s expecting a n/w initiated or terminated call(CS or PS) in future

b.URA PCH:

In Cell PCH UE is still identified on cell level, but now in the cell where UE made its last update,

when it was in CELL_FACH state.At regular time interval, UE would have to move in



Interview Questions

CELL_FACH state to update the location area. If UE is traveling, this update will become

frequent because of changing cells. If UE is traveling continuously from one cell to another. It

would have to transit multiple time to CELL_FACH state for location update. By assigning

URA_PCH state, this condition is avoided, since 1 URA contains one or more cells and vice

versa

100) What is Monitor set and detected set Cells?

a.Cells, which are not included in the active set, but are included in the CELL_INFO_LIST

belong to the Monitored Set.

b.Cells detected by the UE, which are neither in the CELL_INFO_LIST nor in the active set

belong to the Detected Set. Reporting of measurements of the detected set is only applicable to

intra-frequency measurements made by UEs in CELL_DCH state.

101) What are the various types of RNC?

a.Serving RNC

b.Drift RNC

c.Controlling RNC

102) What is the use and difference between CRNC, Drift RNC and SRNC?

RNC from which the UE is currently getting Served or connected to is called SRNC or Serving

RNC.

During a soft handover procedure the RNC UE is moving to, is called Drift RNC.

The RNC which controls the movement of UE from Serving RNC to the Drift RNC is calledControlling RNC.

103) What is initial direct transfer message? What is the use?

The initial direct transfer procedure is used in the uplink to establish a signaling connection. It is

also used to carry an initial upper layer (NAS) message over the radio interface.

104) What is security mode command? Why it is used?

Security mode command is a RRC Message,

The purpose of this procedure :

To trigger the start of ciphering or to command the restart of the ciphering with a new ciphering

configuration, for the radio bearers of one CN domain and for all signalling radio bearers.

It is also used to start integrity protection or to modify the integrity protection configuration for all

signalling radio bearers



Interview Questions

105) What is CCTrCH? (IMP)

A Coded Composite Transport Channel is the result of combining (multiplexing) severalTransport

Channels of the same type; i.e.: we can have a CCTrCh made up of one or several DCHs, a

CCTrCh made up of DSCHs. One exception is that we can have one or several FACHs

multiplexed with a PCH on the same CCTrCh.

A better notation would be for example:

CCTrCh4DCH: A CCTrCh consisting of 4 DCHs

CCTrCh3FACH: A CCTrCh consisting of 3 FACHs

CCTrChPCH,2FACH: A CCTrCh consisting of the PCH and 2 FACHs

106)What is compressed mode? (IMP)

Higher layers can request a UE to monitor cells on other frequencies (FDD, TDD), or other

systems (GSM). This implies that the UE has to perform measurements, on the other

frequencies/systems and thus higher layers command the UE to enter Compressed Mode. The

UTRAN need to send to the UE all the parameters for the Compressed Mode.

Compression Methods:

When in Compressed Mode the information normally transmitted during a 10 ms frame is

compressed in time so that gaps of adequate length are created. To achieve this one of three

mechanisms

PuncturingThis method is NOT applicable to downlink.

Spread Factor Reduction

Actually halving the Spread Factor. So with this method if SF is used in non-compressed frames

then SF/2 is used in the compressed frames.

Higher Layer Scheduling

Higher Layers set restrictions so that only a subset of allowed TFCs are used in Compressed

Mode.

Compressed Mode Parameter:

TGSN (Transmission Gap Starting Slot Number): A transmission gap pattern begins in a radio

frame, called firstradio frame of the transmission gap pattern, containing at least one

transmission gap slot. TGSN is the slot numberof the first transmission gap slot within the firstradio frame of the transmission gap pattern;

• TGL1 (Transmission Gap Length 1): Duration of the first transmission gap within the

transmission gap pattern, expressed in number of slots;

• TGL2 (Transmission Gap Length 2): Duration of the second transmission gap within the

transmission gap pattern, expressed in number of slots. If this parameter is not explicitly set by

higher layers, then TGL2 =TGL1;



Interview Questions

• TGD (Transmission Gap Start Distance): Duration between the starting slots of two

consecutive transmission gaps within a transmission gap pattern, expressed in number of slots.

The resulting position of the second transmission gap within its radio frame(s) shall comply with

the limitations of TS 25.101 (Ref [2]). If this parameter is not set by higher layers, then there is

only one transmission gap in the transmission gap pattern;

• TGPL1 (Transmission Gap Pattern Length): Duration of transmission gap pattern 1; • TGPL2 (Transmission Gap Pattern Length): Duration of transmission gap pattern 2. If this

parameter is not explicitly set by higher layers, then TGPL2 = TGPL1.

The following two parameters (integers) control the transmission gap pattern sequence start and

repetition:

• TGPRC (Transmission Gap Pattern Repetition Count): Number of transmission gap patterns

within the transmission gap pattern sequence; From Figure 10 it seems TGPRC is even,

therefore the number of slots in TG Sequence is: 0.5*TGPRC (TGPL1+TGPL2)

• TGCFN (Tr ansmission Gap Connection Frame Number): CFN of the first radio frame of the

first pattern 1 within the transmission gap pattern sequence.

107) What is FACH measurement occasion?

GAPs in S-CCPCH to perform measurement on Different Frequencies or Interrat resection.

108) Features of Rel99/Rel5/Rel6/7?

Release '99

Bearer services

64 kbit/s circuit switched

384 kbit/s packet switched

Location servicesCall services: compatible with Global System for Mobile Communications (GSM), based on

Universal Subscriber Identity Module (USIM)

Release 4

Edge radio

Multimedia messaging

MeXe levels

Improved location services

IP Multimedia Services (IMS)

Release 5

IP Multimedia Subsystem (IMS)

IPv6, IP transport in UTRAN

Improvements in GERAN, Mexe, etc

HSDPA



Interview Questions

Release 6

WLAN integration

Multimedia broadcast and multicast

Improvements in IMS

HSUPA

109) MIB/SB/SIB details?

MIB: The Scheduling of MIB is Static.

110) What is Channelisation / scrambling?

Spreading is applied to the Physical Channels (except SCH). It consists of two distinct

operations: a. Channelisation and b. Scrambling

Channelisation is performed before scrambling.

Channelisation:

The term spreading is also used to refer to channelisation. Channelisation is the basis for Code

Division Multiple Access (CDMA) encoding. This operation transforms every data symbol of a

signal into a number of chips. The bandwidth of the resulting signal occupies a much larger

bandwidth typically 5 MGHz and therefore the name Wideband-CDMA or W-CDMA. The

number of chips per data symbol is called the Spreading Factor (SF).

Scrambling.

111) When secondary scrambling code used?

When there is any conflict with the Primary Scrambling code then Secondary Scrambling Codewill be used.

112) Why we perform Channelisation or spreading before scrambling?

Before sending a data we need to identify the channel to send the data. A particular

Channelisation code is the “identity” for the channel.

Hence we use Channelisation code to select the channel and then scramble it so as to send the

identity of the UE over the channel

113) When TMSI , P- TMSI , IMSI , IMEI used?

TMSI (GSM-MAP): The TMSI (GSM-MAP) shall be chosen if available. The IE "LAI" in the IE

"Initial UE identity" shall also be present when TMSI (GSM-MAP) is used, for making it unique.

P-TMSI (GSM-MAP): The P-TMSI (GSM-MAP) shall be chosen if available and no TMSI (GSM-

MAP) is available. The IE "RAI" in the IE "Initial UE identity" shall in this case also be present



Interview Questions

when P-TMSI (GSM-MAP) is used, for making it unique.

IMSI (GSM-MAP): The IMSI (GSM-MAP) shall be chosen if available and no TMSI (GSM-MAP)

or P-TMSI is available.

IMEI: The IMEI shall be chosen when none of the above three conditions are fulfilled.

114)What is Auto attached?

Auto Attach

Enabled

Disabled

When Power On

If

MNO I

It will perform a combined Attach

If NMOII

It will do a LAU and Attach.

If UE is Location updated i.e. SIM LAI is same as Cell LAI

Then it will do nothing.

If UE is not Location updated i.e.

SIM LAI differs from the Cell LAI then UE will perform Location Area Update

PS Call

Same RRC connection can be used for the PS call.

Will do a PS attach then the PS call.

115) Why we do a Combined Attach?

If the network operates in Network Mode Of Operation I i.e. NMO I and GPRS MS that operatesin Mode A or Mode B will do a simultaneous IMSI attach for GPRS and non GPRS services and

shall use the Combined GPRS attach, combined and periodic routing area updating procedures

instead of the corresponding MM specific procedures IMSI attach and normal and periodic

Location area updating.

116) Where CK and IK gets generated?

In RRC CK and IK gets generated.

117) Where CK and IK gets stored?

In USIM CK and IK will get stored.

118) Where the Security procedure will start?

Security Procedure will start in RRC but Ciphering will start for AM and UM data security



Interview Questions

procedure will start at RLC layer. And for TM mode security procedure start at MAC layer.

119) What are the mandatory SIB’s which UE has to read before campign?

SIB 1,3,5,7,11(It contains neighbor cell Information.)

120) What are the important RACH parameters and which SIB contains this information?

SIB 7/5

121) Which SIB contains S criteria information?

SIB 3

122) Which NAS message contains the information about Qos and where Qos is been

decide on the network side?

PDP context req.SGSN.

123) What is SHO and HHO? Which are the layers which are impacted with SHO and

HHO?

SHO - RRC,L1

HHO – L1

124) LAI and RAI structure and where is it broadcasted?

MNC+MCC+LAC=LAI

LAI+RAC=RAI

SIB 1.Need to check on MNC can be 2 bit also.

125) What is Multiple PDP and what is meant by Multi RAB and multi call?

2 PS - Multiple PDP

PS+CS – Multi RAB

2CS – Multi Call.

126) Training WCDMA Handover Fundamentals GSM to UMTS

Training WCDMA Handover Fundamentals GSM to UMTS

Learn about the differences between the GSM handover and the WCDMA handover



Interview Questions

-WCDMA handover decision algorithm

-WCDMA handover procedure

-WCDMA handover parameters

-WCDMA blind handover and directed retry

127) What can we try to improve when access failure is high?

Ans:- When access failure is high we can try the following to improve RACH Performance

1.) Inc Maximum UE transmit power allowed: Max_allowed_UL_Tx_Power.

2.) Increase power quickly: power_Offset_P0.

3.) Increase number of preambles

128). What is Eb/No for requirement for HSDPA?

Ans:- It varies with user bit(data) rate , typically 2 for 768 kps & 5 for 2 Mbps.

129). What HS Channels are introduced in HSDPA in L1?

Ans:- HS-PDSCH-High Speed Physical Downlink Shared Channel

HS-SCCH-High Speed Shared Control Channel

130) How power control is implemented in HSDPA?

Ans:- Initial power is set in the same way as open loop power control of DCH & there is no

further power control on HSDPA shared channel HS-DSCH.The channel rate is controlled by

adaptive modulation & coding format.

The principle & functionality of the power control for the HSDPA associated dedicated channels

are the same as for the DPCH power control.

HS-DPCCH power is an offset relative to DPCCH depending upon whether the UE is in soft

handoff or not.

The power of HS-SCCH is fixed.

131). What fixed SF is used for HSDPA?

SF 16, maximum of 5 codes.

132). What do you understand by the CQI Measurements?

Ans:- Channel Quality Estimation for HSDPA sessions only. In order to aid scheduling & TFRC

selection in the RBS, the UE sends a channel quality indicator (CQI) report on the uplink. The

CQI report estimates the number of bits that can be transmitted to the UE using a certain

assumed HS-PDSCH power with a block error rate of 10 %.



Interview Questions

133) What type channel coding is used for the voice & data services?

Ans Voice-Convolution Coding

Data-Turbo Coding

134) What is the relationship of SF & data rate?

Ans) They are inversely proportional, Lower the SF higher the data rate.

135) What is the relationship of SF & power reuired?

Ans:- Lower SF more power required

136):- Idle Mode Behaviour is managed by System information send on which L3

Channel?

Ans:- BCH

137) How many Radio Bearers (RB) are involved in CS voice call?

3

138 ) How many Service Radio Bearers (SRB) are involved in CS voice call?

Ans:- 4

139) SCH channel consists of how many chips?

Ans) 256 Chips

140) Cell Reselection is valid in both Idle and in which Sate in Connected mode?

Ans:- CELL FACH

141) Explain Near far effect?

Ans:- All users use the same bandwidth at the same time and therefore users interfere with

one another. Due to the propagation path loss, the signal received by the base station from a

UE close to the base station will be stronger than the Signal received from another terminal

located at the boundary. Hence, the distant user will be dominated by the close user. This is

called the near-far effect. To achieve a considerable capacity, all signals, irrespective of

distance, should arrive at the base station with the same mean power. A solution to this problemis power control, which attempts to achieve the same mean received power for each user.

142) How is Initial RACH Power is calculated?

Ans:- The initial power on the PRACH - the power of the first preamble - is determined

according to equation

P_PRACH = L_PCPICH + RTWP + constantValueCprach



Interview Questions

Where L_PCPICH is the path loss estimated by UE since it knows transmit & receive CPICH

power

RTWP is received Total Wideband Power(uplink interference) measured by RBS .

constantValueCprach is used by the UE to calculate the initial power on the PRACH . This

parameter is configurable and decides at which level below RTWP preamble ramping will start.

143) What power RACH message Control Part is sent?

Ans:- The power of the control part of the RACH message is determined by the power of the last

transmitted preamble and by a configurable offset power Offset Ppm.

144) However there is a tradeoff between soft/softer handover & system capacity?

Ans:- A UE involved in Soft/Softer Handover uses several radio links, more DL channelization

codes, and more DL power than a single-link connection. Consequently, if all the UEs

connected to a particular RNC are considered, more resources are needed in the RBSs, more

resources over the Iub and Iur interfaces, and more resources in the RNC. For this reason, the

number of radio links involved in the Soft/Softer handover must be limited.

A typical target for soft handover in WCDMA network is less than or equal to 30%

145) Which link is required to perform Inter RNC SHO?

Ans:- Iur

146). What is “noise rise”? What does a higher noise rise mean in terms of network

loading?

For every new user added to the service, additional noise is added to the network. That is, each

new user causes a “noise rise”. In theory, the “noise rise” is defined as the ratio of total received

wideband power to the noise power. Higher “noise rise” value implies more users are allowed

on the network, and each user has to transmit higher power to overcome the higher noise level.

This means smaller path loss can be tolerated and the cell radius is reduced. To summarize, a

higher noise rise means higher capacity and smaller footprint, a lower noise rise means smaller

capacity and bigger footprint.

147) When in 3-way soft handover, if a UE receives power down request from one cell

and power up request from the other 2 cells, should the UE power up or down and why?

Ans. - UE will power down because if a cell is able to sustain a good connection with one cell on

lower power level it will discard power up messages from other cells. It also helps in maintaining

low interference level for other surrounding UE‟s.



Interview Questions

148) Suppose two UE are served by the same cell, the UE with weaker link (poor RF

condition) uses more “capacity”, why does this mean?

Answer -The UE with weaker RF link will require NodeB to transmit higher traffic power in order

to reach the UE, resulting in less power for other UE – therefore consumes more “capacity

149) Under what circumstances can a NodeB reach its capacity? What are the capacity

limitations?

Ans:- Node B reaches its maximum transmit power, runs out of its channel elements, uplink

noise rise reaches its design target, etc

150) What is “cell breathing” and why?

Answer - The cell coverage shrinks as the loading increases, this is called cell breathing.

In the uplink, as more and more UE are served by a cell, each UE needs to transmit higher power to compensate for the uplink noise rise. As a consequence, the UE with weaker link (UE

at greater distance) may not have enough power to reach the NodeB – therefore a coverage

shrinkage.

In the downlink, the NodeB also needs to transmit higher power as more UE are being served.

As a consequence UE with weaker link (greater distance) may not be reachable by the Node B.

151) . Is UMTS an uplink-limited or downlink-limited system?

Answer – Initially, A typical WCDMA network is Uplink Limited. Later a Loaded Network

becomes Downlink Limites.

152) If you have 3 cells in your Active Set and a drop call occurs, which Cell a Drop call

would be pegged?

Answer - Serving Cell in Active Set

153) Briefly describe Capacity Management and its functions?

Answer - Capacity Management is responsible for the control of the load in the cell. It consists

of 3 main functions:

1. Dedicated Monitored Resource Handling: tracks utilization of critical resources of the system.

2. Admission Control: accepts/refuses admission requests based on the current load on the

dedicated monitored resources and the characteristics of the request3. Congestion Control: detects/resolves overload situations

154) What Resources are monitored for Capacity Management?

Answer –

*DL Power

*Received Total Wideband power



Interview Questions

*OVSF Codes

*RBS Channel Elements

154) What Radio Measurements are used for Congestion Monitoring?

Answer –

Downlink Received Power Uplink Received Total Wideband Power

155) How is inter-frequency Handover triggered?

The network decides that inter frequency measurements need to be performed and sends the

MEASUREMENT CONTROL MESSAGE with Measurement type set to Inter-Frequency

measurements. Generally it will set an Event as well along with the measurements. The

following are list of Events that can trigger Measurement Report.

• Event 2a: Change of Best Frequency

• Event 2b: The estimated quality of the currently used frequency is below a certain thresholdand the estimated quality of a non-used frequency is above a certain threshold

• Event 2c: The estimated quality of a non-used frequency is above a certain threshold

• Event 2d: The estimated quality of the currently used frequency is below a certain threshold

• Event 2e: The estimated quality of a non-used frequency is below a certain threshold

• Event 2f: The estimated quality of the currently used frequency is above a certain threshold

The Inter-Frequency Handover Evaluation bases its decision on P-CPICH quality measures on

the currently used frequency and on one or more non-used frequencies. If the evaluation result

is positive, one cell on a non-used frequency is proposed to Inter-Frequency handover

Execution.

Inter-Frequency Handover is a hard handover where the UE is ordered by the network to tune to

another frequency. This means that there will be small interruptions in the data flow to and from

the UE.

156) What kind of Handover takes place in Load Sharing?

It‟s a blind handover to the co-located cell. IFHO

157) What do you understand by Directed Retry?

When there is a co-existing GSM RAN, excess traffic in a WCDMA cell may be off-loaded to



Interview Questions

GSM

If a call is chosen for Directed Retry to GSM, the request for the speech RAB will be rejectedwith cause "Directed retry" and then a request is made to the core network to relocate the UE toa specific GSM cell, using the Inter-RAT handover procedure. This handover is a blind onesince the target cell is chosen not based on UE measurements. Therefore, the target cell must

be co-located with the WCDMA cell. Co-located GSM cells are assumed to have similar coverage and accessibility as their respective WCDMA cells.

158.)What is HSDPA and how is it different from the current 3G/GPRS network available

today?

HSDPA stands for High Speed Downlink Packet Access and is an evolution of 3G technology. It

is sometimes called 3.5G. HSDPA is a technology for improving the download performance of

the current 3G network. It offers higher data transfer speeds, increased efficiency and greater

system capacity for operators.

159.)What is HSUPA?

HSUPA stands for High Speed Uplink Packet Access, and improves the uplink performance in

networks which support HSUPA. 3G networks support a maximum of 384kbps on the uplink.

HSUPA will support a maximum of 1.9Mbps. The average uplink speed will be about 1Mbps.

160.)Do 3.5G/HSPA networks support Voice?

HSPA is purely a data network. All Voice traffic is supported on the current 3G network.

161.)What are the benefits of using 3.5G/HSPA for the end-user?

As a result of the enhancement, 3.5G/HSPA promises a better broadband multimedia

experience. For the business user, HSDPA enables high-speed Internet access and rapid

download of emails with attachments. For the consumers, HSDPA allows faster downloading of

high-resolution digital images, high-quality music downloads, Mobile TV, and mobile multi-player

games.

162.)What do I need to enjoy the enhanced experience using 3.5G/HSPA?

To enjoy 3.5G/HSPA services, you will require a HSDPA-enabled phone and a 3G USIM.

163.)What is the maximum download speed of StarHub's 3.5G/HSPA network?

StarHub's 3.5G/HSPA network currently supports download speeds of up to 21Mbps, and

upload speeds of up to 5.76Mbps.



Interview Questions

(Note: Actual bandwidth and speeds experienced are dependent on a combination of factors

including the mobile equipment, software used, internet traffic and destination server. Presently

only the Mobile Broadband Modem supports up to 21Mbps (download) and up to 5.76Mbps

(upload). Most handsets available today only supports HSDPA, with download speeds of up to

3.6Mbps and upload speeds of 384kbps.)

164.)What affects the download and upload speeds when using the HSPA service?

As with any network, the actual bandwidth experienced will have to take into account network

and protocol overheads and therefore the actual addressable bandwidth might be lower. In

addition to this, performance will also be affected by the following:

1. the mobile equipment that you are using and the bandwidth it supports

2. the websites and servers you are accessing

3. the types of Applications being used

4. the radio conditions

5. level of congestion on the Internet

165). What is the benefit for shorter TTI?

1)After every TTI the resources can be redistributed among the users. Therefore, the resource

usage is more efficient.

2)each UE reports about the channel quality after every TTI by sending the CQI.

3)CQI is sent after the very short period of time of 2 ms, it is possible to effectively perform link

adaptation even in rapidly changing conditions.

166.) Difference between ARQ HARQ?

ARQ (Automatic Repeat Request), which is used in UMTS, a package received in error will be

discarded and a retransmission will be requested. If the retransmission is also erroneous,

another retransmission will be requested.

Using HARQ, an erroneous package will be stored at the receiver and a retransmission will be

requested. Even if the retransmission is faulty, the receiver attempts to combine the two

erroneous packages to reproduce the original package.

166.) What are the main technologies used for HSDPA?



Interview Questions

Main Technology

1)Link adaptation (AMC)

2)Fast Hybrid Automatic Repeat Request (HARQ), and

3)Shorter Transmission Time Interval (TTI)

most likely interview questions

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