Brief UMTS interview questions

[jimjim]

Dears,

I would like to share with you a document which I got recently about interview question for UMTS.


4shared folder - My 4shared (http://www.4shared.com/folder/UxomqpWY/_online.html)


No password, enjoy and do not forget reps and thanks
:)
:)

[koepret]

Dears,

I would like to share with you a document which I got recently about interview question for UMTS.


4shared folder - My 4shared (http://www.4shared.com/folder/UxomqpWY/_online.html)


No password, enjoy and do not forget reps and thanks
:)
:)

please check again your file , need password

[arthenian11]

Friend,
File is password protected..

Please share the password.....

[jimjim]

Sorry fellows,

I uploaded the file password however the default pass is zaza12345

Enjoy and sorry for inconvenience :D

Regards,

BTW "reps and thanks" are highly appreciated.:D

[jimjim]

Dears,

If you are facing any problem just let me know.

:):)

[adewijaya]

detail answer of some questions that already send before :




1. What is a typical NodeB maximum output power?
The maximum NodeB output power is usually 20W or 40W, that is, 43dBm or 46dBm.
2. What is UE maximum transmit power in your link budget?
21dBm.
3. What is a typical antenna gain?
The antenna gain depends on antenna model; in link budget we use around 17dBi.
4. What is difference between dBi and dBd?
dBi is the gain in dB from isotropic source; dBd is the gain from a dipole source.
dBd + 2.15 = dBi.
5. What is the difference between dB and dBm?
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.
6. What is 0dBm?
0dBm = 1 milli-watt.
7. How many types of handovers are there in UMTS?
Soft handover, softer handover, inter-frequency handover, inter-RAT handover, inter-RAT cell change (UE moving out of UMTS coverage into GSM/GPRS/EGDGE coverage).
8. What is soft handover and softer handover?
 Soft handover: when a UE is connected to cells owned by different NodeB.
 Softer handover: when a UE is connected to cells owned by the same NodeB.
9. How does soft/softer handover work?
 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.
10. Brief describe the advantages and disadvantages of soft handover?
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.
11. What is channelization code?
Channelization codes are orthogonal codes used to spread the signal and hence provides channel separation, that is, channelization codes are used to separate channels from a cell.
12. What is scrambling code? How many scrambling codes there are?
Scrambling codes are used to separate cells and UEs from each other, that is, each cell or UE should have a unique scrambling code. There are 512 scrambling codes on the downlink and millions on the uplink.
13. What is scrambling "code group"?
The 512 scrambling codes are divided into 64 code groups – each code group has 8 scrambling codes.
Code group i (i = 0 to 63) has codes from i*8 to (i+1)*8-1, i.e. (0-7) (8-15)…504-511).
14. Do you divide scrambling code groups into subgroups? Please give an example.
Yes, we divide the 64 code groups into subgroups:
 Macro layer group: 24 code groups reserved for macro (outdoor) sites.
 Micro layer group: 16 code groups reserved for micro (in-building) sites.
 Expansion group: 24 code groups reserved for future expansion sites.
15. What is "pilot pollution"?
Simply speaking, when the number of strong cells exceeds the active set size, there is "pilot pollution" in the area. Typically the active set size is 3, so if there are more than 3 strong cells then there is pilot pollution.
Definition of "strong cell": pilots within the handover window size from the strongest cell. Typical handover 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.
16. What is "soft handover" and "softer handover"?
"Soft handover" is when UE has connection to multiple cells on different NodeB.
"Softer handover" is when UE has connection to multiple cells on same NodeB.
In downlink a UE can combine signals from different cells, improving the signal quality. For uplink and soft handover, RNC selects the best signal from different NodeB. For uplink and softer handover, a NodeB combines the signal from different sectors.
17. What is "compressed mode"?
Before UE can perform inter-frequency or IRAT handover, it needs to have some time to lock on to the control channel of the other frequency or system and listen to the broadcast information. Certain idle periods are created in radio frames for this purpose and is called "compressed mode".
18. 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?
Power down. Maintaining one good link is sufficient to sustain a call and having unnecessary stronger links creates more interference.
19. Suppose two UE are served by the same cell, the UE with weaker link (poor RF condition) uses more "capacity", why does this mean?
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".
20. What is "cell breathing" and why?
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 NodeB.
21. Is UMTS an uplink-limited or downlink-limited system?
A UMTS system could be either uplink-limited or downlink-limited depending on the loading. In a lightly loaded system, the UE transmit power sets a coverage limitation therefore it is uplink-limited. In a heavily loaded system, the NodeB transmit power limits the number of UEs it can serve therefore it is downlink-limited.
22. What is the impact of higher data rate on coverage?
Higher data rate has lower processing gain and therefore a NodeB needs to transmit more power to meet the required Eb/No; this means the coverage is smaller for higher data rate.
23. What are the interfaces between each UTRAN component?
Uu: UE to NodeB
Iub: NodeB to RNC
Iur: RNC to RNC
Iu: RNC to MSC
24. Briefly describe the UE to UTRAN protocol stack (air interface layers).
The radio interface is divided into 3 layers:
1. Physical layer (Layer 1, L1): used to transmit data over the air, responsible for channel coding, interleaving, repetition, modulation, power control, macro-diversity combining.
2. Link layer (L2): is split into 2 sub-layers – Medium Access Control (MAC) and Radio Link Control (RLC).
 MAC: responsible for multiplexing data from multiple applications onto physical channels in preparation for over-the-air transmition.
 RLC: segments the data streams into frames that are small enough to be transmitted over the radio link.
3. Upper layer (L3): vertically partitioned into 2 planes: control plane for signaling and user plan for bearer traffic.
 RRC (Radio Resource Control) is the control plan protocol: controls the radio resources for the access network.
In implementation:
1. UE has all 3 layers.
2. NodeB has Physical Layer.
3. RNC had MAC layer and RRC layer.
25. Briefly describe UMTS air interface channel types and their functions.
There are 3 types of channels across air interface – physical channel, transport channel and logical channel:
1. Physical Channel: carries data between physical layers of UE and NodeB.
2. Transport Channel: carries data between physical layer and MAC layer.
3. Logical Channel: carries data between MAC layer and RRC layer.
26. Give some examples of Physical, Transport and Logical channels.
1. Logical Channel:
 Control channel: BCCH, PCCH, CCCH, DCCH.
 Traffic channel: DTCH, CTCH.
2. Transport Channel:
 Common control channel: BCH, FACH, PCH, RACH, CPCH.
 Dedicated channel: DCH, DSCH.
3. Physical Channel:
 Common control channel: P-CCPCH, S-CCPCH, P-SCH, S-SCH, CPICH, AICH, PICH, PDSCH, PRACH, PCPCH, CD/CA-ICH.
 Dedicated channel: DPDCH, DPCCH.
27. What are the RRC operation modes?
Idle mode and connected mode.
28. What are the RRC states?
There are 4 RRC States: Cell_DCH, Cell_FACH, URA_PCH and Cell_PCH.
URA = UTRAN Registration Area.
29. What is OVSF?
Orthogonal Variable Spreading Factor.
30. How many OVSF code spaces are available?
 Total OVSF codes = 256.
 Reserved: 1 SF64 for S-CCPCH, 1 SF256 for CPICH, P-CCPCH, PICH and AICH each.
 Total available code space = 256 – 4 (1 SF64) – 4 (4 SF256) = 248.
31. What are the symbol rates (bits per symbol) for BPSK, QPSK, 8PSK and 16QAM?
 BPSK: 1.
 QPSK: 2.
 8PSK: 3.
 16QAM: 4.
32. How many scrambling codes are there?
There are 512 scrambling codes in the downlink and 16,777,216 codes in the uplink.
33. How many scrambling code groups are there for downlink?
There are 64 code groups, each group has 8 scrambling codes.
34. Can we assign same scrambling codes to sister sectors (sectors on same site)?
No, because scrambling code on the downlink is used for cell identity. As a requirement, scrambling codes have to maintain a safe separation to avoid interference.
35. Can we assign scrambling codes 1, 2 and 3 to sister sectors?
Yes.
36. What are the events 1a, 1b, 1c, etc.?
 e1a – a Primary CPICH enters the reporting range, i.e. add a cell to active set.
 e1b – a primary CPICH leaves the reporting range, i.e. removed a cell from active set.
 e1c – a non-active primary CPICH becomes better than an active primary CPICH, i.e. replace a cell.
 e1d: change of best cell.
 e1e: a Primary CPICH becomes better than an absolute threshold.
 e1f: a Primary CPICH becomes worse than an absolute threshold.
37. 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.
 e3a: the UMTS cell quality has moved below a threshold and a GSM cell quality had moved above a threshold.
 e3b: the GSM cell quality has moved below a threshold.
 e3c: the GSM cell quality has moved above a threshold.
 e3d: there was a change in the order of best GSM cell list.
38. 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.
39. What are the possible causes for an IRAT Failure?
 Missing 2G relations
 Non availability of 2G Resources
 Poor 2G Coverage
 Missing 3G Relations
40. 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
41. 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
42. What is the maximum number of Channelization Codes that can be allocated for HS, as per 3GPP standard?
 15 codes of SF 16.

[c21579]

Very clear and useful information. Did they ask you sth about site testing? Thanks

[adewijaya]

open for all member, to learn more about interview phase

[abdelhayisthebest]

thanks a lot :) :p:p:p:p:p:p:p:p:victory::victory::victory::victory:

[adewijaya]

just put/click menu thanks on bottom right !

[bruury]

thanks for the share...

nice Q&A though...

[shahfu@gmail.com]

detail answer of some questions that already send before :




1. What is a typical NodeB maximum output power?
The maximum NodeB output power is usually 20W or 40W, that is, 43dBm or 46dBm.
2. What is UE maximum transmit power in your link budget?
21dBm.
3. What is a typical antenna gain?
The antenna gain depends on antenna model; in link budget we use around 17dBi.
4. What is difference between dBi and dBd?
dBi is the gain in dB from isotropic source; dBd is the gain from a dipole source.
dBd + 2.15 = dBi.
5. What is the difference between dB and dBm?
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.
6. What is 0dBm?
0dBm = 1 milli-watt.
7. How many types of handovers are there in UMTS?
Soft handover, softer handover, inter-frequency handover, inter-RAT handover, inter-RAT cell change (UE moving out of UMTS coverage into GSM/GPRS/EGDGE coverage).
8. What is soft handover and softer handover?
 Soft handover: when a UE is connected to cells owned by different NodeB.
 Softer handover: when a UE is connected to cells owned by the same NodeB.
9. How does soft/softer handover work?
 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.
10. Brief describe the advantages and disadvantages of soft handover?
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.
11. What is channelization code?
Channelization codes are orthogonal codes used to spread the signal and hence provides channel separation, that is, channelization codes are used to separate channels from a cell.
12. What is scrambling code? How many scrambling codes there are?
Scrambling codes are used to separate cells and UEs from each other, that is, each cell or UE should have a unique scrambling code. There are 512 scrambling codes on the downlink and millions on the uplink.
13. What is scrambling "code group"?
The 512 scrambling codes are divided into 64 code groups – each code group has 8 scrambling codes.
Code group i (i = 0 to 63) has codes from i*8 to (i+1)*8-1, i.e. (0-7) (8-15)…504-511).
14. Do you divide scrambling code groups into subgroups? Please give an example.
Yes, we divide the 64 code groups into subgroups:
 Macro layer group: 24 code groups reserved for macro (outdoor) sites.
 Micro layer group: 16 code groups reserved for micro (in-building) sites.
 Expansion group: 24 code groups reserved for future expansion sites.
15. What is "pilot pollution"?
Simply speaking, when the number of strong cells exceeds the active set size, there is "pilot pollution" in the area. Typically the active set size is 3, so if there are more than 3 strong cells then there is pilot pollution.
Definition of "strong cell": pilots within the handover window size from the strongest cell. Typical handover 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.
16. What is "soft handover" and "softer handover"?
"Soft handover" is when UE has connection to multiple cells on different NodeB.
"Softer handover" is when UE has connection to multiple cells on same NodeB.
In downlink a UE can combine signals from different cells, improving the signal quality. For uplink and soft handover, RNC selects the best signal from different NodeB. For uplink and softer handover, a NodeB combines the signal from different sectors.
17. What is "compressed mode"?
Before UE can perform inter-frequency or IRAT handover, it needs to have some time to lock on to the control channel of the other frequency or system and listen to the broadcast information. Certain idle periods are created in radio frames for this purpose and is called "compressed mode".
18. 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?
Power down. Maintaining one good link is sufficient to sustain a call and having unnecessary stronger links creates more interference.
19. Suppose two UE are served by the same cell, the UE with weaker link (poor RF condition) uses more "capacity", why does this mean?
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".
20. What is "cell breathing" and why?
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 NodeB.
21. Is UMTS an uplink-limited or downlink-limited system?
A UMTS system could be either uplink-limited or downlink-limited depending on the loading. In a lightly loaded system, the UE transmit power sets a coverage limitation therefore it is uplink-limited. In a heavily loaded system, the NodeB transmit power limits the number of UEs it can serve therefore it is downlink-limited.
22. What is the impact of higher data rate on coverage?
Higher data rate has lower processing gain and therefore a NodeB needs to transmit more power to meet the required Eb/No; this means the coverage is smaller for higher data rate.
23. What are the interfaces between each UTRAN component?
Uu: UE to NodeB
Iub: NodeB to RNC
Iur: RNC to RNC
Iu: RNC to MSC
24. Briefly describe the UE to UTRAN protocol stack (air interface layers).
The radio interface is divided into 3 layers:
1. Physical layer (Layer 1, L1): used to transmit data over the air, responsible for channel coding, interleaving, repetition, modulation, power control, macro-diversity combining.
2. Link layer (L2): is split into 2 sub-layers – Medium Access Control (MAC) and Radio Link Control (RLC).
 MAC: responsible for multiplexing data from multiple applications onto physical channels in preparation for over-the-air transmition.
 RLC: segments the data streams into frames that are small enough to be transmitted over the radio link.
3. Upper layer (L3): vertically partitioned into 2 planes: control plane for signaling and user plan for bearer traffic.
 RRC (Radio Resource Control) is the control plan protocol: controls the radio resources for the access network.
In implementation:
1. UE has all 3 layers.
2. NodeB has Physical Layer.
3. RNC had MAC layer and RRC layer.
25. Briefly describe UMTS air interface channel types and their functions.
There are 3 types of channels across air interface – physical channel, transport channel and logical channel:
1. Physical Channel: carries data between physical layers of UE and NodeB.
2. Transport Channel: carries data between physical layer and MAC layer.
3. Logical Channel: carries data between MAC layer and RRC layer.
26. Give some examples of Physical, Transport and Logical channels.
1. Logical Channel:
 Control channel: BCCH, PCCH, CCCH, DCCH.
 Traffic channel: DTCH, CTCH.
2. Transport Channel:
 Common control channel: BCH, FACH, PCH, RACH, CPCH.
 Dedicated channel: DCH, DSCH.
3. Physical Channel:
 Common control channel: P-CCPCH, S-CCPCH, P-SCH, S-SCH, CPICH, AICH, PICH, PDSCH, PRACH, PCPCH, CD/CA-ICH.
 Dedicated channel: DPDCH, DPCCH.
27. What are the RRC operation modes?
Idle mode and connected mode.
28. What are the RRC states?
There are 4 RRC States: Cell_DCH, Cell_FACH, URA_PCH and Cell_PCH.
URA = UTRAN Registration Area.
29. What is OVSF?
Orthogonal Variable Spreading Factor.
30. How many OVSF code spaces are available?
 Total OVSF codes = 256.
 Reserved: 1 SF64 for S-CCPCH, 1 SF256 for CPICH, P-CCPCH, PICH and AICH each.
 Total available code space = 256 – 4 (1 SF64) – 4 (4 SF256) = 248.
31. What are the symbol rates (bits per symbol) for BPSK, QPSK, 8PSK and 16QAM?
 BPSK: 1.
 QPSK: 2.
 8PSK: 3.
 16QAM: 4.
32. How many scrambling codes are there?
There are 512 scrambling codes in the downlink and 16,777,216 codes in the uplink.
33. How many scrambling code groups are there for downlink?
There are 64 code groups, each group has 8 scrambling codes.
34. Can we assign same scrambling codes to sister sectors (sectors on same site)?
No, because scrambling code on the downlink is used for cell identity. As a requirement, scrambling codes have to maintain a safe separation to avoid interference.
35. Can we assign scrambling codes 1, 2 and 3 to sister sectors?
Yes.
36. What are the events 1a, 1b, 1c, etc.?
 e1a – a Primary CPICH enters the reporting range, i.e. add a cell to active set.
 e1b – a primary CPICH leaves the reporting range, i.e. removed a cell from active set.
 e1c – a non-active primary CPICH becomes better than an active primary CPICH, i.e. replace a cell.
 e1d: change of best cell.
 e1e: a Primary CPICH becomes better than an absolute threshold.
 e1f: a Primary CPICH becomes worse than an absolute threshold.
37. 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.
 e3a: the UMTS cell quality has moved below a threshold and a GSM cell quality had moved above a threshold.
 e3b: the GSM cell quality has moved below a threshold.
 e3c: the GSM cell quality has moved above a threshold.
 e3d: there was a change in the order of best GSM cell list.
38. 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.
39. What are the possible causes for an IRAT Failure?
 Missing 2G relations
 Non availability of 2G Resources
 Poor 2G Coverage
 Missing 3G Relations
40. 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
41. 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
42. What is the maximum number of Channelization Codes that can be allocated for HS, as per 3GPP standard?
 15 codes of SF 16.

Thanks man.. Its very useful..

[jimjim]

Dear Shahfu,

Would you kindly add reps and thanks.

Regards,

[spikerow]

Thanks. It's could be in great help.

[spikerow]

Please provide more questions
Thanks