Read the most frequently asked 60 top 3G interview questions and answers for freshers and experienced job interview questions pdf
3G Interview Questions and Answers List
1. Explain 3G?
Third generation (3G) wireless networks will offer faster data transfer rates than current networks. The first generation of wireless (1G) was analog cellular. The second generation (2G) is digital cellular, featuring integrated voice and data communications. So-called 2.5G networks
offer incremental speed increases. 3G networks will offer dramatically improved data transfer rates, enabling new wireless applications such as streaming media.
2. What is International Telecommunication Union?
The International Telecommunication Union is the eldest organization in the UN family still in existence. It was founded as the International Telegraph Union in Paris on 17 May 1865 and is today the leading United Nations agency for information and communication technology issues, and the global focal point for governments and the private sector in developing networks and services.
3. What is Universal Mobile Telecommunications System?
Universal Mobile Telecommunications System (UMTS) is one of the third-generation (3G) mobile telecommunications technologies, which is also being developed into a 4G technology. The first deployment of the UMTS is the release99 (R99) architecture. It is specified by 3GPP and is part of the global ITU IMT-2000 standard.
4. What is 2G?
2G (or 2-G) is short for second-generation wireless telephone technology. Second generation 2G cellular telecom networks were commercially launched on the GSM standard in Finland in 1991. Three primary benefits of 2G networks over their predecessors were that phone conversations were digitally encrypted, 2G systems were significantly more efficient on the spectrum allowing for far greater mobile phone penetration levels; and 2G introduced data services for mobile, starting with SMS text messages.
5. What is 4G?
4G refers to the fourth generation of cellular wireless standards. It is a successor to 3G and 2G standards. The nomenclature of the generations generally refers to a change in the fundamental nature of the service. The first was the move from analogue (1G) to digital (2G) transmission. This was followed by multi-media support, spread spectrum transmission and at least 200 kbit/s (3G) and now 4G, which refers to all IP packet-switched networks, mobile ultra-broadband (gigabit speed) access and multi-carrier transmission.
6. Explain CDMA2000?
CDMA2000 (also known as IMT Multi Carrier (IMT MC)) is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B. All are approved radio interfaces for the ITU’s IMT-2000.
7. Explain Digital Enhanced Cordless Telecommunications?
Digital Enhanced Cordless Telecommunications (DECT), known as Digital European Cordless Telephone until 1995, is an ETSI standard for digital portable phones (cordless home telephones), commonly used for domestic or corporate purposes. It is recognised by the ITU as fulfilling the IMT-2000 requirements and thus qualifies as a 3G system. Within the IMT-2000 group of technologies, DECT is referred to as IMT-2000 Frequency Time (IMT-FT)
8. Explain WiMAX?
WiMAX, an acronym for Worldwide Interoperability for Microwave Access, is a telecommunications protocol that provides fixed and fully mobile internet access. The current WiMAX revision provides up to 40 Mbps with the IEEE 802.16m update expected offer up to 1 Gbit/s fixed speeds. (WiMAX is based on the IEEE 802.16 standard, also called Broadband Wireless Access). The name WiMAX was created by the WiMAX Forum, which was formed in June 2001 to promote conformity and interoperability of the standard.
9. Explain Enhanced Data Rates for GSM Evolution?
Enhanced Data rates for GSM Evolution (EDGE) (also known as Enhanced GPRS (EGPRS), or IMT Single Carrier (IMT-SC), or Enhanced Data rates for Global Evolution) is a backward-compatible digital mobile phone technology that allows improved data transmission rates, as an extension on top of standard GSM. EDGE is considered a 3G radio technology and is part of ITU’s 3G definition.
10. Explain High Speed Packet Access?
High Speed Packet Access (HSPA) is a collection of two mobile telephony protocols, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), that extends and improves the performance of existing WCDMA protocols. A further standard, Evolved HSPA (also known as HSPA+), was released late in 2008 with subsequent adoption worldwide into 2010.
11. Explain Ec/Io and RSCP; on what channel are they measured on?
Ec/Io = energy of carrier over all noise. RSCP = Receive Signal Code Power. In FDD mode (what we normally deal with) they are measured on the CPICH (pilot). Bonus if they know that Io is the sum of all interference: thermal/background noise + interferers + own cell and is wideband. Bonus if they understand that RSCP is actually measured AFTER de-spreading (i.e. narrowband)
12. What does channelization codes do and function?
Channelization codes are used for spreading and de-spreading of the signals, they also create the “channels” making it possible to distinguish between users/connections/channels. Bonus if they know that they have an associated Spreading Factor and are allocated depending on the bandwidth required by the service.
13. What does the scrambling code do and function?
Scrambling Code makes it possible for the UE to distinguish the transmissions from different cells/NodeBs. Bonus if he knows there are 512 primary scrambling codes and that are broken up to 64 groups of 8 codes each.
14. Explain the concept of Cell Breathing. How is the accounted for in the link Budget?
Io or No (the interference part of Ec/Io and Eb/No) increase as the traffic on the network increases since everyone is using the same frequency. Therefore as Io or No increases the UE or BTS needs to use more power to maintain the same Eb/No or Ec/Io. When the power required is more than the maximum power allowed, the connection cannot be made. Users at the cell edge are usually the first to lose service; hence the service area of a cell shrinks. As traffic decreases, the reverse happens and the service area increases. They should say that it is accounted for in the Noise Rise Margin found in the Link Budget.
15. Explain the different Handover types in UMTS
Soft(er) Handover: connected to more than one cell on the same frequency, softer occurs when 2 cells in the active set belong to same Node-B; Intra-frequency Hard Handover: Occurs when Ue moves from one cell in one RNC to a cell in another RNC and the RNCs do not have an Iur link between each other; Inter-Frequency Hard Handover: when UE changes from one frequency to another frequency (usually due to traffic layer management or Quality reasons);Inter-technology (iRAT) Hard Handover: Handover from UMTS to GSM (v.v.) usually at the edge of UMTS service area but also due to quality reasons.
16. What is an active set, monitor set and detected set?
Active Set: the set of cells with which the UE is currently connected/communicating with; Drive test usually show them as SC or Pilots but they are actually cells; Monitored Set: Cells that the UE has detected and is monitoring and are known to the network, they either don’t meet the criteria or the active set is full; Detected Set – Cells that the UE has detected but are not known to the network as yet (missing neighbor likely).
17. What is the major difference in link budgets between UMTS and GSM/TDMA?
In UMTS you generally have a link budget for each service (voice, data, video etc), in GSM you usually only use 1 for voice. Each service has a different Eb/No target. In UMTS you have to consider the target traffic load you will have and add a noise-rise margin, in GSM you may have a slight interference margin but not normally related to traffic. In UMTS some services (like voice) will show up as uplink limited but other services (like HSDPA, 384kbps service) will show as downlink limited. In UMTS you usually have to consider that all users use the same power from the BTS therefore the more number of users the lower the maximum power available per user (maximum power per connection) which is a starting point in the link budget.
18. In the Link Budget, what is a Shadow Fade Margin for and what factors does it depend on?
The shadow fade margin is dependent on the target percentage area coverage, the propagation model, and the standard deviation of the lognormal shadowing (usually the same as the model’s standard deviation if the fast fading effects are removed). The Shadow Fade Margin is a added margin placed in the link budget such that a guaranteed level of service can be offered “in the worst case”.
19. What is the typical maximum active set size and what needs to be considered when setting this?
3 to 4 cells, the larger the active set size the more likely it is that Iub link efficiency is reduced (more than one resource for a single connection due to SHO)
20. What are typically the requirements (criteria) for a cell to be added/removed/replaced to/from/in the active set?
For addition (Event 1a), candidate cell needs to have an Ec/Io value that is within a T_ADD threshold of the primary/reference (usually the best) cell for a specify time hysteresis. For removal (event 1b), cell needs to have Ec/Io lower than T_DROP margin for a specific time hysteresis. For replacement (event 1c), cell needs to have an Ec/Io better than the worst cell in the active set by the T_REPLACE and for a specific time hysteresis.
21. What would you define as a pilot polluter?
Many definitions: A cell that has high signal strength at a location but is not part of the active set. A cell that, meets the criteria for addition into the Active Set but cannot enter because the active set is full.
22. How would you find such cells from a planning tool and from a drive test tool?
Ignoring low signal conditions, if the best cell RSCP is greater than say -85dBm and there are cells not in the active set but are strong enough to be in the active set then they are candidate for pilot polluters. Looking at cells that have a high noise rise, high amount of traffic compared to surrounding cells, may also indicate a pilot polluter, Any location where, high Signal strength for the (Active Set Size + 1) best pilot (like the 4th best pilot if AS size is 3). In DTT, areas with poor Ec/Io but good RSCP, in the monitored set contain a cell with a good Ec/Io but cannot enter the AS because it is full, Areas where scanner shows a strong signal for a far away cell.
23. What would the call flow be for a Mobile Originated Call (major RRC messages)?
RRC Connect Request -> RRC Connection Setup -> RRC Setup Complete -> (SETUP, authentication encryption, TMSI reallocation etc) -> CALL PROCEEDING-> Radio Bearer Setup -> Radio Bearer Setup Complete -> ALERT -> CONNECT -> CONNECT ACK ->DISCONNECT -> RELEASE.
24. What are the general triggers for an iRAT handover?
Ec/Io of best cell below a certain threshold (usually around -16 to -18 dB) or RSCP of best cell below a certain threshold (usually around -100 dBm).
25. What is compressed mode, what is its function, and what impact does it have on the network?
Compressed mode is when the mobile goes into a slotted transmit mode whereby it opens up an idle period (transmission gap) where it can monitor another carrier or technology (GSM). The impact is that to maintain the same bitrates, it halves the SF, and therefore increases power level causing higher interference to the network, If, the SF cannot be halved then the bit rate of the bearer decreases. If they seem knowledgably, ask them if they know what messages and events trigger and configure compressed mode on/off. 2D event for on, 2F for off. Messages would for configuration would be RADIO BEARER RECONFIGURATION, TRANSPORT CHANNEL RECONFIGFURATION or PHYSICAL CHANNEL RECONFIGURATION.
26. Name the 4 RRC Connected Modes (states) and describe the characteristics of each.
Cell-DCH: UE has been allocated a dedicated physical channel in uplink and downlink.
Cell-FACH: UE listens to RACH channel (DL) and is allocated a FACH channel (UL). Small amounts of UL/DL data can be transfers in this state. The RNC tracks the UE down to the cell level and cell reselections are possible with the CELL UPDATE message.
Cell-PCH: UE monitors (using discontinuous reception) a PCH channel (PCH) indicated by the PICH channel. The RNC tracks the UE down to the cell level and cell reselections are possible with the CELL UPDATE message. No data can be transferred in the UL in this state.
URA-PCH: UE monitors (using discontinuous reception) a PCH channel (PCH) indicated by the PICH channel. The RNC tracks the UE down to the URA level.
27. If a UE is on a data call (CELL-DCH state) and there is in no activity for awhile what would you expect to see occur?
UE should go from CELL-DCH to CELL-FACH then if still no activity to either CELL-PCH or URA-PCH (via CELL-FACH). If they talk about inactivity timers and mention that the state goes from CELL-DCH straight to CELL-PCH or URA-PCH that is also possible. Bonus they say they would see RADIO BEARER RECONFIGURATION messages when the states are changing.
28. In Release ’99, how does the network manage the throughput on the Radio Interface for a user/connection?
This question is a little harder to ask, so you may need to work it differently a few times. Perhaps leading questions could be: What parameter/configuration does the network change on the air interface What you are trying to hear from the candidate is that the network assigns a radio bearer with a channelization code with a spreading factor that matches the requested service maximum bit rate.
29. What is the typical/most common bit rate that a voice call uses?
They should say 12.2kbps but may be different if they start talking about AMR and the different rates then the know more. Prod them to see if they know the Spreading Factor (SF) used for the radio bearer, should be 128.
30. Depending on the RF conditions, what can the network do to manage call quality?
AMR – for good conditions use codec will low redundancy/overhead; for poor conditions use codec with lower bit rate requirement but higher overhead, stronger coding and more redundancy.
31. In HSDPA, how does the network manage the throughput on the Radio Interface for a user/ connection?
Modulation (16QAM, QPSK etc), Coding (convolution coding, fire codes etc), number of codes allocated and scheduling (it’s a shared resource)
32. Explain Inner and Outer loop power control and who controls them.
If they start talking about Open and Closed Loop PC, tell them you want Inner/Outer Closed Loop PC. Inner loop power control is performed by the NodeB to set the transmit power of the UE and BTS to compensate for signal variations due to fading or path loss to maintain the set SIR (occurs up to 1500 times per sec). Outer loop power control is performed by the RNC to set the target SIR based on the required BER/BLER for the requested services (occurs up to 100 times per sec).
33. In what cases is Open Loop Power Control used?
1) Idle to Cell-DCH state, when a connection is setup. When UE goes into soft handover, ACTIVE SET UPDATE where the new Radio Link initial power settings use open loop PC.
34. Explain the concept of a Monte Carlo Simulation for UMTS Design
This is a simulator that randomly distributes terminals/users geographically onto the network and then checks the link budget for each terminal/connection to see if they can successfully connect or not. The simulator modifies parameters such has UE Tx Power, BTS Tx Power, requested bearer (in the case that multiple bearers could support the same service) when checking if a connection can be made. In every snapshot the simulator runs through the list of terminals/connections and attempts to make them all connect successfully, it starts a new snapshot when the number of successful connections converges. The process then starts on a new snapshot.
35. In pre-launch optimization, how are missing neighbors usually detected?
Usually you use a scanner and compare the best pilots in Ec/Io from the scanner against that of the active set and monitored set from an active UE. If there is a stronger pilot from a nearby cell that appears on the scanner but not on the UE, there is a possible missing neighbor. One would then verify that the neighbor appears in defined neighbor list from the OSS.
36. What is the CQI in HSDPA?
CQI is the channel Quality Indicator, Which is calculated on the Basis of RF informant, and the code allocation is done on the basis of CQI.
37. What is the HARQ?
This is the Hybrid Automatic repeat request Technique for the retransmission of the lost frame which is used by the HSDPA. That helps to recover the lost frame by two partially lost frames.
38. What is MIMO Antenna System?
This is the multiple inputs multiple output antenna technique system which improves the n/w throughput over the air interface.
39. What are the different RABs in R99?
cs-12.2 for speech,cs-64 for video calls ,ps-16 ps-64 ps 128 ps-384 for data services.
40. What is TTI in WCDMA how it impact?
It is the transmission time interval for sending one frame for WCDMA it is 10ms.
41. How many PSC in WCDMA?
0-511, Total 512
42. What is the Processing Gain?
Ratio of chip rate to bit rate, lower bit rate services will offer higher processing gain
43. What is the Power Control?
Power control is the mechanism of maintaining minimal power level with acceptable QOS for each service
44. What should be the idle CPICH power?
10% of the Total Power
45. What is rake receiver?
A rake receiver is a radio receiver designed to counter the effects of multipath fading. It does this by using several “sub-receivers” called fingers, that is, several correlators each assigned to a different multipath component. Each finger independently decodes a single multipath component; at a later stage the contribution of all fingers are combined in order to make the most use of the different transmission characteristics of each transmission path.
46. What kinds of services are available with WCDMA?
Conversational, Background, Streaming, Interactive.
47. Which modulation schemes are used in WCDMA?
QPSK. HPSK, BPSK, 16QAM, 64QAM
48. What is interleaving?
Interleaving is the technique used to distribute the data so as to make the error correction accurate at the receiver end.
49. What is threshold for adding and deleting a cell from Active Set?
For addition 3 dB and for deletion 6dB
50. What are the types of location & routing is registration update?
1. IMSI attach / detach. 2. Normal LA & RA updating. 3. Periodic LA & RA updating
51. Which timer is involved for periodic LA & RA update?
Timer t3212 is involved and is contained in SIB1.
52. What is the Difference between Ec/No and Ec/Io?
Ec/No- Interference caused by combination of the Non-orthogonality of Codes, thermal noise & all other noises present in the Channel. Ec/Io- Interference caused by only due to non-orthogonality of codes in the channel.
53. What are the no. of scrambling codes used in UL and DL?
Scrambling codes in uplink- 2^24-1. Scrambling codes in Downlink- 2^18-1= 8192.
54. What are the types of compressed mode techniques used in 3G?
Three Types- 1. Puncturing. 2. SF/2. 3.Higher Layer Scheduling.
55. What are MM Procedures?
MM common procedures: 1. TMSI reallocation procedure. 2. authentication procedure. 3. Identification procedure. 4. MM Information Procedure. 5. Abort Procedure. 6. Normal Location Update. 7. Periodic Location update. 8. Imsi attaches.
56. What is URA and URA_PCH state?
URA or UTRAN Registration Area is a collection of cells that are used for fast moving UE’s in connected mode when they are not transferring any data. In this case the UE is in CELL_PCH state. Every time a fast moving UE in CELL_PCH state changes the cell, a CELL UPDATE needs to be performed to let the UTRAN know of the new position of the UE. This is done because in the connected mode (CELL_PCH), UE is known at cell level rather than UTRAN level as in IDLE state. If too many CELL UPDATES are performed, it defeats the purpose of UE being in CELL_PCH. Hence in this case the UE is put in URA_PCH state. Now the UE will perform CELL UPDATE only when the URA is changed for a UE. The drawback is that when UE needs to be paged the paging area is now extended to many cells belonging to the URA.
Also Note that the CELL_PCH state is actually a subset of the URA_PCH state. It is possible to define overlapping URAs to be used in the URA_PCH state. Thus, the UTRAN operator could define that each cell is a separate URA in addition to other larger URAs. Then the operator could assign small one-cell URAs for slow-moving mobiles, and larger URAs for mobiles with greater mobility. The small URAs could nicely perform the task of the CELL_PCH state. However, it has been decided to keep these states separate.
The URAs can be overlapping or even hierarchical. The same cell may belong to several different URAs, and the UEs in that cell may have been registered to different URAs. SIB 2 contains a list of URA identities indicating which URAs this cell belongs to. This arrangement is done to further reduce the amount of location update signaling because now the UEs moving back and forth in the boundary area of two URAs do not have to update their URA location information if the boundary cells do belong to both URAs.
57. Which channel contains Layer 1 information?
DPCCH, Layer-1 contains information regarding Power control, Spreading-De-spreading, Multiplexing-De-multiplexing, Scrambling.
58. Which Channel Contains Layer 3 Information?
59. What are the main KPI’s?
Accessibility, Retainability, Intra Frequency HOSR, Intra Frequency HOSR, IRAT HOSR
60. Which parameter decides the preference between IFHO & IRAT HO Handover type