1. 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).
2. 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.
3. 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.
4. 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.
5. 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.
6. 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/over shooting neighbor likely).
3 cells.
8. 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.
9. What would you define as a pilot polluter?
More then 3 Active set is nothing pilot pollution.
10. 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.
11. 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).
12. 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.
13. 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.
14. 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.they would see RADIO BEARER RECONFIGURATION messages when the states are changing.
15.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)
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).
17. In what cases is Open Loop Power Control used?
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.
