UMTS Questinaire

7/27/2019 UMTS Questinaire

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UMTS Questionnaire

1 Explain Ec/Io and RSCP; on what channel are they measured on?

2 What does channelization codes do and function?

3 What does the scrambling code do and function?

4Explain the concept of Cell Breathing. How is the accounted for in the link

Budget?

5 Explain the different Handover types in UMTS

6 What is an active set, monitor set and detected set?


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7What is the major difference in link budgets between UMTS and

GSM/TDMA?

8In the Link Budget, what is a Shadow Fade Margin for and what factors does

it depend on?

9What is the typical maximum active set size and what needs to be consider

when setting this?

10What is typically the requirements (criteria) for a cell to be

added/removed/replaced to/from/in the active set?

11 What would you define as a pilot polluter?

12How would you find such cells from a planning tool and from a drive test

tool?

13What would the call flow be for a Mobile Originated Call (major RRC

messages)?


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14 What are the general triggers for an iRAT handover?

15 What is compressed mode, what is it's function, and what impact does ithave on the network?

16Name the 4 RRC Connected Modes (states) and describe the characteristics

of each.

17If 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?

18In Release '99, how does the network manage the throughput on the Radio

Interface for a user/connection?

19 What is the typical/most common bitrate that a voice call uses?

20Depending on the RF conditions, what can the network do to manage call

quality?


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21In HSDPA, how does the network manage the throughput on the Radio

Interface for a user/ connection?

22 Explain Inner and Outer loop power control and who controls them.

23 In what cases is Open Loop Power Control used?

24 Explain the concept of a Monte Carlo Simulation for UMTS Design

25 In pre-launch optimization, how are missing neighbors usually detected?


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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/bg noise + interferers + own cell and is wideband. Bonus if they understand that RSCP is actually

measured AFTER despreading (i.e. narrowband)

Channelization codes are used for spreading and despreading 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.

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 the are broken up to 64 groups of 8 codes each.

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 tomaintain 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.

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.

Active Set: the set of cells with which the UE is currently connected/communicating with; DriveTT 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).


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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 powerfrom 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.

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 nadded margin placed in the link budget such that a

guarenteed level of service can be offered "in the worst case"

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)

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.

Many definitions: A cell that has a 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 can not enter because the active set is full.

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.

Areas with 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 contains 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.

RRC Connect Request -> RRC Connection Setup -> RRC Setup Complete -> (SETUP, authentication encyrption,

TMSI reallocation etc) -> CALL PROCEEDING-> Radio Bearer Setup -> Radio Bearer Setup Complete -> ALERT ->

CONNECT -> CONNECT ACK ->DISCONNECT -> RELEASE.


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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)

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 bitrate, it halves the SF, and therefore increases power level causing higher interference to the network. Ifthe SF cannot be halved then the bitrate of the bearer decreases. If they seem knowledable, 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.

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. TheRNC tracks the UE down to the cell level and cell reselections are possible with the CELL UPDATE message. No

data can be transfered 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.

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.

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.

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.

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.


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Modulation (16QAM, QPSK etc), Coding (convolution coding, fire codes etc), number of codes allocated and

scheduling (it's a shared resource)

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 pathloss 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).

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.

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.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 stonger 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.

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