Clear Definition is "A band Channel Element is the resource base required in the Node-B to Provide for one voice channel capacity, including the control plane to Provide for one voice channel capacity, including the control plane signaling, compressed mode, the transmit diversity and softer handover. "signaling, compressed mode, the transmit diversity and softer handover." Factors affecting capacity in WCDMA UL and DL, sorted by descending level of importance as follows: Factors affecting Capacity in WCDMA UL and DL, sorted by descending level of Importance as Follows:
- UL: No. CE support on each Node B, interference, Node B sensivity, .. - UL: No. CE support on the each Node B, interference, Node B sensivity, .. - DL: No. CE support on each Node B Iub interface capacity, power DL, OVSF, ... - DL: No. CE support on the each Node B Iub interface capacity, power DL, OVSF, ... Thus, CE can be viewed as a license for each NodeB capacity (not more). Thus, CE can be viewed as a license for each vertex NodeB capacity (not more). Each service will need some different CE UL and DL, such as: the each service will need some Different CE UL and DL, such as: AMR 12.2 k: Uplink 1; downlink AMR 12.2 k 1: Uplink 1; downlink 1 CS / PS 64 K: 3 Uplink, downlink 2 CS / PS 64 K: 3 Uplink, downlink 2 PS 128/144k: Uplink 5; downlink 128/144k PS 4: Uplink 5; DL 4 384 k PS: Uplink 10, downlink 8384 k PS: Uplink 10; downlink 8
DL: Spreading codes in DL is used to differentiate different UEs under a BTS. It might happen that 2 UEs under a cell use same spreading code. In that case scrambling code is used on top of spreading code to differentiate UEs. UL: In the UL spreading code is used to differentiate different services of UE. It might happen that UE is using RT as well as NRT services simultaneously.
What is cell selection criterion? Cell selection is based on:
Qmean: the average SIR of the target cell. Qmin: minimum required SIR.
Pcompensation: 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.
What is SIR? What is SIR? 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.
What effect is there on signal by spreading and dispreading? Spreading will increase the bandwidth of a as signal. A signal of 10 Kb/s will become 40 Kb/s after spreading and will become 10 Kb/s after dispreading. The processing gain term expresses 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 measured at the RLC level.
Power control In WCDMA
Open Loop Inner Loop
Outer Loop
Open Loop Power control-The Open loop power control technique requires that the transmitting entity measures the channel interference and adjusts its transmission power accordingly. This can be done quickly, but the problem is that the interference estimation is done on the received signal, and the transmitted signal probably uses a different frequency, which differs from the received frequency by the system’s duplex offset. As uplink and downlink fast fading (on different frequency carriers) do not correlate, this method gives the right power values only on average. Inner Loop-In this method the received signal-to interference ratio (SIR) is measured over a 667- microsecond period (i.e., one time slot), and based on that value, a decision 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 is compensated for by making the
measurements over a very short period of time. The transmit 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 decrease command. Outer Loop-The outer loop power control functions within the base station system, and adjusts the required SIR value (SIRtarget), which is then used in the inner loop control. Different channel types, which can be characterized by, for example, different coding and interleaving methods, constitute a channel’s parameters. Different channel parameters may require different SIRtarget values. The final result of the transmission process can only be known after the decoding process, and the resulting quality parameter is then used to adjust the required SIR value. If the used SIR value still gives a low quality bit stream, then the outer loop power control must increase the SIRtarget value. This change in the outer loop will trigger the inner loop power control to increase the mobile station transmission power accordingly.
What is Significance of Eb_No? 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 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
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, the CPICH 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
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)
What is the family of codes used for Chanelization in WCDMA? Family of codes used for Chanelization in WCDMA used is Orthogonal Variable Spreading Factor.(OVSF)
How Initial RACH Power is is calculated? The initial power on the PRACH - the power of the first preamble is determined according to equation P_PRACH = L_PCPICH + RTWP + constantValueCprach 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.
How many time Inner Loop Power Control happens and what type of fading it compensates? How many time Inner Loop Power Control happens and what type of fading it compensates? Ans:1500Hz and compensates Fast Fading.
How many slots are there in a WCDMA Frame? How big is a frame in ms. how many chips 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/s Length of frame = 10 ms Number of chips in a frame = 3840 *10=38400 chips.
Number of chips in a slot = 38400/15= 2560 chips
How much power usually a NodeB 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, including CPICH. However, if HSDPA is deployed on the same carrier then the total power allocated to control channel may go up to 25 to 30% because of the additional HSDPA control channels required.
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
Explain Soft and Softer handover? Give some advantage and disadvantage for soft handover In Soft Handover, the UE connection consists of at least two radio links established with cells belonging to different RBSs. In Softer handover, the UE connection consists of at least two radio 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 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.
Explain the functionality of TPC 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).
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.
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.
Explain Timer T3212? Periodic LA and RA updating is used to notify the network of the UEs availability, and to avoid unnecessary paging attempts for a UE that has lost coverage and is not able to inform the CN that it is inactive.
The periodic LA update procedure is controlled by a timer, called t3212, which gives the time interval between two consecutive periodic location updates. The value is sent by the WCDMA RAN to UEs on the BCCH.
When is System information sent to UE? 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 a system information change indication message.
Simple definition of Pole Capacity The pole capacity is the theoretical maximum capacity of the system. In WCDMA, this capacity is only theoretical since, once reached, the system goes in an instable state that leads to its collapse. However it is still a reference for expressing the load. The uplink noise increases with the loading exponentially. When the uplink noise approaches 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
What are different Idle mode tasks UE performs? In Idle mode, the UE has no connection to the radio network.Keeping UEs in Idle mode minimizes the use of resources both for the UEs and in the network. However, the UEs must still be 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
Location Area (LA) and Routing Area (RA) updating
Paging
System information broadcast
What is Compressed Mode? Compressed mode is a physical layer function that allows the UE to temporarily tune to another frequency, and measure the RF environment of another UMTS frequency (e.g. IFHO) or another technology (e.g. IRAT), while maintaining an existing dedicated channel.
What is typical pole capacity for CS-12.2, PS-64, PS-128 and PS-384? 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.
What is the processing gain for 384 Kb/s service? Processing Gain= 10 log (Chip rate /Bit rate) =10*Log(3840/384) =10*log (10) =10*1 =10
What is the usage of Channelization code in downlink and uplink? Uplink separation of physical data (DPDCH) and control channel (DPCCH) from same terminal. Downlink separation of downlink connections to different users within one cell.
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.
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.
Difference between PICH and PCH? PICH-Paging Indicator Channel PCH-Paging Channel PICH is used to indicate UE to when it should read to SCCPCH (Carries PCH) whereas PCH is used to carry RRC Message “Paging type 1” which contains actual Paging information.
Is UMTS an Uplink limited or Downlink limited System? Initially, A typical WCDMA network is Uplink Limited. Later a Loaded Network becomes Downlink Limites.
Capacity Management and Its functions- WCDMA 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 request 3. Congestion Control: detects/resolves overload situations.
What do you understand by DRX cycle? 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 called Discontinuous Reception (DRX) cycle. Different DRX cycles are used for circuit switched and packet switched services in Idle mode. A separate DRX cycle is also used to page Connected mode UEs in state URA_PCH.
How does UE camp (synchronize) to a NodeB? 1. UE uses the primary synchronization channel (P-SCH) for slot alignment (TS synchronization). 2. After aligning to NodeB time slot, UE then uses secondary synchronization channel (SSCH) to obtain frame synchronization and scrambling code group identification. 3. UE then uses scrambling code ID to obtain CPICH, thus camping to a NodeB.
How Power Control is implemented in HSDPA? 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 formats. The principles and 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 for HS-SCCH is fixed.
What HS Channels are introduced in HSDPA in L1? HS-PDSCH – High Speed Physical Downlink Shared Channel HS-SCCH – High Speed Shared Control Channel
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.
What do you understand by Directed Retry? When there is a co-existing GSM RAN, excess traffic in a WCDMA cell may be offloaded to GSM If a call is chosen for Directed Retry to GSM, the request for the speech RAB will be rejected with cause "Directed retry" and then a request is made to the core network to relocate the UE to a specific GSM cell, using the Inter-RAT handover procedure. This handover is a blind one since 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.
What is a typical UE sensitivity level? 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: PS-64: PS-128: PS-384: HSDPA:
-119 dBm -112 dBm -110 dBm -105 dBm -95 dBm
What is a typical maximum path loss? 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.
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
What are the major differences between GSM and UMTS handover?
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”. UE plays more part in the handover decision.
Definition RSCP,RSSI and Ec/Io Email This BlogThis! Share to Twitter Share to Facebook Share to Google Buzz
Hello Friends Here I try to simplify these three Terms of UMTS RSCP: The “Received Signal Code Power” (RSCP) is the collected RF energy after the correlation / 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 the noise level. Because of the system gain (see 1), the interference level can be higher than the wanted signal level. 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 both RSCP and Ec/I0. It is usually given in dBm and can be calculated as follows: RSSI [dBm] = RSCP [dBm] - Ec/I0 [dB]
Why Ec/Io is negative? Email This BlogThis! Share to Twitter Share to Facebook Share to Google Buzz 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,the value of Ec/Io is always negative. Here is the calculation. Ec = Effect energy Io = Total energy and noise. Ec/Io = 10.log[Effect energy / (total energy+noise)] < 10.log(1) = 0 Ec/Io always negative
Some Question Answer of 3G 1.
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 CELLPCH 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.
2. 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.
3. In what cases is Open Loop Power Control used?
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.
4. 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).
5. 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)
6. 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.
7. 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
8. 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.
9. 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.
10. What is compressed mode, what is it's 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 bit rate, 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.
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 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 can not enter because the active set is full.
13. What is 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.
14. What is the typical maximum active set size and what needs to be considering 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).
15. 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".
16. 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.
17. 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. 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.
18. 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.
19. 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).
20. 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.
21. 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.
22. 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 the are broken up to 64 groups of 8 codes each.
23. What does channelization codes do and function?
Channelization codes are used for spreading and dispreading 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.
