22. What is cell selection criterion?
Answer - 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.
23. Idle Mode Behaviour is managed by System information send on which L3 Channel?
Answer – BCH
24. How many Radio Bearers (RB) are involved in CS voice call?
Answer – 3
25. How many Service Radio Bearers (SRB) are involved in CS voice call?
Answer – 4
26. SCH channel consists of how many chips?
Answer -256 chips
27. What do you understand by DRX cycle?
Answer - 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.
28. Cell Reselection is valid in both Idle and in which Sate in Connected mode?
Answer - CELL FACH
29. Difference between PICH and PCH?
Answer - PICH-Paging Indicator Channel
PCH-Paging Channel
PICH is used to indicate UE to when it should read to S-CCPCH (Carries PCH) whereas PCH is used to carry RRC Message “Paging type 1” which contains actual Paging information.
30. When is System information sent to UE?
Answer - 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.
31. Explain Timer T3212?
Answer -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.
32. Explain Near far effect?
Answer;-All users use the same bandwidth at the same time and therefore users interfere with one another. Due to the propagation path loss, the signal received by the base station from a UE close to the base station will be stronger than the Signal received from another terminal located at the boundary. Hence, the distant user will be dominated by the close user. This is called the near-far effect. To achieve a considerable capacity, all signals, irrespective of distance, should arrive at the base station with the same mean power. A solution to this problem is power control, which attempts to achieve the same mean received power for each user.
33. Name three loops in Power control In WCDMA? Explain them briefly.
Answer; - 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
34. What is SIR?
Answer - 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.
35. How many time Inner Loop Power Control happens and what type of fading it compensates?
Answer - 1500Hz and compensates Fast Fading.
36. What is BLER?
Answer - Block Error Rate
37. How is Initial RACH Power is calculated?
Answer - 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.
38. What power RACH message Control Part is sent?
Answer - The power of the control part of the RACH message is determined by the power of the last transmitted preamble and by a configurable offset powerOffsetPpm
39. Briefly describe why open loop power control is needed and how it works?
Answer -Open Loop power control is used when no feedback mechanism is possible. An estimate of the required power is made from measurements and system information.
This is used for initial network access and finding initial power settings during dedicated mode.
40. Explain the functionality of TPC?
Answer – 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).
41. How many types of handovers are there in UMTS?
Answer –
Soft/Softer Handover
Inter Frequency Handover
Inter RAT Handover
Core Network Hard Handover
Service based handover to GSM
HSDPA Mobility
42. Explain Soft and Softer handover? Give some advantage and disadvantage for soft handover. What is the target for soft handover in WCDMA networks?
Answer - 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
A typical target for soft handover in WCDMA network is less than or equal to 30%
43. Define Active Set? Pros and Cons of having a small or longer Active Set.
Answer - 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
No comments:
Post a Comment