UMTS Interview Questions and Answer

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UMTS Interview Questions and Answer

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PowerPoint Presentation:

3G 1. What are the three sets in handover? The 3 sets in handover are: · Active set – the list of cells which are in soft handover with UE. · Monitored set – the list of cells not in active set but RNC has told UE to monitor. o Detected set – list of cells detected by the UE but not configured in the neighbor list. 2. What are the major differences between GSM and UMTS handover decision? GSM: 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”.

3G -Events:

3G -Events What are the events 1a, 1b, 1c, etc.? · Event 1A – a Primary CPICH enters the reporting range, i.e. add a cell to active set. · Event 1B – a primary CPICH leaves the reporting range, i.e. removed a cell from active set. · Event1C – a non-active primary CPICH becomes better than an active primary CPICH, i.e. replace a cell. · Event 1D - change of best cell. · Event 1E - a Primary CPICH becomes better than an absolute threshold. · Event 1F – a Primary CPICH becomes worse than an absolute threshold. 8. 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. · Event 3A: the UMTS cell quality has moved below a threshold and a GSM cell quality had moved above a threshold. · Event 3B: the GSM cell quality has moved below a threshold. · Event 3C: the GSM cell quality has moved above a threshold. · Event 3D: there was a change in the order of best GSM cell list.

PowerPoint Presentation:

1.What are the typical values for 'reportingrange1a' and 'reportingrange1b'? 3 dB and 5 dB respectively. 2.What will be the impact when you change 'reportingrange1a' from 3 to 4 dB and 'timetotrigger1a' 100 to 320 ms, without changing any other parameters? Reduction in number of Event1a Delayed Event1a trigger Reduction in Average Active Set Size Delay in Event1a could increase DL interference, which could lead to a drop call or increase in Average Power Per User (reduction in cell capacity) 3.What is processing Gain? Ans :- Processing gain is the ratio of chip rate over data bit rate, usually represented in decibel (dB) scale. For example, with 3.84MHz chip rate and 12.2k data rate, the processinggain is: PG12.2k = 10 * log (3,840,000 / 12,200) = 25dB

PowerPoint Presentation:

1…. Eb /No Eb /N0 (the energy per bit to noise power spectral density ratio) is an important parameter in digital communication or data transmission. It is a normalized signal-to-noise ratio (SNR) measure, also known as the "SNR per bit". It is especially useful when comparing the bit error rate (BER) performance of different digital modulation schemes without taking bandwidth into account.

PowerPoint Presentation:

1….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. 2….. 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 3….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]

PowerPoint Presentation:

1…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 2….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.

CPICH- :

CPICH- Common Pilot Indication Channel (CPICH) is broadcast from every cell ▪It carries no information and can be thought of as a “ beacon”constantly transmitting the Scrambling Code of the cell ▪It is this “ beacon”that is used by the phone for its cell measurements for network acquisition and handover purposes ( Ec , Ec /Io). 1.If the UE can’t see the CPICH the UE can’t see the cell 2. Initial 3G network optimisation will be performed purely from CPICH measurements Ec -The Received Signal Level of a particular CPICH ( dBm ) ▪ Io-The Total Received Power ( dBm ) ▪ Ec /Io-The CPICH Quality (The ratio of the above two values)

WCDMA Parameters:

WCDMA Parameters Channel B.W 5 MHz Forward RF Channel Structure Direct Spread Chip Rate 3.84 Mcps Frame Length 10 ms (38400 chips) No. of slots/frame 15 No. of chips/slot 2560chips (Max. 2560 bits) Power Control Open and fast close loop (1.6 KHz) Uplink SF 4 to 256 Downlink SF 4 to 512

Power Control:

Power Control Fast Closed Loop PC – Inner Loop PC Feedback information. Uplink PC is used for near-far problem. Downlink PC is to ensure that there is enough power for mobiles at the cell edge. Two special cases for fast closed loop PC: Soft handover:- how to react to multiple power control commands from several sources. At the mobile, a “power down” command has higher priority over “power up” command. Compressed mode:- Large step size is used after a compressed frame to allow the power level to converge more quickly to the correct value after the break. Open loop PC No feedback information. Make a rough estimate of the path loss by means of a downlink beacon signal. Provide a coarse initial power setting of the mobile at the beginning of a connection. Apply only prior to initiating the transmission on RACH or CPCH.

PowerPoint Presentation:

WCDMA Network Structure GSM /GPRS BSS BTS BSC PCU SS7 SCP SMS SCE PSTN/other PLMN Internet, Intranet MSC/VLR GMSC HLR/AUC SGSN CG BG GGSN PS backbone CN Other PLMN CS domain PS domain NodeB RNC UTRAN RAN Iu-CS Iu-PS A G b

PowerPoint Presentation:

WCDMA Interfaces A Interface A-bis Um MSC BSC BTS UE SGSN Gb GSM Iub Uu MSC RNC NodeB UE SGSN Iu-PS Iu-CS Iub Uu RNC NodeB UE Iur WCDMA UTRAN BSS

PowerPoint Presentation:

Scrambling Code Scrambling codes GOLD sequence. Uplink scrambling codes Uplink scrambling codes are used to distinguish different UEs Downlink scrambling codes F or downlink physical channels, a total of 2 18 -1 = 262,143 scrambling codes can be generated. Only scrambling codes k = 0, 1, …, 8191 are used. 8192 codes are divided into 512 groups, each of which contains 16 scrambling codes. The first scrambling code of each group is called primary scrambling code (PSC), and the other 15 ones are secondary scrambling codes (SSC). Scrambling code :GOLD sequence. Scrambling code period : 10ms ,or 38400 chips. The code used for scrambling of uplink DPCCH/DPDCH may be of either long or short type, There are 2 24 long and 2 24 short uplink scrambling codes. Uplink scrambling codes are assigned by higher layers. F or downlink physical channels, a total of 2 18 -1 = 262,143 scrambling codes can be generated. Only scrambling codes k = 0, 1, …, 8191 are used.

PowerPoint Presentation:

Handover Softer handover Handover between intra-frequency cells under the control of the same BTS Radio link connection to new target cell is created before existing connection being deleted. Soft handover Handover between intra-frequency cells under the control of the different BTSs Hard handover Condition of hard handover: Intra-frequency handover, cells controlled by different RNCs and no Iur interface between them Inter-frequency handover Inter-system handover Interruption in voice or data communication occurs but this interruption does not effect the user communication

PowerPoint Presentation:

Principle of RAKE Receiver RAKE receiver help to overcome on the multi-path fading and enhance the receive performance of the system Receive set Correlator 1 Correlator 2 Correlator 3 Searcher correlator Calculate the time delay and signal strength Combiner The combined signal t t s(t) s(t)

PowerPoint Presentation:

Handover Softer handover Handover between intra-frequency cells under the control of the same BTS Radio link connection to new target cell is created before existing connection being deleted. Soft handover Handover between intra-frequency cells under the control of the different BTSs Hard handover Condition of hard handover: Intra-frequency handover, cells controlled by different RNCs and no Iur interface between them Inter-frequency handover Inter-system handover Interruption in voice or data communication occurs but this interruption does not effect the user communication

PowerPoint Presentation:

Common Pilot Channel (CPICH) Primary CPICH Uses OVSF code - Cch , 256,0 Scrambled by the primary scrambling code Only one CPICH per cell Broadcast over the entire cell The P-CPICH is a phase reference for SCH, Primary CCPCH, AICH, PICH. By default, it is also a phase reference for downlink DPCH. Secondary CPICH An arbitrary channel code of SF=256 is used for S-CPICH S-CPICH is scrambled by either the primary or a secondary scrambling code There may be zero, one , or several secondary CPICH. S-CPICH may be transmitted over part of the cell S-CPICH may be a phase reference for S-CCPCH and downlink DPCH.

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