3G WCDMA Interview Question

3G Question and Answers for Interviews

1) What is a chip rate of WCDMA System? How much is the bandwidth required for WCDMA?

3840 Kc/s. FDD 5 MHZ of paired band. TDD 5 MHz only.

2) What is the minimum number of bins required for a certain propagation model?

The more bins the more likely to come up with a good model. Usually a minimum of 2,000 bines is considered acceptable, but sometimes as low as 500 bins may be accepted.

3) What is the minimum and maximum SF in Downlink and uplink for FDD Mode?

Minimum of 4 and maximum of 512 chips in downlink

Minimum of 4 and maximum of 256 chips in uplink

4) What is the usage of scrambling code in WCDMA for both downlink and uplink?

In the downlink scrambling codes are used to reduce the inter-basestation interference.

In the uplink scrambling codes are used to separate the terminals

5) How many scrambling code groups are there for downlink?

There are 64 code groups, each group has 8 scrambling codes

6) 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.

7) According to GSM recommendations, there is a delay equal to 3 time slots (=1.73 usec) in between uplink & downlink why ?

According to GSM recommendations, there is a delay equal to 3time slots (=1.73 usec) in between uplink & downlink because of the following facts:-

If MS receives & transmit simultaneously, it will chew up the battery of the MS.
The delay of 3 time slot is sufficient enough to detune itself from the downlink frequency & tune to the uplink frequency.
The difference of 3 time slot in between receiving & transmission is used toprocess the normal burst that is just received i.e. the process of AdaptiveEqualization.
The difference of 3 time slot in between receiving & transmission is used to perform measurement, process & store result.

8) What is UMTS chip rate?

3.84Mcps

9) What is Paging Success Ratio? What is the typical PSR that you have seen in a UMTS network?

PSR â€“ Paging Responses to the Paging Attempts
About 90%

10) What is Soft Handover Overhead? What is the typical value in UMTS network?

Soft Handover Overhead is calculated in two ways. 1) Average Active Set Size â€“ Total Traffic / Primary Traffic. 2) Secondary / Total Traffic
Typical Values are like 1.7 (Avg Active Set Size) or 35% (Secondary / Total )

11) What will happen to the Soft Handover Overhead when you apply OCNS on the network? And Why?

With OCNS, the interference (load) increases. This leads to reduction in Ec/Io of a Pilot, which reduces the pilot spillovers. Reduction in Pilot Spillover will reduce the Soft Handover Overhead.

12) What will be the impact when you change reportingrange from 3 to 4 dB and timetotrigger 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)

13) Is there any interface from UMTS core network to other mobile networks?

UMTS specifications do not have any special interface planned for other

mobile networks, but all telephone networks can be connected to UMTS

core network with standard S7 (or other) signalling system using E1s or

T1s. Than enables voice calls to be made to all other telephone

networks. If other networks support additional services like “call

forwarding”, “calling line identity”, fax, slow-speed data ect, technically

that is possible to implement this between networks.

All telephone networks are designed to work with each other and UMTS

networks will use standard interfaces towards all other networks. S7 and

IP (internet protocol) will be the most commonly used interfaces

standard, but all UMTS vendors can offer tens of different countryspecific

interface protocols if required.

14) Is there a transition period between UMTS and the present system?

There will probably not be a “transition” period in that sense, because GSM systems will keep on operating at least next ten years. (some old 1G networks are still running round the world). Only limitations for operators are the GSM license terms and customer preferences. UMTS networks will just be added to mobile landscape.

15) Suppose we are designing a CS network and a PS network, is there a major difference in the design consideration?

Server dominance is the key difference. In a CS network we shall limit the number of strong servers in any given area to no more than the active set size to avoid pilot pollution (in the downlink). In a PS network, however, there isnâ€™t soft handover in the downlink so the server dominance is very important â€“ meaning ideally there should be only one dominant server in a given area.

16) Suppose I have a maximum path-loss of 130dBm, what is the new path-loss if a 5dB body loss is added?

The new path loss will be 125 dB

17) What are the major components in calculating maximum path loss, starting from NodeB?

FOR Downlink

NodeB CPICH transmit power.
Jumper and feeder connector loss.
Antenna gain.
Over-the-air loss.
Building / vehicle penetration loss.
Body loss.

18) What is the practical maximum speed for downloading and uploading in 3g?

In WCDMA HSPA+ the maximum speed theoretically possible is 84 Mbps downlink

and 5.8 Mbps in Uplink, But offcourse this is not practically acheived unless in lab enviorenment

19) What is the typical Call Setup Time for a 3G UE to 3G UE Call? What are the possible RF related causes for a delayed CST in this type of call?

6 to 9 seconds
Multiple RRC Attempts (UE is on poor coverage â€“ need more than Access Attempt)
Delayed Page Responses
High Load on Paging and/or Access Channel
Paging / Access Parameter

20)
What is the differance between TMA and TMB used at cell site?

At first TMA is used to improve uplink gain at the cell site.

On the other hand,TMB uses the booster amplifiers in the downlink path which equally increase the EIRP of the antenna & there-by complement the improved uplink resulting in a balancedcoverage in a cell.

21) How does TMA work?

A TMA reduces system noise, improves uplink sensitivity andleads to longer UE battery life. Sensitivity is the minimum input power needed to get a suitable signal-to-noise ratio (SNR) at the output of the receiver. It is determined by receiver noise figure, thermo noise power and required SNR. Thermo noise power is determined by bandwidth and temperature, SNR is determined by modulation technique, therefore the only variable is noise figure. The cascading noise figure can be calculated by Friis equation (Herald Friis): NFt = NF1 + (NF2-1)/G1 + (NF3-1)/(G1*G2) + â€¦ + (NFi-1)/(G1*G2*â€¦*Gi) As the equation shows, the first block imposes the minimum and the most prominent noise figure on the system, and the following blocks imposes less and less impact to the system provided the gains are positive. Linear passive devices have noise figure equal to their loss. A TMA typically has a gain of 12dB. There are typically top jumper, main feeder and a bottom jumper between antenna and BTS. A TMA placed near antenna with a short jumper from antenna provides the best noise figure improvement â€“ the noise figure will be restricted to the top jumper loss (NF1) and TMA ((NF2-1)/G1), and the remaining blocks (main feeder and bottom jumper) have little effect. To summarize, a TMA has a gain thatâ€™s close to feeder loss.

22) What is Double power transceiver (DPTRX) and in which BTS it can be used?

Double Power TRX and 2-way uplink diversity-Double Power TRX is a licensed capacity application software product. Thelicense filemust be installed in the BSC prior to siteactivation.Double Power TRX (DPTRX) doubles the TRX output power by synchronisously combining the two transmitters of the Dual TRX. The Double Power TRX has a single TRX capacity.
The solution can be supported with one Dual TRX module and two antennas/sector
(TX/RX and RX): one TX antenna for the Double Power TRX in downlink direction, andtwo RX antennas for 2-way uplink diversity. Double Power TRX increases the BTS TX output power typically by 2.5 dB, and 2-way uplink diversity with MHA improves the uplink performance.

23) Double Power TRX and 4-way uplink diversity-For even a bigger improvement in uplink and coverage, the Double Power TRX(DPTRX) can be implemented with 4-way uplink diversity. It requires two Sector modules per sector. This solution requires four antennas: one for transmit/receive and three for receive.
The Double Power TRX with 4-way uplink diversity increases thecoverage area by up to 40%, significantly reducing the number of sites needed.

Hopefully, 3g interview questions and answers help you with your next interview.