UMTS Interview Questions & Answers

Question 1. What Is A Typical Nodeb Sensitivity Level?

Answer :

The service and load determines the NodeB sensitivity; in general, in a no-load condition, the sensitivity is between -115dBm to -125dBm. For Ericsson, the NodeB sensitivity level is calculated at around:

• CS12.2: -124 dBm.
• PS-64: -119 dBm.
• PS-128: -115 dBm.
• PS-384: -115 dBm.

Question 2. What Is A Typical Ue Sensitivity Level?

Answer :

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: -119 dBm.
• PS-64: -112 dBm.
• PS-128: -110 dBm.
• PS-384: -105 dBm.
• HSDPA: -95 dBm.

Question 3. What Is A Typical Nodeb Maximum Output Power?

Answer :

The maximum NodeB output power is usually 20W or 40W, that is, 43dBm or 46dBm.

Question 4. What Is Ue Maximum Transmit Power In Your Link Budget?

Answer :

21dBm.

Question 5. What Is A Typical Antenna Gain?

Answer :

The antenna gain depends on antenna model; in link budget we use around 17dBi.

Question 6. What Is A Typical Maximum Path Loss?

Answer :

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.

Question 7. What Is Difference Between Dbi And Dbd?

Answer :

dBi is the gain in dB from isotropic source; dBd is the gain from a dipole source.
dBd + 2.15 = dBi. 

Question 8. What Is The Difference Between Db And Dbm?

Answer :

dBm is a unit of power level, measured in milli-watts in logarithm scale, that is, dBm = 10 * log(W*1000) where W is the power in Watts dB is not a unit, it is the difference in dBm.

Question 9. What Is 0dbm?

Answer :

0dBm = 1 milli-watt. 

Question 10. What Are The Pros And Cons (advantages And Disadvantages) Of Tma?

Answer :

On the upside, a TMA reduces system noise, improves uplink sensitivity and leads to longer UE battery life. On the downside, TMA imposes an additional insertion loss (typically 0.5dB) on the downlink and increases site installation and maintenance complexity.

Question 11. What Is Typical Tma Gain?

Answer :

TMA typically has a 12 dB gain; however, the effective gain comes from noise figure reduction and the gain is close or equivalent to the feeder loss.

Question 12. Why Tma Are Installed At The Top Near The Antenna And Not The Bottom Near The Nodeb?

Answer :

Based on Friis Equation, having a TMA near the BTS will have the top jumper and main feeder losses (noise figures) cascaded in and a TMA will not be able to help suppress the losses.

Question 13. What Is Umts Chip Rate?

Answer :

3.84MHz. 

Question 14. What Is Processing Gain?

Answer :

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 processing gain is: 
PG12.2k = 10 * log (3,840,000 / 12,200) = 25dB

Question 15. What Are The Processing Gains For Cs And Ps Services?

Answer :

• CS12.2: 25dB
• PS-64: 18dB
• PS-128: 15dB
• PS-384: 10dB
• HSDPA: 2dB 

Question 16. What Is Eb/no?

Answer :

By definition Eb/No is energy bit over noise density, i.e. is the ratio of the energy per information bit to the power spectral density (of interference and noise) after dispreading.

Eb/No = Processing Gain + SIR

For example, if Eb/No is 5dB and processing gain is 25dB then the SIR should be -20dB or better.

Question 17. What Are The Eb/no Targets In Your Design?

Answer :

The Eb/No targets are dependent on the service:

• On the uplink, typically CS is 5 to 6dB and PS is 3 to 4dB – PS is about 2dB lower.
• On the downlink, typically CS has 6 to 7dB and PS is 5 to 6dB – PS is about 1dB lower.

Question 18. Why Is Eb/no Requirement Lower For Ps Than For Cs?

Answer :

PS has a better error correction capability and can utilize retransmission, therefore it can afford to a lower Eb/No. CS is real-time and cannot tolerate delay so it needs a higher Eb/No to maintain a stronger RF link. 

Question 19. What Is Ec/io?

Answer :

Ec/Io is the ratio of the energy per chip in CPICH to the total received power density (including CPICH itself).

Question 20. Sometimes We Say Ec/io And Sometimes We Say Ec/no, Are They Different?

Answer :

• Io = own cell interference + surrounding cell interference + noise density
• No = surrounding cell interference + noise density

That is, Io is the total received power density including CPICH of its own cell, No is the total received power density excluding CPICH of its own cell. Technically Ec/Io should be the correct measurement but, due to equipment capability, Ec/No is actually measured. In UMTS, Ec/No and Ec/Io are often used interchangeably.

Question 21. What Is Rscp?

Answer :

RSCP stands for Received Signal Code Power – the energy per chip in CPICH averaged over 512 chips.

Question 22. 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.

Question 23. What Is The Loading Factor In Your Design?

Answer :

The designed loading typically is 50%; however, sometimes a carrier may want to design up to 75% load. 

Question 24. What Is Typical Pole Capacity For Cs-12.2, Ps-64, Ps-128 And Ps-384?

Answer :

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.

Question 25. How Many Types Of Handovers Are There In Umts?

Answer :

Soft handover, softer handover, inter-frequency handover, inter-RAT handover, inter-RAT cell change (UE moving out of UMTS coverage into GSM/GPRS/EGDGE coverage).

Question 26. What Is Soft Handover And Softer Handover?

Answer :

• Soft handover: when a UE is connected to cells owned by different NodeB.
• Softer handover: when a UE is connected to cells owned by the same NodeB.

Question 27. How Does Soft/softer Handover Work?

Answer :

Soft/softer handover downlink: UE rake receiver performs maximum ratio combining, i.e. UE combines multi-path signals and form a stronger signal.

• Soft handover uplink: RNC performs selection combining, i.e. RNC selects the better signal coming from multiple NodeB.
• Softer handover uplink: NodeB performs maximum ratio combining, i.e. NodeB rake receiver combines signals from different paths and forms a stronger signal.

Question 28. Why Is There “soft Handover Gain”?

Answer :

Soft handover gain comes from the following:

• Macro diversity gain over slow fading.
• Micro diversity gain over fast fading.
• Downlink load sharing over multiple RF links.

By maintaining multiple links each link could transmit at a lower power, resulting in lower interference therefore a gain.

Question 29. Brief Describe The Advantages And Disadvantages Of Soft Handover?

Answer :

Advantages:

• Overcome fading through macro diversity.
• Reduced Node B power which in turn decreases interference and increases capacity.
• Reduced UE power (up 4dB), decreasing interference and increasing battery life.

Disadvantages:

• UE using several radio links requires more channelization codes, and more resources on the Iub and Iur interfaces.

Question 30. What Are Fast Fading And Slow Fading?

Answer :

Fast fading is also called multi-path fading, as a result of multi-path propagation. When multi-path signals arriving at a UE, the constructive and destructive phases create a variation in signal strength.Slow fading is also called shadowing. When a UE moves away from a cell the signal strength drops down slowly.

Question 31. What Are Fast Fading Margin And Slow Fading Margin?

Answer :

To factor in the fast fading and slow fading, we need to have a margin in the link budget and they are called fast fading margin and slow fading margin. In link budget, the fast fading margin is usually set to 2-3; slow fading margin is set to 7-10.

Question 32. What Is A Typical Soft Handover Gain In Your Link Budget?

Answer :

• CS-12.2k: 3dB (UL), 2dB (DL).
• PS-64k: 1dB (UL), 0dB (DL).
• PS-128k: 1dB (UL), 0dB (DL).
• PS-384k: 1dB (UL), 0dB (DL).

Question 33. What Is The Percentage In Time A Ue Is Expected To Be In Soft Or Softer Handover?

Answer :

Typically a UE should be in soft handover mode at no more than 35 to 40% of the time; in softer handover mode at about 5% of the time.

Question 34. What Is A Typical Eirp?

Answer :

The EiRP depends NodeB transmit power, cable and connector loss and antenna gain. With a sample system of 43dBm transmit power, a 3dB cable and connector loss and a 17dBi antenna gain, the EiRP = 43 – 3 + 17 = 57dBm.

Question 35. How Much Power Usually A Nodeb Is Allocated To Control Channels?

Answer :

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.

Question 36. What Is A Typical Cpich Power?

Answer :

CPICH power typically takes about 10% of the total NodeB power. For a 20W (43dBm) NodeB, CPICH is around 2W (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:

• The coverage area is small since users are close to the site.
• More power can be allocated to traffic channels.

Question 37. How Much Is Your Hsdpa (max) Link Power?

Answer :

HSDPA link power is typically 4 to 5dB below the maximum NodeB maximum output power. For example, for 43dBm maximum NodeB power the HSDPA link power is 39dBm.

Question 38. Consider Downlink Only, What Are The Major Components In Calculating Maximum Path Loss, Starting From Nodeb?

Answer :

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

Question 39. What Is Maximum Path-loss?

Answer :

The maximum path-loss is how much signal is allowed to drop from a transmitter to a receiver and maintains as good signal.

Question 40. Simple Link Budget: With A 30dbm Cpich And A -100dbm Ue Sensitivity, Ignoring Anything In Between, What Is The Maximum Path Loss?

Answer :

30 – (–100) = 30 + 100 = 130dB.

Question 41. Suppose I Have A Maximum Path-loss Of 130dbm, What Is The New Path-loss If A 5db Body Loss Is Added?

Answer :

125dB.

Question 42. What Is Channelization Code?

Answer :

Channelization codes are orthogonal codes used to spread the signal and hence provides channel separation, that is, channelization codes are used to separate channels from a cell.

Question 43. How Many Channelization Codes Are Available?

Answer :

The number of channelization codes available is dependent on the length of code. In the uplink the length is defined as between 4 and 256. In the downlink the length is defined as between 4 and 512.

Question 44. Are Channelization Codes Mutually Orthogonal? If So, Why Is “orthogonality Factor” Required In The Link Budget?

Answer :

Yes, channelization codes are mutually orthogonal. Nonetheless, due to multi-path with variable time delay, channels from the same cell are no longer perfectly orthogonal and may interfere with each other. A “Downlink Orthogonality Factor”, typically 50-60%, is therefore needed in the link budget to account for the interference – and hence reduces pole capacity.

Question 45. What Is Scrambling Code? How Many Scrambling Codes There Are?

Answer :

Scrambling codes are used to separate cells and UEs from each other, that is, each cell or UE should have a unique scrambling code. There are 512 scrambling codes on the downlink and millions on the uplink.

Question 46. What Is Scrambling “code Group”?

Answer :

The 512 scrambling codes are divided into 64 code groups – each code group has 8 scrambling codes. Code group i (i = 0 to 63) has codes from i*8 to (i+1)*8-1, i.e. (0-7) (8-15)…(504-511).

Question 47. Do You Divide Scrambling Code Groups Into Subgroups? Please Give An Example.

Answer :

Yes, we divide the 64 code groups into subgroups:

• Macro layer group: 24 code groups reserved for macro (outdoor) sites.
• Micro layer group: 16 code groups reserved for micro (in-building) sites.
• Expansion group: 24 code groups reserved for future expansion sites.

Question 48. Which Service Usually Needs Higher Power, Cs Or Ps?

Answer :

Consider downlink and take CS-12.2 and PS-384k for example. The processing gain is 25 for CS-12.2 and 10 for PS-384. The Eb/No requirement is 7 for CS-12.2 and 5 for PS-384. Therefore the power requirement is higher for CS-12.2 than PS-384.

Question 49. What Is Eb/no Requirement For Hsdpa?

Answer :

The Eb/No requirement for HSDPA varies with user bit rate (data rate), typically 2 for 768kbps and 5 for 2Mbps.

Question 50. What Is “noise Rise”? What Does A Higher Noise Rise Mean In Terms Of Network Loading?

Answer :

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.