CDMA / UMTS RF Interview Questions 4

1. What are the RRC states?
There are 4 RRC States: Cell_DCH, Cell_FACH, URA_PCH and Cell_PCH.

URA = UTRAN Registration Area.
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2. What are transparent mode, acknowledged mode and unacknowledged mode?
· Transparent mode corresponds to the lowest service of the RLC layer, no controls and no
detection of missing data.
· Unacknowledged mode offers the possibility of segment and concatenate of data but no
error correction or retransmission therefore no guarantee of delivery.
· Acknowledged mode offers, in addition to UM mode functions, acknowledgement of
transmission, flow control, error correction and retransmission.

3. Which layer(s) perform ciphering function?
RRC – for acknowledged mode (AM) and unacknowledged mode (UM).
MAC – for transparent mode (TM).

4. What is OVSF?
Orthogonal Variable Spreading Factor.

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5. How many OVSF code spaces are available?
· Total OVSF codes = 256.
· Reserved: 1 SF64 for S-CCPCH, 1 SF256 for CPICH, P-CCPCH, PICH and AICH each.
· Total available code space = 256 – 4 (1 SF64) – 4 (4 SF256) = 248.

6. Can code space limit the cell capacity?
Yes, cell capacity can be hard-limited by code space. Take CS-12.2k for example:
· A CS-12.2k bearer needs 1 SF128 code.
· Total available codes for CS-12.2k = 128 – 2 (1 SF64) – 2 (4 SF256) = 124.
· Consider soft-handover factor of 1.8: 124 / 1.8 = 68 uers/cell.

7. Can a user have OVSF code as “1111”?
No, because “1111…” (256 times) is used by CPICH.

8. What are the symbol rates (bits per symbol) for BPSK, QPSK, 8PSK and 16QAM?
· BPSK: 
.

· QPSK:
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· 8PSK:


· 16QAM: 


9. Briefly describe UMTS frame structure.
· UMTS frame duration = 10ms.
· Each frame is divided into 15 timeslots.
· Each timeslot is divided into 2560 chips.
· Therefore 2560 chips/TS * 15 TS/frame * (1000ms/10ms) frame/sec = 3,840,000
chip/sec.

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10. 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.

11. Briefly describe Capacity Management and its functions:
Capacity Management is responsible for the control of the load in the cell. It consists of 3
main functions:
· Dedicated Monitored Resource Handling: tracks utilization of critical resources of the
system.
· Admission Control: accepts/refuses admission requests based on the current load on the
dedicated monitored resources and the characteristics of the request
· Congestion Control: detects/resolves overload situations

12. What are the major 4 KPIs in propagation model tuning and typical acceptable values?
The 4 KPIs are standard deviation error, root mean square error, mean error and correlation
coefficient. The typical acceptable values are:
· Standard deviation error: the smaller the better, usually 7 to 9dB.
· Mean error: the smaller the better, usually 2 to3.
· Root mean square error: the smaller the better, usually
· Correlation coefficient: the larger the better, usually 70% to 90%.

13. 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.
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14. How many scrambling codes are there?
There are 512 scrambling codes in the downlink and 16,777,216 codes in the uplink.

15. How many scrambling code groups are there for downlink?
There are 64 code groups, each group has 8 scrambling codes.

16. Can we assign same scrambling codes to sister sectors (sectors on same site)?
No, because scrambling code on the downlink is used for cell identity. As a requirement,
scrambling codes have to maintain a safe separation to avoid interference.

17. Are scrambling codes orthogonal?
No, scrambling codes are not orthogonal since they are not synchronized at each receiver.
They are pseudo random sequences of codes.

18. Can we assign scrambling codes 1, 2 and 3 to sister sectors?
Yes.

19. In IS-95 we have a PN reuse factor (PN step size) and therefore cannot use all 512 PN
codes, why isn’t it necessary for UMTS scrambling codes?
Because IS-95 is a synchronized network, different PN codes have the same code sequence
with a time shift, therefore we need to maintain a certain PN step size to avoid multi-path
problem. For example, if two sectors in the neighborhood have a small PN separation then
signal arriving from cell A may run into the time domain of cell B, causing interference.
UMTS, on the other hand, is not a synchronized network and all scrambling codes are
mutually orthogonal so no need to maintain a step size.
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20. What are coverage thresholds in your UMTS design and why?
The coverage thresholds are based on UE sensitivity, fading and penetration loss. Assuming
UE sensitivity of -110dBm, fade margin of 5dB:
· Outdoor: -110dBm sensitivity + 5dB fade margin = -105dBm.
· In-vehicle: -110dBm + 5dB + 8dB in-vehicle penetration loss = -97dBm.
· In-building: -110dBm + 5dB + 15dB in-building penetration loss = -90dBm.

21. What is the Ec/Io target in your design?
The Ec/Io target typically is between -12 to -14dB. However, if a network is designed for
data then the Ec/Io target could go higher to around -10dB because server dominance is more
critical for a data network – since there isn’t software in the downlink.


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