A handover window size is usually between 4 to 6dB.
2. What is “soft handover” and “softer handover”?
“Soft handover” is when UE has connection to multiple cells on different NodeB.
“Softer handover” is when UE has connection to multiple cells on same NodeB.
In downlink a UE can combine signals from different cells, improving the signal quality. For
uplink and soft handover, RNC selects the best signal from different NodeB. For uplink and
softer handover, a NodeB combines the signal from different sectors.
3. During a handover, if one cell sends a power down request and two cells send a power
up request, shall the UE power up or power down?
Power down. As long as a good link can be maintained it is not necessary to power up in
order to maintain multiple links. Maintaining unnecessary multiple links increases noise rise
and shall be avoided.
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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 isn’t soft handover in the downlink so the server
dominance is very important – meaning ideally there should be only one dominant server in a
given area.
5. What is the active set size on your network?
3.
6. How many fingers does a UE rake receiver have?
4.
7. What is “compressed mode”?
Before UE can perform inter-frequency or IRAT handover, it needs to have some time to lock
on to the control channel of the other frequency or system and listen to the broadcast
information. Certain idle periods are created in radio frames for this purpose and is called
“compressed mode”.
8. Describe the power control schemes in UMTS?
· Open loop – for UE to access the network, i.e. used at call setup or initial access to set
UE transmit power.
· Closed outer loop: RNC calculates the SIR target and sends the target to NodeB (every
10ms frame).
· Closed inner loop: NodeB sends the TPC bits to UE to increase or decrease the power at
1,500 times a second.
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9. What is the frequency of power control (how fast is power control)?
· Open loop: depends on parameter setting:
T300 – time to wait between RRC retries (100ms to 8000 ms, typical 1500ms)
· Closed outer loop: 100 times a second.
· Closed inner loop: 1,500 times a second.
10. Briefly describe why open loop power control is needed and how it works?
· When a UE needs to access to the network it uses RACH to begin the process.
· RACH is a shared channel on the uplink used by all UE, therefore may encounter
contention (collision) during multiple user access attempts and interfere with each other.
· Each UE must estimate the amount of power to use on the access attempt since no
feedback from the NodeB exists as it does on the dedicated channel.
· The purpose of open loop power control is to minimize the chance of collision and
minimize the initial UE transmit power to reduce interference to other UE.
Initial UE transmit power = Primary_CPICH_Power – CPICH_RSCP + UL_Interferrnce
+ constant_Value_Cprach
· Instead of sending the whole message, a “test” (preamble) is sent.
· Wait for answer from NodeB.
· If no answer from NodeB increase the power.
· Try and try until succeed or timeout.
11. What is power control “headroom”?
Power control “headroom” is also called “power rise”. In a non-fading channel the UE needs
to transmit a certain fixed power. In a fading chennel a UE reacts to power control
commands and usually increases the transmit power. The difference between the average
power levels of fading and non-fading channels is called “power rise” or “headroom”.
12. When in 3-way soft handover, if a UE receives power down request from one cell and
power up request from the other 2 cells, should the UE power up or down and why?
Power down. Maintaining one good link is sufficient to sustain a call and having unnecessary
stronger links creates more interference.
13. Suppose two UE are served by the same cell, the UE with weaker link (poor RF
condition) uses more “capacity”, why does this mean?
The UE with weaker RF link will require NodeB to transmit higher traffic power in order to
reach the UE, resulting in less power for other UE – therefore consumes more “capacity”.
14. Under what circumstances can a NodeB reach its capacity? What are the capacity
limitations?
NodeB reaches its maximum transmit power, runs out of its channel elements, uplink noise
rise reaches its design target, etc.
15. What is “cell breathing” and why?
The cell coverage shrinks as the loading increases, this is called cell breathing.
In the uplink, as more and more UE are served by a cell, each UE needs to transmit higher
power to compensate for the uplink noise rise. As a consequence, the UE with weaker link
(UE at greater distance) may not have enough power to reach the NodeB – therefore a
coverage shrinkage.
In the downlink, the NodeB also needs to transmit higher power as more UE are being
served. As a consequence UE with weaker link (greater distance) may not be reachable by
the NodeB.
16. Is UMTS an uplink-limited or downlink-limited system?
A UMTS system could be either uplink-limited or downlink-limited depending on the
loading. In a lightly loaded system, the UE transmit power sets a coverage limitation
therefore it is uplink-limited. In a heavily loaded system, the NodeB transmit power limits
the number of UEs it can serve therefore it is downlink-limited.
17. What is the impact of higher data rate on coverage?
Higher data rate has lower processing gain and therefore a NodeB needs to transmit more
power to meet the required Eb/No; this means the coverage is smaller for higher data rate.
18. What is OCNS?
OCNS stands for Orthogonal Channel Noise Simulator. It is a simulated network load
usually by increasing the noise rise figure in the NodeB.
OCNS is used to simulate additional users on the downlink. OCNS is automatically enabled when the sum of the levels of all the enabled downlink physical channels is less than the cell power setting.
19. What are the interfaces between each UTRAN component?
Uu: UE to NodeB
Iub: NodeB to RNC
Iur: RNC to RNC
Iu: RNC to MSC
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20. Briefly describe the UE to UTRAN protocol stack (air interface layers).
The radio interface is divided into 3 layers:
1. Physical layer (Layer 1, L1): used to transmit data over the air, responsible for
channel coding, interleaving, repetition, modulation, power control, macro-diversity
combining.
2. Link layer (L2): is split into 2 sub-layers – Medium Access Control (MAC) and
Radio Link Control (RLC).
· MAC: responsible for multiplexing data from multiple applications onto physical
channels in preparation for over-the-air transmition.
· RLC: segments the data streams into frames that are small enough to be transmitted
over the radio link.
3. Upper layer (L3): vertically partitioned into 2 planes: control plane for signaling and
user plan for bearer traffic.
· RRC (Radio Resource Control) is the control plan protocol: controls the radio
resources for the access network.
In implementation:
1. UE has all 3 layers.
2. NodeB has Physical Layer.
3. RNC had MAC layer and RRC layer.
21. Briefly describe UMTS air interface channel types and their functions.
There are 3 types of channels across air interface – physical channel, transport channel and
logical channel:
1. Physical Channel: carries data between physical layers of UE and NodeB.
2. Transport Channel: carries data between physical layer and MAC layer.
3. Logical Channel: carries data between MAC layer and RRC layer.
22. Give some examples of Physical, Transport and Logical channels.
1. Logical Channel:
· Control channel: BCCH, PCCH, CCCH, DCCH.
· Traffic channel: DTCH, CTCH.
2. Transport Channel:
· Common control channel: BCH, FACH, PCH, RACH, CPCH.
· Dedicated channel: DCH, DSCH.
3. Physical Channel:
· Common control channel: P-CCPCH, S-CCPCH, P-SCH, S-SCH, CPICH, AICH,
PICH, PDSCH, PRACH, PCPCH, CD/CA-ICH.
· Dedicated channel: DPDCH, DPCCH.
23. What are the RRC operation modes?
Idle mode and connected mode.
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