1. How many types of handovers are there in UMTS?
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).
2. What is soft handover and softer handover?
· 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.
3. How does soft/softer handover work?
· 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.
4. Why is there “soft handover gain”?
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.
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5. Brief describe the advantages and disadvantages of soft handover?
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.
6. What are fast fading and slow fading?
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.
7. What are fast fading margin and slow fading margin?
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.
8. What is a typical soft handover gain in your link budget?
· 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).
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9. What is the percentage in time a UE is expected to be in soft or softer handover?
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.
10. What is a typical EiRP?
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.
11. How much power usually a NodeB is allocated to control channels?
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.
12. What is a typical CPICH power?
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, and
· More power can be allocated to traffic channels.
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13. How much is your HSDPA (max) link power?
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.
14. Consider downlink only, what are the major components in calculating maximum path
loss, starting from NodeB?
· NodeB CPICH transmit power.
· Jumper and feeder connector loss.
· Antenna gain.
· Over-the-air loss.
· Building / vehicle penetration loss.
· Body loss.
· Etc.
15. What is maximum path-loss?
The maximum path-loss is how much signal is allowed to drop from a transmitter to a
receiver and maintains as good signal.
16. Simple link budget: with a 30dBm CPICH and a -100dBm UE sensitivity, ignoring
anything in between, what is the maximum path loss?
30 – (–100) = 30 + 100 = 130dB.
17. Suppose I have a maximum path-loss of 130dBm, what is the new path-loss if a 5dB
body loss is added?
125dB.
18. What is channelization code?
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.
19. How many channelization codes are available?
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.
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20. Are channelization codes mutually orthogonal? If so, why is “Orthogonality Factor”
required in the link budget?
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.
21. What is scrambling code? How many scrambling codes there are?
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.
22. What is scrambling “code group”?
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).
23. Do you divide scrambling code groups into subgroups? Please give an example.
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.
24. Which service usually needs higher power, CS or PS?
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.
25. What is Eb/No requirement for HSDPA?
The Eb/No requirement for HSDPA varies with user bit rate (data rate), typically 2 for
768kbps and 5 for 2Mbps.
26. What is “noise rise”? What does a higher noise rise mean in terms of network loading?
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.
27. 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.
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