General
1.
What is the experience and involvement in your current
and previous UMTS design projects?
Talk about your current and previous projects, your
responsibilities, design objectives, tools used, activities involved,
challenges, objectives met, etc.
Link Budget
2.
What is a typical NodeB sensitivity level?
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
3.
What is a typical UE sensitivity level?
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
4.
What is a typical NodeB maximum output power?
The maximum NodeB output power is usually 20W
or 40W, that is, 43dBm or 46dBm.
5.
What is UE maximum transmit power in your link budget?
21dBm.
6.
What is a typical antenna gain?
The antenna gain depends on antenna model; in
link budget we use around 17dBi.
7.
What is a typical maximum path loss?
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.
8.
What is difference between dBi and dBd?
dBi is the gain in dB from
isotropic source; dBd is the gain from a dipole source.
dBd + 2.15 = dBi.
9.
What is the difference between dB and dBm?
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.
10.
What is 0dBm?
0dBm = 1 milli-watt.
11.
How does TMA work?
A TMA reduces system noise,
improves uplink sensitivity and leads 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)
+ ... + (NFi-1)/(G1*G2*...*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 – 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
that’s close to feeder loss.
12.
What are the pros and cons (advantages and
disadvantages) of TMA?
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.
13.
What is typical TMA gain?
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.
14.
Why TMA are installed at the top near the antenna and
not the bottom near the NodeB?
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.
15.
What is UMTS chip rate?
3.84MHz.
16.
What is processing gain?
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
17.
What are the processing gains for CS and PS services?
CS12.2: 25dB
PS-64: 18dB
PS-128: 15dB
PS-384: 10dB
HSDPA: 2dB
18.
How to calculate maximum number of users on a cell?
To calculate the maximum number of users (M) on a cell, we need to know:
W: chip rate (for UMTS
3,840,000 chips per second)
EbNo: Eb/No requirement
(assuming 3dB for CS-12.2k)
i: other-cell to in-cell
interference ratio (assuming 60%)
R: user data rate (assuming
12,200 kbps for CS-12.2k)
η: loading factor (assuming 50%)
Take 12.2kbps as example:
M
= W / (EnNo * (1 + i) * R) * η = 3,840,000 (3 * (1 + 0.6) * 12,200) * 0.5 =
32.8
The number of users could also be
hard-limited by OVSF code space. Take
CS12.2k 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 and loading factor of 50%: 124 / 1.8 *.05 = 34
uers/cell.
19.
What is Eb/No?
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.
20.
What are the Eb/No targets in your design?
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.
21.
Why is Eb/No requirement lower for PS than for CS?
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.
22.
What is Ec/Io?
Ec/Io is the ratio of the energy per chip in CPICH to
the total received power density (including CPICH itself).
23.
Sometimes we say Ec/Io and sometimes we say Ec/No, are
they different?
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.
24.
What is RSCP?
RSCP stands for Received Signal
Code Power – the energy per chip in CPICH averaged over 512 chips.
25.
What is SIR?
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.
26.
What is the loading factor in your design?
The designed loading typically
is 50%; however, sometimes a carrier may want to design up to 75% load.
27.
Give a simple definition of pole capacity?
The uplink noise increases with
the loading exponentially. When the
uplink noise approaches infinity then no more users can be added to a cell – and
the cell loading is close to 100% and has reached its “pole capacity”.
Mathematically, to calculate the
uplink pole capacity we need to know:
W: chip rate (for UMTS
3,840,000 chips per second)
R: user data rate (assuming
12,200 kbps for CS-12.2k)
f: other-cell to in-cell
interference ratio (assuming 65%)
EbNo: Eb/No requirement
(assuming 5dB)
AF: Activity factor (assuming
50%)
Pole Capacity = (W/R) / ((1+f) * AF * 10^(EbNo/10)) = 120.6
To calculate the downlink pole capacity we also need
to know:
α: downlink channels
orthogonality factor (assuming 55%)
Pole Capacity = (W/R) / ((1- α +f) * 10^(EbNo/10)) =
64.06
28.
What is typical pole capacity for CS-12.2, PS-64, PS-128
and PS-384?
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.
29.
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).
30.
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.
31.
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.
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.
33.
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.
34.
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.
35.
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.
36.
What is a typical soft handover gain in your link
budget?
·
PS-64k: 1dB
(UL), 0dB (DL).
·
PS-128k: 1dB
(UL), 0dB (DL).
·
PS-384k: 1dB
(UL), 0dB (DL).
37.
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.
38.
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.
39.
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.
40.
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.
41.
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.
42.
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.
43.
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.
44.
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.
45.
Suppose I have a maximum path-loss of 130dBm, what is
the new path-loss if a 5dB body loss is added?
125dB.
46.
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.
47.
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.
48.
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.
49.
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.
50.
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).
51.
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.
52.
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.
53.
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.
54.
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.
55.
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.
56.
What is a typical handover window size in your
network?
A
handover window size is usually between 4 to 6dB.
57.
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.
58.
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.
59.
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 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.
60.
What is the active set size on your network?
3.
61.
How many fingers does a UE rake receiver have?
4.
62.
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”.
63.
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.
64.
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.
65.
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.
66.
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”.
67.
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.
68.
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”.
69.
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.
70.
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.
71.
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.
72.
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.
73.
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.
UTRAN
74.
What are the interfaces between each UTRAN component?
Uu: UE to NodeB
Iub: NodeB to RNC
Iur: RNC to RNC
Iu: RNC to MSC
75.
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.
76.
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.
77.
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.
78.
What are the RRC operation modes?
Idle mode and connected mode.
79.
What are the RRC states?
There are 4 RRC States: Cell_DCH, Cell_FACH, URA_PCH
and Cell_PCH.
URA = UTRAN Registration Area.
80.
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.
81.
Which layer(s) perform ciphering function?
RRC – for acknowledged mode (AM) and unacknowledged
mode (UM).
MAC – for transparent mode
(TM).
82.
What is OVSF?
Orthogonal Variable Spreading Factor.
83.
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.
84.
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.
85.
Can a user have OVSF code as “1111”?
No, because “1111…” (256 times) is used by CPICH.
86.
What are the symbol rates (bits per symbol) for BPSK,
QPSK, 8PSK and 16QAM?
·
BPSK: 1.
·
QPSK: 2.
·
8PSK: 3.
·
16QAM: 4.
87.
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.
88.
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.
89.
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
Planning
90.
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%.
91.
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.
92.
How many scrambling codes are there?
There are 512 scrambling codes
in the downlink and 16,777,216 codes in the uplink.
93.
How many scrambling code groups are there for downlink?
There are 64 code groups, each group has 8 scrambling
codes.
94.
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.
95.
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.
96.
Can we assign scrambling codes 1, 2 and 3 to sister
sectors?
Yes.
97.
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.
98.
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.
99.
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.
100. What is
“Monte Carlo simulation”?
Since UMTS coverage is dependent
on the loading, static coverage and quality analysis (RSCP and Ec/Io)
represents the network performance in no-load condition. Monte Carlo simulation is therefore used to
illustrate network performance under simulated loading consition.
101. What is the
key difference between a static analysis and a Monte Carlo simulation?
Static analysis can only show
RSCP and Ec/Io in no-load condition.
Monte Carlo simulation not only can show RSCP and Ec/Io in simulated
loading condition but also can show many more others: mean served, cell loading,
uplink and downlink capacity limits reached, etc.
102. What should
be run first (what information should be ready and loaded) before running a
Monte Carlo simulation?
Before running Monte Carlo simulation, the following
should be completed or in place.
·
Run prediction.
·
Spread the
traffic.
·
Define terminal
types.
103. How many
snap shots and iteration do you usually have when running Monte Carlo
simulation?
(Depend on software tool
recommendations).
104. What are the
design KPI’s?
(RSCP, Ec/Io, mean served, soft
handover ratio…)
105. What plots
do you usually check after running Monte Carlo for trouble spots?
(RSCP, Ec/Io, service
probability, reasons for failure…)
106. What are the
typical reasons of failure in Monte Carlo simulation?
·
Downlink Eb/No
failure (Capacity).
·
Downlink Eb/No
failure (Range).
·
Uplink Eb/No
failure.
·
Low pilot SIR.
·
Noise rise limit
reached.
·
Etc.
107. What does
“traffic spread” mean?
“Traffic spread” means spreading
traffic (number of terminals) in a cell coverage area.
108. Do you use
live traffic or even-load traffic in your design?
(Depends).
Optimization
109.
What are the optimization tools you use?
Drive test, analysis, others?
110.
Are System Information Blocks (SIB) transmitted all
the time?
No, system information block is multiplexed with
synchronization channel. Synchronization
channel occupies the first time slot (TS) and SIB occupies the other 9 time
slots.
111.
How does UE camp (synchronize) to a NodeB?
1. UE uses the primary synchronization channel (P-SCH)
for slot alignment (TS synchronization).
2. After aligning to NodeB time slot, UE then uses
secondary synchronization channel (S-SCH) to obtain frame synchronization and
scrambling code group identification.
3. UE then uses scrambling code ID to obtain CPICH, thus
camping to a NodeB.
112.
What could be the cause of soft handover failure?
·
UE issue.
·
Resource
unavailable at target NodeB.
·
Inadequate SHO
threshold defined.
·
Etc.
113. What are the
three sets in handover?
The 3 sets in handover are:
·
Active set – the
list of cells which are in soft handover with UE.
·
Monitored set –
the list of cells not in active set but RNC has told UE to monitor.
·
Detected set –
list of cells detected by the UE but not configured in the neighbor list.
114. What are the
major differences between GSM and UMTS handover decision?
GSM:
·
Time-based mobile
measures of RxLev and RxQual – mobile sends measurement report every SACH
period (480ms).
·
BSC instructs
mobile to handover based on these reports.
UMTS:
·
Event-triggered
reporting – UE sends a measurement report only on certain event “triggers”.
·
UE plays more
part in the handover decision.
115. What are the
events 1a, 1b, 1c, etc.?
·
e1a – a Primary
CPICH enters the reporting range, i.e. add a cell to active set.
·
e1b – a primary
CPICH leaves the reporting range, i.e. removed a cell from active set.
·
e1c – a
non-active primary CPICH becomes better than an active primary CPICH, i.e.
replace a cell.
·
e1d: change of
best cell.
·
e1e: a Primary
CPICH becomes better than an absolute threshold.
·
e1f: a Primary
CPICH becomes worse than an absolute threshold.
116.
What are event 2a-2d and 3a-3d?
Events 2a-2d are for inter-frequency handover
measurements and events 3a-3d are for IRAT handover measurements.
·
e3a: the UMTS
cell quality has moved below a threshold and a GSM cell quality had moved above
a threshold.
·
e3b: the GSM cell
quality has moved below a threshold.
·
e3c: the GSM cell
quality has moved above a threshold.
·
e3d: there was a
change in the order of best GSM cell list.
117. What may
happen when there’s a missing neighbor or an incorrect neighbor?
·
Access failure
and handover failure: may attempt to access to a wrong scrambling code.
·
Dropped call: UE
not aware of a strong scrambling code, strong interference.
·
Poor data
throughput.
·
Poor voice
quality.
·
Etc.
118. What can we
try to improve when access failure is high?
When access failure is high we can try the following
to improve RACH performance:
·
Increase maximum
UE transmit power allowed: Max_allowed_UL_TX_Power.
·
Increase power
quickly: power_Offset_P0.
·
Increase number
of preambles sent in a given preamble cycle: preamble_Retrans_Max.
·
Increase the
number of preamble cycles: max_Preamble_Cycle.
·
Increase number
of RRC Connection Request retries: N300.
119. What are the
conditions you typically set to trigger IRAT handover?
RSCP and Ec/Io are used to trigger IRAT handover:
·
RSCP ≤ -100dBm.
·
Ec/Io ≤ -16dBm.
120. What are the
typical KPIs you use to measure a network and what criteria?
·
Access failure
rate (≤ 2%).
·
Call setup time
(CS: over 95% of the time < 6-second for mobile-to-PSTN, 9-second for
mobile-mobile. PS: over 95% of the time
< 5-second).
·
Dropped call rate
(≤ 2%).
·
BLER: over 95% of
the blocks ≤ 2%.
·
Average DL/UL
throughput for PSD: 210kbps for loaded, 240kbps for unloaded.
121. What is the
typical UE transmit power?
Varies - most of the time below 0dBm.
122. Have your
used Ericsson TEMS? If so:
·
Do you know how
to create command sequence?
·
What are the call
sequences you typically have? CS long
call, CS short call, PSD call, etc.
·
What are the
typical commands you have for CS and PS call?
·
Do you regularly
stop and restart a new log file? Why and
when to stop and start a new file?
·
How do you stop a
log file? Stop command sequence first,
wait and make sure all equipment are in idle mode before stop logging.
123. Did you work
on neighbor prioritization?
124. What is the
typical event sequence of IRAT Handover from 3G to 2G
·
Event 2d –
entering into compressed mode – measurement of 2G candidates – Event 3a –
Verification of 2G resources – Handover from UTRAN Command from 3G RNC to UE
125. What are the
possible causes for an IRAT Failure?
·
Missing 2G
relations
·
Non availability
of 2G Resources
·
Poor 2G Coverage
·
Missing 3G
Relations
126. What is
Paging Success Ratio? What is the typical PSR that you have seen in a UMTS
network?
·
PSR – Paging
Responses to the Paging Attempts
·
About 90%
127. What are the
possible causes for a lower PSR?
·
Non-continuous RF
Coverage – UE going in and out of coverage area frequently
·
Very High
‘Periodic Location Update Timer’ – Keeping UEs in VLR long time after it moved
out of coverage
·
Lower Paging
Channel Power
·
Access Channel
Parameter Issues
·
Delayed Location
Update when crossing the LA / CN Boundaries
128. What are the
possible causes for a Drop Call on a UMTS network?
·
Poor Coverage (DL
/ UL)
·
Pilot Pollution /
Pilot Spillover
·
Missing Neighbor
·
SC Collisions
·
Delayed Handovers
·
No resource
availability (Congestion) for Hand in
·
Loss of Synchronization
·
Fast Fading
·
Delayed IRAT
Triggers
·
Hardware Issues
·
External
Interference
129. A UE is
served by 2 or 3 SC in AS. It is identifying a SC from 3rd tier,
Stronger and meets the criteria for Event1a or Event1c. But SHO did not happen
because of missing neighbor relations? How do you optimize this issue?
·
Study the Pilot
spillover from the 3rd Tier SC and control its coverage
·
Even after
controlling the coverage, if the spillover is there, Add the neighbor.
130. A UE is
served by 2 SC in AS, a SC is coming in to Monitored Set and Event1a is
triggered. But UE is not receiving
Active Set Update from NodeB and the call drops. What could be possible causes
for this drop?
·
Delayed Handover
·
Loss of
Synchronization
·
Fast Fading
·
Pilot Pollution /
Spillover issues
131. What is Hard
Handover in UMTS? When will it happen?
·
Hard Handover in
UMTS is a break before make type Handover
·
It can happen in
the inter RNC boundaries where there is no Iur link.
132. 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 – need more than Access Attempt)
·
Delayed Page
Responses
·
High Load on
Paging and/or Access Channel
·
Paging / Access
Parameters
133. 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 – Total Traffic
/ Primary Traffic. 2) Secondary / Total Traffic
·
Typical Values
are like 1.7 (Avg Active Set Size) or 35% (Secondary / Total )
134. 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.
135. What are the
possible causes for an Access Failure in UMTS?
·
Missing Neighbors
·
Poor Coverage
·
Pilot Pollution /
Spillover
·
Poor Cell
Reselection
·
Core Network
Issues
·
Non –
availability of resources. Admission Control denies
·
Hardware Issues
·
Improper RACH
Parameters
·
External
Interference
136. (FOR
ERICSSON EXPERIENCED) What is RTWP? What is the significance of it?
·
Received Total
Wide-band Power
·
It gives the
Total Uplink Power (Interference) level received at NodeB
137. (FOR
ERICSSON EXPERIENCED) What is the System Reference Point at which all the
Power Levels are measured in Ericsson NodeB?
·
System Ref Point
for E/// NodeB is at the output of TMA (Between TMA and Antenna)
138. What are the
typical values for ‘reportingrange1a’ and ‘reportingrange1b’?
·
3 dB and 5 dB
respectively.
139. What will be
the impact when you change ‘reportingrange1a’ from 3 to 4 dB and
‘timetotrigger1a’ 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)
140. What is
Admission Control?
·
Admission Control
is an algorithm which controls the Resource Allocation for a new call and
additional resource allocation for an existing call. Incase, if a cell is heavily
a loaded and enough resources in terms of power, codes or CEs are not
available, admission control denies permission for the additional resource
requirement.
141. What is
Congestion Control?
·
Congestion
Control monitors the dynamic utilization of specific cell resources and insures
that overload conditions do not occur.
If overload conditions do occur, Congestion Control will immediately
restrict Admission Control from granting additional resources. In addition, Congestion Control will attempt
to resolve the congestion by either down switching, or terminating existing
users. Once the congestion is corrected,
the congestion resolution actions will cease, and Admission Control will be
enabled.
142. What is the
maximum number of Channelization Codes that can be allocated for HS, as per
3GPP standard?
·
15 codes of SF
16.
·
143. What is
‘Code Multiplexing’ in HSDPA?
·
Sharing the HS
Channelization Codes among more than one HS users within the 2ms TTI period.
144. (FOR
ERICSSON EXPERIENCED) In Ericsson System, how is the Power allocated for
HSDPA>
·
Power unutilized by R99 PS, CS and Comman Channels, is used for HS (PHS = Pmax - hsPowerMargin - Pnon-HS)
145. What are
Events that can trigger the HSDPA Cell Change?
·
Event 1d HS –
Change of Best Cell in the Active Set
·
Event 1b or Event
1c – Removal of the Best Cell from the Active Set
146. How is
typically the Call Setup Time of a CSV call calculated in UMTS using L3
messages?
·
CST is calculated
as the time difference between ‘Alerting’ and the first RRC Connection Request
(Call Initiation) messages.
