3G Question & Answer
UMTS
Interview Question and Answers
1)UMTS
Modulation Technique:
Uplink vs. Downlink modulation:
Downlink uses QPSK
Uplink uses Dual BPSK
2)Range of Spreading
Factors:
Downlink
: 4 to 512
Uplink: 4 to 256
3)There are several
Event in 3G
Event 1A:UTRAN will add the new cell in the UE's active cell list and will send an ACTIVE SET UPDATE message.
Event 1B:UTRAN will send ACTIVE SET UPDATE message to remove the cell from UE's
active set.
Event 1C:UTRAN will send ACTIVE SET UPDATE message that will remove one or more
cells and will add one or more cells. The only restriction for 1C is that there
should be atleast one radio link that is not affected by the procedure.
What is difference RRC & RAB?
RRC Setup Success Rate = (RRC Connection Setup Success / RRC Connection
Request) × 100%
RAB Setup Success Rate = RAB Assignment Success / RAB Assignment Request × 100%
RRC
is signalling part and RAB is Data part.
RRC is in layer3 and RAB is in between UE and CN.
RRC is part of control plane and RAB of user Plane.
RRC is the Radio Resource Control which works as the Control Plane in the Layer
3 .
It is basically used for signalling in UMTS.
It can be compared with GSM signalling part such as Immediate Assignment
Process for Allocating SDCCH Resource.
UMTS:- RRC Connection Success Rate
GSM:-Immediate Assignment Success Rate.
RAB:- Radio Access Bearer which works as the User plane for provide Data for
User Request Service.
for Each service which was requested by User there is only 1 RAB.
e.g. if user requested CS-AMR Call then 1 CS RAB will be generated and provided
to user.
similarly for PS Call.
UMTS:-RAB Assignment Success Rate(RNC-CN)
GSM:-Assignment Success Rate(BSC-MSC)
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
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
What
is a typical NodeB maximum output power?
The
maximum NodeB output power is usually 20W or 40W, that is, 43dBm or 46dBm.
What
is UE maximum transmit power in your link budget?
21dBm.
What
is a typical antenna gain?
The antenna gain depends on antenna model;
in link budget we use around 17dBi.
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.
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.
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.
What
is 0dBm?
0dBm
= 1 milli-watt.
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.
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.
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.
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.
What
is UMTS chip rate? Ans : 3.84MHz.
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
What
are the processing gains for CS and PS services?
CS12.2: 25dB, PS-64: 18dB, PS-128: 15dB, PS-384:10dB, HSDPA: 2dB
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.
A CS-12.2k bearer needs 1 SF128 code.
b.
Total available codes for CS-12.2k = 128 – 2 (1 SF64) – 2 (4 SF256) = 124.
c.
Consider soft-handover factor of 1.8 and loading factor of 50%: 124 / 1.8 *.05
= 34 uers/cell.
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.
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.
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.
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).
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.
What
is RSCP?
RSCP
stands for Received Signal Code Power – the energy per chip in CPICH averaged
over 512 chips.
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.
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.
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”.
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.
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).
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.
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.
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.
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.
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.
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.
What
is a typical soft handover gain in your link budget?
a.
CS-12.2k: 3dB (UL), 2dB
(DL). b.
PS-64k:
1dB (UL), 0dB (DL).
c.
PS-128k: 1dB (UL),
0dB (DL). d.
PS-384k: 1dB
(UL), 0dB (DL).
What
is the percentage in time a UE is e xpected 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.
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.
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.
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.
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.
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.
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.
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.
Suppose
I have a maximum path-loss of 130dBm, what is the new path-loss if a 5dB body
loss is added?
125dB.
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.
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.
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.
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.
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).
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.
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.
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.
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.
What
is a typical handover window size in your network?
A
handover window size is usually between 4 to 6dB.
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.
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.
What
is the active set size on your network? Ans
: 3.
How
many fingers does a UE rake receiver have? Ans : 4.
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”.
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.
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.
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.
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”.
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.
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”.
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.
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 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.
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.
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.
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.
What
are the interfaces between each UTRAN component?
Uu: UE to
NodeB
Iub:
NodeB to RNC
Iur: RNC
to RNC
Iu: RNC to MSC
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.
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:
§ Physical
Channel: carries data between physical layers of UE and NodeB.
§ Transport
Channel: carries data between physical layer and MAC layer.
§ Logical Channel: carries data between MAC layer
and RRC layer.
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.
2. 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.
What
are the RRC operation modes?
Idle
mode and connected mode.
What
are the RRC states?
There
are 4 RRC States: Cell_DCH, Cell_FACH, URA_PCH and Cell_PCH.
URA = UTRAN Registration Area.
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.
Which
layer(s) perform ciphering function?
RRC
– for acknowledged mode (AM) and unacknowledged mode (UM).
MAC
– for transparent mode (TM).
What
is OVSF? Ans : Orthogonal Variable Spreading Factor.
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.
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.
Can
a user have OVSF code as “1111”?
Ans : No, because “1111…” (256 times) is used by CPICH.
What
are the symbol rates (bits per symbol) for BPSK, QPSK, 8PSK and 16QAM?
§ BPSK: 1.
§ QPSK: 2.
§ 8PSK: 3.
§ 16QAM: 4.
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.
What
is cell selection criterion?
Cell
selection is based on:
a. Qmean: the average SIR of the target
cell.
b. Qmin:
minimum required SIR.
c.
Pcompensation: a correction value for difference UE classes.
S = Qmean - Qmin - Pcompensation
d.
If S>0 then the cell is a valid candidate.
e.
A UE will camp on the cell with the highest S.
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
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%.
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.
How
many scrambling code groups are there for downlink?
There
are 64 code groups, each group has 8 scrambling codes.
Are
scrambling codes orthogonal? Ans
: No, scrambling codes are not
orthogonal since they are not synchronized at each receiver. They are
pseudo random sequences of codes.
Can
we assign scrambling codes 1, 2 and 3 to sister sectors? Ans : Yes.
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.
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.
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.
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.
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.
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.
What
could be the cause of soft handover failure?
§ UE issue.
§ Resource
unavailable at target NodeB.
§ Inadequate
SHO threshold defined.
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.
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.
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.
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.
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.
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.
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.
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.
What
is the typical UE transmit power?
Varies
- most of the time below 0dBm.
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
What
are the possible causes for an IRAT Failure?
§ Missing 2G
relations
§ Non
availability of 2G Resources
§ Poor 2G
Coverage
§ Missing 3G Relations
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%
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
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
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.
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
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 )
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.
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
(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
(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)
What
are the typical values for ‘reportingrange1a’ and ‘reportingrange1b’?
§ 3 dB and 5
dB respectively.
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)
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.
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.
What
is the maximum number of Channelization Codes that can be allocated for HS, as
per 3GPP standard?
§
15 codes of SF 16.
What
is ‘Code Multiplexing’ in HSDPA?
Sharing
the HS Channelization Codes among more than one HS users within the 2ms TTI period.
(FOR
ERICSSON EXPERIENCED) In Ericsson System, how is the Power allocated for
HSDPA>
Power unutilized by 99 PS, CS and Comman
Channels, is used for HS (PHS = Pmax - hsPowerMargin - Pnon-HS)
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
What
is pre-synchronized handover?
A pre-synchronization method in
which the source cell chooses a signature for the user mobile to use in the
target cell for RACH access, and instructs the user mobile to perform RACH
access and return to the current cell before a handover command (break command)
is issued. This causes parallel execution of the UL synchronization process
with the context transfer process among the two base stations
Brief describe the
advantages and disadvantages of soft handover?
Advantages
Speech quality:
.Power saving:
Lower transmission power level:
No ping-pong effect:
Frequency planning and network expansion:
Disadvantages
Costs of
computation:
Complex to
implement:
ARQ (Automatic Repeat Request), which is
used in UMTS, a package received in error will be discarded and a
retransmission will be requested. If the retransmission is also erroneous,
another retransmission will be requested.
Using HARQ, an erroneous package will be stored at the receiver
and a retransmission will be requested. Even if the retransmission is faulty,
the receiver attempts to combine the two erroneous packages to reproduce the original package.
1)Link adaptation (AMC)
2)Fast Hybrid Automatic Repeat Request (HARQ), and
3)Shorter Transmission Time Interval (TTI)
HSDPA uses link adaptation, which means the way of
transmission is changed according to the quality of the channel conditions.
If a user is in favourable conditions, for example close to the nearest antenna tower, this user will be assigned a high data rate.
When the user moves into worse channel conditions, for example far away from the antenna tower, the transmission parameters
will be changed accordingly and thus the data rate will be decreased.
RF Optimization UMTS,HSDPA
Latency is the time a packet needs to travel from
sender to receiver. While UMTS typically features an
end-to-end latency of approximately 200ms, HSDPA manages to lower the delay
times in transmission to around 100ms.
1)After every TTI the resources can be
redistributed among the users. Therefore, the resource usage is more efficient.
2)each UE reports about the channel quality after every TTI by sending the CQI.
3)CQI is sent after the very short period of time of 2 ms, it is possible to
effectively perform link adaptation even in rapidly changing conditions.
Introduction
to HSDPA
High-Speed Downlink Packet Access (HSDPA)
is also known as High-Speed Downlink Protocol Access. HSDPA is a protocol for
mobile telephones. It is a third-generation (3G) High-Speed Packet Access
technology designed to speed up network capacity and data transmission rate of
cellular phones.
HSDPA is associated with various Universal
Mobile Telecommunications System (UMTS) networks. These
include the Global System for Mobile (GSM)
Communications.
Currently, HSDPA can support speeds ranging
from 1.8 Mbps to 14.4 Mbps. While this is not very fast compared to various
wired networks, it is a major milestone considering the speed available to
cellular technology.
Although HSDPA is mostly identified with
its downlink speeds, it also has the ability to greatly expand transfer
capacity. HSDPA networks can transfer up to 30 GB of data monthly and as much
as 300 minutes of cellular television viewing and 1000 minutes of talk time.
Current HSDPA Technology
At present, there are 100 HSDPA-capable
networks operating in 54 countries worldwide. A large number of these networks
offer 3.6 Mbps downlink speeds. However, more networks are beginning to switch
to faster 7.2 Mbps downlink speeds. Only a number of networks presently offer a
full 14.4 Mbps speed
HSDPA competes with a rival format, the
Evolution Data Optimized (EVDO) provided by Code Division Multiple Access
(CDMA) cellular providers.
HSDPA Capable Devices
Aside from having a cellular plan with a
company offering HSDPA access, the only requirement for logging into the HSDPA
network is having a cellular device capable of using the technology. A current
total of 171 handset models and other mobile devices can use an HSDPA network.
Some of these HSDPA capable devices are famous cellular brand and model names
such as the Nokia N95, the Palm Treo 750, and the
Motorola RAZR as well as many of its various versions.
In addition to the various HSDPA handsets
in the market, several HSDPA notebooks and laptops with access to the data
network are available. Such products are offered by several major computer
manufacturers including Dell, Acer, Hewlett-Packard, Siemens, Fujitsu,
Panasonic, and Lenovo.
Future HSDPA Technology
Although 14.4 Mbps is the current cap on
downlink speed over HSDPA, networks are preparing to be
upgraded with a new type of HSDPA system. Dubbed as
HSPA Evolved, this new HSDPA technology is designed to increase download speeds to 42 Mbps upon release, with the possibility of reaching greater speed values in the future.
Facts
about HSDPA
HSDPA
represents an evolution of the WCDMA radio interface, which uses very similar
methods to those employed by EDGE (Enhanced Data Rates for GSM Evolution)
technology for the GSM radio interface. The fundamental characteristics which
enable the increase in the data throughput and capacity with reduced latency
are summarised below:
Time and code multiplexing of the users
Multi-Code transmission
Fixed Spreading Factor (SF = 16)
Shorter TTI = 2ms
The Paging Channel (PCH) is a downlink transport channel. The
PCH is always transmitted over the entire cell. The transmission of the PCH is
associated with the transmission of physical-layer generated Paging Indicators,
to support efficient sleep-mode procedures.
System
information block type 5 (SIB 5)
Two Paging
Message Types:
PAGING TYPE 1 message is used to send
information on the paging channel. One or several UEs, in idle or connected
mode, can be paged in one message, which also can contain other information
PAGING TYPE 2 message is used to page an UE in
connected mode (CELL_DCH or CELL_FACH state), when using the DCCH for CN
originated paging.
UMTS Drive Parameter:
1. CPICH EC/No- 0 to -34
dB.
2. CPICH RSCP - -15 dBm to -140 dBm for each rake finger.
3. Ms TX power – -44 dBm
to 60 dBm
4. UTRA Carrier RSSI -
-20 dBm to -140 dBm for each rake finger.
5. SQI MOS - 1 to 5
6. SQI - -20dBQ to 30 dBQ
7 . SIR - -30
t0 20 db (signal interfere ratio
measured on DPCCH)
