Capacity Management and Its functions- WCDMA

Capacity Management is responsible for the control of the load in the cell.

It consists of 3 main functions:

1. Dedicated Monitored Resource Handling: tracks utilization of critical resources of the system.

2. Admission Control: accepts/refuses admission requests based on the current load on the dedicated monitored resources and the characteristics of the request

3. Congestion Control: detects/resolves overload situations.

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Is UMTS an Uplink limited or Downlink limited System?

Initially, A typical WCDMA network is Uplink Limited. Later a Loaded Network becomes Downlink Limites.

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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.

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What is Compressed Mode?

Compressed mode is a physical layer function that allows the UE to temporarily tune to another frequency, and measure the RF environment of another UMTS frequency (e.g. IFHO) or another technology (e.g. IRAT), while maintaining an existing dedicated channel.

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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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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 SIR?

Define Active Set? Pros and Cons of having a small or longer Active Set

Active Set consists of group of cells that takes part in soft/softer handover & measure by UE.

Typical size of Active set is 3 or 4 & generally a standard practice in all WCDMA networks.

A small active set size may provide more resources available due to less soft/softer handover but at the expense of handover gain thereby reducing the capacity & link redundancy.

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Explain Soft and Softer handover? Give some advantage and disadvantage for soft handover

In Soft Handover, the UE connection consists of at least two radio links established with cells belonging to different RBSs. In Softer handover, the UE connection consists of at least two radio links established with cells belonging to the same RBS.

It acts as macro diversity since UE is connected to more than one radio link at any given point, adds redundancy and reduces interference. However there is a tradeoff between soft/softer handover & system capacity.

A UE involved in Soft/Softer Handover uses several radio links, more DL channelization codes, and more DL power than a single-link connection.

Consequently, if all the UEs connected to a particular RNC are considered, more resources are needed in the RBSs, more resources over the Iub and Iur interfaces, and more resources in the RNC. For this reason, the number of radio links involved in the Soft/Softer handover must be limited.

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How many types of handovers are there in UMTS?

• Soft/Softer Handover
• Inter Frequency Handover
• Inter RAT Handover
• Core Network Hard Handover
• Service based handover to GSM
• HSDPA Mobility

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Explain the functionality of TPC

During Power Control, Transmit Power control (TPC) commands are used to power up or power down based on SIR target in the step of 0.5 dB ( 1 dB if the connection is made over Iur).

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How is Initial RACH Power is calculated?

The initial power on the PRACH - the power of the first preamble - is determined according to equation

P_PRACH = L_PCPICH + RTWP + constantValueCprach

Where L_PCPICH is the path loss estimated by UE since it knows transmit & receive CPICH power RTWP is received Total Wideband Power(uplink interference) measured by RBS .

constantValueCprach is used by the UE to calculate the initial power on the PRACH . This parameter is configurable and decides at which level below RTWP preamble ramping will start.

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How many time Inner Loop Power Control happens and what type of fading it compensates?

How many time Inner Loop Power Control happens and what type of fading it compensates?
Ans:1500Hz and compensates Fast Fading.


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What is SIR?

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.


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Power control In WCDMA

• Open Loop
• Inner Loop
• Outer Loop

Open Loop Power control-The Open loop power control technique requires that the transmitting entity measures the channel interference and adjusts its transmission power accordingly. This can be done quickly, but the problem is that the  interference estimation is done on the received signal, and the transmitted signal probably uses a different frequency, which differs from the received frequency by the system’s duplex offset. As uplink and downlink fast fading (on different frequency carriers) do not correlate, this method gives the right power values only on average.

Inner Loop-In this method the received signal-to interference ratio (SIR) is measured over a 667- microsecond period (i.e., one time slot), and based on that value, a decision is made about whether to increase or decrease the transmission power in the other end of the connection. Note that the delay inherent in this closed-loop method is compensated for by making the measurements over a very short period of time. The transmit power control (TPC) bits are sent in every time slot within the uplink and the downlink.

There is not a neutral signal; all power control signals contain either an increase or decrease command.

Outer Loop-The outer loop power control functions within the base station system, and adjusts the required SIR value (SIRtarget), which is then used in the inner loop control. Different channel types, which can be characterized by, for example, different coding and interleaving methods, constitute a channel’s parameters. Different channel parameters may require different SIRtarget values. The final result of the transmission process can only be known after the decoding process, and the resulting quality parameter is then used to adjust the required SIR value. If the used SIR value still gives a low quality bit stream, then the outer loop power control must increase the SIRtarget value. This change in the outer loop will trigger the inner loop power control to increase the mobile station transmission power accordingly.


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Explain Timer T3212?

Periodic LA and RA updating is used to notify the network of the UEs availability, and to avoid unnecessary paging attempts for a UE that has lost coverage and is not able to inform the CN that it is inactive.

The periodic LA update procedure is controlled by a timer, called t3212, which gives the time interval between two consecutive periodic location updates. The value is sent by the WCDMA RAN to UEs on the BCCH.


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When is System information sent to UE?

The system information is regularly broadcast to the UE on the BCCH. When a parameter in the system information is changed, all UE in a cell are notified by a paging message or by a system information change indication message.

Difference between PICH and PCH?

PICH-Paging Indicator Channel 

PCH-Paging Channel PICH is used to indicate UE to when it should read to S-CCPCH (Carries PCH) whereas PCH is used to carry RRC Message “Paging type 1” which contains actual Paging information.

What do you understand by DRX cycle?

The UE listens to the PICH only at certain predefined times, reducing power consumption. The periodicity of these searches is set by the system and the time interval is called Discontinuous Reception (DRX) cycle.

Different DRX cycles are used for circuit switched and packet switched services in Idle mode. A separate DRX cycle is also used to page Connected mode UEs in state URA_PCH.

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.

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.

Simple definition of Pole Capacity

The pole capacity is the theoretical maximum capacity of the system. In WCDMA, this capacity is only theoretical since, once reached, the system goes in an instable state that leads to its collapse. However it is still a reference for expressing the load.

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

What is the usage of Channelization code in downlink and uplink?

Uplink separation of physical data (DPDCH) and control channel (DPCCH) from same terminal.

Downlink separation of downlink connections to different users within one cell.

What is the family of codes used for Chanelization in WCDMA?

Family of codes used for Chanelization in WCDMA used is Orthogonal Variable Spreading Factor.(OVSF)

How many slots are there in a WCDMA Frame? How big is a frame in ms. how many chips are there in a slot?

WCDMA Frame is 15 slots wide. It is 10ms in length. There are 2560 chips in one slot. Chip rate is 3840 Kc/s

Length of frame = 10 ms

Number of chips in a frame = 3840 *10=38400 chips.

Number of chips in a slot = 38400/15= 2560 chips

What are different Idle mode tasks UE performs?

In Idle mode, the UE has no connection to the radio network.Keeping UEs in Idle mode minimizes the use of resources both for the UEs and in the network. However, the UEs must still be able to access the system and be reached by the system with acceptable delays. For this, the following procedures need to be

performed.

• PLMN selection and reselection
• Cell selection and reselection
• Location Area (LA) and Routing Area (RA) updating
• Paging
• System information broadcast