Power Control

 

What is power control and why do we need it?

 

Power control is the process of controlling the uplink and downlink power to the minimum level while ensuring/ maintaining good QoS (Quality of Service). It can be done both by UE or UTRAN (NodeB & RNC).

 

The mechanism is to adjust and control the power of transmit signals according to the changes of radio environment and the quality of receive signals.

Improve the system capacity & ensures the QoS.

 

 

There are four types of power control.

 

• Open-loop power control:
• Applicable in both UL and DL.
• It sets the initial uplink and downlink transmit power.
• Open-loop power control is used on physical channel such as PRACH (Physical Random Access Channel), DPCH (Dedicated Physical Channel).
• Inner-loop power control:
• Applicable in both UL and DL.
• It directly adjusts the uplink and downlink transmit power using power control commands.
• The inner loop power control is performed by each UE and NodeB with the frequency of 1500 times per second (1.5 kHz).
• Outer-loop power control:
• Applicable in UL and DL.
• It indirectly controls the uplink and downlink transmit power by increasing or decreasing the target SIR value.
• DL power balance:
• It is used to reduce the downlink power drifting of a given UE during soft handover.

*** 

1. In the uplink, if a UE near the NodeB has too large a transmit power, it may cause great interference to other UEs on the edge of the cell or even block the whole cell. This is called near-far effect. In that case, uplink power control is needed.
2. In the downlink, the system capacity is determined by the total required code power for each connection. Therefore, it is necessary to keep the transmit power at the lowest level while ensuring signal quality at the receiving end for each UE. In that case, the downlink power control is needed.
3. Power control can be used against shadow fading and fast fading.
4. Power control can increase system capacity.
5. Power control for power drifting can improve the soft handover performance in
   the downlink.

 

Later will explain in what type of scenario we use each of them.

 

Logical, Transport & Physical Channels in LTE

All these channels help UE to establish the connection with the eNodeB, maintain the connection and terminate the same.  

Logical channels

• Characterized by the information that is transferred (what kind information).
• Provide services for the Medium Access Control (MAC) layer within the LTE protocol structure.

Transport channels 

• Characterized by how the data are transferred over the radio interface (way/ method data is being transmitted).
• The physical layer transport channels offer information transfer to Medium Access Control (MAC) and higher layers. 

Physical channel

• These are transmission channels that carry user data and control messages.
• Corresponds to a set of resource elements used by the physical layer. Channels are further divided into control channel (signalling) and traffic channel (user/data) at logical channel stage.

Physical Channels

PDSCH - Physical Downlink Shared Channel, mainly used to carry high speed data/multimedia information. Can be either QPSK/16QAM/64QAM.

PDCCH - Physical Downlink Control Channel, mainly used to carry UE specific control information. It will have QPSK modulation used. It is mapped on resource elements (REs) in first 3 OFDM symbols in the first slot of sub-frame.

CCPCH - Common Control Physical Channel, carries cell-wide control information. QPSK is used. CCPCH is transmitted exclusively on 72 sub-carriers centered around DC carrier.
 

Physical Signals

Physical signals do not convey L2/L3 layer information, but mainly used for synchronization and channel estimation purpose. RS is used for estimating channel response. P-SS and S-SS synchronization signals used for determining network frame timing information i.e. start of the information.
 

Transport Channels

• Downlink and Uplink transport channels carry L2/L3 information. 
• It also configures LTE PHY layer. 
• It sends status information such as packet error and CQI to upper layers. 
• Also supports peer-peer signaling between higher layers. 

 

Based on broadcast, unicast or multicast concept different transport channels exist. 

 

Downlink channels include BCH (broadcast channel), DL-SCH( downlink shared channel, to multiple mobile subscribers or UEs), PCH (paging channel, used for UE DRX and broadcasted over entire cell ), MCH (multicast channel, transmitted over entire cell).  

 

Uplink channels include RACH (Random Access Channel), UL-SCH(Uplink Shared Channel).

 

Uplink PRBs (Physical Resource Blocks) are assigned to UE by eNodeB scheduler. 

 

PUSCH is used and shared by multiple UEs to carry upper layer information towards eNodeB. It will employ QPSK/16QAM/64QAM modulation types.

LTE Terminology

eNB or eNodeB 

• It is similar to Base Station in GSM networks and NodeB in WCDMA.
• Prefix "e" stands for evolved.

UE 

• User equipment. Could be handphone, tablet, datacard. It is similar to mobile subscriber (MS) in GSM environment.

OFDMA

• Orthogonal Frequency Division Multiple Access.
• Is a technique for transmitting large amounts of digital data over a radio wave by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver.
• Used in physical layer of LTE Downlink. 
• It is digital modulation scheme & better in dealing with multi-path effect in radio transmission.

SC-FDMA

• Single Carrier Frequency Division Multiple Access.
• Used in physical layer of LTE Uplink due to low PAPR (Peak to Average Power) ratio.
• This increases the efficiency of power amplifier and hence increases the battery life

LTE Frame

• LTE frame are of 2 types FDD (Type 1) and TDD (Type 2). 
• In both the cases, frame is composed of 10 sub frames and each sub frame is made of 2 slots. Frame size is 10ms.

Resource Block (RB)

• The smallest block of resource that can be allocated to UE by eNB; 
• It is 12 sub-carriers for 7 symbols (Normal CP); 6 symbols (Extended CP)
• LTE Bandwidth (1.4KHz=6RB, 3MHz=15RB, 5MHz=25RB, 10MHz=50RB, 15MHz=75RB and 20MHz=100RB) 

Resource Element (RE)

• The smallest unit of radio resources, one sub-carrier for one symbol.

Slot

• 7 consecutive symbols for short Cyclic Prefix (Normal CP), 6 symbols for long cyclic prefix (Extended CP).

Sub-frame

• 2 consecutive timeslots.
• Each timeslot has 0.5ms. 

Reference Signal

• Similar to pilot carrier and is used for channel estimation at the receiver.

PCI

• Physical Cell Identity/ Identifier.
• Provide value for identifying eNodeBs.
• Created from two components; PSS & SSS
• PSS has the value 0, 1, or 2 (total 3)
• SSS can have a value between 0 to 167 (total 168)
• The PCI value is [(3(SSS)+PSS], resulting in a value between 0 - 503 (total is 504, coming from 168*3).
• PCI needs to plan very well to avoid high interruption of the Reference Signal (RS) which may result towards bad signal coverage. 
• Poor planning results in PCI conflicts or collisions which impact network performance.
  

 Synchronization signal

• Is used as preamble sequence in LTE for synchronization purpose.
• There are two synchronization signals, Primary (PSS) and Secondary SSS). 
• Primary Synchronization Signal: Determine Cell ID in a cell group 
• Secondary Synchronization Signal: Determine cell group ID
• Both are transmitted in slot 0 and slot 10 in all the frames. 
• It is same as preamble used in earlier systems and used for time, frequency synchronization purpose.
• 168 physical layer cell group & each group consist of 3 ID. Total is 504. That's make it the PCI (0-503). The PCI value is determined by [(3x SSS)+(PSS)].
• In 3G, PSC consists of 512 (0-511)  

P-SS

• Primary Synchronization Signal
• Is used for initial synchronization
• Determine Cell ID in a Cell Group

S-SS

• Secondary Synchronization Signal
• S-SS is used for frame boundary determination
• First signal transmitted upon access network
• Determine the Cell Group ID

S-GW

• Serving Gateway
• For user plane/ data plane
• Also the interface to MME during CSFB 

MME

• Mobility Management Entity
• For control plane 

X2 Interface

• Interface between eNodeB and eNodeB.

S1 Interface

• Interface between eNodeB and core network interface (MME/S-GW).

Uu Interface

• Interface between UE and eNodeB.

Control channel

• This channel carry the information to control, make, maintain and terminate the connection. 
• Used for the transfer of control plane information in LTE.

Data channel

• This channel carry traffic information. 
• Used for the transfer of user plane information.

PBCH

• Physical Broadcast Channel.
• It carries only MIB (Master Information Block)
• It uses QPSK modulation.

PDSCH

• Physical Downlink Shared Channel. It's RB is shared among all other active connections.
• Used to transmit paging message, RRC signalling messages (Signalling radio bearer will use this channel) and transfer application data.
• It also broadcast information that are not transmitted on the PBCH which include System Information Blocks (SIB). 
• This channel uses QPSK, 16QAM, 64QAM modulation types (based on adaptation algorithm by eNodeB - depending on radio condition & buffer capacity).
• For SIB and Paging, it uses QPSK (the most robust scheme).

PDCCH

• Physical Downlink Control Channel.
• Used to carry DCI (Downlink Control Information) - information about number of OFDMA symbols used after decoding PCFICH.
• Normally, a maximum of three or four OFDM symbols can be used for PDCCH.
• These symbols are always at the start of each sub-frame.
• This channel also used to inform the UE about the resource allocation of PCH (paging channel) & DL-SCH (downlink shared channel).
• Among other function, indicating the modulation, coding and hybrid-ARQ (HARQ) information related to DL-SCH.
• It uses QPSK modulation .

PCFICH

• Physical Control Format Indicator Channel
• Physical Channel that carries CFI (Control Format Indicator)
• CFI is a indicator telling how many OFDM symbols are used for carrying control channel (e.g, PDCCH and PHICH) at each sub-frame.
• If CFI is set to be 1 for a sub-frame, it means one symbol (the first symbol) at the sub-frame is used for PDCCH allocation. If CFI is 2, it means two symbols (the first and the second symbol) are used for PDCCH and so on.
•  

PHICH

• Physical Hybrid ARQ Indication Channel.
• A control channel on LTE downlink.
• Used to indicate the reception of PUSCH transmission from a mobile/UE. 
• The PHICH indicates either "ACK" or "NACK", which the UE will decode to find out if
  Carries Hybrid ARQ ACK/NAKs in response to uplink transmissions (whether re-transmission is required or not). 
• Uplink transmission -> from UE to network.

PCH

• Paging channel
• It carries RRC paging message (RRC procedure) using RLC (radio link control) and PCCH (physical control channel)
• The RRC procedure is a procedure to waking up the UE which is Idle (RRC Idle state) using paging cycle (part of SIB2).
• If there is any Paging for it then it will react to it by triggering a RRC connection request message. If no Paging, then it will cont to sleep (saving resources & UE battery).
• Mean, it support the UE discontinuous reception (DRX) to enable UE power saving (DRX cycle is indicated by the network to the UE).

RS

• Reference Signal, used both in uplink and downlink
• Only exist at PHY layer.
• Used as pilot subcarrier in LTE for channel estimation and tracking.
• Is not for delivering any specific information.
• The main purpose is to deliver the reference point for the downlink power. 
• RS are of two types viz. Demodulation RS (DRS) and Sounding RS (SRS)

SRS

• Sounding Reference Signal, used in uplink only.
• SRS is used for channel quality estimation purpose
• Used to estimate UL channel quality and provide input to UL scheduler, as well as help timing advance.

DMRS

• Demodulation Reference Signal, used both in uplink and downlink.
• DRS is used for sync and channel estimation purpose
• This is a reference signal for PUSCH implying that eNodeB would not be able to decode PUSCH if this PUSCH DMRS is bad.
• PUSCH DMRS always takes up the center symbol of a slot (meaning symbol 3 and symbol 10 of a UL sub-frame). 

PRACH

• Physical Random Access Channel used in uplink
• Carries the random access preamble.

PUSCH

• Physical Uplink Shared Channel
• Carries the UL-SCH.

PUCCH

• Physical Uplink Control Channel
• Carries Hybrid ARQ ACK/NAKs in response to downlink transmission (network to UE);
  - Carries Scheduling Request (SR);
  - Carries CQI reports.
•  

 to be cont..

LTE Frame Structure

LTE Frame Structure:

• Type 1 - FDD
• Type 2 - TDD

Type 1 LTE Frame Structure

• The basic type 1 LTE frame has an overall length of 10ms
• This radio frame consists of 10 sub-frame, for each 1ms each sub-frame.
• Each sub-frames has 2 slots (total 20 slots)
• Each slot is 0.5ms

Type 2 LTE Frame Structure 

• The 10 ms frame (1 radio frame) comprises two half frames, each 5 ms long. 
• First half-frame is 0 - 4, second half-frame is from 5 - 9.
• Each sub-frames has 2 slots (total 20 slots) wt 0.5ms for each slot.
• The half-frames are further split into five sub-frames, each 1ms long.
• The sub-frames may be divided into standard sub-frames of special sub-frames. The special sub-frames consist of three fields;
  
      DwPTS - Downlink Pilot Time Slot
      GP - Guard Period
      UpPTS - Uplink Pilot Time Stot.

What is the difference between FDD and TDD?

  
FDD

• Different freq. assigned to UL and DL.
• Transmission can happen at the same time.
• More for coverage.
• Easy to deploy & less expensive.
• Very popular among major operator.

 
TDD

• Same frequency for both UL and DL.
• Transmission at different time period.
• More for capacity.
• Difficult to deploy & quite expensive.

CQI

What is CQI?

• Channel Quality Indicator/ Index (CQI). It an indicator of channel quality. 
• It is an information that UE send to network based on network quality.
• Physical channel that carry this information/ report measurement is:
• PUCCH (if Periodic CQI)
• PUSCH (if Not Periodic).
• In HSDPA, CQI ranging from (0-30). 
• In LTE it ranges from (0-15). 
• The higher number the better quality; 0, 1 is the poorest quality. 
• If network gets high CQI, it transmits larger transport block size & vice versa.
• Some situation like network send larger transport block size even though UE reports low CQI: This probably due to UE failed to decode it (CRC error on UE side). Due to this error, UE send a NACK message to network & network has to re-transmit it which cause more & more block size transmitted (wasting radio resources).
• Same explanation if UE reports good CQI, but network transmit small block size.

 

  
Factors that play important roles in report measurement: 

• Signal-to-noise ratio (SNR)
• Signal-to-interference plus noise ratio (SINR)
• Signal-to-noise plus distortion ratio (SNDR)

Frequency Reuse

What is frequency reuse?

• Frequency reuse is the process of using the same radio frequencies on the radio transmitter within a geographic area that are separated by sufficient distance to cause minimal interference with each other.

Why do we need to re-use frequency?

• To improve coverage & capacity in cellular systems by reducing interference.

How distance between two cells are calculated?

• D=R*(3N)^1/2

D: Distance
R: Cell radius
N: Number of cell per cluster

• Reuse factor

• D/R=3N^1/2

As user increases, channel capacity decreases. Two techniques to improve channel capacity:

• Cell Splitting 
• Cell Sectoring

What is Cell Splitting?

• Process of subdividing a congested cell into smaller cell 
• Reducing the size of cell, the smaller cell cover less area so that more cells required to cover more area 
• Limitation: Handovers are frequent, channel assignments more difficult 
• Solution: Umbrella approach (making sure all area covered well)
  

What is Cell Sectoring?

• Process of replacing an omni to directional (bi-direction) antenna at the base station 
• Kinda process of implementing ICIC (Inter Cell Interference Coordination) 
• Process to change interference distribution as well as to improve the throughput on the edge of cell.  
• Reason: To overcome some limitation like co-channel interference 
• Advantages: Improve S/I ratio, reduce interference, increase capacity, enable to reduce cluster size, freedom in assigning channel. 
• Limitation: Number of antenna at base station increase, decrease in trunk efficiency, loss of traffic, no of handover also increase (since sectoring reduce the coverage area of particular group of channels. 

 
 * Frequency reuse is part of Cell Planning (Frequency Planning)


The general process of LTE Network Design.   

• Information collection 
• Network dimensioning
• Network detailed design
• Cell Planning
  

Under Cell Planning, there are:

1. Frequency Planning 
2. Cell ID Planning 
3. TA Planning 
4. PCI Planning 
5. Neighbor Cell Planning 
6. X2 Planning 
7. PRACH Planning

to be cont.


**The reason why I choose this topic is because of the interview yesterday :)
 
 

Setting CS + PS Mode on Samsung Galaxy Note 4

How to set PS mode only 

• Go to dialler and type *#9090#
• Tap menu and choose back 
• Tap menu again and choose key input and type Q 
• Tap menu again and choose key input and enter input 00002112113 
• Wait for about 15s, and there should be information that SELECT NET_SVC_DOMAIN_PS_ONLY 
• Tap menu again and choose close.
  

How to set CS+PS mode

• Go to dialler and type *#9090# 
• Tap menu and choose back 
• Tap menu again and choose key input and type Q 
• Tap menu again and choose key input and enter input 00002112111 
• Wait for about 15s, and there should be information that SELECT NET_SVC_DOMAIN_PS_ONLY 
• Tap menu again and choose close.
  

Note**
Please follow instruction given accordingly.
 
Tap menu can be found at top right after entering *#9090#
Please disconnect UE from Nemo Outdoor or Exit Nemo Handy before performing steps above. Restart the UE every each setup either PS only or CS+PS mode.

Thank you.