07_lte physical channels and procedures
DESCRIPTION
Good doc about LTETRANSCRIPT
HSDPA Channels and FeaturesPrimary colours:
Supporting colours:
LTE Air Interface Course
* © Nokia Siemens Networks
At the end of this module, you will be able to:
Outline the relation between the layers (e.g. MAC, RLC) and the channels in LTE
Restate the definition of the physical resource block
Briefly explain the time structure for both FDD and TDD modes of LTE
Introduce in overview all the LTE channels
Understand the cell search procedure and the LTE channels involved
Discuss the random access process in LTE and the physical channels involved
Explain the downlink transmission process and the physical channels involved
Describe the uplink transmission process and the physical channels involved
Underline the TDD physical channels
Module Objectives
Comments regarding chapter 7
The chapter is long and in principle could take 1 day for the presentation. In my opinion this chapter is the key of understanding LTE, because it is discussing the physical channels. As one can see, several issues are to be solved at the physical layer. This is because the complexity of the system has been moved from higher layers to the physical layer.
The chapter is having 197 slides including the TDD physical channels explanation (24 slides). One may compare this size with the chapter from the 3GRPLS which is explaining the UMTS physical channels and which is having 193 slides (without TDD mode).
From the didactical point of view I recommend that the trainer is adapting to the target people from the course. If there are people which like to know only the big picture without details then only the first 5 sections should be presented (including the Overview of the physical channels).
The next level could consider experts which really need to understand the physical channels because this knowledge will be further relevant for understanding the network planning and the parameters planning. In this case some parts could still be dropped from the presentation. For example some target groups may not be interested in the TDD physical channels (China only) or in the sounding reference signals in the UL (which will be probably not implemented at the beginning).
* © Nokia Siemens Networks
Layer and Channels Relationship
Cell Search
Initial Access
DCCH
DTCH
The radio interface is composed of different layers in order to set up, reconfigure and release the radio bearer services.
The protocol layer is composed of physical layer (layer 1), data link layer (layer2), and the network layer (layer3).
In the E-UTRAN layer 2 is divided into two sub-layers: Medium Access Control (MAC) and Radio Link Control (RLC) protocol.
Layer 3 consists of two protocols, called Radio Resource Control (RRC) and Packet Data Convergence Protocol (PDCP).
The top to down arrows represent downlink (DL) channels from eNB to UE..
Down to top arrows are uplink channel (UL) from UE to eNB.
Most of common and dedicated channels transmitted from RLC to the physical layer share the same transport channel. There are no dedicated channels in transport and physical layer.
Some physical channels are existing solely in the physical layer itself, such as PHICH, PCFICH and PDCCH.
* © Nokia Siemens Networks
Layer and Channels Relationship
Cell Search
Initial Access
Physical Resource Block or Resource Block ( PRB or RB):
12 subcarriers in frequency domain x 1 slot period in time domain.
0 1 2 3 4 5 6 0 1 2 3 4 5 6
Subcarrier 1
Subcarrier 12
180 KHz
1 slot
1 slot
Resource Element ( RE):
Theoretical minimum capacity allocation unit.
1 RE is the equivalent of 1 modulation symbol on a subcarrier, i.e. 2 bits for QPSK, 4 bits for 16QAM and 6 bits for 64QAM.
Resource Element
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
* © Nokia Siemens Networks
Physical Resource Blocks
In both the downlink and uplink direction, data is allocated to users in terms of resource blocks (RBs).
A resource block consists of 12 consecutive subcarriers in the frequency domain, that are reserved for the duration of one 0.5 millisecond time slot.
..
..
Resource block
During each TTI, resource blocks for different UEs are scheduled in the eNodeB
* © Nokia Siemens Networks
Layer and Channels Relationship
Cell Search
Initial Access
It provides the basic bit transmission functionality over air
LTE physical layer based on OFDMA downlink and SC-FDMA in uplink direction
This is the same for both FDD and TDD mode of operation
There is no macro-diversity in use
System is reuse 1, single frequency network operation is feasible
No frequency planning required
There are no dedicated physical channels anymore, as all resource mapping is dynamically driven by the scheduler
FDD
LTE Physical Layer Structure – Frame Structure (FDD)
FDD Frame structure ( also called Type 1 Frame) is common to both uplink and downlink.
Divided into 20 x 0.5ms slots
Structure has been designed to facilitate short round trip time
- Frame length =10 ms
- FDD: 10 ms sub-frame for UL and 10 ms sub-frame for DL
- 1 Frame = 20 slots of 0.5ms each
- 1 slot = 7 ( normal CP) or 6 symbols ( extended CP)
SF: SubFrame
s: slot
Sy: symbol
Layer and Channels Relationship
Cell Search
Initial Access
LTE Physical Layer Structure – Frame Structure (TDD)
TDD has a single frame structure: same as FDD but with some specific fields to enable also TD-SCDMA co-existence (China):
Common frame structure and slot duration allows to parameterize the LTE TDD mode of operation so that the site can have compatible UL and DL split (static parameter)
Each half frame carries six subframes and three specialized fields ( inherited from TD-SCDMA): DwPTS, GP, UpPTS
Subframe 0 and DwPTS are reserved for downlink; subframe1 and UpPTS are reserved for UL. Remaining fields are dynamically assigned between UL and DL.
Also called Frame Type 2. TDD may change between UL and DL either with 5 or 10 ms period
DwPTS: Downlink Pilot time Slot
UpPSS: Uplink Pilot Time Slot
GP: Guard Period to separate between UL/DL
Downlink Subframe
Uplink Subframe
subframe 0
subframe 1
Subframe 5
half frame
As the single frequency block is shared in time domain between UL and DL the transmission in TDD is not continuous. All UL transmission need to be on hold while any downlink resource it is used and the other way around.
Switching between transmission directions has a small hardware delay (for both UE and NodeB) and needs to be compensated. To control the switching between the UL and DL a guard period GP is allocated which compensates for the maximum propagation delay of interfering components.
Description of the switching mechanism:
In LTE TDD there is maximally one UL->DL and one DL->UL transition per 5ms period (half frame)
UL-> DL transition
Is done for all intra-cell UEs by the process of time alignment. The NodeB instructs each UE to use a specific time offset so that all UEs signals are aligned when they arrive at Node-B. So UL is synchronous very similar to FDD (intra cell case only!)
DL->UL transition
The interference coming from other DL neighbors has to be avoided. The same methodology like in TD-SCDMA is applied: a special subframe is introduced that is divided into 3 parts:
DwPTS, GP and UpPTS (Uplink Pilot Time slot). This special subframe replace the subframe 1. The individual time duration in OFDM symbols of the special subframe parts are adjustable. The GP implements the DL->UL transition and the GP has to be large enough to cover the propagation delay of DL interferers.
DwPTS is considered as a “normal” DL subframe and carries control information as well as data for those cases when sufficient duration is configured.
UpPTS is primarily intended for sounding reference signals (SRS) transmission from UE
* © Nokia Siemens Networks
There are 7 frame configurations, according to different DL/UL partition
1 frame = 10ms
1 subframe = 10ms
TDD frame structure (2/2)
Downlink / Uplink ratio can vary from 1/3 (Frame configuration = 0) to 8/1 (Frame configuration = 1), depending on the service requirements of the carrier
Frame always starts with a downlink subframe, used for advertising the frame descriptor, PCFICH and PDCCH. UE hence learns the frame structure in this subframe.
3rd frame is always used for uplink.
When switching from downlink to uplink, there is need for a special switching subframe. No special subframe is used when switching from uplink to downlink.
* © Nokia Siemens Networks
Special subframe
UE always needs a guard period in order to switch from receiver to transmitter.
The guard period includes RTD (Round Trip Delay).
eNodeB
UE
PT
PT
SP
Downlink
Downlink
Uplink
Uplink
UE has switched to transmission and has begun UL subframe
Start of UL subframe reaches at eNodeB
PT = Propagation Time
SP = Switching Period
Layer and Channels Relationship
Cell Search
Initial Access
General Comment
Please note that the target of this section is to show the “big picture” of all the physical channels involved in LTE. The details for every channel are shown in the consequent following sections. Therefore it is recommended that here the level of detail is kept as small as possible.
To show all steps and channels involved, one scenario is selected.
* © Nokia Siemens Networks
and would like to download something
from the Internet
What are all the steps and the physical channels involved ??
Scenario:
eNodeB
Find one cell
Global cell id
(Time-slot & Frequency synchronisation
(Frame synchronisation
(MIB* – DL system bandwidth, PHICH configuration)
3. DL Reference Signals
PHICH = Physical HARQ Indicator Channel
The first steps after switching on the mobile are the following:
Primary Synchronization Signal PSS– from which the mobile can acquire frequency and time-slot synchronization. The synchronization is absolutely necessary, otherwise the mobile cannot read the rest of the physical channels. Also from the PSS the mobile is learning the cell identity – which could have values 0,1 or 2 in LTE. The cell identities are used to differentiate between different cells
Secondary Synchronizations Signal SSS – from which the mobile can learn what is the frame structure (10 ms in LTE). Also the physical cell id group with values from 1 to 168 is achieved. The physical cell id together with the group are used to separate the cells in LTE.
DL reference signals – they have almost the same functionality like the CPICH (common pilot channel) in UMTS. Used for channel estimation and measurements. Details of the measurements are provided in chapter 8.
PBCH – Physical Broadcast Channel. From this channel the UE is learning about the system information. Please note that in LTE the PBCH is designed to have minimum possible information (for coverage reasons mainly). Therefore the rest of system information which is organized in MIB = Master information Block and SIBs = System Information Blocks is now sent on the Physical Downlink Shared Channel PDSCH. From PBCH the UE is learning the system bandwidth 1.4, 3, … 20 MHz and the PHICH = Physical HARQ Indication Channel configuration (explained later).
* © Nokia Siemens Networks
Challenge:
The PBCH contains only the MIB (Master Information Block) → the SIBs(System Information Blocks) are on the PDSCH (Physical Downlink Shared Channel)!!
need to read SIBs on PDSCH
The UE should read PDSCH but it doesn't know which resource blocks are reserved for it and where are they placed (in time and frequency)
Solution:
PCFICH (Physical Control Format Indicator Channel) indicates the size of PDCCH (Physical Downlink Control Channel)
the PDCCH is indicating which resource blocks are scheduled and where are located
Cell Search
5. PCFICH Physical Control Format Indicator Channel
(How many symbols (1,2,3) in the beginning of the sub-frame are for PDCCH)
6. PDCCH Physical Downlink Control Channel
(Resource allocation for PDSCH )
(*SIBs: Cell global ID, parameters for cell selection reselection, … )
eNodeB
UE
*SIB = System Information Block
Because the SIBs are placed on the PDSCH then the mobile should now read the PDSCH. The steps are the following:
5. PCFICH (Physical Control Format Indicator Channel) indicates how many symbols in the beggining of each subframe (1 subframe is having 1 ms in LTE) are allocated for the PDCCH. This is beacuse the size of the PDCCH may be changed based on several variables like cell bandwith, cell load ...
6. PDCCH (Physical Downlink Control Channel) – from this channel the mobile can learn: what are the physical resources allocated for the mobile and where are they placed in the time and frequency
7. Finally the UE may read the PDSCH to read the MIB and the SIBs. For a detailed list of SIBs please reffer to the section Downlink Transmission
After the mobile is reading the system information from the PDSCH the next step is the so called cell selection and reselection. The basic ideea is that the UE is measuring several cells and is selecting the best one with the help of the thresholds from the system information
* © Nokia Siemens Networks
The UE has selected one cell → “camping on the cell” procedure
The “camping on the cell” procedure will be explained later
(more details in chapter 8)
The UE can start the initial access
What are the Next Steps?
UE
eNodeB
eNodeB
UE
Challenge:
Several UEs may send the same preamble. How to solve the collision ?
Solution:
For the initial random access the steps are the following
8A – the mobile is selecting randomly one preamble. There are in total 64 preambles available preambles in one cell. In this case with A it is intended to note the first random preamble
8C – If no answer is received from the Node-b then the mobile is repeating the preamble. In this example with C is noted the 3rd preamble. That means that the assumption is that after three preambles the UE is receiving an answer from the Node-B
* © Nokia Siemens Networks
Random Access (2/2)
9. PCFICH Physical Control Format Indicator Channel
(How many symbols (1,2,3) in the beginning of the sub-frame are for PDCCH)
10. PDCCH Physical Downlink Control Channel
(Resource allocation for PDSCH)
(Random Access response,
C-RNTI *)
eNodeB
UE
*C-RNTI = Cell Radio Network Temporary Identity
In the next step the mobile should receive the answer to the preamble. However, the answer is sent on the PDSCH. Therefore the steps are as follows:
9. PCFICH (Physical Control Format Indicator Channel) indicates how many symbols in the beggining of each subframe (1 subframe is having 1 ms in LTE) are allocated for the PDCCH. This is beacuse the size of the PDCCH may be changed based on several variables like cell bandwith, cell load ...
10. PDCCH (Physical Downlink Control Channel) – from this channel the mobile can learn: what are the physical resources allocated for the mobile and where are they placed in the time and frequency
11. PDSCH – containing the random access response. In this message the id of the transmitted preamble should be included. Also the Node-B allocates to the mobile the C-RNTI = Cell Radio Network temporary Identity. C-RNTI is allocated by the eNB serving a UE when it is in active mode (RRC_CONNECTED). This is a temporary identity for the user only valid within the serving cell of the UE. It is exclusively used for radio management procedures.
* © Nokia Siemens Networks
8. PRACH Preamble
11. PDSCH Physical Downlink Shared Channel
12. PUSCH Physical Uplink Shared Channel
(Random Access response, ID of the received preamble, UL resources for TX,
C-RNTI)
(Contention Resolution,
eNodeB
UE
TMSI = Temporary Mobile Subscriber Identity
Please note how the contention resolution is done:
If several UEs are colliding then they receive all of them the same UL grant from the NodeB so they sent the RRC Connection Request (sent on Uplink PUSCH) message with high probablity of error.
If the Node-B cannot decode the RRC (because of high interference), then all the mobiles involved in the colission should restart the random access (restart from message 8A).
If the Node-B detects the message of at least on mobile, it is then sending the answer with the identity of this terminal. All the other mobiles involved in the collision and not receiving the answer will restart the initial access.
The message flow is as following:
11. PDSCH – containing the random access response. In this message the id of the transmitted preamble should be included. Also the Node-B allocates to the mobile the C-RNTI = Cell Radio Network temporary Identity. Also very important – this message is containing the UL grant, that is, indicating to the UE what are the resources that the mobile could use in the UL for PUSCH (Physical UL Shared Channel)
12. PUSCH Physical UL Shared Channel. The mobile is sending the actual higher layer message – RRC Connection Request. The message should include the C-RNTI allocated in 11 and also TMSI = Temporary Mobile Subscriber Identity (or a random number if TMSI not available). This is because IMSI should be never sent on the air interface.
13. PDSCH – contention resolution message. As explained this message is only sent if the Node-B could decode the message number 12 from the mobile. The message should contain the C-RNTI allocated in 11 and also TMSI or a the same random number sent by the mobile.
* © Nokia Siemens Networks
Next steps:
What are the Next Steps?
UE
Now I am connected so I can download the web page from the Internet.
www.nsn.com
eNodeB
(CQI based on DL reference signals measurements)
3. PCFICH Physical Control Format Indicator Channel
(How many symbols (1,2,3) in the beginning of the sub-frame are for PDCCH)
4. PDCCH Physical Downlink Control Channel
(Downlink assignment for PDSCH:
Modulation & coding, resource blocks)
(user data -> initial transmission)
(ACK/ NACK for HARQ)
(user data → eventual re-transmission)
1. DL Reference signals
HARQ = Hybrid Automatic Repeat Request
The message flow for the DL transmission is as following (please note that for simplicity the notation of the messages counter is restarted from 1).
DL reference Signals - Used for channel estimation and measurements
PUCCH – used in this in scenario to indicate the CQI based on the measurements performed in the previous step. Please note that PUCCH or PUSCH could be used depending on whether the mobile is having some UL data transmission or not. For details please refer to the section “UL Transmission”
PCFICH indicates how many symbols in the beggining of each subframe (1 subframe is having 1 ms in LTE) are allocated for the PDCCH. This is beacuse the size of the PDCCH may be changed based on several variables like cell bandwith, cell load ...
PDCCH (Physical Downlink Control Channel) – from this channel the mobile can learn: what are the physical resources allocated for the mobile and where are they placed…
Supporting colours:
LTE Air Interface Course
* © Nokia Siemens Networks
At the end of this module, you will be able to:
Outline the relation between the layers (e.g. MAC, RLC) and the channels in LTE
Restate the definition of the physical resource block
Briefly explain the time structure for both FDD and TDD modes of LTE
Introduce in overview all the LTE channels
Understand the cell search procedure and the LTE channels involved
Discuss the random access process in LTE and the physical channels involved
Explain the downlink transmission process and the physical channels involved
Describe the uplink transmission process and the physical channels involved
Underline the TDD physical channels
Module Objectives
Comments regarding chapter 7
The chapter is long and in principle could take 1 day for the presentation. In my opinion this chapter is the key of understanding LTE, because it is discussing the physical channels. As one can see, several issues are to be solved at the physical layer. This is because the complexity of the system has been moved from higher layers to the physical layer.
The chapter is having 197 slides including the TDD physical channels explanation (24 slides). One may compare this size with the chapter from the 3GRPLS which is explaining the UMTS physical channels and which is having 193 slides (without TDD mode).
From the didactical point of view I recommend that the trainer is adapting to the target people from the course. If there are people which like to know only the big picture without details then only the first 5 sections should be presented (including the Overview of the physical channels).
The next level could consider experts which really need to understand the physical channels because this knowledge will be further relevant for understanding the network planning and the parameters planning. In this case some parts could still be dropped from the presentation. For example some target groups may not be interested in the TDD physical channels (China only) or in the sounding reference signals in the UL (which will be probably not implemented at the beginning).
* © Nokia Siemens Networks
Layer and Channels Relationship
Cell Search
Initial Access
DCCH
DTCH
The radio interface is composed of different layers in order to set up, reconfigure and release the radio bearer services.
The protocol layer is composed of physical layer (layer 1), data link layer (layer2), and the network layer (layer3).
In the E-UTRAN layer 2 is divided into two sub-layers: Medium Access Control (MAC) and Radio Link Control (RLC) protocol.
Layer 3 consists of two protocols, called Radio Resource Control (RRC) and Packet Data Convergence Protocol (PDCP).
The top to down arrows represent downlink (DL) channels from eNB to UE..
Down to top arrows are uplink channel (UL) from UE to eNB.
Most of common and dedicated channels transmitted from RLC to the physical layer share the same transport channel. There are no dedicated channels in transport and physical layer.
Some physical channels are existing solely in the physical layer itself, such as PHICH, PCFICH and PDCCH.
* © Nokia Siemens Networks
Layer and Channels Relationship
Cell Search
Initial Access
Physical Resource Block or Resource Block ( PRB or RB):
12 subcarriers in frequency domain x 1 slot period in time domain.
0 1 2 3 4 5 6 0 1 2 3 4 5 6
Subcarrier 1
Subcarrier 12
180 KHz
1 slot
1 slot
Resource Element ( RE):
Theoretical minimum capacity allocation unit.
1 RE is the equivalent of 1 modulation symbol on a subcarrier, i.e. 2 bits for QPSK, 4 bits for 16QAM and 6 bits for 64QAM.
Resource Element
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
0 1 2 3 4 5 6 0 1 2 3 4 5 6
* © Nokia Siemens Networks
Physical Resource Blocks
In both the downlink and uplink direction, data is allocated to users in terms of resource blocks (RBs).
A resource block consists of 12 consecutive subcarriers in the frequency domain, that are reserved for the duration of one 0.5 millisecond time slot.
..
..
Resource block
During each TTI, resource blocks for different UEs are scheduled in the eNodeB
* © Nokia Siemens Networks
Layer and Channels Relationship
Cell Search
Initial Access
It provides the basic bit transmission functionality over air
LTE physical layer based on OFDMA downlink and SC-FDMA in uplink direction
This is the same for both FDD and TDD mode of operation
There is no macro-diversity in use
System is reuse 1, single frequency network operation is feasible
No frequency planning required
There are no dedicated physical channels anymore, as all resource mapping is dynamically driven by the scheduler
FDD
LTE Physical Layer Structure – Frame Structure (FDD)
FDD Frame structure ( also called Type 1 Frame) is common to both uplink and downlink.
Divided into 20 x 0.5ms slots
Structure has been designed to facilitate short round trip time
- Frame length =10 ms
- FDD: 10 ms sub-frame for UL and 10 ms sub-frame for DL
- 1 Frame = 20 slots of 0.5ms each
- 1 slot = 7 ( normal CP) or 6 symbols ( extended CP)
SF: SubFrame
s: slot
Sy: symbol
Layer and Channels Relationship
Cell Search
Initial Access
LTE Physical Layer Structure – Frame Structure (TDD)
TDD has a single frame structure: same as FDD but with some specific fields to enable also TD-SCDMA co-existence (China):
Common frame structure and slot duration allows to parameterize the LTE TDD mode of operation so that the site can have compatible UL and DL split (static parameter)
Each half frame carries six subframes and three specialized fields ( inherited from TD-SCDMA): DwPTS, GP, UpPTS
Subframe 0 and DwPTS are reserved for downlink; subframe1 and UpPTS are reserved for UL. Remaining fields are dynamically assigned between UL and DL.
Also called Frame Type 2. TDD may change between UL and DL either with 5 or 10 ms period
DwPTS: Downlink Pilot time Slot
UpPSS: Uplink Pilot Time Slot
GP: Guard Period to separate between UL/DL
Downlink Subframe
Uplink Subframe
subframe 0
subframe 1
Subframe 5
half frame
As the single frequency block is shared in time domain between UL and DL the transmission in TDD is not continuous. All UL transmission need to be on hold while any downlink resource it is used and the other way around.
Switching between transmission directions has a small hardware delay (for both UE and NodeB) and needs to be compensated. To control the switching between the UL and DL a guard period GP is allocated which compensates for the maximum propagation delay of interfering components.
Description of the switching mechanism:
In LTE TDD there is maximally one UL->DL and one DL->UL transition per 5ms period (half frame)
UL-> DL transition
Is done for all intra-cell UEs by the process of time alignment. The NodeB instructs each UE to use a specific time offset so that all UEs signals are aligned when they arrive at Node-B. So UL is synchronous very similar to FDD (intra cell case only!)
DL->UL transition
The interference coming from other DL neighbors has to be avoided. The same methodology like in TD-SCDMA is applied: a special subframe is introduced that is divided into 3 parts:
DwPTS, GP and UpPTS (Uplink Pilot Time slot). This special subframe replace the subframe 1. The individual time duration in OFDM symbols of the special subframe parts are adjustable. The GP implements the DL->UL transition and the GP has to be large enough to cover the propagation delay of DL interferers.
DwPTS is considered as a “normal” DL subframe and carries control information as well as data for those cases when sufficient duration is configured.
UpPTS is primarily intended for sounding reference signals (SRS) transmission from UE
* © Nokia Siemens Networks
There are 7 frame configurations, according to different DL/UL partition
1 frame = 10ms
1 subframe = 10ms
TDD frame structure (2/2)
Downlink / Uplink ratio can vary from 1/3 (Frame configuration = 0) to 8/1 (Frame configuration = 1), depending on the service requirements of the carrier
Frame always starts with a downlink subframe, used for advertising the frame descriptor, PCFICH and PDCCH. UE hence learns the frame structure in this subframe.
3rd frame is always used for uplink.
When switching from downlink to uplink, there is need for a special switching subframe. No special subframe is used when switching from uplink to downlink.
* © Nokia Siemens Networks
Special subframe
UE always needs a guard period in order to switch from receiver to transmitter.
The guard period includes RTD (Round Trip Delay).
eNodeB
UE
PT
PT
SP
Downlink
Downlink
Uplink
Uplink
UE has switched to transmission and has begun UL subframe
Start of UL subframe reaches at eNodeB
PT = Propagation Time
SP = Switching Period
Layer and Channels Relationship
Cell Search
Initial Access
General Comment
Please note that the target of this section is to show the “big picture” of all the physical channels involved in LTE. The details for every channel are shown in the consequent following sections. Therefore it is recommended that here the level of detail is kept as small as possible.
To show all steps and channels involved, one scenario is selected.
* © Nokia Siemens Networks
and would like to download something
from the Internet
What are all the steps and the physical channels involved ??
Scenario:
eNodeB
Find one cell
Global cell id
(Time-slot & Frequency synchronisation
(Frame synchronisation
(MIB* – DL system bandwidth, PHICH configuration)
3. DL Reference Signals
PHICH = Physical HARQ Indicator Channel
The first steps after switching on the mobile are the following:
Primary Synchronization Signal PSS– from which the mobile can acquire frequency and time-slot synchronization. The synchronization is absolutely necessary, otherwise the mobile cannot read the rest of the physical channels. Also from the PSS the mobile is learning the cell identity – which could have values 0,1 or 2 in LTE. The cell identities are used to differentiate between different cells
Secondary Synchronizations Signal SSS – from which the mobile can learn what is the frame structure (10 ms in LTE). Also the physical cell id group with values from 1 to 168 is achieved. The physical cell id together with the group are used to separate the cells in LTE.
DL reference signals – they have almost the same functionality like the CPICH (common pilot channel) in UMTS. Used for channel estimation and measurements. Details of the measurements are provided in chapter 8.
PBCH – Physical Broadcast Channel. From this channel the UE is learning about the system information. Please note that in LTE the PBCH is designed to have minimum possible information (for coverage reasons mainly). Therefore the rest of system information which is organized in MIB = Master information Block and SIBs = System Information Blocks is now sent on the Physical Downlink Shared Channel PDSCH. From PBCH the UE is learning the system bandwidth 1.4, 3, … 20 MHz and the PHICH = Physical HARQ Indication Channel configuration (explained later).
* © Nokia Siemens Networks
Challenge:
The PBCH contains only the MIB (Master Information Block) → the SIBs(System Information Blocks) are on the PDSCH (Physical Downlink Shared Channel)!!
need to read SIBs on PDSCH
The UE should read PDSCH but it doesn't know which resource blocks are reserved for it and where are they placed (in time and frequency)
Solution:
PCFICH (Physical Control Format Indicator Channel) indicates the size of PDCCH (Physical Downlink Control Channel)
the PDCCH is indicating which resource blocks are scheduled and where are located
Cell Search
5. PCFICH Physical Control Format Indicator Channel
(How many symbols (1,2,3) in the beginning of the sub-frame are for PDCCH)
6. PDCCH Physical Downlink Control Channel
(Resource allocation for PDSCH )
(*SIBs: Cell global ID, parameters for cell selection reselection, … )
eNodeB
UE
*SIB = System Information Block
Because the SIBs are placed on the PDSCH then the mobile should now read the PDSCH. The steps are the following:
5. PCFICH (Physical Control Format Indicator Channel) indicates how many symbols in the beggining of each subframe (1 subframe is having 1 ms in LTE) are allocated for the PDCCH. This is beacuse the size of the PDCCH may be changed based on several variables like cell bandwith, cell load ...
6. PDCCH (Physical Downlink Control Channel) – from this channel the mobile can learn: what are the physical resources allocated for the mobile and where are they placed in the time and frequency
7. Finally the UE may read the PDSCH to read the MIB and the SIBs. For a detailed list of SIBs please reffer to the section Downlink Transmission
After the mobile is reading the system information from the PDSCH the next step is the so called cell selection and reselection. The basic ideea is that the UE is measuring several cells and is selecting the best one with the help of the thresholds from the system information
* © Nokia Siemens Networks
The UE has selected one cell → “camping on the cell” procedure
The “camping on the cell” procedure will be explained later
(more details in chapter 8)
The UE can start the initial access
What are the Next Steps?
UE
eNodeB
eNodeB
UE
Challenge:
Several UEs may send the same preamble. How to solve the collision ?
Solution:
For the initial random access the steps are the following
8A – the mobile is selecting randomly one preamble. There are in total 64 preambles available preambles in one cell. In this case with A it is intended to note the first random preamble
8C – If no answer is received from the Node-b then the mobile is repeating the preamble. In this example with C is noted the 3rd preamble. That means that the assumption is that after three preambles the UE is receiving an answer from the Node-B
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Random Access (2/2)
9. PCFICH Physical Control Format Indicator Channel
(How many symbols (1,2,3) in the beginning of the sub-frame are for PDCCH)
10. PDCCH Physical Downlink Control Channel
(Resource allocation for PDSCH)
(Random Access response,
C-RNTI *)
eNodeB
UE
*C-RNTI = Cell Radio Network Temporary Identity
In the next step the mobile should receive the answer to the preamble. However, the answer is sent on the PDSCH. Therefore the steps are as follows:
9. PCFICH (Physical Control Format Indicator Channel) indicates how many symbols in the beggining of each subframe (1 subframe is having 1 ms in LTE) are allocated for the PDCCH. This is beacuse the size of the PDCCH may be changed based on several variables like cell bandwith, cell load ...
10. PDCCH (Physical Downlink Control Channel) – from this channel the mobile can learn: what are the physical resources allocated for the mobile and where are they placed in the time and frequency
11. PDSCH – containing the random access response. In this message the id of the transmitted preamble should be included. Also the Node-B allocates to the mobile the C-RNTI = Cell Radio Network temporary Identity. C-RNTI is allocated by the eNB serving a UE when it is in active mode (RRC_CONNECTED). This is a temporary identity for the user only valid within the serving cell of the UE. It is exclusively used for radio management procedures.
* © Nokia Siemens Networks
8. PRACH Preamble
11. PDSCH Physical Downlink Shared Channel
12. PUSCH Physical Uplink Shared Channel
(Random Access response, ID of the received preamble, UL resources for TX,
C-RNTI)
(Contention Resolution,
eNodeB
UE
TMSI = Temporary Mobile Subscriber Identity
Please note how the contention resolution is done:
If several UEs are colliding then they receive all of them the same UL grant from the NodeB so they sent the RRC Connection Request (sent on Uplink PUSCH) message with high probablity of error.
If the Node-B cannot decode the RRC (because of high interference), then all the mobiles involved in the colission should restart the random access (restart from message 8A).
If the Node-B detects the message of at least on mobile, it is then sending the answer with the identity of this terminal. All the other mobiles involved in the collision and not receiving the answer will restart the initial access.
The message flow is as following:
11. PDSCH – containing the random access response. In this message the id of the transmitted preamble should be included. Also the Node-B allocates to the mobile the C-RNTI = Cell Radio Network temporary Identity. Also very important – this message is containing the UL grant, that is, indicating to the UE what are the resources that the mobile could use in the UL for PUSCH (Physical UL Shared Channel)
12. PUSCH Physical UL Shared Channel. The mobile is sending the actual higher layer message – RRC Connection Request. The message should include the C-RNTI allocated in 11 and also TMSI = Temporary Mobile Subscriber Identity (or a random number if TMSI not available). This is because IMSI should be never sent on the air interface.
13. PDSCH – contention resolution message. As explained this message is only sent if the Node-B could decode the message number 12 from the mobile. The message should contain the C-RNTI allocated in 11 and also TMSI or a the same random number sent by the mobile.
* © Nokia Siemens Networks
Next steps:
What are the Next Steps?
UE
Now I am connected so I can download the web page from the Internet.
www.nsn.com
eNodeB
(CQI based on DL reference signals measurements)
3. PCFICH Physical Control Format Indicator Channel
(How many symbols (1,2,3) in the beginning of the sub-frame are for PDCCH)
4. PDCCH Physical Downlink Control Channel
(Downlink assignment for PDSCH:
Modulation & coding, resource blocks)
(user data -> initial transmission)
(ACK/ NACK for HARQ)
(user data → eventual re-transmission)
1. DL Reference signals
HARQ = Hybrid Automatic Repeat Request
The message flow for the DL transmission is as following (please note that for simplicity the notation of the messages counter is restarted from 1).
DL reference Signals - Used for channel estimation and measurements
PUCCH – used in this in scenario to indicate the CQI based on the measurements performed in the previous step. Please note that PUCCH or PUSCH could be used depending on whether the mobile is having some UL data transmission or not. For details please refer to the section “UL Transmission”
PCFICH indicates how many symbols in the beggining of each subframe (1 subframe is having 1 ms in LTE) are allocated for the PDCCH. This is beacuse the size of the PDCCH may be changed based on several variables like cell bandwith, cell load ...
PDCCH (Physical Downlink Control Channel) – from this channel the mobile can learn: what are the physical resources allocated for the mobile and where are they placed…