2 -Air Interface Slides

LTE Air-Interface

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The Physical Layer - OFDM

Orthogonal Frequency Division Multiplexing

Each user is assigned a specific frequency resource

Orthogonal Codes

2

The Physical Layer - OFDMA

0.5mS

OFDMA

LTE

Own Cell What is interference? 3

PRACH PARAMETERS- LTE TROUBLE SHOOTING COURSE Unlikely to get own cell interference: Same time/Same Frequency

Frequency

However If you lose timing advance PRACH

Different Time Timing Av

UE 2

Frequency

UE 1

RRC CONNECTED

Evolved Node B (eNB) Time Slot

Data

Time Slot

Packet Scheduling

PS allocates frequency and Time to the UE

Data

UE 1

UE 2

4

PRACH PARAMETERS- LTE TROUBLE SHOOTING COURSE Unlikely to get own cell interference: Same time/Same Frequency

Frequency

However If you lose timing advance PRAC H

Different Time Timing Av

UE 2

Frequency

UE 1

RRC CONNECTED

Evolved Node B (eNB) Time Slot

Data

Time Slot

Packet Scheduling

PS allocates frequency and Time to the UE

Data

UE 1

UE 2

5

Function Evolved Node B (eNB) Evolved Node B

SINR ave =

S I+N I = Iown + Iother

(eNB)

RRC CONNECTED

UE at cell edge Same time slot Same Frequency

PS allocates frequency and Time to the UE

Packet Scheduling

Data

Other Cell Interference Evolved Node B (eNB) Packet Scheduling

Data

PS allocates frequency and Time to the UE

6

Traffic SINR SINR ave =

S I+N

I = Iown + Iother

There are a number of ways of controlling other cell Interference 7

Channel Quality Indicator The CQI indicates the downlink channel quality

CQI=15 CQI=10

Evolved Node B

RF conditions will change as the user moves

(eNB)

CQI=1 CQI=8

Packet Scheduling

Downlink 16-QAM

User reports CQI index8, it informs the eNB that, for the CQI bandwidth being reported, it can support a transport block using 16-QAM modulation and a coding rate of approximately 0.48 with a block error of less than 10%. 8

Channel Quality Indicator In LTE the Physical Resource Block is made up of 12 subcarriers

The CQI indicates the downlink channel quality

Coding Rate OPSK 2bits/Hz

16QAM 4bits/Hz

Normal Frame 84 OFDM symbols (12x7)

64QAM 6bits/Hz

modulation and coding scheme CQI

CQI

64-QAM

Evolved Node B (eNB)

CQI

Resource Element(RE) : The smallest unit made up of 1 symbol x 1 subcarrier QPSK = 2bits 16 QAM = 4bits 64 QAM = 6bits

QPSK

16-QAM

64-QAM

84 OFDM symbols

84 OFDM symbols

84 OFDM symbols

(12x7)

(12x7)

(12x7)

OPSK 2bits/Hz

16QAM 4bits/Hz

64QAM 6bits/Hz

9

SINR - Signal to Interference & Noise Ratio S: indicates the power of measured usable signals. SINR ave =

S I+N I = Iown + Iother

Path Loss

10

SINR - Signal to Interference & Noise Ratio The components of the SINR calculation can be defined as: S: indicates the power of measured usable signals. I: interference signals from other cells in the current system plus own cell N: indicates background noise, which is related to measurement bandwidths and receiver noise coefficients UEs typically use SINR to calculate the CQI (Channel Quality Indicator) they report to the network

CQI

Modulation

1

QPSK

2

Actual coding rate

S I +0.07618 N I = Iown + Iother

-4.46

QPSK

0.11719

-3.75

QPSK

0.18848

-2.55

4

QPSK

308/1024

-1.15

5

QPSK

449/1024

1.75

6

QPSK

602/1024

3.65

7

16QAM

378/1024

5.2

8 9

16QAM 490/1024 Channel Quality Indicator6.1 16QAM 616/1024 7.55

10

64QAM

466/1024

10.85

11

64QAM

567/1024

11.55

12

64QAM

666/1024

12.75

13

64QAM

772/1024

14.55

14

64QAM

873/1024

18.15

15

64QAM

948/1024

19.25

3

SINR ave =

Required SINR

not defined in the 3GPP specs but defined by the UE vendor.

11

coding rate CQI

Modulation

Efficiency

Actual coding rate

Required SINR

1

QPSK

0.1523

0.07618

-4.46

2

QPSK

0.2344

0.11719

-3.75

3

QPSK

0.3770

0.18848

-2.55

4

QPSK

0.6016

308/1024

-1.15

5

QPSK

0.8770

449/1024

1.75

6

QPSK

1.1758

602/1024

3.65

7

16QAM

1.4766

378/1024

5.2

8

16QAM

1.9141

490/1024

6.1

9

16QAM

2.4063

616/1024

7.55

10

64QAM

2.7305

466/1024

10.85

11

64QAM

3.3223

567/1024

11.55

12

64QAM

3.9023

666/1024

12.75

13

64QAM

4.5234

772/1024

14.55

14

64QAM

5.1152

873/1024

18.15

15

64QAM

5.5547

948/1024

19.25

The coding rate indicates how many real data bits are present out of 1024 while the efficiency provides the number of information bits per modulation symbol. 602/1024 = 0.5879 QPSK = 2bits Efficiency= 2x0.5879=1.1758 data bits per symbol 12

Function Evolved Node B (eNB) SINR = 19db

SINR SINR=-4.46dB S I+N I = Iown + Iother

Evolved Node B

SINR ave =

QPSK 2bits/Hz

(eNB)

64QAM 6bits/Hz

16QAM 4bits/Hz

Data

Packet Scheduling

By improving SINR you will increase coverage and throughput SINR

SINR = 19db

SINR=-4.46dB

QPSK 2bits/Hz

Evolved Node B 16QAM 4bits/Hz

64QAM 6bits/Hz

(eNB)

Packet Scheduling

Data

13

Traffic SINR

Point of interest

14

What is meant by adaptive modulation and coding (AMC)?

15

Link adaptation, or adaptive modulation and coding (AMC) Link adaptation, or adaptive modulation and coding (AMC), is a term used in wireless communications to denote the matching of the modulation, coding and other signal and protocol parameters to the conditions on the radio link

If the base station receives the data correctly

Sends the mobile a positive acknowledgement on the physical hybrid ARQ indicator channel (PHICH).

16

Link adaptation, or adaptive modulation and coding (AMC) If the base station receives the data with errors Two ways for it to respond

1. The base station can trigger a non adaptive re-transmission by sending the mobile a negative acknowledgement on the PHICH. The mobile then re-transmits the data with the same parameters that it used first time around.

Scheduling grant Change parameters like uplink modulation scheme QPSK for noisy channels 2. Alternatively, the base station can trigger an adaptive re-transmission by explicitly sending the mobile another scheduling grant. It can do this to change the parameters that the mobile uses for the re-transmission, such as the resource block allocation or the modulation scheme. 17

What is a time slot? Same Time-Different Frequency Own cell interference zero

Physical Resource Block

You need allot of frequencies

time slot 0 Block of Frequencie s Block of Frequencie s Block of Frequencie s Block of Frequencie s

time slot 1 Same Frequency -Different time Own cell interference zero

SINR ave =

S I+N I = Iown + Iother

Block of Frequencie s

18

What is a time slot?

10ms

0.5ms

You need allot of frequencies

time slot 0

time slot 1

Block of Frequencie s Block of Frequencie s Block of Frequencie s Block of Frequencie s Block of Frequencie s

time slot 2

time slot 19

UE1

TTI = 1ms 10 sub channels in 10mS Physical Downlink Control Channel (PDCCH)

TTI = 1mS UE3

19

Transmission Time Interval In UMTS Release '99 the shortest TTI is 10 ms and can be 20 ms, 40 ms, or 80 ms.

In UMTS Release-5 the TTI for HSDPA is reduced to 2ms.

In LTE the TTI for HSDPA is reduced to 1ms This provides the advantage of faster response to link conditions and allows the system to quickly schedule transmissions to mobiles which temporarily enjoy better than usual link conditions Having decoded the bits the receiver can estimate the bit error rate (BER). And because the shortest de-codable transmission is one TTI the shortest period over which BER can be estimated is also one TTI

20

Physical downlink control channel (PDCCH) 10ms 0.5ms

sub channel

time slot 0 of I need to read theBlock PDCCHFrequencie Is it QPSK, 64 QAM, s 16QAM What is the size of the transport Block? Do I do hopping? What about Power control What is my uplink/Down link resources?

time slot 1

time slot 2

time slot 3

time slot 19

UE1

Physical Downlink Shared Channels

Physical downlink control channel (PDCCH)

I cannot change my MCS till I see another PDCCH

Scheduling grant I need to change MCS

At the start of each subframe, a few symbols are reserved for the control information that the base station transmits on the PCFICH, PDCCH and PHICH. The number of control symbols can vary from one subframe to the next, depending on how much control information the base station needs to

21

What is Physical Resource Block? 100 Physical Resource Blocks

0.5ms

time slot 0

Frequencies 1200

12 subcarriers

Block of Frequencie s

12 subcarriers

Block of Frequencie s

12 subcarriers

Block of Frequencie s

12 subcarriers

Block of Frequencie s

12 subcarriers

Block of Frequencie s

In LTE the Physical Resource Block is made up of 12 subcarriers If there are 100 Physical Resource Blocks you would require 1200 frequencies

22

Master Information Block Bandwidth 1.4 (MHz) # of RBs

6

3

5

10

15

20 LTE-Uu Air-Interface

15

25

50

75

100

MIB

Evolved Node B (eNB)

20MHz 15MHz

10MHz 5MHz 3MHz

Subcarriers

72

180

300

600

900

1200

1.4MHz

Channel Bandwidth in Resource Blocks

6 x 12 = 72 Subcarriers 50 x 12 = 600 Subcarriers 23

Master Information Block Logical BCC

PCC H

H

Transport

CCC H

DCC H

DTC H

MCC H

MTC H

BCH PCH

PHYS .

PBC H

PDSC H

DL-SCH MC H

PMCH

REFERENCE SIGNALS

R R

R R

R

0

24

Physical Resource Block 12 subcarriers

0.5ms

Physical Resource Block

time slot 0

Normal Frame 84 OFDM symbols (12x7)

cyclic prefix In the time domain, a guard interval may be added to each symbol to combat inter-OFDM-symbolinterference due to channel delay spread Normal 7 Extended 6

12 subcarriers

Resource Element(RE) : The smallest unit made up of 1 symbol x 1 subcarrier QPSK = 2bits 16 QAM = 4bits 64 QAM = 6bits

Extended 72 OFDM symbols(12x6) 7 symbols

25

Channel Bandwidth Carrier spacing 15 kHz

QPSK = 2bits 16 QAM = 4bits 64 QAM = 6bits

12 subcarriers = 180 kHz

Frequency Domain

Normal Cyclic Prefix

Normal Frame 84 OFDM symbols

)

(12x7

7 symbols = 0.5 ms

12 subcarriers in the frequency domain x Carrier spacing 15 kHz = 180 kHz

100 x 180khz= 18Mhz

Time Domain

Channel Bandwidth in Resource Blocks

50 x 180khz= 9Mhz

26

Channel Bandwidth Channel Bandwidth (MHz)

1.4

3

5

10

15

20

Transmission Bandwidth Config. (RB)

6

15

25

50

75

100

Number of Subcarriers

72

180

300

600

900

1200

Occupied Bandwidth (MHz)

1.08

2.7

4.5

9.0

13.5

18.0

20 MHz Channel Bandwidth (20MHz) Transmission Bandwidth Configuration (RB)

100 x 180khz= 18Mhz

12 subcarriers in the frequency domain x Carrier spacing 15 kHz = 180 kHz 27

Delay spread

Greater the Delay spread Greater the Guard period Extended

Evolved Node B (eNB)

2 1 3

If we sample here

Direct signal

If we sample here

Reflection 1

Last Reflection

Guard Period

Sampling Window

28

Delay spread radio waves travel at speed of light = 300 000000m/s

For LTE, the normal CP length has been set at 4.69 μs, enabling the system to cope with path delay variations up to about 1.4 km. 300m× 4.69 =1.4km Extended cyclic prefix of 16.7 μs for highly dispersive environments. variations up to about 5km 300mx 16.7 =5km 29

Summary so far

12 subcarriers = 180 kHz

Frequency Domain

Normal Cyclic Prefix

Normal Frame 84 OFDM symbols (12x7)

12 subcarriers = 180 kHz

Resource Element 7 symbols = 0.5 ms 2 bits Time Domain 4 bits 6 bits Extended Cyclic Prefix

Extended 72 OFDM symbols(12x6)

6 symbols = 0.5 ms

Resource Block represents the basic unit of resource for LTE Resource Block is a grid: 12 subcarriers in the frequency domain (180 kHz) 6 or 7 symbols in the time domain

72 or 84 Resource Elements per Resource Block Each Resource Element can accommodate 1 modulation symbol, e.g. QPSK, 16QAM, 64QAM Bandwidth 1.4 (MHz)

3

5

10

15

20

# of RBs

6

15

25

50

75

100

Subcarriers

72

180

300

600

900

1200

30

LTE is about 300Mbps (4x4)

12 subcarriers = 180 kHz

12 subcarriers x 7 OFDMA symbols= 84

How do we get 300Mbps? Assume 20 MHz channel bandwidth, normal CP, 4x4 MIMO. 64 QAM modulation and no coding. 25% 20 MHz Overhead about 25% overhead 64 QAM PDCCH Calculate the number of resource elements (RE) in a sub-frame with 20 MHz channel bandwidth:

7 symbols = 0.5 ms

84x100= 840

Time Domain

12 subcarriers = 180 kHz

20 Mhz-100PRB

Normal Cyclic Prefix

reference signal 12 subcarriers x 7 OFDMA symbols x 100 resource blocks x 2 slots= 16800 REs per sub-frame (One mS). Each RE can carry a modulation symbol: 2bits/4bits/6bits

16800 REs per sub-frame x 6 = 100800bits per ms

840x2=16800 RE’s per subframe

7 symbols = 0.5 ms

100.8 Mbits per second per transmitter (1x1) 4x4 MIMO about 400Mbits/s 31

One sub channel

3 PRB 6 x 180kHz =1.080Mhz

3 PRB

32

Time-Division Duplexing (TDD) Normal / Extended

Multicast-broadcast single-frequency network (MBSFN) is a communication channel defined in Long Term Evolution (LTE). It can deliver services such as mobile TV using the LTE infrastructure

Normal Cyclic Prefix

7 symbols = 0.5 ms 12 subcarriers = 180 kHz

Frequency-division duplexing(FDD) Normal / Extended

12 subcarriers = 180 kHz

Frame Structures

Extended Cyclic Prefix

6 symbols = 0.5 ms Time Domain

33

Physical Downlink Shared Channels is shared

Multicast Traffic Channel (MTCH) Dedicated Traffic Channel (DTCH) Dedicated Control CHannel Logical

BCCH

PCCH

DCCH

DTCH MCCH

MTCH

SIB’ s

MIB

Transport

CCCH

BCH PCH

Physical Downlink Control Channel

DL-SCH

MCH

SIB’ s

PDCCH

PHYS.

PBCH

PDSCH

PMCH

REFERENCE SIGNALS

Physical Downlink Shared Channels 7 symbols = 0.5 ms 7 symbols = 0.5 ms

DCCH SIB’ s 7 symbols = 0.5 ms 7 symbols = 0.5 ms

34

12 subcarriers = 180 kHz

Frame Structures

7 symbols = 0.5 ms

For LTE, the normal CP length has been set at 4.69 μs, enabling the system to cope with path delay variations up to about 1.4 km. 35

Frame Structures

12 subcarriers = 180 kHz

Extended Cyclic Prefix

Extended cyclic prefix of 16.7 μs for highly dispersive environments. variations up to about 5km

6 symbols = 0.5 ms Time Domain

36

Frequency selective packet scheduling

• Frequency domain scheduling uses those resource blocks

•

that are not faded Not possible in CDMA based system Carrier bandwidth Resource block

5 Mhz

CDMA based system Frequency

Transmit on those resource blocks that are not faded 37

CQI Reporting  CQI reporting can be: Wideband Reporting

Channel Bandwidth

Configured Sub-band Reporting

Sub-band

CQI

Sub-band

CQI

Sub-band

CQI

Sub-band

CQI

CQI

38

Power control in LTE CQI Modulation

Actual Required coding rate SINR

1

QPSK

0.07618

2

QPSK

0.11719SINR -3.75

3

QPSK

4

QPSK

308/1024

-1.15

5

QPSK

449/1024

1.75

6

QPSK

602/1024

3.65

7

16QAM

378/1024

5.2

8

16QAM

490/1024

6.1

9

16QAM

616/1024

7.55

10

64QAM

466/1024

10.85

11

64QAM

567/1024

11.55

12

64QAM

13

64QAM

14

64QAM

NO POWER CONTROL IN 666/1024 12.75 DOWNLINK. SINR WILL 772/1024 14.55 REDUCE MOVING 873/1024 TOWARDS18.15 CELL EDGE

15

64QAM

948/1024

19.25

SINR 0.18848

-4.46

SINR

SINR

SINR

-2.55

SINR ave =

S I+N I = Iown + Iother

39

Power control in LTE(UPLINK) Cell Edge

Power of the UE

Power control commands

Goes behind a building power control slow to react

Ideal

Distance Practical Errors

SINR

40

Power control in LTE(UPLINK)

41

Power control in LTE(UPLINK) In LTE, UE specific TPC commands can be sent in two modes: • Accumulative TPC commands • Absolute TPC commands In Accumulative TPC command mode, each TPC command signals a power step relative to the previous level and is suitable for fine tuning of transmission power. Two sets of power step values (+1 dB,-1 dB) and (-1 dB,0 dB,+1 dB,+3 dB) are available in this mode. Accumulative TPC command type is available for PUSCH & PUCCH In Absolute TPC command mode, each TPC command signals a power step which is independent of previous power step level. The set of power offset signaled using absolute TPC command mode is (-4 dB,-1 dB,+1 dB,+4 dB ). An absolute TPC command allows UE to adjust its transmission power in a single step. Absolute TPC command available only for PUSCH.. 42

Power control in LTE(UPLINK) In LTE, UE specific TPC commands can be sent in two modes: • Accumulative TPC commands • Absolute TPC commands

PUSCH is TPC command carried by DCI format 0/3/4

Accumulative - Two sets of power step values (+1 dB,-1 dB) and (-1 dB,0 dB,+1 dB,+3 dB) are available in this mode. Accumulative TPC command type is available for PUSCH & PUCCH

DCI format 3A

43

Power Control & Downlink Control Indicator (DCI) It is DCI which carries those detailed information like "which resource block carries your data ?" and "what kind of demodulation scheme you have to use to decode data ?" and some other additional information including power control. .

Format 3 - TPC Commands for PUCCH and PUSCH with 2 bit power adjustment - Power Control Only

Format 3A - TPC Commands for PUCCH and PUSCH with 1 bit power adjustment Power Control Only DCI format 3A

Breathing

PUCCH PUSCH

DCI format 3A

44

Control Channel (PDCCH) It means the reciever first have to decode DCI and based on the information you got from the DCI you can decode the real data. It means without DCI, decoding the data delivered to you is impossible

Cannot read the PDCCH

Path Loss

45

Power Headroom Report eNB uses Power Headroom Report (PHR) sent by the UE to schedule uplink transmission resources to different UEs in an appropriate manner. With PHR, eNB gets an idea about how much more bandwidth UE is capable of using in a subframe How much uplink bandwidth can a UE can use for a specific subframe. Power Headroom Report

Power headroom indicates how much transmission power left for a UE to use in addition to the power being used by current transmission 46

Downlink Control Indicator (DCI)

47

Questions

48

Questions 1. How many PRB’s (Physical Resource Block)? do you have in 0.5ms in 10 MHz bandwidth? a. b. c. d.

10 100 50 None of the above

2. How many sub channels at there in one radio frame a. b. c. d.

10 20 5 None of the above

49

Questions 3. TTI refers to the duration of a transmission on the radio link? TRUE FALSE

4. What is the bandwidth of one PRB (Physical Resource Block)? a. 20Mhz b. 10Mhz c. 15khz d. 180khz

50

Questions 3.In an extended frame you have 84 Resource Elements per Resource Block? TRUE FALSE

51

In Closing

52