Lte Air Interface 2

8/3/2019 Lte Air Interface 2

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Welcome to the Directed Remote Learning session of

LTE Air Interface Basics

During this course, we will:

Compare the LTE air interface in terms of Frequency and Time withGSM and UMTS

Compare the capacity of the LTE air interface with GSM and UMTS


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In GSM, the Radio Network portion of the mobile telephone network is called the BSS

(Base Station Subsystem).

2G (GSM) and 2.5G (GERAN)

It contains:

BTS BSC

BTS

BTS

TRC To Core

BTSs (Base Transceiver Stations)

BSCs (Base Station Controllers) TRCs (TRanscoder Controllers)

BSS


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3G UMTS/WDMA

It contained:

RNC

Node-B

To Core

Node-Bs

RNCs (Radio Network Controllers)

In UMTS, the Radio Network portion is called the UTRAN or, more simply, the RAN

(Radio Access Network).

Node-B

Node-B

RAN


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4G (almost) EPS

It contains:

e-Node B

To Core

eNBs

In EPS (Evolved Packet System), the Radio Network portion is called the e-UTRAN

(where e stands for evolved) or LTE (Long Term Evolution).

e-Node B

e-node B

LTE or e-UTRAN


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Time Domain

All three technologies utilize Time Slots (TSs)

GSM

WCDMA

LTE

667 sec

577 sec

500 sec

Drawings not to scale.


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Frames - GSM

In GSM, there are 8 TSs in a frame. Normally, eachconnection gets its own TS on a carrier of 200KHz

1 frame = 8 TSs4.616 ms

Carrier 1

Carrier 2


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Frames - UMTS

In UMTS, there are 15 TSs in a frame. Everyone has a connection on every TS.There is a single carrier having a bandwidth of 5 MHz

1 frame = 15 TSs1 msec

Carrier 1


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Frames LTE

In LTE, two TSs make a subframe and 10 subframes make a frame.

1 frame = 10 Subframes = 20 TSs10 msec

TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS

10

Subframe

987654321


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TS

Frames LTE



TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS

Subframe

The minimumallocation for aconnection is onesubframe. Both TSsmust be allocated.

10987654321


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Frames LTE




Subframe

A connection may useseveral subframes

during a frame.

10987654321


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Frames LTE




Subframe

Not all subframeshave to be allocated

10987654321


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1. How many subframes can be transmitted in a second?

Exercise


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Frequency GSM and UMTS

In GSM and UMTS, a subscriber is limited to using a single carrier at any point in time.

Carrier 1

Carrier 2

X


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Frequency LTE 2TSs1 subframe

1 msec

Frequency Block12 subcarriers

@ 15 kHz each

In LTE, the smallest possiblefrequency allocation is aFrequency Block

A Frequency Block consists of12 contiguous subcarriers at15 kHz each

freq


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Frequency LTE

A subscriber may beallocated more thanone Frequency Blockduring a subframe*

2TSs1 subframe

1 msec


@ 15 kHz each



freq


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Frequency LTE

A subscriber may beallocated more thanone Frequency Blockduring a subframe*

2TSs1 subframe

1 msec


@ 15 kHz each



freq

*On the UL, the Frequency Blocks for a

single connection must be contiguous. Onthe DL, they do not have to be contiguous


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Frequency LTE 2TSs1 subframe

1 msec

Not all Frequency Blockshave to be allocated.


@ 15 kHz each



freq


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2. What is the bandwidth of a Frequency Block? How does this compare with a single GSMcarrier? How does this compare with a UMTS carrier?

Exercise


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Operator Spectrum

freq

An operator hasflexibility in how

much spectrum theycan allocate for LTE.

Possible allocationsare: 1.4 MHz

3 MHz 5 MHz 10 MHz 15 MHz 20 MHz

1.43

5

10

15

20


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3. Calculate the number of Frequency Blocks in each allocation that an operator is allowed to use(i.e., 1.4, 3, 5, 10, 15 and 20 MHz). In each case, round down and remove one Frequency Blockfor guard band purposes.

Exercise


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Scheduling Block LTEIn LTE, the smallest possible allocationfor a connection is a SchedulingBlock

Frequency Block12 contiguous subcarriers@ 15 kHz each

2 TSs = 1 subframe = 1 msec

Subcarrier 1

Subcarrier 2

Subcarrier 3

Subcarrier 4

Subcarrier 5

Subcarrier 6

Subcarrier 7

Subcarrier 8

Subcarrier 9

Subcarrier 10

Subcarrier 11

Subcarrier 12

Subcarrier 1

Subcarrier 2

Subcarrier 3

Subcarrier 4

Subcarrier 5

Subcarrier 6

Subcarrier 7

Subcarrier 8

Subcarrier 9

Subcarrier 10

Subcarrier 11

Subcarrier 12

time

freq

Each subcarrier is15kHz wide

`

A Scheduling Block consists of : 1 Frequency Block 1 subframe


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Frequency LTEA possible allocation for 4 subscribers over 5 subframes

time

freq

2 TSs = 1 subframe = 1 msec

Frequency Block12 subcarriers@ 15 kHz each

Subscriber 1

SchedulingBlock

Subscriber 2

Subscriber 3

Subscriber 4

Unallocated


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A closer look at a Time Slot

12 subcarriersFrequency Block

1 Timeslot = 1/2 subframe = 0.5 msec

time

freq

Symbol 1 Symbol 2 Symbol 3 Symbol 4 Symbol 5 Symbol 6 Symbol 7

A Time Slot on one SubCarrier has 7 Resource ElementsEach Resource Element carries one symbol (see next slide)


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Resource Elements

12 subcarriersFrequency Block

1 msec = 1 subframe = 2 TSs

time

freq

The smallest unit of data transmission is a Resource Element

SchedulingBlock

It consists of: 1 symbol on 1 subcarrier


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4. How many Resource Elements are in a Scheduling Block?5. For a single Frequency Block, how many Resource Elements can be transmitted in one

second?

Exercise

Bi R d M d l i


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Bit Rates and Modulation

Symbol

1

A symbol (can carry: 2 bits (if the modulation is QPSK) or 4 bits (if the modulation is 16QAM) or 6 bits (if the modulation is 64QAM)

Symbol

2Symbol

3Symbol

4Symbol

5Symbol

6Symbol

7Symbol

8Symbol

9Symbol

10Symbol

11Symbol

12Symbol

13Symbol

14

1 msec = 1 subframe = 2 TSs

One subcarrier


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6. If QPSK is used, how many bits per second can be transmitted on a single Frequency Block?7. If 16QAM is used, how many bits per second can be transmitted on a single Frequency Block?8. If 64QAM is used, how many bits per second can be transmitted on a single Frequency Block?9. The highest bit-rate possible with EDGE in GSM is approximately 60 kbits/sec per TS. Using all

8 TSs, what bit-rate could EDGE achieve for a single carrier? How does this compare with thethree bit-rates of a single Frequency Block on EPS-LTE?

10. Using your answer from question 3, what bit rate could EPS-LTE achieve with 5 MHz ofbandwidth using QPSK? Using 16QAM? Using 64QAM?

11. A single DL carrier in UMTS (P5) can achieve a bit-rate of approximately 14 Mbits/sec. Using

your answer from question 10, how do the 3 bit-rates compare with the bit-rate of UMTS?12. A voice call, including all error-coding and signaling, requires approximately 33 kbits/sec to

support. Approximately how many voice calls could be supported on a single Frequency Blockin EPS-LTE? Assume QPSK modulation.

13. How does the answer in question 12 compare with the number of voice calls supported by asingle GSM carrier?

14. How many voice calls could be supported with 5 MHz of bandwidth in EPS-LTE? How does thatcompare with UMTS?

Exercise

Lte Air Interface 2

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