Lt e Air Interface 2

7/28/2019 Lt e 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 with

GSM 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, each

connection gets its own TS on a carrier of 200KHz

1 frame = 8 TSs

4.616 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 TSs

1 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 TSs

10 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



10 msec

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

Subframe

The minimumallocation for a

connection is one

subframe. Both TSs

must be allocated.

10987654321


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Frames LTE



10 msec


Subframe

A connection may use

several subframes

during a frame.

10987654321


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Frames LTE



10 msec


Subframe

Not all subframes

have 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 Block

12 subcarriers

@ 15 kHz each

In LTE, the smallest possible

frequency allocation is a

Frequency Block

A Frequency Block consists of

12 contiguous subcarriers at

15 kHz each

freq


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Frequency LTE

A subscriber may be

allocated more than

one Frequency Block

during a subframe*

2TSs

1 subframe

1 msec

Frequency Block

12 subcarriers

@ 15 kHz each



Frequency Block



15 kHz each

freq


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Frequency LTE

A subscriber may be

allocated more than

one Frequency Block

during a subframe*

2TSs

1 subframe

1 msec

Frequency Block

12 subcarriers

@ 15 kHz each



Frequency Block



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 Blocks

have to be allocated.

Frequency Block

12 subcarriers

@ 15 kHz each



Frequency Block



15 kHz each

freq


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

carrier? How does this compare with a UMTS carrier?

Exercise


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Operator Spectrum

freq

An operator has

flexibility in how

much spectrum theycan allocate for LTE.

Possible allocations

are:

1.4 MHz

3 MHz 5 MHz

10 MHz

15 MHz

20 MHz

1.4

3

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 Block

for guard band purposes.

Exercise


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Scheduling Block LTEIn LTE, the smallest possible allocation

for a connection is a Scheduling

Block

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 is

15kHz 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 Block

12 subcarriers

@ 15 kHz each

Subscriber 1

Scheduling

Block

Subscriber 2

Subscriber 3

Subscriber 4

Unallocated


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A closer look at a Time Slot

12 subcarriers

Frequency 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 Elements

Each Resource Element carries one symbol (see next slide)


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Resource Elements

12 subcarriers

Frequency Block

1 msec = 1 subframe = 2 TSs

time

freq

The smallest unit of data transmission is a Resource Element

Scheduling

Block

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 the

three 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 of

bandwidth 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 Block

in EPS-LTE? Assume QPSK modulation.

13. How does the answer in question 12 compare with the number of voice calls supported by a

single GSM carrier?

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

compare with UMTS?

Exercise

Lt e Air Interface 2

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