SFN

Copyright 2009 NHK1

June,2011 Montevideo, Uruguay

Tomofumi Yoshimi

Senior Manager

Japan Broadcasting Corporation (NHK)

Copyright 2011 NHK

Single Frequency Network

Digital Terrestrial Television Broadcasting

Copyright 2009 NHK

SFN technology

・The SFN technology is one of the key features of ISDB-T.

・The SFN technology is used in ISDB-T to save frequencies.

The broadcasting network can be constructed with MFN and SFN.

SFN is especially useful where the available frequencies are scarce.

★SFN: Single Frequency network

2 Copyright 2011 NHK

Copyright 2009 NHK

Principle of OFDM Modulation

Carrier1

OFDM TIME WAVEFORM

+

+

+

=

+

OFDM FREQUENCY SPECTRUM

Frequency

+

+

+

+=

Carrier2

Carrier k

Guard interval Effective symbol duration

OFDM transmission symbol

Transmitting signal

OFDM signal time waveform and frequency spectrum

Guard interval application

GuardInterval

GuardIntervalEffective symbol Effective symbol

IFFT output data IFFT output data

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Copyright 2009 NHK

GI Effective Symbol GI Effective Symbol

GI Effective Symbol GI Effective Symbol

FFT Window

td

(a)

Time Axis

(b)

(c)

(a) : Direct wave from transmitter, (b) : Reflected wave (multi-path wave)

(c) FFT window of receiver GI: Guard Interval , td: delay time of multi-path

FFT window of receiver cuts a signal with Ts (effective symbol ) length, this signal is

fed to FFT to demodulate OFDM signal. If FFT window can be set within the interval

of “transmitted OFDM symbol”, Inter Symbol Interference (ICI) is not occurred.

As a result, if multi-path delay time is no longer than GI, multi-path interference is

almost compensated.

Transmitted OFDM symbol

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Guard Interval ～Effect of guard interval～

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What is SFN ?

f2

f3f1

f1

f1f1

Master station

Relay station Relay station

Analog Network

SFN Network

Master station

Relay station Relay station

f1

f2f1

Master station

Relay station Relay station

SFN and MFN Network

f1, f2, f3 ⇒ Frequency channels

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ANALOG SYSTEM DIGITAL SYSTEM

Copyright 2009 NHK

Conditions for SFN construction

• Network transmission timing synchronization

At the receiving point, the time difference between two or more transmitter signals must be within the guard interval length (GI※).

Studio

Master station

Relay station

Broadcast-wave

Household

T1

T2

｜T2－T1｜ ＜ GI

6

Copyright 2009 NHK

Conditions for SFN construction

Studio

Master station

Relay station

HouseholdT1

T2

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D/U≧Protection ratio・・・OK

D/U＜ Protection ratio・・・NGIf: ｜T2－T1｜ ≧ GI

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Interference protection ratio

Preferred Interfering Lower channel Co-channel Upper channel

Analog Digital 0dB(-6/-3dB) 45dB(39/44dB) 10dB(-6/-3dB)

Digital

Analog -21dB(-29dB) 20dB (14dB) -24dB( -31dB)

Digital -26dB(-24dB) 28dB( 23dB) -29dB(-27dB)

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Interference protection ratio in Japan (64QAM,7/8)

( ) :ITU-R BT.1368-3

Copyright 2009 NHK

Transmission Network

• Two Types of transmission network

- Microwave: STL, TTL (in case of SFN with delay control)

- Broadcast Wave

Studio

Main station

Relay station

Household

Relay station

Relay station

Relay station

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Copyright 2009 NHK

Delay Adjustment system for SFN

TS-STL

TransmitterStudioSystem

TS-STL

Reciever

Tranmitter

IF-TTL

Transmitter

Tranmitter

MODTSDelay

OFDM　ModulatorIFDelay

IFDelay

IF-TTL

Reciever

Studio Main station Relay station

TS transmission network IF transmission network

TS

Delay

IF

Delay

IF

Delay

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Copyright 2009 NHK

Ｅｘａｍｐｌｅ：SFN in Ibaraki Prefecture

Mito Station,

Hitachi Station, and

Yamagata Station

comprise the SFN.

Master Sta.

(Tokyo Tower)

Shoubu-Kuki Fx.

Kaba-san Fx.

Mito Studio

47.3km

53.6km

24.2km

Hitachi-kamine Fx

Juo Sta.

Yamagata Sta.

Mito Sta.

Hitachi Sta.

Tokyo Studio

Copyright 2011 NHK11

GI=126μsec・・・37.8km

Copyright 2009 NHK

Simulation Results

Adjusted DelayUnadjusted Delay

SFN interference area

TTL２４ｋｍ

TTL２４ｋｍ

Yamagata Sta.

Hitachi Sta.

Mito Sta.

Yamagata Sta.

Hitachi Sta.

Mito Sta.

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Copyright 2009 NHK

Tx

Adjusting the OFDM Modulator’s Output

RemuxTS-STL

TX

TS-STL

RX

TS DelayOFDM

MODTx

IF

Delay

TxIF

DelayTS DelayOFDM

MOD

measurement point

350msec

(Set value)

Adjust to 410 msec

Adjust to 412 msec

Mito

Station

Hitachi

Station

43.976 msec

40.176 msec

1.994 msec

350msec

(Set value) 1.997 msec

TS-TTL

TX

TS-TTL

RXTS-TTL

TX

TS-TTL

RXTS-TTL

TX

TS-TTL

RX

TS-TTL

TX

TS-TTL

RX

TS DelayOFDM

MODTx

IF

Delay

Yamagata

Station40.175 msec350msec

(Set value) 2.347 msec

(412 – 410.003 + 0.35)

TS-TTL

TX

TS-TTL

RX

Toff(0.35msec)

Tokyo Studio

Copyright 2011 NHK13

Copyright 2009 NHK

Muchas Graciaspor su atención…

Copyright 2011 NHK15