Addressing Transmitters in a Single Frequency Network (SFN)

Addressing Transmitters in a Single Frequency Network Download the pdf version here.

Introduction

Optimizing spectrum/bandwidth is becoming more and more crucial as digital services (DVB-T, DVB-H, DTMB...) are deployed. A new topology has thus been defined, namely Single Frequency Network (SFN) topology: all the transmitters broadcast at the same time, the same content, over the same frequency (contrary to MFN: Multiple Frequency Network). This document is focused on DVB-T and DVB-H SFN network. The principle is however the same in DTMB (chinese standard, also known as DMB-T) broadcast network.

SFN and MFN topology - Addressing transmitters in a Single Frequency Network

SFN adapter: the keystone for setting up one Single Frequency Network

In order to achieve this very accurate task, transmitters need transmission and synchronization information. All this is embedded within one TS packet called MIP (Megaframe Initialization Packet) and inserted into TS stream by one Single Frequency Network SFN adapter before distribution to transmitters (SFN adapter performs baseband processing over TS stream).

MIP: Megaframe Initialization Packet

Due to strong constraints brought while setting up one Single Frequency Network, additional information have been defined (apart from modulation and synchronization parameters). These additional information (namely Optional functions) will enable the broadcasters to get control over DVB-T DVB-H transmitters.

Whatever the modulation type (DVB-T, DVB-H, or DTMB), MIP packet can be viewed this way:

Transmission parameters (modulation parameters and TPS signalling)
Synchronization
Optional functions data

- Addressing transmitters in a Single Frequency Network

SFN Optional functions

In DVB, five main optional functions have been defined:

tx_time_offset()
tx_frequency_offset()
cell_id()
tx_power()
private_data()

Why do we need to address individually DVB-T DVB-H transmitters ? The idea is basically to optimize RF coverage, correct some transmitters inacuracy... Since each transmitter is uniquely identified with one TxID (from 0 to 65,535), it enables the SFN adapter to address individually each transmitter belonging to one SFN cell.

tx_time_offset() Optional function

Within an Single Frequency Network, DVB-T DVB-H transmitters are to radiate synchronously... however, depending on the environment, distance, transmitter power, or clock accuracy, it can be interesting, let say mandatory for optimal SFN RF coverage, to delay the transmission of one or several transmitters. This is achieved by specifying for each TxID, the corresponding delay.

Use case:
2 transmitters with TxID = 1 and 2, with different transmission power. Propagation time t1 > t2. Signal will thus not be in-phase. In order to get SFN +3db gain, one can with interest delay transmitter TxID=2 !

tx_time_offset() optional function - Addressing transmitters in a Single Frequency Network

tx_frequency_offset() Optional function

Within one Single Frequency Network cell, some dozens of transmitters may co-exist. Each transmitter has a certain accuracy that may vary in time... Frequency accuracy is relevant: in COFDM: TS signal is modulated over a large number of carriers. If all the transmitters are not radiating exactly the same carriers, handset will not be able to receive live signal due to Inter-Carrier Interferences (ICI). As for tx_time_offset() function, one can easily configure frequency offset using ENENSYS NN6-MIP^TM SFN Adapter: for a list of given TxID, one can specify for each of them the frequency offset to be applied.

cell_ID() Optional function

In SFN mode, transmitters may need to know which cell they belong to (otherwise TS will not be processed !!).

tx_power() Optional function

Depending on the environment, transmitter accuracy... the power radiated may need to be adjusted. Data can also be sent individually to transmitters in order to configure their power (it ranges from 0 to -500 dbm).

Conclusion

Addressing transmitters using optional functions is useful, let say necessary! Among other applications, it enables the broadcasters to:

Optimize RF coverage
Correct transmitters inaccuracy
Address transmitters so that they could work in SFN mode
...

NN6-MIP Inserter / SFN Adapter - Addressing transmitters in a Single Frequency Network

ENENSYS Technologies designed and widely deployed its Single Frequency Network SFN adapter: NN6-MIP^TM SFN Adapter is a flexible, highly reliable and robust MIP insertion solution, matching any broadcaster demand (full SNMPv2 based: get + set + trap, any redundancy scheme supported...).