Dvb-T Time Interleaving

traTime interleaving in DVB-T2
Chris Nokes 9th October 2008

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Time interleaving in DVB-T2
• • • • • Architecture recap Interleaving frames Capacity/interleaving compromise Time interleaver – the details Use of time interleaver memory as frame buffer (modulator) • Efficient de-interleaver implementation

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Architecture recap
PLP based processing Input services (PLPs) Timeinterleaving Time interleaving Time interleaving Frame builder

PLP based processing

OFDM generation

PLP based processing

MPEG-2 TS/GSE to BBFRAME/FEC blocks; cell interleaving

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Interleaving frames

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Interleaving frames
• “The smallest unit of the frame structure used for signalling and mapping interleaved FEC blocks” • Corresponds to one or more T2-frames • Applies separately toeach PLP

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Interleaving frames (1)
• Data rate of each PLP can vary
– variable number of FEC blocks in each interleaving frame – modulator must signal this number to receiver – signalling unit is always “interleaving frame”

• In simplest case, FEC blocks are collected for one T2-frame, and time-interleaved
– interleaving frame same as T2-frame

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Interleaving Frames – simplecase

• Limits maximum data rate because of limit on maximum time interleaver memory
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Increasing data rate
• To increase the data rate, we need to use the time interleaver more than once per T2-frame
– described as “multiple TI-blocks per T2-frame” – mainly applies to the single PLP case

• But we still want to signal the total number of FEC blocks in the T2-frame
– in this caseinterleaving frame still corresponds to T2-frame – use a simple rule to decide how many FEC blocks are in each of the TI-blocks
• equal split, with any remainder in the later blocks • e.g. interleaving frame (=T2-frame for this case) has 202 FEC blocks, using 3 TI-blocks each with 67, 67, 68 FEC blocks
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Several TI-blocks in one interleaving frame

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Interleaving over 3 T2-frames
FEC blocksTI-blocks Interleaving Frames

TI block Interleaving Frame

T2-frames

T2-frame 0

T2-frame 1

T2-frame 2

T2-frame 3

T2-frame 4

• Only 1 TI-block per interleaving frame allowed
– allows low data-rate services to have longer time interleaving with same memory
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Interleaving over multiple T2-frames
• In this case ONLY, number of cells in each T2-frame not required tocorrespond to an integer number of FEC blocks
– e.g. 68 FEC blocks in an interleaving frame mapped to 3 T2-frames (“PI=3”)
• • • • total cells in interleaving frame = 68×Ncells cells in each T2-frame = 68xNcells/3 PI must be a factor of Ncells e.g. Ncells = 8100 → 183600 cells in each T2-frame

• Remember: interleaving frames are applied separately for each PLP
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Interleaving frames –summary
Interleaving frame Simple case; 1 TI-block Multiple TI-blocks Interleaving over multiple TI-frames T2-frame Comments

1 whole Integer FEC blocks per T2interleaving frame frame; limited data rate 1 whole interleaving frame; multiple TI-blocks Part of interleaving frame Allows max data rate (single PLP mode) Only 1 TI-block in interleaving frame; longer interleaving for low data rate services12

Capacity/interleaving compromise

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Capacity/interleaving compromise
Capacity of T2-frame (32KE, SISO, PP7, 256-QAM, CR=3/5, GI=1/128, Pi=1, no TR, Nldpc=64800)
37 36 30 25

Data rate (Mbit/s)

35 34 33 Data rate 32 31
0 10 20 30 40 50 60

“Sweet-spot” for this mode

20 15 10 5 0

Time interleaving depth
70 80

Number of symbols per frame (Lf)

Time interleavingdepth (symbols)

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Time interleaver – the details

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Time interleaver – basics
• Time interleaver operates on cells of input FEC blocks • Simple row/column interleaver
– write in columns, read in rows – de-interleave by writing in rows, reading columns

• Number of rows (Nr) = Ncells/5
– each FEC block is divided into 5 columns

• Number of columns (NC) varies with data rate...