OFDM Numerology - IEEE · PDF fileIEEE 802.3bn San Antonio, TX November 12-15, 2012 3 Downstream Numerology Overview

1 IEEE 802.3bn San Antonio, TX November 12-15, 2012 IEEE 802.3bn San Antonio, TX November 12-15, 2012 1

OFDM Numerology

Christian Pietsch (Qualcomm)

Juan Montojo (Qualcomm)

2 IEEE 802.3bn San Antonio, TX November 12-15, 2012

Outline

Downstream Numerology Overview

Frame Structure and Pilot Structure

CP Impact Analysis

FEC Proposal

Time Domain Interleaving


Downstream Numerology Overview

OFDM Numerology

Subcarrier spacing 12.5 kHz

FFT size = 16384 with sampling frequency of 204.8 MHz

– 15200 available subcarrier in 190 MHz of OFDM block

Cyclic prefix: configurable size between 1.25 s and 5 s

Constellation size: Even constellations from 256QAM to 4096QAM

Pilots

1 pilot symbol every 64 subcarrier in each OFDM symbol

Pilots spread over a frame of 32 OFDM symbols

Pilot overhead is 1/64

Interleaving

Time domain interleaving is configurable: different levels or none

Frequency domain interleaving

– Across code blocks within one OFDM symbol

– Each code block sees similar SNR conditions


Frame Structure

• Two OFDM symbols compose a subframe

• 16 subframes compose a frame

• 100 frames compose a super frame

• Frame 0 contains a preamble


Pilot Structure (Details)

1 pilot subcarrier every 64 subcarriers in each OFDM symbol (except around center frequency)

Same pilot locations on two consecutive OFDM symbols (defining a subframe)

Pilot positions symmetric with respect to center carrier frequency (DC subcarrier in baseband)

Staggered in frequency over time to yield a regular pilot pattern with 1 pilot every 4 subcarriers

32 OFDM symbols compose a frame

Effectively, OFDM symbols with 16 different pilot positions

Single channel estimate every 32 OFDM symbols (i.e. one per frame)

No need for additional pilots

Subcarrier idx38

37

36

35

34

33

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

32

31

-3

-4

-5

-6

-7

-8

-9

-10

-11

-12

-13

-14

-15

-16

-17

-18

-19

-20

-21

-22

-1

-2

0 1

0

2 3

1

4 5

2

6 7

3

8 9

4

10 11

5

12 13

6

14 15

7

16 17

8

18 19

9

20 21

10

22 23

11

24 25

12

26 27

13

28 29

14

30 31

15

0 1

0

2 3

1

4

2

Symbol idx

Subframe idx

Frame idx Frame 0 Frame 1

DC subcarrier

Time

Frequency


Pilot Structure (Details)

Staggered Pilots:

Reduces pilot overhead to 1/64 = 1.56%

Same Pilot position in a subframe:

Enables continuous tracking of phase noise and carrier frequency offset

Symmetric pilots round carrier frequency:

Enables correction of Tx IQ mismatch and RX IQ mismatch correction

Effective pilot pattern with pilots on every fourth subcarrier:

With a maximal CP length (delay spread) of 4us, the minimal coherence bandwidth is 250kHz

5 pilots in the coherence bandwidth (i.e. with 50 kHz spacing) is a reasonable choice


ReDeSign Channel Models Case 1 and Case 2

ReDeSign Channel Model Case 1

ReDeSign Channel Model Case 2


Frequency Domain Channel Gain for ReDeSign

Frequency domain channel gain in dB for ReDeSign Case 1

𝐺 𝑓 = 10 ∙ 𝑙𝑜𝑔10 𝐻 𝑓 2

0 2000 4000 6000 8000 10000 12000 14000 16000 18000-10

-5

0

5

Subcarrier Index

Fre

quency D

om

ain

Channel G

ain

ReDesign Channel, 7.5 kHz Subcarrier Spacing

8700 8710 8720 8730 8740 8750 8760 8770 8780 8790 8800

-10

-9.5

-9

-8.5

-8

-7.5

-7

-6.5

Subcarier Index

Fre

quency D

om

ain

Channel G

ain


SINR at Demodulator Output – ReDeSign Case 1

0 0.5 1 1.5 2 2.5 3 3.5 4 4.530

32

34

36

38

40

42

SIN

R/d

B

CP / µs

SINR vs CP Length (SNR = 40 dB)

7.5 kHz Spacing

12 kHz Spacing

50 kHz Spacing


0 0.5 1 1.5 2 2.5 3 3.5 4 4.526

28

30

32

34

36

38

40

SIN

R/d

B

CP / µs

SINR vs CP Length (SNR = 40 dB)

7.5 kHz Spacing

12 kHz Spacing

50 kHz Spacing

SINR at Demodulator Output – ReDeSign Case 2


Forward Error Correction

Downstream proposal: DVB-C2 LDPC codes

Common MCS per group of users enables the aggregation of Ethernet frames dedicated to multiple users of such a group into a single code word. (equivalent to multiple profiles approach)

It is anticipated that longer codes are more efficient when users are grouped

Applying the LDPC DVB-C2 codes is the preferred approach since they are well known and fully specified

– Final decision will depend on the channel model

Upstream proposal: IEEE 802.11n LDPC codes

The IEEE LDPC codes support short code word lengths that fit well with OFDMA

Analysis for AWGN and time dispersive channels has shown that performance is superior compared to RS codes of similar length

Code word lengths are optimized for Ethernet frame lengths


BER Curves for DVB-C2 LDPC Code – Example

Code word length 16200 bits w/o outer BCH code

Gray mapping in I and Q

Floating point LLR

Note: 8192 QAM is plotted for information. There is little benefit of using 8192 QAM over 4096 QAM and 16384 QAM

-10 0 10 20 30 40

10-5

10-4

10-3

10-2

10-1

100

BER in AWGN for QAM, LDPC, 20 decoder iterations

SNR / dB

BE

R

16,7/9

16,8/9

64,2/3

64,7/9

64,9/10

256,11/15

256,37/45

256, 8/9

1024,11/15

1024,37/45

1024,8/9

4096,37/45

4096,8/9

8192,37/45

8192,8/9

16384,37/45

16384,8/9


QAM Modulation and Outer Coding Proposal

Downstream QAM Modulation

Preferred modulation alphabets are (16QAM), (64QAM), 256QAM, 1024QAM, and 4096QAM

Usage of odd modulation orders (32QAM), (128QAM), 512QAM, and 2048QAM is not preferred since it complicates processing like LLR computation. Similar performance can be achieved by choosing even modulation orders and appropriate coding rates.

Downstream Outer Coding

Preference is not applying an outer BCH code

This proposal may be revised of performance needs for an outer code are shown

– Performance to be verified in time dispersive channels, burst noise, etc.


Direct Convolutional Time Interleaving

Convolutional interleaving is applied at subcarrier level

Convolutional interleaving delays each subcarrier in time

For a time-invariant channel, interleaving across MCS profiles is possible

But: Delay and memory consumption are excessive for direct interleaving

Required number of memory elements: 16k (16k – 1) / 2 = 131064k

Burst Error

Channel

Time

Fre

quency

Interleaving De-Interleaving

MCS profiles

1

2

FFT Size -

1

1

2

FFT -1

IFF

T

FF

T

Memory elements Memory elements


Parallel Convolutional Time Interleaving

Interleaver depth depends on the burst noise model

Interleaver depth is expected to be at most 16 OFDM symbols (similar to DVB-C2)

For a 16k FFT, 16k/16 parallel interleavers are required

Required number of memory elements: 1025 16 (16 – 1) / 2 = 120k

Burst Error

Channel

Time

Fre

quency

Interleaving Depth

Parallel De-Interleaving

MCS profiles

Parallel Interleaving

IFF

T

FF

T


Parallel Convolutional Interleaving Structure E

rroneous c

ode s

ym

bols

in fre

quency d

om

ain

Synchronous Burst Noise

w/o Time Interleaving Synchronous Burst Noise w/

Time Interleaving Depth D = 6

Asynchronous Burst Noise w/

Time Interleaving Depth D = 6

D-1

err

or

free c

ode s

ym

bols

D-2

err

or

free c

ode s

ym

bols

OFDM Symbols OFDM Symbols OFDM Symbols


26 28 30 32 34 36 38 4010

-7

10-6

10-5

10-4

10-3

10-2

10-1

100

Bit e

rror

rate

SNR (dB)

LDPC Bit Error Rate

OFDM symbol 80 s, sync Burst


OFDM symbol 80 s, async Burst


No Burst Noise



Performance when Asynchronous Burst Noise is Present

Data Rate

4096QAM

DVB-C2 LDPC code

– Code length n = 16200 bits

– Code rate R = 8/9, 20 Iterations

OFDM Symbol Duration

20, 40, 80 s

AWGN Channel Model

Interleaver depth D = 16

Burst Noise Assumptions

CIR = 20 dBc, duration = 20 s

Gaussian distributed

Synchronous and symmetrically asynchronous to OFDM symbols

Loss ~ 1.2 dB for interleaver depth D = 16 and 80 s OFDM symbol




Conclusions

A frame structure was proposed with 1.6% pilot overhead

Pilot density supports channels with up to 4 s delay spread

Pilot pattern allows for estimation of phase noise and I/Q imbalance

The impact of CP length has been analyzed for ReDeSign channels

ReDeSign like channels require CP durations of almost 4 s and long OFDM symbol for optimum performance

OFDM symbol duration of 80 s is required for channels with long delay spread

The DVB-C2 LDPC codes should be used in downstream

Main advantage is that they are fully specified and field-proven

The need for time interleaving depends on the burst model and is for further study

Required interleaver depth depends on burst noise model and OFDM symbol duration

Longer OFDM symbols provide better protection against burst noise than shorter symbols


thank you

Documents

OFDM Numerology - IEEE · PDF fileIEEE 802.3bn San Antonio, TX November 12-15, 2012 3 Downstream Numerology Overview