ofdm numerology - ieeegrouper.ieee.org/groups/802/3/bn/public/jan13/pietsch_01_0113.pdf · ofdm...

1 IEEE 802.3bn Phoenix, AZ January 23-25, 2012 1 IEEE 802.3bn Phoenix, AZ January 23-25, 2012

OFDM Numerology

Christian Pietsch (Qualcomm)

2 IEEE 802.3bn Phoenix, AZ January 23-25, 2012

Outline

Downstream Numerology Overview

Frame Structure and Pilot Structure

CP Impact Analysis

Modulation and FEC Proposal

Time Domain Interleaving


Downstream Numerology Overview

OFDM numerology

Subcarrier spacing: 50 kHz and 25 kHz

FFT sizes: 4096 and 8192 with sampling frequency of 204.8 MHz

– 3800 or 7600 available subcarrier in 190 MHz of OFDM block

Cyclic prefix: configurable: 1.25 s, 2.5 s, 3.75 s, and 5 s

Constellation size: Odd and even constellations from 256QAM to 4096QAM

Frame structure

A frame consists of 128 (4k FFT) and 64 (8k FFT) subframes

A subframe consists of 2 OFDM symbols

Pilots

Regular pilots only in subframe 0; used for full blown channel estimation

Continual pilots in all subframes; used for tracking

Pilot overhead: 1-2% based on FFT size

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


Pilot Structure: 25 kHz Spacing

Symmetric pilots around center subcarrier

Regular pilot symbols:

One pilot symbol on every second subcarrier

Two consecutive OFDM symbols with regular pilots. These two symbols define subframe 0.

Repetition of regular pilots every 64 subframes

Used to obtain a reliable one shot estimate of the channel response

Continual pilots:

One pilot symbol on every 256 subcarriers

Used to track/update the channel estimate that was obtained from the regular pilots until a new full blown channel estimate becomes available

Pilot overhead:

Regular pilots: 1/128

Continual pilots: 1/256

Combined pilot overhead: 3/256 = 1.17%

385384383

133132131130129128127126125124123

76543210-1-2-3-4-5-6-7

-123-124-125-126-127-128-129-130-131-132-133

-383-384-385

Time

Fre

qu

en

cy

DC subcarrier

Frame idx 0 1

Symbol idx 0 1 2 3 4 5 6 7 126 127 0 1 2 3 4 5 6 7

Subframe idx 0 1 2 3 63 0 1 2 3

Re

gula

r p

ilot

Co

nti

nu

al P

ilot


Pilot Structure: 50 kHz Spacing 193192191

6968676665646362616059

76543210-1-2-3-4-5-6-7

-59-60-61-62-63-64-65-66-67-68-69

-191-192-193

Time

Fre

qu

en

cy

DC subcarrier

Re

gula

r p

ilot

Co

nti

nu

al P

ilot

Co

mb

ine

d P

ilot

Frame idx 0 1

Symbol idx 0 1 2 3 4 5 6 7 254 255 0 1 2 3 4 5 6 7

Subframe idx 0 1 2 3 127 0 1 2 3

Symmetric pilots around center subcarrier

Regular pilot symbols:

One pilot symbol on every subcarrier

Two consecutive OFDM symbols with regular pilots. These two symbols define subframe 0.

Repetition of regular pilots every 128 subframes

Used to obtain a reliable one shot estimate of the channel response

Continual pilots:

One pilot symbol on every 128 subcarriers

Used to track/update the channel estimate that was obtained from the regular pilots until a new full blown channel estimate becomes available

Pilot overhead:

Regular pilots: 1/128

Continual pilots: 1/128

Combined pilot overhead: 1/64 = 1.56%


Pilot Structure (Details)

Subframe pilot structure:

The pilots are all symmetric with respect to the center frequency (DC), i.e. if there is a pilot on frequency f there is also a pilot on frequency –f

If the pilot symbol is ‘a’ at frequency f and ‘b’ at frequency –f in the first OFDM symbol of the subframe the second OFDM symbol of the subframe carries pilot symbol ‘b’ at frequency f and ‘-a’ at frequency –f.

This pilot structure provides excellent properties to estimate impairments like carrier frequency offset, phase noise, sampling frequency offset, and IQ mismatch

Regular pilot density in subframe 0:

With a maximal delay spread of about 4us, the minimal coherence bandwidth is 250kHz

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

a b

b

f

1DC-1

-f -a


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


Modulation and Forward Error Correction

QAM Modulation

Preferred modulation alphabets are (16QAM), (32QAM), (64QAM), (128QAM), 256QAM, 512QAM, 1024QAM, 2048QAM, and 4096QAM

Downstream proposal: DVB-C2 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 DVB-C2 LDPC and BCH codes is the preferred approach since they are well known and fully specified

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


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: 4k (4k – 1) / 2 = 8386560

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 4k FFT, 4k/16 parallel interleavers are required

Required number of memory elements: 256 16 (16 – 1) / 2 = 30720

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




Comparison Interleaver Depth 8 and 16

26 28 30 32 34 36 38 4010

-5

10-4

10-3

10-2

10-1

100

Bit e

rror

rate

SNR (dB)

LDPC Bit Error Rate

Burst Noise = 20 dBc, 16 s, 16 OFDM symbols interleaving

Burst Noise = 20 dBc, 16 s, 8 OFDM symbols interleaving

No Burst Noise

Data Rate

4096QAM

DVB-C2 LDPC code

– Code length n = 16200 bits

– Code rate R = 8/9, 20 Iterations

OFDM Symbol Duration: 80 s

AWGN Channel Model

Interleaver depth D = 16

Burst Noise Assumptions

CIR = 20 dBc, duration = 20 s

Gaussian distributed

Symmetrically asynchronous to OFDM symbols

Interleaving across 16 symbols performs 2dB better than interleaving across 8 OFDM symbols when moderate burst noise is present

Interleaving across very few symbols shows little benefits


Conclusions

A frame structure was proposed with 1-2% 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 longer OFDM symbol for optimum performance

The DVB-C2 codes should be used in downstream direction

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

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

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

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


thank you

ofdm numerology - ieeegrouper.ieee.org/groups/802/3/bn/public/jan13/pietsch_01_0113.pdf · ofdm...

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