Line Coding for Vlc Wireless

8/10/2019 Line Coding for Vlc Wireless

1/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009

Rick Roberts [Intel]Slide 1

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Submission Title:Line Coding for VLC Wireless

Date Submitted: September 2009Source:Rick Roberts [Intel]

Address

Voice:503-712-5624, E-Mail: [email protected]

Re:

Abstract:

Purpose:

Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for

discussion and is not binding on the contributing individual(s) or organization(s). The material in this

document is subject to change in form and content after further study. The contributor(s) reserve(s) the right

to add, amend or withdraw material contained herein.

Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE

and may be made publicly available by P802.15.


2/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


The subject of line coding for VLC has been discussed within the committee during the

last few meetings. This contribution offers the authors view points.

We need to be careful in selecting a code that was intended for a wired connection (i.e.

high SNRs and low BERs) for use with a wireless technology (i.e. low SNRs and high

BERs). Some codes do not perform well in the presence of bit errors.

Some of the desirable characteristics of the code are:

1. No DC component (spectral shift away from DC)

2. Does not propagate errors (an error in one part of the code should not cause an error in another part of the code)

3. The passband bandwidth of the code is not excessive (no long runs of zeros or ones)

4. Easy clock recovery

5. Code levels must map efficiently to VLC modulation techniques (e.g. OOK)


3/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


So lets consider some common line codes:

1. AMI

2. Modified AMI B8ZS

3. 4B5B

4. 6B8B

5. 8B10B

6. Manchester


4/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


AMI and Modified AMI B8ZS

AMI has no run length constraints on the number of zero symbols that can be transmitted in a

row, which can be problematic. To circumvent this problem B8ZS coding constraints the run

length of zero symbols by replacing each string of 8 consecutive zeros with the special pattern

"000VB0VB". Depending on the polarity of the preceding mark, that could be 000+0+ or

000+0+. There are other variants of this same idea such as B6ZS and B3ZS.

Problems with AMI

1. The AMI ternary coding (3 modulation levels) does not map well to OOK modulation (twomodulation levels)

2. The AMI code itself has no constraint on the run length of the zero symbol which causes

spectral problems and clock recovery problems

3. Modifications to constraint the run length, such as B8ZS, can be error propagating since an

error in the received replacement string can cause the decoder to not take the correct action,

resulting in one error into the decoder producing multiple errors out of the decoder.

1:1 coding rate


5/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


4B5B Code

4B5B maps groups of four bits onto groups of 5 bits;hence, it is a rate 4/5 code

1. While 4B5B coding helps clock recovery, it is not a

DC balanced code.

2. One code word bit error can result in multipledecoder errors.


6/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


6B8B

Coding Rate: 3/4

6b/8b is a line code that expands 6-bit codes to 8-bit symbols for the purposes of

maintaining DC-balance in a communications system. Each 8-bit output symbol

contains 4 zero bits and 4 one bits, so the code can, like a parity bit, detect all

single-bit errors (i.e. it can not correct the error). Unfortunately, a single bit error

can lead to 6 decoding errors.

Note that no data symbol contains more than four consecutive matching bits, or

begins or ends with more that three identical. Thus, the longest run of identical

bits that will be produced is 6 (i.e. requires a low frequency response to handle the

long string of symbols).


7/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


8B10B Coding

Code Rate: 4/5

8b/10b is a line code that maps 8-bit symbols to 10-bit symbols to achieve DC-

balance and yet provide enough state changes to allow reasonable clock recovery.

There are not more than five 1s or 0s in a row (which again means there must be

sufficient low frequency response in the analog).

The lower 5 bits of data are encoded into a 6-bit group (the 5b/6b portion) and the

top 3 bits are encoded into a 4-bit group (the 3b/4b portion). These code groups are

concatenated together to form the 10-bit symbol that is transmitted.

Unfortunately a single bit error on the received code word can cause the decoder tomake multiple bit errors during the decoding process.


8/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


Manchester Coding

Manchester coding belongs to a family of codes that use mid-bit transitions as part

of the encoding scheme. Manchester is a rate code with a max run length of two.

The encoder can be implemented by XORing the clock with the data.

The code is not error propagating.


9/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


Bandpass Filtering and Run Length Intersymbol Interference

There is a relationship between the code run length (number of ones and/or zeros

in a row) and the bandpass filtering requirements. And note that any code that

does not have a DC component by definition is a bandpass signal whose spectrum

for all practical purposesexist between some low cut off frequency and some

high cut off frequency.

spectrum

ambient light noise of the photo detector spectrally falls

around DC and can be blocked with capacitive coupling.

Spectrum of Manchester Coding

The reason we care about the spectral properties of the code has to do with the

modulation domain spectral interference; that is, at the output of the photodetector we

have a spectrum that contains interference (i.e. CFL) and desired signals.


10/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


From document 15-09-0136-00-0007 modulation domain spectrum for a halogen

light bulb with a dimmer.

Typical spectral output from the photodetector. This particular spectrum is from a

halogen light bulb with the dimmer at 75% (100% is full brightness).


11/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


Simulation:Bandpass filter bandwidth and Intersymbol Interference due to Run Length

Simulation Scenario

Chip Rate: 1 Mchip/sec

Test Pattern: Run Length embedded in a 101010 pattern

Run Lengths Tested: 2, 4, 6

High Cutoff Frequency: 1 MHz

Low Cutoff Frequency: 5 kHzFilter Response: Bessel

Test Patterns:

Run Length 2: 101010 1010101101010

Run Length 4: 101010 101010111101010 Run Length 6: 101010 10101011111101010


12/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


Time (us)

Waveform

-2

-1.6

-1.2

-0.8

-0.4

0

0.4

0.8

1.2

1.6

2

Time (us)

90 94 98 102 106 110 114 118 122 126 130

Run Length 25 kHz lower cutoff

-1.5 dB droop


13/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009



Time (us)

Waveform

-2

-1.6

-1.2

-0.8

-0.4

0

0.4

0.8

1.2

1.6

2

Time (us)

90 94 98 102 106 110 114 118 122 126 130

-2.6 dB droop


14/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009



Time (us)

W

aveform

-2

-1.6

-1.2

-0.8

-0.4

0

0.4

0.8

1.2

1.6

2

Time (us)

90 94 98 102 106 110 114 118 122 126 130

-3.8 dB droop

The longer the run length, the more the droop.


15/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


The higher the lower corner frequency, the more the droop.


Time (us)

Waveform

-2

-1.6

-1.2

-0.8

-0.4

0

0.4

0.8

1.2

1.6

2

Time (us)

90 94 98 102 106 110 114 118 122 126 130

-10.3 dB droop


16/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


Code Rate DC Comp onent Error

Propagating

Run

Length

Clock

Recovery

Bi- level

Code

AMI 1/1 No No No

Limit

Poor No

Tri-level

B8ZS 1/1 No Yes 10 Good No

Tri-level

4B5B 4/5 Yes bit pattern

dependent

Yes 8 Good Yes

6B8B 3/4 No Yes 6 Good Yes

8B10B 4/5 No Yes 5 Good Yes

Manchester 1/2 No No 2 Good Yes

Summary of the discussed line codes

While we are willing to consider other coding schemes, at this time we are

supporting Manchester coding. While Manchester coding is a rate code, its bi-

level nature, its short run length and its non-error propagating characteristics make

it the desirable choice at this time.


17/17

doc.: IEEE 802.15-09-0637-00-0007

Submission

September 2009


Concatenated FEC and Line Code A Possibility

FEC

Encoder

Line

Code

Encoder

Line

Code

Decoder

FEC

Decoder

Chip

Errors

Line Code decoder can

cause a of burst errors

FEC decoder correctsthe error burst

Wed need to investigate this carefully. There are a number of issues that

need to be taken into consideration.

Line Coding for Vlc Wireless

Documents

Transcript of Line Coding for Vlc Wireless