WO2013001408A1

WO2013001408A1 - Methods for encoding and decoding coded light

Info

Publication number: WO2013001408A1
Application number: PCT/IB2012/053073
Authority: WO
Inventors: Constant Paul Marie Jozef Baggen
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2011-06-29
Filing date: 2012-06-19
Publication date: 2013-01-03
Anticipated expiration: 2013-12-29

Abstract

The present invention relates to a method for encoding an information sequence to be emitted as coded light by at least one light source, the method comprising the steps of receiving an input string representing the information sequence, dividing the input string into a plurality of pairs of bits, mapping each of the plurality of pairs of bits into one of four symbols based on a predetermined schedule, thereby forming a sequence of channel symbols; and providing the sequence of channel symbols to be emitted by the light source, wherein a pulse width of an active portion of each of the symbols of the sequence of channel symbols are based on a predetermined dimming level of light emitted by the light source. The present invention provides advantages in relation to an increased bit rate of transmitted information.

Description

Methods for encoding and decoding coded light

FIELD OF THE INVENTION

The present invention relates to coded light, specifically in relation to methods for encoding and decoding coded light. The invention also relates to corresponding information encoders and decoders.

BACKGROUND OF THE INVENTION

Recently, much progress has been made in increasing the brightness of light emitting diodes (LEDs). As a result, LEDs have become sufficiently bright and inexpensive, to serve as a light source in for example illumination arrangements such as lamps with adjustable color. By mixing differently colored LEDs any number of colors can be generated, e.g. white. An adjustable color lighting system is typically constructed by using a number of primary colors, and in one example, the three primaries red, green and blue are used. The color of the generated light is determined by the LEDs that are used, as well as by the mixing ratios. To generate "white", all three LEDs have to be turned on.

By using LEDs it is possible to decrease the energy consumption, a

requirement, which is well in line with the current environmental trend. For further decreasing the energy consumption of the illumination arrangement it is possible to include light sensors and presence detectors, which will detect changes in ambient lighting and approaching persons, respectively. Such additions may in turn lead to a decrease in the time the illumination arrangement is active, as well as an intensity decrease by taking into account the ambient lighting.

In a lighting system comprising a plurality of the above described illumination arrangements, it may be useful to include a hardware component with the illumination arrangement for allowing some sort of communication between the different illumination arrangements, thereby further decreasing the energy consumption. For such communication, a number of wireless technologies have been developed, including for example RF transmission circuitry supporting IEEE 802.11. However, a problem with such wireless technologies are that they lead to a great increase of the cost of the illumination arrangement and thus the resulting lighting system. There has also been proposed to enable a more intuitive and simpler control of the light sources, and to create scenes, by the embedding of information sequences such as invisible identifiers in the light output of luminaires. This embedding of identifiers can be based on unique modulation of the visible light of the luminaire or by placing of an additional infra-red (IR) light source in the luminaire and uniquely modulate this IR light. The

Manchester code is commonly used as a means to modulate coded light.

However, even though the above mentioned embedding of information sequences such as invisible identifiers in the light output of luminaires provides some improvements and flexibility to the lighting systems, there is always a desire to further increase the bit rate of the information communicated by means of for example invisible identifiers for further expand the functionality of the lighting system.

SUMMARY OF THE INVENTION

According to an aspect of the invention, the above is at least partly met by a method for encoding an information sequence to be emitted as coded light by at least one light source, the method comprising the steps of receiving an input string representing the information sequence, dividing the input string into a plurality of pairs of bits, mapping each of the plurality of pairs of bits into one of four symbols based on a predetermined schedule, thereby forming a sequence of channel symbols, and providing the sequence of channel symbols to be emitted by the light source, wherein a pulse width of an active portion of each of the symbols of the sequence of channel symbols are based on a predetermined dimming level of light emitted by the light source.

The invention is based on the understanding that it may be possible to, in comparison to a Manchester type coding scheme, drastically increase the bit rate of transmitted information for a given bandwidth of the transmitted signal. This is achieved by upgrading the mapping scheme for each pair of bits by including an additional two waveforms/symbols, resulting in a total of four waveforms/symbols as compared to the only two waveforms/symbols used in Manchester coding. The increased bit rate is provided by synchronizing the transmitter with the receiver and thereby allowing for information to be sampled at all clock transitions. As a comparison, when using Manchester code, the transitions at the period boundaries do not carry information, they exist only to place the signal in the correct state to allow the mid-bit transition.

The four different waveforms/symbols each have a form that is based on a predetermined/desired dimming level of light that is to be emitted by the light source. As mentioned above, each pair of bits are mapped into one of four symbols based on a predetermined schedule; however the form of the waveform corresponding to a specific symbol is based on a dimming level of light to be emitted by the light source. In relation to pulse width modulation (PWM) this is accomplished by selecting an active portion of the mapped symbol/waveform (where the waveform has a fixed duration, i.e. symbol time) to correspond to the diming level of light to be emitted by the light source. As understood by the skilled addressee, this provides the advantage of allowing for both robust transmission of data and as well as for controlling the average illumination level of emitted light.

The predetermined mapping schedule is preferably achieved by configuring each of the four symbols such that each comprises four binary symbol elements. As an example, in an embodiment, the mapping is implemented as 11— > -1,1,1,-1; 00— > 1,-1,-1,1; 01— > -1,-1,1,1; and 10— > 1,1,-1,-1. The mapping as well as the encoding of the information sequence will be discussed in detailed below in the detailed description of the invention. Accordingly and in relation to an exemplifying embodiment, if each waveform has a fixed duration (symbol time), the time each Ί ' is "active" (duty cycle) is dependent on the desired diming level of light emitted by the light source.

According to another aspect of the invention, there is provided a method for decoding an information sequence from a signal received from a light detector, the signal being indicative of coded light as emitted from at least one light source, the method comprising the steps of receiving a sequence of channel symbols, calculating a first inner product between the sequence of channel symbols and a first matched filter, calculating a second inner product between the sequence of channel symbols and a second matched filter, the second matched filter being different from the first matched filter, and determining a pair of bits representing the portion of the information sequence based on the first and the second inner product.

The disclosed decoding method preferably matches the transmission of the information sequence being encoded using the above discussed encoding method and emitted by the light source. As mentioned, two separate inner products are calculated using two different matched filters, and the results of these calculations are used for determining a corresponding symbol, preferably comprising a pair of bits forming part of a binary string.

As the information sequence preferably comprises four binary symbol elements, so do the first and the second matched filters. More specifically, the first matched filter is preferably configured to have an essential a sinusoidal form and the second matched filter is preferably configured to have an essential cosinusoidal form. According to still another aspect of the invention, the encoding method may be implemented in an information encoder preferably comprising a control unit configured to receive an input string representing the information sequence, divide the input string into a plurality of pairs of bits, map each of the plurality of pairs of bits into one of four symbols based on a predetermined schedule, thereby forming a sequence of channel symbols, and provide the sequence of channel symbols to be emitted by the light source, wherein a pulse width of an active portion of each of the symbols of the sequence of channel symbols are based on a predetermined dimming level of light emitted by the light source.

Similarly and according to a still further aspect of the invention, the decoding method may be implemented in an information decoder preferably comprising a receiver for receiving a sequence of channel symbols, a control unit configured to calculate a first inner product between the sequence of channel symbols and a first matched filter, calculate a second inner product between the sequence of channel symbols and a second matched filter, the second matched filter being different from the first matched filter, and determine a pair of bits representing the portion of the information sequence based on the first and the second inner product.

These further aspects provide similar advantages as discussed above in relation to the encoding and decoding methods.

It should be noted that the information encoder and/or decoder may be provided as a separate unit, but may also be incorporated in with a light source of the lighting system or included in a socket of a light source. Also, to achieve a high energy efficiency the light source is preferably selected from a group comprising light emitting diodes (LEDs), organic light emitting diodes (OLEDs), polymeric light emitting diodes (PLEDs), inorganic LEDs, cold cathode fluorescent lamps (CCFLs), hot cathode fluorescent lamps (HCFLs), plasma lamps. As mentioned above, LEDs have much higher energy efficiency in

comparison to conventional light bulbs which generally deliver at best about 6% of their electric power used in the form of light. The skilled person would appreciate that it of course would be possible to use a standard incandescent light source, such as an argon, krypton, and/or xenon light source.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled addressee realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

The various aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which:

Fig. 1 illustrates a lighting system according to an embodiment;

Fig. 2 illustrates a light source according to an embodiment;

Fig. 3 illustrates an information decoder according to an embodiment;

Fig. 4 - 6 illustrates exemplary Quadrature Manchester Encoding waveforms according to the invention at differently set duty cycles;

Figs. 7a - 7d illustrate pulse trains for source symbols and channel symbols on the transmitter side according to currently preferred embodiments of the invention;

Figs. 8a - 8d illustrate pulse trains for portion of the sequence of channel symbols and exemplary matched filters for the calculating inner products for the portion of the sequence of channel symbols.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled addressee. Like reference characters refer to like elements throughout.

Referring now to the drawings and to Fig. 1 in particular where it is shown a lighting system 1 comprises at least one light source, schematically denoted by the reference numeral 2. The at least one light source 2 may be a luminaire and/or be part of a lighting control system, thus the lighting system 1 may be denoted as a coded lighting system. A luminaire may comprise at least one light source 2. The term "light source" means a device that is used for providing light in a space, for purpose of illuminating objects in the space. A space is in this context typically an apartment room or an office room, a gym hall, a room in a public place or a part of an outdoor environment, such as a part of a street. Each light source 2 is capable of emitting coded light, as schematically illustrated by the arrow 6. The emitted light thus comprises a modulated part associated with coded light comprising information sequences. The emitted light may also comprise an un-modulated part associated with an illumination contribution. Each light source 2 may be associated with a number of lighting settings, inter alia pertaining to the illumination contribution of the light source, such as color, color temperature and intensity of the emitted light. In general terms the

illumination contribution of the light source may be defined as a time-averaged output of the light emitted by the light source 2. The light source 2 will be further described with reference to Fig. 2.

As noted above the at least one light source 2 emits information sequences via the visible light 6. Before the information sequences are emitted via the visible light 6, it is mapped to a sequence of channels symbols to form a modulated signal. This modulated signal may then act as a control signal to drive the at least one light source. The control signal may thereby determine the pulse train which switches the at least one light source 2 between emitting light (in an "ON"-state) and not emitting light (in an "OFF"-state).

The lighting system 1 further comprises an apparatus, termed an information decoder 4. The information decoder 4 is arranged to decode an information sequence from coded light emitted by the at least one light source 2. The information decoder 4 will be further described with reference to Fig. 3.

The lighting system 1 may further comprise other devices 10 arranged to control and/or provide information to the at least one light source 2.

Fig. 2 schematically illustrates, in terms of a number of functional blocks, a light source 2. The light source 2 comprises an emitter 14 for emitting coded light. The emitter 14 may comprise one or more LEDs, but it could as very well comprise one or more FL or HID sources, etc. In general, the coding schemes may utilize multiple light sources. For example, a 3-level coding scheme may have two LEDs using the mappings (OFF, OFF) for the level "-A", (ON, OFF) for the level "0", and (ON, ON) for the level "+A". How the levels are determined is disclosed below. The emitter is controlled by a light driver 18. The light driver 18 may comprise or be part of a processing unit 16 such as a central processing unit (CPU). As such the light driver 18 comprises a receiver 20 and a transmitter 24. The receiver 20 may be arranged to receive settings, control information, code parameters and the like. The receiver 20 may be a receiver configured to receive coded light. The receiver 20 may comprise an infrared interface for receiving infrared light. Alternatively the receiver 20 may be a radio receiver for receiving wirelessly transmitted information. Yet alternatively the receiver 20 may comprise a connector for receiving information transmitted by wire. The wire may be a powerline cable. The wire may be a computer cable. Information pertaining to settings, control information, code parameters and the like may be stored in the memory 22. The light driver 18 may receive information via the receiver 20 pertaining to an information sequence to be transmitted by means of coded light by the light source 2. By e.g. utilizing the processing unit 16 the light driver 18 may change the encoding of the coded light such that the coded light emitted by the emitter 14 comprises (an encoded version of) the information sequence. In order to achieve such a transmission the light driver 18 may be arranged to perform a number of functionalities. For example the receiver 20 is arranged to receive a sequence u = [ui, . . . , ¾, . . . , UK] of source symbols ¾ representing an information sequence of an information source. The processing unit 16 is arranged to determine, from the sequence of source symbols, a sequence z = [z_{l s} . . . , Zk, . .. , z_K] of channel symbols Zk forming a control signal. This is further explained in relation to Figs. 5a-d.The transmitter 24 is arranged to provide the light source 2 with the control signal and thereby drive the light source 2. Alternatively, the light source 2 does not comprise a light driver. The light driver 18 may then be part of the lighting system 1.

The information decoder 4 may be arranged to detect and receive light, such as coded light, comprising information sequences emitted by the at least one light source 2 as well as the light emitted by light sources outside the lighting system 1 (not shown). From the detected and received light the receiver 4 is arranged to determine information sequences transmitted by the at least one light sources 2. A functional block diagram for an information decoder 4 according to an embodiment of the present invention is given in Fig. 3. The information decoder 4 comprises a receiver 34 arranged to receive the signal from a light detector 32, the signal being indicative of the sequence z = [z_{l s} . . . , Zk, . .. , z_K] of channel symbols Zk. The information decoder 4 further comprises a processing unit 26 arranged to determine, from the signal, a plurality of bit pairs each forming part of the decoded information sequence. This is further explained in relation to Figs. 6a-d. The information decoder 4 may further comprise a memory 28 and a transmitter 30. The memory 28 may store instructions pertaining to the functionalities to estimate an information sequence. The transmitter 30 may be utilized in order to communicate information to the at least one light source 2 in the lighting system 1.

As noted above, before the information sequences are emitted via the visible light 6 it is mapped to a sequence of channels symbols to form a modulated signal. The mapping involves determining a modulation scheme with which the sequence of channels symbols is to be modulated with. According to the invention, a Quadrature Manchester Encoding (QME) is used as a means to modulate the coded light. In QME, each pair of bits from of the source sequence u, of length 2n, are encoded into one of four QME symbols, each symbol, i, comprising a sequence of four binary symbol elements z_4i+j, where j ranges from 0 to 3. The sequence of QME symbols makes an output symbol string y of length An.

As mentioned, for encoding the binary source sequence u, each binary pair [«2i«2i+i] is mapped onto QME symbol [z₄i, ζ₄₁₊₁, z₄i₊₂, z₄i₊₃] as follows:

11→-l, 1,1,-1

00→ 1,-1,-1,1

01→ -1,-1,1,1

10→ 1,1,-1,-1.

Correspondingly, each QME symbol corresponds to one of the four basic waveforms shown in Fig.4 for modulating the visible light 6. As can be seen, the light intensity atft), corresponding to symbol z_; is such that the average light level a at a detector placed at an arbitrary but fixed distance from the transmitter is constant dc and the average variation of a is zero. In the illustrated example shown in Fig.4a - 4d, the duty cycle for the waveforms are shown to be 50%. As understood, it is possible and within the scope of the invention to vary the duty cycle of the waveforms based on a desired dimming level of light to be emitted by the light source 14 as shown Fig 2. Further examples illustrating different duty cycles are given in Figs 5a - d and 6a - d where the duty cycles of the different symbols are set to 10% and 75% respectively.

Figs.7a-d gives an example of an application of the proposed mapping. Fig. 7a illustrates a source sequence ¾ = [-1, 1, 1,-1, 1, 1,-1,-1, ...] which inter alia may represent the information sequence [0, 1, 1,0, 1, 1, 0, 0, ...]. A waveform representation for the source sequence u is illustrated in Fig.7b where each source bit has a time duration of T seconds. For illustrative purpose only the amplitude of the waveform is confined to the interval from -1 to +1, but in general it may be from -A to +A if amplitude modulation with amplitude A is used. Fig.7c illustrates the corresponding sequence of channel symbols z, i.e. Zk = [-1,-1,1,1, 1,1,-1,-1, -1,1,1,-1, 1,-1,-1,1, ...]. A waveform representation for the channel sequence z is illustrated in Fig.7d.

Figs.8a-d gives an example of the reception of an exemplary sequence of channel symbols z, i.e. Zk = [-1,-1,1,1, 1,1,-1,-1,-1,1,1,-1, 1,-1,-1,1, ...] being received by the receiver 34. The sequence of channel symbols z is divided into groups of four binary symbol elements, each group forming a portion of the sequence of channel symbols z , i.e. z_a= [-1,- 1,1,1], Zb = [ 1,1,-1,-1], etc (shown in Figs.8a and 8b). An inner product is then calculated between each of the groups of the sequence of channel symbols, e.g. z_a, Zb, and two matched filters (shown in Figs. 8c and 8d) being denoted as an inside-outside (i-o) filter and a right- left (r-1) filter, respectively. Each of the matched filters are in turn each represented by the sequence i-o = [-1, 1, 1, -1] and r-1 = [-1, -1, 1, 1]. In the current example in regards to z_a= [- 1,-1,1,1], Zb = [ 1,1,-1,-1], the calculations are implemented as:

Inner product Zai = z_a * i-o = [-1,-1,1,1] * [-1, 1, 1, -if = (-1 ) + (-1 * 1) + (1 * 1) + (1 *-1) = 0

Inner product z_a2 = z_a * l-r = [-1,-1,1,1] * [-1, -1, 1, 1] = (-1 ) + (-1 ) + (1 *1) + 1 * 1 = 4

Inner product z_M = z_b * i-o = [1,1,-1,-1] * [-1, 1, 1, -1] = (1 *-1) + (1 * 1) + (- l * l) + (-l *-l) = 0

Inner product z_b2 = z_b * 1-r = [1,1,-1,-1]* [-1, -1, 1, 1] = (-1 *-1)+ (1 *-1) + (- 1 * 1) + (-1 * 1) = -4

As will be readily understood, it is based on the result of the inner product calculations possible to identify a corresponding bit pair of the information sequence.

According to the example, the identification of the bit pair corresponding to a given group is determined by analyzing which inner product (i.e. the "inside-outside" inner product or the "left-right" inner product) that has got the largest absolute value as well as the sign of the inner product having the largest absolute value. In the current example, the "left-right" inner product for z_a, i.e. z_a2, is identified to have the largest absolute value, i.e. 4, and the sign is positive, i.e. +. This in turn maps to a specific bit pair, in this case being "01". Similarly, for Zb also the "left-right" inner product, i.e. Zb₂ is identified to have the largest absolute value, i.e. 4, however the sign is negative, i.e. -. This correspondingly maps to a predefined bit pair, in this case being "10". Of course a similar analysis and calculation is valid for mapping of the two remaining bit pairs (i.e. "00" and "11"). It should be noted that the matched filters disclosed in Figs. 8a - d are designed to work independently of any duty cycle selected by the transmitter 24, e.g. the duty cycles as illustrated in Figs. 4 - 6. Additionally, the matched filters are also robust to amplitude modulation of the transmitted signal.

Based on the design of the mapping scheme and the matched filters the communication of information sequences from the transmitter 24 to the receive 34 may be made robust at the same time as it is the mapping scheme allows for both pulse width and amplitude modulation of the signals for the purpose of allowing the visible light 6 to be dimmable. Preferably and for the purpose of maximizing the robustness of communication of the information sequences, amplitude modulation of the signals are used for dimming levels between e.g. 10% and 100% of the maximal light emission levels and pulse width modulation is used for dimming levels below 10%>.

Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Variations to the disclosed embodiments can be understood and effected by the skilled addressee in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

Claims

CLAIMS:

1. A method for encoding an information sequence to be emitted as coded light by at least one light source, the method comprising the steps of:

receiving an input string representing the information sequence; dividing the input string into a plurality of pairs of bits;

mapping each of the plurality of pairs of bits into one of four symbols based on a predetermined schedule, thereby forming a sequence of channel symbols; and

providing the sequence of channel symbols to be emitted by the light source, wherein a pulse width of an active portion of each of the symbols of the sequence of channel symbols are based on a predetermined dimming level of light emitted by the light source.

2. Method according to claim 1, wherein each of the four symbols comprises four symbol elements.

3. Method according to claim 2, wherein the mapping is implemented as:

11→-l, 1,1,-1;

00→ 1,-1,-1,1;

01→ -1,-1,1,1; and

10→ 1,1,-1,-1.

4. A method for decoding an information sequence from a signal received from a light detector, the signal being indicative of coded light as emitted from at least one light source, the method comprising the steps of:

receiving a sequence of channel symbols;

calculating a first inner product between the sequence of channel symbols and a first matched filter;

calculating a second inner product between the sequence of channel symbols and a second matched filter, the second matched filter being different from the first matched filter; and determining a pair of bits representing the portion of the information sequence based on the first and the second inner product.

5. Method according to claim 4, wherein the sequence of channel symbols comprises four binary symbol elements, the first matched filter has a sinusoidal form, and the second matched filter has a cosinusoidal form.

6. An information encoder for encoding an information sequence to be emitted as coded light by at least one light source, the information encoder comprising a control unit configured to:

receive an input string representing the information sequence; divide the input string into a plurality of pairs of bits;

map each of the plurality of pairs of bits into one of four symbols based on a predetermined schedule, thereby forming a sequence of channel symbols; and

provide the sequence of channel symbols to be emitted by the light source, wherein a pulse width of an active portion of each of the symbols of the sequence of channel symbols are based on a predetermined dimming level of light emitted by the light source.

7. An information decoder for decoding an information sequence from a signal received from a light detector, the signal being indicative of coded light as emitted from at least one light source, the information decoder comprising:

a receiver for receiving a sequence of channel symbols;

a control unit configured to:

calculate a first inner product between the sequence of channel symbols and a first matched filter;

calculate a second inner product between the sequence of channel symbols and a second matched filter, the second matched filter being different from the first matched filter; and

determine a pair of bits representing the portion of the information sequence based on the first and the second inner product.

8. A luminaire for emitting light having an adjustable characteristic, the luminaire comprising at least one light source, light detector and an information decoder according to claim 7.