Modulation for coded light transmission
View Patent ↗Coded light has been proposed to enable advanced control of light sources and transmit information using light sources. Sequences of channel symbols to drive the light source are determined from sequences of source symbols such that no visible flicker is present in coded light emitted by the light sources. Each source symbol is mapped to a composite channel symbol comprising at least one first channel symbol which may be identical to the current source symbol and at least one second channel symbol which may be a function of the current source symbol and at least one future and/or past source symbol.
1. A light driver for driving at least one light source arranged to emit coded light based on a control signal, the light driver comprising:
a receiver arranged to receive a sequence u=[u1, . . . , uk, . . . , uK] of source symbols uk representing an information sequence of an information source;
a processing unit arranged to determine, from the sequence of source symbols, a sequence z=[z1, . . . , zk, . . . , zK] of channel symbols zk forming the control signal;
a transmitter arranged to provide the at least one light source with the control signal thereby driving the at least one light source;
wherein the processing unit is arranged to determine the sequence of channel symbols z by mapping each source symbol uk at time k to a composite channel symbol zk=(zk,1, zk,2) comprising at least one first channel symbol zk,1, one of which is identical to uk, and at least one second channel symbol zk,2 having a value chosen from a set M, wherein the value of zk,2 is determined according to a weighting function having the source symbol uk and at least one future and/or past source symbol ui, i≠k as input arguments, the weighting function comprising at least two non-zero weighting factors, the mapping resulting in that the power spectral density at frequency zero of the sequence z of channel symbols is equal to zero, and wherein
the control signal thereby enables no visible flicker to be present in coded light emitted by the at least one light source.
2. The light driver according to claim 1 , wherein the number of elements in the set M is odd.
3. The light driver according to claim 1 , wherein the set M is symmetric around zero.
4. The light driver according to claim 1 , wherein a zero element is included in the set M.
5. The light driver according to claim 1 , wherein zk,2 is a signed summation of weighted averages of L terms uk−l+1+uk+1 for l=1, . . . L, resulting in that the power spectral density of the sequence z of channel symbols decays as (fT)(4L), where f denotes frequency in Hz and where T denotes the time in seconds for transmitting uk.
6. The light driver according to claim 5 , wherein a weighting factor a L,l for the L terms is determined such that
1
-
∑
l
=
1
L
a
L
,
l
cos
(
(
2
l
-
1
)
π
fT
)
=
O
(
(
fT
)
2
L
)
holds.
7. The light driver according to claim 6 , wherein the weighting factor a L,l is further determined such that
A
2
T
(
1
2
(
1
-
∑
l
=
1
L
a
L
,
l
cos
(
(
2
l
-
1
)
π
fT
)
)
sin
c
(
fT
/
2
)
)
2
=
O
(
(
fT
)
4
L
)
holds for amplitude modulation with amplitude A of the sequence of channel symbols.
8. The light driver according to claim 6 , wherein zk,2=−(uk+uk+1)/2.
9. The light driver according to claim 6 , wherein the at least one first channel symbol zk,1 is a sequence of first channel symbols, zk,1=[zk,11 . . . zk,1j . . . zk,1N] where the j:th symbol is given by zk,1j=(−1)j−1uk, for j=1, . . . , N, and wherein zk,2=−(zk,1N+zk+1,11)/2.
10. A luminaire comprising at least one light source, modulation means and a light driver according to claim 6 , the modulation means being arranged to modulate, according to the control signal provided by the light driver, coded light to be emitted by the at least one light source.
11. 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 driven by a light driver according to claim 6 , comprising:
a receiver arranged to receive the signal from the light detector, the signal being indicative of the sequence z=[z1, . . . , zk, . . . , zK] of channel symbols zk=(zk,1, zk,2);
a processing unit arranged to determine, from the signal, a sequence û=[û1, . . . , ûk, . . . , ûK] of decoded source symbols ûk forming the decoded information sequence;
wherein the processing unit is arranged to determine the sequence of decoded source symbols û whereby the decoded source symbol ûk is determined from a difference between zk,1 and a weighted average of at least the channel symbol zk,2 and one future and/or past channel symbol zi, i≠k.
12. The information decoder according to claim 11 , wherein the processing unit comprises a hard decision decoder, and wherein the hard decision decoder is arranged to determine the sequence of decoded source symbols û as the sign of said difference.
13. The information decoder according to claim 11 , wherein ûk is determined according to ûk=sign (zk,1−(zk−1,2+zk,2)/2).
14. The information decoder according to claim 11 , wherein û k is determined according to
u
^
k
=
sign
(
z
k
,
1
-
∑
l
=
1
L
a
L
,
l
z
k
-
1
,
2
+
z
k
+
1
-
1
,
2
2
)
.
15. 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 driven by a light driver according to claim 6 , comprising:
a receiver arranged to receive the signal from the light detector, the signal being indicative of the sequence z=[z1, . . . , zk, . . . , zK] of channel symbols zk=(zk,1, zk,2);
a processing unit arranged to determine, from the signal, a sequence û=[û1, . . . , ûk, . . . , ûK] of decoded source symbols ûk forming the decoded information sequence;
wherein the processing unit is arranged to determine the sequence of decoded source symbols û whereby the decoded source symbol ûk is determined by taking linear combinations of zl,1, and half symbols zl,2,L and zl,2,R, for values of l denoting a set of channel symbols from said sequence z, where zk,2,L is the first half of zk,2 and zk,2,R is the second half of zk,2.
16. The information decoder according to claim 15 , wherein the processing unit is arranged to determine the sequence of decoded source symbols û by using a matched filter having a set of coefficients comprising the sequence [−1,0,1,1].
17. The information decoder according to claim 15 , wherein the matched filter has 2N+4 coefficients, starting with the sequence [−1,0], followed by N pairs of coefficients alternating between [1,1] and [−1,−1] until the N-th pair which equals [(−1)N−1, (−1)N−1], and ending with the sequence [0, (−1)N].
18. A method for driving at least one light source arranged to emit coded light based on a control signal, the method comprising:
receiving a sequence u=[u1, . . . , uk, . . . , uK] of source symbols uk representing an information sequence of an information source;
determining a sequence z=[z1, . . . , zk, . . . , zK] of channel symbols zk forming the control signal from the sequence of source symbols;
providing the at least one light source with the control signal, thereby driving the at least one light source;
wherein the sequence of channel symbols z is determined by mapping each source symbol uk at time k to a composite channel symbol zk=(zk,1, zk,2) comprising at least one first channel symbol zk,1, one of which is identical to uk, and at least one second channel symbol zk,2 having a value chosen from a set M, wherein the value of zk,2 is determined according to a weighting function having the source symbol uk and at least one future and/or past source symbol ui, i≠k as input arguments, the weighting function comprising at least two non-zero weighting factors, the mapping resulting in that the power spectral density at frequency zero of the sequence z of channel symbols is equal to zero, and wherein
the control signal thereby enables no visible flicker to be present in coded light emitted by the at least one light source.