GB2498933A

GB2498933A - Forwarding a mobile telephone signal using light

Info

Publication number: GB2498933A
Application number: GB1201552.5A
Authority: GB
Inventors: Roger Julian Green; Arnesh Vijay
Original assignee: University of Warwick
Current assignee: University of Warwick
Priority date: 2012-01-30
Filing date: 2012-01-30
Publication date: 2013-08-07
Also published as: WO2013114103A1; GB201201552D0

Abstract

Forwarding a mobile telephony signal to a mobile device 110 using light, by means o an apparatus 200 configured to receive an analogue signal 204 comprising the mobile telephony signal and to produce a digital signal 306 based upon the analogue signal, the apparatus 200 further configured to produce a continuous phase modulated signal 310 based upon the digital signal 306, wherein the continuous phase modulated signal 310 is for modulating the output of a light source 206.

Description

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Communication apparatus Description

The application relates to communication apparatus and, in particular, to apparatus 5 for forwarding a mobile telephony signal to a mobile device using light.

In visible light communication systems, data signals are modulated onto visible light. At practical data rates, the modulation is too rapid to be sensed by the human eye. The visible light can thus be used for both illumination and communication. 10 Light emitting diodes (LEDs) are often used as the light source because, amongst other things, their output can be modulated sufficiently rapidly. The use of white LEDs for illumination is also becoming increasingly widespread. Nevertheless, many ways of exploiting the potential of visible light communications and of implementing practical systems remain unexplored.

15

According to a first aspect of the invention, there is provided apparatus for forwarding a mobile telephony signal to a mobile device using light, the apparatus configured to receive an analogue signal comprising the mobile telephony signal and to produce a digital signal based upon the analogue signal, the apparatus further 20 configured to produce a continuous phase modulated signal based upon the digital signal, wherein the continuous phase modulated signal is for modulating the output of a light source.

Thus, the apparatus can enable a mobile device to receive a mobile telephony signal 25 by a means other than a radio signal. This can be particularly useful for reception indoors where the radio signal may be attenuated and where artificial lighting may be needed in any case. Furthermore, the mobile telephony signal can be forwarded effectively and efficiently and in way which enables the mobile device to process the signal in substantially the same way as a received radio signal.

30

As used herein, the term light is used to mean, for example, visible light or infrared light.

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The light may be visible light.

The mobile telephony signal may comprise a Global System for Mobile Communications signal.

5

The continuous phase modulated signal may comprise a minimum shift keyed signal. The continuous phase modulated signal may comprise a Gaussian minimum shift keyed signal.

10 Producing the digital signal may comprise producing a first digital signal representative of the analogue signal and producing a second digital signal which is a continuous phase digital signal based upon the first digital signal.

The apparatus may further comprise the light source, the light source may comprise 15 a light emitting diode, and the continuous phase modulated signal may be used to modulate the intensity of the light emitted by the light emitting diode.

The light emitting diode may comprise one or more light emitting diodes configured to produce white light.

20

The apparatus may be configured to receive a radio signal comprising the mobile telephony signal from a base station of a mobile telephone network.

The apparatus may be configured to receive a further light signal comprising a 25 further mobile telephony signal from each of one or more mobile devices and to produce a further analogue signal based upon the further light signal, wherein the further analogue signal is for radio transmission to a base station of a mobile telephone network.

30 The further light signal may have different wavelengths from the light output by the light source.

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Thus, the apparatus can also enable mobile devices to transmit mobile telephony signals to a base station by means other than radio signals and to do so in such a way that interference between the transmitted and received light signals is minimised.

5

The further light signal may be an infrared light signal.

According to a second aspect of the invention, there is provided apparatus for receiving a mobile telephony signal transmitted using light, the apparatus configured 10 to detect a light signal comprising the mobile telephony signal and to produce a signal based upon the light signal, wherein the light signal is modulated such that the signal comprises a continuous phase modulated signal, the apparatus further configured to demodulate the signal, to produce a digital signal based upon the demodulated signal, and to produce an analogue signal based upon the digital signal, 15 wherein the analogue signal is for processing by a mobile device in substantially the same way as a received radio signal comprising a mobile telephony signal.

The light may be visible light.

20 The mobile telephony signal may comprise a Global System for Mobile Communications signal.

The continuous phase modulated signal may comprise a minimum shift keyed signal. The continuous phase modulated signal may comprise a Gaussian minimum shift 25 keyed signal.

The apparatus may be configured to receive a further analogue signal comprising a further mobile telephony signal, to produce a further digital signal based upon the further analogue signal and to produce a further continuous phase modulated signal 30 based upon the further digital signal, wherein the further continuous phase modulated signal is for modulating the output of a light source.

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The light output by the light source may have different wavelengths from the light signal which the apparatus is configured to detect.

The light source may be an infrared light source.

5

There may be provided a mobile device comprising apparatus according to the second aspect of the invention. There may be provided a communication system comprising apparatus according to the first aspect of the invention and at least one of the mobile devices, wherein the apparatus is configured to forward the mobile 10 telephony signal to the mobile device using light and the mobile device is configured to transmit the further mobile telephony signal to the apparatus using further light having different wavelengths to the light.

According to a third aspect of the invention, there is provided a method for 15 forwarding a mobile telephony signal to a mobile device using light, the method comprising: receiving an analogue signal comprising the mobile telephony signal; producing a digital signal based upon the analogue signal; and producing a continuous phase modulated signal based upon the digital signal, wherein the continuous phase modulated signal is for modulating the output of a light source.

20

Producing the digital signal may comprise: producing a first digital signal representative of the analogue signal; and producing a second digital signal which is a continuous phase digital signal based upon the first digital signal.

25 The method may comprise: receiving a radio signal comprising the mobile telephony signal from a base station of a mobile telephone network.

The method may comprise: receiving a further light signal comprising a further mobile telephony signal from each of one or more mobile devices; and producing a 30 further analogue signal based upon the further light signal, wherein the further analogue signal is for radio transmission to a base station of a mobile telephone network.

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According to a fourth aspect of the invention, there is provided a method for receiving a mobile telephony signal transmitted using light, the method comprising: detecting a light signal comprising the mobile telephony signal and producing a signal based upon the light signal, wherein the light signal is modulated such that 5 the signal comprises a continuous phase modulated signal; demodulating the signal; producing a digital signal based upon the demodulated signal; and producing an analogue signal based upon the digital signal, wherein the analogue signal is for processing by a mobile device in substantially the same way as a received radio signal comprising a mobile telephony signal.

10

The method may comprise: receiving a further analogue signal comprising a further mobile telephony signal; producing a further digital signal based upon the further analogue signal; and producing a further continuous phase modulated signal based upon the further digital signal, wherein the further continuous phase modulated 15 signal is for modulating the output of a light source.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a block diagram of a communication system embodying aspects of the 20 invention;

Figure 2 is a block diagram of parts of the light transceiver and the mobile device shown in Figure 1;

Figure 3 is a block diagram of the visible light transmitter shown in Figure 2;

Figure 4 is a flow diagram showing example operations of the visible light 25 transmitter shown in Figure 3;

Figure 5 is a block diagram of the visible light source shown in Figure 4;

Figure 6 is a block diagram of an alternative to the continuous phase generator block shown in Figure 3;

Figure 7 is a block diagram of the visible light receiver shown in Figure 2; and 30 Figure 8 is a flow diagram showing example operations of the visible light receiver shown in Figure 3.

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Communication System 100

Figure 1 shows a communication system 100 embodying aspects of the invention. The communication system 100 comprises a radio transceiver 102, an optional signal distributor 104, and one or more light transceivers 106. The elements 102 5 and 104 and the elements 104 and 106 are preferably operatively interconnected via respective fibre-optic links 108. However, they may be operatively interconnected via any suitable means. The communication system 100 further comprises one or more mobile devices 110 which may communicate with the light transceivers 106 and/or one or more base stations 112 associated with one or more mobile networks 10 (not shown).

As will be explained in more detail below, the radio transceiver 102 can exchange radio frequency (RF) mobile telephony signals with the base stations 112. The RF mobile telephony signals are passed to and from the light transceivers 106 which 15 can exchange the signals with one or more of the mobile devices 110 using light as the communication means. The signals to the mobile devices 110, i.e. the downlink signals, are preferably transmitted using visible light; the signals from the mobile devices 110, i.e. the uplink signals, are preferably transmitted using infrared light. The mobile devices 110 can receive a visible light signal and convert this signal to 20 an RF mobile telephony signal, which can then be processed in the usual way. The mobile device 110 can also convert an RF mobile telephony signal, which can be generated by the mobile device 110 in the usual way, to an infrared light signal and transmit this signal to the light transceiver 106.

25 The communication system 100 is shown in a building 111. Flowever, the communication system 100 may be used in other settings, particularly settings where mobile telephone reception is weak and where lighting may be provided. Such settings include, for example, structures such as tunnels, vehicles such as trains, etc. The communication system 100 may also be used in settings where, regardless of 30 the reception strength, radio communications are to be avoided for a reason such as interference with electronic devices. Such settings include, for example, hospitals. Furthermore, the communication system 100 need not be used indoors but may be used outdoors, for example in illuminated urban areas.

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The radio transceiver 102 is preferably located such that it has a strong connection to the one or more base stations 112 and hence to one or more mobile telephone networks (not shown).

5

The exchange of the light signals between the light transceiver 106 and the mobile device 110 preferably requires a line-of-sight therebetween. Thus, the light transceivers 106 may be located such that there are one or more overhead light transceivers 106 in each of a plurality of rooms of the building 111, as shown in the 10 figure. However, the light transceivers 106 may be located in any suitable way. In some embodiments, there may be only one light transceiver 106.

One or more mobile devices 110 may be located in each room of the building 111, as shown in the figure. However, there may any number of mobile devices 110 and 15 these may be located anywhere. Moreover, mobile devices 110 may be moved into, around, and out of the building 111.

Radio transceiver 102 and signal distributor 104

Referring still to Figure 1, the radio transceiver 102 is configured to transmit a radio 20 signal to, and receive a radio signal from, the one or more base stations 112. The radio signals comprise mobile telephony signals, which preferably comprise Global System for Mobile Communications (GSM) signals. However, other, similar communication protocols may also be used. The radio transceiver 102 includes one or more antennas 113 for converting the radio signal to an RF electrical signal and 25 vice versa. The antenna 113 may take any suitable form and may be omnidirectional or directional. The radio transceiver 102 may include circuitry (not shown) configured to process the RF electrical signals.

The radio transceiver 102 is further configured to exchange the RF signals 30 comprising the mobile telephony signals with the one or more light transceivers 106. As will be explained below, if there is more than one light transceiver 106,

then the signals are preferably exchanged via the signal distributor 104. Since the connections between the radio transceiver 102 and the light transceivers 106

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preferably comprise the fibre-optic links 108, the radio transceiver 102 preferably includes circuitry (not shown) configured to convert the RF electrical signal to an optical signal and vice versa. This may be done in any suitable way.

5 The signal distributor 104 is configured to distribute the signal received from the radio transceiver 102 to each of the plurality of light transceivers 106 and to provide the signal received from each of the light transceivers 106 to the radio transceiver 102. The signal distributor 104 may comprise an optical-to-electrical converter (not shown) at each of its inputs and a corresponding electrical-to-optical converter (not 10 shown) at each of its outputs. The signal distributor 104 may further comprise circuitry (not shown) configured to divide the signal received from the radio transceiver 102 and to combine the signals received from the light transceivers 106.

Light transceiver 106

15 Referring still to Figure 1, the light transceiver 106 preferably includes circuitry 114 to convert the optical signal received via the fibre-optic link 108 to an RF electrical signal (Fig. 2, feature 204) and vice versa. The circuitry 114 preferably corresponds to that at the radio transceiver 102 and hence the RF electrical signals (before and after the conversion) preferable correspond to each other. In the figure, only one 20 of the light transceivers 106, i.e. light transceiver 106l5 is shown as comprising the circuitry 114. However, all of the light transceivers 106 are preferably the same.

Referring to Figure 2, the light transceiver 106 further comprises a visible light transmitter 200 and an infrared light receiver 202.

25

As will be explained in more detail below, the visible light transmitter 200 is provided with the RF electrical signal 204 from the circuitry 114. The visible light transmitter 200 is configured to cause a light emitting diode (LED) 206 to transmit a modulated visible light signal 208 derived from the RF electrical signal 204. The 30 visible light signal 208 can be received by the mobile devices 110.

The infrared light receiver 202 is configured to receive an infrared light signal 210 from one or more mobile devices 110. The infrared light signal 210 is detected

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using a photodiode 212. The infrared light receiver 202 is further configured to generate an RF electrical signal 214 derived from the received infrared light signal 210. The RF electrical signal 214 is provided to the circuitry 114, which passes the signal to the radio receiver 102 which, in turn, transmits the signal to the one or 5 more base stations 112 as explained above.

Mobile device 110

Referring still to Figure 2, the mobile device 110 comprises a further transceiver 216 configured to enable the mobile device to communicate with the light transceiver 10 106. The further transceiver 216 may be an integral part of the mobile device 110, as shown in the figure, or it may be a separate part which is connectable to the mobile device 110.

The further transceiver 216 comprises a visible light receiver 218 and an infrared 15 light transmitter 220.

As will be explained in more detail below, the visible light receiver 218 comprises a photodiode 221 configured to detect the visible light signal 208 from the light transceiver 106 and to generate an RF electrical signal 222 derived from the visible 20 light signal 208. The RF electrical signal 222 corresponds to the signal received by the radio transceiver 102. Thus, the RF electrical signal 222 can be processed by the mobile device 110 in the usual way, that is to say using at least some of the same components of the mobile device 110 that are used for processing RF electrical signals received directly from the base station 112 using an antenna 224.

25

The infrared light transmitter 220 is provided with an RF electrical signal 226. The RF electrical signal 226 may have been generated by the mobile device 110 using at least some of the same components of the mobile device 110 that are used for generating RF electrical signals to be transmitted directly to a base station 112 using 30 the antenna 224. For example, the RF electrical signal 226 may be the same as the RF electrical signal that is provided to the antenna 224 during radio communications. The infrared light transmitter 220 is configured to cause an infrared light emitting diode 228 to transmit the modulated infrared light signal 210

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derived from the RF electrical signal 226. The infrared light signal 210 can be received by the light transceiver 106.

In Figure 2, a mobile device 110 is communicating with a light transceiver 106.

5 However, as will be appreciated, there may be a plurality of mobile devices 110 in communication with the light transceiver 106. In this case, the signals associated with the uplink, e.g. the radio signal eventually transmitted to the base station 112, may comprise a plurality of mobile telephony signals transmitted by the plurality of mobile devices 110. Also, the downlink signals, e.g. visible light signal 208 10 transmitted by the light transceiver 106, may comprise a plurality of mobile telephony signals, each of which may be received and processed by one or more mobile devices 110.

Using visible light for the downlink from the light transceiver 106 to the mobile 15 device 110 and using infrared light for the uplink from the mobile device 110 to the light transceiver 106 can have the advantage of reducing interference, e.g. inter-symbol interference, between the downlink and the uplink. Nevertheless, as will be explained in more detail below, the transmitters 200 and 220 and the receivers 202 and 218 are preferably substantially the same except for the light emitting and 20 detecting means. This can make it easier and more efficient to implement the communication system 100.

Since the mobile device 110 is preferably also configured to communicate directly with the base station 112 using radio signals, it may further comprise hardware 25 and/or software (not shown) for switching between communicating using light and/or using radio signals. This may be performed in any suitable way. For example, the mobile device 110 may switch from one of the communication means to the other in dependence upon signal strength.

30 Thus, the communication system 100 can provide uninterrupted mobile telephony connectivity, for example when radio signals become attenuated indoors.

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Furthermore, since power consumption may be higher when communicating using radio signals compared to using light, particularly when reception is weak, the communication system 100 can help to increase the battery life of the mobile devices 110.

5

Visible light transmitter 200

Figure 3 shows the visible light transmitter 200 in more detail.

The visible light transmitter 200 preferably comprises an analogue-to-digital 10 converter 300 which is configured to convert the RF electrical signal 204 into a digital signal 302. The visible light transmitter 200 further comprises a continuous phase generator block 304 which is provided with the digital signal 302 and is configured to generate a continuous phase digital waveform 306. The visible light transmitter 200 further comprises a modulation block 308 which is configured to 15 modulate the continuous phase digital waveform 306 onto an analogue carrier thereby generating a continuous phase modulated (CPM)/minimum shift keyed (MSK) signal 310. The CPM/MSK signal 310 is provided to a lighting block 311 which is configured to cause the light emitting diode 206 to transmit the correspondingly-modulated visible light signal 208. These features will now be 20 described in more detail.

The analogue-to-digital converter 300 preferably comprises an amplitude compressor 312, a uniform quantiser 314 and an encoder 316.

25 The amplitude compressor 312 is configured to reduce the dynamic range of the RF electrical signal 204. This can improve the signal-to-distortion ratio of the signal and the coding efficiency.

The uniform quantiser 314 is configured to convert the signal from the amplitude 30 compressor 312 into a discrete signal. The properties of the uniform quantiser 314 are selected such that the fidelity of the signal is retained while unnecessary precision is eliminated and the dynamic range is kept within practical limits.

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The encoder 316 is configured to encode the signal from the uniform quantiser 314 into a digital signal 302 comprising a stream of digital symbols ak, wherein k = 0, 1, 2, etc. This may be done in any suitable way.

5 The continuous phase generator block 304 preferably comprises a digital filter 318 and a phase generator 320. The digital filter 318 is preferably a finite impulse response filter. The digital filter 318 is configured to convert the digital symbols ak from the analogue-to-digital converter 300 into a digital message waveform, e.g.

kp(t — kT), wherein p is the delay of the standard rectangular pulse and T

10 is the time period of the symbols ak. This digital message waveform, which has a discontinuous phase, is then provided to the phase generator 320 which is configured to produce a continuous phase digital message waveform 306, e.g. Mx (/) = iTCh^a^t -kT). Here, h is the modulation index which is set to a value of 0.5.

15

The modulation block 308 preferably comprises a signal generator 322 and a modulator 324. The signal generator 322 is configured to generate an analogue sinusoidal carrier signal. The carrier frequency is preferably around several hundred MHz but may be more than or less than this. The modulator 324 is configured to 20 modulate the continuous phase digital message waveform 306 onto the carrier signal using frequency-shift keying (FSK). This may be performed in any suitable way. Thus, the modulator 324 produces the CPM/MSK signal 310.

The lighting block 311 preferably comprises a summing element 325, a driving 25 circuit 326 and the LED 206. The summing element 325 is configured to add a direct current (DC) offset signal 327 to the CPM/MSK signal 310. Thus, a signal with a constant polarity is produced, which is provided to the driving circuit 326. The driving circuit 326 is configured to generate a signal which is suitable for causing the LED 206 to emit a visible light signal 208 which is intensity-modulated 30 in a way which corresponds to the CPM/MSK signal 310. This may be done in any suitable way. The power of the light emitted by the LED 206 is typically

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proportional to the input current. The LED 206 may form part of the visible light transmitter 200 or it may be provided separately.

Operation of the visible light transmitter 200 5 Figure 4 shows operations which can be performed by a visible light transmitter according to an embodiment of the invention.

At step S400, an analogue signal comprising a mobile telephony signal is received. This analogue signal may be the RF electrical signal 204 described above. The 10 signal may be treated as analogue, although, as will be appreciated, it comprises digital (mobile telephony) data. At step S402, a digital signal representative of the analogue signal is produced. This digital signal may be the signal 302 described above. At step S404, a continuous phase digital signal based upon the digital signal is produced. This continuous phase digital signal may be the waveform 306 15 described above. At step S406, a continuous phase modulated signal based upon the digital signal is produced. The continuous phase modulated signal may be produced using minimum shift keying or Gaussian minimum shift keying in any suitable way. At step S408, the continuous phase modulated signal is used for modulating the output of a visible light source, such as the LED 206.

20

As will be appreciated, the operations S400 to S408 are preferably performed continuously.

By using continuous phase modulation, particularly MSK, the side lobes which are 25 present in the spectrum of non-continuous phase modulated signals can be reduced and so data can be carried in a more compact spectrum. In other words, the bandwidth efficiency of the communication system 100 can be improved. This can be particularly important since, as will be explained below, the light signal provided, for example, by the LED 206 may have a modulation bandwidth which is 30 comparable to that needed to carry the mobile telephony or GSM signals. The use of continuous phase modulation can also improve the power efficiency of the communication system 100. This can be particularly important since the system 100 may comprise mobile devices 110 with limited battery life.

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The use of continuous phase modulation can also make the communication system 100 less affected by noise and/or by interference from sources such as other infrared and visible light communication systems. For example, constant-amplitude 5 CPM signals are less affected by varying-amplitude noise or interference.

Light emitting diode 206

Referring to Figure 5, the LED 206 associated with the visible light transmitter 200 is preferably a phosphor-based white LED. The LED 206 thus comprises a blue 10 LED 500 and one or more yellow phosphors 502 and may further comprise an optical filter 504. The blue LED 500 and the yellow phosphors 502 are arranged such that the blue light emitted by the blue LED 500 passes through the yellow phosphors 502, each of which absorbs some of the blue light and emits Stoke-shifted yellow light. The overall spectrum of the LED 206 thus appears white.

15

The optical filter 504 may be included in order to increase the modulation bandwidth of the visible light signal 208. Due to their finite response times, the yellow phosphors 502 reduce the modulation bandwidth of the light from the blue LED 500, which is typically around 15 MHz, to around 2 to 6 MHz The optical 20 filter 504 is configured to remove most of the light emitted by the yellow phosphors 502, thereby restoring the modulation bandwidth to around 15 MHz.

The bandwidth of the primary GSM transmission band is 25 MHz. This band is divided into 124 channels with a spacing of 200 kHz. Each channel is divided into 25 8 slots or sub-bands for 8 respective users. Without the optical filter 504, the modulation bandwidth of the light from the LED 206 may correspond to around a quarter of the primary GSM transmission band bandwidth. This may be sufficient in some instances. The modulation bandwidth may be increased by including the optical filter 504 as explained above. In some embodiments, the modulation 30 bandwidth may be increased in various other ways. For example, various signal processing techniques may be used in order to increase the modulation bandwidth, including suitable pre and post equalisation and filtering techniques. Furthermore, in some embodiments, the LED 206 may comprise multiple LEDs, each of which is

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provided with a fraction of the spectrum of the signal to be transmitted and is configured to emit light over a different range of wavelengths. In this way, the modulation bandwidth of the communication system 100 can be increased by increasing the number of LEDs 206. For example, the LED 206 may comprise red, 5 green and blue LEDs, in which case it may have around three times the modulation bandwidth. In this case, the red, green and blue LEDs may form an LED 206 which is configured to produce white light. In such embodiments, the communication system 100 may further comprise apparatus (not shown) configured to monitor and control channel and/or the LED 206 allocation of multiple mobile 10 devices 110.

Similar considerations apply in relation to the bandwidths of the infrared LED 228 and the primary GSM reception band.

15 Alternative continuous phase generator block 304'

Figure 6 shows an alternative continuous phase generator block 304' which may be used in place of the continuous phase generator 304 shown in Figure 3.

The continuous phase generator 304' comprises a Gaussian low-pass filter 600 in 20 place of the digital filter 318. The digital signal 302 from the analogue-to-digital converter 300 is passed through the filter 600 before being provided to the phase generator 320, which is as described above. The filter 600 has a transfer function H(f) = exp (- (//B4)2(ln2/2)), where Bh is the 3-dB bandwidth. The filter 600 is configured to produce a digital message waveform M(/) = ^ akg{t - kT), where 2$ gU~kT) is the basic frequency Gaussian pulse delayed by the function kT, where T is the symbol period. The waveform 306' from the phase generator 320 is used as the modulating signal by the modulator block 308, thereby generating a Gaussian minimum shift keyed (GMSK) signal 310. In this way, the phase trajectory of the waveform 306' can be smoothed. Thus, the spectral side lobes in the signal 310 can 30 be further reduced and so the bandwidth and/or power efficiency of the communication system 100 can be further improved.

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Visible light receiver 218

Figure 7 shows the visible light receiver 218 in more detail. As explained above, the visible light receiver 218 is included at the mobile device 110 so that the mobile device 110 can receive the visible light signal 208 from the visible light transmitter 5 200 and produce the RF electrical signal 222 corresponding to the signal 204

provided to the visible light transmitter 200. The visible light receiver 218 is thus configured to reverse some of or all of the operations performed on the signal 204 by the visible light transmitter 200.

10 The visible light receiver 218 preferably comprises an optical-to-electrical converter 700, a demodulator 702, a sampling and decoding block 704, and a digital-to-analogue converter 706.

The optical-to-electrical converter 700 preferably comprises an optical filter 708, the 15 photodiode 221 and an amplifier 709.

The optical filter 708 is an optical band-pass filter configured to pass light in a frequency band corresponding to that emitted by the LED 206. This can help to reduce interfering signals. In some embodiments, the filter 708 is configured to 20 pass only the blue light emitted by the blue LED 500. In this way, the reduction in modulation bandwidth due to the yellow phosphors 502 can be lessened.

The optical filter 708 and the photodiode 221 are arranged such that the filtered light is incident upon the photodiode 221. The photodiode 221 is preferably an 25 avalanche photodiode or a PIN photodiode, that is to say a photodiode with p-type, intrinsic and n-type regions. The photodiode 221 preferably operates by direct detection. The photodiode 221 is configured to convert the received light to an electrical signal.

30 The electrical signal from the photodiode 221 is provided to the amplifier 709,

which is configured to generate an amplified signal 710. The amplifier 709 is preferably either a transimpedance amplifier or a high-impedance amplifier.

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The demodulator 702 is configured to demodulate the signal 710. The demodulator 702 is preferably a coherent, serial MSK demodulator, which will be described below. However, the demodulator 702 may be configured to operate in any suitable way. For example, the demodulator 702 may be a parallel demodulator.

5

The demodulator 702 is configured to split the signal 710 into two signals 710j, 7102. The signals 710^ 7102 are passed through respective local mixers 712l5 7122. The local mixers 712l5 7122 are configured to multiply the signals 710l5 7102by respective signals 714l5 7142 which are proportional to cos(2^ <JC -|T)/) and 10 —sin(2n{jc -jT)/) respectively, wherein fc is the frequency of the carrier signal and Tis the symbol period (see above). This results in demodulated signals 716l5 7162 which are proportional to cos(<p(t,a)+(m/2T)) and sin(<p(t,a)+(m/2T)) respectively, wherein (p{t,a) is the phase function of the signals. The signals 716l3 7162 are passed through respective low-pass filters 718l5 7182. Filter 718a has an impulse 15 responses hx (t) = cos2(,t//2T) for |/| <T and hl{t) = 0 otherwise. Filter 7182 has an impulse responses h2(t) = -j;sm(ffl/2T) for |/|<T and h2(t) = 0 otherwise. Finally, the signals 720l5 7202 from the filters 718l5 7182 are summed at block 722 to produce a demodulated signal 723.

20 The sampling and decoding block 704 comprises a sampler 724 and a decoder 726. The signal 723 is provided to the sampler 724, which is configured to sample the signal 723 with the symbol period T. The signal from the sampler 724 is provided to the decoder 726 which is configured to produce a decoded digital signal 727. The decoding is preferably the reverse of the encoding performed by the visible 25 light transmitter 200 and so the respective digital signals are preferably the same.

The digital-to-analogue converter block 706 comprises a digital-to-analogue converter 728 and an expander 730. The digital signal 727 is provided to the digital-to-analogue converter 728 which is configured to produce an analogue signal based 30 upon the digital signal 727. The analogue signal is provided to the expander 730 which is configured to increase the dynamic range of the analogue signal, thereby producing the RF electrical signal 222. In other words, the expander 730 is

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configured to reverse the operation performed by the compressor 312 at the transmitter 200.

Thus, the RF electrical signal 222 produced by the visible light receiver 218 5 corresponds to the RF electrical signal 204 provided to the visible light transmitter 200. This, in turn, corresponds to the radio signal received from the base station 112 by the radio transceiver 103 as described above. Accordingly, the RF electrical signal 222 can be provided to the mobile device 110 and processed by it in substantially the same way as if it had been received directly from the base station 10 112 using the antenna 224.

The demodulator 702 may comprise a synchronisation block (not shown)

configured to synchronise the transmitter 200 and receiver 218 and, amongst other things, to determine the appropriate phase of the signals 714l5 7142.

15

The visible light receiver 218 and, in particular, the demodulator 702 may be the same or similar regardless of whether the visible light transmitter 200 is configured to use MSK or GMSK modulation.

20 Operation of the visible light receiver 218

Figure 8 shows operations which can be performed by a visible light receiver according to an embodiment of the invention.

At step S800, a visible light signal comprising a mobile telephony signal is detected 25 and, at step S802, a corresponding electrical signal is produced. The visible light signal may be the signal 208 described above. The mobile telephony signal may comprise a GSM signal. The visible light signal is modulated such that a continuous phase modulated signal results from the detection. The signal may be a minimum shift keyed or Gaussian minimum shift keyed signal. At step S804, the continuous 30 phase modulated signal is demodulated. At step S806, a digital signal based upon the demodulated signal is produced. This digital signal may be the signal 727 described above. At step S808, an analogue signal based upon the digital signal is produced. This analogue signal may be the signal 222 described above. This

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analogue signal preferably corresponds to an RF signal received from a mobile telephone network and can thus be processed in the usual way in order to obtain the mobile telephony traffic data, e.g. voice, packet data or short messaging service messages, and/or control data.

5

As will be appreciated, the operations S800 to S808 are preferably performed continuously.

Infrared light transmitter 220 10 Referring again to Figures 2 and 3, the infrared light transmitter 220 at the mobile device 110 is preferably largely the same as the visible light transmitter 200. However, the infrared light transmitter 220 comprises the LED 228, which is configured to emit infrared light, in place of the LED 206. Furthermore, the infrared light transmitter 220 is configured to receive the RF electrical signal 226 15 from the mobile device 110 rather than the signal 204 from the circuitry 114.

Infrared light receiver 202

Referring again to Figures 2 and 7, the infrared light receiver 202 at the light transceiver 106 is preferably largely the same as the visible light receiver 218. 20 However, the infrared light receiver 202 comprises the photodiode 212, which is configured to detect infrared light, in place of the photodiode 221. Furthermore, the infrared light receiver 202 may comprise a filter (not shown) configured to pass infrared light in place of the filter 708. Furthermore, the infrared light receiver 202 is configured to provide the RF electrical signal 214 to the circuitry 114 rather than 25 to the mobile device 110.

Further embodiments

Numerous alternative embodiments will be apparent to the skilled person, and all such embodiments that fall within the scope of the claims are within the scope of 30 this invention.

For example, in some embodiments, the transmitter 200 and/or the transmitter 228 may comprise more than one LED, each of which is configured to transmit the

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signal. Furthermore, in some embodiments, the receiver 202 and/ the receiver 220 may comprise more than one photodiode, each of which is configured to detect the signal.

5 In some embodiments, rather than a phosphor-based white LED 206, the LED may comprise red, green and blue LEDs for generating the white visible light signal 208.

In some embodiments, the LED 206 may emit one or more different colours rather than white. In this case, besides the communication of the signal, the LED 206 may 10 be for decoration and/or entertainment rather than for illumination. The optical-to-electrical converter 700 at the visible light receiver 218 may be configured accordingly.

In some embodiments, the LED 206 may be configured to emit infrared light rather 15 than visible light. In this case, the mobile device 110 may be provided with an infrared light receiver similar to receiver 202 in place of the visible light receiver 221. The visible light transmitter 200 can thus allow the mobile telephony signal to be forwarded without providing illumination. In some embodiments, the communication system 100 may be switchable from a state in which the mobile 20 telephony signal is forwarded using visible light to one in which it is forwarded using infrared light, for example depending upon whether illumination is required. In this case, the further transceiver may be configured so that it can receive visible and/or infrared light signals.

25 In some embodiments, other types of light sources may be used in place of the LED 206 and/or the LED 228.

In some embodiments, the transmitter 200 and/or the transmitter 218 may be configured to use another form of continuous phase modulation in place of the 30 minimum shift keying or the Gaussian minimum shift keying described above.

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In some embodiments, the communication system 100 may communicate directly with other parts of the mobile telephone network rather than via the base station 112.

5 In some embodiments, the communication system 100 may include apparatus configured to perform various processing operations in relation to the signals. For example, as explained above, there may be provided apparatus for monitoring and controlling the channel and/or the LED allocation of the mobile devices 110. Alternatively or additionally, such apparatus may be configured to manage 10 handovers between different LEDs 206 or between different light transceivers 200 when a mobile device 110 is moved around.

Any of the circuitry and functional blocks described above may be implemented using any combination of digital and/or analogue circuits, processing means such as 15 microprocessors, and so forth. Furthermore, any two or more of the above-described elements may be combined, in which case operations said to be performed by the separate parts are performed by the combined parts.

It will be appreciated that the above described embodiments are purely illustrative 20 and are not limiting on the scope of the invention. Other variations and modifications will be apparent to persons skilled in the art upon reading the present application.

For instance, the communication system 100 can be readily modified so that it may 25 alternatively or additionally forward other types of data signals using visible light.

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Claims

1. Apparatus for forwarding a mobile telephony signal to a mobile device using light, the apparatus configured to receive an analogue signal comprising the mobile 5 telephony signal and to produce a digital signal based upon the analogue signal, the apparatus further configured to produce a continuous phase modulated signal based upon the digital signal, wherein the continuous phase modulated signal is for modulating the output of a light source.

10

2. Apparatus according to the preceding claim, wherein the light is visible light.

3. Apparatus according to either preceding claim, wherein the mobile telephony signal comprises a Global System for Mobile Communications signal.

15

4. Apparatus according to any preceding claim, wherein the continuous phase modulated signal comprises a minimum shift keyed signal.

5. Apparatus according to the preceding claim, wherein the continuous phase modulated signal comprises a Gaussian minimum shift keyed signal.

20

6. Apparatus according to any preceding claim, wherein producing the digital signal comprises producing a first digital signal representative of the analogue signal and producing a second digital signal which is a continuous phase digital signal based upon the first digital signal.

25

7. Apparatus according to any preceding claim, comprising the light source, wherein the light source comprises a light emitting diode, and wherein the continuous phase modulated signal is used to modulate the intensity of the light emitted by the light emitting diode.

30

8. Apparatus according to the preceding claim, wherein the light emitting diode comprises one or more light emitting diodes configured to produce white light.

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9. Apparatus according to any preceding claim, configured to receive a radio signal comprising the mobile telephony signal from a base station of a mobile telephone network.

5 10. Apparatus according to any preceding claim, configured to receive a further light signal comprising a further mobile telephony signal from each of one or more mobile devices, and to produce a further analogue signal based upon the further light signal, wherein the further analogue signal is for radio transmission to a base station of a mobile telephone network.

10

11. Apparatus according to claim 10, wherein the further light signal has different wavelengths from the light output by the light source.

12. Apparatus according to claim 10 or 11, wherein the further light signal is an 15 infrared light signal.

13. Apparatus for receiving a mobile telephony signal transmitted using light, the apparatus configured to detect a light signal comprising the mobile telephony signal and to produce a signal based upon the light signal, wherein the light signal is

20 modulated such that the signal comprises a continuous phase modulated signal, the apparatus further configured to demodulate the signal, to produce a digital signal based upon the demodulated signal, and to produce an analogue signal based upon the digital signal, wherein the analogue signal is for processing by a mobile device in substantially the same way as a received radio signal comprising a mobile telephony 25 signal.

14. Apparatus according to claim 13, wherein the light is visible light.

15. Apparatus according to claim 13 or 14, wherein the mobile telephony signal 30 comprises a Global System for Mobile Communications signal.

16. Apparatus according to any one of claims 13 to 15, wherein the continuous phase modulated signal comprises a minimum shift keyed signal.

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17. Apparatus according to claim 16, wherein the continuous phase modulated signal comprises a Gaussian minimum shift keyed signal.

18. Apparatus according to any one of claims 13 to 17, configured to receive a further analogue signal comprising a further mobile telephony signal, to produce a further digital signal based upon the further analogue signal and to produce a further continuous phase modulated signal based upon the further digital signal, wherein the further continuous phase modulated signal is for modulating the output of a light source.

19. Apparatus according to claim 18, wherein the light output by the light source has different wavelengths from the light signal which the apparatus is configured to detect.

20. Apparatus according to claim 18 or 19, wherein the light source is an infrared light source.

21. A mobile device comprising apparatus according to any one of claims 13 to 20.

22. A communication system comprising:

apparatus according to any one of claims 1 to 12; and at least one mobile device according to claim 21,

wherein the apparatus is configured to forward the mobile telephony signal to the mobile device using light and the mobile device is configured to transmit the further mobile telephony signal to the apparatus using further light having different wavelengths to the light.

23. A method for forwarding a mobile telephony signal to a mobile device using light, the method comprising:

receiving an analogue signal comprising the mobile telephony signal; producing a digital signal based upon the analogue signal; and

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producing a continuous phase modulated signal based upon the digital signal, wherein the continuous phase modulated signal is for modulating the output of a light source.

24. A method according to claim 23, wherein producing the digital signal comprises:

producing a first digital signal representative of the analogue signal; and producing a second digital signal which is a continuous phase digital signal based upon the first digital signal.

25. A method according to claim 23 or 24, comprising:

receiving a radio signal comprising the mobile telephony signal from a base station of a mobile telephone network.

26. A method according to any one of claims 23 to 25, comprising:

receiving a further light signal comprising a further mobile telephony signal from each of one or more mobile devices; and producing a further analogue signal based upon the further light signal, wherein the further analogue signal is for radio transmission to a base station of a mobile telephone network.

27. A method for receiving a mobile telephony signal transmitted using light, the method comprising:

detecting a light signal comprising the mobile telephony signal and producing a signal based upon the light signal, wherein the light signal is modulated such that the signal comprises a continuous phase modulated signal;

demodulating the signal;

producing a digital signal based upon the demodulated signal; and producing an analogue signal based upon the digital signal, wherein the analogue signal is for processing by a mobile device in substantially the same way as a received radio signal comprising a mobile telephony signal.

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28. A method according to claim 27, comprising:

receiving a further analogue signal comprising a further mobile telephony signal;

producing a further digital signal based upon the further analogue signal; and 5 producing a further continuous phase modulated signal based upon the further digital signal, wherein the further continuous phase modulated signal is for modulating the output of a light source.