WO2010035010A1

WO2010035010A1 - Apparatus and method for transporting multiple signals over optical fiber

Info

Publication number: WO2010035010A1
Application number: PCT/GB2009/002306
Authority: WO
Inventors: Chin-Pang Liu; Alwyn John Seeds
Original assignee: Ucl Business Plc
Priority date: 2008-09-26
Filing date: 2009-09-25
Publication date: 2010-04-01
Also published as: GB0817734D0

Abstract

A method and apparatus for transporting three or more radio signals of the same frequency, such as multiple input multiple output (MIMO) radio signals, over optical fiber on a single optical carrier by frequency mixing two such MIMO radio signals at a time with a single low-frequency local oscillator (LO) signal is disclosed.

Description

Apparatus and Method for Transporting Multiple Signals Over Optical Fiber

The present invention relates to the field of telecommunications and more particularly, to transporting three or more radio signals of the same frequency, such as multiple input, multiple output (MIMO) radio signals, over optical fiber.

A radio signal is defined as a signal whose carrier frequency corresponds to the part of the electromagnetic spectrum in the frequency range between 3 kHz and 300 GHz. This encompasses the part of the communications spectrum that may also be referred to as microwave. Radio signals of the same frequency concerned are to be understood to have the same carrier frequency. Baseband signal has nonzero spectral amplitude between 0 Hz (DC) and an upper frequency which is dependent on the modulation rate. Optical signal is to be understood to be any electromagnetic radiation in the visible light and infrared regions with the wavelength range between 380 nm and 3000 nm which corresponds to the frequency range between 789 THz and 100 THz.

The rapid rise in information and telecommunication services has resulted in the need for ever more efficient and higher capacity wireless services to provide very high speed wireless multimedia communication services. This is being achieved through the use of a combination of optical communication and wireless communication technologies where a wired communication technology is combined with a wireless communication technology to provide a high speed transmission system along with a mobile wireless technology - such a technology being called Radio over Fiber (ROF).

Such an ROF technology uses an optical link apparatus and a radio link apparatus as the basic components. The optical link apparatus modulates a baseband transmission signal into a radio frequency band signal, converts the radio frequency band signal into an optical signal, and then transmits the optical signal through an optical fiber. This optical signal is then converted back into the electrical domain and the wireless link apparatus wirelessly carries a signal which has been received through the optical fiber.

In an environment in which various wireless services for voice, broadcasting, data, etc., are provided, it is inefficient to construct a remote antenna link for every type of service. Hence methods for combining multiple signals and transmitting them over a single optical fiber are of significant value and the focus of research.

The distribution of radio signals over optical fibers reduces the complexity of remote antenna units (AUs) and helps centralise communication equipment in the central offices which facilitates routine system maintenance and reduces costs. Compared to the signal distribution using conventional coaxial cables, the low transmission loss characteristic of optical fibers extends the distance the remote antenna units can be located from the central offices. Another advantage of using optical fibers compared to using conventional coaxial cables is the much wider transmission bandwidth which allows a combination of cellular, wireless local area network (WLAN) and other wireless service signals at different frequencies to be distributed together over a single optical fiber. Commercial radio over fiber (ROF) products are already on sale for carrying a multitude of existing wireless services, such as the GSM, UMTS and IEEE 802.1 lb/g WLAN, using optical fibers.

Considering the wireless network services continued demand for higher data transmission rates without a corresponding increase in the allocated radio channel bandwidth has led to the development of the Multiple Input Multiple Output (MIMO) system. MIMO is a multi-antenna radio transmission technique using multiple antennas for transmitting and multiple antennas for receiving. Compared to the Single Input Single Output (SISO) technique, MIMO has two major performance enhancements.

Firstly, the use of multiple antennas in a MIMO system increases the transmission range and/or improves the radio link reliability through the well-known spatial diversity technique. Secondly, MIMO systems can deliver higher data transmission rates over the airwave than the SISO counterparts for the same channel bandwidth. This transmission rate enhancement is achieved using a technique known as spatial multiplexing. Spatial multiplexing can be compared to solving a set of simultaneous equations where the transmitted radio signals from different transmitting antennas are analogous to the independent variables, and the received radio signals by different receiving antennas are analogous to the dependent variables. Once the different radio transmission path characteristics between each of the transmitting antennas and each of the receiving antennas have been established, the coefficients describing the set of simultaneous equations can be determined. Each of the received radio signals is dependent on all of the different transmitted radio signals. With the received radio signals and the coefficients describing the different radio path characteristics, each of the different transmitted radio signals can then be obtained mathematically, analogous to solving for the independent variables of a set of simultaneous equations. As a result all the different transmitted radio signals, despite having the same frequency occupying the same channel bandwidth, can be radiated to and received by the receiver individually with a higher aggregate transmission rate compared to an SISO system, as if a number of parallel radio channels existed between the transmitter and the receiver.

^"New and emerging wireless standards increasingly employ MIMO for greater data throughputs as well as for improving transmission range/reliability. Examples of wireless standards employing MIMO include the IEEE 802.1 In WLAN, the IEEE 802.16e WiMAX and all future 4^th generation (4G) cellular systems.

While optical fiber is well suited for carrying radio signals of different frequencies, it is not straightforward to use the same radio over fiber technique for transmitting a group of signals of the same frequency, such as the MIMO signals feeding multiple antennas, over an optical fiber.

This difficulty arises because to transmit multiple radio signals, they have to be combined first in, for example, a power combiner prior to transmission over an optical fiber. If the radio signals involved have different frequencies, they can be easily separated and recovered using simple electrical filters after transmission over fiber. If, on the other hand, the radio signals are of the same frequency, it will be impossible to separate and recover the individual radio signals without any form of signal processing or multiplexing/demultiplexing technique before and after transmission over an optical fiber.

A conventional solution for tackling this problem is to employ as many individual optical fibers as there are radio signals. However, it would mean substantially increasing the cost of constructing such MIMO radio over fiber system as each transmitting antenna would need its own optical fiber and associated components.

A number of techniques have been reported by others for transporting radio signals having the same frequency over a single optical fiber.

With wavelength division multiplexing (WDM), each of the radio signals to be transmitted is modulated onto and carried by one optical carrier of a different wavelength. Optical wavelength dependent filters are used to multiplex and demultiplex the optical carriers of different wavelengths before and after transmission over the fiber, respectively.

With sub-carrier multiplexing (SCM), also referred to as frequency division multiplexing (FDM), the radio signals are first frequency shifted into different frequency bands using electrical mixers and local oscillator sources so that they can be modulated onto and carried by a single optical carrier for transmission over an optical fiber. After the fiber, the frequency shifted radio signals are separated using electrical filters and then frequency shifted again to their original frequency band.

Allert van Zelst in US Patent Application Publication US 200400177S5 Al described, in very broad terms, using the well-known WDM and FDM techniques a method for transporting MIMO radio signals over an optical fiber. Ichiro Seto et al. in "Optical Subcarrier Multiplexing Transmission for Base Station With Adaptive Array Antenna", IEEE Transactions On Microwave Theory And Techniques, Vol. 49, No. 10, pp. 2036-2041, October 2001, proposed transporting a multitude of radio signals destined for a group of adaptive array antennas over a single optical fiber using SCM. Adaptive array antennas require accurate and stable relative phase relationship between the radio signals radiated by individual antennas. Since all the radio signals are transported over a single optical fiber, any disturbance to the fiber resulting in fluctuating path length will affect all the signals equally but their relative phase relationship remains unaffected. Although such an SCM technique has been proposed for carrying adaptive array antenna signals which are different between them only in the phase and amplitude but not the actual radio content, the same technique can be applied to carry MIMO radio signals.

P. Ritosa et al. in "Optically steerable antenna array for radio over fibre transmission", Electronics Letters, Vol. 41, No. 16, pp. 47-48, 4^th August 2005, reported transmitting the received radio signals at 2.1 GHz from the three elements of a smart antenna array over an optical fiber using WDM. The relative signal phase relationship was controlled by the choice of the laser wavelengths since the signal transmission speed over fiber is wavelength dependent. The relative signal amplitude was adjusted using external Mach-Zehnder modulators. Although the system was set up simply to demonstrate its ability to create different radiation patterns by varying the relative signal phase amplitude, it can easily be adapted for carrying MIMO radio signals using the same WDM technique.

There are a numbeT of drawbacks associated with the WDM and SCM techniques for transporting MIMO signals over a single optical fiber.

If WDM is used, then as many optical sources of different wavelengths and the corresponding photodetectors as there are radio signals to be transported will be required since each signal will need its own optoelectronic components for the electrical-to-optical and optical-to-electrical conversion processes. Since many more components are required, the cost of constructing a WDM based ROF system for MIMO will increase substantially and almost linearly with the number of antennas.

If SCM is used, all but one of the MIMO radio signals need to be initially frequency translated to different frequency bands before fiber transmission, and subsequently frequency translated back to the original frequency band afterwards. Therefore, if there are a total of n MIMO radio signals to be transported, « - 1 different LO sources are required. Moreover, it is common in SCM to frequency translate those

MIMO signals to an intermediate frequency (IF) band which is substantially different from the original radio frequency, e.g. from 2.4 GHz to 100 MHz, therefore separate electrical amplifiers covering different frequency bands are required for the frequency translated and non-translated MIMO signals.

It is an object of the prevent invention to alleviate, at least partially, any of the above problems.

The present invention provides an apparatus for transporting three or more radio signals of the same frequency over at least one optical fiber, on a single optical carrier, the apparatus comprising: a first frequency translator arranged to frequency translate a first radio signal to a lower sideband by mixing the first radio signal with a first LO signal, and arranged to frequency translate a second radio signal to an upper sideband by mixing the second radio signal with the same LO signal; wherein, when the number of radio signals is greater than three, the apparatus comprises a further respective frequency translator for each furtheT pair of signals, arranged to operate in the same way as the first frequency translator to frequency translate each respective pair of radio signals to a different respective lower and upper sideband around the original radio signal frequency, but using a different LO frequency signal for each said further pair of the radio signals; wherein, when the total number of radio signals is an odd number, the apparatus is arranged such that the last single radio signal that is not part of any of the radio signal pair or pairs is not frequency translated; one or more combiners arranged to combine together all resulting lower and upper sidebands and said last single radio signal, when present, into a single electrical signal; an optical source arranged to generate an optical carrier signal modulatable by said single electrical signal; at least one optical fiber arranged to transport the modulated optical carrier signal generated by the optical source; a photodetection unit arranged to detect the optical signal after transmission over the at least one optical fiber and produce a corresponding received electrical signal; wherein, when the total number of radio signals is an odd number, the apparatus comprises at least one filter arranged to separate and recover the last single radio signal from the other lower and upper sidebands in the received electrical signal; at least two filters arranged to separate individually all the lower and upper sidebands, contained in the received electrical signal; first and second mixers arranged to mix the respective separated lower and upper sidebands, generated by the first frequency translator, with an LO signal having the same LO frequency as used in the first frequency translator to recover the first and second radio signals at their original radio frequency; and wherein, when the number of radio signals is greater than three, the apparatus comprises further mixers, arranged to operate in the same way as the first and second mixers to recover each other pair of radio signals at the original radio frequency by mixing the respective separated lower and upper sidebands generated from each respective pair of signals, with a respective LO signal having respective LO frequency as used in the respective further frequency translator.

The invention also provides a method for transporting three or more radio signals of the same frequency over at least one optical fiber, on a single optical carrier, the method comprising: frequency translating a first radio signal to a lower sideband by mixing the first radio signal with an LO signal, and frequency translating a second radio signal to an upper sideband by mixing the second radio signal with the same LO signal; wherein, when the number of radio signals is greater than three, frequency translating each further pair of radio signals, in the same way as the first and second radio signals, to a different respective lower and upper sideband around the original radio signal frequency, using a different LO frequency signal for each further pair of radio signals; wherein, when the total number of radio signals is an odd number, the last single radio signal that is not part of any of the radio signal pair or pairs is not frequency translated; combining together all resulting lower and upper sidebands and said last single radio signal, when present, into a single electrical signal; modulating an optical carrier signal using said single electrical signal; transporting the modulated optical carrier signal over at least one optical fiber; detecting the optical signal after transmission over the at least one optical fiber and producing a corresponding received electrical signal; wherein, when the total number of radio signals is an odd number, separating and recovering the last single radio signal from the lower and upper sidebands in the received electrical signal by filtering; separating individually all the lower and upper sidebands, contained in the received electrical signal, by filtering; recovering the first and second radio signals at their original radio frequency by mixing the respective separated lower and upper sidebands, generated by the first frequency translating step, with an LO signal having the same LO frequency as used in the first frequency translating step; and wherein, when the number of radio signals is greater than three, recovering each other pair of radio signals at the original radio frequency by mixing the respective separated lower and upper sidebands generated from each respective pair of signals, with a respective LO signal having respective LO frequency as used in the respective frequency translation. Accordingly the invention enables the transport of three or more radio signals of the same frequency, over an optical fiber or fibers on a single optical carrier. Embodiments of the invention require fewer local oscillator sources or frequencies compared to those employing SCM and fewer optical sources of different wavelengths compared to those employing WDM, and therefore can be substantially cheaper to construct compared to other existing alternatives. Embodiments of the invention also have the advantage that it is not necessary to maintain any phase relationship between the local oscillator (LO) signal at the transmitter and the LO signal at the receiver.

In one embodiment of the invention two MIMO radio signals at a time are processed using one low-frequency local oscillator (LO) source. In the transmitter the first of the two MIMO radio signals is frequency translated to a lower sideband by mixing with the LO source followed by bandpass filtering at the lower sideband frequency while the other MIMO radio signal, being the second of the two, is frequency translated to an upper sideband by mixing with the same LO source followed by bandpass filtering at the upper sideband frequency. As a consequence, the two MIMO radio signals now respectively occupy a lower and an upper sideband which are respectively below and above their original radio frequency in the frequency domain by an offset equal to the LO source frequency. Since the original MIMO radio signal frequency band between the lower sideband and the upper sideband is not used by these two MIMO radio signals at this point after frequency translation, it can be occupied by and used for transmission of a third MIMO radio signal without frequency translation. The loweτ sideband, the uppeT sideband and the thiid MIMO radio signal are then combined using an electrical diplexer/duplexer or power combiner or other components performing similar function before electrical-optical conversion into an optical signal and transmission over an optical fiber.

After transmission over fiber and optical-electrical conversion back to the electrical domain in the receiver, the third MIMO radio signal is separated from the lower and upper sidebands using an electrical diplexer or bandpass filter or other components performing similar function. The lower and upper sidebands are also separated using electrical diplexers or bandpass filters or other components performing similar function. To recover the first MIMO radio signal and return it to the original frequency, the filtered lower sideband is mixed with an LO source whose frequency is the same as the LO source in the transmitter. To recover the second MIMO radio signal and return it to the original frequency, the filtered upper sideband is mixed with the same LO. One advantageous feature of the invention is that it is not necessary to have a separate low-frequency LO in the receiver. The LO signal generated by the LO in the transmitter can be sent over the same fiber to the receiver for the mixing processes. Bandpass filtering will be required to remove unwanted mixed products.

For transporting three MIMO radio signals, the present invention requires only one LO source or frequency. In comparison, a system employing SCM will require at least two LO sources or frequencies in order to frequency translate two of the three MIMO radio signals from their original frequency before transmission over an optical fiber.

The present invention can be adapted for carrying a greater number of MIMO radio signals requiring a smaller number of LO sources or frequencies compared to systems employing SCM performing the same tasks. To transport four or five MIMO radio signals over an optical fiber, the present invention requires only two LO sources or frequencies. An SCM based system will require three and four LO sources or frequencies to transport four and five MIMO radio signals, respectively. Similarly systems employing WDM will require more optical sources of different wavelengths compared to the invention. Systems implemented with the present invention are therefore substantially cheaper to construct compared to other existing alternatives. This reduces the power burden of the system.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 shows mixing of the first MIMO radio signal with an LO signal followed by bandpass filtering to generate the resultant frequency translated lower sideband;

Fig. 2 shows mixing of the second MIMO radio signal with the same LO signal followed by bandpass filtering to generate the resultant frequency translated upper sideband;

Fig. 3 illustrates the process of combining the third MIMO radio signal with the frequency translated lower and upper sidebands from the first and second MIMO radio signals, respectively;

Fig. 4 illustrates separation of the third MIMO radio signal by bandpass filtering; Fig. 5 illustrates separation of the lower sideband due to the first MIMO radio signal by bandpass filtering;

Fig. 6 illustrates separation of the upper sideband due to the second MIMO radio signal by bandpass filtering;

Fig. 7 shows recovery of the original first MIMO radio signal by mixing the lower sideband with an LO signal followed by bandpass filtering;

Fig. 8 shows recovery of the original second MIMO radio signal by mixing the upper sideband with an LO signal followed by bandpass filtering;

Fig. 9 is a schematic representation of an apparatus embodying the present invention for transporting three MIMO radio signals over optical fiber; and Fig. 10 is a schematic representation of an apparatus embodying the present invention for transporting five MIMO radio signals over optical fiber.

The invention can be understood by considering the following schematic descriptions and embodiments of the invention. In a first embodiment the radio signals to be transmitted over optical fiber are MIMO radio signals though it is understood that the invention applies to any radio signals of the same frequency.

Fig. 1 shows a first MIMO radio signal 101, with a radio frequency carrier of frequency /_RF. Fig. 2 shows a second MlMO radio signal 201, with a radio frequency carrier of frequency /_RF. The two MIMO radio signals 101, 201 occupy the same frequency band around fa which has the result that they cannot simply be combined together, transported over an optical fiber and separated again afterwards without frequency translation being employed.

The transportation of a multiplicity of MIMO radio signals having the same frequency over an optical fiber is achieved by means of frequency translation using

LO sources whose frequencies are substantially lower than those of the MIMO radio signals. Such a technique is employed and applied to two MIMO radio signals at a time by mixing the two signals with a single low-frequency LO source followed by bandpass filtering in order to select for each MIMO signal one of the two sidebands generated.

Fig. 1 shows a MIMO radio signal 101 of frequency /^ being mixed with a low- frequency LO 102 that has a frequency of fw- This mixing results in a lower sideband 103 with frequency (f_Rf -fw) and an upper sideband 104 with frequency

Similarly Fig. 2 shows another MIMO radio signal 201 of frequency /R_F being mixed with the same low-frequency LO 102 at frequency f_Lo- This mixing results in a lower sideband 202 with frequency V_RF-fw) and an upper sideband 203 with frequency

Only one of the two sidebands generated from each MIMO radio signal is required and needs to be retained. The retained sidebands must be chosen so that they do not overlap in the frequency domain. Therefore if the lower sideband 103 from the first MIMO signal 101 is retained, then the opposite upper sideband 203 from the second MIMO signal 201 must be retained. Similarly, if the upper sideband 104 from the first MIMO signal 101 is retained, then the opposite lower sideband 202 from the second MIMO signal 201 must be retained. In Fig. 1, a bandpass filter 105a with a center frequency of if RF - fiό) is used to select and retain the lower sideband 103 while rejecting the upper sideband 104 from MIMO signal 101 following the mixing. Similarly, in Fig. 2 another bandpass filter 204a with a center frequency of VRF ⁺ILO) is used to select and retain the upper sideband 203 while rejecting the lower sideband 202 from MIMO signal 201 following the mixing.

Following the mixing processes, the original frequency band at fpj? is no longer occupied by any of the sidebands following the mixing processes with the two

MIMO signals. The vacant frequency band at fw can now accommodate a third

MIMO signal without frequency translation. Fig. 3 shows the combining of a third

MIMO signal 301 with the lower sideband 103 from the first MIMO signal 101 and the upper sideband 203 from the second MIMO signal 201. The resultant signals are now suitable for transmission over fiber.

Following transmission over fiber, the original three MIMO signals need to be recovered and separated.

Firstly, the third MIMO signal 301 can be recovered by simply passing the resultant radio signals through a bandpass filter 401a aroundy^ as shown in Fig. 4.

To recover the two MIMO radio signals 101 and 201, the lower sideband 103 and the upper sideband 203 first need to be separated. Fig. 5 shows the resultant radio signals being passed through a bandpass filter 105b around (/ΛF— ./Zo) which selects the lower sideband 103 generated from MIMO signal 101. Similarly Fig. 6 shows the resultant radio signals being passed through a bandpass filter 204b around (fø + fio) which selects the upper sideband 203 generated from MIMO signal 201.

In Fig. 7 the bandpass filtered lower sideband 103 is then mixed with an LO signal 701 which is of the same frequency as 102. This mixing process restores the original first MIMO signal 101 and in addition produces a new lower sideband 702 at frequency {fκ_F-2fw). A bandpass filter 401b around /m? then selects the dcsircujiist MIMO signal 101 and rejects the unwanted sideband 702. In Fig. S the bandpass filtered upper sideband 203 is similarly mixed with the same LO signal 701. The mixing process restores the original second MIMO signal 201 and in addition produces a new upper sideband 801 at frequency (/Jyr +2fιo)- A bandpass filter 401c around fw selects the desired second MIMO signal 201 and rejects the unwanted sideband 801.

An embodiment of the present invention for transporting three MIMO radio signals over fiber is shown in Fig. 9. A low-frequency LO signal 904 is first split into two identical signal components in a combiner/splitter 905a which are then used to drive the two mixers 906a and 906b. The first MIMO signal at input 901 is mixed with one of the LO signal components in mixer 906a. The output from mixer 906a is then passed through a bandpass filter 907a around (fRF-fiό) in order to select and retain the lower sideband signal at the same frequency while rejecting other mixer output signals at other frequencies. The second MIMO signal at input 902 is mixed with the other component of the LO signal in mixer 906b. The output from mixer 906b is then passed through a bandpass filter 908a around (f/u? + fw) in order to select and retain the upper sideband signal at the same frequency while rejecting other mixer output signals at other frequencies.

The lower sideband signal from bandpass filter 907a and the upper sideband signal from bandpass filter 908a are then combined in the power combiner/splitter 909a.

This mixing and any other signal mixing described herein can be performed using analogue multipliers, variable gain amplifiers and/or electrical mixers examples of which include the following: single diode mixers, double balanced diode mixers, triple balanced diode mixers, active bipolar transistor mixers, active field effect transistor mixers or active transistor mixers configured in a Gilbert cell.

The LO source frequency f_w is preferably chosen to be substantially lower than the

MIMO radio frequency f^ . For transmission of three MIMO radio signals, there are two criteria for choosing the LO frequency used. Firstly the LO frequency chosen should be high enough so that the lower and upper sidebands generated are sufficiently apart in the frequency domain so that a third non-frequency translated MIMO radio signal can be inserted between them without overlap. Secondly the LO frequency chosen should be low enough so that the lower and upper sidebands generated do not interfere or overlap with other co-transported signals in other frequency bands. Using the IEEE 802.1 In WLAN standard operating in the 2.4 GHz ISM band (2.4 GHz to 2.483 GHz) as an example, each MIMO signal occupies 40 MHz channel bandwidth. In order for the frequency translated lower and upper sidebands to be sufficiently apart, an LO frequency of at least 40 MHz, preferably even higher, should be used. However the same optical fiber may commonly carry other radio signals for other wireless services such as the UMTS. The UMTS in the frequency division duplex mode has a nearby downlink frequency band allocated between 2.110 GHz and 2.170 GHz. In order for the two sidebands not to interfere with the UMTS system, the LO frequency should be lower than 230 MHz (The difference between 2.4 GHz and 2.170 GHz). Therefore a 100 MHz LO frequency is a suitable choice in this example. Further criteria affecting the choice of the exact LO frequency are the bandwidth and order of the filters employed.

A third MIMO radio signal from input 903 is combined with the lower and upper sidebands in power combiner/splitter 910a. Since this third MIMO radio signal from input 903 is at a different frequency from the lower and upper sidebands, the combiner/splitter 910a can also be implemented with an electrical diplexer or duplexer or any suitable combination of electrical filters.

The resultant signals from 910a are converted to the optical domain in electrical- optical converter 911 (the electrical-optical converter operating using known electrical-optical transmission techniques). Examples of suitable electrical-optical converter 911 include: a directly electrically modulatable laser source; or an external optical intensity modulator in conjunction with a continuous-wave external laser source.

The output of the electrical-optical converter 911 is a modulated optical carrier signal which is launched or coupled into an optical fiber 912. The optical fiber 912 may be of a singlemode type or of a multimode type. In one optional example, an optical power divider or optical filter is used to split the modulated optical signal between two or more optical fibers in order to feed a number of separate receivers. Once the optical signal has been transmitted over fiber 912 to a receiver at the required destination, the optical signal can be converted back into the electrical domain using an optical-electrical converter 913 (the optical-electrical converter operating using known electrical-optical transmission techniques). The optical- electrical converter 913 can be a photodetector, such as a PIN photodiode, photoconductive photodetector, avalanche photodiode, metal-semiconductor-metal photodiode, Schottky photodiode, bipolar phototransistor, field effect phototransistor or any such photodetector integrated with or directly connected to an electrical amplifier.

The non-frequency translated third MIMO signal can be recovered from the received resultant signals by simple bandpass filtering. In Fig. 9, the received resultant signals from 913 is first split into two outputs in a combiner/splitter 910b and one such output is then passed through a bandpass filter 914a around /R_F which selects and retains the third MIMO signal while rejecting all other sidebands at other frequencies. The original third MIMO signal is now available at output 918.

To recover the first and second MIMO signals, the lower and upper sidebands need to be individually mixed with an LO signal whose frequency is the same as the LO used in the transmitter. The LO signal in the receiver can be generated using an independent signal generator. Alternatively, a fraction of the LO signal power in the transmitter can be sent over the same fiber to the receiver which, after further amplification, is used as the LO signal for recovering the first and second MIMO signals. In Fig. 9, an LO signal 915, whose frequency is the same as 904, is split into two identical components which are then used to drive two mixers 906c and 906d.

The other output from 910b containing the resultant signals is further split into two identical output signals in combiner/splitter 909b. To recover the first MIMO signal, one of the two output signals from 909b is passed through a bandpass filter 907b around (f_RF-fω) which selects and retains the lower sideband signal while rejecting other signals at other frequencies. The filtered lower sideband is subsequently mixed with one component of the LO signal 915 in mixer 906c which restores the original first MIMO signal at /R_F but also produces an additional sideband at (/}yr - 2/j,ø). Another bandpass filter 914c around /RF is used to filter and select the original first MIMO signal at/jur. The original first MIMO signal is now available at output 916.

To recover the second MIMO signal, the other output signal from 909b is passed through a bandpass filter 908b around (/RF +S_LO) which selects and retains the upper sideband signal while rejecting other signals at other frequencies. The filtered upper sideband is subsequently mixed with the other component of the LO signal 915 in mixer 906d which restores the original second MIMO signal SA/RF but also produces an additional sideband at (fø ₊ 2fιo). Another bandpass filter 914b around /_RF is used to filter and select the original first MIMO signal at ^. The original second MIMO signal is now available at output 917.

So far the working principles and the implementation of the present invention have been described for transmission of three MIMO radio signals over optical fiber. However, the present invention can also be adapted for transporting four, five and even a greater number of MIMO radio signals over optical fiber.

If five MIMO radio signals are to be transported over optical fiber using the present invention, for example, the embodiment in Fig. 9 can be expanded to that shown in Fig. 10. In Fig. 10, the fourth and the fifth MIMO radio signals present at inputs 1001, 1002 are mixed with a second LO signal 1003 and the resulting lower and upper sidebands are then filtered and combined in the same manner as for the first and second MIMO radio signals from inputs 901 and 902. AU the lower and upper sidebands from the first, the second, the fourth and the fifth MIMO radio signals are combined with the non-frequency translated third MIMO radio signal from input 903 in a power combiner/splitter 1004 in the transmitter before transmission over fiber.

In the embodiment in Fig. 10, the second LO frequency used for mixing with the fourth and fifth MIMO signals in the transmitter is chosen so that the generated lower and upper sidebands do not overlap with those generated by the first and second MIMO signals. Preferably, the second LO frequency used for mixing with the fourth and fifth MIMO signals is twice that for mixing with the first and second MIMO signals.

Following transmission over optical fiber, the fourth and the fifth MIMO signals can be recovered in the same manner as for the first and the second MIMO signals by first bandpass filtering and then mixing the corresponding lower and upper sidebands with an LO signal 1005 having the same frequency as the second LO 1003 in the transmitter. Following further bandpass filtering, the fourth and the fifth MIMO signals are presented at outputs 1006 and 1007, respectively.

If an even greater number of MIMO radio signals are to be transported over optical fiber using the present invention, each additional pair of MIMO radio signals can be mixed with another LO signal having a higher frequency than the previous LO frequencies, for example at successive harmonics of the original LO frequency. If an even number of signals is to be used, then one option is simply to omit the central un- shifted signal at frequency fw and input 903; the even number of signals are transmitted as pairs of upper and lower sidebands.

Claims

1. An apparatus for transporting three or more radio signals of the same frequency over at least one optical fiber, on a single optical carrier, the apparatus comprising: a first frequency translator arranged to frequency translate a first radio signal to a lower sideband by mixing the first radio signal with a first LO signal, and arranged to frequency translate a second radio signal to an upper sideband by mixing the second radio signal with the same LO signal; wherein, when the number of radio signals is greater than three, the apparatus comprises a further respective frequency translator for each further pair of signals, arranged to operate in the same way as the first frequency translator to frequency translate each respective pair of radio signals to a different respective lower and upper sideband around the original radio signal frequency, but using a different LO frequency signal for each said further pair of the radio signals; wherein, when the total number of radio signals is an odd number, the apparatus is arranged such that the last single radio signal that is not part of any of the radio signal pair or pairs is not frequency translated; one or more combiners arranged to combine together all resulting lower and upper sidebands and said last single radio signal, when present, into a single electrical signal; an optical source arranged to generate an optical carrier signal modulatable by said single electrical signal; at least one optical fiber arranged to transport the modulated optical carrier signal generated by the optical source; a photodetection unit arranged to detect the optical signal after transmission over the at least one optical fiber and produce a corresponding received electrical signal; wherein, when the total number of radio signals is an odd number, the apparatus comprises at least one filter arranged to separate and recover the last single radio signal from the other lower and upper sidebands in the received electrical signal; at least two filters arranged to separate individually all the lower and upper sidebands, contained in the received electrical signal; first and second mixers arranged to mix the respective separated lower and upper sidebands, generated by the first frequency translator, with an LO signal having the same LO frequency as used in the first frequency translator to recover the first and second radio signals at their original radio frequency; and wherein, when the number of radio signals is greater than three, the apparatus comprises further mixers, arranged to operate in the same way as the first and second mixers to recover each other pair of radio signals at the original radio frequency by mixing the respective separated lower and upper sidebands generated from each respective pair of signals, with a respective LO signal having respective LO frequency as used in the respective further frequency translator.

2. An apparatus according to claim 1 , wherein the frequency for the first LO signal for frequency translating the first pair of radio signals is such that the lower and upper sidebands generated are sufficiently apart in the frequency domain so that a non-frequency translated radio signal can be placed between the said lower and upper sidebands in the frequency domain without overlap.

3. An apparatus according to claim 1 or 2, wherein the frequency for any subsequent LO signal is selected so as to produce lower and upper sidebands sufficiently far apart that they can accommodate, between them in the frequency domain, any lower and upper sidebands generated earlier and any non-frequency translated radio signal, without overlap.

4. An apparatus according to claim 1, 2 or 3, wherein the frequency for any LO signal is selected so as to produce such lower and upper sidebands that in the frequency domain will not interfere or overlap with other lower and upper sidebands generated using other respective LO frequencies, and will not interfere or overlap with other co-transported signals in other frequency bands.

5. An apparatus according to any one of the preceding claims, wherein the mixers for recovering the radio signals are arranged to receive LO signals generated by one or more independent phase-locked LOs or derived and sent over the optical fiber or fibers from one or each original LO signal used by the frequency translators.

6. An apparatus according to any one of the preceding claims, wherein the radio signals of the same frequency are MIMO radio signals.

7. An apparatus according to any one of the preceding claims, wherein the first frequency translator further comprises: a first filter for selecting only the lower sideband resulting from the mixing of the first radio signal with the first LO signal; and a second filter for selecting only the upper sideband resulting from the mixing of the second radio signal with the first LO signal, and wherein any further frequency translator further comprises: a lower sideband filter for selecting only the lower sideband resulting from the mixing of the first of a pair of radio signals with an LO signal; and an upper sideband filter for selecting only the upper sideband resulting from the mixing of the second of a pair of radio signals with the LO signal.

8. A method for transporting three or more radio signals of the same frequency over at least one optical fiber, on a single optical carrier, the method comprising: frequency translating a first radio signal to a lower sideband by mixing the first radio signal with an LO signal, and frequency translating a second radio signal to an upper sideband by mixing the second radio signal with the same LO signal; wherein, when the number of radio signals is greater than three, frequency translating each further pair of radio signals, in the same way as the first and second radio signals, to a different respective lower and upper sideband around the original radio signal frequency, using a different LO frequency signal for each further pair of radio signals; wherein, when the total number of radio signals is an odd number, the last single radio signal that is not part of any of the radio signal pair or pairs is not frequency translated; combining together all resulting lower and upper sidebands and said last single radio signal, when present, into a single electrical signal; modulating an optical carrier signal using said single electrical signal; transporting the modulated optical carrier signal over at least one optical fiber; detecting the optical signal after transmission over the at least one optical fiber and producing a corresponding received electrical signal; wherein, when the total number of radio signals is an odd number, separating and recovering the last single radio signal from the lower and upper sidebands in the received electrical signal by filtering; separating individually all the lower and upper sidebands, contained in the received electrical signal, by filtering; recovering the first and second radio signals at their original radio frequency by mixing the respective separated lower and upper sidebands, generated by the first frequency translating step, with an LO signal having the same LO frequency as used in the first frequency translating step; and wherein, when the number of radio signals is greater than three, recovering each other pair of radio signals at the original radio frequency by mixing the respective separated lower and upper sidebands generated from each respective pair of signals, with a respective LO signal having respective LO frequency as used in the respective frequency translation.

9. A method according to claim 8, wherein the frequency for the first LO signal for frequency translating the first pair of radio signals is selected such that the lower and upper sidebands generated are sufficiently apart in the frequency domain so that a non-frequency translated radio signal can be placed between the said lower and upper sidebands in the frequency domain without overlap.

10. A method according to claim 8 or 9, wherein the frequency for any subsequent LO signal is selected so as to produce lower and upper sidebands sufficiently far apart that they can accommodate, between them in the frequency domain, any lower and upper sidebands generated earlier and any non-frequency translated radio signal, without overlap.

11. A method according to claim 8, 9 or 10, comprising selecting the frequency for any LO signal so that it is below an upper limit so as to produce such lower and upper sidebands that in the frequency domain will not interfere or overlap with other lower and upper sidebands generated using other respective LO frequencies, and will not interfere or overlap with other co-transported signals in other frequency bands.

12. A method according to any one of claims 8 to 11, wherein the LO signals for recovering the radio signals are generated by one or more independent phase-locked LOs or are derived and sent over the optical fiber or fibers from one or each original LO signal used for the frequency translating.

13. A method according to any of claims 8 to 12, wherein the radio signals of the same frequency are MIMO radio signals.

14. A method according to any one of the claims 8 to 13, wherein the frequency translating operations further comprise: filtering to select only the lower sideband resulting from the mixing of the first radio signal with the LO signal; and filtering to select only the upper sideband resulting from the mixing of the second radio signal with the same LO signal, and wherein any further frequency translating further comprises: filtering to select only the lower sideband resulting from the mixing of the first of a pair of radio signals with an LO signal; and filtering to select only the upper sideband resulting from the mixing of the second of a pair of radio signals with the LO signal.