WO2011070448A2

WO2011070448A2 - Radio-over-fiber communication system

Info

Publication number: WO2011070448A2
Application number: PCT/IB2010/003537
Authority: WO
Inventors: Graham Town
Original assignee: Graham Town
Priority date: 2009-11-30
Filing date: 2010-11-30
Publication date: 2011-06-16
Also published as: WO2011070448A3

Abstract

A system for optical transmission of radio signals modulated for the purpose of communicating information without the use of radio-frequency local oscillators or radio-frequency electro-optic modulators.

Description

Title of Invention: Radio-over-Fiber Communication System Technical Field

[1] This invention concerns a communication system design, for the generation,

modulation, transmission, and detection of broadband microwave, mm-wave and terahertz radio signals using optical carriers. Such systems are intended for optical transmission (over optical fibre) of information modulated onto radio-frequency carriers to points where they are retransmitted or distributed wirelessly as a radio frequency signal, for instance in a local area or mobile communication network.

Background Art

[2] Systems for the optical transmission of broadband wireless signals are required for low-loss distribution of radio-frequency signals to points at which they are retransmitted wirelessly. Such systems are particularly necessary when the radio- frequency carrier is in the microwave frequency range, or higher, due to high propagation losses at these frequencies. Radio-over-fibre (RoF) systems are a common example of such systems. See [Mikk98, Smoc02] for a review of radio-over- fibre system concepts and design considerations.

[2] A considerable number of approaches for realising the various components and combinations for realising systems capable of optical transmission of mm-wave and other radio-frequency signals have been proposed. See [Seed06^~] for reviews of the component technologies, and [Smit97, Brau98, Kuri99, Joha03] for typical system designs.

Disclosure of Invention

Technical Problem

[4] Radio-over-fiber systems require conversion of optical signals to radio-frequency signals (for radiation by antennas), and vice-versa. Existing systems typically rely on complicated (e.g. phase- locked) laser sources and/or complicated modulation and filtering techniques, and usually require expensive microwave oscillators and/or high performance optical modulators designed to operate at the mm-wave carrier frequency. Furthermore, at THz radio frequencies components do not exist to implement many of the functions required in existing RoF and related system designs.

Technical Solution

[5] The invention is an optical transmission system for broadband radio frequencies, such as. microwave and above (e.g. for use in Radio over Fibre (RoF) applications), comprising:

• A source (or sources) of two optical carrier signals separated in frequency by an amount that defines the wireless carrier frequency.

A modulator to modulate the optical carriers with an information signal.

• An optical transmission system to distribute the modulated carrier signals.

An optical detector to receive the transmitted signals and heterodyne them together to generate a radio-frequency carrier modulated by the information signal.

• A radio frequency transmission system (at a base station) to wirelessly

transmit the radio-frequency signal containing the information signal.

Modulation may involve the use of a Mach-Zehnder (MZM) modulator to modulate the combined optical carriers with the baseband information.

At the base station, the signal may be detected by a PIN photodetector, and then the radio-frequency carrier with the modulated sidebands containing the information may be sent to a base station antenna via an equalising amplifier.

A mobile device may receive the mm-wave signal from the wireless transmission system and detect or homodyne that signal to recover the data. Low pass filtering is typically used to extract the information signal.

In a second aspect the invention is a method for operating an optical transmission system for broadband radio frequencies, comprising the steps of:

• Generating two optical carrier signals separated in frequency by an amount that defines the wireless carrier frequency.

• Modulating the optical carriers with an information signal.

• Distributing the modulated carrier signals over an optical transmission system.

Detecting the transmitted optical signals and heterodyning them together to generate a radio-frequency carrier modulated by the information signal.

Wirelessly transmitting the radio-frequency signal containing the information signal over a radio frequency transmission system.

A further step involves receiving the radio-frequency signal and converting it to baseband and then retrieving the information signal.

Advantageous Effects

The proposed scheme offers simplified and cost-effective implementation of such systems, which has great potential to be competitive with other wireless access technologies . It also makes THz wireless systems much more practical than is possible with the current state of the art.

This system avoids the need for microwave synthesizers and high speed optical modulators. In this technique, the radio carrier frequency is determined by the separation between two continuous-wave optical carriers, which, for example, may be generated by two separate lasers, or as two longitudinal modes of a single laser cavity.

[13] Simulation results shows that the proposed system can successfully avoid all the stringent requirements of phase locking, high speed modulators or local oscillators, or both, at the central office and the base stations.

Description of Drawings

[14 ] An example of the invention will now be described with reference to the accompanying drawings, in which:

• Fig- 1 is a schematic diagram of a millimeter-wave (mm-wave) Radio over Fibre (RoF) system.

• Fig. 2 is a graph of the optical power spectrum of the modulated mm-wave carrier.

• Fig. 3 is a graph of the back to back Bit-Error-Rate (BER) both before (a) and after optical transmission (b).

• Fig. 4 is a schematic diagram of an alternative millimeter- wave Radio over Fibre system.

Fig. 5 is a graph of the Bit-Error-Rate for different frequency spacing between two laser beams.

• Fig. 6 is a graph of the power penalty for Relative Intensity Noise (RIN) variation in both the laser beams

• Fig. 7 is a graph of the Bit-Error-Rate for different data rates after optical transmission.

Best Modes of the Invention

[15] Fig. 1 presents a first RoF system 10 in which two optical tones are generated in a single laser cavity, these are heterodyned together. After optical and wireless transmission they arc self mixed/homodyned to recover the baseband data in a mobile device. System 10 comprises, a laser source 12 realized using a Fabry-Perot cavity incorporating a short length of highly doped erbium-fibre amplifier 14 with a gold mirror 16 butt-coupled at one end, and a dual-channel Bragg grating comb-filter 18 spliced to the other end to select the lasing modes around 1550 nm.

[16] Two lasing modes, one at 1550.34 nm and the other at 1550.54 nm are separated by the mm-wave frequency at approximately 25 GHz. The two modes from the laser carrying the mm-wave carrier frequency are then modulated with data 25 using a Mach-Zehnder (MZM) modulator 20. The MZM bias voltage and modulation voltage are -2.8 V and 1.5 V respectively.

[17] The resulting modulated signal is then transmitted over forty kilometers of single- mode fibre (SMF) 28 to base station 30. At the base station, the signal is detected by a PIN photodetector 32, and then the muo-wave carrier with the modulated sidebands containing information data are sent to the base station antenna 34 via equalising amplifier 36.

[18] After reception at a user's mobile unit 40, via antenna 42, the signal is heterodyned at mixer 44 to extract the baseband, which then passes another equalizing amplifier 46 before low pass filtering 48 to extract the data 25.

[19] Now we will present numerical simulations of the performance of a 1 Gbps data stream on a 25GHz carrier generated by a dual wavelength laser.

[20] Fig. 2 presents the modulated dual- wavelength laser power spectrum with 25 GHz mm-wave carrier separation after the MZM in our system.

[21] The bit-crror-rate (BER) curves for the recovered signals for both back-to-back

(BTB) connection (a) and after transmission over 40 km of SMF (b), respectively are shown in Fig. 3. A power penalty of 0.3 dB at a BER of 10-9 is observed.

[22] An alternative implementation of the Rof system 100 will now be described with reference to Fig. 4. This system generates the mm-wave signal by optically heterodyning two off-the-shelf, low cost light single mode distributed feedback (DFB) lasers 102 and 104 operating at 1550 and 1550.32 respectively, and nm coupled together by a 3 dB coupler 106.

[23] Simulation was performed using VPI Transmission maker 7.6™. Linewidths for both lasers are defined as 5 MHz with RIN of -130 dBc/Hz. The difference between the lasers is a 40 GHz mm-wave signal in our case. A 2.5 Gbps ASK NRZ data with a PRBS length of 2^T- 1 is used to dri ve the Mach Zehnder modulator (MZM) 20' with 30 dB extinction ratio. The double sideband modulated output js then carried to a 25 km single-mode fiber (SMF) 28'.

[24] Photodetector 32' senses the signal output from the optical fibre, that is an RF carrier at. 40 GHz along with the data, and after passing equalizing amplifier 36" they are wirelessly transmitted vja antenna 34'.

[25] After wireless transmission they are received at antenna 42' of a mobile device 40, self mixed/bomodyned 44' to recover the baseband data. A low pass filter of 5 GHz bandwidth 48' is then used to recover the data 25'.

[26] Fig. 5 shows the Bit-Error-Rate (BER) penalty for the recovered signals for both back-to-back (BTB) cases (a), (b) and (c) and after transmission over 25 km of SMF (d) at a BER of lO⁹ which is 0.1 dB in our simulation. Fig. 5 also presents the BTB BER for different frequency separations between two lasers. It can be seen that power penalty is increased as frequency difference between two lasers is decreased from 40 GHz to 10 GHz due to spectral overlapping at lower frequency separations.

[27] Phase noise effects and Relative Intensity Noise (RIN) play an important role in

microwave photonic systems, since the BER penalty increases due to increased phase noise and RIN. Though the RIN is inherent and cannot be improved after a certain limit, jt is important to keep the phase noise as low as possible. Therefore, we observed both the phase noise and RIN effects on BER for our system. We also investigated the effect of the laser linewidths on the BTB BER performance and found there to be no dependence for linewidths between 10kHz and 50MHz (i.e. much less than the data rate).

[28] In order to see the effects of RIN, we used different sets of RIN values for both the lasers (from - 130 to -120 dBc/Hz); see Fig. 6. About 4.9 dB power penalty is experienced as the RIN values are changed from - 130 to -120 dBc/Hz. Inset (a) and (b) shows eye diagrams of recovered data at -120 and -130 dBc/Hz respectively.

[29] This confirms that the system with higher RIN might degrade the system performance. As DFB lasers with RIN value of -140 dBc/Hz are already available commercially, in our system we used lasers with RIN of -130 dBc/Hz to observe system performance.

[30] Data rates of our system were varied from 155 Mbps to 2.5 Gbps. For 155 Mbps data, the linewidths of the both the lasers are increased from 5 MHz to 50 MHz. 0.35 dB BTB power penalty is observed for 155 Mbps data compared to almost no power penalty for 2.5 Gbps data as in Fig. 7. The power penalty is found due to the fact that for a given linewidth, data rate should be chosen large enough so that phase correlation in successive pulses is maintained. Apart from that, increasing the data rate from 155 Mbps to 2.5 GHz increases the BTB penalty by approximately 4.5 dB when lasers of 5 MHz linewidths are used.

[31] Although the invention has been described with reference to two particular examples it should be appreciated that many variations and modifications fall within the scope of the invention.

[32] References

[33] [Brau98] R. P. Braun, et al._r X.ow phase noise millimeterwave generation at 64 GHz and data transmission using optical side band injection locking,' IEEE Photon. Tech. Lett., 10, (1998)

[34] [Kuri99] T. Kuri, et al, 'Fiber-Optic Millimeter- Wave Downlink System Using 60

GHz-Band External Modulation,' J. Light. Technol. 17, 799 (1999)

[35] [Joha03] L. Johansson, A. Seeds, ' Generation and Transmission of Millimeter- Wave

Data-Modulated Optical Signals Using an Optical Injection Phase-Lock Loop', J.

Light. Technol., 21, 511 (2003)

[36] [Mikk98] J. Mikkonen, 'Emerging wireless broadband networks,' IEEE Comm. Mag.,

36(2), 112, (1998)

[37] [Seed06] A. J. Seeds, and K. J. Williams, "Microwave photonics", J. Lightwave Technol., 24, 4628-4641 (2006) [38] [Srnoc02] L. Smoczynski, et al, Ά comparison of different radio over fibre system concepts with regard to applications in mobile Internet and multimedia,' Proceedings of 20024th International Conference on Transparent Optical Networks, 1 , 211-213 (2002)

[39] [Smit97] G. H. Smith, D. Novak, and Z. Ahmed, Overcoming chromatic dispersion effects in fibre-wireless systems incorporating external modulators, ' IEEE Trans, on Microwave Theory and Tech., 45, 1410-1415 (1997)

Claims

An optical transmission system for broadband radio frequencies, such as microwave and above, comprising:

a source of two optical carrier signals separated in frequency by an amount that defines the wireless carrier frequency;

a modulator to modulate the optical carriers with an information signal;

an optical transmission system to distribute the modulated carrier signals;

an optical detector to receive the transmitted signals and heterodyne them together and generate a radio-frequency carrier modulated by the information signal;

a radio frequency transmission system to wirelessly transmit the radio-frequency signal containing the information signal.

An optical transmission system according to claim 1 , further comprising a mobile device wherein a radio-frequency detector and baseband signal processing system convert received radio-frequency signals to baseband and retrieve the information signal.

An optical transmission system according to claim 1, further comprising a Mach-Zehnder (MZM) modulator to modulate the combined optical carriers with the baseband information.

An optical transmission system, according to claim 1 , wherein the optical detector is a PIN photodetector.

An optical transmission system, according to claim 4_r wherein the radio-frequency signal containing the information signal is a carrier modulated by the information signal, and it is passed to a base station antenna via an equalising amplifier.

A method for operating an optical transmission system for broadband radio frequencies, comprising the steps of:

generating two optical carrier signals separated in frequency by an amount that defines the wireless carrier frequency;

modulating one or both the optical carriers with an information signal;

distributing the modulated carrier signals over an optical transmission system;

detecting the transmitted optical signals and heterodyning them together to generate a radio-frequency carrier modulated by the information signal; and,

[Claim 7] A method according to claim 6, further comprising the steps of

receiving radio-frequency signal, converting it to baseband and then retrieving the information signal.