Utility model content
The utility model aims to provide a kind of hardware construction of ultrahigh speed optical fiber radio MIMO transmission system, by coordinating control method to provide a kind of coding, modulation, transmission, demodulation, connecting system and method to high speed fibre microwave transmission between adjacent base station and base station, base station and terminal room high-speed radio ambulatory transceiver in the 5G (the 5th third-generation mobile communication) of following wireless mobile operator, 6G (the 6th third-generation mobile communication) future mobile communication system.The purpose of this utility model is realized by following technical scheme:
A kind of ultrahigh speed optical fiber radio MIMO transmission system, is characterized in that, comprising: modulus/D/A conversion unit, is connected with base station; Also/parallel serial conversion unit of string, is connected with described modulus/D/A conversion unit; Four road 4QAM sequential coding decoding units, with described string also/parallel serial conversion unit is connected; Four road OFDM modem modules, are connected with described four road 4QAM sequential coding decoding units; Four road radio frequency orthogonal modem modules, are connected with described four road OFDM modem modules; Descending optics two-way orthogonal coherent modulating unit, is connected with the downstream interface of described four road radio frequency orthogonal modem modules; Descending optical coherence demodulation photodetection unit, is connected with described descending optics two-way orthogonal coherent modulating unit by optical fiber; Four rf filtering magnification processing, its downstream interface is all connected with described descending optical coherence demodulation photodetection unit; Four road radio-frequency receiving-transmitting antennas, are connected with described four rf filtering magnification processing respectively; Up optical coherence light modulated electrical resistivity survey measurement unit, is connected with the upstream Interface of described four rf filtering magnification processing; Up optical coherence demodulation photodetection unit, is connected with described up optical coherence light modulated electrical resistivity survey measurement unit by optical fiber, and then is connected with the upstream Interface of described four road radio frequency orthogonal modem modules.
As concrete technical scheme, described descending two-way orthogonal coherent light modulated electric unit is identical with the structure of up two-way orthogonal coherent light modulated electric unit, includes: CW laser, cross-polarization beam splitter, two two-way orthogonal coherent electrooptic modulators and cross-polarization bundling device; Cross-polarization beam splitter is connected with laser, and the two-way output 45 degree polarization spectros of cross-polarization beam splitter are connected respectively a two-way orthogonal optical electric modulator with 135 degree polarization spectros, and the output of two two-way orthogonal optical electric modulators connects cross-polarization bundling device; Described two-way orthogonal optical electric modulator adopts phase place to differ the structural design of pi/2, and each two-way orthogonal optical electric modulator accesses You Si road radio frequency orthogonal modem module provides or two parallel code streams of two-way 4QAM-OFDM-radio frequency orthogonal that rf filtering magnification processing provides.
As concrete technical scheme, described descending optical coherence demodulation photodetection unit is identical with up optical coherence demodulation photodetection cellular construction, includes: the first cross-polarization beam splitter, local oscillator laser, the second cross-polarization beam splitter, the first optical coupler, the second optical coupler, two pi/2 optical phase shift devices, four 1:1 optical splitters, eight photodetectors, four electronic subtraction devices; The first cross-polarization beam splitter connects descending two-way orthogonal coherent light modulated electric unit or up two-way orthogonal coherent light modulated electric unit, and its two-way output 45 degree polarization spectros and 135 degree polarization spectros access respectively an optical coupler; The second cross-polarization beam splitter connects local oscillator laser, and its two-way output 45 degree polarization spectros and 135 degree polarization spectros access respectively an optical coupler; The output of each optical coupler Yi road connects two photodetectors after a 1:1 optical splitters, and the parallel code stream of a road 4QAM-OFDM-radio frequency orthogonal is exported in the output of these two photodetectors behind electronic subtraction Qi He road; Another road output of each optical coupler connects two photodetectors after pi/2 optical phase shift device after a 1:1 optical splitters again, and the parallel code stream of another road 4QAM-OFDM-radio frequency orthogonal is exported in the output of these two photodetectors behind electronic subtraction Qi He road.
As concrete technical scheme, the structure of described four rf filtering magnification processing is identical, comprises separately: the first filter, first duplexer, power amplifier, LNA, the second duplexer and the second filter; Wherein, the first filter has lower upstream Interface, connects respectively descending optical coherence demodulation photodetection unit and up optics two-way orthogonal coherent modulating unit; The first time division duplex device is connected with the first filter; The first time division duplex device and then connect respectively power amplifier and LNA, power amplifier is with LNA and then be connected the second time division duplex device; The second time division duplex device is also connected with the second filter, the second filter and then connect described radio-frequency receiving-transmitting antenna.
The ultrahigh speed optical fiber radio MIMO transmission system that the utility model provides, it adopts 3.5GHz radio-frequency carrier, S/P-4QAM information source coding format, secondary cascade orthogonal radio frequency carrier modulation technology, optics orthogonal polarisation state deciliter technology, optics two-way orthogonal coherent photoelectricity modulation technique, optical coherence demodulation detecting technique, 4x4MIMO technology, the 5G (five third-generation mobile communication) of this system to following wireless mobile operator, high speed fibre microwave transmission between adjacent base station and base station in 6G (the 6th third-generation mobile communication) future mobile communication system, base station and terminal room high-speed radio ambulatory transceiver provide a kind of coding, modulation, transmission, demodulation, the hardware system of access.
Embodiment
Below in conjunction with accompanying drawing and embodiment, the utility model is described in further detail.
(1) as shown in Figure 1,, for realize the structural map of ultrahigh speed optical fiber radio MIMO transmission system of 56Gbit/s based on 3.5GHz carrier wave, it comprises: modulus/D/A conversion unit, is connected with base station; Also/parallel serial conversion unit of string, is connected with described modulus/D/A conversion unit; Four road 4QAM sequential coding decoding units, with described string also/parallel serial conversion unit is connected; Four road OFDM modem modules, are connected with described four road 4QAM sequential coding decoding units; Four road radio frequency orthogonal modem modules, are connected with described four road OFDM modem modules; Descending optics two-way orthogonal coherent modulating unit, is connected with the downstream interface of described four road radio frequency orthogonal modem modules; Descending optical coherence demodulation photodetection unit, is connected with described descending optics two-way orthogonal coherent modulating unit by optical fiber; Four rf filtering magnification processing, its downstream interface is all connected with described descending optical coherence demodulation photodetection unit; Four road radio-frequency receiving-transmitting antennas, are connected with described four rf filtering magnification processing respectively; Up optical coherence light modulated electrical resistivity survey measurement unit, is connected with the upstream Interface of described four rf filtering magnification processing; Up optical coherence demodulation photodetection unit, is connected with described up optical coherence light modulated electrical resistivity survey measurement unit by optical fiber, and then is connected with the upstream Interface of described four road radio frequency orthogonal modem modules.
The downlink transfer link of above-mentioned ultrahigh speed optical fiber radio MIMO transmission system: at wireless access network base station machine room end, base station stochastic simulation baseband signal is input to A-D/D-A modulus D/A conversion unit, analog baseband signal is converted to digital baseband signal, this railway digital baseband signal (for example: digital baseband signal 01100101110001110101110100000111 is converted into four railway digital signals through also/parallel serial conversion unit of string ... extract and be converted to four railway digital signals every 4-digit number, the first via extracts 1, 5, 9, 13, 17 ... bit digital: 00100100 the second tunnel extracts 2,6,10,14,18 ... bit digital: 11111101 Third Road extracts 3,7,11,15,19 ... bit digital: 10010001 Si road extracts 4, 8, 12, 16, 20 ... bit digital: 01011101 ...), four railway digital signals after string conversion process are implemented coding digital information by four road 4QAM sequential coding decoding units respectively, four railway digital information after coding enter respectively four road OFDM modem modules four railway digital information after encoding are implemented to OFDM modulation processing, OFDM (OFDM) signal after modulation is by two pairs---and four road radio frequency orthogonal modem modules carry out 3.5GHz high frequency carrier quadrature modulation, orthogonal radio frequency carrier signal after modulation is by optics two-way orthogonal coherent modulating unit, orthogonal radio frequency carrier signal is loaded on photon carrier wave, photon carrier signal after modulation is transferred to wireless signal by standard single-mode fiber (SSMF) and covers far-end, demodulate the parallel code stream of four road S/P-4QAM-OFDM radio frequency orthogonals through optical coherence demodulation photodetection unit, the parallel code stream of four road radio frequency orthogonals carries out rf filtering by four rf filtering magnification processing to it respectively, amplify and process, pass through respectively again four road antenna transmissions to 4x4MIMO wireless terminal device.
The uplink link of above-mentioned ultrahigh speed optical fiber radio MIMO transmission system: the corresponding Si of four part rf filtering magnification processing modules road dual-mode antennas receive after the unlimited carrier information that 4x4MIMO wireless terminal device sends over, carry out photon carrier modulation through the parallel code stream of optics two-way orthogonal coherent modulating unit Dui Si road S/P-4QAM-OFDM radio frequency orthogonal, photon carrier signal after modulation is sent at wireless access network base station machine room end by standard single-mode fiber (SSMF), demodulate the parallel code stream of four road S/P-4QAM-OFDM radio frequency orthogonals through optical coherence demodulation photodetection unit, radio frequency orthogonal parallel code stream in Gai Si road is by two pairs---and four road radio frequency orthogonal modem modules demodulate OFDM chip, four road chips demodulate four road 4QAM coded messages through four road OFDM modem modules, four road 4QAM coded messages are through the four parallel digital signals in 4QAM sequential coding decoding unit decodes Chu Si road, road, four railway digital signals are repacked processing by also/parallel serial conversion unit Jiang Si road signal of string again, convert a railway digital baseband signal to, this railway digital baseband signal is input to wireless access network base station machine room end equipment after digital baseband signal being transformed into analog baseband signal by A-D/D-A modulus D/A conversion unit.
(2) as shown in Figure 2, descending two-way orthogonal coherent light modulated electric unit is identical with the structure of up two-way orthogonal coherent light modulated electric unit, includes: CW laser, cross-polarization beam splitter, two two-way orthogonal coherent electrooptic modulators and cross-polarization bundling device; Cross-polarization beam splitter is connected with laser, and the two-way output 45 degree polarization spectros of cross-polarization beam splitter are connected respectively a two-way orthogonal optical electric modulator with 135 degree polarization spectros, and the output of two two-way orthogonal optical electric modulators connects cross-polarization bundling device; Described two-way orthogonal optical electric modulator adopts phase place to differ the structural design of pi/2, and each two-way orthogonal optical electric modulator accesses You Si road radio frequency orthogonal modem module provides or two parallel code streams of two-way 4QAM-OFDM-radio frequency orthogonal that rf filtering magnification processing provides.
Wherein, cross-polarization beam splitter is divided into 45 degree and 135 degree two-way crossed polarized lights by signal post with CW laser.The parallel code stream of four road 4QAM-OFDM-radio frequency orthogonals is modulated 45 degree and 135 degree two-way crossed polarized lights by two optics two-way orthogonal coherent electrooptic modulators.Optics two-way orthogonal coherent electrooptic modulator adopts phase place to differ pi/2 (being 90 ° of phase phasic differences) structural design.Parallel code stream-1 of 4QAM-OFDM-radio frequency orthogonal and parallel code stream-2 of 4QAM-OFDM-radio frequency orthogonal pair 45 degree polarised lights are modulated, its phase place of photon carrier signal after modulation differs pi/2 (being 90 ° of phase phasic differences), form orthogonally, mutually can not produce interference; Same principle, parallel code stream-3 of 4QAM-OFDM-radio frequency orthogonal and parallel code stream-4 of 4QAM-OFDM-radio frequency orthogonal pair 135 degree polarised lights are modulated, its phase place of photon carrier signal after modulation differs pi/2 (being 90 ° of phase phasic differences), forms orthogonally, mutually can not produce interference.Modulated 45 degree, 135 degree polarized photon carrier signals are spent the photosynthetic roads of polarization by cross-polarization bundling device by orthogonal 45 degree, 135 again.
(3) as shown in Figure 3, descending optical coherence demodulation photodetection unit is identical with up optical coherence demodulation photodetection cellular construction, includes: the first cross-polarization beam splitter, local oscillator laser, the second cross-polarization beam splitter, the first optical coupler, the second optical coupler, two pi/2 optical phase shift devices, four 1:1 optical splitters, eight photodetectors, four electronic subtraction devices; The first cross-polarization beam splitter connects descending two-way orthogonal coherent light modulated electric unit or up two-way orthogonal coherent light modulated electric unit, and its two-way output 45 degree polarization spectros and 135 degree polarization spectros access respectively an optical coupler; The second cross-polarization beam splitter connects local oscillator laser, and its two-way output 45 degree polarization spectros and 135 degree polarization spectros access respectively an optical coupler; The output of each optical coupler Yi road connects two photodetectors after a 1:1 optical splitters, and the parallel code stream of a road 4QAM-OFDM-radio frequency orthogonal is exported in the output of these two photodetectors behind electronic subtraction Qi He road; Another road output of each optical coupler connects two photodetectors after pi/2 optical phase shift device after a 1:1 optical splitters again, and the parallel code stream of another road 4QAM-OFDM-radio frequency orthogonal is exported in the output of these two photodetectors behind electronic subtraction Qi He road.
Wherein, 45 degree, the orthogonal polarised light of 135 degree are isolated through cross-polarization beam splitter after receiving the photon carrier signal that optical fiber transmission sends here in optical coherence demodulation photodetection unit.Another local oscillator laser (with the CW laser same centre frequency in optics two-way orthogonal coherent modulating unit) is isolated 45 degree, the orthogonal polarised light of 135 degree by cross-polarization beam splitter.
Spend for two group 45 after polarised light is coupled by optical coupler and implement two-way optical coupling output.Wherein a road coupling output optical signal carries out shunt by 1:1 optical splitters, light signal along separate routes carries out opto-electronic conversion by two photodetectors respectively, and the two path signal after conversion is again by parallel code stream-1 of electronic subtraction device output 4QAM-OFDM-radio frequency orthogonal modulation signal; Wherein an other road coupling output optical signal is implemented pi/2 (being 90 °, phase place) skew by pi/2 optical phase shift device to its phase place, carry out shunt by 1:1 optical splitters again, light signal is along separate routes respectively by two photodetector opto-electronic conversion, and the two path signal after conversion is again by parallel code stream-2 of electronic subtraction device output 4QAM-OFDM-radio frequency orthogonal modulation signal.
Same principle, spends after polarised light is coupled by optical coupler for two group 135 and implements two-way optical coupling output.Wherein a road coupling output optical signal carries out shunt by 1:1 optical splitters, light signal along separate routes carries out opto-electronic conversion by two photodetectors respectively, and the two path signal after conversion is again by parallel code stream-3 of electronic subtraction device output 4QAM-OFDM-radio frequency orthogonal modulation signal; Wherein an other road coupling output optical signal is implemented pi/2 (being 90 °, phase place) skew by pi/2 optical phase shift device to its phase place, carry out shunt by 1:1 optical splitters again, light signal along separate routes carries out opto-electronic conversion by two photodetectors respectively, and the two path signal after conversion is again by parallel code stream-4 of electronic subtraction device output 4QAM-OFDM-radio frequency orthogonal modulation signal.
(4) as shown in Figure 4, the structure of described four rf filtering magnification processing is identical, describe as an example of rf filtering magnification processing 01 example, comprise separately: the first filter, first duplexer, power amplifier, LNA, the second duplexer and the second filter; Wherein, the first filter has lower upstream Interface, connects respectively descending optical coherence demodulation photodetection unit and up optics two-way orthogonal coherent modulating unit; The first time division duplex device is connected with the first filter; The first time division duplex device and then connect respectively power amplifier and LNA, power amplifier is with LNA and then be connected the second time division duplex device; The second time division duplex device is also connected with the second filter, the second filter and then connect described radio-frequency receiving-transmitting antenna.
Downlink transfer link: parallel code stream-1 of 4QAM-OFDM-radio frequency orthogonal modulation signal is by filter 001 filtering, goes out by antenna transmission after entering duplexer 001, power amplifier 001, duplexer 001, filter 001; Uplink link: antenna reception arrives after the wireless signal from mobile terminal, after filtering device 001, duplexer 001, LNA 001, duplexer 001, parallel code stream-1 of the rear output of filter 001 4QAM-OFDM-radio frequency orthogonal modulation signal.In addition, the monitor portion of rf filtering magnification processing is implemented modules Detection & Controling, and power pack carries out Power supply to each active module embedded therein.
The working theory and processing explanation of above-mentioned ultrahigh speed optical fiber radio MIMO transmission system:
(1) four road photon carrier information orthogonality principle and method
Optical information photon is by a lot of direction of vibration, its direction of vibration is perpendicular to photon spread direction, cross-polarization beam splitter is divided into 45 orthogonal degree of two-way, 135 degree two-way polarised lights (the phase place moment all differs 90 °) by signal post with CW laser, this two-way polarised light is loaded after rf modulations information is closed road and in same optical fiber, transmit and be independent of each other.
The photon information phase place moment after the modulation of optics two-way orthogonal coherent electrooptic modulator Qi Mei road all differs 90 °, is that the two-way photon carrier signal moment after modulation is all occurring orthogonal.
Therefore, it is all orthogonal that the parallel code stream modulation signal of four road 4QAM-OFDM-radio frequency orthogonals loads on CW laser light wave the tetra-road photon carrier signal moment of Hou by cross-polarization beam splitter, two optics two-way orthogonal coherent electrooptic modulators, it propagates and is independent of each other in optical fiber, the generation that this has also been avoided transmission to disturb.Reduce optical signal transmission by optics orthogonal polarisation state, the modulation of optical information quadrature in phase and warble and dispersion, the noise contribution of containment optical noise to up radio-frequency (RF) power amplification, reduces the interference of radio frequency uplink signal noise to base station.
(2) four road photon carrier information optical coherence demodulation photodetection principle and methods
45 degree, the orthogonal polarised light of 135 degree are isolated through cross-polarization beam splitter after receiving the photon carrier signal that optical fiber transmission sends here in optical coherence demodulation photodetection unit.Another local oscillator laser (with the CW laser same centre frequency in optics two-way orthogonal coherent modulating unit) is isolated 45 degree, the orthogonal polarised light of 135 degree by laser polarization beam splitter.
Spend for two group 45 after polarised light is coupled by optical coupler and implement two-way optical coupling output.Wherein a road coupling output optical signal carries out shunt by 1:1 optical splitters, light signal is along separate routes respectively by two photodetector opto-electronic conversion, and the two path signal after conversion is exported wherein parallel code stream-1 of first via 4QAM-OFDM-radio frequency orthogonal modulation signal by electronic subtraction device again; An other road coupling output optical signal is implemented pi/2 (being 90 °, phase place) skew by pi/2 optical phase shift device to its phase place, carry out shunt by 1:1 optical splitters again, light signal is along separate routes respectively by two photodetector opto-electronic conversion, and the two path signal after conversion is again by wherein parallel code stream-2 of the second road 4QAM-OFDM-radio frequency orthogonal modulation signal of electronic subtraction device output.
Same principle, spends after polarised light is coupled by optical coupler for two group 135 and implements two-way optical coupling output.Wherein a road coupling output optical signal carries out shunt by 1:1 optical splitters, light signal is along separate routes respectively by two photodetector opto-electronic conversion, and the two path signal after conversion is exported wherein parallel code stream-3 of Third Road 4QAM-OFDM-radio frequency orthogonal modulation signal by electronic subtraction device again; An other road coupling output optical signal is implemented pi/2 (being 90 °, phase place) skew by pi/2 optical phase shift device to its phase place, carry out shunt by 1:1 optical splitters again, light signal is along separate routes respectively by two photodetector opto-electronic conversion, and the two path signal after conversion is exported wherein parallel code stream-4 of Si road 4QAM-OFDM-radio frequency orthogonal modulation signal by electronic subtraction device again.
(3) also/parallel serial conversion unit method for designing of string
Going here and there and changing is that digital baseband signal is converted into four railway digital signals, conversion method is for example: digital baseband signal 01100101110001110101110100000111 ... extract and be converted to four railway digital signals every 4-digit number, the first via extracts 1,5,9,13,17 ... bit digital: 00100100 The second tunnel extracts 2,6,10,14,18 ... bit digital: 11111101 Third Road extracts 3,7,11,15,19 ... bit digital: 10010001 Si road extracts 4,8,12,16,20 ... bit digital: 01011101 ...
In like manner parallel-serial conversion is that four railway digital signals are changed into a railway digital signal, conversion method for example: first via numeral: 00100100 ..., the second railway digital: 11111101 ..., Third Road numeral: 10010001 ..., the 4th railway digital: 01011101 ...Using the first via as 1,5,9,13,17 ... position, the second tunnel is as 2,6,10,14,18 ... position, Third Road 3,7,11,15,19 ... position, Si road is as 4,8,12,16,20 ... position, composition one railway digital signal 01100101110001110101110100000111 ...
Also/parallel serial conversion unit of string, keeps strict synchronous in time series, string converting unit input data rate 56Gbit/s, and every road parallel signal data output rate is 56/4Gbit/s, Ji Mei road parallel signal speed is 14Gbit/s.
(4) four road 4QAM sequential coding decoding unit methods for designing
The four identical QAM encoders in tunnel keep strict synchronous in time series, and each symbol takies 2, are that 22=4 kind changes, i.e. 4QAM.4QAM encoder output Q and I two-way multi-system signal.4QAM encoder to go here and there and change after four railway digital baseband signals through row recompile, generate four road 4QAM pattern sequences, amount to eight road Q and I multi-system signal.
Four road QAM decoders, its identical QAM encoder process in principle design method Yu Si road is reciprocal, and Principle Method design process is not repeated.
(5) four road OFDM modem module methods for designing
Four road OFDM modulators keep strict synchronous in time series, and Qi Mei road OFDM modulated process method for designing and parameter designing are identical.For example first via OFDM modulation,------> inverse Fourier transform------> parallel-serial conversion---> digital-to-analogue conversion---> filter shape---the >OFDM signal output that > adds redundancy prefix that > goes here and there and changes of its data OFDM modulating transformation processing procedure: base band data input; Parameter designing: 512 subcarriers, 256 position arrays, 1024 Fourier transform points, redundancy prefix point is 0.
Four road ofdm demodulators keep strict synchronous in time series, and Qi Mei road OFDM demodulating process method for designing and parameter designing are identical.For example first via OFDM demodulation,---> filter shape---> analog-to-digital conversion---------> Fourier transform---> parallel-serial conversion---the > base band data output that > adds redundancy prefix that > goes here and there and changes of its data OFDM demodulation conversion process: ofdm signal input; Parameter designing: reference rate is 14/2Gbit/s, reference rate is 7Gbit/s, 512 subcarriers, 256 position arrays, 1024 Fourier transform points, redundancy prefix point is 0.
(5) four road radio frequency orthogonal modem module methods for designing
Four road rf quadrature modulators keep strict synchronous in time series, and Qi Mei road radio frequency orthogonal modulated process method for designing and parameter designing are identical.Every road parameter designing: carrier modulation frequency 3.5GHz, phase place is 0 degree.
Four road radio frequency orthogonal demodulators keep strict synchronous in time series, and Qi Mei road radio frequency orthogonal demodulating process method for designing and parameter designing are identical.Every road parameter designing: carrier wave demodulation frequency 3.5GHz, phase place is 0 degree, and cut-off frequency is 14G/2, is 7GHz, and filtering type is cosine rolling cut-off filtering.
The beneficial effects of the utility model: (1) provides a kind of wireless 4x4MIMO transmission system of ultrahigh speed optical fiber that realizes 56Gbit/s based on 3.5GHz carrier wave; (2) a kind of method that provides high speed of four road 4QAM sequential coding two-stage different-format orthogonal radio frequency modulation to receive and dispatch; (3) provide a kind of 45 degree and 135 degree laser polarization deciliter Shu Shuanlu orthogonal coherent modulator approaches; (4) provide a kind of 45 degree and 135 degree laser polarization beam splitting coherent demodulation photo-detection methods; (5) provide a kind of method of noise jamming that automatically reduces except making an uproar; (6) provide a kind of system and method for a kind of Beyond4G (5G) future mobile communications wireless access; (7) provide a kind of method that between a kind of Beyond4G (5G) future mobile communications base station and base station, high speed fibre transmits; (8) use optical fiber as transmission link, reduce difficulty of construction, reduce construction cost; (9) provide simple optical fiber transmitted in both directions, reduce the use of optical fiber and optical transceiver, greatly reduce costs; (10) 3.5GHz antenna opening power be can effectively control, indoor and outdoor network rate and communication quality improved; (11) according to business demand, can remote adjustment 3.5GHz antenna opening power, control coverage, accomplish to save energy and reduce the cost; (12) provide intelligent real-time monitoring, reduce attendant and turn out for work, lower maintenance cost; (13) use composite fiber optical cable, remote power feeding function is provided, solve the difficult problem of rf filtering magnification processing power taking.