Radio over Fiber technologies for in-building networks

Transcription

1 Radio over Fiber technologies for in-building networks Davide Visani 29 October 2010 Bologna

2 Summary Reason for a Distributed Antenna Systems (DAS) Radio over Fiber technologies Multimode Fiber for in-building environment IM-DD link with multimode fiber Modal noise and its impact in IM-DD link: Link gain Harmonic and Intermodulation products Conclusions

3 Reason for DAS Services in the in-building scenario state of the art Services Wired solution

4 Reason for DAS Evolution of wireless standard Wireless service are going in the direction of 100 Mbps to mobile user in the near future GSM: Global Service for Mobile communications GPRS: Global Packet Radio Service UMTS: Universal Mobile Telecommunications System HSPA: High Speed Packet Access LTE/SAE: Long Term Evolution/System Architecture Evolution BWA/WiMAX: Broadband Wireless Access/Worldwide Interoperability for Microwave Access

5 Reason for DAS New wireless standard can in principle achieve more than Mbps per user, but using: Pico-cell MIMO technologies This to avoid users interference and maximize the use of the free space channel This need a backhaul network able to connect all the pico-cell and to transfer a multi-gigabit traffic Need of Distributed Antenna Systems

6 Reason for DAS DAS architecture for in-building scenario Remote Antenna Unit (RAU) High-speed link: - Coaxial cable - UTP - Optical Fiber Central Office (CO)

7 Reason for DAS DAS basic link - downlink Radio signal over Fiber/Coax CO Main advantage: Multi-services, Multi-operator applications RAU Main disvantage: require high linearity and dynamic range Digitized radio signal over Fiber/Coax/UTP CO ADC Main advantage: Simple integration with digital network DAC RAU Main disvantage: it can support only few frequency bands

8 Radio over Fiber systems Definition: Radio over Fiber (RoF) refers to a technology whereby light is modulated by a radio signal and transmitted over an optical fiber link. (Wikipedia) Main advantages of RoF technologies with silica fiber: Big bandwidth of the fiber Low losses of the fiber Electromagnetic immunity Possible applications: TV broadcasting (CATV - USA) Radio astronomy Distributed Antenna Systems

9 Fiber Distributed Antenna Systems (F-DAS) Building Tunnel Hybrid Fiber Coax (HFC): fiber to the floor, coax to the antenna Radio cell Scenario: Radio Remote Antenna Unit (RAU) are connected to a Central Office (CO) by fiber links in different network solutions (point-to-point, ring, star,...) Main advantages: Out-door and In-door coverage Possibility of multi-service and multi-operator use on the same fiber Flexible radio network interface

10 Fiber Distributed Antenna Systems Central Office (CO) to Remote Antenna Unit (RAU) link uplink E/O RF to/from BS downlink O/E RF RAU

11 Fiber Distributed Antenna Systems Case of study: Intensity Modulated Direct Detection (downlink and uplink) C in RF I LD (t) Ligth I PD (t) RF C out P TX P RX Direct intensity Modulation of a Laser (no external modulator) Direct Detection with a PIN or a APD photodiode Use of Single (SMF) or Multimode fiber (MMF) Non ideality: - Frequency dependent response - Laser non linearity - Frequency chirping - Intensity and Phase noise Non ideality: - Chromatic and modal dispersion - Brillouin and Rayleigh scattering More with MMF Non ideality: - Frequency dependent response - Shot and thermal noise

12 Fiber Distributed Antenna Systems Multimode Fiber for In-building Network Why do we consider MMF? The problem of SMF for in-building coverage is related to the deployment. The 9-10 um core is critical for coupling with transceiver, receiver, connector, and need to be deployed carefully (more costs) Since the distance we want to reach are less than 500 m, the bandwidth is not a problem for our applications MMF is already deployed in some areas and will be deployed for future short-range 40/100 Gbps LAN Acces fiber (SMF) Distribution fiber (MMF)

13 Multimode Fiber - introduction For Multimode Fiber (MMF) we mean optical fiber which carries more than one mode The limiting factor of this kind of fiber is intermodal dispersion The simplest approach to understand how intermodal dispersion works is to think at the different modes as to rays with different trajectory (geometrical optics approximation)

14 Multimode Fiber - introduction Numerical aperture Maximum acceptance angle n cladding Numerical aperture: θ max For Step index MMF: n core We can also show that the product bandwidth-length for Step index MMF is related to NA

15 Multimode Fiber Standard Silica Multimode Fiber have a core dimension of 50 or 62.5 um with a parabolic (graded) refractive index n core n cladding Step index multimode fiber has a low bandwidth, because the different modes have too much different group delay n core (0) n cladding The graded index profile can provide a smaller difference between group delay of the modes (less intermodal dispersion) More bandwidth

16 Multimode Fiber Multimode fiber improved during the year and has been standarized in ITU G.651 and ISO/IEC 11801

17 Multimode Fiber A common classification from ISO based on bandwidth Bandwidth at 850 nm Bandwidth at 1310 nm OM1 200 MHz*km 500 MHz*km OM2 500 MHz*km 500 MHz*km OM MHz*km 500 MHz*km OM3+ (no standard) 3500 MHz*km 500 MHz*km OM MHz*km 500 MHz*km These fibers are specially considered for LAN application together with a 850 nm VCSEL We will instead consider them at 1310 nm: why?

18 Multimode Fiber - Transceiver The typical source at 850 nm is Vertical Cavity Surface Emitting Laser (VCSEL), while at 1310nm is Fabry Perot (FP) or Distributed Feed-back (DFB) Laser (both lateral emitting) I INJ I INJ Lateral emission The light reflect and amplify in a long horizontal cavity Vertical emission The light pass through several vertical layer

19 Multimode Fiber - Transceiver VCSEL at 850 nm are a well established technology for digital communication, however the maximum emitted power is typically under 1-3 mw DFB (and FP) at 1310 nm are available both for digital and analog application and can provide output power typically between 4 and 10 mw

20 Multimode Fiber - Transceiver The main disadvantage of VCSEL is the high dependence on temperature of its own characteristics DFB LD has very good performance in term of distortion and it s the most chosen solution for direct analogue modulation (DFB RoF TX are already designed) For this reason we will consider this type of transmitter

21 Multimode Fiber - Transceiver Fabry-Perot LD DFB LD Most cheaper More expensive Multi Mode Emission Highest performance Several Noise Contributions Lower noise, superior linearity High Modulation Bandwidth

22 Multimode Fiber IM-DD link C in RF Ligth I PD (t) RF C out SMF MMF MMF Central Launch Beam Expander Offset Launch They both excite the Fundamental Mode (MAINLY) plus other modes with τ g s much different from its one (τ g mode group delay) It excites with ~ equal weight higher order modes which have similar τ g s

23 Multimode Fiber IM-DD link C in C out SMF MMF MMF τ g Δτ g high P mode /P tot (CL) Modes Power mainly in the core center

24 Multimode Fiber IM-DD link C in C out SMF MMF MMF τ g.4 P mode /P tot (OL) Δτ g small Modes Power mainly in the core border

25 Multimode Fiber IM-DD link Phenomenon related to the multimode nature of the link: Modal noise Pratical explanation: The received optical (and RF) power vary during time like in a fading radio channel

26 Modal noise Modal noise physical explanation Noise generated by the interference between modes of the MMF by the combination of mode-dependent optical losses and the fluctuation in the modal distribution. Main Ingredients : Source (laser) coherence Non ideally connectors Finite area photodiode Temperature changes Mechanical stresses Maximum Interference Mode-dependent loss Fluctuation of the modal distribution

27 Modal Noise Example: perfect central launch in perfect circulary fiber 6 main modes considering polarization -> 3 different intensity ϕ 2 (t 1 ) t 1

28 Modal Noise Example: perfect central launch in perfect circulary fiber 6 main modes considering polarization -> 3 different intensity ϕ 2 (t 2 ) t 2

29 Modal Noise Example: perfect central launch in perfect circulary fiber 6 main modes considering polarization -> 3 different intensity ϕ 2 (t 3 ) t 3

30 Modal Noise Example: perfect central launch in perfect circulary fiber t 1 t 2 t 3 Different power distribution on the same finite photodiode area leads to different received power In this vision the phenomenon depend on the optical power distribution leading to a similar effect on the received optical power and radio signal power (link gain) That s not true

31 Link gain impact Analytical model Input field before going into the MMF Field spatial distribution Intensity modulation Laser frequency (coherence) Frequency chirping Input field in the MMF m-th mode coefficient m-th mode spatial distribution It depends on the launching condition

32 Gain link impact Analytical model Output field in the MMF m-th mode Phase shift m-th mode random phase fluctuation m-th mode group delay We have considered the coherence of the source and the random fluctuation generated by environmental changes. Now we need mode selective losses: In this model we consider mode selective losses generated by the finite area of the photodiode

33 Gain link impact Analytical model Received Optical Intensity Finite photodiode (PD) area DC current: RF amplitude:

34 Gain link impact Analytical model Overlap integral between m-th and n-th modes Slow phase fluctuation difference between m-th and n-th modes Adiabatic dependent chirp factor It is the cause of the different temporal (and statistical) behavior of the received DC current and the RF amplitude

35 Gain link impact Analytical model result DC and RF appear to be in a quadrature relationship

36 Gain link impact Analytical model result explanation J 1 (x) m I J 0 (x) (m I = 0.2) For increasing values of x mn (that means frequency chirp factor K f ) J 1 (x) prevails over m I J 0 (x)

37 Gain link impact Experimental setup Vector Network Analyzer MMF RoF TX Launch Condition Climatic Chamber RoF RX Temperature meter Multimeter V

38 Power supply RoF TX Multimeter VNA MZM RoF RX

39 Power supply Data acquisition board RoF TX Multimeter MMF VNA Temperature sensor MZM RoF RX

40 Gain link impact Experimental results DC and RF are really in quadrature! Medium chirp DFB laser

41 Gain link impact Experimental results High chirp DFB laser Enhanced amplitude fluctuation External modulator (MZM) (very low chirp factor) No quadrature effect and lower RF fluctuations

42 Gain link Impact Performance parameter ΔI RFk Average Value: <I RF >= (1/N) Σ k I RFk Standard Deviation from the Average value: σ I_RF = [(1/N) Σ k (ΔI RFk2 )] 1/2

43 Gain link impact Meaning of Γ Standard deviation of the link gain σ G (db)

44 Gain link impact Performance results High chirp Medium chirp MZM

45 Gain link impact Experimental results: launching conditions laser light multimode fiber Offset launch single mode fiber Central launch Comparison of two types of launch technique: central launch (offset < 3 um) offset launch (offset ~ um) Central launch performs better than offset launch

46 Gain link impact Central launch vs Offset launch modes distribution Central launch Offset launch Offset launch excite high-order modes, which are less confined in the fiber core and more sensitive to the finite area of the photo detector and to misalignments

47 Gain link impact Experimental results: launching conditions and fiber length Offset launch Central launch

48 Modal noise in MMF IM-DD link Experimental results: photodiode area We compare two photodiode with different active area. Smaller area Bigger area

49 Intermodulation impact Link Gain it s not the only quantity to be taken under consideration when developing the systems Since Radio over Fyber systems are designed as analogue systems, we usually take under consideration some parameter like Intercept Point of the Second and the Third order (IP2, IP3) As the Link Gain also the power of the intermodulation products vary with time

50 Intermodulation impact The entire RoF link can be seen as an RF non-linear two port network i i i o RoF TX MMF Climatic Chamber RoF RX i i Non-Linear Time variant Two-Port Network i o Thus, we can make the one and two tone tests Also the new harmonic generated by the system are subjected to modal noise (therefore fiber length)

51 Intermodulation impact Harmonic distortion test i i Non-Linear Time variant Two-PortNetwork i o Link Gain variation around average value Second harmonic variation around average value

52 Intermodulation impact Harmonic distortion test We consider the average value of the second harmonic (normalized to the first harmonic power) versus fiber length High chirp Medium chirp We achieve similar result of the Link Gain study, but with a stronger dependence on the fiber length

53 Intermodulation impact Intermodulation distortion test i i Non-Linear Time variant Two-PortNetwork i o

54 Intermodulation impact Intermodulation distortion test Link Gain variation around average values Intermodulation product around average value

55 Intermodulation impact Intermodulation distortion test We consider IIP3 average value versus fiber length High chirp Medium chirp We achieve similar result of the Link Gain study, but with a stronger dependence on the fiber length MZM

56 Conclusions Radio over Fiber is an attractive solution for in-building wireless distribution (Distributed Antenna System) Multimode fiber is a competitive medium for in-building coverage and should be exploited in Radio over Fiber technologies We did an extensive experimental campaign to characterize an IM-DD Radio over Multimode Fiber link employing DFB laser We developed a model to determine modal noise impact on link gain and intermodulation products. Some operative rules are obtained to diminish modal noise impact with appropriate choices of the components