Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection

Transcription

1 Optimisatio d'ue Chaîe de Commuicatio Numérique Adaptée aux Systèmes de Trasmissio Optique Sas Fil : Exemple de la Modulatio OFDM/OQAM Modifiée Wireless Optical Networks with Modified OFDM/OQAM ad Direct Detectio 1, 2, Patrick Tortelier 1, Ramesh Pydiah 2, Olivier Bouchet 1 1 Frace Telecom, Orage Labs 2 IT-TELECOM TELECOM Bretage, SC Departmet mamdouh.eltabach@orage-ftgroup.com Labs, 4 rue du Clos Courtel, Cesso-Sévigé Cedex BP 52, Frace Departmet, Techopôle Brest-Iroise, CS 83818, Brest Cedex 3, Frace Sémiaire des doctorats TELECOM Bretage 28 28/3/28

2 p 2 Wireless optical etworks Propagatio types Trasmissio model Chael characteristics Outlie Itroductio to OFDM/OQAM Priciples of OFDM/OQAM Similarities/differeces betwee CP-OFDM ad OFDM/OQAM OFDM/OQAM i equatios Modified OFDM/OQAM for wireless optical etworks Hermitia symmetry Normalized offset value System performace Coclusio

3 p 3 Wireless Optical Networks (WON) - Why? Serious alterative to radio frequecy trasmissio Uregulated ad ulicesed optical spectrum No iterferece with existig radio systems Ease of home etworks iterface Potetial of high bit rate Optical wave ad walls Trasmissio security Frequecy reuse Eye safety

4 p 4 System modules: WON - Propagatio types Trasmitter LED or Laser diode Electrical to Optical (E/O) coversio Divergece optics, Half Power agle (HP) Average optical trasmitted power Pt Receiver Optical filter Cocetratio Optics, Field Of View (FOV) Optical to Electrical (O/E) coversio PIN or APD Photodiode Optical chael Geometrical atteuatio Multiple reflectios, Iter Symbol Iterferece (ISI) Noises

5 p 5 WON - Propagatio types Lie Of Sight (LOS) Wide Lie Of Sight (WLOS) Diffusio (DIFF) LOS: high directivity, poit-to-poit systems, like IrDA WLOS: ecessity of aligmet, wider divergece (HP) ad Field Of View (FOV) agles, like remote cotrollers DIFF: obstructed or ot, always full-duplex coectio - Multiple optical reflectios - Most user-friedly for the customer - Need of high trasmitted optical power

6 p 6 WON - Trasmissio model Wireless optical etwork model, IM/DD: Trasmissio Itesity Modulatio (IM): OOK, PPM, Optical power carries modulated useful iformatio Receptio Output photocurret proportioal to iput optical power Direct Detectio (DD) Quadratic detectio

7 p 7 WON - Trasmissio model h(t) the optical chael impulse respose N(t) the differet oise cotributios Y(t) the received photocurret (A) Costraits o the trasmitted sigal Istataeous optical trasmitted power X ( t) Average optical trasmitted power P t 1 = lim T + 2T T T X ( t)dt P Y ( t) = R. X ( t) h( t) + N( t) X(t) the trasmitted optical power (W) R the receiver resposivity (A/W) max Equivalet Basebad Model!

8 p 8 WON - Trasmissio model DC chael gai, geometrical atteuatio Average optical received power ( ) H ( f ) = = + H = f h( t) dt P r P r P r = = = 1 lim T 2T 1 lim T 2T u t + T T u ( X ( t) h( t) ) h( u) h( u) P du = P + T T t u X ( t dt h( u) du = 1 = lim T 2T u) dt du = PH t ( ) P r = u + T T u h( u) X ( t 1 h( u) lim T 2T PH t ( ) u) du dt + T T X ( t u) dt du

9 p 9 WON Chael characteristics h(t): the optical multipath chael impulse respose Real positive Atteuatio o each path Delay o each path No phase compoet [Source: Barry & Kah 97] Photodetecor diameter >>> Optical wavelegth Absece of multipath fadig, but Presece of multipath distortio, ISI Y(t) is a spatial mea of X(t) icludig all the values of fadig

10 p 1 WON Chael characteristics N(t) depeds o: Ambiet light Photodiode type Receiver desig Used wavelegth Trasmissio eviromet Domiat Periodic electric oise Thermal oise Shot oise Ambiet light ca be miimized by optical filterig, but it still adds shot oise! Due to its high itesity, this shot oise ca be modeled as white, Gaussia, ad idepedet of X (t) Additive White Gaussia Noise (AWNG) Power spectral desity: shot ( Iicidet + Id ) = 2q( Iuseful + Iambiet + Id ) qiambiet DSP = 2q 2 I icidet : the total curret correspodig to icidet power q: the electro charge I ambiet : the ambiet backgroud curret I d : the dark curret

11 p 11 Wireless optical etworks Propagatio types Trasmissio model Chael characteristics Outlie Itroductio to OFDM/OQAM Priciples of OFDM/OQAM Similarities/differeces betwee CP-OFDM ad OFDM/OQAM OFDM/OQAM i equatios Modified OFDM/OQAM for wireless optical etworks Hermitia symmetry Normalized offset value System performace Coclusio

12 p 12 OFDM/OQAM basic priciples Aim: to icrease OFDM spectral efficiecy by removig the guard iterval (cyclic prefix) How: waveform modulatig sub-carriers as much localized as possible i the time ad frequecy domai robustess to both time ad frequecy selectivity itroduced by the chael The waveform must guaratee orthogoality betwee subcarriers ad multi-carrier symbols Orthogoality i the real domai OffsetQAM o each subcarrier Example: IOTA waveform (quasi optimally localized i time ad i frequecy)

13 p 13 OFDM/OQAM modulatio ad CP-OFDM * key features Parameters Symbols Guard Iterval Symbol duratio Orthogoality Prototype Implemetatio CP-OFDM Complex (QAM) Yes T +GI Complex Rectagular FFT OFDM/OQAM Real (PAM) No T /2 Real Various possibilities (IOTA, ) FFT + polyphase filter * CP-OFDM = Cyclic Prefix OFDM

14 p 14 OFDM/OQAM i equatios Trasmitted sigal i OFDM / Trasmitted sigal i OFDM/OQAM M 1 2i mft s( t) = π cm, e p( t T ) m= s = m m+ 2iπmν ( ) t t a i e f ( t τ ) m, where: c m, are complex 1 F = T where: a m, are real ν = 1 2τ s( t) M = m= 1 c m, pm, ( t) s M = 1 m= ( t) a f ( t) m, m, m, 2iπF t ( t) = p( t T ) e p f m, m 2iπmν ( ) t t = i + e f ( t τ )

15 p 15 Without chael: Received sigal Demodulatio I (m, M 1 + s ( t ) = a m, f m, ( t ) s m = = * ( t) f ( t) m, dt = a Itrisic iterferece is pure imagiary With chael: m, is orthogoal to the real symbol Received sigal Demodulatio r OFDM/OQAM i equatios r * ( t) f ( t) m, M m, ( m, ) ( m, ) ( I m, a m, ( t ) = h a f ( t ) dt = m = = h m, a m m,, m, + m, h a ( m, ) ( m, ) I m, f m, f * m, : itrisic iterferece m, a m, dt Real Orthogoality m, * m, : itrisic iterfere ce f Depeds o m, f dt

16 p 16 Wireless optical etworks Propagatio types Trasmissio model Chael characteristics Outlie Itroductio to OFDM/OQAM Priciples of OFDM/OQAM Similarities/differeces betwee CP-OFDM ad OFDM/OQAM OFDM/OQAM i equatios Modified OFDM/OQAM for wireless optical etworks Hermitia symmetry Normalized offset value System performace Coclusio

17 p 17 Modified OFDM/OQAM for WON Hermitia symmetry I WON equivalet basebad model X(t) is a optical power It must be real ad positive I OFDM/OQAM basebad model Chael iput is complex!! OFDM/OQAM iherits from may properties related to classical OFDM Hermitia Symmetry? Due to Fourier trasform properties, for classical OFDM, if the discrete iput sequece of the modulator has its first ad ceter coefficiets ull, ad if it presets Hermitia symmetry with respect to its ceter, the the OFDM time modulated sigal is real. Hermitia symmetric coditios could be rewritte for OFDM/OQAM

18 p 18 Modified OFDM/OQAM for WON Hermitia symmetry OFDM/OQAM i discrete domai s + M 1 m D 2iπ k m+ 2N 2 [ k] = a i e f [ k N ] = m= m, For all k, s[k] is real if a, if am m =, N m =, D N, ( 1) m k: istat time coefficiet Lp: prototype filter legth, D=Lp-1 N=M/2 is the half umber sub-carriers, otherwise + 2iπ k + [ ] = [ ] m 2 N 2 Real s k 2 f k N Re a i e Positive?? = N 1 m= 1 m, m D

19 p 19 Modified OFDM/OQAM for WON Normalized offset value To esure positivity, a offset (DC) is required Discrete chael iput will be x [ k] = s[ k] + Trasmitted symbols a m, are cetered P t = DC DC We reshape the sigal to ormalize Pt ad DC Photodiode discrete output is ( ) DC [ ] y [ k] = s[ k] h[ k] + H + k Offset cotributio is removed by subtractig H()DC Ad, the OFDM/OQAM demodulatio is performed Hermitia symmetry Normalized offset value Modified OFDM/OQAM For WON

20 p Modified OFDM/OQAM for WON System performace Modified OFDM, zero CP Modified OFDM/OQAM LOS/WLOS typology Oe direct path (LOS/WLOS) Type OFDM IM/DD OFDM/OQAM IM/DD Prototype Rectagular IOTA BER 1-2 Modulatio Symbol rate 4-QAM 1.5 Gbaud 4-QAM 1.5 Gbaud 1-3 FOV Distace d m m FFT size 2*N Eb/No Cyclic prefix Modified OFDM/OQAM ad OFDM Icrease the bit rate Use complex multi level modulatios Modified OFDM/OQAM Is similar to Modified OFDM, zero CP

21 p 21 BER Modified OFDM/OQAM for WON System performace Modified CP-OFDM Modified OFDM/OQAM Eb/No Modified OFDM/OQAM Outperforms Modified CP-OFDM DIFF typology, Gfeller example Oe optical reflectio h τ τ t, otherwise 2 ( t) = si ( FOV ) miimum delay 2, τ t 3 Modified OFDM/OQAM ad CP-OFDM Mitigate ISI effects Icrease the bit rate τ cos( FOV ) Modified CP-OFDM: CP duratio: 25% Same other parameters Modified OFDM/OQAM Optimal spectral efficiecy

22 p 22 Wireless optical etworks Propagatio types Trasmissio model Chael characteristics Outlie Itroductio to OFDM/OQAM Priciples of OFDM/OQAM Similarities/differeces betwee CP-OFDM ad OFDM/OQAM OFDM/OQAM i equatios Modified OFDM/OQAM for wireless optical etworks Hermitia symmetry Normalized offset value System performace Coclusio

23 p 23 Coclusio Wireless optical etworks WON A promisig alterative to radio etworks Based o direct detectio ad OFDM/OQAM Modified OFDM/OQAM for WON is proposed Modified OFDM/OQAM scheme Fights agaist ISI problems Icreases the bit rate Permits the use of complex QAM modulatios Beefits from optimal spectral efficiecy Could outperform modified CP-OFDM

24 p 24 Thak you!

25 p 25 OFDM/OQAM time-frequecy represetatio Time-frequecy lattice: τ ν f OQAM symbol (real) QAM symbol (complex) t ν τ = 1 /desity desity = 1, complex orthogoality desity = 2, real orthogoality