Opportunistic unsynchronized cognitive radio networks Maurice Bellanger g COST-TERRA meeting– 30-31/08/2010 Contents • Cognitive radio and opportunistic networks • A physical layer based on FBMC • « Good Neighbour » strategy for DDSA • Applications and perspectives Cognitive radio networks characterized by spectrum access • Centralized spectrum access - cognitive pilot channel (primary+secondary users / databases) - cognitive g control channel ((between CRS-coexistence of secondary systems / collaborative) • Decentralized dynamic spectrum access (DDSA) - local decision based on spectrum sensing - no coordination with other systems opportunistic unsynchronized networks Network configuration Base stations and their users – different providers – spectrum sharing SS21 SS11 SS12 BS1 SS31 SS13 BS3 BS: base station SS: subscriber station SS23 SS33 BS2 SS22 SS32 Opportunistic network concept • Detection of an unoccupied frequency band (at a particular time and geographical area) • Local access decision and building of the capacities requested by the users • Spectrum monitoring and adaptation to adjust capacity Objective: global optimization of the spectrum usage Opportunistic terminal Functionalities of the terminal user interface spectrall resources spectrum sensing/monitoring g g opportunistic protocol adaptive transceiver physical layer importance of spectrum sensing/monitoring Some requirements for PHY • Capability to handle unsynchronized users with minimal loss in spectral use (primary ( i andd secondary d users cannott be b synchronized) h i d) • Guaranteed protection of other users (coexistence) • Capability to exploit fragmented spectrum (broadband) (b db d) • Capability to establish a link without preliminary distant alignment • Real time spectrum sensing/monitoring (resolution-latency) • Maximum spectral efficiency • Reasonable computational complexity A physical layer (Phydyas) • Based on the filter bank multicarrier (FBMC) technique - high bit rate and efficient use of the spectrum - high resolution real time spectrum analysis/synthesis • Independence of sub sub-channels channels - flexible spectrum access by unsynchronized users - fine bandwidth granularity g y • Features for cognitive radio - efficient spectrum sharing (minimum distance between users) - guaranteed separation of users (essential for coexistence) - simultaneous spectrum sensing and transmission C Comparing i OFDM andd FBMC Sub channel frequency response Sub-channel amplitude 1.2 FILTER BANK 1 0.8 0.6 OFDM 0.4 02 0.2 0 -0.2 4 5 6 7 8 9 10 11 sub-channel 12 no sidelobes with the filter bank no cyclic prefix : increased spectral efficiency Prototype filter • • Coefficients: ( Overlap factor K=4 K 4 - filter length=4xFFT length 4xFFT size ) hi=1-1.94392cos(πi/512)+1.414cos(πi/256)-0.47029cos(πi3/512) ; i=1,….,1023 Frequency response fre q ue nc y re s p o ns e 0 S C i+ 2 S C i+ 1 SC i -10 -20 -30 -40 -50 -60 -70 70 -80 -90 -100 100 0 0.5 1 1.5 2 2.5 3 3.5 fre q ue nc y (unit: s ub -c ha nne l s p a c ing ) 4 Filter bank amplitude li d 1.2 1 0.8 0.6 0.4 0.2 0 -0 2 -0.2 5 6 7 8 9 10 sub-channel 11 odd/even subchannel do not overlap a subchannel overlaps with neighbours only M lti Multi-user multicarrier lti i transmission t i i • FBMC: independence of subchannels - disjoint user spectra • Several users can have different transmission parameters (b d idth power, ti (bandwidth, timing, i carrier i ffrequency offset…) ff t ) A m p li t u d e 1 .2 use r 2 1 0 .8 0 .6 use r 1 0 .4 use r 3 0 .2 0 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 s u b - c h a n n e l n u m b e r ( fr e q u e n c y ) 4 5 E li ti capability Equalization bilit Withh FBMC, Wi FBMC time i andd frequency f alignment li off the h distant di user is i not a prerequisite to begin transmission Sub-channel equalizer H(Z) x(n) P P N F F T data equalizer coefficients sub-channel equalizer - frequency offset compensation - fractionally spaced transversal equalizer (channel distortion distortion, timing offset compensation) Duplexing aspects • Frequency division duplex (FDD) p of s.c. 2 sub-channels or ggroups • Time division duplex (TDD) - 1 sub-channel or group of s.c. - simple spectrum allocation - burst transmission - the filter impulse response introduces transitions Burst transmission • Impact of prototype filter impulse response a m p litud e 60 iinitial iti l trans ition finall fi trans ition data trans m is s ion 40 20 0 -20 -40 -60 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 tim e the FBMC burst can be shortened to (Ns+1) symbols Sensing / monitoring with FBMC • Performance determined by the prototype filter • Sensing techniques applied at sub-channel level ( energy detection, …..) • Continuous monitoring ( 3 subchannels left idle the center subchannel can sense during transmission ) Access procedure • Identification of available sub-channels ( SINR estimation in every s.c. + available capacity) • Capacity p y requested q byy the users • Allocation of the sub-channels ( groups of contiguous s.c. ) • Adaptation of transmission parameters ( bits per s.c., modulation, burst length, packet length, …) Allocation issues • Available estimated capacity exceeds user needs d ii decision: number b off sub-channels b h l / power • Available capacity below user needs multiantenna processing to mitigate interference and increase the number of available s.c. (space-time spectrum sensing) • Competition for the resource - no rules : instability y of the allocation pprocess - some rules are needed « Good neighbour » concept • Claim the minimum of resources (sub-channels, power) to meet the user demand ( at start and during transmission ) • Self regulation: limit the capacity allocated to a single user to ensure spectrum sharing challenge of the protocol: distribute the total spectral capacity in real time and minimum delayy « Good Neighbour » strategy • Threshold based spectrum allocation - a capacity C0 is defined - no band extension or new band allocation for a user if its capacity is above C0 • Minimum number of band extensions or new band allocations to achieve the given threshold ( minimize interference non-stationarities to other systems ) Threshold determination • Objectives: - approach the maximum total spectral capacity - achieve fast convergence • Local decision by each base station • Based on SINR measurements in available sub-channels • Simple i l approach: h fraction f i off the h estimated i d capacity i pp has been proposed p p in the Phydyas y y • An efficient approach project ( needs validation and optimization ) Application characteristics • Opportunistic access dependent on sensing reliability and performance limited band (ex. digital dividend: 790-862 MHz and 2500-2690 MHz) • Spectrum fragmentation broadband capability • Quick Q i k access to t the th spectrum t andd vacation ti p to the instantaneous user needs • Adaptation Models • Functions of the base station - local l l (high (hi h capacity) i ) networkk - gateway to general networks (fixed or cellular) • Cognitive WiFi-like network (LAN) • Cognitive WiMAX-like network (RAN) Applications • • • • High speed local complement to cellular systems B db d networks Broadband k in i rurall areas Easy and cheap access in sparsely populated areas Temporary high hi h capacity i networks k for f emergency, events or moving events • Peer-to-peer P t b db d communications broadband i ti (ex. machine-to-machine) Challenges of opportunistic networks • Technical challenges - physical layer: FBMC-based proposal (Phydyas) / tool-box available - protocol: « good neighbour » proposal ( need for experimentation and full scale validation ) • Standardization and regulation - specification of key technical parameters (coexistence/access) - reserved/authorized bands + access rules • Business model Want to know more ? An FBMC primer is available on the project website www.ict-phydyas.org