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Improving Wireless Access Technologies Adam Wolisz Professor of EE&CS, Technische Universität Berlin, TKN Adjunct Professor, EE&CS Dept, UC Berkeley, BWRC.

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Presentation on theme: "Improving Wireless Access Technologies Adam Wolisz Professor of EE&CS, Technische Universität Berlin, TKN Adjunct Professor, EE&CS Dept, UC Berkeley, BWRC."— Presentation transcript:

1 Improving Wireless Access Technologies Adam Wolisz Professor of EE&CS, Technische Universität Berlin, TKN Adjunct Professor, EE&CS Dept, UC Berkeley, BWRC Sept.26,2007

2 TKN Telecommunication Networks Group 2 Overview Short introduction of the TU Berlin research environment Towards Dynamic OFDM (OFDMA) Dynamic spectrum usage with Cognitive Radios

3 TKN Telecommunication Networks Group 3 The contribution of my collaborators/students notably TUBerlin: Dr. Gross, Mathias Bohge, Oscar Punal, Daniel Willkomm, Murad Abusbeih UCBerkeley: Prof. Brodersen, Dr. Cabric, Mubaraq Mishra ST Microelectronics: Wendong HU, Dr. George Vlantis Is gratefully acknowledged. Acknowledgements:

4 TKN Telecommunication Networks Group 4 Main-Campus Berlin University of Technology

5 TKN Telecommunication Networks Group 5 Established in April 2001 as a pilot fusion of EE and CS. 43 associate or full professors (German C4 or C3) + a numerous Professors in Residence 3 curricula (Number of beginners/Year, trend) EE (260+), CE (150+), CS (300+-) Communication Technology is one of the major focus areas – for the whole TUB. TU Berlin: The Faculty of EE&CS

6 TKN Telecommunication Networks Group 6 Communications Research relevant environment COOPERATION: 21 joint faculty appointments; incl.17 fully funded by partners The Institutes in 2004 total budget of 165m, (110m in external grants) 1800 employees, incl. 380 employees with a PhD TU Berlin

7 TKN Telecommunication Networks Group 7 Telecommunication Networks Group (TKN) People: Full professor (Chair) A. Wolisz + (2-4) Post-Docs / Assistant Professors + (20-25) Research Assistants/PhD students + 3 Technical Staff + 2 administrative assistants Supported by grants from: EU, BMBF, DFG, DAAD, Siemens AG, Ericsson, DoCoMo,.... External Funding – approx. 1.5 Mio /Year Som-2006e numbers for the time post-doc research associates/Phd Graduates/adjunct lectureres have been appointed professors. Degrees granted: Doctorate: 20, Diplomae: Over 90 Papers: ca. 50 in journals, magazines or book chapters ca. 150 in refereed conferences/workshops see

8 TKN Telecommunication Networks Group 8 Research Topics (selection) General Direction: Architectures and Protocols for Networks Optical backbone/ Optical Metro Networks... Wireless access...(QoS + Capacity). Mobility incl. Group mobility and High speed mobility Sensor networks Cognitive Radio

9 TKN Telecommunication Networks Group 9 The Fundamental Problem Wireless dominates last hop (s?) Because cable is always a constraint... Two fundamental features of wireless communication: Interference; i.e. influence of any transmission on other ones calling for proper Separation (space, Frequency, time, code) Dynamic change of the received signal strength in spite of constant transmission power even without interference. These features result in challenges: Limited Capacity, given frequency spectrum and space. Difficulties in proper QoS support even on a single link

10 TKN Telecommunication Networks Group 10 Introduction: OFDM Orthogonal Frequency Division Multiplexing (OFDM) splits the bandwidth into narrowband sub-carriers Parallel symbol transmission (reduces intersymbol interference) Orthogonal sub-carriers (no intercarrier interference) Fading produces a strong variation in the sub-carrier gains: always some sub- carriers in a bad state Orthogonal Frequency Division Multiplexing This changes in the time domain as well…

11 TKN Telecommunication Networks Group 11 Basic Scenario Terminals Access Point Backbone Downlink Data Queues at Access Point OFDM as transmission scheme!

12 TKN Telecommunication Networks Group 12 Link Adaptation IEEE a/g : Time division Multiple Access and adaptive modulation/coding - the same over all sub-carriers. Average channel gain adaptation: the few sub-carriers with the lowest gain dominate the BER & PER [Awoniyi06] Same Modulation and Coding Scheme on all Sub- carriers, which are assigned to one station

13 TKN Telecommunication Networks Group 13 Adaptive Modulation Scheme For each individual sub-carrier the selected modulation schema assures the highest bit-rate for upper-bounded BER.

14 TKN Telecommunication Networks Group 14 Dynamic OFDM: Adaptive Modulation Adapts the modulation type to the current gain of each sub-carrier subject to a bit error probability target, performing adaptation on a per- packet base. Theoretically this procedure has been shown to outperform Link Adaptation [Czylwik98] Assignments: NO MOD. SNR x X i BPSK X i < SNR x < X j QPSK X j SNR x < X k … BPSK Sub-Carrier 1 16-QAM Sub-Carrier 2 BPSK Sub-Carrier 3 BPSK Sub-Carrier 4 64-QAM Sub-Carrier 43 QPSK Sub-Carrier QAM Sub-Carrier 45 BPSK Sub-Carrier QAM Sub-Carrier 47 QPSK Sub-Carrier 48 STA 1

15 TKN Telecommunication Networks Group 15 Dynamic OFDMA Performance Improvement [Wong99] The whole set of sub-carriers is split into sub-groups, which are then assigned to different stations in a FDM fashion. STA 2 STA 1 STA 3 MULTI-USER DIVERSITY BPSK Sub-Carrier 1 16-QAM Sub-Carrier 2 BPSK Sub-Carrier 3 BPSK Sub-Carrier 4 64-QAM Sub-Carrier 43 QPSK Sub-Carrier QAM Sub-Carrier 45 BPSK Sub-Carrier QAM Sub-Carrier 47 QPSK Sub-Carrier 48

16 TKN Telecommunication Networks Group 16 Now: Optimization approach Open question: How to generate the subsets of sub-carriers serving individual flows? Two-step approach (Yin et al. 2000) per time SLOT. First: sub-carrier allocation - Determine the number of sub-carriers for each subset (meeting the demand) - IDEA: Utilize packet queue… Second: sub-carrier assignment - Choose sub-carriers according to the allocated number for each subset - IDEA: Utilize channel-related information for the assignment (goal: increase the capacity, assure fairness)

17 TKN Telecommunication Networks Group 17 Allocation: satisfying the demand... Let us assume ONE Queue per terminal (i.e. flow) The queue will built up if bad transmission... Packets should be dropped if waiting excessively. Observation: Not all frames in an MPEG video are equally important (I, B, P frames) Drop packets based on importance: I frames on deadline, P frames 25% earlier, B frames 50% earlier. Allocate the bit-rates (simplified: NUMBER of sub-carriers) based on weighted lengths of queues The size of important packets is given a larger weight... Queue length is the sum of these weights

18 TKN Telecommunication Networks Group 18 Assignment: Integer Programming Formulation More flexibility by formulating assignment as Integer Program {1,...,J} : Set of wireless terminals, {1,...,S} : Set of sub-carriers g j,s : CNR of terminal j on sub-carrier s p s : power assignment for sub-carrier s F(): Mapping of subcarrier SNR to applied modulation type c j,s :(= 0,1): Assignment of sub-carrier s to terminal j z j : Subcarrier allocation for terminal j Basic formulation: Maximize capacity Each subcarrier only assigned once Number of sub-carriers per terminal fixed out of Allocation!

19 TKN Telecommunication Networks Group 19 Optimal solution (constant power assignments) Assignment problem maps to a graph theoretical problem Maximum weight bipartite matching problem An optimal algorithm for this problem exists – the Hungarian algorithm with complexity of O(S 3 ) Measured run times of the algorithm: ~2ms are too long compared to the Allocation/Assignment TIME SLOTS < coherence time of wireless channels in our test data Good heuristics exist for this problem, solving the problem with average parameter setting within 500µs Performance loss: 10%

20 TKN Telecommunication Networks Group 20 Assigned Sub-Carrier Set of User 3 Assigned Sub-Carrier Set of User 2 Assigned Sub-Carrier Set of User 1 A heurisitc for the Assignment G B B G G G G G G G G B B G G G G G G G G G G G G G B G B B B G G B G B Sub-Carriers User 1 User 2 User 3 Single Sub-Carrier States towards: Sub- Carrier Weight: Dynamic FDM assignment of sets of sub-carriers to individual terminals

21 TKN Telecommunication Networks Group 21 Numerous results addressing: Joint optimization of power (loading) and assignment. Including the overhead for signaling (how should the receiver now which sub-carriers have been assigned?) Allocation including priorities See our URL… =================================================================== But: does this require completely new systems? We suggest the Adaptive Modulation in the up-link and Dynamic OFDMA in the down-link… A proposal for including this to IEEE … as backward compatible solution. - Technical Report: TKN Submissions to IEEE , VHT SG

22 TKN Telecommunication Networks Group 22 Required changes to use Dynamic OFDM In order to choose an (optimal) modulation/coding per sub- carrier, we need to - - estimate the channel gain per sub-carrier for each transmission Obligatory RTS/CTS - inform the receiver about modulation/coding used per sub-carrier extended header in data packets - adjust the NAV settings after the transmission For the multi-user case (parallel transmission of packets), we have to add Multiple CTS per RTS and ACKS per data packet - multtiple CTS per RTS, and ACKS per data packet; signal the assignment of sub-carrier sets, Higher Performance but also higher processing power.

23 TKN Telecommunication Networks Group 23 Schemata of the changes Single user Multi-user

24 TKN Telecommunication Networks Group 24 Some performance data (simulation!) Simulation Settings: - Saturation mode, - 4 and 8 stations, - downlink traffic Scenarios: – Multi-user mode – Single-user mode with Round Robin Scheduling – a/g with RTS/CTS and Round Robin Scheduling Metrics: – Goodput, PER and PHY Efficiency

25 TKN Telecommunication Networks Group 25 This approach has potential… Goodput - 8 STAs - Large Packets (1564 Byte) a/g with RTS/CTS Goodput - 4 STAs - Large Packets (1564 Byte) a/g with RTS/CTS

26 TKN Telecommunication Networks Group 26 Ongoing work … We have proposed recently a proposal for the interface definition and API between MAC-PHY 3FPP LTE simulator (Ericsson) – used for sensitivity of the system efficiency on the Control Channel error due to interference from neighbor cells… The real overhead, inaccuracies of sub-channel estimations, etc. require experimental investigation. Several PHY developments (FPGA based) in preparation. Execution of the algorithms on FPGA platforms is to be considered.

27 TKN Telecommunication Networks Group 27 The underlying philosophy: Using the best opportunity. Why not use the same philosophy for grabbing more spectrum… This is Cognitive Radio based spectrum utilization…

28 TKN Telecommunication Networks Group 28 Nutrition facts: The bad news... The amount of non-licensed spectrum (in interesting frequencies) is limited GHz Allocation in the 3GHz-6GHz

29 TKN Telecommunication Networks Group 29 Nutrition facts: The good news... Licensed users do not REALLY use ALL THEIR spectrum ALL the time in EACH place the license holds The estimates are like 80%? 90%? 99%? unused... A snapshot from Berkeley: real measurements…

30 TKN Telecommunication Networks Group 30 Re-Usage of sectrum: What are the options? The licensed users – called PRIMARY USERS – could be screened, and Be permitted to sell or lease their licenses…. Loose licenses (for some area) if spectrum not used properly Have licenses limited to some times of the day recycled assignments could be opened as ISM bands The primary user (or a broker on their behalf) could be obliged to announce unused frequencies (again in space and time domain); usage as ISM bands is possible. A new category of users - SECONDARY USERS - possess the ability to assess autonomously the temporarily unused spectrum and grab it for specific usage without primary users being aware of this kidnapping

31 TKN Telecommunication Networks Group 31 A Primary User X … legally owns some frequency band … can tolerate a maximal interference time t x each time he resumes channel usage A secondary user system has t x time units to detect a primary user and clear the corresponding Sub-Channels (time domain!) t x is dependent on the primary user system and may vary from system to system … is not (cognitive) secondary radio aware (i.e. does not provide specially signaling of activity – especially no preparation to re-gain his frequency band) NOTE: IF primary user would use a carrier sensing protocol, neglecting the (unknown!) secondary user MUST be assured (operation below the carrier sense sensitivity!)

32 TKN Telecommunication Networks Group 32 Cognitive radio for Secondary Users

33 TKN Telecommunication Networks Group 33 Sensing while sending??? (a system view) A fundamental challenge … three possible answers Interrupted sending… (see e.g. IEEE basic…) Quick sensing needed. Bad for QoS of the secondary's Part of the primary band not re-used (see Corvus) Coordination of frequency usage required Band only partially available for sensing…

34 TKN Telecommunication Networks Group 34 Interleaved frequency usage (see CORVUS) Active primary user Hz Sub-channel Primary user frequency band (F-Band) Secondary user link (SUL) Bandwidth B [Hz] SUs compose SU-Links out of free sub-channels Sub-channels of active Primary users (Pus) can't be used by SUs PU F-Band covers multiple sub-channels New sub-channels should be acquired Divided into N sub-channels of bandwidth b=B/N [Hz] (Re-)appearance of PU All affected Sub-Channels have to be cleared

35 TKN Telecommunication Networks Group 35 This resembles OFDMA… sure: Efficiency: Potential for usage of the spatial diversity (anyway in downlink) Available actual bandwidth of each sub-carrier (depending on the channel conditions) fully utilized. Constant Sub-carrier Spacing Robust to channel positioning (offset) and bandwidth changes Interesting options for usage for the secondary devices in the Interleaved transmission modus.

36 TKN Telecommunication Networks Group 36 Sensing while sending??? (a system view) cont. A fundamental challenge … three possible answers Interrupted sending… (see e.g. IEEE basic…) Quick sensing needed. Bad for QoS of the secondaries Part of the primary band not re-used (see Corvus) Coordination of fre-quency usage required. Band only partially available for sensing… Regular hopping (see e.g. IEEE DFH mode) Changing frequency bands regular event (collision in 802.3) Relaxed sensing in free(?) band – assuming coordination

37 TKN Telecommunication Networks Group 37 IEEE (DFH) Worldwide first draft of a Cognitive Radio standard Provide wireless broadband Internet access using TV-bands Ensure non interference with incumbents (grace period 2s) through spectrum sensing - Philosophy following WiMax (IEEE ) Basic Mode: Assure respecting the grace period by sensing during TRANSMISSION INTERRUPTION DFH: Perform data transmission and sensing in parallel Transmit data on channel X and perform sensing on channel Y After 2 seconds channel Y is used for data transmission and the next channel is sensed. Might be X again...2 channels/cell Thus an cell hops through a set of working channels

38 TKN Telecommunication Networks Group 38 DFHC (DFH Communities) – some ideas Each community has a community leader Leader is selected through leader election Leader calculates a hopping pattern for each member of the community (i.e. cell) Leader is responsible for accepting / rejecting new members Neighborhood discovery One-hop broadcast of used frequencies and current interference situation by all cells Used to create and maintain communities It is possible to support N Cells with (N+1) frequencies…

39 TKN Telecommunication Networks Group 39 DFHC hopping pattern W. Hu, D. Willkomm, L. Chu, M. Abusubaih, J. Gross, G. Vlantis, M. Gerla, and A. Wolisz, "Dynamic Frequency Hopping Communities for Efficient IEEE Operation", IEEE Communications Magazine, Special Issue: "Cognitive Radios for Dynamic Spectrum Access", May 2007 W. Hu, D. Willkomm, L. Chu, M. Abusubaih, J. Gross, G. Vlantis, M. Gerla, and A. Wolisz Cells need to shift their operation periods by one quiet time Quiet time is the minimum time needed to sense a channel WRAN2WRAN1 WRAN3

40 TKN Telecommunication Networks Group 40 Protocol sketch for schedule maintenance Hopping patterns can change Due to incumbent appearance on a used channel Due to a member leaving / joining a community Community leader needs to calculate new hopping pattern and distribute it in the community Consistency issue: How to assure that all members receive the new hopping information If not all members switch to the new hopping pattern simultaneously there might be collisions between the old and the new hopping pattern Solution: Hopping pattern lifetime and sequential switching

41 TKN Telecommunication Networks Group 41 Hopping pattern lifetime Hopping patterns have a specific lifetime After expiration of the lifetime the hopping pattern cannot be used anymore The leader thus has to periodically renew the hopping pattern Upon renewal the pattern can be changed, new members can be added, etc. This ensures consistency: even if some members do not receive the new pattern, they cannot use the old one anymore But what if a hopping pattern needs to be changed in the middle of a lifetime (i.e. due to appearance of an incumbent)? Solution: Sequential Switching

42 TKN Telecommunication Networks Group 42 Sequential Switching The leader sequentially switches all members one by one to the new hopping pattern New hopping pattern is collision free with the pattern of all members not switched yet Implicit acknowledgement: sensing on the newly assigned channel (implicit confirmation by acting) Even if somebody fails to follow - all members already switched can use the new pattern without any collisions

43 TKN Telecommunication Networks Group 43 In any case: Detection of (possibly) Weak Signals Cognitive radio observes (senses) primary system signals Those might be strongly attenuated While the transmission of the CR(Tx) towards Rx is not… Primary User Cognitive Radio users must guarantee non-interference requirement distance Distance and channel not known Tx Rx CR(Tx) CR(RX) Decoding SNR Sensing SNR

44 TKN Telecommunication Networks Group 44 Solution: Network Spectrum Sensing [BWRC,Cabric] Prob. of false alarm Prob. of detection 1 radio 5 radios If spacing >> λ/2 a few cooperative radios give big improvements

45 TKN Telecommunication Networks Group 45 We need a signaling channel: Cooperation is needed… at least in proximity For assuring that no other secondary is transmitting during the sensing For assuring network spectrum sensing. Who should be subject to coordination? How to organize the exchange of information for coordination??

46 TKN Telecommunication Networks Group 46 But: Control Channel needed (logically) for: Universal Control Channel (UCC) Globally unique Used to get necessary information for creation of new groups and to announce them Used by new users to choose and join a specific Group Group Control Channel (GCC) Each SUG has own control channel Used for exchange of sensing information – recognition of primary users. Used for data channel establishment (out of temporarily available resources) and its maintenance in spite of re- appearing primary users. Numerous claims in favor of a specific Control Channel are recently being made…

47 TKN Telecommunication Networks Group 47 Options for the control channel implementation An Universally/regionally pre-assigned frequency…. Globally unique Will be difficult…. ISM band… Globally available… What about possible (strong??) interference? UWB Not interfering Tradeoff: distance vs. bit-rate might be very useful Cost? Deployment? One of the available channels Convention for selection needed IEEE considers this variant….

48 TKN Telecommunication Networks Group 48 BWRC Platform [ BWRC: Cabric, Tkaczenko] Sensing PHY real-time processor : 4 FPGAs ~ 10M gates ASIC at 250 MHz On-chip memory: Soft+Hard > 10 Mbits Dynamic Partial Reconfiguration Dedicated DSP blocks: 18b mult + MAC Architecture optimization for ASIC - Parallelism/Pipelining/Interleaving - Bitwidth optimization - Area estimate: 10,000 slice = 1mm 2 Sensing MAC embedded processor: Central FPGA: Linux + Full IP Embedded processors: PPC+ARM On-chip Ethernet MAC Bus connection to 4 other FPGAs Radio interfaces: 16 high speed radio links (10 Gbps) 4 interfaces per FPGA Fiber optic cable compatible

49 TKN Telecommunication Networks Group 49 Reconfigurable Wireless Radio Modem [BWRC] 10 Gbps infiniband connection supports fiber optic cables Sensing radio processor Antenna A/D 12b/64MHz D/A 14b/128MHz 2.4 GHz radio (85 MHz) ISM band Suitable for sensing and transmission in TDD mode

50 TKN Telecommunication Networks Group 50 Why 2.4GHz? Very crowded spectrum with unlicensed devices. IEEE b/g cards within laptops, are quite programmable and allow users to control their transmission parameters. Easy to implement protocols on these cards Hardware and software support for the 2.4GHz bands is already developed within BWRC BWRC cards can be programmed to the complete 80MHz band

51 TKN Telecommunication Networks Group 51 First Experimental setup at 2.4GHz [BWRC/TKN] BEE2 radios perform sensing Laptops perform transmission Laptops connect to the BEE2 via standard TCP/IP

52 TKN Telecommunication Networks Group 52 What about control channel ?? In first set-up: Internal communication between sensing boards Ethernet communication between the laptops… One selected wireless channel, ot IEEE a could also have been used. This might be enough for SOME of the experiments… ================================== Let us also keep in mind some (available) other option, the IEEE a CHIRP solution (Nanotron, Berlin) Promises: A pretty robust transmission with range sufficient for WLAN type deployment…. + inherent (precise) LOCATION Samples being in possession of TUBerlin/TKN for evaluation….

53 TKN Telecommunication Networks Group 53 Functions of Classical Wireless Systems We will consider here: Cellular, infrastructure WLANs, ad-hoc networks… What has been typical for all of them? (history) Usage of specific frequency bands optimized transmission Exclusive usage of frequency bands by regulation Channel structure within this bands - using of a selected /negotiated channels for the whole transmission assigned at the beginning of the data exchange Basic steps Finding the partner mostly by beaconing (notably base stations!) Exchanging set-up data separate signaling channel or in- band Selecting a channel to work on ditto

54 TKN Telecommunication Networks Group 54 What does change in CR ? (potentially) Usage of arbitrary frequency ranges (for much better spectrum utilization) Approach: Consulting data basis + distributed sensing No fixed channel structure (for flexible adaptation to the traffic needs…) Approach: Negotiating variable channel structure Imposed high dynamics in changing the used frequencies (if the primary user pops-up!) Approach: sense while communicating (how?) Usage of arbitrary transmission schemata Approach: Use OFDMA (see )

55 TKN Telecommunication Networks Group 55 Looks like a revolution? Not entirely… Usage of specific frequency bands ? GSM: 900/1800/1900 …. IEEE : 2.4GHz, 5.4 GHz, 5.9 GHZ (DSRC)… WiMAX… Specialized transmission schemata? General trend towards OFDM /OFDMA!!! Exclusive usage of frequency bands? Take …. Coexistence with radars (in 5 GHZ/Europe), microwave ovens, Bluetooth, , etc Persistent usage of assigned channel? Some frequency hopping during the communication ( FH, Bluetooth, Some GSM) Channel change in (e.g. Mobility…)

56 TKN Telecommunication Networks Group 56 Lessons learned: There is a trend to increase the dynamics of adaptive resource usage on each time scale and granularity. OFDMA seems to be especially attractive transmission schema Available systems/set-ups allow for investigation of multiple features for the future Cognitive Radio

57 TKN Telecommunication Networks Group 57 Thank You !

58 TKN Telecommunication Networks Group 58 IEEE a Chirp … some facts [ ] Chirp technology operating in 2.4 GHz ISM band, 20 MHz, 7 channels (3 non overlapping) Data rates 2, 1 Mbps; 500, 250, 125, 62.5, kbps Power/sensitivity Tx: up o dBm (plus ext. amplifier for long range) Rx: 250kbps; BER= (with FEC) Symmetrical double-sided two way ranging 128 bit hardware encryption. RS 232 interface (USB expected…)

59 TKN Telecommunication Networks Group 59 IEEE a Chirp … early data [IEEE Documents] Indoor (European office building) measurement results for a data rate of 1 Mbps (over air interface) and BER = are as follows: Output power (EIRP) = -30 dBm (1 µW), distance = 5 m, 1 wall Output power (EIRP) = -15 dBm (32 µW), distance = 23 m, 4 walls Outdoor measurement results for a data rate of 1 Mbps (over air interface) and BER = are as follows: Output power (EIRP) = +7 dBm (5 mW), distance = 739 m (+/-10 m) Both transceivers use equivalent isotropic antenna (gain = 0 dBi) For long ranges the transmit power may be allowed to rise to each country s regulatory limit For example the US would allow 30 dBm of output power with up to a 6 dB gain antenna The European ETS limits would specify 20 dBm of output power with a 0 dB gain antenna Ranging: 2m indoors – 1 m outdoors…. (data sheet)

60 TKN Telecommunication Networks Group 60 Communication Stack Secondary users create communicating GROUPS (SUGs) A universal control channel as well as group control channels are assumed... PHY Layer Link Layer UCCGCC Data Transfer Channel Spectrum Sensing Channel Estimation Data Transmission MAC Group Management Link Management Data Transmission Data Transmission MAC Control channels....

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