Outline. Wireless Channel Characteristics. Multi-path Fading. Opportunistic Communication - with a focus on WLAN environments -
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1 Outline Opportunistic Communication - with a focus on WLAN environments - Jong-won Lee Background? Wireless Channels? Opportunistic communication? Examples? Basics of WLAN Previous Works? MAD (MobiCom 04)? OSMA (WCNC 04) Wireless Distributed Opportunistic Scheduling (WDOS) Concluding Remarks References 2 Wireless Channel Characteristics Multi-path Fading Path loss d0 Pr = Pt K,2 γ 8 d Shadowing Pr ( db) = ψ P t Multi-path fading γ db, ψ db ~ N (0, Received Signal Power (db) 2 σ ψ ) path loss shadowing Rayleigh fading log (distance) Fast time-scale fading due to constructive and destructive interference between multipaths? Non line-of -sight: Rayleighfading? Line-of -sight: Rician fading Classical viewpoint? main cause of performance degradation? Make a channel as close to AWGN channels as possible By time interleaving and/or antenna diversity? Classical diversity techniques Improve the reliability of communication 3 4
2 Rayleigh vs. AWGN Opportunistic Communication The term of Opportunistic communication? Transmit when and where the channel is good? Denotes the ability to exploit channel fluctuation Opportunistic scheduling? Multi-user diversity? Takes the channel quality into consideration when choosing which user to schedule.? The scheduler usually select the best user according to how the algorithm solves the trade-off between rateoptimality and QoS/fairness. Opportunistic beamforming 5 6 Multi-user Diversity Multi-user Diversity Gain Improves system performance by exploiting channel fluctuation due to fading.? while the classical diversity techniques pertain to a point-topoint link Based on the high probability that there is a user with a channel strength much larger than the mean level. SNR=0dB κ=5 7 8
3 Application to CDMA/HDR (1xEVDO) Downlink Channel Measurement and Feedback in HDR Multiple access is TDMA via scheduling. Each user is rate-controlled rather than power-controlled. Access terminals can accurately and rapidly estimate its own channel condition via the pilot bursts transmitted by BS. The channel state information (CSI), in the form of data rate request, is fed back to the AP via the reverse link data request channel (DRC) and updated every 1.67ms SINR and Rate in HDR Symmetric and Asymmetric Channels In case of symmetric channels, the simple strategy of communicating with the user having the best channel maximizes the total throughput and guarantees the fairness in throughput. In case of asymmetric channels, a user having relatively poor channel conditions can get little chance to be served
4 Proportional Fair Scheduler Provides good trade-off between the throughput and fairness Algorithm? Select a user i with the highest ratio of DRCi ( t) argmax i R ( t) DRC ( t) : 1 1 Ri ( t) = 1 Ri( t) + DRC i( t) φ i tc tc 1, φi = 0, i i maximum possible transmissi on selected by a scheduler otherwise rate at time t. Opportunistic Beamforming In environments where the channel fluctuations are small,? Why not purposely induce faster and larger fluctuations? Method: The information bearing signal at each of the transmit antenna is multiplied by a random complex gains Opportunistic Beamforming Opportunistic Beamforming Before After If the information bearing signal at each of the transmit antenna is multiplied by a random complex gains, dumb antennas create a beam in random time-varying direction
5 Opportunistic Beamforming IEEE Network Types BSS (Basic Service Set)? A group of stations that communicate with each other Infrastructure BSS Independent BSS? communication via access point (AP)? Direct communication (no AP) Not allowed to communicate directly? No relay? AP provides? Ad hoc BSS or ad hoc network Connection to wired network Relay function SNR=0dB κ= IEEE MAC Protocol DCF CSMA/CA (mandatory)? collision avoidance via randomized back-off mechanism? ACK packet for acknowledgements (not for broadcasts) DCF w/ RTS/CTS (optional)? avoids hidden/exposed terminal problem, provides reliability PCF (optional)? access point polls terminals according to a list DCF: Distributed Coordination Function PCF: Point Coordination Function IEEE DCF CSMA/CA Stations ready to send starts sensing the medium If the medium is busy, wait. When the medium is free, the station has to wait for a free additionally, then the station must additionally wait a random back-off time (collision avoidance) If another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) When a back-off time elapses, it transmits a packet
6 IEEE DCF CSMA/CA IEEE DCF CSMA/CA station 1 station 2 busy bo e bo e bo r busy bo e bo r bo e busy Sending unicast packets? station has to wait for before sending data? receivers acknowledge at once (after waiting for SIFS) if th e packet was received correctly (CRC)? automatic retransmission of data packets in case of trans mission errors station 3 station 4 bo e bo r bo e busy bo e busy bo e bo r bo e bo r sender receiver data SIFS ACK station 5 t busy medium not idle (frame, ack etc.) bo e elapsed backoff time 21 other stations waiting time contention data 22 t IEEE with RTS/CTS Virtual Carrier Sensing? Sender sends RTS with NAV (Network allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium) after waiting for? Receiver acknowledges via CTS after SIFS (if ready to receive)? Sender can now send data at once, acknowledgement via ACK? Other stations store NAV distributed via RTS and CTS By using NAV, other stations can not interrupt the current transmission. => virtual carrier sensing STATION A STATION B IEEE with RTS/CTS RTS CTS DATA ACK... ACK NAV ( RTS) RTS... OTHER STATIONS BACKOFF SIFS SIFS SIFS NAV ( CTS) BACKOFF 23 24
7 IEEE PHY IEEE : Multi-rate Capabilities IEEE a/b/g data rates g a b Standard approved June 2003 July 1999 July Frequency band 2.4 GHz 5 GHz 2.4 GHz 2.4 GHz Data rate Upto 54 Mbps Upto 54 Mbps 5.5 Mbps, 11 Mbps 1 Mbps, 2 Mbps Modulation OFDM, DSSS/CCK OFDM DSSS/CCK DSSS, FHSS Multi-user Diversity in WLANs Previous Works MAD How to exploit multi-user diversity in WLAN environments?? Problem: There is no feedback channel to report CSI in WLANs. Procedure? Channel probing AP sends query to randomly selected 3 receivers for channel condition probing All inquired receivers reply on their own slot? Scheduling AP does scheduling based on 3 receivers channel qualities MAD with OAR Exploiting Medium Access Diversity in Rate Adaptive Wireless LAN s (Mobicom 04) 27 28
8 Previous Works - OSMA OSMA Procedure? A channel probing message which includes a list of 4 candidate receivers? Prioritized CTS Candidate receivers with channel quality better than a certain threshold are allowed to access media by replying a CTS To avoid collisions, receivers are prioritized. The nth receiver can send a CTS at SIFS+(n-1)slot. Limitation? OSMA limits the number of candidate receivers to 4. Channel probing procedure MRTS SIFS CTS 1 SIFS CTS 2 SIFS CTS 3 SIFS CTS M Sender 1 st receiver 2 nd receiver 3 rd receiver M th receiver Opportunistic Packet Scheduling and Media Access Control for Wireless LANs and Multi-hop Ad Hoc Networks, WCNC Wireless Distributed Opportunistic Scheduling Considerations? How to increase the number of candidate receivers without too much overhead? Contention-based CTS Use the back-off time based on the relative channel quality To reduce collision probability. To reduce channel probing time Basic idea? When a receiver is in a relatively better channel condition, the backoff delay for a CTS will be smaller.? Only the receiver with the relatively best channel condition will response. The others should give up sending their CTS. WDOS Basic channel proving procedure 31 32
9 WDOS: Channel-aware Backoff WDOS: Channel-aware Backoff Assume that all the channels experience Rayleigh fading. Divide the p.d.f into L*N intervals equally.? The number of intervals is proportional to the number of active receivers. 5,QWHUYDO Backoff = random(0,m-1) If current channel quality is within i-th interval, backoff = (i-1)*m + random(0,m-1) [slot time] where M is an adjustable parameter &KDQQHOFRQGLWLRQ 5,QWHUYDO Backoff = 2M+random(0,M-1) &KDQQHO FRQGLWLRQ Optimal Value of L Optimal Value of L and M Optimal values of L and M at a long coherence time Insensitive to the number of users Insensitive to the average SNR L=4 (near optimal value) 35 36
10 Additional Time for Channel Proving Throughput Throughput Comparison Temporal Fairness and Throughput Rayleigh fading, Doppler shift = 1Hz All receivers have the same channel conditions, 10^ -7.2mW 39 40
11 Effect of Channel Model Approximation Concluding Remarks WDOS fully exploit multi -user diversity without the excess overhead for channel probing in WLAN environment.? Proposed channel-aware backoff algorithm provides effective channel probing scheme even though the number of active users increase The huge surge of research activities in physical layer wireless communication theory? Opportunistic communication techniques? MIMO? Cooperative communication References [1] Pramod Viswanath, David N. C. Tse, Rajiv Laroia, Opportunistic Beamforming Using Dumb Antennas, IEEE TRANSACTIONS ON INFORMATION THEORY, VOL. 48, NO. 6, JUNE 2002 [2] Z. Ji, Y. Yang, J. Zhou, M. Takai, R. Bagorodia, Exploiting Medium Access Diversity in Rate Adaptive Wireless LANs, MobiCom 2004 [3] J. Wang, H. Zhai, Y. Fang, Opportunistic Packet Scheduling and Media Access Control for Wireless LANs and Multi-hop Ad Hoc Networks, WCNC [4] David Tse and Pramod Viswanath, Fundamentals of Wireless Cmmunications, Cambridge Univ. Press, Thank You! 43 44
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