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Ofdma 1

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This document proposes an iterative subcarrier allocation algorithm for OFDMA systems that aims to improve system performance and provide quality of service (QoS). It discusses: 1) The motivation for adaptive subcarrier allocation in OFDMA to improve transmission rates by assigning more bits to subcarriers with higher gains. 2) An iterative algorithm that assigns bits based on modulation levels and iteratively redistributes bits and exchanges subcarriers to reduce transmit power while meeting QoS requirements. 3) Simulation results showing the iterative algorithm achieves higher average bit SNR over time compared to existing algorithms and improves performance as RMS delay spread and number of users increases.

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QoS Aware AdaptiveQoS Aware Adaptive Subcarrier AllocationSubcarrier Allocation in OFDMA Systemsin OFDMA Systems Mustafa Ergen & Sinem Coleri {ergen,csinem}@eecs.berkeley.edu University of California Berkeley
Introduction ? Motivation ? Orthogonal Frequency Division Multiple Access(OFDMA) ? OFDMA System ? Resource Allocation Problem ? Algorithms ? Optimal ? Suboptimal ? Simulation ? Conclusion
Motivation ? Broadband Wireless Access ?Ex: IEEE 802.16, Wireless MAN ? OFDM ?Eliminates InterSymbol Interference ? OFDMA
OFDM Diagram
Multiuser OFDM ? OFDM-TDMA ? OFDM-FDMA ? OFDMA User 1 User 2 User 3 Subcarrier Time OFDM-TDMA Subcarrier Time OFDM-FDMA Subcarrier Time OFDMA ¡­ ¡­
Resource Allocation Goals: ? Dynamic subcarrier selection ? Improve system performance with adaptive modulation ? More bits transmitted in large channel gain carriers ? Provide QoS ? Rate and BER
Resource Allocation Assumptions: ? Base station knows the channel ? Base station informs the mobiles for allocation Base Station subcarrier user
System Application Network Resource Allocation Physical Layer rQoS=[rR,rBER] oQoS=[oR,oBER,oCoS] oCoS=Ptotal for downlink oCoS=Pu for uplink [User x Subcarrier]
OFDMA
Resource Allocation 64-QAM 16-QAM 4-QAM Channel Subcarrier User RATE: [12 6 6 8 ] BER: [1e-2 1e-2 1e-4 1e-4] QoS Resource Allocation
Notation )12() 4 ( 3 )(: 2 , : },...,1,0{ , : },...,1{: },...,1{: )( : 2 1 2 , , , ?? ? ? ? ? ? =? ¡Ê ¡Ê ¡Ê = ? c nk nkkc nk BER Q No cfQAMM nk gainchannel M nk cbitassigned Nnsubcarrier Kkuser cf PPowerTransmit ¦Á ¦Á
Optimal ? Integer Programming ¡Æ¡Æ ¡Æ¡Æ ¡Æ¡Æ¡Æ = = = = = = = ¡Ü¡Ü = ¡Ê K k M c cnk cnk N n M c nkk cnkcnk K k N n M c nk nk nallforand kallforcRtosubject for cf cnk 1 1 ,, ,, 1 1 , ,,,, 1 1 1 2 , , .,10 ,. }1,0{ )( min ,, ¦Ã ¦Ã ¦Ã¦Ã ¦Á¦Ã Subcarrier User Subcarrier User Subcarrier User Subcarrier Use Pc2 Pc3 Pc1
Motivation for Sub-optimal Algorithms ? IP is complex ? Allocation should be done within the coherence time ? Time increases exponentially with the number of constraints
Current Suboptimal Algorithms 2-step: ? Subcarrier Allocation ?Assume the data rate for all subcarriers ?Assume modulation rate is fixed ?Assign the subcarriers ? Bit Loading ?Greedy approach to assign the bits of user
Current Suboptimal Algorithms ? Subcarrier Allocation ? Hungarian algorithm ? Optimal, very complex ? LP approximation to IP problem ? Close to optimal ? Bit Loading Subcarrier User Subcarrier User Subcarrier User).( 1 )(minarg :, ,, ,, ,, nknk nknk nknk Sn k cPevaluate cc cPn timesRfollowingtherepeatkeachFor k ? += ?= ¡Ê
Problems in Current Suboptimal Algorithms ? Subcarrier assignment and bit loading are separated ?Users with bad channels may need higher number of subcarriers ? Not iterative subcarrier assignment
Iterative Algorithm Iterative algorithm based on ? Assignment of bits according to highest modulation ? Finding the best places ? Distributing the assigned bits to other subcarriers or to non-assigned subcarriers ? Exchanging the subcarriers among user pairs for power reduction.
Iterative Algorithm ? Fair Selection(FS) ? Greedy Release(GR) ? Horizontal Swaping(HS) ? Vertical Swaping(VS)
Iterative Algorithm FAIR SELECTIONPtotal<Pmax GREEDY RELEASE Start Modulation-- HORIZONTALSWAP VERTICALSWAP ASSIGNMENT ITERATION
Simulation Environment Build the OFDMA system ? Modulations:4-QAM,16-QAM,64-QAM ? Independent Rayleigh fading channel to each user ? Number of subcarriers =128 ? Nodes are perfectly synchronized
CDF of total transmit power without Pmax constraint
CDF of total transmit power with Pmax constraint
Average bit SNR vs. RMS delay spread As RMS delay spread increases, the fading variation increases hence higher gains are obtained by adaptive allocation
Average bit SNR vs. number of users As the number of users increases, the probability of obtaining good channel at a subcarrier increases
Instantaneous Average bit SNR vs Time Iterative Algorithm improves its Average Bit SNR by the time.
Conclusion ? OFDMA ? Broadband Wireless Access ? Resource Allocation ? Channel Information ? QoS Requirement ? Optimal Algorithms ? complex ? Iterative Algorithms

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Ofdma 1

Editor's Notes

  • #3: The analysis, Research and consultancy (ARC) group forecasts that the fixed wireless deployments in both homes and business will reach 28 million by 2005. 802.16 comiitee broadband wireless access based on OFDMA.
  • #4: BWA is alternative to DSL technologies Physical layer should mitigate non LOS environments in indoor. 802.16 10-66GHz 802.16a 2-11GHz
  • #7: Instead of assigning a fixed frequency or time slot to each user, the performance will increase. The users will not use the subcarriers that are in deep fade. The performance will increase since it is quite unlikely that this subcarriers will be in deep fade for all the other users.