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Insights from CSMA with Multipacket Reception: Achieving >1 Gbps Aggregate Throughput with a Multiuser-based Ph PowerPoint Presentation
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Insights from CSMA with Multipacket Reception: Achieving >1 Gbps Aggregate Throughput with a Multiuser-based Ph

Insights from CSMA with Multipacket Reception: Achieving >1 Gbps Aggregate Throughput with a Multiuser-based Ph

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Insights from CSMA with Multipacket Reception: Achieving >1 Gbps Aggregate Throughput with a Multiuser-based Ph

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  1. Insights from CSMA with Multipacket Reception: Achieving >1 Gbps Aggregate Throughput with a Multiuser-based Physical Layer Date: 2009-07-16 Authors: D. Chan and T. Berger

  2. Latest CSMA-MPR research indicates multiuser PHY is a direct and effective method for reaching >1 Gbps aggregate throughput and satisfy 11ac PAR • In a soon-to-be submitted paper [1], we investigate the effects on CSMA from a multipacket reception physical layer • Insights from the paper indicate strong potentials of applying a MU-PHY layer that is capable of simultaneous transmissions for 802.11ac • In particular, if the multiuser PHY is created via building on resources that previously cannot be utilized, then we can expect to obtain significant throughput gains • 802.11ac’s current resources: raw frequency bandwidth (20, 40, 80 MHz), MIMO-OFDM, multipath, modulation and coding • New resources: spatial diversity multiple access (MU-MIMO) via more antennas, multiuser coding, etc. • Due to SDMA gain proportional to scaling of AP antennas only, MU-MIMO is very promising • Our simulations and analysis indicate exploiting SDMA (MU-MIMO) with more antennas provide should be a feasible way to satisfy beyond 11ac PAR: achieve >1 Gbps aggregate throughput, backwards compatibility, etc. • A three-user capable MU-MIMO with 2 s.s. 64 QAM 5/6 at 80 MHz can achieve this minimum requirement • This submission is the first attempt to provide analysis on this aspect D. Chan and T. Berger

  3. Intro to Multipacket Reception (1): Current CSMA network research and 802.11 channel models assume only one user transmits at a time • Prevalent network research and current WLAN technologies assume a collision channel model • Only one transmission can occur over channel at any instance • Likely reasons: • Analysis more tractable • Reflects PHY layers and technologies available at the time • Worst case scenario D. Chan and T. Berger

  4. Intro to Multipacket Reception (2): An abstraction of the multiuser PHY layer for analyzing network performances Example of a MPR channel • It’s well known that PHY layer technologies today have the capability to separate and decode multiple packets transmitted simultaneously • Eg., by CDMA, spread spectrum modulation, spatial diversity multiaccess, multiuser coding and decoding techniques • This is called multiple packet reception (MPR) channel in the literature • Equivalently, simply another name for a multiuser-based PHY layer • The multiuser-capability of the channel is characterized by Cn, the average no. of correctly decoded packets with n user transmitting • Can be calculated via PHY simulations/measurements D. Chan and T. Berger

  5. Multiuser PHY has been proposed and considered for 802.11 and theoretical CSMA network research • Our paper of 2004 [2] is the first paper on the topic of CSMA with MPR • Results from this paper has been extended to the previously mentioned to-be-submitted paper [1] • Submission 05/0946 at the Sept 2005 WNG session is an early attempt to propose multiuser to 802.11 • Scheme proposes adding a Multiuser Detection (MUD) layer above the MAC layer to perform multiuser detection and coding, as an attempt to maintain backwards compatibility • Suggested possible MPR techniques, like via multiuser-LDPC codes… • Early on in VHT SG, Marc de Courville et al. have suggested multiuser diversity as a new resource for 802.11 to exploit (07/2187) • De Courville at al. later also proposed a multichannel MAC that allows simultaneous transmissions • Ultimately the idea of provisioning multiuser is mentioned in the 802.11ac PAR and 5C (08/807) as a potential candidate for meeting the 1 Gbps and other requirements • As 11ac enters its current stage, it is evident that TG members are quite supportive of the idea of multiuser PHY • Usage models and channel measurement and geared towards multiuser environment • Recent sessions have had many submissions on multiuser PHY concepts D. Chan and T. Berger

  6. Insights for 802.11ac from multiuser-CSMA theory (1): Multiuser PHY implies shorter backoffs (Excerpt from paper [1]) • Main result above is aggregate throughput for large population networks and it can provide general trends and characteristics we can expect for MU-802.11: • Multiuser CSMA throughput eventually approaches multiuser S-ALOHA and approaches “send anytime you want” • Need for carrier sensing or random access diminishes as more simultaneous users are supported by the PHY • MAC efficiency increases as MPR strength increases • Peculiarly, CSMA is 100% efficient in collision channel as well! • As we support more users simultaneously, stations should transmit after waiting smaller backoffs (i.e., CW should decrease as MPR strength increases) D. Chan and T. Berger

  7. Increasing multiuser support ultimately means obtaining and allocating “degrees of freedom” in PHY Example of a MPR channel • Assumed in the MPR model is that each increase in support for multiuser transmissions do not sacrifice each user’s transmission rate • That is assumed to remain unchanged to have any effective performance gains • In general, to improve MPR strength or multiuser capability: • Increase and allocate some of the current PHY resources for multiuser • Find and use new PHY resources for multiuser • Examples: • FDMA – Obtain more channels and allocate them to different users • OFDMA – Obtain more channels and allocate subcarriers from these channels for different users • SDMA – Obtain more spatial diversity (eg. more antennas) and allocate some of that diversity gain different users • Ultimately boils down to obtaining more degrees of freedom to afford the orthogonality required for multiplexing different users’ transmissions D. Chan and T. Berger

  8. Insights for 802.11ac from multiuser-CSMA theory (2): To obtain throughput gain from new multiuser PHY, it must build on a resource that cannot improve current single user PHY • For example, consider this MPR channeland apply Theorem 4: • Current SU-PHY: • Cn= [1], η= 1 packet / slot • New MU-PHY: • Cn= [1, 2, 3, 4, 5], η= 2.64 packet / slot • If the new MU-PHY is created by finding 4 new channels, where each channel can support 600 Mb/packet • Cn= [1], η= 3 Gb/slot • Cn= [1, 2, 3, 4, 5], η= 1.56 Gb/ slot • MU-PHY performs even worse! • But if the new MU-PHY is a resource that cannot improve the current level of MU-PHY, we have a 264% gain! • This effect is due to the cost incurred from randomand distributedmultiple access • To obtain significant MU-gain in aggregate throughput , the new MU-PHY must be created with a resource that cannot significantly improve the current level of MU-PHY. D. Chan and T. Berger

  9. Implications of Insight No. 2: 11ac should only moderately widen channel and seriously consider SDMA via more antennas • We can only afford moderate widening of channel • C = B log(1 + S/N); B is the most direct way to increase raw PHY rate • Cannot be sacrificed for MU-PHY • FDMA is out of the question • Limited channels in 5 GHz and OBSS coex are also issue • OFDMA maybe better reserved for other usages • Cannot maintain current single user PHY rate (assuming same channel width) –will not provide MU-gain in aggregate throughput • Limited channels in 5 GHz and OBSS coex are also issue • SDMA via more antennas has tremendous MU-gain potential • Adding spatial stream does increase PHY rate, but >4 s.s. not robust • This is a resource that can’t really help current PHY! • MU-MIMO’s multiaccess capacity gain proportional to no. of AP antennas [4] • Spatial multiplexing at the AP does not require additional antennas at STAs [4] • AP doesn’t mind having more and more antennas… • Require channel state information at transmitter; adds significant overhead • Multiuser via coding? (No comments) D. Chan and T. Berger

  10. Investigating multiuser aggregate throughput: Simulation scenario for a possible MU-MIMO 11ac • MU-MIMO PHY • Expressed in terms of SINR: If higher than a certain threshold given by number of simultaneous transmissions, then no successful receiving of packets • Consider the new 80 MHz MCS and apply the PER @ SNR=40 dB provided in 08/535 • Uses 802.11 BW mechanism and MAC overheads • 11n Greenfield preamble; but control frames -- legacy PLCP and rates • No. of HT-LTFs depends on no. of spatial streams • CSI essentially useless after 20 ms [Eldad et al.] • MU-MIMO MAC • Every 20 ms some form of training session (“TRQ”) • Vanilla CSMA also plotted for reference (“Basic CSMA”) • Fully load equilength A-MPDU in a TXOP for all users • Asymptotic (or saturated) user traffic • Stress system to find asymptotic performances • Emulates current user traffic scenarios ←[Excerpted from 09/0303] D. Chan and T. Berger

  11. MU-MIMO aggregate throughput at 80 MHz with MCS 15 (622 Mbps) and 23 (933 Mbps) [Channel D] 11n MCS 15 @ 40 MHz D. Chan and T. Berger

  12. MU-MIMO MAC efficiency at 80 MHz with MCS 15 (622 Mbps) and 23 (933 Mbps) [Channel D] 11n MCS 15 @ 40 MHz MM and GF Appears efficiency goes down as MPR improves. D. Chan and T. Berger

  13. MU-MIMO aggregate throughput at 80 MHz with MCS 15 (622 Mbps) and 23 (933 Mbps) [Channel D] TXOP 3 x “No. of multiuser supported” D. Chan and T. Berger

  14. MU-MIMO MAC efficiency at 80 MHz with MCS 15 (622 Mbps) and 23 (933 Mbps) [Channel D] “Tuned” TXOP 3 x “No. of multiuser supported” Multiuser scenario’s efficiency still not brought up. D. Chan and T. Berger

  15. MU-MIMO aggregate throughput at 80 MHz with MCS 31 (1.12 Gbps) [Channel D] (PER=5.5%) PER expended a lot of throughput. D. Chan and T. Berger

  16. Insight No. 1 -- Cross Layer Design : Next step in bringing up throughput and MAC efficiency, MAC parameters adapted to a multiuser PHY by shortening backoff • The MAC’s backoff shouldn’t be the same for each MU-PHY layer • While exponential backoff works, it’s not matched with the underlying PHY layer • Investigate for using the optimal transmission probability p. • This essentially implies adapting different backoff window values • Simulations still running; but initial results show performance improvements when MU-PHY’s MAC shortened backoff time D. Chan and T. Berger

  17. Recent simulations and measurements indicate the channel should be able to support the minimum spectral efficiency per user for >1 Gbps aggregate throughput In recent sessions, contributors from ETRI, NTT and Qualcomm, etc. have conducted many simulations and measurements on the PHY capacity for multiuser transmissions We work backwards from the minimums established by the aggregate throughput simulations and see if the channel can support these assumptions In particular, in 09/0574, they found that at SNR of 24 dB, an AP with 16 antennas can transmit simultaneously to eight 2-antenna STAs at 2 streams per user and achieve 35 bps/Hz. [Excerpted from 09/0574] • In recent sessions, contributors from ETRI, NTT and Qualcomm, This is a 16 stream system [5], so at 24 dB, we can expect ~2.2 bps/Hz/stream • In other words, at 24 dB, to reach the minimum PAR requirements for 500 Mbps, we can use 3 streams per user (~528 Mbps per user) • This gives ~6.6 bps/Hz/user and we can support 5.3 users • Since we saw in the sim, 1 Gbps aggregate throughput is achieved with 3 users at ~6.5 bps/Hz/user (MCS15) or ~2 users with ~3.25 bps/Hz/user, we can conclude from the recent PHY channel measurements that MU-MIMO with CSMA should be able to reach higher than 1 Gbps aggregate throughput D. Chan and T. Berger

  18. ~ Acknowledgements ~ We would like to thank the following individuals for their helpful comments and suggestions: • Brian Hart • Len Cimini D. Chan and T. Berger

  19. References [1] D. S. Chan, T. Berger, et al., “Carrier Sense Multiple Access with Multipacket Reception: Theory and Applications to Wireless Networks”, to be submitted to IEEE Trans. Communications. (Please contact D. Chan for preprint when it becomes available.) [2] D.S. Chan, T. Berger and L. Tong, "On the Stability and Optimal Decentralized Throughput of CSMA with Multipacket Reception Capability," Proc. of Allerton Conference on Communication, Control, and Computing, Sept 2004. [3] D. S. Chan, P. Suksompong, J. Chen and T. Berger, "Improving IEEE 802.11 Performance with Cross-Layer Design and Multipacket Reception via Multiuser Iterative Decoding," IEEE 802.11-05/0946r0, Sept 2005. [4] W. Yu and W. Rhee, “Degrees of Freedom in Wireless Multiuser Spatial Multiplex Systems with Multiple Antennas,” IEEE Trans. Communications, Oct 2006. [5] Greg Breit, conversation, July 2009. D. Chan and T. Berger