doc.: ieee 802.11-05/0946r0 submission september 2005 douglas s. chan et al., cornell...
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September 2005
Douglas S. Chan et al., Cornell University
Slide 1
doc.: IEEE 802.11-05/0946r0
Submission
Improving IEEE 802.11 Performance with Cross-Layer Design and Multipacket Reception via Multiuser Iterative Decoding
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Date: 2005-09-20
Name Company Address Phone email Douglas S. Chan [email protected]
Prapun Suksompong
Jun Chen
607-254-8818
Toby Berger
Cornell University
School of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853
607-255-1447 [email protected]
Authors:
September 2005
Douglas S. Chan et al., Cornell University
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Abstract
Receivers today have the ability to decode more than one packets from multiple users. Such a physical layer can deliver significant improvements to network performances. Thus, the classical collision model is no longer realistic and a cross-layer approach should be employed when designing multiple access protocols. This is especially the case for CSMA communications, which previously have not been implemented with a multipacket reception (MPR) model. We propose applying recent information theoretic results in multiuser iterative decoding to help improve IEEE 802.11 wireless LAN standards’ performances. Our method also preserves the underlying physical layer’s implementation.
September 2005
Douglas S. Chan et al., Cornell University
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Collision Channel Model
• Traditionally, network research done over collision channel model – Only one transmission can occur over channel at any instance
• Analysis is more tractable, reflects PHY layers at the time, worst case scenario
BackoffCollision
STA B
Channel activities
STA A
Success
September 2005
Douglas S. Chan et al., Cornell University
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Multipacket Reception Channel Model
• Receivers today can correctly separate and decode multiple packets transmitted simultaneously– e.g., by signal processing or spread spectrum modulation
• This is referred to as a multiple packet reception (MPR) channel [Ghez]
• Better performance than collision channel and more realistic
BackoffTwo successes
STA B
Channel activities
STA A
Success
September 2005
Douglas S. Chan et al., Cornell University
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CSMA with Multipacket Reception • CSMA over an MPR-capable PHY layer has not been
previously studied
• First proposed recently by Chan, Berger and Tong [Chan1] in 2004– Showed that MPR improves performance of CSMA over Collision
Channel
– Performance also improves over S-ALOHA
– However, as MPR strength becomes stronger, carrier sensing and scheduling becomes unnecessary• But requires a lot of resource for MPR strength to be perfect
• So this is a balance between MPR-PHY strength and scheduling
September 2005
Douglas S. Chan et al., Cornell University
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CSMA Performance Improvement with MPR
September 2005
Douglas S. Chan et al., Cornell University
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Current PHY layers for 802.11• Currently, their PHY implementations actually can support multiuser
detection– 802.11: frequency hopping, direct sequence spread spectrum– 802.11b: DSSS with higher rate codes (and optional PBCC)– 802.11a: OFDM– 802.11g: OFDM (and optional PBCC)– 802.11n(WG): MIMO
• Used these MPR implementations to– 1. Combat narrowband interference in the unlicensed radio-bands
where these standards and other RF applications co-exist– 2. Mitigate interferences between nearby WLANs in case their chosen
channels’ bandwidths partially or wholly overlap
• The MPR abilities are used to separate transmissions in unrelated nearby WLANs as opposed to increasing the multiaccess transmission capacity of a single WLAN.
September 2005
Douglas S. Chan et al., Cornell University
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CSMA Performance Improvement with MPR
September 2005
Douglas S. Chan et al., Cornell University
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Implementing MPR on 802.11• MPR can improve performance of 802.11
• Consider adding MPR capability to next generations of 802.11
• But MPR usually involves modifications of PHY layer– Makes it harder to maintain backwards compatibility
– Or ready application to current PHY proposals
• Ideally we would like an MPR solution that can preserve the underlying PHY layer
September 2005
Douglas S. Chan et al., Cornell University
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Survey of methods for MPR• Direct Sequence Spread Spectrum with (pseudo-)orthogonal codes
– PHY centric/Requires changes in PHY
• MIMO – Space-Time Block Coding– Embedding known symbols in packets and successive cancellation
– PHY centric/Requires changes in PHY
• Successive Cancellation– 1. Signaling level
• (Pseudo-) Orthogonal waveforms– PHY centric/Requires changes in PHY
– 2. Modulation/Coding level• Treat other users’ signals as noise and successively decode
– Coding centric/May not require changes in PHY
• Joint Decoding with Multiaccess codes:– Iterative detection and decoding– Iterative joint decoding
– Coding centric/May not require changes in PHY
September 2005
Douglas S. Chan et al., Cornell University
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Network model for multiaccess codes• User i encodes source data via his/her assigned codebook as Xi
– This codebook is part of an overall codebook for L users
– Central receiver has overall codebook
• Central receiver receives channel affected version of symbols sent by multiple users:
– Y is received signal
– Xi is symbol transmitted by user i
– Hi is channel coefficients for user I
– N is noise
• Central receiver decodes the data sent by each user via:– Joint decoding or successive cancellation
• Scheme works as long as signaling scheme over channel is additive
September 2005
Douglas S. Chan et al., Cornell University
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Signaling for multiaccess codes• Examples of additive signaling schemes
– Amplitude modulation
– Phase modulation: BPSK, QPSK, n-QAM (!!)
• Not additive: If information encoded in differential of phase (eg. DPSK)• 802.11 PHYs all employ non-differential phase modulation, i.e. additive
1 1 -1 1
-1 1 1 1
-1 -1 1 1
-1 1 1 3
X1
X2
X3
Channel
Noise
Hard/SoftDecision
MUD
Receiver
X1= 1 1 -1 1
X2= -1 1 1 1
X3= -1 -1 1 1
September 2005
Douglas S. Chan et al., Cornell University
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New MUD layer• Propose adding new “Multiuser
Decoding” (MUD) layer to perform multiuser detection and decoding
• Can employ any underlying PHYs to achieve multiaccess coding
• MAC still needed for scheduling re-transmissions
• Can turn on and off as wished
• Joint decoding performs at least as good as successive decoding– Recent results on iterative joint
decoding show very good performance
– Can focus implementation on iterative joint decoding codes
MUD
PHY
MAC
Performs Multiuser detection and decoding
Current 802.11Proposed 802.11
September 2005
Douglas S. Chan et al., Cornell University
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MUD 1: Graph-based jointly decoded codes• Introduced in Palanki, Khandekar and McEliece’s 2001 paper [Palanki]
• Each user encodes with their own distinct low density parity check LDPC codebook
• Use belief propagation (BP) to perform iterative joint decoding – Each edge is an independent estimate of each Xi
– Contains joint channel information
– Iteration stops until all parity check satisfied or after certain iterations
Variablenodes
Variablenodes
Checknodes
September 2005
Douglas S. Chan et al., Cornell University
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MUD 1: Graph-based jointly decoded codes• Design of multiuser LDPC codes can be achieved with a graph splitting and density evolution
techniques
• Advantages:– Each user’s code can be of its distinct rate
• Achieving priority scheduling (QoS)
– BP decoding of LDPC can be done in parallel
• Implementation issues:– Requires fixed length codewords– Codes designed only for a fixed number of users
• We can put limit to the number of users associating to AP
– AP needs to tell each user which codebook to use during association– Requires good codes to combat code expansion (CE)– No MPR when AP transmits
September 2005
Douglas S. Chan et al., Cornell University
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MUD 2: Iterative detection and decoding• Sanderovich, Peleg and Shamai’s 2005 paper [Sanderovich]
• Each user encodes with same LDPC code
• Performs user-specific random interleaving to codeword
10
00
Inte
rlea
ver
Inte
rlea
ver
LDPC
LDPC
QPSK 1 1 -1 1
-1 -1 1 1
-1 -1 1 1
Inte
rlea
ver
11
LDPC
September 2005
Douglas S. Chan et al., Cornell University
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MUD 2: Iterative detection and decoding• Then performs iterative but separate multiuser detection and single-user
decoding– Multiuser detection can be an optimal soft-decision LLMSE
– Single user detection is single LDPC BP decoder
– Iteration stops until all parity check satisfied or after certain iterations
Mul
tius
er D
etec
tor
LDPC SISO Decoder 1 1 -1 1
-1 -1 1 1
-1 -1 1 1
-1 1 1 3
10
00
11
September 2005
Douglas S. Chan et al., Cornell University
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doc.: IEEE 802.11-05/0946r0
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MUD 2: Iterative detection and decoding• Advantages:
– Each user’s code are the same• Simplifies multiaccess LDPC design
• A simpler implementation
– BP decoding of LDPC can be done in parallel
• Implementation issues:– Requires fixed length codewords– Codes designed only for a fixed number of users
• We can put limit to the number of users associating to AP
– AP needs to tell each user what interleaving scheme to use during association– Requires good codes to combat code expansion (CE)– No MPR when AP transmits
September 2005
Douglas S. Chan et al., Cornell University
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Performance of MUD enhanced 802.11
September 2005
Douglas S. Chan et al., Cornell University
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Simulation details
• Used 802.11a overheads in PHY preamble and MAC headers
• Asymptotic (or saturated) user traffic– Stress system to find asymptotic performances
– Emulates current user traffic scenarios
• Assume every user’s packet is corrupted if only one bit of a single user is corrupted– Worse case performance for MPR-enabled 802.11a
September 2005
Douglas S. Chan et al., Cornell University
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Cross Layer Design• The MAC’s protocol is the same for each PHY layer
• While exponential backoff works, it’s not matched with the underlying PHY layer– [Calì] showed that the current MAC is not optimal for 802.11
• We show in the next slide the goodput for using the optimal transmission probability p
Backoffslots
Transmission
p p p p
September 2005
Douglas S. Chan et al., Cornell University
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Performance of MUD enhanced 802.11 with Cross-layer (matched) MAC
September 2005
Douglas S. Chan et al., Cornell University
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Conclusion
• A MPR-enabled 802.11 architecture is proposed
• Two MPR methods that preserve the underlying PHY layer are discussed
• Pointed out the importance of cross-layer design via a matched MAC
• Showed significant performance improvements for MPR enabled 802.11 and with matched MAC
September 2005
Douglas S. Chan et al., Cornell University
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Acknowledgements
• Bruce Kraemer
• Teik-Kheong “TK” Tan
• Anuj Batra
September 2005
Douglas S. Chan et al., Cornell University
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References• [Calì] F. Calì, M. Conti and E. Gregori, “Dynamic Tuning of the IEEE 802.11
Protocol to Achieve a Theoretical Throughput Limit,” IEEE/ACM Trans. Networking, vol. 8, pp. 785-799, 2000.
• [Chan1] D.S. Chan, T. Berger and L. Tong, “On the Stability and Optimal Decentralized Throughput of CSMA with Multipacket Reception Capability”, Allerton Conference on Communication, Control, and Computing, 2004.
• [Chan2] D.S. Chan and T. Berger, “Performance and Cross-Layer Design of CSMA for Wireless Networks with Multipacket Reception,” Proc. of Asilomar Conf. on Signals, Systems and Computers, 2004.
• [Ghez] S. Ghez, S. Verdú, and S. C. Schwartz, “Optimal decentralized control in the random-access multipacket channel,” IEEE Trans. Automat. Contr., 1989.
• [Palanki] R. Palanki, A. Khandekar and R. McEliece, “Graph based codes for synchronous multiple access channels,” Allerton Conference on Communication, Control, and Computing, 2001.
• [Sanderovich] A. Sanderovich, M. Peleg and S. Shamai, “LDPC Coded MIMO Multiple Access With Iterative Joint Decoding,” IEEE Trans. Info. Theory, 2005.