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Multi-User Capacity and Opportunistic Communication

Multi-User Capacity and Opportunistic Communication 40
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Dr.ShivJindal,India,Teacher
Published Date:19-07-2017
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Multi-User Capacity and Opportunistic Communication Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 1Outline  Reference: Chapter 6 (and 5): Tse/Viswanath  Multiple access (or multi-user) channels are different from pt- pt channels  New concepts/techniques: successive interference cancellation (SIC), superposition coding, multi-user diversity.  AWGN multiuser uplink: CDMA + SIC  AWGN multiuser downlink: superposition-coding (CDMA- like) + SIC  Fast Fading: ability to track channel at sender (CSI) + opportunistic more important due to multi-user diversity  Gains over CSIR for full range of SNR (not just low SNR)  Opportunistic beamforming, IS-856 (1x EV-DO) etc… Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 2Pt-pt channel Capacity  A slow fading channel is a source of unreliability: very poor outage capacity. Diversity is needed.  A fast fading channel with only receiver CSI has a capacity close to that of the AWGN channel. Delay is long compared to channel coherence time.  A fast fading channel with full CSI can have a capacity greater than that of the AWGN channel: fading now provides more opportunities for performance boost.  The idea of opportunistic communication is even more powerful in multiuser situations. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 3Fundamental Feature of Wireless Channels: Time Variation  multipath fading  large-scale channel variations  time-varying interference Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 4Traditional Approach to (Multi-user) Wireless System Design Compensates for channel fluctuations. I.e. treats a multi-user channel like a set of disjoint single-user (or pt-pt) channels. Examples: interference averaging; near-far power control, fixed coding/modulation rates Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 5Example: CDMA Systems Two main compensating mechanisms: 1. Channel diversity:  frequency diversity via Rake combining  macro-diversity via soft handoff  transmit/receive antenna diversity 2. Interference management:  power control  interference averaging Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 6What Drives this Approach? Main application is voice, with very tight latency requirements. Needs a consistent channel. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 7Opportunistic Communication: A Different View Transmit more when and where the channel is good. Exploits fading to achieve higher long-term throughput, but no guarantee that the "channel is always there". Appropriate for data with non-real-time latency requirements (file downloads, video streaming). Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 8Recall: Point-to-Point Fading Channels Capacity-achieving strategy is waterfilling over time. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 9Variable rate over time: Target BER  In the fixed-rate scheme, there is only one code spanning across many coherence periods.  In the variable-rate scheme, different codes (distinguished by difference shades) are used depending on the channel quality at that time.  For example, the code in white is a low-rate code used only when the channel is weak. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 10Adaptive Modln/Coding vs Shannon Limit  Optionally turbo-codes or LDPC codes can be used instead of simple block/convolutional codes in these schemes Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 11Performance over Pt-Pt Rayleigh Channel Not much bang-for-buck for going to CSI from CSIR high SNR Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 12Performance: Low SNR At low SNR, capacity can be greater (w/ CSI) when there is fading. Flip side: harder to get CSI at low SNR  Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 13Hitting the Peaks Low SNR: Hard in Practice (High SNR) Fixed power almost as good as waterfilling (Low SNR) Waterfilling helps, But CSI harder & users pay delay penalties At low SNR, one can transmit only when the channel is at its peak. Primarily a power gain. In practice, hard to realize such gains due to difficulty in tracking the channel when transmitting so infrequently. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 14Multiuser Opportunistic Communication Multiple users offer new diversity modes, just like time or frequency or MIMO channels Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 15Performance AWGN Increase in spectral efficiency with number of user at all SNR’s, not just low SNR Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 16Multi-user w/ CSI: Low SNR case Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 17Multiuser Diversity Total average SNR = 0 dB.  In a large system with users fading independently, there is likely to be a user with a very good channel at any time.  Long-term total throughput can be maximized by always serving the user with the strongest channel. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 18Sum Capacity: AWGN vs Ricean vs Rayleigh  Multiuser diversity gain for Rayleigh and Ricean channels ( = 5); KP/N0 = 0 dB.  Note: Ricean is less random than Rayleigh and has lesser sum capacity Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 19Multiuser Diversity: A More Insightful Look  Independent fading makes it likely that users peak at different times.  In a wideband system with many users, each user operates at low average SNR, effectively accessing the channel only when it is near its peak.  In the downlink, channel tracking can be done via a strong pilot amortized between all users. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 20