tailieunhanh - Opportunistic Communication and Multiuser Diversity

Channel diversity: frequency diversity via Rake combining, time diversity via interleaving and coding, macro-diversity via soft handoff, transmit/receive antenna diversity. | 6. Opportunistic Communication and Multiuser Diversity Fundamental Feature of Wireless Channels: Time Variation multipath fading large-scale channel variations time-varying interference Traditional Approach to Wireless System Design Compensates for channel fluctuations. Example: CDMA Systems Two main types of compensating mechanisms: Channel diversity: frequency diversity via Rake combining time diversity via interleaving and coding macro-diversity via soft handoff transmit/receive antenna diversity 2. Interference management: power control interference averaging What Drives this Approach? Main application is voice, with very tight latency requirements. Needs a consistent channel. Opportunistic 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, . | 6. Opportunistic Communication and Multiuser Diversity Fundamental Feature of Wireless Channels: Time Variation multipath fading large-scale channel variations time-varying interference Traditional Approach to Wireless System Design Compensates for channel fluctuations. Example: CDMA Systems Two main types of compensating mechanisms: Channel diversity: frequency diversity via Rake combining time diversity via interleaving and coding macro-diversity via soft handoff transmit/receive antenna diversity 2. Interference management: power control interference averaging What Drives this Approach? Main application is voice, with very tight latency requirements. Needs a consistent channel. Opportunistic 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). Point-to-Point Fading Channels Capacity-achieving strategy is waterfilling over time. Performance over Rayleigh Channel Performance: Low SNR At low SNR, capacity can be greater when there is fading. Hitting the Peaks 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. Multiuser Opportunistic Communication Performance Increase in spectral efficiency with number of user at all SNR’s, not just low SNR. AWGN Multiuser 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. Multiuser Diversity: A More Insightful Look Independent fading makes it likely that users peak at different times. In a .

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