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EE360: Lecture 7 Outline Adaptive CDMA Techniques

EE360: Lecture 7 Outline Adaptive CDMA Techniques. Introduction CDMA with power control Adaptive techniques for interference reduction Rate and power adaptation Adaptive techniques to meet QOS. Introduction.

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EE360: Lecture 7 Outline Adaptive CDMA Techniques

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  1. EE360: Lecture 7 OutlineAdaptive CDMA Techniques • Introduction • CDMA with power control • Adaptive techniques for interference reduction • Rate and power adaptation • Adaptive techniques to meet QOS

  2. Introduction • Adaptive CDMA varies the rate and/or power of the signal relative to the channel and interference • Traditional power control used to compensate for the near-far problem • More sophisticated techniques vary rate and power to maximize throughput • Adaptive techniques can be used to meet QOS constraints for different types of media

  3. Near-Far Problem and Traditional Power Control • On uplink, users have different channel gains • If all users transmit at same power (Pi=P), interference from near user drowns out far user • “Traditional” power control forces each signal to have the same received power • Channel inversion: Pi=P/hi • Increases interference to other cells • Decreases capacity • Degrades performance of successive interference cancellation and MUD • Can’t get a good estimate of any signal h3 h1 P3 P1 h2 P2

  4. Adaptive Techniques for Interference Reduction • Global Power Control (Bambos/Pottie) • Power of all users is controlled such the each user’s SIR meets his required constraint • There is no unique solution • Want all users to operate at minimum power • Distributed algorithms converges to optimum • Ideas used for call admission control • Variable spreading codes • Varying the spreading gain of a code varies the interference rejection P2 P1

  5. Rate and Power Adaptation* • In CDMA the max. possible data rate depends on • The required Eb/N0 for a given BER target (Pb=f(Eb/N0,R)) • The channel gains (Gi)and powers (Si) of all users • Power and rate adaptation schemes • Data rate fixed for Gi>g0, transmit power adjusted to meet Pb target. For Gi<g0 power is fixed and data rate is adjusted to meet Pb target. • Rate is adapted by varying the bit time: spreading code fixed. • Can also truncate rate or power below cutoff g0 • The cutoff avoids wasting power on bad channels • Significant power savings for given throughput target Assumes interference modeled as AWGN *S. Kim, 2000

  6. CDMA Rate Adaptation • Vary bit rate for binary modulation • Use multiple codes per user • Orthogonal or semiorthogonal • Vary the constellation size • Variable bit rate typically most effective • Multicode with orthogonal codes equivalent • Can optimize rate and power allocation to maximize total throughput for all these strategies • Jafar/Goldsmith paper

  7. Power Control given Rate Requirements • Mark/Zhu paper (Lola’s presentation) • Multirate CDMA • Power control to meet QOS (SIR) constraints • Adaptation depends on spread BW, required data rates, and SIR requirements • Rate and power allocation strategy proposed • What about fairness • Song/Mandayam paper (Tim’s presentation) • Many algorithms will give more resources to user with the best channel: unfair resource allocation • Incorporate fairness into resource allocation strategy using utility functions.

  8. Main Contributions(Student Paper Presentations) • Adaptive WCDMA (Mark/Zhu) • Power and rate adaptation • More users make adaptation less useful • Variations in traffic helps • Raising power removes effect of errant interferers • The “traffic demands” parameter • Rates times SIR per channel • Utility functions in adaptive CDMA (Song/Mandayam ) • Utility functions as a flexible tool to impose fairness • Changing the SIR on each retransmission • Separating computations between the mobile and base

  9. Real-time vs. Non Real-time* • Adapting power and rate based on random channel variations may lead to large delays. • Can devise adaptive strategies that transmits real-time data at a fixed rate • Non real-time data transmitted at a variable rate based on channel conditions • Similar to capacity analysis with minimum rate requirements. *S. Kim, 2000; Oh and Wasserman, 1999, others

  10. Summary • There are multiple degrees of freedom in adaptive CDMA techniques • Power, Rate, Spreading codes, QOS, etc. • Adaptation techniques must be global • Every user affects all other users • Complex optimization problem • Adaptive CDMA is an active research area as 3G systems have adopted a WCDMA standard

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