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A 4G System Proposal Based on Adaptive OFDM

This project focuses on designing a low-cost, high-mobility packet data system with 100 Mbit/s performance and high spectral efficiency. Adaptive resource allocation, smart antennas, and cross-layer interaction are key design concepts to enhance system capabilities. The proposal includes an example of an adaptive OFDM downlink design for improved throughput in varying channel conditions. Throughput analysis and future work considerations aim to optimize system performance, intercell interference, and quality of service.

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A 4G System Proposal Based on Adaptive OFDM

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  1. A 4G System Proposal Based on Adaptive OFDM Mikael Sternad

  2. The Wireless IP Project Part of SSF PCC, 2000-2002 A SSF funded project2002-2005 +Vinnova funding www.signal.uu.se/Research/PCCwirelessIP.html

  3. Visions and Goals • A flexible, low-cost general packet data system allowing wide area coverage and high mobility (vehicular velocities) • Perceived performance of 100 Mbit/s Ethernet • High spectral efficiency (10 fold increase vs. 3G) • Quality of service and fairness Leads to an extreme system based on adaptive resource allocation

  4. Design concepts • Use short term properties of the channelinstead of averaging (predictive link adaptation) • Interference control (smart antennas etc.) • Scheduling among sectors and users (combined MAC and RRM) • Cross-layer interaction(soft information)

  5. Short-term Channel Properties • Typical time-frequency channel behavior (6.4 MHz, ~50 km/h) • Data from Stockholm, Sweden @1900MHz (by Ericsson) Accurate channel prediction is needed Coherence bandwidth 0.6 MHz Coherence bandwidth 4.9 MHz

  6. Channel Prediction

  7. Adaptive Modulation and Prediction Errors Modify thresholds to keep BER constant (single-user)

  8. Smart Antennas: Simplest Case Fixed lobes (sectors, cells) at base stations MRC in mobile stations (MS) Advantages BS: Efficient use of space (robust) Low interference levels MS: Improvement of SNR (robust)

  9. Scheduling Among Users in a Sector • Feedback info from each mobile: Appropriate modulation level for each bin in a time slot. • Perform scheduling based on predicted SNR in bins  • For each bin let the “best” user transmit; use adaptive modulation and ARQ scheme • Modify to take QoS and fairness into account 1 4 3 5 2 user freq time

  10. Minimizing Interference Among Sectors • Exclusive allocation of time-frequency bins to users within border zones between sectors of a base station. • Frequency reuse 1 in inner parts of sectors • Frequency reuse 3 in outer parts of sectors • Multi-antenna terminals (IRC) • (Power control) • Slow resource reallocation between sites and sectors, based on traffic load f 1 1 1 2 2 2 2 1 1 1 2 2 time

  11. Design Example: An Adaptive OFDM Downlink • Maximize throughput. Ignore fairness and QoS • Target speed 100 km/h +large cells  Frequency-selective fading • WCDMA frequency band (5 MHz bandwidth, 1900 MHz carrier) • Adaptive modulation. Fixed within a bin (BPSK, 4-QAM, 8-QAM, 16-QAM, 32-QAM, 64-QAM, 128-QAM, 256-QAM) • Simple ARQ • No channel coding

  12. Physical Layer • OFDM system with cyclic prefix yielding low inter-channel interference • Symbol period is 111 ms (100+11 cyclic prefix) • 10 kHz carrier spacing (500 subcarriers in 5 MHz) • Time-frequency grid 0.667 ms x 200 kHz (120 symbols/bin; 5 are pilots) • Channel ~ constant within each bin • Design target speed is 100 km/h • Broadband channel predictor • Accurate over λ/4 - λ/2  2 - 4 slots @ 1900 MHz and 100 km/h

  13. Analysis of Throughput Simplifying assumptions: • Flat AWGN channel within each bin; Independent fading between bins • MRC with L antennas at mobiles (one sector of BS) • Average SNR  = 16 dB / receiver antenna and info symbol (same for all users; slow power control) • Adaptive modulation. Selection based on perfect channel prediction • K users. Fairness between users, QoS requirements, and delay constraints are neglected

  14. Analysis of Throughput (cont.) Spectral efficiency (L antennas, K users):Cyclic prefix: Pilots:

  15. Thresholds Select the modulation level i as

  16. Spectral Efficiency and Throughput(one sector, 16 dB) 25 20 Throughput [Mbit/s] 15 10

  17. Observations • Scheduling gives multiuser selection diversity (from both time and frequency selectivity of the channels) • MRC leads to good initial SNR • Good spectral efficiency improvement already at low to moderate load (#users) • Not all bins can be used in every sector due to interference • Uplink control information is required to signal modulation level

  18. Work in Progress • Evaluation of system level performance • Intercell interference, QoS, and fairness • First indications give a reuse of 1.7, average SIR  16dB • Results in 1.25 bits/s/Hz/sector at K=1 user/sector (Reuse 1 combined with reuse 3, ”area-fair scheduling”, interference limited, full load, Rayleigh+path loss, L=1 ant.) • Improved adaptive modulation systems • TCM (See presentation by Sorour Falahati) • Prediction errors ( - ” -) • Feedback information • MIMO ( 2 x 2 MIMO quite reasonable) • Development of a network simulator • Study of TCP/IP interaction • Design of uplink system • Single carrier modulation or OFDM?

  19. Work in Progress (cont.) • Optimize scheduler • QoS and fairness • Maximum Entropy Scheduler (using information about buffer influx). Minimize average buffer contents. • Intercell scheduling • Soft information • Passing PHY soft information to application • JPEG 2000 application • Modifications to TCP and UDP • Format for soft information Server Network

  20. Thank you! Questions?

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