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A 4G System Proposal Based on Adaptive OFDM. Mikael Sternad. The Wireless IP Project. Part of SSF PCC, 2000-2002 A SSF funded project 2002-2005 +Vinnova funding www.signal.uu.se/Research/PCCwirelessIP.html. Visions and Goals.
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A 4G System Proposal Based on Adaptive OFDM Mikael Sternad
The Wireless IP Project Part of SSF PCC, 2000-2002 A SSF funded project2002-2005 +Vinnova funding www.signal.uu.se/Research/PCCwirelessIP.html
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
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)
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
Adaptive Modulation and Prediction Errors Modify thresholds to keep BER constant (single-user)
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)
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
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
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
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
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
Analysis of Throughput (cont.) Spectral efficiency (L antennas, K users):Cyclic prefix: Pilots:
Thresholds Select the modulation level i as
Spectral Efficiency and Throughput(one sector, 16 dB) 25 20 Throughput [Mbit/s] 15 10
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
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?
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
Thank you! Questions?