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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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the wireless ip project
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
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
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
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
Adaptive Modulation and Prediction Errors

Modify thresholds to keep BER constant (single-user)

smart antennas simplest case
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
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
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
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
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
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
Analysis of Throughput (cont.)

Spectral efficiency (L antennas, K users):Cyclic prefix:

Pilots:

thresholds
Thresholds

Select the modulation level i as

observations
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
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
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

slide20

Thank you!

Questions?