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A 4G System Proposal Based on Adaptive OFDM

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Interference control (smart antennas etc. ... Smart Antennas: Simplest Case. MRC in mobile stations (MS) ... 16 dB / receiver antenna and info symbol (same ... – PowerPoint PPT presentation

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Title: A 4G System Proposal Based on Adaptive OFDM


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 _at_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 Improv
ement 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
1
2
2
2
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 (10011 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 ?/4 - ?/2 ? 2 - 4 slots _at_ 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
Modulation
0 BPSK 1
1 4-QAM 2 8.70
2 8-QAM 3 13.53
3 16-QAM 4 16.89
4 32-QAM 5 20.46
5 64-QAM 6 23.59
6 128-QAM 7 26.86
7 256-QAM 8 29.94
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 K1
    user/sector
  • (Reuse 1 combined with reuse 3,
    area-fair scheduling,
  • interference limited, full load,
    Rayleighpath loss, L1 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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