Guard Time

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Wideband OFDM Technologies for Next Generation
Mobile Communication
Presented for Semi-annual Review for CWC/UCSD
November 3, 2000
Chung Gu Kang, Ph.D Visiting Scholar,
CWC/UCSD ccgkang_at_popsmail.com
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Scope

• System Requirements for 4G

• OFDM Issues

- Channel estimation for OFDM
- Pilot symbol arrangement for rapid dispersive
fading channels

• OFDM/SDM

• Space division multiplexing (SDM)

- Optimal receiver structure for OFDM/SDM

• Applications of Wideband OFDM

- Wireless LAN
- ACIS (Advanced Cellular Internet Service)
- IST ADAMS
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4G Family Systems Evolution
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Traffic Demand for 4G or beyond 3G .

• Emergence of VHMM (Very High Multimedia)
  128kbps (UL) 10Mbps (DL)
• gt Required bandwidth at 2015 1830MHz

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System Requirements
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Wideband OFDM for 4G?
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OFDM - Basic Concept (1)
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OFDM - Basic Concept (2)

• Basic OFDM Receiver

- If we are able to obtain the following
relationship for the received signal
then the transmitted symbol can be recovered
independently for each subcarrier
gt Cyclic Extension
FFT samples (1024 samples)
Cyclic Prefix (96)
Cyclic Suffix (96)
Ramp Down (32)
Ramp Up (32)
T
One OFDM block (1280 samples)
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Channel Estimation (1)
References 1 Seung Y. Park and Chung G.
Kang, Performance Analysis of Pilot Symbol
Arrangement for OFDM System under
Time-varying Multi-path Rayleigh Fading
Channels, To appear in IEICE Transactions
on Communications, January 2000. 2 S.Y. Park
and C.G. Kang, "Performance of Pilot-Assisted
Channel Estimation for OFDM System under
Time-varying Multi-path Rayleigh Fading with
Frequency Offset Compensation,
Proceedings of IEEE VTC '00, May 2000
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Channel Estimation (2)
Transmitted Signal
- N-points IFFT
- Guard interval inserted to prevent ISI
Pilot Symbol Arrangement - At every
th burst th subcarrier, pilot symbol is
inserted
i.e.,
where
e.g.,
Mf 1 Mt gt 1 ? Block Type
Mt 1 Mf gt 1 ? Comb Type
- In the current analysis,
Mt gt 1 Mf gt 1 ? Hybrid Type
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Channel Estimation (3)

• Time Varying Dispersive Channel Model

- WSSUS Rayleigh Fading Channel with L Paths
- Power delay profile

• Correlation function of the channel gain

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Channel Estimation (4)

• Correlation Function of Channel Gain

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Channel Estimation (5)

• Correlation Function of Channel Gain

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Channel Estimation (6)

• Effect of pilot symbol spacing

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Channel Estimation (7)

• Effect of mismatch

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OFDM/SDMA
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OFDM/SDMA Uplink (1)

• OFDM/SDMA System Transmitter

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OFDM/SDMA Uplink (2)

• OFDM/SDMA - Uplink

- Performance (A 4)
- MMSE-OFDM/SDMA Receiver
where ELSn is given by
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OFDM/SDMA - Uplink (3)

• OFDM/SDMA - Uplink

- Performance (A 4)
- MLSE-OFDM/SDMA Receiver
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OFDM/SDMA Downlink

• Downlink
• System Model

- Predistorting each subcarrier for all users,
- Detection Scheme Channel Inversion
If U A, Epren H-1n such that
If U lt A, Epren is the pseudo-inverse of HTn
s.t.

gt Each user sees a single-user AWGN channel per
carrier!
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Iterative Detection for OFDM/SDMA (1)

• Receiver Design for OFDM/SDMA System

• Iterative Scheme for Joint MAP Equalization
  Channel Estimation
• (based on Turbo Principle)

• Useful for suppressing co-channel interference
  in the ISM band or cellular systems

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Iterative Detection for OFDM/SDMA (3)

• Performance (Preliminary Results)

- A single user performance

• Convolution code (K3, R1/2)
• Number of coded bits
• ( of information bits tail bits) 2
  
• (618 2) 2 1240 bits
• QPSK modulation
• N 64
• Number of subcarriers used 48
• Guard Interval 16
• One slot 13 OFDM blocks
• Pilot symbol arrangement Mt Mf 4
• Eb/No 5dB
• Number of multipaths L 10 samples
• Normalized Doppler frequency fdT 2E-4
• Path loss exponent alpha 0

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Applications

• IEEE 802.11 TGa ETSI HIPERLAN/2 Standards

• Wireless LAN for 5GHz band
• Multi-rate OFDM (N64) with data rate up to
  54Mbps

• ACIS Advanced Cellular Internet Service

• Proposed by AT T Lab
• To provide wide-area Internet service to mobile
  subscribers gt 4G
• Combines OFDM with multiple transmitter/receiver
  antennas and coding
• Peak data rate greater than 3Mbps with a
  bandwidth of about 4MHz
• High bandwidth Efficiency with OFDM-BDMA DPA
• (2-3Mbps per cell with an average delay of 60
  - 120ms)

• European Union IST Project ADAMAS

• Adaptive Multicarrier Access System
• Adaptive OFDM Point-to-Multipoint outdoor
  broadband fixed wireless access system
• Licensed (10.5GHz) and unlicensed (5.8GHz)
  scenarios
• Dynamic TDMA/TDD with bit rates of 64kbps -
  25Mbps

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Concluding Remarks

• High-bit-rate transmission beyond 2Mbps for
  mobile communication?

- OFDM is essential for combating the multipath
distortion
- Robust channel estimation scheme is required

• Bandwidth efficiency?

• To achieve the frequency reuse of 1 with TDMA

(1) Dynamic packet assignment with interference
avoidance
gt Interference measurement can be efficiently
implemented using FFT
(2) Space diversity OFDM gt OFDM/SDMA
gt Optimal receiver structure can be derived
using the turbo principle

• Future Research

• Design of iterative detection scheme for the
  multi-user communication

- Design of dynamic packet assignment protocol
for guaranteeing QoS
- Comparison with a multi-user detection scheme
for CDMA