Multiple Access OFDM for High Bit Rate Indoor Wireless Systems

1
Multiple Access OFDM for High Bit Rate Indoor
Wireless Systems

• Andy Klein
• Professor Robert W. Brodersen
• Dept. of EECS
• University of California
• Berkeley
• June, 1999

2
Why OFDM?

• Reduces ISI and effects of frequency selective
  fading, eliminating the need for equalization
• Time domain lengthened symbol period is larger
  than the channel time dispersion.
• Frequency domain each subchannel has
  sufficiently small width and can be considered
  ideal (i.e. flat).
• To completely remove ISI and ICI, its necessary
  to add a cyclic prefix, which causes negligible
  rate loss.
• Spectrally efficient
• Less sensitive to sample timing offsets than
  single carrier systems

3
OFDM Basics

• Main idea split data stream into N parallel
  streams of reduced data rate and transmit each on
  a separate subcarrier.
• When the subcarriers have appropriate spacing to
  satisfy orthogonality, their spectra will
  overlap. OFDM modulation is equivalent to the
  IDFT

...
...
Parallel to Serial (P/S)
...
S

IDFT
4
System Overview
OFDM spectrum
Parallel to Serial (P/S)
Serial to Parallel (S/P)
Add cyclic prefix
...
...
...
Transmit filter GT(w)
IDFT
s(t)
f
Transmitter
vs. conventional FDM spectrum
Channel H(w)
Channel
n(t)
Receiver
Serial to Parallel (S/P)
Parallel to Serial (P/S)
Remove cyclic prefix
...
...
...
Receive filter GR(w)
DFT
r(t)
f
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A Multiple Access Solution

• Can combine concepts of CDMA and OFDM
• Reap the benefits of both techniques
• In 1993, three slightly different schemes were
  independently proposed
• MC-CDMA (Yee, Linnartz, Fettweis, and others)
• multicarrier DS-CDMA (DaSilva and Sousa)
• MT-CDMA (Vandendorpe)
• Same structure as multicarrier DS-CDMA, but uses
  longer spreading codes in proportion to the
  number of carriers.
• This violates orthogonality of subcarriers, which
  minimizes MAI at expense of adding ICI

6
MC-CDMA

• Spreads the data, then modulates a different
  subcarrier with each chip (can be thought of as
  spreading in frequency domain)
• If c(t)c1, c2, , cN represents the spreading
  code for an arbitrary user, the transmitter
  structure is

c1
IDFT, P/S convert
...
Copier
s(t)
cN
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Multicarrier DS-CDMA

• Spreads the S/P converted data, then modulates a
  different subcarrier with each stream (spreading
  in time domain).
• If c(t)c1, c2, , cN represents the spreading
  code for an arbitrary user, the transmitter
  structure is

c(t)
IDFT, P/S convert
...
S/P convert
s(t)
c(t)
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Problems with OFDM

• Adds some complexity, but much work has been done
  in computationally efficient DFT algorithms (e.g.
  FFT)
• Signal has a noise-like amplitude with a very
  large dynamic range. Thus, OFDM systems requires
  RF amplifiers with a high peak to average power
  ratio.
• More sensitive to carrier frequency offset and
  drift than single carrier systems

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Wrap Up

• Presented arguments for use of hybrid OFDM/CDMA
  schemes.
• Next investigate performance aspects of each
  scheme for the indoor environment, as well as the
  implementation costs.
• Other alternatives filter bank implementations
  (instead of DFT), frequency domain equalization,
  etc.
• Design a system