FDM versus OFDM

1
FDM versus OFDM

• FDM
• Frequency Division Multiplexing
• Frequency guard bands
• OFDM
• Orthogonal FDM
• Overlapping, but orthogonal bands (e.g. sinc
  functions)
• Sample at appropriate points
• Much denser than FDM

2
Digital Modulation/Multiplexing

• Only values at sample points are important and
  contain data
• IFFT generates the time samples corresponding to
  these frequency points
• IFFT generates an infinite periodic time
  sequence, but only the first N samples are sent
  over the channel (these samples contain exactly
  the same information as the frequency points)
• This windowing to N samples results in a
  convolution with a sinc function in the frequency
  domain (which has the orthogonality property)

3
Data Transmission Sequence

• Data is grouped into blocks and each block is
  treated sequentially
• Each block i consists of N symbols which are
  transformed into N time samples using the IFFT

4
Upconverting and Multiplexing

ej.2?.F.t
p
s

cN-1
xN-1
5
Fast Fourier Transform

p

cN-1
xN-1
x0


xk
N-IFFT


xN-1
6
Single Carrier versus Multicarrier

• Single carrier and multicarrier both
• send N symbols in NT, or 1/T symbols/second
• have a total single sided bandwidth of about 1/T

Single carrier
7
Inter Symbol Interference (ISI)

• Single carrier system
• Multipath components cause interference between
  the desired symbol and previous ones

?

• Multicarrier system
• A cyclic prefix is added, which consists of C
  duplicate symbols
• The length of the cyclic prefix is chosen such
  that C.T gt ?
• Behaves as a cyclic convolution with the channel
  response

8
Frequency Response

• Cyclic convolution results in multiplication of
  frequency data points with the FFT of the channel
  response ( channel frequency response)
• Each data point is thus multiplied by a single
  factor

9
Overall System View

• Typically QAM (quadrature amplitude modulation)
  is used to modulate the bits onto symbols, but
  any modulation is possible

b 4 bits/symbol
b 2 bits/symbol
b 6 bits/symbol
16-QAM
4-QAM
64-QAM
10
Equalizing

• 1-tap equalizer, channels with small ?k may be
  treated as erasures at the receiver
• Adaptive loading takes channel info into account
  at the sender

Without adaptive loading
With adaptive loading
11
Adaptive Loading

• Assign bi and Pi such that Ptot is minimized
• Send more information when channel is good
• Channel needs to be estimated (as for
  equalization)

• Loading information needs to be fed back to the
  transmitter
• The channel must remain quasi-stationary between
  estimation updates (low Doppler rate)

12
Things to Consider About OFDM

• Dynamic range at output of IFFT is much larger
  than at input (or single carrier systems) large
  peak-to-average ratio (PAR)
• Very good frequency synchronization is crucial to
  maintain orthogonality (otherwise ISI)
• Example use OFMA as multiple access technique

13
Different Flavors of Multicarrier Systems

• Discrete Multi Tone (DMT)
• Terminology used in xDSL systems
• In what is it different from OFDM? Nobody knows.
• DMT baseband ? OFDM passband / DMT loaded ? OFDM
  unloaded ??
• Different Variations of OFDM (none of them use
  adaptive loading)
• W-OFDM or Wideband OFDM (WiLAN)
  http//www.wilan.com
• Basis for 802.11a and new 802.16
• Data rate up to 30 Mbps at 70 mph
• Reed-Solomon coding with erasures
• large carrier spacing (wideband) to simplify
  frequency synchronization
• V-OFDM or Vector OFDM (CISCO) http//www.cisco.co
  m
• MMDS microwave band, non-line-of-sight
• 2-fold receive antenna diversity (vector), up
  to 44 Mbps
• Flash-OFDM (Flarion) httpwww.flarion.com
• Fast frequency hopping on OFDM (flash)
• Data rate 384 kbps 3 Mbps at highway speeds,
  aimed at 3G systems
• Market trail in one big city this year,
  commercial availability in 2002