1. Multi Carrier Modulation and OFDM

2. Transmission of Data Through Frequency Selective Time Varying Channels
We have seen a wireless channel is characterized by time spread and frequency spread.
Time Spread
Frequency Spread

3. Single Carrier Modulation in Flat Fading Channels
if symbol duration >> time spread then there is almost no Inter Symbol Interference (ISI).
channel
time 1 1
phase still recognizable
Problem with this: Low Data Rate!!!

4. … in the Frequency Domain
this corresponds to Flat Fading
channel
Frequency
Frequency
Flat Freq. Response
Frequency

5. Single Carrier Modulation in Frequency Selective Channels
if symbol duration ~ time spread then there is considerable Inter Symbol Interference (ISI).
channel
time ? ? 1
phase not recognizable

6. One Solution: we need equalization
channel
equalizer
time
time 1 1
Channel and Equalizer
Problems with equalization:
it might require training data (thus loss of bandwidth)
if blind, it can be expensive in terms computational effort
always a problem when the channel is time varying

7. The Multi Carrier Approach
let symbol duration >> time spread so there is almost no Inter Symbol Interference (ISI);
send a block of data using a number of carriers (Multi Carrier) 1
time
channel
“symbol”

8. Compare Single Carrier and Multi Carrier Modulation
Frequency
channel
Block of symbols
subcarriers
Each subcarrier sees a Flat Fading Channel: Easy Demod MC 1
One symbol
Flat Fading Channel: Easy Demod SC

9. Structure of Multi Carrier Modulation
In MC modulation each “MC symbol” is defined on a time interval and it contains a block of data
OFDM Symbol
data
data
data
data
data
guard interval
data interval
with
MAX channel time spread

10. NO Inter Block Interference!
Guard Time
We leave a “guard time” between blocks to allow multipath TX RX
Guard Time
the “guard time” is long enough, so the multipath in one block does not affect the next block
Data Block
Data Block
data+guard RX TX
NO Inter Block Interference!

11. MC Signal
Transmitted Signal:
Baseband Complex Signal:

12. “Orthogonal” Subcarriers and OFDM
guard interval
data interval
Choose:
Orthogonality:

13. Orthogonality at the Receiver
transient response
Transmitted subcarrier
Channel (LTI)
Received subcarrier
steady state response
still orthogonal at the receiver!!!

14. OFDM symbols in discrete time
Let
be the sampling frequency;
be the number of data samples in each symbol;
the subcarriers spacing
Then:
with the guard time.

15. Summary OFDM Symbol TIME: FREQUENCY: # samples # subcarriers
Sampling Interval
guard
data
TIME:
Freq spacing
FREQUENCY:
# samples
# subcarriers

16. OFDM Symbol and FFT Where: positive subcarriers negative subcarriers
unused subcarriers

17. Guard Time with Cyclic Prefix (CP)
CP from the periodicity
IFFT{ X } CP

18. OFDM Demodulator
See each block:
No Inter Block Interference
with

19. Overall Structure of OFDM Comms System
IFFT
+CP
P/S
FFT
-CP
S/P

20. Simple One Gain Equalization
To recover the transmitted signal you need a very simple one gain equalization:
transm.
noise
received
channel
Use simple Wiener Filter:

21. OFDM as Parallel Flat Fading Channels
Significance: a Freq. Selective Channel becomes N Flat Fading Channels
OFDM Mod
OFDM Demod
Frequency Selective channel
N Flat Fading Channels

22. Summarize basic OFDM Parameters: sampling rate in Hz
N length of Data Field in number of samples
L length of Cyclic Prefix in number of samples
total number of Data Subcarriers
data
time
frequency
guard

23. IEEE a:
Frequency Bands: GHz and GHz (12 channels)
Modulation OFDM
Range: 100m
IEEE g
Frequency Bands: GHz
Modulation: OFDM
Range: 300m

24. Channel Parameters: FCC
Example: the Unlicensed Band 5GHz U-NII (Unlicensed National Information Infrastructure)
8 channels in the range GHz
4 channels in the range GHz

25. Channel Parameters: Example IEEE802.11
In terms of a Transmitter Spectrum Mask (Sec in IEEE Std a-1999)
Typical Signal Spectrum
Typical BW~16 MHz

26. In either case:
Sampling frequency
FFT size
Cyclic Prefix
DATA CP

27. Sub-carriers: (48 data + 4 pilots) + (12 nulls) = 64
Frequency
Time
IFFT
Pilots at: -21, -7, 7, 21

28. Frequencies:
Subcarriers index
DATA

29. Time Block:

30. Overall Implementation (IEEE 802.11a with 16QAM).
1. Map encoded data into blocks of 192 bits and 48 symbols:
data
Encode Interleave
Buffer (192 bits)
Map to 16QAM
… … …
1101
+1+j3
-1+j
+3-j3 …
+1-j …
4x48=192 bits

31. Overall Implementation (IEEE 802.11a with 16QAM).
2. Map each block of 48 symbols into 64 samples
time domain
frequency domain
null
+1+j3 …
-3-j
+3-j3
+1-j
24 data 2 pilots
null
24 data 2 pilots
IFFT

32. Channel Parameters: Physical
Frequency Spread
Time Spread
Constraints on OFDM Symbol Duration:
roughly!!!
to minimize CP overhead
for channel Time Invariant

33. Summary of OFDM and Channel Parameters
Max Time Spread sec
Doppler Spread Hz
Bandwidth Hz
Channel Spacing Hz
OFDM (design parameters):
Sampling Frequency
Cyclic Prefix
FFT size (power of 2)
Number of Carriers

34. Example: IEEE802.11a
Channel:
Max Time Spread
Doppler Spread
Bandwidth
Channel Spacing
OFDM (design parameters):
Sampling Frequency
Cyclic Prefix
FFT size (power of 2)
Number of Carriers

35. Applications: various Area Networks
According to the applications, we define three “Area Networks”:
Personal Area Network (PAN), for communications within a few meters. This is the typical Bluetooth or Zigbee application between between personal devices such as your cell phone, desktop, earpiece and so on;
Local Area Network (LAN), for communications up 300 meters. Access points at the airport, coffee shops, wireless networking at home. Typical standard is IEEE (WiFi) or HyperLan in Europe. It is implemented by access points, but it does not support mobility;
Wide Area Network (WAN), for cellular communications, implemented by towers. Mobility is fully supported, so you can move from one cell to the next without interruption. Currently it is implemented by Spread Spectrum Technology via CDMA, CDMA-2000, TD-SCDMA, EDGE and so on. The current technology, 3G, supports voice and data on separate networks. For (not so) future developments, 4G technology will be supporting both data and voice on the same network and the standard IEEE (WiMax) seems to be very likely

36. More Applications
1. WLAN (Wireless Local Area Network) standards and WiFi. In particular:
IEEE a in Europe and North America
HiperLAN /2 (High Performance LAN type 2) in Europe and North America
MMAC (Mobile Multimedia Access Communication) in Japan
2. WMAN (Wireless Metropolitan Network) and WiMax
IEEE
3. Digital Broadcasting
Digital Audio and Video Broadcasting (DAB, DVB) in Europe
4. Ultra Wide Band (UWB) Modulation
a very large bandwidth for a very short time.
5. Proposed for IEEE (to come) for high mobility communications (cars, trains …)