Unformatted text preview: ) Fall 2011 100
frequency (Hz) October 22, 2012 150 27 / 31 Lecture Notes 13 OFDM Signal and Spectrum (M =16)
M=16 M=16 15 40
30 OFDM Power Spectrum (dB) OFDM Signal and Envelope 10
rms value
5 0 −5 20
10
0
−10
−20
−30 −10
−40
−15
0 0.5
time (s) EECS 455 (Univ. of Michigan) 1 Fall 2011 −50
50 100
frequency (Hz) October 22, 2012 150 28 / 31 Lecture Notes 13 OFDM Signal and Spectrum (M =64)
M=64 M=64
40 20 30 15 OFDM Power Spectrum (dB) OFDM Signal and Envelope 25 rms value
10
5
0
−5
−10
−15 10
0
−10
−20
−30
−40 −20
−25
0 20 0.2
0.4
time (s) EECS 455 (Univ. of Michigan) 0.6 Fall 2011 −50
50 100
frequency (Hz) October 22, 2012 150 29 / 31 Lecture Notes 13 OFDM Summary
OFDM has narrow spectrum (relative to single carrier with same
data rate)
OFDM has larger peaktoaverage power ratio
Requires larger backoff of ampliﬁer to avoid nonlinear distortion
compared to single carrier
Larger backoff usually implies a smaller power efﬁciency for the
ampliﬁer Frequency selective fading can be mitigated in OFDM by
appropriate channel estimation, coding and interleaving
OFDM has lower sidelobes (smaller high frequency components)
and thus can not make fast transitions of the amplitude or phase
compared to unﬁltered BPSK/QPSK. EECS 455 (Univ. of Michigan) Fall 2011 October 22, 2012 30 / 31 Lecture Notes 13 OFDM in Wireless Systems Parameter 802.11a
(WiFi) UWB (Multiband) LTE (10 MHz) WiMax
(10MHz) Bandwidth
Sample Time
Number of Active Carriers
Number of Guard Carriers
Data Carriers
Pilots Carriers
FFT Size
Subcarrier frequency spacing 20MHz
50ns
52
12
48
4
64
0.3125MHz
(20MHz/64)
3.2 µs
0.8 µs
(16
samples)
4.0 µs 528 MHz
1.8939ns
112
10
100
12
128
4.125MHz
(528MHz/128)
242.42ns
70.08ns (37
samples)
312.5ns 10MHz
65.104 ns
601
423 1024
15kHz 10MHz
1024
up to 914
–
1024
–
1024
10.9375kHz 66.67 µ s
4.695.21 µ s 91.429 µ s
11.428 µ s 71.3571.88
µs 102.857 µs IFFT period
Guard Interval
OFDM Symbol Duration EECS 455 (Univ. of Michigan) Fall 2011 October 22, 2012 31 / 31...
View
Full
Document
This note was uploaded on 02/12/2014 for the course EECS 455 taught by Professor Stark during the Fall '08 term at University of Michigan.
 Fall '08
 Stark
 Frequency

Click to edit the document details