PERFORMANCE OF OFDM IN NAKAGAMI FADING

Report
Performance Evaluation of WiMax
Systems
Metehan Dikmen and Mehmet Şafak
Hacettepe University
Dept. of Electrical and Electronics Engineering
06532 Beytepe, Ankara, Turkey
Outline
•
•
•
•
Motivation
Physical layer
IEEE 802.16 extensions
WiMax System Model
–
–
–
–
–
Randomization
FEC
Modulation
Interleaving
OFDM
• Simulation
– AWGN channel
– Rayleigh fading channel
• Results and Conclusions
Motivation
• WirelessMAN provides network access to buildings through
exterior antennas communicating with BSs
• Offers an alternative to cabled access networks,
– e.g., fiber optic links, coaxial systems using cable
modems, and digital subscriber line (DSL) links
• Have the capacity to provide high-rate network connections
over broad geographic areas without costly infrastructure
required in deploying cable links to individual sites
• Likely to support nomadic and increasingly mobile users
Motivation
• Supports different transport technologies, including
IPv4, IPv6, Ethernet
• The standard achieves high data rates in part via FEC
and OFDM techniques
• Has a long transmission range because
– regulations allow high power transmissions
– the use of directional antennas produces focused signals
• Transmission range and data rate vary significantly
depending on the frequency bands used
The Physical Layer
• Three types of physical layers are defined:
– WirelessMAN-SCa
– WirelessMAN-OFDM
• A 256-carrier OFDM.
• Multiple access of different mobile terminals: TDMA
– WirelessMAN-OFDMA
• A 2048-carrier OFDM.
• OFDMA: Multiple access is provided by assigning a
subset of the carriers to an individual user.
IEEE 802.16 Extensions
• IEEE 802.16a
– used in licensed and ISM bands from 2 to 11 GHz.
• At the lower ranges, the signals can penetrate barriers and
thus do not require LOS between transmitter and receiver
• IEEE 802.16b
– increases the spectrum in 5 and 6 GHz frequency bands
– provides QoS to ensure priority transmission for real-time
voice and video
– offers differentiated service levels for different traffic types
IEEE 802.16 Extensions
• IEEE 802.16c
– represents a 10 to 66 GHz system profile
• IEEE 802.16d (also called as WiMax)
– includes minor improvements and fixes to 802.16a.
– creates system profiles for compliance testing of 802.16a devices
• IEEE 802.16e
– standard for networking between fixed BSs and MSs.
– would enable the high-speed signal handovers necessary for
communications with users moving at vehicular speeds.
IEEE 802.l6d
Wireless MAN-OFDM
• Designed for NLOS operation
• Operating frequencies:
– 2-11GHz
• Channel bandwidths:
– 20 or 25 MHz (U.S.)
– 28 MHz (Europe)
Randomization
• Randomization is performed on data transmitted on
the downlink and uplink.
• For each OFDM symbol randomizer shall be used
independently
• Pseudo Random Binary Sequence (PRBS)
generator : 1  X 14  X 15
Randomization
PRBS for data randomization
FEC
• FEC:
– Reed-Solomon, with variable block size and error
correction capabilities
– RS concatenated with punctured inner convolutional
code
– Turbo coding is optional
– Space-time block codes are optional
• Interleaving is also employed
FEC
• Reed Solomon Code
– Derived from a systematic RS (n=255, k=239 T=8) code
using GF(m=8)
– primitive polynomial specified as: [1 0 0 0 1 1 1 0 1]
• Convolutional Code
– Coding rate is ½
– Constraint length is 7
– Generator polynomials:
• G1=171oct [1111001]
• G2=133oct [1011011]
Convolutional Encoder
Puncturing
• Puncturing is applied after CC
• 1 means a transmitted bit 0 means a removed bit
Channel coding schematic
• Modulation schemes: BPSK, QPSK, 16-QAM and 64-QAM
• Pulse shaping: Square-root raised-cosine with a rolloff
factor of 0.25
Interleaving
• Interleaving ensures that adjacent coded bits are
mapped onto nonadjacent subcarriers
• Block Interleaver with a two step permutations:
– mk= (Ncbps/12).k mod12+floor(k/12) k=0,1..Ncbps-1
– jk= s.floor(mk/s)+(mk+Ncbps-floor(12.mk/Ncbps)) mod(s)
• De-interleaver is also defined by two step permutations
– mj=s.floor(j/s)+(j+floor(12.j/Ncpbs)) mod(s) j=0,1..Ncbps-1
– kj=12.mj-(Ncbps-1).floor(12.mj/Ncbps)
OFDM
• OFDM symbol is made up from 256 subcarriers:
– 192 data subcarriers
– 8 pilot subcarriers
– 56 null subcarriers: 28 lower, 27 higher frequency subcarriers
for guard bands and one DC subcarrier
• Pilot subcarriers are used to aid the receiver with
synchronization and channel estimation.
• Designated ratios of cyclic prefix time to useful time are
1/4, 1/8, 1/16, 1/32
Simulations
data
In1
Binary Input
RS Encoder
Out1
Randomizer
Convolutional
Encoder
Viterbi Decoder
Binary Output
RS Decoder
32,24
32,24,4
Puncture
General
Block
Interleaver
General
Block
Interleaver
General
Block
Deinterleaver
General
Block
Deinterleaver
2/3
Interleaver1
Interleaver2
Deinterleaver2
Deinterleaver1
QPSK
QPSK
Modulator
In1
Out1
OFDM Transmitter
Channel
Models
In1
output
Out1
DeRandomizer
In1
Out1
insert zero
Out1
QPSK
In1
Out2
OFDM Receiver
QPSK
Demodulator
AWGN Channel - QPSK
AWGN Channel
0
10
Theoretical
QPSK1
QPSK2
-1
10
-2
10
-3
BER
10
-4
10
-5
10
-6
10
-7
10
0
2
4
6
E b/N0 (dB)
8
10
12
AWGN Channel - 16 QAM
AWGN Channel
0
10
Theoretical
16QAM1
16QAM2
-1
10
-2
BER
10
-3
10
-4
10
-5
10
0
2
4
6
8
Eb/N0 (dB)
10
12
14
Rayleigh Fading Channel - QPSK
Rayleigh Channel
0
10
Theoretical
QPSK1
QPSK2
-1
BER
10
-2
10
-3
10
0
2
4
6
8
10
E b/N0 (dB)
12
14
16
18
Comparison
Coherence Time
Eb/N0
1bit duration
1 OFDM symbol duration
System without OFDM
nearly 0
0.0076
18dB
System with OFDM
0.00022
0.006
18dB
QPSK without OFDM
0.0038
0.0037
18dB
QPSK with OFDM
0.016
0.0027
18dB
System with OFDM
0.00022
0
inf
QPSK with OFDM
0.015
0
inf
• As we did not use pilot carriers for channel estimation,
the use of the OFDM has harmful effects to the system
varying with coherence time
Rayleigh Fading Channel – 16 QAM
Rayleigh Channel
0
10
Theoretical
16QAM1
-1
10
-2
10
0
2
4
6
8
10
12
14
16
18
Results and Conclusions
• If we use OFDM without channel estimation we
are encounter with serious problems in Rayleigh
fading channels
• Increasing the coherence time makes this worse
because of the burst errors
• It is obvious that QAM is useless in a Rayleigh
fading because of its dependence on amplitude
• Our next step will be channel estimation

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