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OFDMA Networks
By Mohamad Awad
OutlineOutline
Wi l h l i i t d th i ff t on Wireless channel impairments and their effect on wireless communicationChannel modelingg
Sounding techniqueOFDM as a solutionOFDMA as an improved solutionOFDMA as an improved solutionMIMO-OFDMAConclusion
Propagation Mechanisms in wireless channels
Reflection
Three propagation mechanisms in two types of propagation channels
Diffraction Scattering
Indoor wireless channel Outdoor wireless channel
T iTransmitter
Receiver
Large Scale FadingLarge Scale Fading
Topographic and elevation data were obtained from Naturalresources Canada.
Large-Scale Fading Vs. Small-Scale Fading
Large-Scale Fading
Small Scale Fading
Coherent Summation of
2 multipaths
Incoherent Summation
Partly- Coherent Summation of 2
multipaths Im
Im
ed P
ower
Small-Scale Fading of 2 multipaths
ReIm
Re
Rec
eive
Re
Figure taken from :”Wireless communication, principle and practice” by T. S. Rappaport
Transmitter – Receiver Separation
Channel-Sounding
?Propagation mechanisms:- Reflection.
?
- Diffraction.- Scattering.
Measurement setting:- frequency. e Po
wer
frequency.- line-of-sight vs no-line-of-sight.- environment.- transmitter - Receiver separation.- human presence.
fl A
Cha
nnel
Impu
lse
Res
pons
e
pow
er
time- floor Area.- extent of Clutters.- detection Threshold.
P Ch l
ReceiverTransmitter
Propagation Channel
Channel Impulse-Response
( ) ( )nex
N
nex ah ττδτ −=∑1
22
Channel Impulse Response
n=1
Total number of multipathsNB)
na n-th multipath amplitude
ved
Pow
er (
dB
ex
ττ excess delay
Receiver Detection Threshold
mal
ized
Rec
eiv
nτ n-th multipath excess delay
Excess Delay (ns)
Nor
m
maxτ
Review: Convolution Review: Convolution http://www.jhu.edu/signals/convolve/
Frequency-Flat Vs. Frequency-Selective Fading
ChannelTransmitted Signal
Received SignalSignal
No Time DispersionISITime Dispersion and ISI
Ts @ Tx
time
maxτ
Ts >> maxτ
@ RxTs @ Tx
time
maxτ
maxτTs <<
Frequency-Flat FadingFrequency-Selective Fading q y g
FrequencyBs @ Tx Bs @ RxBw
q y g
BwBs @ Tx Bs @ Rx Frequency
Bs << Bw
Frequencyw
Bs >> Bw
s @ s @
Multicarrier Transmission
History of Multicarrier transmission History of Multicarrier transmission A principle for multi-channel transmission over a bandlimited channel was proposed in 1966 !!bandlimited channel was proposed in 1966 !!Why did take that long to be implemented?
Combine ?
DFT !
What is OFDM?What is OFDM?Orthogonal Frequency Division MultiplexingMethod to multiplex many “narrowband” signals into Method to multiplex many narrowband signals into an aggregate “wideband” data streamNormally the subcarriers would need sufficient Normally, the subcarriers would need sufficient separation to avoid adjacent channel interferenceOFDM uses orthogonal (1/T ) subcarrier spacing to OFDM uses orthogonal (1/Tsym) subcarrier spacing to maintain subcarrier separation
OrthogonalityOrthogonality
Orthogonal Carrier SpacingOrthogonal Carrier Spacing
1/Tsym
1
1.2
0.6
0.8
0
0.2
0.4
0 5 10 15 20 25 30-0.4
-0.2
OFDM System ModelOFDM System Model
OFDM-TDMA MAC
Sub-1
Frequency
User 2
User 1
05
-0.4
-0.2 0 0.2
0.4
0.6
0.8 1 1.2
Sub-2
Sub 1
S b 4
Sub-3
User 2
U 4
User 3
510
1520
Sub-4
Sub-5
Time
User 42530
Disadvantages
High latency # users large
Advantages
Easy implementation High latency, # users large
In-effecient power usage
In-effecient in frequency utilization
Easy implementation
Simple resource allocation
Low processing requirement In-effecient in frequency utilizationp g q
Low signaling overhead (channel gain)
ApplicationsApplications
Multi-user diversityMulti user diversity
Time Varying Channel
•The channel is generally time varying due to:•User changes location.•Object gets between the transmitter and receiver (bus)
τex
Transmitter Time τex
τex
OFDMAOFDMAOrthogonal Frequency Division Multiple Access.A subcarrier is exclusively assigned to a userA subcarrier is exclusively assigned to a user.Exploits multiuser diversity.Dynamic subcarrier assignmenty g
Frequency
Disadvantages of OFDM
High latency, # users large
Sub-2
Sub-1
S b 3
User 2
User 1 In-effecient power usage
In-effecient in frequency utilization
Sub-4
Sub-3
Sub-5 User 4
User 3
Time
OFDMA Resource Allocation Internet
Channel gain
QoS (rate, dropping rate, etc..)
Base Station ( BS )
•Maximize ThroughputTime
Freq
•Maximize Throughput •Assign power < Max•Each subcarrier is exclusively assigned
•New•gainse c us ve y ss g ed
Guarantee QoSgains
Allocated subcarriers
Trans Power
User # 1 User # 2
Trans Power
OFDMA MAC
Sub-1
FrequencyUser 1
Sub-2
Sub-4
Sub-3
User 2
User 3Sub-4
Sub-5
Time
User 4
Disadvantages
Complicated Res Alocation
Advantages
Simple implementation Complicated Res. Alocation
Signaling overhead (adaptive scheme)
Simple implementation
Resource allocation flexibility
Adaptation to channel characteristics (adaptive scheme)
Better BER performance (adaptive scheme)
Space Diversity
Two antennas separated by D
Two antennas separated by several wavelengths will not generally experience fades at h the same time
“Space Diversity” can be obtained by using two obtained by using two receiving antennas and switching instant-by-instant
Signal received by Antenna 1
Si l i d b to whichever is bestSignal received by Antenna 2
C bi d Si lCombined Signal
MIMO systemsMIMO systems
MIMOMIMO
Basic idea of MIMO: Improve quality (BER) and/or datarate (bits/sec) by using multiple TX/RX antennas( ) y g p
Core scheme of MIMO: space-time coding (STC)Two main functions of STC: diversity & multiplexingy p g
MIMO - OFDMAMIMO OFDMA
ConclusionConclusionWireless channel suffers from large scale fading and small scale fadingscale fadingAfter modeling the channel, communication systems are matched to the channel.OFDMA is one of advanced technologies to overcome the wireless channel impairments.OFDMA can be combined with MIMO to improve performance .
QuestionsQuestions????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????Questions???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????
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