LTE FDD OFDM Technology
ZTE University
Contents
Basic Principle Benefits and Drawbacks Key Technologies Application in DL Application in UL
Radio Channel Characteristics
Multi-Path Effect Time Dispersion causes
ISI (InterSymbol Interference
When Transmitter sends a Pulse signal, the receiver receives:
Time-Variant Channel Doppler Shift is one
form Frequency
Dispersion causes ICI (InterChannel Interference)
Frequency
Frequency
Transmitter
Receiver
Power Power
Time
What is OFDM ?
OFDM: Orthogonal Frequency Division Multiplexing, is a kind of multi-carrier transmission;
In frequency domain, OFDM divides channel into some sub-channels overlapped between adjacent sub-channels. These sub-channels are orthogonal.
Implement of CP can undermine ISI caused by Delay Spread
OFDM Modulation and Demodulation
Modulation
Demodulation
Multi-Path Effect
GI: guarding interval
Path 2
The second symbol in Path 1 interferes with the first symbol in Path 2
Path 1
ISI (InterSymbol Interference) will occur because of multi-path effect
If GI is Inserted
GI: Guarding Interval
Path 2
Path 1
GI
GI is inserted in order to eliminate ISI When GI is longer than max delay spread of channel, the multi-path component of
a symbol doesn’t interfere with next symbol
ICI Brought by GI
subcarrier 1
Delayed subcarrier 2
GI FFT IntegrationTime = 1/subcarrier spacing
Part of subcarrier 2 causing ICI on subcarrier 1
CP: Cyclic Prefix
FFT integral time length can’t contain the entire waves because GIs are included. The orthogonality between subcarriers is detroyed.
If CP is Inserted Amplitude
Time
GI FFT Integral Time Length
OFDM Symbol Length
CP refers to the prefixing of a symbol with a repetition of the end, in order that the orthogonality between subcarriers is not undermined due to GI.
FFT integral time length can contain integer number of waveforms of subcarriers in multi-path, as long as time delay of every path is less than GI.
CP: Cyclic Prefix
CP
OFDM Principle Diagram
OFDM Frequency-Time Structure
Each column corresponds to one OFDM symbol Each row corresponds to one OFDM subcarrier
OFDMA: Downlink multiple access
Up to 64 QAM can be used Resistance to multi-path interference by Cyclic Prefix. Friendly to MIMO.
Sub-carriers
Sub-frame
Frequency
Time
Time frequency resource for User 1
Time frequency resource for User 2
Time frequency resource for User 3
System Bandwidth
OFDM vs OFDMA
Contents
Basic Principle Benefits and Drawbacks Key Technologies Application in DL Application in UL
OFDM Benefits Dividing large bandwidth into small subcarriers, can be effectively
against frequency-selective fading
It can be effectively against ISI, so it is suitable for high-speed data transmission in multi-path environment.
Spectrum efficiency is maximized. Because of the orthogonality between subcarriers, adjancent subchannels overlap.
Modulation/demodulation can be achived by IFFT/FFT. Calculation is efficient and simple.
It can achieve the different UL and DL transmission data rate by using different number of subchannels.
It can take full advantage of high SNR subchannels to increase system throughput by dynamic subchannel allocation.
OFDM Drawbacks
Sensitive to frequency deviation: Frequency shift occurred in transmission process(such as Doppler Shift),
or the frequency difference between a receiver local oscillator and the carrier frequency of transmitter, would undermine the orthogonality between subcarriers of OFDM system, leading to inter-channel signal interference.
High PAPR (Peak-to-Average Power Ratio): The output of OFDM modulation is the superposition of multiple sub-
channel. If the signal phases in multiple sub-channels are the same at a certain time, superimposed signal instantaneous power will be far greater than the signal average power, resulting in a high PAPR. High PAPR not only brings higher requirements on transmitter Power Amplifier (PA) linearity, but also reduces the amplifier efficiency.
Contents
Basic Principle Benefits and Drawbacks Key Technologies Application in DL Application in UL
18
Channel Estimation and Reference Symbols
To estimate the frequency-domain channel tap directly by inserting reference symbols(RS).
Using knowledge about the RS, the receiver can estimate the frequency-domain channel around the location of the RS.
The reference symbols should have a sufficiently high density in both the time and the frequency domain to estimate entire time/frequency grid.
0l0R
0R
0R
0R
6l 0l0R
0R
0R
0R
6l
One
ant
enna
por
t
TS36.211 DL RS, normal CP,1TX
One RB
19
Frequency Interleaving
The consequence bits are spread entire the frequency band.
AMC
AMC: Adaptive Modulation and Coding OFDM can achieve dynamic adaption of the modulation
sheme, coding scheme and bit amount on every subcarrier according to the fading situation of frequency-selective channel.
Thus great performance enhancements can be achived in OFDM system.
Contents
Basic Principle Benefits and Drawbacks Key Technologies Application in DL Application in UL
OFDM Time-Frequency Structure
Resource Element The smallest resource unit
Resource Block Contains 12 subcarriers in
frequency domain Contains 7 OFDM symbols in
time domain(6 OFDM symbols when Extended CP is used)
DLsymbN OFDM symbols
One downlink slotslotT
0l 1DLsymb Nl
RB
scD
LR
BN
N
subc
arri
ers
RB
scN su
bcar
rier
s
RBsc
DLsymb NN Resource block
resource elements
Resource element ),( lk
0k
1RBsc
DLRB NNk
1 RB : • 180kHz (i.e. 12 x 15KHz )in
frequency domain• 0.5 ms (i.e. one slot)in time
domain
Bandwidth Analysis
Bandwidth = Sub-frame x No. of sub-frame in each RB x No. of RB Bandwidth of one sub-frame = 15KHz No. of sub-frame each RB = 12
Nominal
Bandwidth
(MHz)
1.4 3 5 10 15 20
Number of RB 6 15 25 50 75 100
Frequency
Domain Real
Bandwidth
(MHz)
1.08 2.7 4.5 9 13.5 18
Contents
Basic Principle Benefits and Drawbacks Key Technologies Application in DL Application in UL
Why SC-FDMA?
OFDM: High PAPR Lower PA (Power Amplifier) efficiency
Lower battery efficiency Not suitable for terminal
SC-FDMA (Single Carrier FDMA): Lower PAPR Higher PA efficiency Longer battery
life Suitable for terminal DFT-S-OFDM (DFT-Spread-OFDM) is the
implementation method of SC-FDMA in the frequency domain, which performs DFT-based precoding before IFFT modulation in the OFDM modulation process.
SC-FDMA: Uplink multiple access
Up to 16 QAM can be used
Single carrier modulation achieves lower Peak to Average Ratio (PAPR)
FDMA is efficiently achieved through FFT operation
0
Single Carrier Sub-frame
Frequency
Time
Time frequency resource for User 1
Time frequency resource for User 2
Time frequency resource for User 3
System Bandwidth
OFDMA and SC-FDMA Comparison
DFT-S-OFDM Process
OFDM vs DFT-S-OFDM
• OFDM PAPR is far higher than DFT-S-OFDM• The higher modulation mode DFT-S-OFDM adopts, the higher PAPR it gets.
Zero Fill
The output of the DFT to equidistant inputs of the IDFT with zeros inserted in between.
The output of the DFT is mapped to consecutive inputs of the IFFT.
Localized DFTS-OFDMLocalized DFTS-OFDM
SD-FDMA Multiple Access
Distributed DFT-S-OFDM Benefit/Drawback
Benefit of distributed DFT-S-OFDM Possibility for additional frequency diversity as even a
low-rate distributed DFTS-OFDM signal (small DFT size M) can be spread over a potentially very large overall BW.
Drawback of distributed DFT-S-OFDM No single sub-carrier property More sensitivity to frequency errors and has higher
requirements on power control
• Localized DFT-S-OFDM is adopted in LTE UL.
Summary
What is OFDM/OFDMA? OFDM Benefits/Drawbacks? Related Key Technologies? Why SC-FDMA? Difference between DFT-S-OFDM & OFDM? DFT-S-OFDM Types?