ECE 6332, Spring, 2014
Wireless Communications
Zhu Han
Department of Electrical and Computer Engineering
Class 23April 16th, 2014
OFDM Basic IdeaOFDM Basic Idea
Orthogonal frequency-division multiplexing
Divide a high bit- rate stream into several low bit- rate streams ( serial to parallel)
Robust against frequency selective fading due to multipath propagation
Orthogonal frequency-division multiplexing Orthogonal frequency-division multiplexing
Special form of Multi-Carrier Transmission.
Multi-Carrier Modulation.– Divide a high bit-rate digital stream into several low bit-rate
schemes and transmit in parallel (using Sub-Carriers)
-6 -4 -2 0 2 4 6
-0.2
0
0.2
0.4
0.6
0.8
Normalized Frequency (fT) --->
Norm
alized A
mplitu
de -
-->
OFDMOFDM
Transmitted SymbolTransmitted Symbol
Guard Time and Cyclic Extension...Guard Time and Cyclic Extension...
A Guard time is introduced at the end of each OFDM symbol for protection against multipath.
The Guard time is “cyclically extended” to avoid Inter-Carrier Interference (ICI) - integer number of cycles in the symbol interval.
Guard Time > Multipath Delay Spread, to guarantee zero ISI & ICI.
M ultipath component that does not cause IS I
guard Symbol guard
guard Symbol guard
guard Symbol guard
M ultipath component that causes IS I
Mathematical description
Mathematical description
OFDM Timing ChallengeOFDM Timing Challenge
OFDM bit loadingOFDM bit loading Map the rate with the sub-channel condition
Water-filling
OFDM Time and Frequency GridOFDM Time and Frequency Grid
Put different users data to different time-frequency slots
OFDM Transmitter and ReceiverOFDM Transmitter and Receiver
OFDMOFDM
Multiband OFDM Multiband OFDM
- Simple to implement
- Captures 95% of the multipath channel energy in the Cyclic Prefix
- Complexity of OFDM system varies Logarithmically with FFT size i.e.
- N point FFT (N/2) Log2 (N) complex multiplies for every OFDM symbol
Pro and ConPro and Con Advantages
– Can easily be adopted to severe channel conditions without complex equalization
– Robust to narrow-band co-channel interference – Robust to inter-symbol interference and fading caused by multipath propagation – High spectral efficiency – Efficient implementation by FFTs – Low sensitivity to time synchronization errors – Tuned sub-channel receiver filters are not required (unlike in conventional
FDM) – Facilitates Single Frequency Networks, i.e. transmitter macro-diversity.
Disadvantages– Sensitive to Doppler shift. – Sensitive to frequency synchronization problems – Inefficient transmitter power consumption, since linear power amplifier is
required.
OFDM ApplicationsOFDM Applications ADSL and VDSL broadband access via telephone network copper wires.
IEEE 802.11a and 802.11g Wireless LANs.
The Digital audio broadcasting systems EUREKA 147, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB.
The terrestrial digital TV systems DVB-T, DVB-H, T-DMB and ISDB-T.
The IEEE 802.16 or WiMax Wireless MAN standard.
The IEEE 802.20 or Mobile Broadband Wireless Access (MBWA) standard.
The Flash-OFDM cellular system.
Some Ultra wideband (UWB) systems.
Power line communication (PLC).
Point-to-point (PtP) and point-to-multipoint (PtMP) wireless applications.
ApplicationsApplications
WiMax
Digital Audio Broadcast (DAB)
Wireless LAN
ApplicationsApplications
High Definition TV (HDTV)
4G Cellular Communication systems
Flash -OFDM
Proprietary OFDM FlavoursProprietary OFDM Flavours
Wideband-OFDM(W-OFDM) of Wi-LAN
www.wi-lan.com
Flash OFDMfrom Flarion
www.flarion.com
Vector OFDM(V-OFDM) of Cisco, Iospan,etc.
www.iospan.com
Wireless Access (Macro-cellular)
-- 2.4 GHz band-- 30-45Mbps in 40MHz
-- large tone-width (for mobility, overlay)
-- Freq. Hopping for CCI reduction, reuse
-- 1.25 to 5.0MHz BW -- mobility support
-- MIMO Technology-- non-LoS coverage,
mainly for fixed access-- upto 20 Mbps in MMDS
Wi-LAN leads the OFDM Forum -- many proposals submitted to IEEE 802.16 Wireless MAN
Cisco leads the Broadand Wireless Internet Forum (BWIF)
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OFDM based StandardsOFDM based Standards
Wireless LAN standards using OFDM are– HiperLAN-2 in Europe
– IEEE 802.11a, .11g
OFDM based Broadband Access Standards are getting defined for MAN and WAN applications
802.16 Working Group of IEEE– 802.16 -- single carrier, 10-66GHz band
– 802.16a, b -- 2-11GHz, MAN standard
Key Parameters of 802.16a Wireless MANKey Parameters of 802.16a Wireless MAN
• Operates in 2-11 GHz• SC-mode, OFDM, OFDMA, and Mesh support
• Bandwidth can be either 1.25/ 2.5/ 5/ 10/ 20 MHz• FFT size is 256 = (192 data carriers+ 8 pilots +56 Nulls)
• RS+Convolutional coding • Block Turbo coding (optional)
• Convolutional Turbo coding(optional)• QPSK, 16QAM, 64QAM
• Two different preambles for UL and DL
Calculations for 802.16a -- Example: 5MHzCalculations for 802.16a -- Example: 5MHz
Carrier frequency 2-11 GHz Channel Bandwidth 5 MHz Number of inputs to IFFT/FFT 256 Number of data subcarriers 192 Number of pilots 8 Subcarrier frequency spacing f 19.53125 KHz (5 MHz/256) Period of IFFT/FFT Tb 51.2 s (1 / f) Length of guard interval 12.8 s (Tb / 4) Length of the preamble for Downlink 128 s (640 sub-carriers) Length of the preamble for Uplink 76.8s (384/5 MHz) Guard interval for Uplink preamble 25.6 s (128/5 MHz) OFDM symbol duration 64 s (320/5 MHZ)
IEEE 802.16(10 to 66 GHz)
Broadband Access Standards -- contd. Broadband Access Standards -- contd.
IEEE LAN and MAN standards
IEEE 802.11a or.11b, or .11g
IEEE 802.16a,b(2 to 11 GHz)
2-5 miles, LoS(> 11GHz)
1-3 miles, non-LoS
The IEEE 802.11a/g StandardThe IEEE 802.11a/g Standard
Belongs to the IEEE 802.11 system of specifications for wireless LANs.
802.11 covers both MAC and PHY layers.
802.11a/g belongs to the High Speed WLAN category with peak data rate of 54Mbps
FFT 64, Carrier 2.4G or 5G. Total bandwidth 20 MHz x 10 =200MHz
The IEEE 802.11 StandardThe IEEE 802.11 Standard
Evolution of Radio Access TechnologiesEvolution of Radio Access Technologies
LTE (3.9G) : 3GPP release 8~9
LTE-Advanced :3GPP release 10+
802.16d/e
802.16m
In Nov. 2004, 3GPP began a project to define the long-term evolution (LTE) of Universal Mobile Telecommunications System
(UMTS) cellular technology
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LTE vs. LTE-AdvancedLTE vs. LTE-Advanced
DS-CDMA versus OFDMDS-CDMA versus OFDM
channelInput (Tx signal)
Output(Rx signal)
ImpulseResponse h(t)
time
3
0
freq.
Frequency Response H(f)
DS-CDMA can exploit
time-diversity
OFDM can exploitfreq. diversity
Comparing Comparing ComplexityComplexity of TDMA, DS- of TDMA, DS-CDMA, & OFDM TransceiversCDMA, & OFDM Transceivers
Timing Sync.
Freq. Sync.
Timing Tracking
Freq. Tracking
ChannelEqualisation
Analog Front-end(AGC, PA, VCO, etc)
TDMA OFDM
Very elegant, requiringno extra overhead
CDMA
Easy, but requiresoverhead (sync.) bits
Difficult, and requiressync. channel (code)
Easy, but requiresoverhead (sync.) bits
More difficult than TDMAGross Sync. Easy
Fine Sync. is Difficult
Modest Complexity Usually not requiredwithin a burst/packet
Requires CPE Tones(additional overhead)
RAKE Combining in CDMA usually more complex than
equalisation in TDMA
Modest Complexity(using dedicated correlator)
Easy, decision-directedtechniques can be used
Frequency DomainEqualisation is very easy
Complexity or cost is very high (PA back-off
is necessary)
Very simple(especially for CPM signals)
Complexity is high inAsynchronous W-CDMA
Modest to High Complexity(depending on bit-rate and
extent of delay-spread)
Fairly Complex(power control loop)
Comparing Comparing PerformancePerformance of TDMA, DS- of TDMA, DS-CDMA, & OFDM TransceiversCDMA, & OFDM Transceivers
Fade Margin(for mobile apps.)
Range
Re-use & Capacity
FEC Requirements
Variable Bit-rateSupport
Spectral Efficiency
TDMA OFDM
Required for mobileapplications
CDMA
Required for mobileapplications
Modest requirement(RAKE gain vs power-
control problems)
Range increase by reducing allowed noise rise (capacity)
Difficult to support large cells (PA , AGC limitations)
Modest (in TDMA) andHigh in MC-TDMA
Re-use planning iscrucial here
FEC is vital even forfixed wireless access
FEC is usually inherent (to increase code decorrelation)
FEC optional for voice
Powerful methodsto support VBR
(for fixed access)
Very High(& Higher Peak Bit-rates)
Modest
Modest
Low to modest support
Poor to Low
Very elegant methodsto support VBR & VAD
Very easy to increasecell sizes
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LTE vs. LTE-AdvancedLTE vs. LTE-Advanced
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LTE vs. LTE-AdvancedLTE vs. LTE-Advanced