1. ECE 6332, Spring, 2014 Wireless Communications
                                                           
Zhu Han
Department of Electrical and Computer Engineering
Class 23
April 16th, 2014

2. OFDM 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

3. 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)

4. OFDM

5. Transmitted Symbol To have ISI-free channel, Tsymbol>>τ
In OFDM, each symbol has T =Ts L >> τ
Guard interval between OFDM symbols Tg>> τ ensures no ISI between the symbols.

6. 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.

7. Mathematical description

8. Mathematical description

9. OFDM Timing Challenge

10. OFDM bit loading Map the rate with the sub-channel condition
Water-filling

11. OFDM Time and Frequency Grid
Put different users data to different time-frequency slots

12. OFDM Transmitter and Receiver

13. OFDM

14. 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

15. Pro 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.

16. OFDM Applications
ADSL and VDSL broadband access via telephone network copper wires.
IEEE a and g 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 or WiMax Wireless MAN standard.
The IEEE 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.

17. Applications
WiMax
Digital Audio Broadcast (DAB)
Wireless LAN

18. Applications High Definition TV (HDTV)
4G Cellular Communication systems
Flash -OFDM

19. Proprietary OFDM Flavours
Wireless Access (Macro-cellular)
Wideband-OFDM
(W-OFDM) of Wi-LAN
Flash OFDM
from Flarion
Vector OFDM
(V-OFDM) of Cisco, Iospan,etc.
-- Freq. Hopping for
CCI reduction, reuse
to 5.0MHz BW
-- mobility support
GHz band
Mbps in 40MHz
-- large tone-width
(for mobility, overlay)
-- 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 Wireless MAN
Cisco leads the Broadand Wireless Internet Forum (BWIF)

20. OFDM based Standards Wireless LAN standards using OFDM are
HiperLAN-2 in Europe
IEEE a, .11g
OFDM based Broadband Access Standards are getting defined for MAN and WAN applications
Working Group of IEEE
single carrier, 10-66GHz band
802.16a, b GHz, MAN standard

21. Key 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

22. Calculations for 802.16a -- Example: 5MHz

23. Broadband Access Standards -- contd.
IEEE LAN and MAN standards
IEEE
(10 to 66 GHz)
IEEE a,b
(2 to 11 GHz)
1-3 miles, non-LoS
IEEE a or
.11b, or .11g
2-5 miles, LoS(> 11GHz)

24. The IEEE a/g Standard
Belongs to the IEEE system of specifications for wireless LANs.
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

25. The IEEE Standard

26. Evolution of Radio Access Technologies
In Nov. 2004, 3GPP began a project to define the long-term evolution (LTE) of Universal Mobile Telecommunications System (UMTS) cellular technology
802.16m
802.16d/e
LTE (3.9G) : 3GPP release 8~9
LTE-Advanced : 3GPP release 10+

27. LTE vs. LTE-Advanced

28. DS-CDMA versus OFDM DS-CDMA can exploit a0 time-diversity Impulse
Response h(t) a3
time
channel
Input
(Tx signal)
Output
(Rx signal)
Frequency
Response H(f)
OFDM can exploit
freq. diversity
freq.

29. Comparing Complexity of TDMA, DS-CDMA, & OFDM Transceivers
Easy, but requires
overhead (sync.) bits
Difficult, and requires
sync. channel (code)
Very elegant, requiring
no extra overhead
Timing Sync.
Easy, but requires
overhead (sync.) bits
Gross Sync. Easy
Fine Sync. is Difficult
Freq. Sync.
More difficult than TDMA
Complexity is high in
Asynchronous W-CDMA
Usually not required
within a burst/packet
Timing Tracking
Modest Complexity
Freq. Tracking
Easy, decision-directed
techniques can be used
Modest Complexity
(using dedicated correlator)
Requires CPE Tones
(additional overhead)
Channel
Equalisation
Modest to High Complexity
(depending on bit-rate and
extent of delay-spread)
RAKE Combining in CDMA
usually more complex than
equalisation in TDMA
Frequency Domain
Equalisation is very easy
Analog Front-end
(AGC, PA, VCO, etc)
Complexity or cost is
very high (PA back-off
is necessary)
Very simple
(especially for CPM signals)
Fairly Complex
(power control loop)

30. Comparing Performance of TDMA, DS-CDMA, & OFDM Transceivers
Fade Margin
(for mobile apps.)
Modest requirement
(RAKE gain vs power-
control problems)
Required for mobile
applications
Required for mobile
applications
Range
Very easy to increase
cell sizes
Range increase by reducing
allowed noise rise (capacity)
Difficult to support large
cells (PA , AGC limitations)
Modest (in TDMA) and
High in MC-TDMA
Re-use planning is
crucial here
Re-use & Capacity
Modest
FEC Requirements
FEC is usually inherent (to
increase code decorrelation)
FEC is vital even for
fixed wireless access
FEC optional for voice
Variable Bit-rate
Support
Powerful methods
to support VBR
(for fixed access)
Very elegant methods
to support VBR & VAD
Low to modest support
Very High
(& Higher Peak Bit-rates)
Spectral Efficiency
Modest
Poor to Low

31. LTE vs. LTE-Advanced

32. LTE vs. LTE-Advanced