1. Advanced Wireless Networks
Lecture 08
OFDM (A), SC-FDMA
Mr. Mohsin Khan
MS (Telecommunication & Networking)
BS (Telecommunication & Networking)
DAE (Electrical Engineering )
telenetcoding.blogspot.com
Mr. MOHSIN UOL Sargodha

2. Orthogonal frequency division multiplexing (OFDM)
multiplexing (OFDM) is a practical
broadband signaling technique for use
in multipath fading channels.
Incorporated with MIMO, OFDM is
promising technology for higher capacity
multi-hop networks.
One of the main challenges of
incorporating OFDM into multi-hop
networking is distributed subcarrier
allocation among multiple users.
Mr. MOHSIN UOL Sargodha

3. FDM  OFDM
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4. WHY OFDM(A) System???
OFDM(A) has become technology of the choice for current and future generation wired and wireless networks like wideband digital communication, digital television, audio broadcasting, DSL broadband internet access, wireless networks, and 4G mobile communications (LTE-A, Mobile WiMAX etc).
OFDM(A) is spectrally efficient
IFFT/FFT operation ensures that sub-carriers do not interfere with each other.
OFDM(A) has an inherent robustness against narrowband interference.
Narrowband interference will affect at most a couple
of subchannels.
Information from the affected subchannels can be erased and recovered via the forward error correction (FEC) codes.
Equalization is very simple compared to Single-Carrier systems
OFDM(A) has excellent robustness in multi-path environments
Cyclic prefix preserves orthogonality between sub-carriers.
Mr. MOHSIN UOL Sargodha

5. Basic OFDM System Model
The OFDM system splits the high speed data stream into a number of parallel low data rate streams and these low rates data streams are transmitted simultaneously over a number of orthogonal subcarriers.
The complex baseband OFDM signal
with N subcarriers can be written as
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6. OFDM
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7. OFDM Modulation and Demodulation using FFTs
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8. Basic OFDMA System OFDMA is a multiple access version of OFDM systems.
The key difference among OFDM and OFDMA is that, instead of being allocated all of the available subcarriers, the base station assigns a subset of carriers to each user in order to accommodate several transmissions at the same time.
Mr. MOHSIN UOL Sargodha

9. There are two different approaches to do subcarrier mapping in OFDMA systems, localized subcarrier mapping and distributed subcarrier mapping.
Distributed subcarrier mapping can be further divided in to two modes, interleaved mode and random interleaved mode.
User 1
User 2
User 3
User 4
User 1
User 2
User 3
User 4
User 1
User 2
User 3
User 4
Mr. MOHSIN UOL Sargodha

10. OFDM(A) Drawbacks Sensitivity to inter-carrier interference( ICI)
The OFDM time-domain signal has a relatively high Peak-to-Average Power Ratio (PAPR)
Tends to reduce the power efficiency of the RF amplifier
Non-linear amplification destroys the orthogonality of the OFDM signal and introduced out-of-band radiation
Conclusion:-OFDM is spectral efficient, but not power efficient (due to linearity requirements of power amplifier=the PAPR-problem).
Mr. MOHSIN UOL Sargodha

11. Example of PAPR
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12. PAPR Problems
A large PAPR corresponds to a high probability of the OFDM
signal being clipped when passing through a power amplifier
at the end of the transmitter.
Clipping reduces the signal power, degrading bit error rate
(BER) performance (in-band distortion) and causing
nonlinear phenomena such as spectral spreading.
Spectral spreading causes degradation of spectral efficiency
(out-of-band radiation).
High PAPR increases complexity of A to D and D to A
convertors and also reduces the efficiency of RF power
amplifier.
Mr. MOHSIN UOL Sargodha

13. PAPR Problems OFDM Requires
Linear Amplifiers and high dynamic range in the transmitter hardware.
Expensive
Inefficient - Consumes more power
If non-linear amplifiers are used
Spectral spread (Out of band radiation)
In-band distortion
Mr. MOHSIN UOL Sargodha

14. So How Can we Fix (Alleviate) It?????
Many methods to reduce the PAPR have been proposed. Each has their own advantages and drawbacks.
Certain properties are desirable
No loss in data rate or side information
Distortionless
Low complexity
The Tradeoffs of any PAPR reduction method
Complexity
Distortion introduced
Reduced Bandwidth/Data Rate
Increased Average Power
Mr. MOHSIN UOL Sargodha

15. PAPR Problems and Solutions
A large PAPR corresponds to a high probability of the OFDM signal being
clipped when passing through a power amplifier at the end of the
transmitter.
Clipping reduces the signal power, degrades bit error rate (BER)
performance (in-band distortion) and causes nonlinear phenomena such
as spectral spreading.
Spectral spreading causes degradation of spectral efficiency (out-of-band
radiation).
High PAPR also increases complexity of A to D and D to A
convertors and reduces the efficiency of RF power amplifier.
One solution is to use Linear Amplifiers with high dynamic range in the
transmitter hardware but these are Expensive and Inefficient (i.e.
Consumes more power). Also, the use of non-linear
amplifiers results in Spectral spread (Out of band radiation) and In-band
distortion.
Another solution is to use any PAPR reduction schemes BUT…
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16. Popular PAPR Reduction Methods
Clipping and filtering and non-linear distortion
In-band distortion is mostly negligible. But out of band distortion is more serious
Multiple signal representation
Partial transmit signalling.
Selected mapping.
Constellation optimization
Tone Reservation.
Tone injection.
Active constellation extension: Similar to TI
Mr. MOHSIN UOL Sargodha

17. Popular PAPR Reduction Methods
Coding
The idea is to select a codeword with less PAPR. It still is an open problem to construct codes with both low PAPR and short Hamming distance
Golay and Reed-Muller codes have shown PAPR reduction properties, but at a significant rate reduction.
Mr. MOHSIN UOL Sargodha

18. Selected Mapping
Define U distinct fixed phase vectors of type of length N.
Data vector is multiplied with U vectors resulting U, statistically independent alternative OFDM symbols.
Each symbol is transformed into time domain by taking IDFT.
The symbol with lowest PAR is selected for the transmission.
No analytical method is present to find proper set of vectors
Problem: Reliable side information is required at the receiver
Mr. MOHSIN UOL Sargodha

19. SLM Based OFDM System
The SLM is a PAPR reduction technique which is based on phase rotations. In SLM-OFDM system, a set of V different data blocks are formed at the transmitter representing the same information and a data block with minimum PAPR is selected for transmission
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20. Precoder Based SLM-OFDM System
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21. Partial Transmit Sequences
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22. Precoding Based OFDM System
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23. Single Carrier FDMA (SC-FDMA)
A major problem with OFDM and OFDMA is high peak-to-average power ratio (PAPR)
Transmitted amplitude with large variation
Requires a linear amplifier at transmitter
Linear amplifies consumes high power
For mobile station, this consumes battery
LTE uses a solution for UL: SC-FDMA
Single carrier transmission
Mr. MOHSIN UOL Sargodha

24. SC-FDMA Process After modulation, apply FFT
Each symbol is on a subcarrier
Put the subcarriers on selected location and apply IFFT
Back to single carrier transmission
Now add CP
Receiver will do the reverse
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25. Hybrid MC/SC Radio Access System
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26. Space-Time Block Code (STBC)
The STBC achieves a full diversity gain by performing a simple maximum-likelihood (ML) decoding algorithm. The (2Tx × 2Rx) orthogonal STBC can be defined as:-
Alamouti encoded signal is transmitted from the two transmit antennas over
two symbol periods.
During the first symbol period, two symbols and are simultaneously
transmitted from the two transmit antennas.
During the second symbol period, these symbols are transmitted again, where,
is transmitted from first transmit antenna and is transmitted from
second transmit antenna.
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27. Basic STBC MIMO-OFDM System Model
OFDM may be combined with multiple-input multiple-output (MIMO) to
increase the diversity gain and system capacity over the time-variant
frequency-selective channels.
However, a major drawback of MIMO-OFDM system is that the
transmitted signals on different antennas might exhibit high peak to
average power ratio (PAPR).
Mr. MOHSIN UOL Sargodha