Advanced Wireless Networks Lecture 08

1. Advanced Wireless Networks Lecture 08 OFDM (A), SC-FDMA Mr. Mohsin Khan MS (Telecommunication & Networking) BS (Telecommunication & Networking) DAE (Electrical Engineering ) E-mail:mohsin_btn@yahoo.com HTTP:/ telenetcoding.blogspot.com Mr. MOHSIN KHAN @ UOL Sargodha

2. Orthogonal frequency division multiplexing (OFDM) • Orthogonal frequency division • 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 KHAN @ UOL Sargodha

3. FDM  OFDM Mr. MOHSIN KHAN @ UOL Sargodha

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 KHAN @ 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 simultaneouslyover a number of orthogonal subcarriers. • The complex baseband OFDM signal • with N subcarriers can be written as Mr. MOHSIN KHAN @ UOL Sargodha

6. OFDM Mr. MOHSIN KHAN @ UOL Sargodha

7. OFDM Modulation and Demodulation using FFTs Mr. MOHSIN KHAN @ UOL Sargodha

8. Basic OFDMA System • OFDMAis 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 KHAN @ UOL Sargodha

9. User 1 User 2 User 3 User 4 User 1 User 2 User 3 User 4 User 1 User 2 User 3 User 4 • 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. Mr. MOHSIN KHAN @ 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 KHAN @ UOL Sargodha

11. Example of PAPR Mr. MOHSIN KHAN @ UOL Sargodha

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 KHAN @ 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 KHAN @ 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 KHAN @ 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 solutionis to use any PAPR reduction schemesBUT… Mr. MOHSIN KHAN @ UOL Sargodha

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 KHAN @ 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 KHAN @ 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 KHAN @ 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 Mr. MOHSIN KHAN @ UOL Sargodha

20. Precoder Based SLM-OFDM System Mr. MOHSIN KHAN @ UOL Sargodha

21. Partial Transmit Sequences Mr. MOHSIN KHAN @ UOL Sargodha

22. Precoding Based OFDM System Mr. MOHSIN KHAN @ UOL Sargodha

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 KHAN @ 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 Mr. MOHSIN KHAN @ UOL Sargodha

25. Hybrid MC/SC Radio Access System Mr. MOHSIN KHAN @ UOL Sargodha

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. Mr. MOHSIN KHAN @ UOL Sargodha

27. Basic STBC MIMO-OFDM System Model STBC MIMO-OFDM (2×2) system • 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 KHAN @ UOL Sargodha