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- 1. WIMAX –PHY Layer 802.16-2004 air interface Mehul Bhandari
- 2. Outline: Worldwide Interoperability for Microwave access ( WIMAX ) <ul><li>Introduction </li></ul><ul><li>Technical Specifications </li></ul><ul><li>Channel characteristics </li></ul><ul><li>OFDM </li></ul><ul><li>Diversity (Space-time) </li></ul><ul><li>Simulations </li></ul><ul><li>Conclusions </li></ul><ul><li>Future work </li></ul>
- 3. Introduction <ul><li>Wimax is an IEEE standard that stands for Worldwide Interoperability for Microwave Access. Nowadays, it is normally associated with the mobile version while the fixed version is represented with the 802.16-2004 air interface which was ratified in the year 2004. </li></ul><ul><li>It is a broadband wireless access that promises to work under NLOS conditions in the range of 2-11 GHz in both licensed and unlicensed bands. The current most popular band is 3.5 GHz. </li></ul><ul><li>It is supported by IEEE and WIMAX Forum, a group comprising of companies like Intel, Siemens, Alvarion etc that are the key driving forces behind this certification of standard. </li></ul>
- 4. Technical Specifications Parameters implemented FFT-length N FFT 256 No. of subcarriers Nc 256 Bandwidth B 6 MHz Subcarrier spacing f sc B/ Nc = 23.437 kHz OFDM symbol duration T OFDM 1/∆ f sc = 42.7 µsec Sampling time T s T OFDM / N FFT = 0.166 µsec Guard interval length 16 samples = 2.66 µsec Modulation M = 2,4 BPSK,QPSK Detection MRC Channel coding FEC RS(255,239,8) outer coded concatenated with (133,171) 8 convolutional code, rate ½, Viterbi decoding Delay profile 3-tap, SUI-3,power decay Max. delay profile max 0.9 µsec Doppler frequency ƒ D 6.6 Hz (3km/hr@ 2.4GHz) Channel estimation Perfect
- 5. TECHNICAL SPECIFICATIONS
- 6. Channel Characteristics (Channel Response) <ul><li>Space–Time–Energy </li></ul><ul><li>Wireless communication phenomena experiences scattering of electromagnetic waves from surfaces or diffraction over and around buildings. </li></ul><ul><li>The design goal is to make the received power adequate to overcome background noise over each link, while minimizing interference to other more distant links operating at the same frequency. </li></ul>
- 7. Attenuative and Distortive agents : Path loss Fading <ul><li>Path Loss- </li></ul><ul><li>Natural phenomenon </li></ul><ul><li>P=10 n log (D)+C </li></ul><ul><li>Where </li></ul><ul><li>n->path loss exponent varying from 2 to 4 according to the environment </li></ul><ul><li>D->Distance between the Tx and Rx </li></ul><ul><li>C->constant accounting for penetrative losses in various obstacles in the path </li></ul><ul><li>Fading- </li></ul><ul><li>More of a distortive agent </li></ul><ul><li>Narrowband Fading </li></ul><ul><li>Wideband Fading </li></ul>
- 8. <ul><li>Narrow Band Fading (Flat Fading) </li></ul><ul><li>Makes the signal experience </li></ul><ul><li>a similar fading across the </li></ul><ul><li>Narrow bandwidth. </li></ul><ul><li>Flat fading is caused by </li></ul><ul><li>absorbers between the two </li></ul><ul><li>antennae and is countered </li></ul><ul><li>by antenna placement </li></ul><ul><li>and transmit power level. </li></ul>
- 9. <ul><li>Wide Band Fading Frequency-Selective/Multi path fading </li></ul><ul><li>Frequency selective fading is </li></ul><ul><li>caused by various scattering </li></ul><ul><li>elements between the </li></ul><ul><li>transmitter and receiver </li></ul><ul><li>creating Multi path effects. </li></ul><ul><li>Different frequency bands </li></ul><ul><li>(in a wideband channel) </li></ul><ul><li>experiences different fading </li></ul><ul><li>affects due to multiple </li></ul><ul><li>scattering elements, hence </li></ul><ul><li>called Frequency – Selective. </li></ul>
- 10. Realisation of Fading <ul><li>Coherence Time : </li></ul><ul><li>The time Interval in </li></ul><ul><li>which the propagating </li></ul><ul><li>wave may be </li></ul><ul><li>considered coherent </li></ul><ul><li>or the fade </li></ul><ul><li>experienced by the </li></ul><ul><li>signal is predictable. </li></ul><ul><li>Often used to describe </li></ul><ul><li>time variant and time </li></ul><ul><li>invariant channel. </li></ul><ul><li>High, the better. </li></ul><ul><li>Coherence Frequency </li></ul><ul><li>The maximum bandwidth over which </li></ul><ul><li>the channel is considered to be flat </li></ul><ul><li>or predictable. </li></ul><ul><li>Within the Coherence bandwidth, it </li></ul><ul><li>shows correlation of 0.9 </li></ul><ul><li>(statistical measure) , high enough </li></ul><ul><li>to induce ISI. </li></ul><ul><li>Low, the better. </li></ul>
- 11. Realisation of Fading <ul><li>Time Domain </li></ul><ul><li>Flat: </li></ul><ul><li>T s > Delay Spread </li></ul><ul><li>Freq-Selective </li></ul><ul><li>T s <Delay Spread </li></ul><ul><li>T s ->Symbol time </li></ul><ul><li>Delay Spread-> </li></ul><ul><li>The time difference between the arrival </li></ul><ul><li>moment of the first multi path </li></ul><ul><li>Component and the last one, is called </li></ul><ul><li>delay spread. </li></ul><ul><li>Frequency Domain </li></ul><ul><li>Flat: </li></ul><ul><li>W>Bc </li></ul><ul><li>Freq- Selective </li></ul><ul><li>W<Bc </li></ul><ul><li>W->Symbol rate </li></ul><ul><li>Bc->Coherence Bandwidth </li></ul>
- 12. OFDM <ul><li>The main idea is to </li></ul><ul><li>divide available </li></ul><ul><li>wideband frequency </li></ul><ul><li>into many narrow </li></ul><ul><li>bands, so as to </li></ul><ul><li>maintain or create . </li></ul><ul><li>Spectrum efficiency </li></ul><ul><li>Propagation in NLOS conditions </li></ul><ul><li>Propagation in Multi path fading. </li></ul><ul><li>Scalability </li></ul>
- 13. OFDM <ul><li>Requirement for OFDM? </li></ul><ul><li>Alternative FDM? </li></ul><ul><li>Wimax Standard </li></ul>
- 14. OFDM <ul><li>Orthogonality </li></ul><ul><li>Flat Fading </li></ul><ul><li>Multi-path Fading </li></ul>
- 15. OFDM <ul><li>ISI </li></ul>
- 16. OFDM
- 17. OFDM <ul><li>Power Spectrum </li></ul><ul><li>CYCLIC PREFIX </li></ul>
- 18. OFDM <ul><li>Fourier Transform </li></ul><ul><li>IFFT </li></ul><ul><li>The inverse DFT ensures that the spacing and the spectral shape of </li></ul><ul><li>the sub carriers are such that the spectra of the individual sub </li></ul><ul><li>carriers is zero at the other sub carriers. That is, the sub channels </li></ul><ul><li>are orthogonal. </li></ul><ul><li>Always Muliplicative of 2^4 because Radix-FFT is efficient. So, </li></ul><ul><li>Normally, it is 64 or 256, depending on data rate, channel size, </li></ul><ul><li>channel delay spread. </li></ul><ul><li>FFT </li></ul><ul><li>Inverse of IFFT at the receiver. Conversion of time domain into frequency </li></ul><ul><li>domain. </li></ul>
- 19. OFDM Some other aspects <ul><li>Reed Solomon coding :FEC technique </li></ul><ul><li>Interleaving: </li></ul><ul><li>Preamble and pilot inserting techniques can help in estimation of channel. </li></ul><ul><li>Randomization: The scrambled data (padding) is added at the end of each transmission block and repeated for each new block. </li></ul>
- 20. OFDM Problems associated <ul><li>PAPR: Peak to Average Power Ratio </li></ul><ul><li>Synchronisation: </li></ul>
- 21. DIVERSITY <ul><li>SIMPLE DIVERSITY (2-BRANCH) TECHNIQUE- </li></ul><ul><li>NO B/W EXPANSION REQUIRED </li></ul><ul><li>COMPLEXITY AT BS IS REQUIRED </li></ul><ul><li>MRRC TECHNIQUE IS APPLIED except for the combiner </li></ul><ul><li>RESULTING DIVERSITY GAIN IS SIMILAR TO MRRC WITH ONE RECIEVER EQUAL TO THAT OF 2-BRANCH MRRC. </li></ul>
- 22. DIVERSITY
- 23. SIMULATIONS: FLAT FADING VS FREQUENCY SELECTIVE
- 24. SIMULATIONS: 2 TRANSMIT VS 1 TRANSMIT ANTENNA(DIVERSITY)
- 25. SIMULATIONS: EFFECT OF CYCLIC PREFIX
- 26. SIMULATIONS: EFFECT OF CONVOLUTIONAL CODE
- 27. SIMULATIONS: SNR v/s FER Reed Solomon and BPSK
- 28. SIMULATIONS: SCHEME OF MODULATION HAS NO EFFECTS
- 29. SIMULATIONS: WIMAX SUI 2 TRANSMIT VS 1 TRANSMIT
- 30. CONCLUSIONS <ul><li>THE WIMAX PLATFORM HAS BEEN EFFECTIVELY IMPLEMENTED BY EVALUATING THE PERFORMANCE AT EACH STAGE ESPECIALLY INVOLVING THE OFDM AND STBC IN MULTIPATH FADING. </li></ul>
- 31. FUTURE WORK <ul><li>Implementation for different rates and </li></ul><ul><li>modulation is needed to evaluate the </li></ul><ul><li>Performance for different modes to </li></ul><ul><li>analyze the performance and see the </li></ul><ul><li>adaptivity and scalability affects. </li></ul>
- 32. References <ul><li>IEEE Std 802.16-2004- Air interface for fixed broadband wireless access systems </li></ul><ul><li>A simple Transmit Diversity Technique for Wireless Communications- Sivash M Alamouti </li></ul><ul><li>Antennas and Propagation for Wireless Communications- Simon Saunders </li></ul>
- 33. THANK YOU

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