ECE 6332, Spring, 2014 Wireless Communications

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# ECE 6332, Spring, 2014 Wireless Communications - PowerPoint PPT Presentation

ECE 6332, Spring, 2014 Wireless Communications. Zhu Han Department of Electrical and Computer Engineering Class 23 April 16 th , 2014. OFDM Basic Idea. Orthogonal frequency-division multiplexing Divide a high bit- rate stream into several low bit- rate streams ( serial to parallel)

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## ECE 6332, Spring, 2014 Wireless Communications

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1. ECE 6332, Spring, 2014Wireless 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

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 802.11a and 802.11gWireless 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.

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) Flash OFDM from Flarion www.flarion.com Vector OFDM (V-OFDM) of Cisco, Iospan,etc. www.iospan.com Wideband-OFDM (W-OFDM) of Wi-LAN www.wi-lan.com -- Freq. Hopping for CCI reduction, reuse -- 1.25 to 5.0MHz BW -- mobility support -- 2.4 GHz band -- 30-45Mbps 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 802.16 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 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

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 802.16 (10 to 66 GHz) IEEE 802.16a,b (2 to 11 GHz) 1-3 miles, non-LoS IEEE 802.11a or .11b, or .11g 2-5 miles, LoS(> 11GHz)

24. The 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

25. The IEEE 802.11 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 • LTE (3.9G) : 3GPP release 8~9 • LTE-Advanced :3GPP release 10+ 802.16m 802.16d/e