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, 2014Wireless Communications Zhu Han Department of Electrical and Computer Engineering Class 23 April 16th, 2014
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
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)
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.
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.
OFDM bit loading • Map the rate with the sub-channel condition • Water-filling
OFDM Time and Frequency Grid • Put different users data to different time-frequency slots
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
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.
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.
Applications • WiMax • Digital Audio Broadcast (DAB) • Wireless LAN
Applications • High Definition TV (HDTV) • 4G Cellular Communication systems • Flash -OFDM
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)
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
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
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)
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
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
Output (Rx signal) Input (Tx signal) channel DS-CDMA versus OFDM DS-CDMA can exploit time-diversity a0 Impulse Response h(t) a3 time Frequency Response H(f) OFDM can exploit freq. diversity freq.
Comparing Complexity of TDMA, DS-CDMA, & OFDM Transceivers TDMA CDMA OFDM Difficult, and requires sync. channel (code) Very elegant, requiring no extra overhead Easy, but requires overhead (sync.) bits 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 Complexity or cost is very high (PA back-off is necessary) Analog Front-end (AGC, PA, VCO, etc) Very simple (especially for CPM signals) Fairly Complex (power control loop)
Comparing Performance of TDMA, DS-CDMA, & OFDM Transceivers TDMA CDMA OFDM Fade Margin (for mobile apps.) Modest requirement (RAKE gain vs power- control problems) Required for mobile applications Required for mobile applications Range increase by reducing allowed noise rise (capacity) Range Very easy to increase cell sizes 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 Poor to Low Modest