OFDM Physical Layer – Fundamentals, Standards, & Advances

OFDM Physical Layer – Fundamentals, Standards, & Advances

Contents • Wireless Propagation -- Overview • OFDM Fundamentals • Comparing TDMA, CDMA, and OFDM • OFDM Standards • Case Study: IEEE 802.11a OFDM WLAN • Key Advances in Wireless Technology • Space-Time Processing for OFDM • Summary

Basics of Radio Propagation Exponential Power 0.1 -1 m (10-100 msecs) Short-term Fading Long-term Fading 10-100 m (1-10 secs) Distance

Multi-path Propagation r(t) = a0 s(t-t0) + a1 s(t-t1) + a2 s(t-t2) + a3 s(t-t3)

Multi-path Propagation -- contd. r(t) = a0 s(t-t0) + a1 s(t-t1) + a2 s(t-t2) + a3 s(t-t3) a0 t3 - t0 Impulse Response h(t) a3 time channel Output (Rx signal) Input (Tx signal) Frequency Response H(f) : Frequency Selective Fading Channel freq.

Gain (in volts) Fading Delay Spread rms = 5msecs 3.0 secs 2.0 secs 2.5 secs Frequency Selective Fading Time Frequency Selective Fading Channels can provide -- time diversity (can be exploited in DS-CDMA) -- frequency diversity (can be exploited in OFDM)

TDMA, CDMA, and OFDM Wireless Systems • Time Division Multiple Access (TDMA) is the most prevalent wireless access system to date • GSM, ANSI-136, EDGE, DECT, PHS, Tetra • Direct Sequence Code Division Multiple Access (DS-CDMA) became commercial only in the mid 90’s • IS-95 (A,B, HDR,1x,3x,...), cdma-2000 (3GPP2), W-CDMA (3GPP) • Orthogonal Frequency Division Multiplexing (OFDM) is perhaps the least well known • can be viewed as a spectrally efficient FDMA technique • IEEE 802.11A, .11G, HiperLAN, IEEE 802.16 OFDM/OFDMA options

TDMA (with FDMA) Principle Carriers Power Freq. Time-slots Time

Direct Sequence CDMA Principle(with FDMA) User Code Waveforms Power Freq. Time

OFDM (with TDMA & FDMA) Principle Tones Carriers Power Freq. Time-slots Time

Other Multiple Access Techniques • Multi-Carrier TDMA • DECT, PACS (picture archiving and communication systems) • Frequency Hopped Spread Spectrum • Bluetooth • CSMA/CA • IEEE 802.11 (1 or 2 Mbps standard) • DS-CDMA with Time Slotting • 3GPP W-CDMA TDD (Time Division Duplex) Packet Switched Air Interface is vital for high bit-rates and high capacity (for data users) -- GPRS, EDGE, etc.

What is an OFDM System ? • Data is transmitted in parallel on multiple carriers that overlap in frequency

Generic OFDM Transmitter OFDM symbol bits Serial to Parallel Pulse shaper FEC LinearPA IFFT & DAC Very expensive ! fc add cyclic extension view this as a time to frequency mapper Complexity (cost) is transferred back from the digital to the analog domain!

Add Cyclic Prefix Parallel/ Serial Serial/ Parallel IFFT OFDM Transmitter -- contd. • S/P acts as Time/Frequency mapper • IFFT generates the required Time domain waveform • Cyclic Prefix acts like guard interval and makes equalization easy (FFT-cyclic convolution vs channel-linear convolution)

Remove Cyclic Prefix Parallel/ Serial Serial/ Parallel FFT OFDM Receiver • Cyclic Prefix is discarded • FFT generates the required Frequency Domain signal • P/S acts like a Frequency/Time Mapper

Generic OFDM Receiver Slot & Timing AGC Sync. Error P/S and Detection Sampler FFT Recovery fc gross offset VCO Freq. Offset Estimation fine offset (of all tones sent in one OFDM symbol)

OFDM Basics • To maintain orthogonality where • = sub-carrier spacing • = symbol duration • If N-point IDFT (or FFT) is used • Total bandwidth (in Hz) = • = symbol duration after CP addition

Time T Condition for Orthogonality Base frequency = 1/T T= symbol period

OFDM Basics -- contd. • If the Cyclic Prefix > Max. Delay Spread, then the received signal after FFT, at the nth tone for the kth OFDM block can be expressed as • where • is additive noise • is channel frequency response

Tx Waveform over a OFDM Symbol(magnitude values, for 802.11a)

Sync Basis Functions(of equal height for single-ray channel) Shape gets upset by (a) Fine Frequency Offset (b) Fading

OFDM -- PHY layer tasks • Signals is sent throughwireless channel and encounter one or more of the following distortions: • additive white noise • frequency and phase offset • timing offset, slip • delay spread • fading (with or without LoS component) • co-channel interference • non-linear distortion, impulse noise, etc • OFDM is well suited for high-bit rate applications

Frequency Offset • Carrier recovery and tracking critical for OFDM • Offsets can be comparable to sub-carrier spacing in OFDM • Non-coherent detectors possible with differential coding • Residual freq. offset causes • constellation rotation in TDMA • loss of correlation strength over integration window in CDMA (thereby admitting more CCI or noise) • increased inter-channel interference (ICI) in OFDM • OFDM can easily compensate for gross freq. offsets (offsets which are an integral multiple of sub-carrier width)

Timing Synchronization • Timing recovery (at symbol level) is easily achieved in OFDM systems • Can easily overcome distortions from delay spread • Can employ non-coherent timing recovery techniques by introducing self-similarity • => very robust to uncompensated frequency offsets • If cyclic prefix is larger than the rms delay spread, range of (equally good) timing phases become available • => robust to estimation errors

Slot and Timing Synchronization in OFDM Example: 4 tones per slot (OFDM symbol) T Traffic Slot IFFT PA t T secs T/2 T Preamble/Control Slot t IFFT PA self-symmetry can be exploited for non-coherent timing recovery T secs zero tones Very easy synchronization !

Effect of Delay Spread • Typical rms delay spread in macro-cells • Urban : 1-4 msecs, • Sub-urban : 3-6 msecs • Rural (plain, open country) : 3-10 msecs • Hilly terrain : 5-15 msecs • TDMA requires equalization (even if rms delay spread is only 20-30% of symbol duration) • higher bit-rates would imply more Inter-Symbol Interference (ISI) • therefore, equalization complexity increases with bit rate

Effect of Delay Spread -- contd. 1 • Effect of delay spread on DS-CDMA is multi-fold • On the Uplink, the time diversity inherent in the delay spread can be used to mitigate fading • On the Downlink, multipath delay spread upsets channelization (short) code orthogonality • Sectorisation vital in CDMA to reduce CCI on the Uplink • However, sectorisation reduces delay spread as well, thereby reducing the RAKE performance

Effect of Delay Spread in OFDM • Delay spread easily compensated in OFDM using : • Cyclic Prefix (CP) which is longer than the delay spread • Thereby, converting linear convolution (with multipath channel) to effectively a circular convolution • enables simple one-tap equalisation at the tone level Example: IEEE 802.11 A (and also in HiperLAN) Data Payload CP 3.2msecs 0.8msecs However, the frequency selectiveness could lead to certain tones having very poor SNR=> poor gross error rate performance

Delay Spread Compensation in OFDM • Two basic ideas to combat freq. selectivity in OFDM • Feed-forward only techniques • Temporal FEC and interleaving • Transmit diversity and space-time coding • Feed-back based techniques (similar to approaches used in Multi-Carrier Modulation in the ADSL modems) • Water-pouring (bit-loading) • Pre-equalisation or pre-distortion • Sectorisation in macro-cell OFDM can help reduce delay spread

OFDM Receiver Algorithms -- Recap AGC Error P/S and Detection Sampler DFT Recovery Freq. -- Gross Freq. Offset -- Channel Estimation and Equalization -- Fine Freq. Offset -- Timing Estimation

Frequency Domain Equalisation -- Conventional OFDM Add CP Symbol Mapping & S/P Tx Mod. Conventional OFDM IDFT Frequency Domain Equaliser Remove CP Rx Algos. Detection & P/S DFT

Frequency Domain Equalisation -- Single Carrier FDE (SC-FDE) Add CP (of symbols) Tx Mod. Symbol Mapping Tx -- low-complexity, TDMA Rx -- implements SC-FDE; Linear Equaliser or DFE to permit FDE Frequency Domain Equaliser Remove CP Rx Algos. IDFT DFT Detector

Time & Frequency Domain Equalisation -- for OFDM in large delay spread channels Add CP Symbol Mapping & S/P TDE + FDE for OFDM Tx IDFT Frequency Domain Equaliser Remove CP Time- Domain Equaliser Detection & P/S DFT Rx

Fading and Antenna Diversity • Short-term fading exhibits spatial correlation • Two antennas, spaced l/4 meters or greater apart, fade independently • Spatial diversity combining can mitigate fading • Switch diversity (least complex, modest improvement) • Selection diversity • Equal gain combining • Maximal ratio combining (most complex, optimal) • TDMA, CDMA, and OFDM systems will invariably require antenna diversity to overcome fading • Antenna diversity is very good for the fading channel.

Fading and Channel Estimation • Use of midamble in GSM and EDGE to avoid channel tracking within the slot duration • Unlike in TDMA and OFDM, fading affects not only signal quality, but alsosystem capacity in DS-CDMA. - Countermeasures: • Fast closed-loop power control is required to track the short-term fading • For RAKE combining, multipath delays and gains are required to be estimated and tracked • By using orthogonal signaling, IS-95 uplink does not need gain estimation, but requires delay estimation. This is problem. • In OFDM systems, the long symbol duration makes channel estimation and tracking very important

Channel Estimation in OFDM -- Example Frame (e.g., 4 slots) • Control slot may also contain MAC messages. • Traffic slots may contain a few equally spaced tones for phase correction (due to residual freq. offset, phase noise, fading) Control + Training Slot Control + Training Slot Traffic Slot 3 Traffic Slot 2 Traffic Slot 1 Training Tones (for channel identification) Phase Correction Tones in traffic slot MAC message (broadcast) in control slot

Fading Compensation in OFDM • OFDM using a FDE, observes only “flat” fading at the sub-carrier level • Fading manifests as ICI terms in the Frequency Domain • In OFDM Phy Layer, two basic ways to reduce ICI 1) Reduce OFDM symbol duration (increase sub-carrier width) • 802.16 has FFT sizes ranging from 256 to 4096 2) Transmit pulse shaping can reduce ICI • (by providing excess “time-width”)

Other PHY Issues in OFDM • High peak-to-average ratio of the signal envelope • Linear Power Amp., with 5-8dB back-off required (costly) • To support mobility (fast fading), it will require • More training tones per symbol and also in every slot • Tx diversity and/or ST coding support • Exploit time, frequency, and space diversity / processing

Phy Layer Issues in Macro-cell OFDM • Macrocells will require larger cyclic extensions / prefix • Microcells may not be economical during initial deployment • GPS-locked base stations required • To control ACI from neighbor BS sites (at cell edge) • CCI can be estimated / controlled only if it is tone-aligned • Strict power control required may be required on uplink • To minimize cross-talk between tones of different users sharing the same OFDM symbol (time slot) • To avoid uplink power control • allocate only one user per uplink slot • or, make uplink a pure TDMA (not OFDM)

Phy Layer Issues in OFDMA • Strict power control required is required on uplink(OFDMA) • To minimize cross-talk between tones of different users sharing the same OFDM symbol (time slot) • To avoid uplink power control • allocate only one user per uplink slot (OFDM) • or, make uplink a pure TDMA (single-carrier)

MAC Layer Issues in Macro-Cell OFDM • Many proprietary broad-band FWA based on OFDM are configured as primarily data networks providing • Bridging functionality (Ethernet packets on air) • Routing functionality (IP packets on air) • Some of the key issues then are • How many modes (scheduling options) should MAC support? • How is voice and other streaming data to be handled? • Indeed, mixing of voice and data not good for statistical multiplexing • CDMA example – the new cdma2000 / HDR standard, where distinct voice-only and data-only base stations are proposed

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 Modest high Modest

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 , .11n • 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

OFDM Physical Layer – Fundamentals, Standards, & Advances