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Wireless Local Networks are Emerging

Wireless LAN. Wireless Local Networks are Emerging. Hiperlan-2, IEEE802.11a, MMAC. Wireless OFDM Transceivers. Luc Deneire deneire@i3s.unice.fr Laboratoire I3S http://www.i3s.unice.fr/. “Future” Broadband Wireless Networks will be OFDM based. 802.11a Hiperlan-II MMAC. 100M 10M 1M

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Wireless Local Networks are Emerging

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  1. Wireless LAN Wireless Local Networks are Emerging Hiperlan-2, IEEE802.11a, MMAC

  2. Wireless OFDM Transceivers Luc Deneire deneire@i3s.unice.fr Laboratoire I3S http://www.i3s.unice.fr/

  3. “Future” Broadband Wireless Networks will be OFDM based 802.11a Hiperlan-II MMAC 100M 10M 1M 100k 10k OFDM link adaptation 6 to 54 Mbit/s for multimedia communication 802.11b Bluetooth HomeRF Spread spectrum time 1999 2000 2001 2002

  4. What you will learn ... • Indoor Propagation • Basic OFDM concepts • OFDM performance • Adaptive loading • The Hiperlan-2 OFDM system • Implementation of Hiperlan-2 Transceivers • Crest factor reduction

  5. Indoor Propagation Model

  6. Multipath TX Direct path RX Metal cupboards Office tables Multipath channel in an office room.

  7. The channel : a collection of delayed, attenuated and dephased diracs Channel Power delay profile P t

  8. Channels differ in time and frequency behavior • Coherence bandwith - delay spread • spreading in time • widening of impulse response due to multipath • Coherence time - doppler spread • spreading in frequency • doppler effect of moving transmitter and/or receiver

  9. Delay spread

  10. Delay spread measures the “length” of the channel • RMS delay spread is measure of the amount of dispersion • 10 to 100ns correspond to paths 3 to 30m

  11. Coherence bandwidth is where the channel is “similar”(correlated) • Autocorrelation of channel response • Bcoh is defined as Df for which

  12. Coherence bandwith is inversely proportional to delay spread

  13. Frequency selective fading…. where bandwith is large …. Compared to the coherence bandwith • W Bcoh frequency selective channel • W Bcoh frequency nonselective channel Bcoh W Bcoh W

  14. Frequency selective channelsintroduce Inter Symbol Interference Incoming signal Channel impulse response Outcoming signal

  15. Coherence Time (10-50 ms indoor): time in which a channel is “stable” S(f) Fourier Signals sent at these instants see uncorrelated channels

  16. Subsequent symbols see different channels in fast fading • Tb  (t)c  fast-fading channel • Tb  (t)c  slow-fading channel

  17. This is wherewe will try tofit the sub-carriers inOFDM. Propagation overviewSummary of channel properties Time dispersion Frequency selectiveLong channel Frequency flatShort channel Using the previousmeasures oncharacteristics wecan place radiochannels in fourgroups. NOTE that theclassificationis in relation tothe transmissionbandwidth/symbol-time. ISI-free and flat-fading channel ISI and flat-fading channel Slow fadingLow Doppler Frequency dispersion ISI-free and fast-fading channel ISI and fast-fading channel Fast fadingHigh Doppler

  18. Question • Assume a wireless system making use of BPSK modulation at 10Mbps. • The system is used indoor. There are two signal paths between Tx and Rx with a relative distance of 10m. • How many symbols are affected by the channel? • What happens if the relative distance becomes 100m? What if the datarate becomes 100Mbps?

  19. Answer • Datarate 10Mbps • Tsymbol=100ns • Distance 10m • delay = distance / c = 10 / 3.108 s = 30ns • delay / Tsymbol = 0.3 • For 100m • delay / Tsymbol = 3 • For 100m, 100Mbps • delay / Tsymbol = 30

  20. What to do against ISI? • Wideband signals: • channel delay = many symbol periods • heavy distortion of the received signal. • Several techniques can be applied to reduce or get rid of ISI in wideband signal transmission • equalization, • spread-signal modulation, • OFDM

  21. f f f An Equalizer is a costly filter Signal (channel) spectrum t Equalizer t Equalized signal t

  22. OFDM avoids ISI by slowing pace needs linear amp + sync • Symbols of high bit rate signal are distributed over a large number of subcarriers. • Low symbol rate per carrier. • Individual carrier signals see flat fading (no ISI). • Promising technique for future high bit-rate applications. • However, it suffers from a number of problems: • a very linear amplifier in the transmitter is required to prevent signal distortion, • accurate synchronization in the receiver is needed, • in the transmitter and receiver real-time discrete Fourier transform (DFT) operations have to be computed.

  23. OFDMbasic principles

  24. f1 f2 fn OFDM is Multi-Carrierand lowers the symbol rate : less ISI ... T / sec f T/n / sec

  25. OFDM : Overlapping spectra to save bandwith

  26. Overlapping spectra are orthogonalto enable proper reception of individual carriers Orthogonality to avoid inter carrier interference: signal design + frequencies

  27. Recent applications of OFDM • high-bit-rate digital subscriber lines (HDSL; 1.6 Mbps), • asymmetric digital subscriber lines (ADSL; up to 6 Mbps), • very-high-speed digital subscriber lines (VDSL; 100 Mbps), • digital audio broadcasting (DAB), • high definition television (HDTV) terrestrial broadcasting, • WLAN (6-54Mbps) indoor communication (IEEE802.11a/g, ETSI Hiperlan/2)

  28. Advantages of OFDM • OFDM deals with multipath At low COST (implementation) • OFDM enables adaptive loading : Bit rate RISES with SNR ON EACH carrier • OFDM is robust against narrowband interference, Inteference affects only part of the carriers.

  29. Disadvantages of OFDM • sensitive to frequency offset and phase noise. • large peak-to-average power ratio, ==> low power efficiency of the RF amplifier.

  30. Parameters for designing an OFDM System • number of subcarriers, • guard time, • symbol duration, • subcarrier spacing, • modulation type per subcarrier, • the type of forward error correction coding

  31. Choice of parameters is influenced by system requirements • available bandwidth, • required bit rate, • tolerable delay spread and • Doppler values

  32. OFDM modulation can be realized with IFFT • An OFDM signal consists of a sum of subcarriers which are modulated by using Phase Shift Keying (PSK) or Quadrature Amplitude Modulated (QAM). OFDM modulator block diagram

  33. Time domain view of OFDM • All subcarriers have the same phase and amplitude, but in practice the amplitudes and phases may be modulated differently for each subcarrier. Example of 4 subcarriers within one OFDM symbol.

  34. The OFDM spectrum fulfills Nyquist’s criterium for an inter-symbol interference free pulse shape

  35. Impact of channel on OFDM Reception • Multipath channel spreads energy of one symbol into adjacent symbol. Results in ISI between symbols • Solutions • make symbols longer by using more carriers, ISI neglegible. But, negative impact due to coherence time, FFT size and latency • use guard interval between symbols

  36. Principle of guard interval

  37. Transmitters and receivers... through the channel ... Channel Noise } CP CP CP CP } } As long as the CP is longer than the delay spread of thechannel, the CP will absorb the ISI.

  38. Guard time reduces ISI • The most important reasons to do OFDM is the efficient way it deals with multipath delay spread. By dividing the input data stream in Ns subcarriers, the symbol duration is made Ns times larger, which also reduces the relative multipath delay spread - relative to the symbol time - by the same factor. • To eliminate intersymbol interference almost completely, a guard time is introduced for each OFDM symbol.

  39. What to transmit during guard interval? • guard time > delay spread • multipath components from one symbol cannot interfere with the next symbol. • The guard time could consist of no signal at all. However, in that case the problem of inter carrier interference (ICI) would arise. ICI is cross-talk between different subcarriers, which means they are no longer orthogonal.

  40. Effect of multipath with zero signal in the guard time; the delayed subcarrier #2 causes inter carrier interference (ICI) on subcarrier #1 and vice-versa.

  41. Cyclic extension in guard Delayed replicas of OFDM symbols have integer number of cycles in FFT interval No ICI if guard is longer than signal delay Guard time with cyclic extension

  42. Example of an OFDM signal with 3 subcarriers in a 2-ray multipath channel. The dashed line represents a delayed multipath component. No crosstalk (ICI) between carriers, but distortion per carrier. Freq domain equalization needed.

  43. Implementation complexity of OFDM vs single carrier modulation • OFDM has the ability to deal with large delay spreads with a reasonable implementation complexity. Frequency domain equalizer needed. • In a single carrier system, the implementation complexity is dominated by equalization, which is necessary when the delay spread is larger than about 10% of the symbol duration.

  44. Implementation complexity of OFDM vs single carrier modulation (cont.) • For Single carrier systems with equalizers, the performance degrades abruptly if the delay spread exceeds the value for which the equalizer is designed and because of error propagation, the raw bit error probability increases so quickly that introducing lower rate coding or a lower constellation size does not significantly improve the delay spread robustness. • For OFDM, there are no such nonlinear effects as error propagation, and coding and lower constellation sizes can be employed to provide fallback rates that are significantly more robust against delay spread. This enhances the coverage area and avoids the situation that users in bad spots cannot get any connection at all.

  45. OFDM Performance

  46. OFDM Performance: Assumptions • The impulse response of the channel is shorter than the cyclic prefix • Transmitter and receiver are perfectly synchronised • Channel noise is additive, white and Gaussian • The fading is slow enough to consider the channel constant during one OFDM symbol

  47. OFDM Performance: Transmitter s0,k x0,k IDFT P to S Add Cyclic Prefix s(t) s1,k x1,k Htr xN-1,k sN-1,k xk

  48. + OFDM Performance: Channel n(t) s(t) Hch r(t) Channel Input Cyclic prefix IFFT Channel Output Tch

  49. OFDM Performance: Receiver q(N+v)T+pT r0,q y0,q Remove Cyclic Prefix S to P D F T r(t) y1,q r1,q Hre r’(t) yN-1,q rN-1,q yq

  50. OFDM Performance: Combined Model • Combine transmit, channel and receive filters • Received Signal:

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