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Wireless Access Systems: Introduction and Course Outline

Wireless Access Systems: Introduction and Course Outline. Wireless Access Systems provide short to medium range tetherless access to a backhaul network, a central unit or peer nodes Examples include Bluetooth WLAN Vehicular Networks WiMax RFID WBAN WPAN. Some Wireless Access Systems.

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Wireless Access Systems: Introduction and Course Outline

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  1. Wireless Access Systems:Introduction and Course Outline

  2. Wireless Access Systems provide short to medium range tetherless access to a backhaul network, a central unit or peer nodes Examples include Bluetooth WLAN Vehicular Networks WiMax RFID WBAN WPAN Some Wireless Access Systems Communication Technology Laboratory Wireless Communication Group

  3. Wireless Access Systems are Ubiquitous Internet Communication Technology Laboratory Wireless Communication Group

  4. Some More Applications intelligent home ambient intelligence security wearable computing shopping defence surveillance traffic security surveillance access control supply chain management environment Internet logistics industrial communications instant messaging enterprise communication information exchange pervasive computing virtual reality health care home care Communication Technology Laboratory Wireless Communication Group

  5. Heterogeneous nodes • RFID tags, readers • sensors, actors • communication appliances • information access • information processing • backhaul access points RFID WLAN Sensor network WPAN Bluetooth Heterogeneous standards cellular: GSM UMTS Internet backhaul • IEEE 802.11 WLAN • IEEE 802.15 WPAN • IEEE 802.16 WMAN • (Hiperlan) • Bluetooth • DECT • various RFID standards WMAN Lots of spectrum (approx.) • 100MHz@2.45GHz (ISM) • 150MHz@5.8GHz (ISM) • 200MHz@17.2GHz (ISM) • 250MHz@24.125GHz (ISM) • >3GHz@5GHz (UWB) Pervasive wireless access Characteristics of Wireless Access Systems Communication Technology Laboratory Wireless Communication Group

  6. WPAN 100M UWB Body Area Networks 15.3a 11g link throughput [bps] 11a WLAN 11b 10M 15.3 1M Sensor Networks RFID 15.1 15.4 Bluetooth 100k 10k ZigBee 1k UWB region (conceptional) 1 3 10 30 100 range [m] The Throughput - Range Tradeoff Communication Technology Laboratory Wireless Communication Group

  7. Fundamentals Fundamentals of short/medium range wireless communication 1 digital transmission systems equivalent baseband model digital modulation and ML-detection Fundamentals of short/medium range wireless communication 2 fading channels diversity MIMO wireless Fundamentals of short/medium range wireless communication 3 Multicarrier modulation and OFDM Systems I: OFDM based broadband access WLAN 1: IEEE 802.11g, a WLAN 2: IEEE 802.11n WMAN: (mobile) WiMAX Vehicular Networks Systems II: Wireless short range access technolgies and systems UWB 1: Promises and challenges of Ultra Wideband Systems UWB 2: Physical Layer options Wireless Body Area Network case study: UWB based human motion tracking The IEEE 802.15x family of Wireless Personal Area Networks (WPAN): Bluetooth, ZigBee, UWB Systems III: RF identification (RFID) and sensor networks RFID 1 RFID 2 Outline of Course Communication Technology Laboratory Wireless Communication Group

  8. Exercises: Motivation • Simulate, practice, verify, learn and have fun  • We will simulate the theoretical ideas/methods/techniques that we learn throughout the lecture. • MATLAB (matrix laboratory) will be used for simulations. • In general we will simulate • Single carrier transmission • Multi-carrier transmission • Wireless Channel • Channel coding • Simple UWB transceiver Communication Technology Laboratory Wireless Communication Group

  9. Exercises: Organization • Students organize in 2 or 4 groups • There will be three exercises with two tasks each during the semester. • Each group will perform one of the two tasks and then present the results. • The general schedule of tasks: • Introduction of tasks. • Working period (2 weeks). Present afterwards. • Each group will work individually. • Combining period (1 week). Present afterwards. • Two groups will work in collaboration. • For further details will be presented in the first exercise lecture next week 8:15 Communication Technology Laboratory Wireless Communication Group

  10. Schedule: Communication Technology Laboratory Wireless Communication Group

  11. Wireless Access Systems:Fundamentals of Short Range Wireless Communications

  12. Fundamentals of Short Range Wireless: Outline • Digital transmission and detection on the AWGN channel • digital transmission systems • equivalent baseband model • digital modulation and ML-detection • Fading channels • fading channels • diversity • MIMO wireless • Modulation schemes for frequency selective channels • multicarrier modulation • Orthogonal Frequency Division Multiplexing (OFDM) Communication Technology Laboratory Wireless Communication Group

  13. Equivalent Baseband Representation Communication Technology Laboratory Wireless Communication Group

  14. Notes f0 and are called the reference frequency and phase of the BB model for f0 = f1 the BB model is time-invariant (a filter) Narrowband Case: Equivalent Baseband Model with Bandpass Channel, Different TX and RX Lowpasses and Frequency/Phase Offset + Notation: Narrowband case: Communication Technology Laboratory Wireless Communication Group

  15. Re{} Im{} Re{} Im{} Narrowband Case: Relation of Physical Signals and Their Complex Baseband Representation • The spectrum of the analytic signal in terms of the physical signal is given by Re{} Re{} Names and Notation: Communication Technology Laboratory Wireless Communication Group

  16. Transmission of Digital Information I: Generation of Finite Signal Sets (Modulation) Communication Technology Laboratory Wireless Communication Group

  17. The information bit vector contains N bit It is mapped onto a message index (i) We use a look-up table with 2N transmit waveforms The transmit signal is selected according to the message index The process of selecting a transmit signal according to an information bit vector is called modulation For finite N this structure models a block transmission General Block Diagram of a Digital Modulator Mapper Communication Technology Laboratory Wireless Communication Group

  18. The signal space is defined by a set of orthonormal basepulses The basepulses are stacked to form the basepulse vector orthonormality implies The signal space representation of the transmit signals is obtained by the projection we refer to as transmit symbol vector Signal Space Representation of Digital Modulator Look-up table Mapper Communication Technology Laboratory Wireless Communication Group

  19. For linear modulation schemes the transmit symbol vector is obtained by a linear transformation of the input symbol vector precoding matrix GTX Dramatically reduces the size of the look-up table general modulation: exponential growth with the number N of information symbols linear modulation: linear growth Linear Modulation Communication Technology Laboratory Wireless Communication Group

  20. Some Popular Linear Modulation Schemes name symbol alphabet 2-PAM 4-QAM (QPSK) Communication Technology Laboratory Wireless Communication Group

  21. Nyquist basepulses (orthonormal) Nyquist criterion Filter Implementation of Linear Modulator: Nyquist Basepulses t T T t Communication Technology Laboratory Wireless Communication Group

  22. Transmission of Digital Information II: Transmission and Detection Communication Technology Laboratory Wireless Communication Group

  23. Channel is modelled as additive noise source In many cases of practical interest the noise can be characterized as zero mean stationary Gaussian random process w(t) any set of samples is jointy normally distributed autocorrelation function power density spectrum For analytical tractability usually a white noise process is assumed for physical system models (real-valued signals) we have for complex baseband representations as used herein we have Additive White Gaussian Noise (AWGN) Channel Communication Technology Laboratory Wireless Communication Group

  24. Bank of correlators generates the decision vector the decision vector is a sufficient statistics (for additive white Gaussian noise; AWGN) contains all information for the optimal estimation of the transmit symbol vector With the impulse correlation matrix we obtain the symbol discrete system model the elements of the noise vector are statistically independent and identically distributed Gaussian random variables Sufficient Statistic and Symbol Discrete System Model Continuous time system model Bank of correlators Symbol discrete system model Communication Technology Laboratory Wireless Communication Group

  25. Frequency Selective Channel • The channel is represented by a filter h(t) and AWGN • A channel matched filter is required prior to the correlator bank in order to obtain a sufficient statistics • These filters may affect the resulting impulse correlation matrix • intersymbol interference (ISI) channel channel matched filter Communication Technology Laboratory Wireless Communication Group

  26. Form-Invariant Basepulses Continuous time system model h(t) impulse modulator g(t) h*(-t) g*(-t) P S kT Symbol discrete system model Communication Technology Laboratory Wireless Communication Group

  27. Transmission of Digital Information III: Decoding Communication Technology Laboratory Wireless Communication Group

  28. With orthonormal basepulse vector the impulse correlation matrix becomes the identity matrix The decoder observes the decision vector and generates an estimate of the transmit symbol vector To minimize the probability of error the decoder selects the hypothesis, which has the minimum Euclidean distance to the decision vector (Maximum Likelihood (ML) decoder) decision regions in the signal space Maximum Likelihood Decoder and Decision Regions decoder Communication Technology Laboratory Wireless Communication Group

  29. Example: Error Performance of QPSK Decision regions Gray mapping (bit 1, bit 2) (0,1) (1,1) (0,0) (1,0) Communication Technology Laboratory Wireless Communication Group

  30. Fundamentals of Short Range Wireless: Outline • Digital transmission and detection on the AWGN channel • digital transmission systems • equivalent baseband model • digital modulation and ML-detection • Fading channels • fading channels • diversity • MIMO wireless • Modulation schemes for frequency selective channels • multicarrier modulation • Orthogonal Frequency Division Multiplexing (OFDM) Communication Technology Laboratory Wireless Communication Group

  31. Fading I: Time Selective (Narrowband) Fading Channels Communication Technology Laboratory Wireless Communication Group

  32. Path Loss and Short Term Fading TX RX distance (log(x)) power [dB] urban 40 dB/dec rural 30 dB/dec free space 20 dB/dec distance (log(x)) Communication Technology Laboratory Wireless Communication Group 32

  33. Doppler Shift I: 1800 Angle of Arrival • Received signal in complex passband notation • For (small scale effects) we obtain the complex envelope of the receive signal • depends only on the displacement . In the spectral domain we obtain the (spatial) Doppler shift Communication Technology Laboratory Wireless Communication Group 33

  34. For a linear movement of the receiver the spatial variations translate linearly into equivalent temporal variations the corresponding frequency shift follows as Complex envelope of received signal in the spectral domain we obtain the spatial Doppler shift Doppler Shift II: Arbitrary Angle of Arrival Communication Technology Laboratory Wireless Communication Group 34

  35. Complex envelope of the received signal Due to the different frequency shifts of the components, the magnitude of the received signal varies with the displacement: fading Example: note the spaced zero crossings Multipath Propagation and Fading 0.5 1 Communication Technology Laboratory Wireless Communication Group 35

  36. infinite number of scatterers under average receive power constraint: continuous PSD of fading process Scattering coefficients cn modelled as uncorrelated random variables with variance fading described as random process Power spectral density (PSD) of fading process for note the relation between Doppler shift fxD,n and the angle of arrival Doppler Spectrum: Power Spectral Density of Fading Process PSD fxD Communication Technology Laboratory Wireless Communication Group 36

  37. Cumulative power distribution versus frequency Power spectral density "Jake's Spectrum" Jake's Doppler Spectrum • Relation of angle of arrival and Dopper shift uniform continuous scattering around receiver cumulative power distribution Communication Technology Laboratory Wireless Communication Group

  38. Multiplicative fading Speed of movement: v Jake's Channel Model for Linear Movement complex white Gaussian noise process fD Communication Technology Laboratory Wireless Communication Group

  39. Fading II: Frequency Selective Fading Communication Technology Laboratory Wireless Communication Group

  40. Channel measurement with a short impulse h(t) (broadband) All scatterers, which lead to a given path delay are located on an ellipse Typical received signal: Broadband Channel Measurement Communication Technology Laboratory Wireless Communication Group 40

  41. Scattering Function Doppler shift fx • The scattering function describes the average power spectral density of the received signal as a function of Doppler shift fx and delay S4 S1 S3 S3 S2 S1 S4 S2 Communication Technology Laboratory Wireless Communication Group 41

  42. rms Doppler spread with the mean Doppler shift Scattering function 2nd order statistics of the spatio-temporal fading process a narrow band system can not resolve the multiple paths narrowband fading with Doppler spectrum Doppler Spectrum of Narrowband System Doppler shift S4 S1 S3 Doppler Spectrum Communication Technology Laboratory Wireless Communication Group 42

  43. Scattering function 2nd order statistics of the spatio-temporal fading process Delay power spectrum rms delay spread with the mean delay Delay Power Spectrum of Broadband System Doppler shift S4 S1 S3 Delay power spectrum Communication Technology Laboratory Wireless Communication Group 43

  44. The signal bandwidth and the duration of the transmit burst determine the fading model flat: no significant variation over the interval of interest selective: varies significantly over the interval of interest Narrowband systems experience frequency-flat fading Broadband systems experience frequency-selective fading A block fading model is suitable in the time-flat regime may be either frequency-flat or frequency selective Systems below the red curve are not physically implementable Note the role of Doppler spread and delay spread Classification of Multipath Channels burst duration TBURST time-selective frequency-selective time-selective frequency-flat time-flat frequency-flat time-flat frequency-selective signal bandwidth B TBurst=1/B Communication Technology Laboratory Wireless Communication Group

  45. A discrete delay power spectrum is specified paths (delays) are usually clustered For each path (k) (i.e. delay ) a Doppler spectrum is specified default: Jake's spectrum if specified in terms of spatial frequency fx, substitute f x = f/v for linear movement with velocity v The filter coefficients are filtered complex normal random processes in line of sight (LoS) situations: nonzero mean Typical Time-Selective/Frequency Selective Channel Model Specification Structure delay power spectrum delay + Generation of fading processes white complex normal random process Communication Technology Laboratory Wireless Communication Group 45

  46. Note that the coefficients are random variables (not processes) For each channel realization a new set of random variables is generated non LoS: Special Cases Frequency-flat fading Block fading channel + • Generation of fading processes white complex normal random process Communication Technology Laboratory Wireless Communication Group 46

  47. Fading III: Impact on Error Performance Communication Technology Laboratory Wireless Communication Group 47

  48. Frequency-Flat Fading Channel • block fading: fading variable instead of fading process • note multiplication with magnitude of fading variable due to • channel matched "filter" • normalization of decision vector channel matched "filter" fading channel Symbol discrete system model with block fading Communication Technology Laboratory Wireless Communication Group

  49. In frequency-flat block fading the error performance of QPSK is determined by the instantaneous value of the fading variable We can define various figures of merit. Frequently used are outage probability: probability, that the instantaneous bit error probability is above a target value fading averaged bit error probability Clearly these figures of merit depend on the probability density function (pdf) of the fading amplitude here is a chi2 random variable with 2 degrees of freedom Error Performance of QPSK in Frequency-Flat Block Fading fading averaged bit error probability Communication Technology Laboratory Wireless Communication Group

  50. Special case L=1 the fading variable z is complex normally distributed; is the sum of two statistically independent squared real-valued normal random variable If , is Rayleigh distributed; Rayleigh fading if , is Rician distributed. K-factor: General case L=N: N-fold diversity For , is the sum of 2L squared real-valued Gaussian random variables chi2-distribution with 2L degrees of freedom e.g. achievable with L receive antennas Approximation: BER (c/SNR)L Diversity fading averaged bit error probability Communication Technology Laboratory Wireless Communication Group

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