Ultra-Wideband (UWB 2): Physical Layer Options and Receiver Structures

2 Communication Technology Laboratory Wireless Communication Group Outline of Course Fundamentals 1.Fundamentals of short/medium range wireless communication 1 –digital transmission systems –equivalent baseband model –digital modulation and ML-detection 2.Fundamentals of short/medium range wireless communication 2 –fading channels –diversity –MIMO wireless 3.Fundamentals of short/medium range wireless communication 3 –Multicarrier modulation and OFDM Systems I: OFDM based broadband access 4.WLAN 1: IEEE g, a 5.WLAN 2: IEEE n 6.Vehicular Networks Systems II: Wireless short range access technolgies and systems 7.UWB 1: Promises and challenges of Ultra Wideband Systems 8.UWB 2: Physical Layer options 9.Wireless Body Area Network case study: UWB based human motion tracking 10.The IEEE x family of Wireless Personal Area Networks (WPAN): Bluetooth, ZigBee, UWB Systems III: RF identification (RFID) and sensor networks 12.RFID 1 13.RFID 2 14.RFID 3 15.Summary and Conclusions

3 Outline Physical Layer Options –UWB Impulse Radio –Direct Sequence UWB –UWB Multiband Receiver Structures –RAKE Receiver –Transmitted Reference Receiver –Energy Detector Appendix –UWB Multiband –IEEE a Multipath Model

4 Ultra-Wideband Impulse Radio (UWB-IR)

5 UWB-IR: Modulation and MA Options - Modulation schemes: PPM, BPSK (BPAM), PAM, OOK, … - MA schemes: TH-MA, DS-MA, …

6 Peer-to-Peer Scenario: In the following, we discuss UWB-IR modulation schemes in peer-to-peer communication: Only one transmitter and one receiver No interferer  No need for a MA scheme. One transmitter One receiver Picture from [Weisenhorn, IZS, 2004]

7 Similarities Among UWB-IR Systems: Application of very short duration pulses with, occupying a very large bandwidth of. In contrast to UWB-MB, the whole band is used in one block. Each symbol consists of pulses  Repetition coding One pulse per frame ( ) Very low duty cycle Time

8 Most Popular UWB-IR Modulations: Binary Pulse Position Modulation (BPPM) Binary Pulse Amplitude Modulation (BPAM) (Binary Phase Shift Keying (BPSK)) Time Symbol ‘  ’ Symbol ‘-1’ Time Symbol ‘1’ and Modulation of pulse position Extension to any M-ary PPM possible with: Modulation of pulse polarity Pictures from [Giannakis, CEWIT, 2003] Symbol ‘  ’

9 Other Types of PAM: Pulse Amplitude Modulation (PAM) Time Symbol ‘  ’ Symbol ‘  ’ and On-Off Keying (OOK) Time Symbol ‘  ’ Symbol ‘  ’ and Pictures from [Giannakis, CEWIT, 2003] Modulation of amplitude Extension to any M-ary PAM possible

10 Example of BPPM: g(t) transmitting TsTs t  s(t) TsTs TfTf t TfTf

11 Example of BPAM: TfTf TsTs t transmitting TsTs t g(t) s(t) TfTf

12 Uncoordinated Multiple Access Scenario: [Weisenhorn, IZS, 2004]  Multiple access (MA) scheme required to reduce interference!  Multiple access (MA) scheme required to reduce interference!

13 Direct Sequence Spread Spectrum (DSSS): Conventional Principle Data signal Pseudo-Random sequence Spread data signal Time domainFrequency domain Data signal „Chip“ sequence DSSS signal * convolution

14 Direct Sequence in UWB-IR (1): Data signal Pseudo-Random sequence Randomized data signal Time domainFrequency domain Data signal „Chip“ sequence DS data signal * Spectral Lines due to Rep. Coding

User B specific binary pseudo-random sequence (PN) of length User A specific binary pseudo-random sequence (PN) of length 15 Direct Sequence in UWB-IR (2): Time... User AUser B Pictures from [Giannakis, CEWIT, 2003] Note: can also be combined with PPM

16 DS-UWB Compared to DSSS: DS in UWB-IR is very similar to DSSS in conventional systems: –Data bit is spread over multiple consecutive pulses. –Pseudo-random code is used to separate users (MA). –Spectrum is smoothed very efficiently. but: –In UWB-IR-DS the code rate equals the pulse rate.  Spectrum is not significantly spread by the DS.

17 Time-Hopping Multiple Access: Time... User AUser B User A specific -ary pseudo-random sequence (PN) of length User B specific -ary pseudo-random sequence (PN) of length Pictures from [Giannakis, CEWIT, 2003] Note: can also be combined with PPM

18 Time-Hopping Properties: Data bit is spread over multiple consecutive pulses. Pseudo-random code is used to separate users (MA). User separation also possible in non-coherent receivers such as the energy detector. Spectral smoothening not as effective as with DS.

UWB Receivers

Communication Technology Laboratory – Wireless Communications Group 20 Outline  Matched filter  Receiver structures  Rake  Transmitted reference  Energy detector

Communication Technology Laboratory – Wireless Communications Group 21 Introduction  Pulse based UWB  Transmitter and receiver for UWB are said to be very simple due to no need of Mixers, RF Oscillators and PLLs  For transmitters this assumption holds probably  But receivers are probably more complex as often assumed since energy has to be captured from all multipaths

Communication Technology Laboratory – Wireless Communications Group 22 Matched Filter single pulse  Transmission of a single pulse s(t) with duration T  n(t) is a white Gaussian noise process of zero mean and power spectral density N 0 /2  Receiver consists of a linear time-invariant filter g(t) and a sampler receiver The matched filter g(t) = s(-t+T) is a time reversed and delayed version of the input signal s(t). It maximizes the SNR at the sampling instant T.

Communication Technology Laboratory – Wireless Communications Group 23 UWB System with Multipath Channel TX RX

Communication Technology Laboratory – Wireless Communications Group 24 Matched Filter for Multipath Channel I  Optimum receiver: correlator or matched filter +

Communication Technology Laboratory – Wireless Communications Group 25 Matched Filter for Multipath Channel II + + correlator in each branch „RAKE“

Communication Technology Laboratory – Wireless Communications Group 26 ARAKE (All RAKE)  Optimum receiver with unlimited resources  Combines all N resolved multipath components  Number of resolvable components N increases with bandwidth => large number of RAKE fingers

Communication Technology Laboratory – Wireless Communications Group 27 SRAKE (selective RAKE)  Also referred as selection combining (SC)  Only subset of resolved multipath components is processed  Selects the L strongest paths  Better performance than a single path receiver  Requires the knowledge of the instantaneous values of all multipath components L = 6

Communication Technology Laboratory – Wireless Communications Group 28 Impact on the design of WBAN´s Number of RAKE fingers using a SRAKE  Antennas placed on the front side of the body in 15cm steps  Collecting 75% of the whole energy (front side measurements)  2 15cm  20 90cm Short distance multihop increases the energy that can be captured with a simple RAKE Short distance multihop increases the energy that can be captured with a simple RAKE

Communication Technology Laboratory – Wireless Communications Group 29 PRAKE (Partial RAKE)  Sometimes also referred as nonselective combining (NSC)  Collects the energy from the M first multipath components  These multipath components must not be the best, e.g. in NLOS environment  Compared to SRAKE no selection mechanism is required  Needs only to find the first M multipath components => complexity reduction M = 6

Communication Technology Laboratory – Wireless Communications Group 30 Selective nonselective Combining (SC-NSC)  Only the strongest path is tracked  The K-1 paths following the strongest path are chosen for the remaining path delays  SC-NSC is better suited for NLOS channels (where the direct path with the shortest delay, i.e. the first path, is attenuated) than PRAKE/NSC since the strongest path can be tracked K = 6

Communication Technology Laboratory – Wireless Communications Group 31 Conclusions on Rake Receivers  ARAKE is an optimum receiver  Realization of a matched filter  High complexity  Complexity reduction by using only a fraction of all paths  Performance degradation  Channel estimation necessary  Amplitudes and delays have to be known  Simpler receiver structures without channel estimation would be desirable

Communication Technology Laboratory – Wireless Communications Group 32 Transmitted Reference Receiver

Communication Technology Laboratory – Wireless Communications Group 33 Principles of Transmitted Reference Systems  2 pulses (=1 doublet) are transmitted for one symbol  1st pulse is the reference pulse, which is used as template  2nd pulse is the data pulse  Implicit channel estimation since both pulses pass the same channel  Channel has to be invariant over 1 doublet only  BPF required for noise reduction  Noisy template for correlation  Information rate usually drops by 50 % since half of the pulses are used as reference

Communication Technology Laboratory – Wireless Communications Group 34 TR PAM I Reference pulses Data pulses  Information in the amplitude of the data pulse

Communication Technology Laboratory – Wireless Communications Group 35 TR PAM II  TX energy higher since two pulses are needed for 1 bit  Correlation of reference and data pulse BPF  Performance depends on the time of integration  p  Performance degradation if inter-pulse interference exists

Communication Technology Laboratory – Wireless Communications Group 36 Integration Duration I  BER performance depends on the integration duration  If integration duration is too short, not enough energy can be captured  If integration duration is too long, the CIR is decayed so much that the noise term gets dominant  Channel models from IEEE a  LOS (Line of Sight)  NLOS (Non-Line of Sight) LOSNLOS

Communication Technology Laboratory – Wireless Communications Group 37 Integration Duration II  Body area network measurements around the torso  In general, shorter integration duration for LOS links than for NLOS

Communication Technology Laboratory – Wireless Communications Group 38 Energy Detector

Communication Technology Laboratory – Wireless Communications Group 39 Energy Detector I  Energy detector (ED) collects energy from multipaths  Integrates the energy of the receive signal  Non-coherent receiver structure  No antipodal signaling possible, e.g. BPSK  Usually used with pulse position modulation (PPM)  No explicit channel estimation is necessary  Begin and end of the integration interval has to be known

Communication Technology Laboratory – Wireless Communications Group 40 Energy Detector II BPF Pulse Position Modulation (2 PPM)  Integration of noise on the position where no pulse is located

Communication Technology Laboratory – Wireless Communications Group 41 Performance Comparison  Real measured channels around the human body (15cm distance  quasi LOS)  Performance of TR and ED similar but about 6 dB worse than MF

Communication Technology Laboratory – Wireless Communications Group 42 Conclusions  TR and ED are much simpler than an ARAKE  No explicit channel estimation necessary  Position of the receive signal in time domain has to be known accurately  Performance is worse than ARAKE  Performance strongly depends on integration duration  Integration duration is for LOS channels usually shorter than for NLOS channels

43 Appendices UWB Multiband (Certified Wireless USB) IEEE a Multipath Model

44 Ultra-Wideband Multi-Band (UWB-MB)

45 UWB-Multiband OFDM Spectrum is divided into sub-bands Serial transmission over the sub-bands Application of TF codes for piconet separation Strongly promoted by industry (Wireless USB, WiMedia, MBOA)

46 ECMA-368 (MB-OFDM Standard): Basic Idea Split overall spectrum into 14 bands of 524MHz bandwidth Serial transmission of OFDM symbols over the bands OFDM symbol: –128 point FFT/IFFT independent of data rate –Modulation: QPSK or DCM Information is coded across several bands (TF codes) to achieve frequency diversity and piconet separation. Zero-padded suffix: –Robustness against multi-path –Time to switch band

47 ECMA-368: Bandplan Overall band of 7.5GHz is split into14 bands: –Bandwidth: 524MHz –Separation: 524MHz Bands are grouped into 5 band groups Several TF codes for each band group  several piconets Band groups are managed by FDMA: –Better SOP performance [ECMA-386, 2006]

48 Code Map of Band Group 1: Fixed Frequency Interleaved Channels (FFI) Time Frequency Interleaved Channels (TFI) [ECMA-386, 2006]

49 EMCA-368: Rate Independent Parameters 8 OFDM tones are set to zero. [ECMA-386, 2006]

50 ECMA-368: Rate Dependent Paramters [ECMA-386, 2006]

51 QPSK versus DCM: Quadrature Phase Shift Keying (QPSK) Dual Carrier Modulation (DCM): –Two different 16-QAM mappings –Two different carriers –Frequency diversity Subcarrier 1 Subcarrier 50 [ECMA-386, 2006] 4 bits

52 Should One Go Multi-Band? Pros –Flexible band selection Easy to fit to spectral masks NBI mitigation –Power efficient –Implementation by COTS –Suitable for IC integration –Suited and strongly promoted for HDR systems (e.g. Wireless USB) Cons –Not low complexity –Not low power –High rate sampling –Small advantage over other systems, e.g n [Giannakis, CEWIT, 2003]

53 IEEE a Multipath Model

54 IEEE a Multipath Model (1) The proposed model uses the following definitions: T l = the arrival time of the first path of the l-th cluster  k,l = the delay of the k-the path within the l-th cluster relative to the first path arrival time T l  = cluster arrival rate = ray arrival rate, i.e., the arrival rate of path within each cluster.

55 IEEE a Multipath Model (2) Log-normal (rather than Rayleigh) distribution for the multipath gain magnitude Independent fading assumed for each cluster as well as each ray within the cluster Real valued passband model Target channels: CM 1 (LOS 0-4m), CM 2 (NLOS 0-4m), CM 3 NLOS 4-10m, CM 4 (Extreme NLOS) Discrete time impulse responses: Multipath gain coefficients Delay of the l th cluster Delay of the k th multipath component relative to the l th cluster arrival time Shadowing coefficient

56 IEEE a Multipath Model (3) Shadowing coefficient Multipath gain coefficients –Multipath amplitude sign –Ray power Exponential decay Poisson cluster and ray arrival Cluster fading Ray fading mean energy of the first path of the first cluster

57 IEEE a Multipath Model (4)