UWB Synchronization

1. UWB Synchronization Hui-Min Yeh

2. Outline • Introduction • Code synchronization • Typical acquisition scheme • Typical search strategies • Definition of ‘Hit set’ • Transmitter design • Hybrid TH/DS signal format • Receiver design • Proposed search algorithms • Joint BRS-NCS search • Joint RPS-NCS search • Proposed acquisition scheme • Simulation results • Conclusions

3. Power delay profile The objective of the signal synchronization is to properly align the received signal by correctly estimating the random delay Time Synchronization ?

4. Introduction • Code synchronization (TH code; DS code): • To enable the lower BER of data demodulation, the template signal should be aligned with the received signal, the code alignment is so-called “code synchronization” process. • Code synchronization can be spilt into two parts • Code acquisition • It is a coarse code acquisition, to resolve the code phase error within certain range. • Code tracking • It is a fine tuning process, to guarantee the timing error below an acceptable level.

5. Acquisition strategy: The search for acquisition is based on the auto-correlation properties of the applied the codes, the auto-correlation is high if the receiver is synchronized and low in other situations [5]. • MAX criterion (Maximum selective): • TC criterion (Threshold Crossing): • MAX/TC criterion:

6. Preamble Data Sequence for synchronization • Before any data can be received in an UWB communication, the receiver must synchronize on the transmission. • Receiver structures • Noise template signal • Reference template signal • Packet data • Synchronization on (known) code • Reception and processing data Data can have specific coding-> communication can not be tapped

7. Code synchronization • Find the best fit between the code and the received signal

8. Typical acquisition system • The receiver’s reference signal generator will shift and guess the code boundary of received signal, and the AWGN noise will effect the performance. Correlator 4 & 5 3

9. Why use RAKE receiver? • More than one hypothesized phase can be considered as an estimated timing delay for a coarse acquisition

10. Definition of ‘Hit set’ • Hit set definition: • The definition of ‘Hit set’ is a critical issue . When the threshold setting is low, the noise effect on the false alarm probability will increase. Oppositely, when the threshold setting is high, the mean acquisition time will increase.

11. Ex: The periodicity of TH code =4, frame duration , that is Nh=128, Tc = 1 nsec, normalized pulse energy =1,CM1(noise free), search space = 512 cells, Th=2, EGC receiver (20 fingers) Peak value Partial correlation energy Hit set N 1 2 True phase

12. Rake fingers = 20, search space = 512 cells, CM1

13. Search strategies • What is the efficient search algorithm? • Serial or Parallel • Typical serial search algorithms [1] • Linear Search (LS) • Random Permutation Search (RPS) • Bit Reversal Search (BRS) • Ideal mean stopping time:

14. Bit Reversal Search • The search algorithm is described by assuming that N is a power of 2. • The order of search positions in the bit reversal search algorithm is decided by “bit reversing” • For example: Let the integer of N is equal to 8 (2^3) Search locations for N = 23 ‘Bit Reversing’ Reorder search locations by ‘bit reversing’

15. The ideal and normalized mean stopping time vs. the parameter H/N for three serial search algorithms (N=512 cells)

16. Bit Reversal Search algorithm , code length=4, Nh=128, CM1(noise free), search space = 512 cells, EGC receiver (20 fingers)

17. Outline • Introduction • Code synchronization • Typical acquisition scheme • ‘Hit’ definition • Typical search strategies • Transmitter design • Hybrid TH/DS signal format • Receiver design • Proposed search algorithms • Joint BRS-NCS search • Joint RPS-NCS search • Proposed acquisition scheme • Simulation result • Conclusions

18. Transmitter design: Hybrid TH/DS signal format Next symbol 1’s Symbol (Nds*Tf) (TH-sequence) (Repeat NT times) Frame (CNth-1) Frame (C0) Frame (C1) Tf

19. The variation of power spectral density (PSD) between the pure time hopping sequences and hybrid TH/DS signals • No TH coding • The periodicity of TH code is equal to the number of pulses per symbol • Hybrid TH/DS signal • The hybrid TH/DS signal format method not only speeds up the code acquisition, but also improves the interference to other co-existing systems [7].

20. UWB signals with no Time-Hopping coding • Parameters: 10 symbols; number of pulses per symbol ( ) =64;pulse repetition time ( ) =10 nsec; chip duration ( ) = 1 nsec • The transmitter power is concentrated at multiples of pulse repetition frequency. Amplitude Time (ns)

21. UWB signals with Time-Hopping coding • Consider the same parameters • The periodicity of TH code ( ) is equal to the number of pulses per symbol, that is , • The PSD of the signals is composed of spectral lines at the distances of • The transmitted power is concentrated at multiples of symbol repetition frequency.

22. UWB signals with hybrid TH/DS signals • Consider the same parameters • a) Short TH code with ; repeat NT =8 • It will increase a lot of spectral lines due to the repeat operation • b) Short TH code with ; repeat NT =8; • The polarity of pulses per symbol is scrambles by DS code

23. Outline • Introduction • Code synchronization • Typical acquisition scheme • ‘Hit’ definition • Typical search strategies • Transmitter design • Hybrid TH/DS signal format • Receiver design • Proposed search algorithms • Joint BRS-NCS search • Joint RPS-NCS search • Proposed acquisition scheme • Simulation result • Conclusions

24. Proposed Joint Search Algorithms • Joint BRS-NCS Search • Joint RPS-NCS Search • Non consecutive search (NCS) criterion: • The non consecutive search strategy is proposed owing to more than one HIT phase in multi-path channel models. • The non consecutive search can achieve a rapid acquisition by testing search locations with a step size D which is greater than one cell. • The cells in total search space are so called uncertainty region that can be divided into blocks • This will reduce the search time required to acquire the location of hit set from an initial search cell, but it could also increase the miss probability.

25. LI-NCS (Linear Increasing NCS) • In order to avoid the miss search, we propose a linear increasing NCS (LI-NCS) approach. • The search equation can be expressed as • The LI-NCS approach can be described as following: • Set initial loop =1, and check blocks. The number of test cells are • If the correlation output of each block in this loop is lower than threshold, then go to the step (3). If the correlation output of one block in this loop is higher than threshold. then the search procedure is DONE. • Set loop = loop + 1, and also check blocks, the number of cells are , then go to step (2)

26. An example of the LI-NCS search approach Assume that search space N=12, hit set =[7 8 9 10 11], block size D = 4, 0 1 2 3 4 5 6 7 8 9 10 11

27. Joint BRS-NCS search procedure a) search space N=48, hit set H=8, b) divided into 8 blocks, each block with D = 6 Search space N Correct Symbol Boundary (a) =: New search location D cells (0) (4) (6) (1) (3) (5) (7) (2) (b)

28. Receiver design : Acquisition scheme • Two-stage acquisition scheme • TH code acquisition • DS code acquisition Find DS-boundary (get symbol boundary) ACQ2 (Second stage) ACQ1 Coarse TH-boundary acquisition Fine acquisition Received signal (First stage)

29. First Stage (RAKE receiver with parallel correlators) Z Hit Roughly find TH-boundary TC #1 No Hit Set new search location based on Joint Search Algorithm (new shift ) Template signal generator (Coarse Acquisition) Maximum path amplitude selective One branch correlator Second Stage (Fine Acquisition)

30. Fine Acquisition Procedure: • Step1:The terminating point of coarse acquisition process is the starting point of fine acquisition. • Step2: The new search window in fine acquisition process is equal to the double Hit set size. • Step3: The fine acquisition process searches both right and left directions from the starting pointof fine acquisition with one branch correlator to acquire the strongest path. The search range of each direction is equal to the size of hit set. • Step4: Weutilize the maximum selective criterion to select the strongest path amplitude • Step5: Finally, the fine acquisition process passes the parameter on to the second stage. Left direction Right direction Starting point

31. Second Stage (RAKE receiver with parallel correlators) Found DS-boundary (Found symbol boundary simultaneously) Z Hit TC #2 No Hit Set step size=: time-hopping code length and new search location based on Joint Search Algorithm location (new shift ) Template signal generator

32. Outline • Introduction • Code synchronization • Typical acquisition scheme • ‘Hit’ definition • Typical search strategies • Transmitter design • Hybrid TH/DS signal format • Power spectral density (PSD) • Receiver design • Proposed search algorithms • Joint BRS-NCS search • Joint RPS-NCS search • Proposed acquisition scheme • Simulation results

33. Simulation cases • Two-stage vs. one stage • Comparison of search strategies • Two-step approach • MAI case

34. Channel model 1&4 • The first path of CM1 we consider has the strongest amplitude gain, and the tenth path of CM4 has the strongest amplitude gain in our simulation. The strongest path The strongest path

35. SNR vs. Detection probability for the second stage in CM1 • Assume that the first stage finds the correct boundary.

36. Mean stopping time of two acquisition schemes as a function of threshold parameter A • Simulation parameters:

37. The mean stopping time of Joint BRS-NCS /Joint RPS-NCS method versus threshold setting for step size D=4, D=8, and D is approximated to hit set.

38. Comparison of performance in terms of Mean stopping time versus threshold setting for proposed search algorithms and bit reversal search algorithm.

39. A statistical number as function of offset value for 5000 trials, a) SNR = 10 dB, coarse acquisition result, b) considering fine acquisition result, in CM1 Single-user Multi-user

40. SNR = 20 dB, 5000 trials, CM4 Single-user Multi-user

41. MAI vs Detection probability (CM1) • Simulation results for detection probability of the UWB channel model (CM1), as a function of the number of interfering users Nu-1

42. MAI vs Detection probability (CM4) • Simulation results for detection probability of the UWB channel model (CM4), as a function of number of interfering users Nu-1

43. Simulation results of detection probability for different lengths of in UWB channel model (CM1), as a function of interfering users (Nu-1), SNR=20dB

44. Conclusions • The mean stopping time of two-stage acquisition scheme is simulated and it is evident to see that the two-stage acquisition scheme outperforms the conventional one stage acquisition scheme. • By simulation results, we can see that the “Joint Search Algorithms” have lower mean stopping time. • We propose a two-step approach which is composed of coarse step and fine step to achieve a fine code acquisition in first stage. • Adding more users to the systems is investigated by computer simulations.

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