FHTW. PSSS - Parallel Sequence Spread Spectrum A Potential Physical Layer for OBAN? Horst Schwetlick. Fachhochschule für Technik und Wirtschaft Berlin

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1 FHTW Fachhochschule für Technik und Wirtschaft Berlin University of Applied Sciences PSSS - Parallel Sequence Spread Spectrum A Potential Physical Layer for OBAN? Horst Schwetlick

2 Content PSSS for OBAN? Aspects of OBAN - Requirements Principle of PSSS - Transmission Performance Investigations Potential for OBAN Conclusion 2

3 OBAN Requirements Problems in Wave Propagation OBAN faces the well known problems of wave propagation and interference, which can be seen in every wireless application Technically this refers to the following aspects: 1. Link Budget Starting with a certain transmission power it describes all contributions of attenuation during the path of travel until arriving at the receiver and hence the resulting signal to noise ratio at the receiver side 2. Multipath Propagation Due to reflection, diffraction and refraction the paths from transmitter to receiver do not travel on only one straight line, rather than of a multiple of propagation paths 3. Co- and adjacent channel interference - Since the ISM bands are to be used freely, the received signal might be disturbed from transmitters on same channel or on an arbitrary near neighboring frequencies 3

4 OBAN Requirements Frequency Dependence for Freespace Propagation Essential for the Link Budget Loss in Freespace m db (Picture Huber & Suner) 4

5 OBAN Requirements Frequency Dependence of Material Attenuation Material: at 2,4 GHz at 5 GHz Brick 11,5 cm - 7 db - 10 db Brick 36 cm - 26 db - 50 db Light Concrete 11,5 cm - 12 db - 19 db Light Concrete 30 cm - 26 db - 35 db Limestone 11,5 cm - 22 db - 36 db Reinforced Concrete 16 cm - 20 db - 32 db Increased attenuation by walls at higher frequencies, Hence, in some cases lower frequencies might have advantages like the 900 MHZ range 5

6 OBAN Requirements Multipath Propagation and Delay Spread The combination of indoor- and outdoor environment LOS and NLOS Propagation places difficult conditions on the physical layer Special design of the Physical Layer might reduce these effects Spreading OFDM (NLOS - Non Line of Sight, LOS - Line of Sight) 6

7 Simulation at 2.4 GHz Room Building Walls Transmitter Position Simulation by REMCOM At 7

8 OBAN Requirements Co- and Adjacent Channel Interference The ISM-Bands are not restricted within certain limits So interference can be induced by networks, working at neighboring frequencies and working at the same frequency Mitigation the effects by Directing antennas Processing of baseband signals Using other, less heavy used frequencies 8

9 OBAN Requirements Discussion Can OBAN work with other systems than x in the 2.4 GHz Band? The future might bring other systems to WLANs for serving of facilities in households and offices, eg. systems at the xx might leave free capacities Different interfaces at the host PC, working simultaneously with different systems and at different frequency ranges could serve the availability of network access depending on the presence of wireless networks in the certain environment Spacial-, System- and Frequency-Roaming One of these additional systems might be PSSS? 9

10 PSSS- Principle Parallel Sequence Spread Spectrum PSSS uses the CDMA-principle and sends in parallel a superposition of orthogonal sequences with M-ary modulation It combines code multiplex of cyclically shifted M- sequences with block transfer of short data segments This approach combines scalable the advantages of spreading with the transmission of higher data rates 10

11 PSSS- Principle Basic Principle Special correlation properties of M-sequences are utilised: Cyclic Correlation of a bipolar version with an unipolar version of the same M-sequence yields a discrete delta impulse Bipolar Sequence Resulting Delta Impulse Unipolar Sequence 11

12 PSSS- Principle Basic Principle Hence one data bit is spreaded by the M-sequence Send through the transmission channel Received Despreaded to a single pulse using cyclic correlation Cyclic shifted impulses are orthogonal and can be superpositioned to a multivalent sequence No sidelobes due to the cyclic correllation appear - Therefore no increase of intersymbol interference 12

13 PSSS- Principle Cyclic Correlation M-sequence of length L Unipolar representation Cyclical correlation { } A = a... 1 a2 ai... a { } B = b... 1 b2 bi... b C = A B L L L i= 1 ab i b k + 1 a k = -1 a 1 1 k + = = 2 0 ( i+ k)mod L = c k Results in a delta impuls c k L + 1 for k =0 = 2 0 else 13

14 PSSS- Principle Cyclic Correlation If the sequence is cyclically shifted by a displacement i a( ) + 1 k+ i mod L 1 Bi = { bi,1, bi,2,... bi, k... bi, L}, bi, k = = A= { a1, a2,... ak... al}, ak = 1 then, after the correlation C = A B i i the resulting delta impulse is also shifted by the same displacement C i L + 1 for k = = 2 0 else i 14

15 PSSS- Principle Cyclic Correlation Orthogonality of the shifted sequences B i is maintained when sequences B i are multiplied by a data word, i.e. +1 or -1 and by superposition of sequences The presence or polarity of a certain sequences in number of superpositioned sequences is detected by the cyclic correlation and threshholding a delta function Hence, the presence or polarity of a certain sequence can represent one bit of information 15

16 PSSS- Principle Superposition of the Shifted Sequences The payload information is represented by the data sequence D of length K D = d... 1 d2 di... d C { } = Bd i i i K + 1 for a 1-Bit di =, 0 i K-1 1 for a 0-Bit a { } ( + 1 k+ i)mod L 1 Bi = bi,1, bi,2,... bi, k... bi, L, bi, k = = 2 0 The sequence to be transmitted X is multivalent and of length L L i= 1 C i = X 16

17 PSSS- Principle PSSS-Sequence The code sequence X (referred to as PSSS-sequence) is the base band signal of the PSSS-procedure After modulation, RF-transmission, reception and demodulation the sequence X is received The cyclical correlation of the multivalent sequence X leads to a sequence of soft bits D The data sequence is reconstructed by threshholding these soft bits 17

18 Transmission of a PSSS-Sequence d 1 B 1 d 2 B 2 d j B j Σ Multivalued Code Sequence C Cyclic Correlation C with A Selection of Data Bits d 1 d 2 d j d k d k B k Transmitter Receiver Summarization of the process of spreading, superposition and correlation 18

19 PSSS- Principle The PSSS Data Stream Channel Impulse Response Cyclic Correlation PSSS-Signal Guard Interval 19

20 PSSS- Principle Transmission of a PSSS-Sequence d 1 B 1 d 2 B 2 d j B j Σ Multivalued Code Sequence C Guard Interval Appended Cyclic Correlation C with A Selection of Data Bits d 1 d 2 d j d k d k B k Transmitter Receiver Summarization of the process of spreading, superposition and correlation 20

21 PSSS- Principle Generatiom of the Sequences at the Transmitter Side d 1 B 1 Multivalued Code Sequence C d 2 B 2 d k B k Σ Transmitter Receiver Cyclic Correlation C with A Selection of Data Bits d 1 d 2 d j d k Assume that one block of PSSS-data includes K data bits Each of these K data bits is multiplied with one cyclically shifted spreading sequence 21

22 PSSS- Principle Superposition of Spreading Sequences at the Transmitter Side d 1 B 1 Multivalued Code Sequence C d 2 B 2 d k B k Σ Transmitter Receiver Cyclic Correlation C with A Selection of Data Bits d 1 d 2 d j d k All K sequences are added and form one PSSSsequence The resulting multivalent sequence is modulated and transmitted 22

23 PSSS- Principle Appending a Guard Interval d 1 B 1 Guard Interval Appended d 2 B 2 d k B k Σ Transmitter Receiver Cyclic Correlation C with A Selection of Data Bits d 1 d 2 d j d k A guard interval is appended to avoid interference between subsequent PSSS-Blocks 23

24 PSSS- Principle Reconstruction at the Receiver Side d 1 B 1 Transmitted Sequence d 2 B 2 d k B k Σ Transmitter Receiver Cyclic Correlation C with A Selection of Data Bits d 1 d 2 d j d k The PSSS-sequence is reproduced by the demodulation The cyclic correlation of the received sequence with the original spreading sequence yields the original data bits 24

25 PSSS- Principle Reassembling Data Bits at the Receiver Side d 1 B 1 Transmitted Sequence d 2 B 2 d k B k Σ Transmitter Receiver Cyclic Correlation C with A Selection of Data Bits d 1 d 2 d j d k These data bits are reassembled to the original data stream 25

26 PSSS- Principle PSSS Overlap of 31 Sequences and Reconstruction The Example with of K = L bits shows the spreading sequence of length 31, the original data sequence, the multivalent PSSSsequence and the reconstructed sequence 26

27 PSSS- Principle Comparison: OFDM versus PSSS Guard Interval Guard Data Code Data Time Frequency OFDM Frequency PSSS 27

28 PSSS- Principle Increasing Multipath Fading Resistance by Scaling PSSS With the multivalent sequence X of length L a maximum number of K bits can be transmitted L = K For a scaled usage, not all L shifted sequences must be used Since the gaps between the reconstructed delta functions get larger, this increases the resistance to multipath fading Hence, a tradeoff between interference resistance and data rate is achieved 28

29 Modulation Amplitude Distribution of PSSS- Sequences Every element of the PSSS-sequence forms a multivalent chip, which is transmitted with one modulation symbol It is noticed that the amplitude distribution of the PSSSsequence is data-dependent and not uniformly distributed approximately a Gauss distribution The maximum of the amplitude distribution is at zero small values appear more often as higher values 29

30 PSSS- Principle Non-Uniform Amplitude Distribution of the PSSS - Baseband Signal 30

31 Performance MODULATION Different Modulation Methods can transmit the PSSS sequence X, e.g.: M-PAM M-QAM M-QPSK 31

32 Performance PSSS with M-PAM The simplest modulation method is given with M-PAM. The PSSS-signal is transmitted by a double side band modulation Transfer rates up to 1 bit/symbol are obtained by this procedure If not all possible sequences are overlapped, lower data rates are possible with a corresponding spreading gain 32

33 Performance PSSS with M-PAM 33

34 Performance PSSS with M-PAM 34

35 Performance PSSS with M-QAM An increased data rate can be achieved with M-QAM In this case two PSSS-Sequences as I- and Q-Signal are fed to the modulator The corresponding curves for M-QAM are plotted for comparison in the diagram M was chosen 17*17=289, were the higher amplitude values are rare or do not appear 35

36 Performance Constellation diagram and BER(EbN0) with M-QAM 36

37 Performance PSSS with M-PSK M-PSK as a modulation method has the advantage, sending with a largely constant power since the PSSSsequence controls the phase angle Due to the amplitude distribution a 16-PSK was used Bit error performance is slightly less than with QAM 37

38 Performance Constellation diagram and BER(EbN0) with M-PSK 38

39 Performance PSSS-Transceiver Hardware MAXIM Eval-Board Here as transmitter 32 - QAM 39 DAC - Board CESYS - Spartan2 FPGA (Virtex)

40 Performance PSSS Performance and Results In the current version of investigation data rates from 2 bit/s/hz scalable down to lower rates with the advantages of signal spreading Bit Error Rate is comparable to unipolar BPSK Advantageous bit error structure favorably compatible with an outer channel coding Implementation advantages simpler hardware Rake Receiver is not necessary Hence low electricity consumption 40

41 Conclusion Application Areas of PSSS General Radio Link Cable Substitute for General Applications Areas with Multipath Fading WLAN / WPAN Wireless industrial control Home automatisation and control Audio-/ video-/ general data transmission in home applications 41

42 Conclusion Integration of PSSS into IEEE b PSSS is in the ongoing discussion for IEEE b Potential Physical Layer for ZigBee Data Rate of 250 kbit/s Frequency Range: Low Band 868 MHz and 915 MHz M-Sequence of Length 31 ( ) At 20 Superpositioned Sequences 42

43 Conclusion Future Potential Subjects of application and further investigations are Integration of channel equalization Integration of MIMO Point to multi-point connections with different channels Pre-Coding using the amplitude statistics Dynamic assignment of transfer capacities to adapt to different channel requirements including data rate multipath fading and delay spread 43

44 Conclusion Conclusion When an increasing number of networks, also other than WLAN in the 2.4 GHz Range become present, additional capacities might become available These networks might not be heavily used all time With a proper priorisation between internal and guest usage these capacities could be made available PSSS might become one of this technlogies, particular if extended to higher data rates These capacities might serve OBAN as well 44

45 FHTW Fachhochschule für Technik und Wirtschaft Berlin University of Applied Sciences Thank You This work was partly sponsored by the BMBF AIF FH 3 Program

46 References A. Kuzminskiy, H.R. Karimi, E. Edvardsen, J. C. Francis, Interference Scenarios in Future Wireless Open Access Networks, WWRF 11th Meeting, Oslo-Norway (2004) A. Wolf, German Patentschrift zu PSSS (2003) H. Schwetlick, A. Wolf: PSSS (Parallel Sequence Spread Spectrum) Application in RF-Communication, 8th IEEE International Symposium on Consumer Electronics, Reading, UK (2004) 46