Wireless SS.pptx

Wireless

1. Wireless - Spread Spectrum PPT Wireless Communication Prof. Devarajan Gopal

2. INTRODUCTON A signal that occupies a bandwidth of B, is spread out to occupy a bandwidth of B ss All signals are spread to occupy the same bandwidth B ss Signals are spread with different codes so that they can be separated at the receivers. Signals can be spread in the frequency domain or in the time domain.

3. Introduction In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth Our goals in SS are to prevent eavesdropping and jamming To achieve these goals, SS techniques add redundancy There are two types of common SS popular in practice Frequency Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum (DSSS)

4. Spread Spectrum (SS) Used to have much wider BW than the std. signal BW. To achieve objectives of overcoming multipath / ISI. For better signal quality in low SNR & to encounter multipath. TWO Major Spread Spectrum: Frequench Hopping Spread Spectrum (FHSS). Direct Sequence Spread Spectrum (DS=SS) – Both types use PRN code periodic sequence. FHSS: changes center (carrier) frequency with hoppingperiod T h. Blue Tooth is a FHSS system. It uses 90 MHz spectrum in 79 different center frequency.Gopping Period T R = T h = 1/1000 sec per hop. Modulation similar to FSK / DPSK/ QPSK/ etc.

5. Spread Spectrum Advantages ( i ) Tnterferene Avoidance : Noise interference in selected center frewuency – avoid interference at Receiver. (ii) Multipath access : Two devices can operate in the same frequency spectrum in a medium. At Rx.we can receive with out failue (iii) Stealth : Randomly hopping among frequency, when an eavesdropper does not know your hopping pattern  Military Communication. DS-SS (Direct Sequence Spread Spectrm ) : Uses pulse shape that has wide BW. Pulse shape is known as Pseudo Random (PR) noise signal which is BPSK modulated signal. PR rate is known to Transmitter & Receiver. Modulated signal s k (t) = a k (t) p k (t) cos (2 п f c t) Where a k (t) is bit signal +1 & -! Which indicates data bits to be sent. p k (t) pulse shape with higher BW noise signal . The bits PR Sequence is called ‘Chips’ to distinguish from bit signal i.e. number of chips in p k (t). The period of chip is T c . Program Gain = T s / T c = R c / R s ,  PG for 15 bit (shift Register) = 2 15 = 32,762.

6. Spread Spectrum Advantages – (1) when there is N/B interference it is multiplied by PR signal & spread over wide band. Receiving end N/B signal is spread over W/B . Receiver dispreading with a filter of N/ B.Received signal is some what free from/B interference. (2) DS-SS  multiple access PN sequence used in DS-SS are near orthogonal. Hence many signal in the mediun can be detected. Frequency Division Spread Spectrum (FDSS) Each user – unique frequency band per channel (approx. about 30 KHz ). Channels are assigned on demand to users. During call no other user can share. One phone / circuit at a time. Channel not in use - it is idle, others can not be used. Resources wasted.

7. FDMA Number of channels simultaneously supported in FDMA Where B t = Total spectrum allocation B c = channel BW B guard = channel guard bandwidth

8. FDMA / TDMA Problem: Solution: TDMA: -- N slots per frame Divide the radio spectrum into time slot Each slot one user to either transmit or receive Buffers and Transmit in burst mode Transfer to any user – non continuous, each slot total Bits = preamble + info (data).

9. TDMA Preamble contains address and synchronization that both base station and the subscriner use to identify each other. TDMA Frame structure

10. TDMA No. of channels in TDMA Where m = max. no. of TDMA users supported on user radio channel Problem: Solution:

11. TDMA Problem Solution:

12. CDMA CDMA Full Bandwidth to all users – wide bandwidth. PRN unique code to each user. Immunity to multipath. All users can access full bandwidth all times.

13. Frequency Hoping Spread Spectrum (FHSS)

14. FREQUENCY SELECTION - FHSS

15. FHSS Cycles

16. FDM Vs FHSS

17. Equalization In the equalization process the errors introduced by the transmission medium is compensated An equalizer should have a frequency characteristic that is the inverse of that of the transmission medium For digital signals, however, complete equalization is really not necessary, because a detector has to make relatively simple decisions‐such as whether the pulse is positive or negative (or whether the pulse is present or absent) A judicious choice of the equalization characteristics is a central feature of all digital communication systems.

18. Need For Equilization A pulse train is attenuated and distorted by the transmission medium. So the signal loses its original shape and position. The distortion is in the form of dispersion (spreading of pulses) which is caused by an attenuation of high‐frequency components of the pulse train. This is why it has to be corrected for recovery of the signal at receiver.

19. Classification

20. Zero‐Forcing Equalizer Basic Principles: It eliminates or minimizes interference with neighboring pulses (inter-symbol interference) at their respective sampling instants. This can be accomplished by the transversal‐filter equalizer which forces the equalizer output pulse to have zero values at the sampling (decision‐making) instants.

21. A pulse train is attenuated and distorted by the transmission medium. So the signal loses its original shape and position. The distortion is in the form of dispersion (spreading of pulses) which is caused by an attenuation of high‐frequency components of the pulse train. This is why it has to be corrected for recovery of the signal at receiver. If the pulses spread beyond their allocated time interval, ISI occurs and the pulses are spread in both the sides in time. Equalization compensates for inter-symbol interference (ISI) created by multipath within time dispersive channels. An equalizer within a receiver compensates for the average range of expected channel amplitude and delay characteristics. Equalizers must be adaptive Since the channel is generally unknown and time varying.

22. Eqilization To eliminate ISI, we must have an equalizer which is an inverse filter of the channel: If H eq (f) is the characteristics of the channel, then F(f) should be the characteristics of the equalizer In frequency selective channel, it enhances the frequency components with small amplitudes, and attenuates the strong frequencies Therefore provides a flat, composite, received frequency response and linear phase response.

23. Block Diagram - Zero Forcing Eqilizer

24. Zero Forcing Equilizer – delay Lines

25. Block Diagram if Zeo Forcing with Feedback

26. Space Division Multiple Access (SDMA) SDMA Control the modulated energyfor each user. Here same carrier frequency Three different directions

27. Rake Receiver Mobile signals- relected delay- self interference Ds-SS is less vulnerable Rake Receiver : Optimum energy combining over different path. Received signal – delayed, weightage as per SNR. It consists of multiple correlarors . Here received sig. is multiplied by time shifted code sequence which is locally generated. Each arm /finger signal coming from resolvable single path. Spreading code (CDMA) will have very small autocorrelation value for any non zero time offset avoids cross talk between fingers.

28. Rake Receiver To separate signals - each finger sees signal coming in over single path. Low autocorrelation of CDMA spread sequence assures multipath components nearly uncorrelated – each finger. Assumes M correlator- captures strongest multipath component. Weighted N/W, linear combination of correlator for bit detection. Correlation 1 syncronizes to the strongest multipath m 1 . Multipath components m 2 arriveslater by τ than m 1 . m 2 is synronized but low correlation with m 1 . Output single correlator corrupted by fading. Bit decision based on single correlator, may produce large BER. Rake Recever , if one correlator corrupted by fading others may not be coupled and corrupted signal may be discounted through weighting process.

29. Rake Receiver Decision based on combination of m separate decision statistics on Rake Rx to overcone fading & to improve diversity. Ref Fig. The summation output Weighting coefficient α m are normalized to the output power of the correlator

30. Conclusion Concluding Remark Supports soft hand off using CDMA. Interference freey system. Atenuation / amplification co efficient is proportional to received signal. RAKE receiver attempts to collect the time-shifted versions of the original signal by providing a separate correlation receiver for each of the multipath signals • RAKE receiver uses several baseband correlators to individually process several signal multipath components • The correlator outputs are (MRC) combined to achieve improved communications reliability and performance • RAKE receiver is used in CDMA based systems such as IS-95 and WCDMA