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Spread Spectrum Multiple Access

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  1. 1. SSMA Spread Spectrum Multiple Access AJAL.A.J Assistant Professor –Dept of ECE,Federal Institute of Science And Technology (FISAT) TM    MAIL:
  2. 2. SSMA Spread spectrum systems : The desired signal is transmitted over a bandwidth which is much larger than the Nyquist bandwidth. It is first developed for military applications for – Security – Undetectability: minimum probability of being detected – Robust against intentional jammers   
  3. 3. Applications Security Robust against unintentional interference It is not bandwidth efficient when used by a single user but has the capability to overcome narrowband jamming signals (cannot overcome AWGN or wideband jamming signal) and multi-path. Providing multiple access If many users can share the same spread spectrum bandwidth without interfering with one another, bandwidth efficient improved but will affect the capability to overcome jamming.   
  4. 4. Spread Spectrum Access Two techniques – Frequency Hopped Multiple Access (FHMA) – Direct Sequence Multiple Access (DSMA)  Also called Code Division Multiple Access – CDMA   
  5. 5. Frequency Hopping (FHMA) Digital muliple access technique A wideband radio channel is used. – Same wideband spectrum is used The carrier frequency of users are varied in a pseudo-random fashion. – Each user is using a narrowband channel (spectrum) at a specific instance of time. – The random change in frequency make the change of using the same narrowband channel very low.   
  6. 6. Frequency Hopping (FHMA) The sender receiver change frequency (calling hopping) using the same pseudo- random sequence, hence they are synchronized. Rate of hopping versus Symbol rate – If hopping rate is greather: Called Fast Frequency Hopping  One bit transmitted in multiple hops. – If symbol rate is greater: Called Slow Frequency Hopping  Multiple bits are transmitted in a hopping period   GSM and Bluetooth are example systems  
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  9. 9. Code Division Multiple Access (CDMA) In CDMA, the narrowband message signal is multiplied by a very large bandwidth signal called spreading signal (code) before modulation and transmission over the air. This is called spreading. CDMA is also called DSSS (Direct Sequence Spread Spectrum). DSSS is a more general term. Message consists of symbols – Has symbol period and hence, symbol rate
  10. 10. Code Division Multiple Access (CDMA) Spreading signal (code) consists of chips – Has Chip period and and hence, chip rate – Spreading signal use a pseudo-noise (PN) sequence (a pseudo- random sequence) – PN sequence is called a codeword – Each user has its own cordword – Codewords are orthogonal. (low autocorrelation) – Chip rate is oder of magnitude larger than the symbol rate. The receiver correlator distinguishes the senders signal by examining the wideband signal with the same time- synchronized spreading code The sent signal is recovered by despreading process at the receiver.
  11. 11. CDMA Advantages Low power spectral density. – Signal is spread over a larger frequency band – Other systems suffer less from the transmitter Interference limited operation – All frequency spectrum is used Privacy – The codeword is known only between the sender and receiver. Hence other users can not decode the messages that are in transit Reduction of multipath affects by using a larger spectrum
  12. 12. CDMA Advantages Random access possible – Users can start their transmission at any time Cell capacity is not concerete fixed like in TDMA or FDMA systems. Has soft capacity Higher capacity than TDMA and FDMA No frequency management No equalizers needed No guard time needed Enables soft handoff
  13. 13. CDMA Principle Represent bit 1 with +1 Represent bit 0 with -1 One bit period (symbol period) 1 1 Data 0 1 1 1 0 1 0 1 1 1 1 1 0 1 0 1 1CodedSignal Chip period Input to the modulator (phase modulation)
  14. 14. Processing Gain• Main parameter of CDMA is the processing gain that is defined as: Bspread Bchip Gp = = R R Gp: processing gain Bspread: PN code rate Bchip: Chip rate R: Data rate • IS-95 System (Narrowband CDMA) has a gain of 64. Other systems have gain between 10 and 100. – 1.228 Mhz chipping rate – 1.25 MHz spread bandwidth
  15. 15. Near Far Problem and Power Control• At a receiver, the signals may come from various B pr(M) (multiple sources. – The strongest signal usually captures the M modulator. The other signals are considered M as noise – Each source may have different distances to M the base station M
  16. 16. Near Far Problem and Power Control In CDMA, we want a base station to receive CDMA coded signals from various mobile users at the same time. – Therefore the receiver power at the base station for all mobile users should be close to eacother. – This requires power control at the mobiles. Power Control : Base station monitors the RSSI values from different mobiles and then sends power change commands to the mobiles over a forward channel. The mobiles then adjust their transmit power.
  17. 17. DSSS TransmitterMessage Baseband sss(t) + m(t) BPF Transmitted p(t) Signal PN Code Generator Oscillator fc Chip Clock 2 Es sss (t ) = m(t ) p (t ) cos(2πf c t + θ ) Ts
  18. 18. DSSS Receiver s1 (t ) m(t ) IF Wideband Phase Shift Keying Filter Demodulator Received Datasss (t ) p (t )Received PN Code SynchronizationDSSS Signal Generator Systemat IF 2 Es s1 (t ) = m(t ) cos(2πf c t + θ ) Ts
  19. 19. Spectra of Received Signal Spectral Interference Spectral Density Density Signal InterferenceSignal Frequency Frequency Output of Wideband filter Output of Correlator after dispreading, Input to Demodulator
  20. 20. CDMA Example R Receiver (a base station) Data=1011… Data=0010… A B Transmitter (a mobile) Transmitter Codeword=010011 Codeword=101010Data transmitted from A and B is multiplexed using CDMA and codeword.The Receiver de-multiplexes the data using dispreading.
  21. 21. CDMA Example – transmission from two sources A Data 1 0 1 1 A 0 1 0 0 1 1 0 1 0 0 1 1 0 1 0 0 1 1 0 1 0 0 1 1CodewordData ⊕ Code 1 0 1 1 0 0 0 1 0 0 1 1 1 0 1 1 0 0 1 0 1 1 0 0 A Signal B Data 0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 BCodewordData ⊕ Code 1 0 1 0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 1 0 1 0 1 0 B SignalTransmitted A+B Signal
  22. 22. CDMA Example – recovering signal A at the receiver A+B Signal received A Codeword at receiver(A + B) ∗ Code Integrator OutputComparator Output 0 1 0 0 Take the inverse of this to obtain A
  23. 23. CDMA Example – recovering signal B at the receiver A+B Signal received B Codeword at receiver(A + B) ∗ Code Integrator Output Comparator Output 1 1 0 1 Take the inverse of this to obtain B
  24. 24. CDMA Example – using wrong codeword at the receiver A+B Signal received Wrong Codeword Used at receiver Integrator OutputComparator Output X 0 1 1 Noise Wrong codeword will not be able to decode the original data!
  25. 25. Hybrid Spread Spectrum Techniques FDMA/CDMA – Available wideband spectrum is frequency divided into number narrowband radio channels. CDMA is employed inside each channel. DS/FHMA – The signals are spread using spreading codes (direct sequence signals are obtained), but these signal are not transmitted over a constant carrier frequency; they are transmitted over a frequency hopping carrier frequency.
  26. 26. Hybrid Spread Spectrum Techniques Time Division CDMA (TCDMA) – Each cell is using a different spreading code (CDMA employed between cells) that is conveyed to the mobiles in its range. – Inside each cell (inside a CDMA channel), TDMA is employed to multiplex multiple users. Time Division Frequency Hopping – At each time slot, the user is hopped to a new frequency according to a pseudo-random hopping sequence. – Employed in severe co-interference and multi-path environments.  Bluetooth and GSM are using this technique.
  27. 27. Capacity of CDMA Systems• Uplink Single-cell System Model User 2 Assumptions • Total active users Ku • The intra-cell MAI can be ... modeled as AWGN User 1 User k • Perfect power control is assumed . . • Random sequences . . . . ... User n User Ku
  28. 28. Capacity of CDMA Systems  Coarse estimate of the reverse link (uplink) capacity  Assumptions:  Single Cell.  The interference caused by other users in the cell can be modeled as AWGN.  Perfect power control is used, i.e. the received power of each user at the base station is the same.  If the received power of each user is Ps watts, and the background noise can be ignored (ex: micro-cells), then the total interference power (MAI) at the output of the desired user’s detector is where Ku is the total number of equal energy users in the cell. Suppose each user can operate against Gaussian noise at a bit- energy-to-noise density level of Eb/Io. Let W be the entire spread bandwidth, then the interference spectral density can be expressed as: I ≅ ( K u − 1) Ps I0 = I Watts / Hz (one − sided ) W
  29. 29. Capacity of CDMA Systems { Ps Also, the bit energy Eb is Eb = Interference Rb limited I I ⋅W W Rb Thus, K u −1 = = 0 = Ps E b ⋅ R b E b I 0★Now, if we consider the factors of voice activity (G v), sectorizedantenna gain (GA), and other-cell interface factor (f), whereGv ≈ 1/v = 2.67GA (three sectors) ≅ 2.4f = (Interference form other cells)/(Interference from given cell) ≅ 0.6
  30. 30. Capacity of CDMA Systems W R b Gv ⋅ GA In this case, Ku u ≅ be approximated by K can ⋅ E b I 0 (1 + f ) 4 ⋅ (W R ) Ex: If Gv ≅ 2.67, GA ≅ 2.4, f ≅ b0.6 ⇒ Ku ≅ ( Eb Io ) If (Eb/Io) required is 6 dB (i.e. Eb/Io = 4) W ⇒ Ku ≅ Rb which will be larger than the TDMA or FDMA systems in the cellular environment.
  31. 31. SDMA• Use spot beam antennas• The different beam area can use TDMA, FDMA, CDMA• Sectorized antenna can be thought of as a SDMA• Adaptive antennas can be used in the future (simultaneously steer energyin the direction of many users) spot beam antenna
  32. 32. Features: A large number of independently steered high-gain beams can be formed without any resulting degradation in SNR ratio. Beams can be assigned to individual users, thereby assuring that all links operate with maximum gain. Adaptive beam forming can be easily implemented to improve the system capacity by suppressing co channel interference.