Introduction to spred spectrum and CDMA
Download as PPTX, PDF3 likes250 views
The document provides an introduction to spread spectrum communication including: - The origins of spread spectrum dating back to the 1940s and its use in military applications. - The two main types of spread spectrum techniques: direct-sequence spread spectrum and frequency hopping spread spectrum. - The advantages of spread spectrum including reduced interference, security, and resistance to jamming. - Key parameters like processing gain, jamming margin, and PN codes that are used in spread spectrum systems. - Common PN code generation techniques including maximal length linear feedback shift registers, Gold codes, and Walsh codes.
![Example : 3 Bit MLFSR
• Consider
– n=3 bit shift register,
– Initial State: [1 0 1]
• Maximal Length of the code , L =2n -1 =7.
– At the 7th state ( starting from 0th state ) the
sequence repeats itself.
– For maximal length , the output sequence cannot
be all zeros.
• PN code structure is determined by the
feedback logic and the initial register states
• The output of the sequence will be [1011100]](https://image.slidesharecdn.com/ch1-150218083750-conversion-gate01/85/Introduction-to-spred-spectrum-and-CDMA-27-320.jpg)
![PN codes in Multiple Access
• Consider three users with messages
– m1 =[ +1 -1 +1]
– m2 = [ +1 +1 -1]
– m3 = [ -1 +1 +1]
• And PN codes assigned to them be :
– P0 = [+1 -1 +1 +1 +1 -1 -1]
– P3 = [+1 -1 -1 +1 -1 +1 +1]
– P6 = [-1 +1 +1 +1 -1 -1 +1]](https://image.slidesharecdn.com/ch1-150218083750-conversion-gate01/85/Introduction-to-spred-spectrum-and-CDMA-32-320.jpg)
Introduction to spred spectrum and CDMA
- 1. Introduction to Spread Spectrum Communication • Origins of Spread Spectrum communication •Direct-Sequence Spread Spectrum •Frequency Hop Spread Spectrum •Advantages of Spread Spectrum •Application of Spread Spectrum Systems •Processing Gain and Other Parameters •Jamming •Linear FeedBack Shift Register Sequence Generation •M-Sequences and their Statistical Properties •Correlation Properties •Non Linear Sequences •Gold Codes •Walsh Codes •Kashami Sequences •Code Tracking Loops
- 2. Spread Spectrum communication • Communication system with the transmission of signals in a substantially greater radio frequency bandwidth than the information bandwidth is called spread spectrum communication • The message is first modulated by traditional amplitude, Frequency and Phase Modulation and then with Code (PN-Code) is used to spread the modulated waveform to a relatively wider bandwidth. • Spread spectrum generally makes use of a sequential noise-like signal structure to spread the normally narrowband information signal over a relatively wideband frequencies • The ratio of the Spread Spectrum Bandwidth to the Information Bandwidth is called Processing Gain of the System G = W/ R Where, G= Processing Gain W=Spread Bandwidth R= Bandwidth of Original Signal
- 3. Fig :Block representation of Spread Spectrum communication System
- 5. Multiple Spreading and De-spreading
- 6. Traditional Vs Spread Spectrum Comm.
- 7. Spread Spectrum System Narrowband Energy Spread to large BW Energy from the wideband is recovered to Narrowband
- 8. Origin of Spread Spectrum Communication • In 1948 Claude Shanon, The father of Information Theory, had put forth the channel Capacity Theorem: C = BW * Log (1 + SNR) = BW * Log (1 + S/N) • Shannons Work Suggests that Every bit of Information requires certain BW for transmission over the channel • First described by an actor(Hedy Lamarr) and musician (George Antheil) as a secure radio link to control torpedos (marine weapon) on a paper in 1941 and received us patent Number: 2.292.387 • After 1980s Used by the US Army for transmitting signal over harsh environmental conditions : Military and Navigation purpose – Used for GPS (Global Positioning System), – WCDMA (GSM 3G), – WLAN (IEEE -802.11b 802.11g and 802.11bgn ) – Bluetooth Communication
- 9. Origin of spread Spectrum The most celebrated invention of frequency hopping was that of actress Hedy Lamarr and composer George Antheil in 1942. Lamarr had learned at defense meetings she had attended with her former husband Friedrich Mandl that radio-guided missiles signals could easily be jammed. The concept of frequency hopping was introduced by Nikola Tesla in July 1900. Tesla came up with the idea after demonstrating the world's first radio-controlled submersible boat in 1898, when it became apparent the wireless signals controlling the boat needed to be secure from "being disturbed, intercepted, or interfered with in any way." - Multi frequency controlled transmitter
- 10. Spread … • Three Types of Spread Spectrum can be implemented – Frequency Hopping Spread Spectrum(FHSS) – Direct Sequence Spread Spectrum(DSSS) – Time Hopping Spread Spectrum(THSS) – Hybrid Spread Spectrum (FHDSS) • Two Types of Spread Spectrum Techniques are commonly used: – 1. Direct Sequence Spread Spectrum – 2. Frequency Hopped Spread Spectrum • Direct Spread Spectrum: • In the transmitting side, signal from the user is spread using a single frequency over broad Bandwidth and in the receiving side same code used for de-spreading • The carrier of the direct-sequence radio stays at a fixed frequency. • Example : CDMA – IS 95 System • Frequency Hopping Spread Spectrum • Whole bandwidth is divided into a smaller bandwidths • Each users narrowband signal hops among discrete frequencies and the receiver follows in sequence • The frequency-hopping technique does not spread the signal, as a result, there is no processing gain • the frequency hopper needs to put out more power in order to have the same SNR as a direct-sequence radio • Examples : Fixed Broadband Wireless Access (FBWA)
- 11. FHSS Vs DSS
- 12. DSSS Vs FHSS • DSSS has the advantage of providing higher capacities than FHSS, but it is a very sensitive technology, influenced by many environment factors (mainly reflections). • The best way to minimize such influences is to use the technology in either – (i) point to multipoint, short distances applications – (ii) long distance applications, but point to point topologies. • In both cases the systems can take advantage of the high capacity offered by DSSS technology, without paying the price of being disturbed by the effect of reflections. • As so, typical DSSS applications include – Indoor wireless LAN in offices – building to building links – Point of Presence (PoP) to Base Station links (in cellular deployment systems) (ii), etc.
- 13. DSSS Vs FHSS • FHSS is a very robust technology, with little influence from noises, reflections, other radio stations or other environment factors. • the number of simultaneously active systems in the same geographic area (collocated systems) is significantly higher than the equivalent number for DSSS systems. • FHSS technology is the one to be selected for installations designed to cover big areas where – a big number of collocated systems is required – where the use of directional antennas in order to minimize environment factors influence is impossible. • Typical applications for FHSS include cellular deployments for fixed Broadband Wireless Access
- 16. Typical Example of Multiple Access
- 17. ADVANTAGES OS SS Comm. 1. Reduced crosstalk interference 2. Better voice quality/data integrity and less static noise 3. Lowered susceptibility to multipath fading 4. Inherent security 5. Co-existence 6. Longer operating distances 7. Hard to detect 8. . Hard to intercept or demodulate 9. Harder to jam
- 18. PARAMETERS • Processing Gain or Spreading Factor • Jamming • Anti-jamming Margin • PN Codes • SNR • Capacity • Frequency Loading factor • Cell Loading
- 19. Processing Gain • Processing gain is the ratio of spread bandwidth to the un- spread baseband bandwidth. It is also defined as the ratio of chip rate to the baseband rate. • Processing Gain is also called spreading Gain. • Processing gain defines how many users the system can accommodate. The more the processing gain , the more users can be accommodated. • For a typical communication system, the vocoder output of 9.6 Kbps is spread over 1.2288MCps. Find the processing gain of the system. G = W/ R= 1228.8/9.6=128 In dB, 10 log (128)=21 dB Using a convention that the user number doubles every decrease in 3dB, and the minimum SNR is 6dB the total numbers users =32 per sector
- 20. Antijam Margin • It is the numerical figure that shows how much interference it can tolerate before the signal begins to be unintelligible. • The minimum SNR is 6dB. So this anti-jamming Margin indicates how much more noise it can tolerate.
- 21. PN Codes • Pseudo random Noise Codes – Random signals cannot be predicted where as PN codes are known to transmitter and receiver and are deterministic and periodic – PN codes appear as samples of white noise • Noise Like wideband spread-spectrum signals are generated using PN Sequence • PN sequence are deterministically generated, however they almost like random sequences to an observer. • The time waveform generated from the PN Sequences also seem like random noise • In DSSS a PN spreading waveform is a time function of a PN Sequence • IN FHSS , frequency Hopping Patterns can be generated from a PN code
- 22. PN Codes • PN codes can be constructed using one of following types: 1. Maximal Length Linear Feedback Shift Register (MLFSR) 2. Gold Codes 3. Walsh –Hadamard Codes 4. Kashami Sequences
- 23. Properties of Randomness of PN codes 1. Balance Property – The number of “1s” is nearly equal to the no of “0s” (in fact exceeded by one at most) 2. Run Property – Run is defined as sequence of single type of digit occurrences in the sequence. – The appearance of another type of digit is beginning of new run. – Among the runs of ones and zeros in each period , it is desirable that the half of the runs of each type are of length 1, one fourth of length 2, one eighth of length 3 and so on. 3. Correlation Property – Autocorrelation Property • How much a signal (Seq.) is related to the time shifted versions of itself • Higher the better • Physically realized by XORing the Sequence • If the result is all 0s then correlation is +1 • If the result is all 1s, then the correlation is -1 • If the result is equal number of 1s and 0s then the correlation is 0 (orthogonal Sequences ) – Cross correlation Property • How much a signal (sequence) is related to the other signals(sequence) • Lesser the better.
- 24. Example :Properties of PN codes using MLFSR •Take 5 Stage Linear Feedback Shift Register with the output sequence as shown . •The length period of non zero occurrence is L= 2n -1 =32-1=31 •There are •15 zeros and 16 ones • (2n -1 +1)/2= 2n -1 =24 =16 sequences with consecutive ones and zeros each •2n -2 = 8 sequences with consecutive 11s and 00 s •2n -3 =4 Sequences with consecutive 111s and 000s •2n -4 = 2 sequences with consecutive 1111s and 0000s •2n -5= 1 sequence with consecutive 11111s and 00000s •The correlation of the sequence S with its shifted sequence S(L) , denoted by < S, S(L) > = -1
- 25. PN sequence Generation with MLFSR • PN sequence can be generated using Linear feedback shift register • Binary Bits are shifted through the different stages of the register • The output of the last stage and the output of one intermediate stage are combined and fed as input to the first stage • The register starts with an initial sequence of bits, initial state stored in its stages • The register is then clocked and bits are moved through the stages • The output bits of the last stage form the PN Code
- 26. PN sequence Generation with MLFSR Initial State of Registers
- 27. Example : 3 Bit MLFSR • Consider – n=3 bit shift register, – Initial State: [1 0 1] • Maximal Length of the code , L =2n -1 =7. – At the 7th state ( starting from 0th state ) the sequence repeats itself. – For maximal length , the output sequence cannot be all zeros. • PN code structure is determined by the feedback logic and the initial register states • The output of the sequence will be [1011100]
- 28. Properties of M Sequences 1. An m bit register produces an m-seq of period 2m - 1 2. An m sequence contains exactly 2(m-1) ones and 2(m-1) -1 zeros 3. The modulo 2 sum (XOR) of an m sequence and another phase of same sequence yields another sequence 4. Each stage of an m-sequence generator runs through some phase of the sequence (Fibonaci LFSR not with Galois LFSR) 5. A sliding window of length m passed along an m-seq for 2m - 1 positions will span every possible m bit number (except all zeros) once and only once
- 29. M-Sequence… 5. Define a run of length r to be a sequence of r consecutive identical numbers bracketed by non equal numbers, then in any m sequence, there are: – 1 run of ones of length m – 1 run of zeros of length m-1 – 1 run of ones and one rum of zeros , each of length m-2 – 4 runs of ones and 4 runs of zeros, each of length m-3 – 2(m-3) Runs of ones and2(m-3) runs of zeros each of length 1 6. If (1 0 ) mapping to (+1 , -1) , then autocorrelation =0 for no delay autocorrelation = -1/(2m -1 ) for other case 7. If the feedback taps order is reversed, the sequence will be time reversed and still be an m- Sequence 8. The feedback set for any m-sequence consists of an even number of taps
- 30. PN…
- 31. Verify… It can be verified that 1. The cross correlation should be zero or very small 2. Each sequence in the set has an equal number of 1s (+1) minus 0s (-1) >=1 3. The scalar dot product of each code should be equal to 1.
- 32. PN codes in Multiple Access • Consider three users with messages – m1 =[ +1 -1 +1] – m2 = [ +1 +1 -1] – m3 = [ -1 +1 +1] • And PN codes assigned to them be : – P0 = [+1 -1 +1 +1 +1 -1 -1] – P3 = [+1 -1 -1 +1 -1 +1 +1] – P6 = [-1 +1 +1 +1 -1 -1 +1]
- 33. User 1 User 2 User 3 Composite:
- 35. Walsh Codes • Proposed by Joseph L. Walsh in 1923 • Walsh matrix is a specific square matrix, with dimensions a power of 2 • the entries of which are +1 or −1 or 1 and 0s • Each row of a Walsh matrix corresponds to a Walsh function • Walsh code are orthogonal codes • 64 x64 Walsh codes are used for channelization in CDMA systems • Each bit of the Walsh code is called a chip • The chip rate of Walsh code is 1.2288 Mcps • Walsh codes are used to identify users in the Forward Link • Variable Walsh codes are used in CDMA 2000 and WCDMA • In the reverse link Walsh code is used to define the type of channel • Walsh Functions are generated using Hadamard Matrix • The Hadamard matrices of dimension N are given by the recursive formula
- 36. Walsh … W1 = (0) W2n Wn Wn Wn Wn
- 37. Properties of Walsh Codes – Every row is orthogonal to every other row and to the logical not of every other row – Requires tight synchronization • Cross correlation between different shifts of Walsh sequences is not zero
- 39. Autocorrelation and Cross Correlation • When two sequences are correlated for given (fixed) phase shifts, a correlation value is obtained. • When two sequences are correlated for all phase shift, the plot of correlation values as a function of phase shift is obtained, which is called correlation function • When two sequences – have positive correlation value, they are somehow related – Have zero (or negative for bipolar wave ) correlation value, they are orthogonal – Can be measured by XORing the sequences : • If the result is all 0s, they are 100% correlated • If the result is all 1s , they are orthogonal – For Bipolar , 0-> -1 and 1s-> +1, if the product sum is • -ve, then they are negatively co related • +ve, then they are positively corelated • Zero, then they are othogonal 1. Autocorrelation • The study of the measure of the relation between a sequence and its own shifted versions is called autocorrelation • For binary sequences, the number of bit by bit position agreements minus the number of disagreements is called the Weight (W) • Example : correlation between two PN codes generated by a single generator
- 40. Cross correlation 2. Cross Correlation: • When the correlation is studied for different sequences from different sources, then it is called cross correlation • Example: the correlation between two sequences of MLFSR in Gold Codes
- 43. Example-1
- 45. For Bipolar Wave Sequences:
- 47. Example-2
- 48. Gold Codes • Introduced by Robert Gold in 1967 and 1968 • Gold codes are widely used in CDMA and GPS (Satellite navigation) and security keys in marine communication • Gold codes have bounded small cross correlations within a set which is useful when multiple devices are broadcasting in the same frequency range • A set of Gold Code sequences consists of 2n -1 sequences each one with a period of 2n -1 • Gold codes are produced by XOR-ing two m-sequences of the same length • If the LFSRs are chosen appropriately, Gold sequences have better cross- Correlation properties than MLFSR sequences • Gold and Kasami showed that for certain well-chosen m-sequences, the cross correlation only takes on three possible values,{-1,-t,t-2 } Where : t (correlation) depends solely on the length of the LFSRs used t=2(n+1)/2 + 1 if n is odd t=2(n+2)/2 + 1 if n is even n : is the number of elements or stage of shift register
- 49. • Gold sequence is an arbitrary phase of the sequence in the set G(u,v) defined by G(u,v)={u, uʘv, u ʘ Tv, u ʘ T2 v, u ʘ T(N-1) v } Where, • Tk denotes the operator which shifts vectors cyclically to the left by K places • ʘ denotes the XOR operator • u and v are two preferred-pair of m- sequences (MLFSR) of period generated by different primitive binary polynomials • The partial cross correlation values can be altered by changing the phases of the code sequences • It is then possible to find optimal phases which minimize the interference in the desired data signal • For K users each employing a sequence of period N, there are a total of N K different sets of sequence phases possible. • For a realistic system, direct computation becomes intractable. • Even when direct computation is performed, the reduction in interference of the optimal set of phases over the worst set of phases is 30%.
- 50. For n=3 MLFSR
- 53. Kasami Sequences • Binary sequences of length m= 2n -1 where n is an even integer • Kasami Sequences have good cross- correlation values • There are two classes of Kasami Sequences – The small set – The Large Set • The small set
Editor's Notes
- #2: Spread Spectrum Communications