Part A: Spread Spectrum Systems

1 Telecommunication Systems and Applications (TL - 424) Part A: Spread Spectrum Systems Dr. ir. Muhammad Nasir KHAN Department of Electrical Engineering Swedish College of Engineering and Technology March 6, 2014

Introduction Spectrum is a general term used to encompass the spatial and temporal properties of any medium, including fiber optic cable, coaxial cable, and ambient air. From the view of a strict constructionist, spectrum is a type of division based on frequency or wavelength. Generally, we think of signals in the real-world setting as being functions of time. 1 0.8 0.6 0.4 0.2 sin(t) 0-0.2-0.4-0.6-0.8-1 0 2 4 6 8 10 12 14 time T Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 2 / 1 2

Introduction (Cont d) It is an interesting exercise to discover how frequency is related to the time domain. In the time domain, it becomes evident that the sine wave in las slide has similar characteristics after the time T. In fact, the function repeats itself after this point. The point labeled T is a special point in that it marks the periodicity of the function (i.e., the shortest length of time after which the function repeats itself). The frequency, f, of the function is defined as the reciprocal of the period T, i.e., f = 1/T. From this expression, it is demonstrated that the frequency domain is the inverse of the time domain. This is seemingly at odds with the original explanation of spectrum. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 3 / 1 3

Narrowband Systems Traditionally spectrum efficiency (measured in bits per seconds per Hz) in digital communication system is obtained by transmitting as much information as possible using a narrow frequency bands. Narrowband Systems: Bandwidth is close to the minimum necessary required to transmit the information. In contrast to NB, wideband systems or spread spectrum (SS) systems use a bandwidth much wider than minimum necessary for the application Motivation Motivations for the SS initially from the military context Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 4 / 1 4

Narrowband Systems (Cont d) In general, signal transmission is enabled through some means of modulation. In the past, systems have relied primarily on narrow-band modulation schemes. In these systems, all of the power in a transmitted signal is confined to a very narrow portion of the frequency bandwidth. As a result of these narrow frequencies, an interfering frequency at or near the transmitting frequency can cause interference, render the signal unrecoverable. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 5 / 1 5

Narrowband Systems Examples Amplitude Modulation is one example of a narrow-band modulation scheme in which the amplitude of the carrier signal is made stronger or weaker based on the information in the signal to be transmitted. The large amounts of power that are associated with Amplitude Modulation allow the signal to travel large distances before it attenuates to an undetectable level. A second popular form of modulation is Frequency Modulation, in which the phase of the carrier frequency is adjusted in accordance with the signal being transmitted. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 6 / 1 6

Narrowband Systems (Cont d) Narrow-band modulation schemes are not the only implementations available to broadcasters. Broadcasting entities may take advantage of the fact that a defined spectral power density may be achieved not only through high power over a very narrow frequency range, but also through lower powers spread over much larger frequency ranges. Power Spectr al Narrow Band Signal Wide Band Signal Frequency Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 7 / 1 7

Spread Spectrum A transmission technique in which a pseudo-noise code, independent of the information data, is employed as a modulation waveform, to spread the signal energy over a bandwidth much greater than the information signal bandwidth. At the receiver the signal is de-spread using a synchronised replica of the pseudo-noise code. Most commercial spread spectrum systems transmit an RF signal bandwidth in the neighbourhood of one to two orders of magnitude greater than the bandwidth of the information that is being sent. Transmitted bandwidth can be as large as three orders of magnitude above the bandwidth of the information. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 8 / 1 8

Principles of SS Communications The SS system is the one in which the transmitted signal is spread over a wide frequency band, much wider than the minimum bandwidth required to transmit the information being sent. A system is said to be a SS system, if it fulfils the following requirements; The signal occupies a bandwidth much larger than the minimum required bandwidth. Spreading is accomplished by means of a code which is independent of the data. Spreading code is pseudo-random, thus the spreading signal resembles noise. A synchronised reception with the code at the receiver is used for de-spreading the information before data recovery. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 9 / 1 9

Principles of SS Communications Block Diagram Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 10 / 1 10

Principles of SS Communications Block Diagram Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 11 / 1 11

Why Spread Spectrum In general the motivations for the use of SS can be categorized as follow, Anti-jamming/Anti-interference capability Spread Spectrum makes use of spreading code which is known only by the intended receiver. Signal de-spreading can then reject strong undesired signals. Low Probability of Intercept (LPI) Signals spread uniformly in the frequency domain. Masks the transmitted signal in the background noise, making the detection very difficult. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 12 / 1 12

Why Spread Spectrum (Cont d) Multiple Access Multiple SS links can share the same band with minimal co-channel interference. Spread spectrum methods can be used a multiple access technique, in order to multiplex (share) a communication resource among numerous users. The technique, termed code division multiple access (CDMA), since each simultaneous user employ a unique spread spectrum signalling code. One of the by products of this type of multiple access is the ability to provide communication privacy between users with different spreading signals. An un-authorized user cannot easily monitor the communications of the authorised users. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 13 / 1 13

Why Spread Spectrum (Cont d) Low Probability of Detection (LPD) Since in SS System, the signal is spread over many more signalling components than conventional modulation schemes, the resulting signal power is spread uniformly in the spread domain. Thus the received signal is small, very difficult to detect by anyone except the desired receiver (or intended receiver) systems designed for this special task are known as low probability of detection (LPD) or low probability of intercept (LPI). Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 14 / 1 14

Basis of SS Technology The basis of SS technology is expressed by Shannon theorem in the form of channel capacity as, where C = B log 2 (1 + γ) C is the capacity in bps, γ = S N bandwidth in Hz. is the signal to noise ratio and B is the The above equation shows the relationship between the ability of a channel to transfer error-free information, compared with the signal to noise ratio existing in the channel and the bandwidth used to transmit the information. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 15 / 1 15

Basis of SS Technology (Cont d) Letting C be the desired system information rate and by changing bases of Eq. in above slide, we find C B = 1.44 log e (1 + γ) For very small SNR, log e (1 + γ) γ, then we get B = NC 1.44S This shows that for any given noise to signal ratio, we can have a low information error rate by increasing the bandwidth used to transfer the information. Example If we want a system to operate in a link in which the interfering noise is 100 times greater than the signal and if C = 3kbps, then, B = 2MHz. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 16 / 1 16

Spread Spectrum Techniques The following techniques are used in SS systems Direct Sequence (DS) Frequency Hopping (FH) Time Hopping (TH) Pulse frequency modulation (chirp) Hybrid forms [DS/FH, FH/TH and DS/FH/TH] Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 17 / 1 17

DS Spread Spectrum (DS-SS Each bit in the original signal is represented by multiple bits in the transmitted signal Spreading code spreads signal across a wider frequency band Spread is in direct proportion to the number of bits used One technique combines digital information steam with the spreading code bit stream using exclusive-or Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 18 / 1 18

Spread Spectrum (Cont d) DS-SS modulation was originally developed for military communication systems between 1950-1980 to ensure robust and covered communications. In DS-SS modulation, the information signal is spread over a much wider bandwidth that required based on the symbol rate. A narrowband signal is made wideband! How? Signal spreading over a much wider bandwidth is achieved by multiplying the signal at the transmitter by a wideband code signal. Why? Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 19 / 1 20

Spread Spectrum (Cont d) DS-SS modulation was originally developed for military communication systems between 1950-1980 to ensure robust and covered communications. In DS-SS modulation, the information signal is spread over a much wider bandwidth that required based on the symbol rate. A narrowband signal is made wideband! How? Signal spreading over a much wider bandwidth is achieved by multiplying the signal at the transmitter by a wideband code signal. Why? To create robustness against: multi-path fading and dispersion, interference, detection, eavesdropping. Because of the low PSD, its presence is difficult to detect. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 19 / 1 21

DS-SS Transmitter In DS-SS spreading of the signal over a much wider bandwidth is achieved by multiplying the signal at the transmitter by a code sequence c(t) { 1, +1} consisting of code chips. The transmitted signal is: x c (t) = A c d(t)c(t) cos ω c t Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 20 / 1 22

DS-SS Transmitter (Cont d) Signals in a DS-SS transmitter for a BPSK data modulation signal. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 21 / 1 23

DS-SS Transmitter (Cont d) data signal 2Rs f Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 22 / 1 24

DS-SS Transmitter (Cont d) data signal d(t) d(t)c(t) 2R s f PN-code signal c(t) 2R c f Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 22 / 1 25

DS-SS Transmitter (Cont d) data signal The PSD of the spread signal is 1/G p less than the PSD of the un-spread signal 2R s f d(t) d(t)c(t) DS-SS signal 2R c f PN-code signal c(t) 2R c f The chip rate R c = 1 T c The ratio G p = BWDS SS BW data factor. symbol rate R s = 1 T s = Rc R s = Ts T c is called the processing gain or spreading Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 22 / 1 26

DS-SS Reception At the DS-SS receiver, the reverse operation is performed We can recover the signal only if: c(t)c (t)d(t) = d(t) The code is known Perfect synchronization data signal DS-SS signal 2R c f d(t)c(t) d(t) 2R s f PN-code signal c (t) f 2R c 27 Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 23 / 1

DS-SS Reception (Cont d) DS-SS is very robust against interference. When de-spreading the desired signal, the interference is spread. Only a fraction 1/G p of the spread interference overlaps with the de-spread data signal spectrum. Effective Interference power: I eff = P I BW data BW spread interference 2R = P s I 2R c = PI G p Narrowband Interference de-spread data signal DS-SS data signal d(t)c(t) d(t) Spread Interference DS-SS code signal c (t) Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 24 / 1 28

DS-SS Reception (Cont d) DS-SS is also robust against other DS-SS interference signals. When the SS-code or code phase is different, the signal remains spread. Also in this situation, only a fraction 1/G p of the interference spectrum overlaps with the de-spread data signal spectrum. de-spread data signal DS-SS data signal 1 and 2 Spread data signal 2 d 1(t)c 1(t) + d 2(t)c 2(t) d 1(t) + c 1(t)c 2(t)d 2(t) DS-SS code signal c 1(t) Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 25 / 1 29

Pseudo-noise code generators A DS-SS code is a pseudo random sequence of { 1, 1}. There are several ways to generate DS-SS codes depending on the required properties. One way is to use Maximum length sequences which are also called Pseudo-Noise (PN) codes because of their noise like spectral properties. Maximum Length (ML) sequences can be digitally generated in a simple way using a shift register with a feedback. A ML-code generator using a five stage shift register with code length 31. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 26 / 1 30

Desired Properties of PN codes Auto-correlation Low auto-correlation of a PN-code outside the main peak is required: to achieve reliable synchronisation to lock on the main resolvable path and reduce interference from other paths. cross correlation Low cross correlation between PN-codes is required: to be able to separate signals of different users. to obtain good detection performance because of low interference from other users Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 27 / 1 31

Spectral Efficiency of DS-SS For DS-SS, with M-level modulation, the spectral efficiency is given by: η R b B T = lrs B T = lrs 2R c = l 2G p [bit/s/hz] The price, we pay for the increased robustness against interference and multi-path fading (large G p ) is very low spectral effaciency. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 28 / 1 32

Spectral Efficiency of DS-SS For DS-SS, with M-level modulation, the spectral efficiency is given by: η R b B T = lrs B T = lrs 2R c = l 2G p [bit/s/hz] The price, we pay for the increased robustness against interference and multi-path fading (large G p ) is very low spectral effaciency. This can be improved by letting multiple users share the same bandwidth! Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 28 / 1 33

Code Division Multiple Access (CDMA) In code division multiple access (CDMA) the spectral responses are shared among multiple users based on different DS-SS code. Each user is defined by its code. The cross-correlation between the codes has to be low (ideally zero orthogonal codes). de-spread data signal DS-SS data signal 1 and 5 Spread DS-SS signals 2 to 5 5 c j(t)d j(t) j=1 DS-SS code signal c 1(t) d 1(t) + 5 c 1(t)c j(t)d j(t) j=2 Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 29 / 1 34

Pseudo-Noise Code Generator Only a few combinations of feedback taps result in an ML-code: the number of ML-codes is limited. When a large number of codes is required, i.e., to assign to different users in a multiple access communication system, other techniques for code generation are applied. Gold codes are generated beaded on two ML-codes: pro: many different codes. con: worse auto-correlation and cross correlation properties among the codes. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 30 / 1 35

Bit Error Rate in DS-SS systems The BER in DS-SS systems is deteriorated by interference produced by other users due to non-orthogonal codes. ( ) For BPSK: P e = Q 2Eb N with N o o = N o + N I In case of many users, the interference can be assumed to be Gaussian distributed. The power spectral density for single interfere is: N I = PI 2R c when synchronised and N I = PI 3R c when not synchronised. For K equal-power DS-SS( users: P e = Q 1 K 1 3N + No 2E b ) Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 31 / 1 36

DS-SS (Cont d) In DS-SS modulation the bandwidth of the signal is increased by multiplying the data with a code known to the receiver. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 32 / 1 37

DS-SS (Cont d) To retrieve the data at the receiver, the DS-SS signal is de-spread: The system is robust against interference and multiple paths, since: signals with a different code remain spread. narrowband signals are spread in the frequency domain. multi-path signals can be separated if the delay difference > T c The robustness is determined by the processing gain. G p = BWDS SS BW data = Ts T c Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 33 / 1 38

DS-SS (Cont d) To retrieve the data at the receiver, the DS-SS signal is de-spread: requires the same code. requires perfect synchronisation. The system is robust against interference and multiple paths, since: signals with a different code remain spread. narrowband signals are spread in the frequency domain. multi-path signals can be separated if the delay difference > T c The robustness is determined by the processing gain. G p = BWDS SS BW data = Ts T c Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 33 / 1 39

Employing DS-SS By applying DS-SS, spectral efficiency is traded for increased robustness due to the introduced processing gain. Different trade-offs can be made, e.g., the spectral efficiency can be increased by: 1 Increasing the symbol rate at the same chip rate. the processing gain is decreased. less robust. 2 Increasing the modulation level at the same symbol and chip rate. robustness is maintained: same processing gain. increase of signal power may be required. 3 Allow multiple users to share the same bandwidth: CDMA Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 34 / 1 40

Exercise Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 6, 2014 35 / 1 41