Objectives Define spread spectrum technologies and how they are used Process gain and jamming margin Direct sequence system

WHAT IS SPREAD SPECTRUM lSpread spectrum is a communication technique that spreads a narrowband communication signal over a wide range of frequencies for transmission then de-spreads it into the original data bandwidth at the receive. l Spread spectrum is characterized by: 4 wide bandwidth and 4 low power l Jamming and interference have less effect on Spread spectrum because it is: 4 Resembles noise 4 Hard to detect 4Hard to intercept

WHY SPREAD SPECRUM Advantages: Resists intentional and non-intentional interference Has the ability to eliminate or alleviate the effect of multipath interference Can share the same frequency band (overlay) with other users Privacy due to the pseudo random code sequence (code division multiplexing) Disadvantages: Bandwidth inefficient Implementation is somewhat more complex.

power Bandwidth

ISM frequency bands UHF ISM Mhz S-Band2 - 4 Ghz S-Band ISM (802.11b) Ghz C-Band4 - 8 Ghz C-Band Satellite downlink Ghz C-Band Radar (weather) Ghz C-Band ISM (802.11a) Ghz C-Band satellite uplink Ghz X-Band8-12 Ghz X-Band Radar (police/weather) Ghz Ku-band12-18 Ghz Ku-band Radar (Police) Ghz

General types of techniques for SS 1.Direct sequence modulated: Modulation of the carrier by a digital code sequences whose chip rate is much higher than information signal bandwidth. 2.Carrier frequency shifting in discrete increments in a pattern dictated by code sequence. This is called frequency hoppers. 3. Pulsed FM or chirp modulation in which a carrier is swept over a wide band during a given pulse interval. 4. Time hopping (TH) 5. Hybrid forms [DS/FH, FH/TH and DS/FH/TH]

Spread spectrum technique The spread spectrum system (SSS) is 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 defined to be a speared spectrum system if it fulfills the following requirements: 1.The signal occupies a bandwidth much in excess of the minimum bandwidth necessary to send the information. 2.Spreading is accomplished by means of a spreading signal, often called a code signal, which is independent of the data. 3 At the receiver, de spreading (recovering the original data) is accomplished by the correlation of the received spread signal with a synchronized replica of the spreading signal used to spread the information. Standard modulation such as FM and PCM also spread the spectrum of an information signal, but they do not qualify as spread spectrum systems since they do not satisfy all the conditions above.

GENERAL MODEL OF SPREAD SPECTRUM SYSTEM

Basis of spread spectrum technique The basis of spread spectrum technology is expressed by Shannon theorem in the form of channel capacity where C=capacity in bit/sec, N=noise power W=bandwidth in Hz, S=Signal power This 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.

Eq (14.5) 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. For example if we want a system to operate in a link in which the interfering noise is 100 time greater than the signal and if C=3k bit/s

The advantages of spread spectrum systems Interference suppression. White Gaussian noise is a mathematical model has infinite power spread uniformly over all frequencies. The communication is possible with this interfering noise (white Gaussian noise) of infinite power because only the finite power noise components that are present within the signal bandwidth can interfere with the signal.For a typical narrowband signal, this means that only the noise in the signal bandwidth degrade performance. The idea behind a spread spectrum anti-jam (AJ) system is as follows: consider that many orthogonal signal components are available to a communication link and that only a small subset of these signal coordinates are used at any time.

b-The noise from interferer (jammer) with a fixed finite power and with uncertainty as to where in the signal space the signal components (coordinates) are located, the jammer’s choices are limited to the following:- 1-jam all the signal components (coordinates) of the system, with equal amount of power in each one, with the result that a little power is available for each component (coordinate). 2-jam a few signal components (coordinates) with increased power in each of the jammed coordinates. Fig on next slide compares the effect of spreading in the presence of white noise with spreading in the presence of interferer (jammer). The power spectral density of the signal is denoted G(F) before spreading and Gss(f) after spreading.

Idea behind anti jamming

2.Energy density reduction. Since in SSS, the signal is spread over many more signaling 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). 3.Fine time resolution Spread spectrum signals can be used for ranging or determination of position location. Distance can be determined by measuring the time delay between transmitted and received signal. Uncertainly in the delay measurements is inversely proportion proportional to thebandwidth of the signal pulse as shown in fig (14.2). The uncertainty of the measurement,  t, is proportional to the rise time of pulse, is given by

Multiple access -Multiple access refers to techniques that enable sharing a common communication channel between multiple users. -There is a difference between multiplexing and multiple access. With multiplexing users requirements (or plans) are fixed, or at most, slowly changing. The user allocation is assigned a priori and the multiplexing (sharing) is usually a process that takes place within the confines of a local site (e.g a circuit board). With multiple access, usually involves the remote multiplexing (sharing) of a user and users requirements are changed (e.g satellite communications). -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 signaling 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 unauthorized user cannot easily monitor the communications of the authorized users.

PN SEQUENCES All the techniques mentioned above require a pseudo random noise (PN) code generator for bandwidth spreading. A (PN) generator produces a binary sequence which is apparently random but can be reproduced deterministically by the intended recipients. A random signal cannot be predicted, its future variations can only be described in a statistical sense. However, pseudorandom signal is not random at all, it is deterministic, periodic signal that is known to both transmitter and receiver.

- Why the name Pseudonoise or pseudorandom? Even though the signal is deterministic, it appears to have the statistical properties of sampled white noise. It appears, to an unauthorized listener, to be a truly random signal.

Direct Sequence Spread Spectrum (DSSS) Each bit represented by multiple bits using spreading code Spreading code spreads signal across wider frequency band – In proportion to number of bits used – 10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code One method: – Combine input with spreading code using XOR – Input bit 1 inverts spreading code bit – Input zero bit doesn’t alter spreading code bit – Data rate equal to original spreading code Performance similar to FHSS

Direct Sequence Spread Spectrum l The PN is also called a Chipping Code (eg., the Barker chipping code) l The bits resulting from combining the information bits with the chipping code are called chips - the result- which is then transmitted. * The higher processing gain (more chips) increases the signal's resistance to interference by spreading it across a greater number of frequencies. * IEEE has set their minimum processing gain to 11. The number of chips in the chipping code equates to the signal spreading ratio. * Doubling the chipping speed doubles the signal spread and the required bandwidth.

Direct sequence spread spectrum systems Direct sequence is the name given to the spectrum spreading technique whereby a carrier wave is modulated with a data signal. χ(t), then the data modulated signal is again modulated with a high speed (wideband) spreading signal g(t). The ideal suppressed carrier binary phase shift keying BPSK modulation results in instantaneous changes either 0 or π radians according to the data.

Signal Spreading 4The Spreader employs an encoding scheme (Barker or Complementary Code Keying (CCK). 4 The spread signal is then modulated by a carrier employing either Differential Binary Phase Shift Keying (DBPSK), or Differential Quadrature Phase Shift Keying (DQPSK). 4 The Correlator reverses this process in order to recover the original data.

Copyright 2005 All Rights Reserved l Fourteen channels are identified, however, the FCC specifies only 11 channels for non-licensed (ISM band) use in the US. l Each channels is a contiguous band of frequencies 22 Mhz wide with each channel separated by 5 MHz. 4 Channel 1 = – (2.412 plus/minus 11 Mhz). 4 Channel 2 = – (2.417 plus/minus 11 Mhz). l Only Channels 1, 6 and 11 do not overlap DSSS Channels

February 2005Copyright 2005 All Rights Reserved24 Spectrum Mask l A spectrum Mask represents the maximum power output for the channel at various frequencies. l From the center channel frequency, 11 MHz and 22 MHZ the signal must be attenuated 30 dB. l From the center channel frequency, outside 22 MHZ, the signal is attenuated 50 dB. ± ± ±

February 2005Copyright 2005 All Rights Reserved25 DSSS Frequency Assignments Channel GHz Channel GHz Channel GHz 25 MHz l The Center DSSS frequencies of each channel are only 5 Mhz apart but each channel is 22 Mhz wide therefore adjacent channels will overlap. l DSSS systems with overlapping channels in the same physical space would cause interference between systems. 4 Co-located DSSS systems should have frequencies which are at least 5 channels apart, e.g., Channels 1 and 6, Channels 2 and 7, etc. 4 Channels 1, 6 and 11 are the only theoretically non-overlapping channels.

February 2005Copyright 2005 All Rights Reserved26 DSSS Non-overlapping Channels 4 Each channel is 22 MHz wide. In order for two bands not to overlap (interfere), there must be five channels between them. 4 A maximum of three channels may be co-located (as shown) without overlap (interference). 4 The transmitter spreads the signal sequence across the 22 Mhz wide channel so only a few chips will be impacted by interference.

February 2005Copyright 2005 All Rights Reserved27 DSSS Encoding and Modulation

February 2005Copyright 2005 All Rights Reserved28 DSSS Encoding and Modulation l DSSS (802.11b) employs two types of encoding schemes and two types of modulation schemes depending upon the speed of transmission. l Encoding Schemes 4Barker Chipping Code: Spreads 1 data bit across 11 redundant bits at both 1 Mbps and 2 Mbps 4Complementary Code Keying (CCK): * Maps 4 data bits into a unique redundant 8 bits for 5.5 Mbps * Maps 8 data bits into a unique redundant 8 bits for 11 Mbps. l Modulation Schemes 4 Differential Binary Phase Shift Keying (DBPSK): Two phase shifts with each phase shift representing one transmitted bit. 4 Differential Quadrature Phase Shift Keying (DQPSK): Four phase shifts with each phase shift representing two bits.

February 2005Copyright 2005 All Rights Reserved29 Barker Chipping Code l adopted an 11 bit Barker chipping code. l Transmission. 4 The Barker sequence, , was chosen to spread each 1 and 0 signal. * The Barker sequence has six 1s and five 0s. 4 Each data bit, 1 and 0, is modulo-2 (XOR) added to the eleven bit Barker sequence. * If a one is encoded all the bits change. * If a zero is encoded all bits stay the same. l Reception. 4 A zero bit corresponds to an eleven bit sequence of six 1s. 4 A one bit corresponds to an eleven bit sequence of six 0s.

February 2005Copyright 2005 All Rights Reserved30 Barker Sequence One Bit Chipping Code (Barker Sequence) Original Data Spread Data Six 0s = 1 Six 1s = 0 One Bit

February 2005Copyright 2005 All Rights Reserved31 Differential Binary Phase Shift Keying (DBPSK) 0 Phase Shift 4A Zero phase shift from the previous symbol is interpreted as a 0. 4A 180 degree phase shift from the previous symbol is interpreted as a degree Phase Shift Previous carrier symbol

February 2005Copyright 2005 All Rights Reserved32 Differential Quadrature Phase Shift Keying (DQPSK) 4A Zero phase shift from the previous symbol is interpreted as a 00. Previous carrier symbol 0 Phase Shift 4A 90 degree phase shift from the previous symbol is interpreted as a 01. 4A 180 degree phase shift from the previous symbol is interpreted as a 11. 4A 270 degree phase shift from the previous symbol is interpreted as a Phase Shift 180 Phase Shift 270 Phase Shift

February 2005Copyright 2005 All Rights Reserved33 DSSS Summary 1Barker Coding 11 chips encoding 1 bitDBPSK 2Barker Coding 11 chips encoding 1 bit DQPSK 5.5CCK Coding 8 chips encode 8 bitsDQPSK 11CCK Coding 8 chips encode 4 bitsDQPSK Data Rate Encoding Modulation

BLOCK DIAGRAM OF A DS SS SYSTEM

RECEIVER OF DSSS

In the absence of noise and interference, the output signal from the correlator can be written as

Direct Sequence Spread Spectrum Example

Direct Sequence Spread Spectrum Using BPSK Example

Approximate Spectrum of DSSS Signal

RADIO FREQUENCY BANDWIDTH IN DIRECT SEQUENCE SYSTEMS RADIO FREQUENCY (RF) bandwidth available, directly affects system capabilities. For ex. If Bandwidth available is 20 mhz, then maximum process gain which might be possible is 20 mhz. As processing gain is proportional to RF bandwidth, as well as power reduction requires wide bandwidth. But losses does not permit a very high bandwidth, as given by Shannon theorem. Main lope bandwidth is function of wave shape and the code rate used. The signal lost as a result of accepting only the main lope is small. Only 10% of the power in a BPSK or qpsk signal is contained in the sidelopes. Signal power loss is not the only effect of bandwidth restriction, however sidelopes contain much of the harmonic power of the modulation. Thus restriction to a narrow RF bandwidh is equivalent of restricting the rise and fall times of the modulating code..

The choice of modulation and code rate is highly dependent on the system for which they Are to be used. One must consider Bandwidth availability Process gain required basic data rates Direct sequence process gain RF banwidth is that of the main lope of the [sinx/x]^2 direct sequence spectrum., which is.88 times the bandwidth spreading code clock rate. So for code clock rate 10 Mcps and 1 kbps information rate process gain will be.88*10^3 or 39 db Quadrature phase shift keying is one method of restricting RF bandwidth when a given code rate is used. It halves the required bandwidth. But in turns it also reduces the process gain

limitations that exist with respect to expanding the bandwidth ratio, for increased process gain RF bandwidth 1. RF bandwidth is dependent upon the code rate used. Doubling the present state of art code rates would increase process gain by only 3 db. Modest gain as compare to effort required to double the operating speed of present circuits. 2. Codes rates are inversely related to operating errors. But high speed logic circuits tend towards noise sensitivity and more susceptible to error. 3. High speed digital circuits consume large amount of current and power dissipation. 4. Spectrum occupancy. Equipment implementation and propagation constraints.

Data rate reduction Local oscillator phase noise or instability in the propagation medium become significant and cause errors. Willingness of user to slow information transfer.

PROCESS GAIN

Processing gain and jamming margin A fundamental subject in spread spectrum systems is how much protection spreading can provide against interfering signals with finite power. The process gain of a processor is defined as the difference between the output (S/N) ratio of the processor and the input (S/N) of the processor. Spread spectrum develops its process gain in a sequential signal bandwidth spreading and despreading operation. The process gain of a spread spectrum processor can be defined as

Processing gain and jamming margin THE DIFFERENCE IN OUTPUT AND INPUT SIGNAL TO NOISE RATIOS IN ANY PROCESSOR IS ITS NOISE GAIN. IN SPREAD SPECTRUM PROCESSOR THE PROCESS GAIN CAN BE ESTIMATED BY

PROCESS GAIN Where is the spread spectrum bandwidth (the total bandwidth used by the spreading technique) R is the data rate. -For direct sequence systems, Wss is approximately the code chip rate Rch and the processing gain can be expressed as

Jamming margin This process gain does not mean that the processor can perform satisfactorily when faced with an interfering signal having a power level larger than the desired signal by the amount of the available process gain. For this reason, jamming Margin (Mj ) is used to express the capability of the spread spectrum system under interference conditions and can be expressed as

EXAMPLE Processing gain=30db s/n (out)= 10db Lsys= 2db Jamming margin? Jamming margin is always less than process gain.