SPREAD SPECTRUM COMMUNICATIONS Chapter 6 SPREAD SPECTRUM COMMUNICATIONS

Chapter overview Spread spectrum systems Spread spectrum direct sequences Pseudo-noise sequences Frequency hopping spread spectrum CDMA Applications

SPREAD SPECTRUM SYSTEM Spread spectrum system requirements The signals occupies a bandwidth much in excess of the minimum bandwidth necessary to send the information. Spreading is accomplished by means of a spreading signal, often called a code signal, which is independent of the data. At Rx, dispreading is accomplished by the correlation of the received spread signal with a synchronized replica of the spreading signal used to spread the information.

Cont’d... Principles of spread spectrum Direct sequence technique Two stages of modulation Incoming data sequence is used to modulate a wideband code. The code transform the narrowband data sequence into a noise-like wideband signal. Resulting wideband signal undergoes the second modulation using a PSK technique Frequency hopping technique The spectrum of a data-modulated carrier is widened by changing the carrier frequency in a pseudorandom manner.

Cont’d... Effect of spread spreading Presence of white noise The single-sided power spectral density of white noise, No is unchanged even by expanding the signal BW from W to Wss. The average power is infinite No performance improvement by using spreading

Cont’d... Presence of intentional jammer Signal BW is spread, the jammer can make one of the two choices Jam all the signal coordinates of the system with an equal amount of power in each one, with the result that little power is available for each coordinate. Results in a reduction in jammer noise spectral density by a factor (W/Wss) across the spread spectrum. Noise spectral density - Broadband jammer noise spectral density. Jam a few signal coordinates, with increased power in each of the jammed coordinates. - Results in a reduction in the number of signal coordinates that the jammer occupies.

Cont’d... Effect of spread spreading Compares the effect of spectrum spreading in the presence of white noise with spreading in the presence of an intentional jammer. Presence of white noise The single-sided power spectral density of white noise, No is unchanged even by expanding the signal BW from W to Wss. The average power is infinite No performance improvement by using spreading Presence of intentional jammer Signal BW is spread, the jammer can make one of the two choices Jam all the signal coordinates of the system with an equal amount of power in each one, with the result that little power is available for each coordinate. Results in a reduction in jammer noise spectral density by a factor (W/Wss) across the spread spectrum. Noise spectral density - Broadband jammer noise spectral density. Jam a few signal coordinates, with increased power in each of the jammed coordinates. - Results in a reduction in the number of signal coordinates that the jammer occupies.

Cont’d... Fine time resolution Spread spectrum signals can be used for ranging or determination of position. Distance Measuring the time delay of a pulse as it traverses the channel. Delay measurement is inversely proportional to the BW of the signal pulse. Larger bandwidth, precise the measurement range Over Gaussian channel, a one-shot measurement on a single pulse is not very reliable.

Cont’d... Multiple access Spread spectrum can be used as MAT in order to share a communication resources among numerous users in a coordinated manner – CDMA

DIRECT-SEQUENCE SPREAD-SPECTRUM Basic Spread Spectrum Technique

Cont’d... Direct sequence The spectrum spreading technique whereby a carrier wave is modulated with a data signal x(t). the data modulated signal is again modulated with a high speed spreading signal g(t).

Cont’d...

Cont’d... Example of Direct Sequencing

Direct Sequence Pseudo Noise

PSEUDO NOISE (PN) SEQUENCES Transmitted Reference (TR) can utilize a truly random code signal for spreading and dispreading. Stored Reference (SR) used pseudonoise or pseudorandom code signal. Random signal – cannot predicted, future variations can only be described in a statistical sense. Pseudorandom – deterministic, periodic signal that is known to both Tx & Rx.

Cont’d... PN sequence is defined as a coded sequence of ones & zeros with certain auto correlation properties.

Cont’d... Randomness properties Balance property In each period of a maximum-length sequence, the number of 1s & is always one more than the number of 0s. Run property Among the runs of 1s & 0s in each period of a maximum –length sequence, ½ the runs of each kind are of length 1, ¼ are of length 2, ⅛ are of length 3 and so on as long as these fractions represent meaningful numbers of runs. Correlation property If the period of the sequence is compared term by term with any cyclic shift of itself, it is best if the number of agreements differs from the number of disagreements by not more than one count. The autocorrelation function is periodic & binary valued.

FREQUENCY HOPPING SYSTEMS Frequency Hopping as a spread spectrum technique used by M-ary Frequency Shift Keying (MFSK). the position of the M-ary signal set is shifted pseudorandomly by the frequency synthesizer over a hopping bandwidth. Information bits used to determine which one of M frequencies is to be transmitted

Example A hopping bandwidth Wss of 4 MHz and a frequency step size Δf of 100 Hz are specified. What is the minimum number of PN chips that are required for each frequency word?

Example - solution Number of tones contained in Wss = Wss Δf Minimum number of chips = log2 M

Cont’d... Frequency hopping using 8-ary FSK modulation

FHSS: change carrier frequency at “random” Time

Frequency Hopping Spread Spectrum Transmitter

FHSS Receiver

Cont’d... Robustness Characterize a signal’s ability to withstand impairments from the channel. A signal configured with multiple replicate copies, each transmitted on a different frequency – greater likelihood of survival than does a single such signal with equal total power. Greater diversity – the more robust the signal against random interference

Cont’d... Fast Hopping versus Slow Hopping Fast-frequency hopping. Several frequency hops per modulation symbol. The shortest uninterrupted waveform : the hop. Slow Hopping Slow-frequency hopping. Several modulation symbols per hop. The shortest uninterrupted waveform: the data symbol.

Cont’d... In general SFH in which the symbol rate, Rs of the MFSK signal is an integer multiple of the hop rate, Rh. Several symbols are transmitted on each frequency hop. FFH in which the hope rate, Rh is an integer multiple of the MFSK symbol rate, Rs. The carrier frequency will change or hop several times during the transmission of one symbol.

Cont’d... DS versus FH Direct-Sequence Frequency Hopping For mobile applications – DS represents a reliable mitigation method, signaling renders all multipath signal copies that delayed by more than one chip time. DS radios encounter more randomly distributed errors that are continuous & lower level. Frequency Hopping For mobile applications – FH provide the same mitigation if the hopping rate faster than the symbol rate, hopping BW is large. SFH radios typically suffer occasional strong burst errors. Used for providing diversity in fixed wireless access applications

Spread spectrum advantages Prevent jamming (destroying the signal by another party) Covert communications (prevents eavesdropping or unauthorized listening), as signal is below noise floor!  Military Less multi-path fading (fading is frequency dependent) Multiple access in wireless applications Cordless, mobile phones sharing a small geographical area Code Division Multiple Access (CDMA) : modern mobile telephony

Spread Spectrum issues Receive much wider range of frequencies now  Low-noise design imperative How to generate the same random string at two different locations? Agree on a protocol for exchange of the “key” (or seed) Ex: MATLAB: RAND('state',sum(100*clock)) resets RAND to a different state each time Ex: Bluetooth Algorithms: Barker, M-Sequence, Gold, Hadamard-Walsh Complex sequence  more robust SS link Higher “wasted” BW offset by the fact that more than one user can transmit in the same BW  CDMA

Processing Gain (PG) PG: figure of merit of spread spectrum systems PG=[SNR]out/[SNR]in FH systems: PG = number of FH channels (typical PG ~ 100) Slow FHSS  one hop every few bits Fast FHSS  many hops during each bit DS systems: PG = number of bits in a chip sequence (also called spreading factor: typical PG ~ few thousands) The general principle behind DS-CDMA is that the information signal with bandwidth Bs is spread over a bandwidth B, where B >> Bs . The processing gain is specified as PG = B/Bs The higher the processing gain, the lower the power density one needs to transmit the information. If the bandwidth is very large, the signal can be transmitted such that it appears like a noise. Here, for instance ultra wide band (UWB) systems (see Chapter 3) can be mentioned as a example. One basic design problem with DS-CDMA is that, when multiple users access the same spectrum, it is possible that a single user could mask all other users at the receiver side if its power level is too high. Hence, accurate power control is an inherent part of any DS-CDMA system.

Hybrid Schemes FFH is more resistant to jamming but it is more complex to implement since fast frequency synthesizers are required. In order to reduce complexity, a hybrid DS/FH scheme can be considered. Here, the signal is first spread over a bandwidth as in DS-CDMA and then hopped over a number of channels, each with bandwidth equal to the bandwidth of the DS spread signal. This allows one to use a much larger bandwidth than with conventional DS spreading by using low cost available components. For instance, if we have a 1 GHz spectrum available, a PN code generator producing 109 chips/s or hopping achieving 109 hops/s might not be practicable. Alternatively, we could use two code generators: one for spreading the signal and the other for producing the hopping pattern. Both codes could be generated using low cost components.

Applications Satellite-positioning systems (GPS) 3G mobile telecommunications W-LAN IEEE802.11a IEEE802.11b IEEE802.11g Bluetooth Reducing EMI in digital electronics (SS clock)! Clock is “dithered” by 2 ~ 4%  no more peaks in PSD

CDMA Code Division Multiple Access All users occupy the same bandwidth at the same time! Users are differentiated by the spreading (PN) code used Each data bit pair (0,1) is represented by a unique code pair (Pn0, Pn1) for each specific (transmitter, receiver) pair To each pair, the signal emanating from other pairs will look like noise Ex: IS-95, IS-98, WLANs,…

CDMA APPLICATIONS SSMA techniques allow multiple signals occupying the same RF BW to be transmitted simultaneously without interfering with one another. CDMA using direct sequence – DS/CDMA The use of CDMA provides resistance to external interference and requires no synchronizing mechanism.

cont’d... CDMA – code division multiple access The user transmissions are permitted to overlap in the frequency & time coordinate. The individual users are identified by assigning distinct signaling codes to them. The use of CDMA provides resistance to external interference and requires no synchronizing mechanism.

CDMA: Code Division Multiple Access Allow each station to transmit over entire frequency spectrum all the time. Multiple simultaneous transmissions are separated using coding theory. Colliding frames may not be totally garbled. There are techniques to separate signals sent by different senders. Similar to a party where different conversations use different language. Extract desired signal and reject others as random noises.

Simple Analysis of CDMA Assume 1 MHz band for 100 stations Use FDM, one station has 10kHz and 10 kbps (assume 1 bit per Hz) Use CDMA, one station has 1MHz, and 1Mchips per seconds. If CDMA uses less than 100 chips per bit then CDMA will be more efficient.

CDMA – cont’d… Each bit time is subdivided into m short intervals called chips, typically 64-128 chips per bit. Each station is assigned a unique m-bit code or chip sequence. To send a bit 1, a station sends its chip sequence. To send a bit 0, a station sends the complement of its chip sequence. For m=8, A is assigned 00011011. A sends 00011011 as bit 1, and 11100100 as bit 0.

CDMA – Code Division Multiple Access (a) Binary chip sequences for four stations (b) Bipolar chip sequences (c) Six examples of transmissions (d) Recovery of station C’s signal

CDMA Code Production Walsh function 2 codes: 1 1 1 0 4 codes: 1 1 1 1 1 0 1 0 1 1 0 0 1 0 0 1 Root Copy Copy Inv

CDMA – Code Division Multiple Access 1 1 1 1 8 codes 1 1 1 1 1 1 1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 1 0 0 1 1 0 0 1 1 0 0 1 0 0 1 1 0 0 1 1 0 0 1 1 1 1 1 0 0 0 0 1 0 1 0 0 1 0 1 1 1 0 0 0 0 1 1 1 0 0 1 0 1 1 0

Differentiation by pseudo-random code used only CDMA Differentiation by pseudo-random code used only

A typical DS/CDMA block diagram Cont’d... A typical DS/CDMA block diagram

Exercise A Code Division Multiple Access (CDMA) uses direct-sequence modulation with a data bandwidth of a 10 kHz and a spread bandwidth of a 10 MHz. With only one signal being transmitted, the received Eb/No is 16 dB. If the required (Eb/No + Io) is 10 dB, calculate the equal power users that can share the band. If each user’s transmitted power is reduced by 3 dB, calculate the equal power users that can share the band. If the received Eb/No for each receiver, calculate the maximum number of users that can share the band. Formula/Equation in Section 12.8

End of Chapter 6