This article is about FHSS (Frequency Hopped Spread Spectrum) and one method suggested for Security improvement in it. In this report you can find an in depth information on FHSS from the basic level to the aim of the project. A detailed explanation is given about the Modulation, Frequency Hopping, PN -sequence generation, Fr equency table w.r.t PN sequence which form crucial components in generation of the FH -spread signal. And finally demodulation is done and the obtained waveforms are analyzed. The whole process is simulated in the well -known simulator MATLAB. Working code i s attached with this report. One method is suggested to improve the security in the FHSS. Introduction : DEFINITION: It is the repeated switching of frequencies during radio transmission, often to minimize the effectiveness of "electronic warfare" - that is, the unauthorized interception or jamming of telecommunications.  Generate a bit pattern.  The original message modulates the carrier, thus generating a narrow band signal.  The frequency of the carrier is periodically modified (hopped) following a specific spreading code.  In FHSS systems, the spreading code is a list of frequencies to be used for the carrier signal.  The amount of time spent on each hop is known as dwell time.  Redundancy is achieved in FHSS systems by the possibility to execute re - transmissions on frequencies (hops) not affected by noise. FHSS Sender Block Diagram: - Figure 1:Simplified Diagram for Transmitter for FHSS 3 FHSS Receiver Block Diagram: Figure 2: Simplified diagram for Receiver for FHSS Implemented Block Diagram of FHSS Transmitter: - Figure 3:Transmitter of proposed Technique 4 Explanation: The above block diagram is the sender block that is used in FHSS. The improvised block diagram is given above where all the odd bits use the PN sequence and the even bits use the encrypted PN-sequence (GRAY CODED) for frequency synthesizing. This makes the jammer harder to find the PN –sequence because the repetition occurs after several iterations th an the un improvised one. Realization in MATLAB: With the help of the block diagram we are able to carry out step by step process. Firstly a bit sequence is generated, and then follows the modulation of the signal (BPSK). Then PN -sequence is generated and the even bits are replaced with its gray code and thus now we have an Improvised PN -sequence. This follows the frequency synthesizing with the help of Improvised PN-sequence. Thus finally multiply the frequencies with the modulated signal to give out the FH- spread signal. Using the grid, plot commands we managed to get the plot between frequencies and PN -sequence. Uses:  Important form of encoding for wireless communication.  Transmit either analog or digital data  Analog signal (transmission)  Developed init ially for military and intelligence requirements  Spread data over wider bandwidth  Makes jamming and interception harder 5 MATLAB CODE: clc clear all close all even=1; odd=1; % generate pn sequence cp=randint(1,20,[1 7]); % for k=1:20 if(mod(k,2)) dualcode(1,k)=cp(1,k); evenbits(1,even)=cp(1,k); even=even+1; else oddbits(1,odd)=cp(1,k); odd=odd+1; const= bin2gray(cp(1,k), 'psk',16); dualcode(1,k)=const; end end s=round(rand(1,20)); signal=[]; carrier=[]; 6 t=[0:2*pi/119:2*pi]; for k=1:20 if s(1,k)==0 sig=-ones(1,120); else sig=ones(1,120); end c=cos(t); carrier=[carrier c]; signal=[signal sig]; end subplot(4,1,1); plot(signal); axis([-100 2500 -1.5 1.5]); title('\bf\it Original Bit Sequence' ); bpsk_sig=signal.*carrier; subplot(4,1,2); plot(bpsk_sig) axis([-100 2500 -1.5 1.5]); title('\bf\it BPSK Modulated Signal' ); t0=[0:2*pi/4:2*pi]; t1=[0:2*pi/9:2*pi]; t2=[0:2*pi/19:2*pi]; 7 t3=[0:2*pi/29:2*pi]; t4=[0:2*pi/39:2*pi]; t5=[0:2*pi/59:2*pi]; t6=[0:2*pi/119:2*pi]; c0=cos(t0); c0=[c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0 c0]; c1=cos(t1); c1=[c1 c1 c1 c1 c1 c1 c1 c1 c1 c1 c1 c1]; c2=cos(t2); c2=[c2 c2 c2 c2 c2 c2]; c3=cos(t3); c3=[c3 c3 c3 c3]; c4=cos(t4); c4=[c4 c4 c4]; c5=cos(t5); c5=[c5 c5]; c6=cos(t6); % Random frequency hopps to form a spread signal spread_signal=[]; for n=1:20 c=dualcode(1,n); switch(c) case(1) spread_signal=[spread_signal c0]; case(2) spread_signal=[spread_signal c1]; case(3) spread_signal=[spread_signal c2]; case(4) spread_signal=[spread_signal c3]; case(5) spread_signal=[spread_signal c4]; case(6) spread_signal=[spread_signal c5]; case(7) spread_signal=[spread_signal c6]; end end subplot(4,1,3) plot([1:2400],spread_signal); axis([-100 2500 -1.5 1.5]); title('\bf\it Spread Signal with 7 frequencies' ); % Spreading BPSK Signal into wider band with total of 12 frequencies freq_hopped_sig=bpsk_sig.*spread_signal; subplot(4,1,4) % plot([1:2400],freq_hopped_sig); axis([-100 2500 -1.5 1.5]); title('\bf\it Frequency Hopped Spread Spectrum Signal' ); % Expressing the FFTs rec=freq_hopped_sig.*spread_signal; lpf=fdesign.lowpass( 'n,fc',50,0.0168,119); 9 d=design(lpf); j=filter(d,rec); figure,plot(j); figure,subplot(2,1,1) plot([1:2400],freq_hopped_sig); axis([-100 2500 -1.5 1.5]); title('\bf\it Frequency Hopped Spread Spectrum signal and its FFT'); subplot(2,1,2); plot([1:2400],abs(fft(freq_hopped_sig))); cc=zeros(size(dualcode)); cc=dualcode; figure fit=[]; for k=1:20 sig=(cc(1,k) - 0.5)*ones(1,10); fit=[fit sig] ; end plot(fit, 'oblack' ,'linewidth' ,8); axis([-10 200 0 8]); grid on hold on for k=10:20:190 stem(k,9, '--','black') hold on end 10 OUTPUTS: Bit Sequence: 11 SPREAD SIGNAL WITH SEVEN FREQUENCIES: FH-SPREAD SIGNAL: 12 FFT OF FH -SPREAD SIGNAL: DEMODULATED WAVE: 13 FREQUENCY vs PN -SEQUENCE: BENEFITS OR ADVANTAGES OF FHSS: Following are the disadvantages of FHSS:  It provides very robust transmission path in the presence of interferences such as multipath, noise and other wireles s transmissions etc. due to support of wide bandwidth.  It can be employed in point to multipoint applications.  It supports about ten nearby WLAN compliant APs (Access Points) without any significant interference issues.  It provides security against any kind of intrusion as only transmitter and receiver are aware of PN codes. DRAWBACKS OR DISADVANTAGES OF FHSS: Following are the disadvantages of FHSS:  As FHSS relies on carrier frequencies to transmit information bits, it leads to strong bursty errors due to frequency selective fading mainly. 14  It supports lower data rate of 3 Mbps compare to 11 Mbps supported by DSSS.  It supports lower coverage range due to high SNR requirement at receiver.  The modulation scheme has become obsolete due to use of emerging wireless technologies in the wireless products. RESULT and CONCLUSION: Thus we have tried hard to reach the aim in getting all the plots as desired and the results match with the theoreti cal background. FHSS is implemented in MATLAB and its fft plot gave us a clear insight on it regarding the spreading of the signal over the entire spectrum. And the improvised PN sequence is implemented to improve the security in the FHSS transmitted signa ls and is achieved .frequency vs PN -sequence plot gave a clear picture on how the frequency is allotted for the given PN sequence. Glossary PN Sequence: (A pseudo noise code (PN code) or pseudo random noise code (PRN code) is one that has a spectrum similar to a random sequence of bits but is deterministically generated.) Gray Code: a numerical code used in computing in which consecutive integers are represented by binary numbers differing in only one digit. Binary Phase Shift Keying (BPSK) is a two p hase modulation scheme, where the 0’s and 1’s in a binary message are represented by two different phase states in the carrier signal: θ=0 ∘ 16 for binary 1 and θ=180 ∘ for binary 0.