PoS(ISCC 2017)047. Fast Acquisition for DS/FH Spread Spectrum Signals by Using Folded Sampling Digital Receiver

Transcription

1 Fast Aquisition for DS/FH Spread Spetrum Signals by Using Folded Sampling Digital Reeiver Beijing Institute of Satellite Information Engineering Beijing, , China Bo Yang Beijing Institute of Satellite Information Engineering Beijing, , China Guangnan Zou Beijing Institute of Satellite Information Engineering Beijing, , China A fast aquisition method is proposed to improve the detetion speed of diret sequene/ frequeny hopping spread spetrum (DS/FHSS) ommuniation system. A novel digital wideband reeiver by using folded sampling is presented to redue the searhed hopping frequenies in oarse aquisition. There are two seletive filters onneted with the RF downonverter in the new sheme, one of whih is wideband and the other is narrow and similar as the traditional one. The aquisition is ahieved on two stages: firstly, the folded sampling is used when the wideband filter is seleted and the hopping frequenies on multiple Nyquist zones are folded together on the first one, then fine aquisition is performed while the narrow band filter is seleted. The detetion performane of the propose method is dedued through state flow graph and verified by simulation. The results show the new method performs better when ompared with the traditional one. It an effetively promote the detetion speed with few detetion performane loss. ISCC Deember, 2017 Guangzhou, China 1 Speaker This work is supported by Natural Siene Foundation of China under Grant Corresponding Author 2 Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommerial-NoDerivatives 4.0 International Liense (CC BY-NC-ND 4.0). 1,2,3

2 Fast Aquisition for DS/FHSS 1. Introdution 2. Proposed Folded Sampling Digital Reeiver and Aquisition Sheme Figure 1 :Struture of Folded Sampling Digital Wideband Reeiver The reeiver infrastruture of the proposed method is shown in Fig.1. Exept for the power supply, the reeiver is omposed of RF front end, A/D onverter and baseband proessor. The reeived signal at the RF front end of the reeiver in one hop dwell is x (t)= 2S ( t ζ T )os [2π( f + f i)t +θ ]+ N (t) 2 (2.1) DS/FHSS is the prefered ommuniation method while LPI and LPJ apabilities are required. Fast synhronization is the essential preondition for the transeiver and the aquisition speed is essential espeially when the DS/FHSS signal is transmitted in a burst manner. The aquisition of DS/FH spread spetrum is an exiting researh topi [1-8]. The searh proess is generally performed from two dimensions [1,2]: FH pattern and PN ode phases. The full parallel method, whih detets the two dimensions simultaneously, is infeasible while PN ode is long for its extremely large hardware onsumption [3]. The serial searh tests the FH pattern and PN ode phases one by one [4,5]. A two-step aquisition tehnique was introdued [6] to searh the FH pattern at the first step and PN ode phase at the seondary step. It is onvenient in implementation but slow in aquisition espeially when the FH and PN ode is long. To overome this demerit, a fast sanning and waiting method is thus provided [2]. The performane is measured with detetion probability and mean aquisition time. A folded sampling digital reeiver is presented herein to further improve the searh speed. In the traditional methods, the signal delivered by RF (Radio Frequeny) downonverter is filtered in hopping arrier spaing [1,2,4~9] and outputs IF (Intermediate Frequeny) signal before aquisition proess. as there are two seletive filters onneted with the downonverter in the new sheme, one of whih is wideband and the other is narrow and similar as the traditional one. The folded sampling is used when the wideband filter is seleted and the hopping frequenies on multiple Nyquist zones are folded together on the first one. Thus, the searhed hopping frequenies are dereased while the noise floor is higer in aquisition proess. The detetion performane of the propose method is dedued and verified. The results show the proposed sheme outperforms than previous work, beause it an effetively promote the detetion speed.

3 Fast Aquisition for DS/FHSS S In (2.1), is the transmitter signal power, length L and ode interval T (t ζ T ) is the PN sequene with ζ T is the relative delay between input PN seonds, ode and loal one, f is the reeiver arrier, f i is the frequeny of hop pattern f 1, f 2, f 3,, f N }, N is the size of hopping pattern, θ is the phase offset, and N (t ) is the noise introdued by AWGN hannel. The hopping arrier spaing (hop slot) is B FH, and the total hopping bandwidth is Bw =NB FH. If digitizing the hopping signal in whole bandwidth, Nyquist sampling is and narrowband filter is Bw/ and frequeny in the proposed method is is folding fator. The bandwidth of broadband B FH respetively. The frequeny synthesizer works in three modes: (1) waiting mode: output fixed frequeny f ; (2) sanning mode: hop over f +(k 1) f s /2, k =1,2,, } ; (3) normal mode: hop synhronizely with the reeiver signal. In oarse aquisition, the frequeny synthesizer works in waiting mode and the mixer output is filtered through the broadband filter. From Fig. 2, it is seen that hopping frequenies f j, 1,f j,2,, f j, } are folded together within the output frequeny range (0, f s /2). Thus, the hopping frequenies to be searhed iderease from N to N '=N /, whih an be expressed as f 1, f 2, f 3,, f N ' }. The FH pattern is alternated simultaneously. Figure 2 : The Nyquist Folded Sampling Graph The signal sampled by A/D onverter is: r (n)= 2S ( n/ f s ζ T )os [2πn ( f i _ folded )/ f s +θ ]+N ( n/ f s ) i _ folded =1,2,, N ', and given by fs f i _ folded = f i 2 (k 1), k odd kf s /2 f i, k even (2.2) (2.3) The r (n) is onverted to baseband by the DDC (Digital Down Converter) and orrelated with loal PN ode. The searh method in oarse aquisition an be the same as traditional algorithm. Without loss of generality, a fast aquisition algorithm without any prior knowledge is used as in [2]. Let f j denote DDC s loal osillator s frequeny, and the aquisition algorithm proposed as follows: Step 1: set loal osillator s frequeny to the first hop frequeny 3 f j= f 1. 2Bw. The sampling f s=2b/, =1,2,3,, where frequeny

4 Fast Aquisition for DS/FHSS T seonds. Find the peak of the orrelation Rmax. Step 3: if Rmax >η, reord f i _ folded = f j and go to Step 4; otherwise go to Step 5. Step 4: if at least B of A independent orrelation values pass the validation, the Step 2: san f for j searh of input folded hop frequeny delares suess. Step 5: if aquisition is failed in Step 3 or Step 4 and the hopping pattern is not searhed totally ( j<n ' ), then j= j+1 (swith to Step 2) and ontinue searhing until the j=n ' (swith to step 1). After suessful oarse aquisition, there are frequenies f j, 1,f j,2 } j, frequeny f mapped to synthesizer f +( k 1) f s /2, k =1,2, } is. Fine aquisition is performed as follows. j set in sanning mode and hops over, while the DDC s loal osillator frequeny is: f DDC, k = f j, k odd f s /2 f j, k even (2.4) The orrelation urve is obtained in every sanned hop, whose peak is preserved. Find the maximum of peaks and reord the orresponding k. Then the exat frequeny of x (t) is f + f j.k }. If =1, the proposed method an be seen the speial ase and similar to the traditional one while the fine aquisition is no longer demanded. 3. Performane Analysis of the Proposed ethod 3.1 Correlation Properties of Folded Sampled Signal The in-phase and quadrature results of orrelator in Fig.1 an be expressed as : k=1 k=1 e I = S T os(θ )δ ( f i _ folded f j ) p+k k + N ( q+k ) k = yos (θ )δ ( f i _ folded f j )+N I (3.1) And k=1 k=1 e Q= S T sin(θ )δ ( f i _ folded f j ) p+ k k + N s (q+k ) k = ysin(θ )δ ( f i _ folded f j )+ N Q NI and (3.2) 2 N Q are AWGN with N (0, σ n ), σ n=n 0 L T /2, δ ( ) is given by δ ( z)= 1, z=0 0, else In (3.1) and (3.2), y is equal to y m1 = s T, H 1 m 0=0, H 0 The orrelation (3.3) R= e I 2 +e Q2 is subjet to Rie distribution : }( ) R 2+mi2 R mi R f (R H i )= 2 exp I, R 0, i=0,1 0 σn 2 σ n2 σ n2 The umulative distribution of R an be given by 4 (3.4) The,, f

5 Fast Aquisition for DS/FHSS F ( R H i )=1 Q 1 ( Q1 ( ) mi R, ) σ n σn (3.5) is the arum funtion. 3.2 ean Aquisition Time Assuming there is no prior knowledge of the reeived signal s hopping frequeny, in the new sheme, there are N '=N / hopping frequenies to be searhed at one aquisition proess. The mean aquisition time an be obtained with the state flow diagram [10,11]. In Fig. 3, the state S 1, S 2, S 3,, S N ' } orresponds to the tested N ' hopping S N' is the state the input folded signal mathes the loal hopping frequeny and the suessful aquistion is possible, and the false alarm state ( FA ) is a ontrary state. Coarse Aq and Fine Aq represent the folded reeived signal that is deteted and the unfolded reeived signal is searhed respetively. Figure 3 : State Flow Diagram of the Aquisition with A Priori Distribution π j } In Coarse Aq proess, the detetion is ompleted when the input folded hop frequeny is searhed suessfully, and the probability is given by : L 1 P d1= f ( x H 1 )( F ( x H 0 ) ) dx (3.6) η S N' The missing probability in State orresponding to that none of the orrelations exeed the detetion threshold: P m1=f ( η H 1)( F (η H 0 )) L 1 If the searhed hop frequeny is inorret, the state swith from (3.7) S N' to FA and the probability is P fa1=1 P d1 P m1 The detetion probability and the false alarm probability in heking an be expressed as umulative binomial distribution A () P d2 = A P d n (1 P d ) A n n= B n 5 (3.8) frequenies. State

6 Fast Aquisition for DS/FHSS A () P tfa = A P fa n (1 P fa )A n n= B n (3.9) P d = f ( x H 1)( F ( x H 0)) L 1 dx 0 P fa = f ( x H 0) F ( x H 1 )( F ( x H 0 )) L 2 dx 0 P nfa0 The P nfa1 and S 1, S 2, S N ' 1 } orresponding to is obtained as P nfa0 =( F ( η H 0 )) A L (3.10) () P nfa1=1 A P fa0n ( 1 P fa0 ) A n n= B n P nfa0 =( F (η H 0 )) (3.11). frequenies are searhed and the detetion probability an In Fine Aq proess, be alulated by 1 P d3= f ( x H 1 )( F ( x H 0 ) ) dx (3.12) 0 The false alarm probability is P fa3=1 P d3 When the priori distribution π j } is uniform, the generating funtion is (3.13) N' H D ( z )( 1 H 0 ( z )) P ACQ ( z)= N ' 1 N ' (1 H ( z )H 0 ( z ))(1 H 0 (z )) H D (z ) represents the transfer funtion from state (3.14) S N' to Coarse H (z ) represents the transfer funtion onneting S N ' with S 1 while H 0 ( z ) represents the transfer funtion between (S i, S i +1 ); i=1,, N ' 1. Aquisition, They are obtained as follows: H D (z )=P d1 z LT P d2 z ALT P d3 z LT H ( z )=P m1 z LT +P d1 z LT P fa2 z AL T + P fa1 z LT ( P tnfa z ALT +P tfa z ALT z PLT ) +P d1 z LT P d2 z ALT P fa3 z L T H 0 ( z )=P nfa0 z LT +P fa00 z LT [ P nfa01 z AL T +P fa01 z z ( A+P ) LT ] The mean aquisition time is: E (T ACQ )= d P ACQ (z ) z =1 d z LT ( N ' 1) 1+ A(P d1+p fa1 )+P P tfa P fa1+ P d1 P d2+ P0 P d1 P d2 P d3 2 P 0=[1+ A P fa00 +P P fa00 P fa01 ]( 2 P d1 P d2 P d3 ) = 6 } (3.15) follows:

7 Fast Aquisition for DS/FHSS 4. Results and Disussion In this Setion, the detetion performane of the proposed algorithm is evaluated by simulation. A traditional one is adopted for omparison, where a fast PN ode phase sanning and serial searh for FH pattern method [2] is adopted as a omparison. I n evaluating the 6 performane, the following parameters were onsidered: PN ode rate 1 10 hip/s and L=64, number of hopping frequenies N =64, hoping frequeny set length f 1, f 2, f 3,, f 64} = 1,3,5, ;, 127} Hz, verifiation pair A=3, B=2 P=100. Hz, the orrelation peak by using proposed method an still be ahieved in Fig. 5. Figure 4 : Correlation Curves for Two ethods when =2, f i =17 Hz and f j =17 Hz The detetion performanes of the traditional method and the proposed sheme with different folding fator are presented in Fig. 6. The hopping frequeny and the loal osillator s frequeny are the same here, while the situation f i f j is not disussed as the traditional method that annot detet signal. The detetion of folded hop frequeny and the aurate one delared suess one the single orrelation peak exeeds the threshold. The false 5 alarm probability is 10 here. It is seen that there is a loss of detetion performane of hopping frequeny and spread ode after folded sampling. When the folding fator grows, the performane degradation beomes severe due to the noise floor inreases while the folded sampling is applied. Figure 5 :Correlation Curves for Two ethods When =2, f i =111 Hz and f j =17 Hz In Fig.7 the detetion speed is ompared between the proposed method and the traditional one. The threshold η is seleted to maximize the detetion speed. In the new algorithm, the 7 Fig. 4 shows the orrelation urves for two aquisition methods with the reeived signal SNR=5dB and the folding fator =2 in the proposed method. The loal osillator s frequeny f j=17 Hz. When the hopping frequeny f i=17 Hz and f i=111

8 Fast Aquisition for DS/FHSS unmathed hopping frequenies are exluded quikly and searhed hopping frequenies are dereased, while the detetion performane suffers degradation. While the SNR is high and the dettion performane has smaller effets than the tested hopping frequenies, the proposed algotithm an effetively promote the detetion speed and outperform the traditional method. =2 osts about 50% of the traditional aquisition time and The proposed method with =4 osts about 25% respetively. Figure 7 : ean Aquisition Time of Two ethods 5. Conlusions In this paper, the folded sampling digital reeiver sheme to derease the aquisition time of DS/FHSS ommuniation system has been proposed. The detetion performane of the propose method is dedued through the state flow graph and verified by simualtion. The numerial results show the new method performs better when ompared with the traditional one when the SNR is high, it an effetively promote the detetion speed with few detetion performane loss. Referenes [1] W. Yang, S. eng, J. Wang, J. Liu. Aquisition performane analysis of a synhronization sheme of DS/FH hybrid spread spetrum signals for TT&C[C]. Eletroni easurement & Instruments, Beijing: [2] X. Wen, W. Song, Y. Zhang, D. Shao and S. Li, Performane Analysis of a Digital Aquisition Ciruit for DS/FH Spread Spetrum Signal[C] International Symposium on Computer Network and ultimedia Tehnology, Wuhan: 1-4. [3] X. Gou B. Zhang Q. Wu, et al. A Fast Synhronization ethod of DS/FH Hybrid Spread Spetrum TT&C System[J]. Energy Proedia, 2011, 13: [4] J. Chen, X. Zhou. A Study of Syhronization and anti-jamming for DS/FH Hybrid Spread Spetrum Signal[D]. University of Eletroni Siene and Tehnology of China. ay.2009.(inchinese) [5] N. Benvenuto, G. Guidotti and S. Pupolin. Performane of a digital aquisition iruit for hybrid FH-DS spread spetrum systems[c] ilitary Communiations Conferene, 1988: [6] F. Wang; D. Zhou. Casade onnetion serial parallel hybrid aquisition synhronization for DSFHSS in air-ground data link[c].seond International Conferene on Spae Information Tehnology Figure 6 :Detetion Probability of Two ethods

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