6 IEEE Ninth International Symposium on Spread Spetrum Tehniques and Appliations An Aquisition Method Using a Code-Orthogonalizing Filter in UWB-IR Multiple Aess Shin ihi TACHIKAWA Nagaoka University of Tehnology, Nagaoka-shi, 94-188 Japan e-mail: tah1@vos.nagaokaut.a.jp Yoshihiro KAMEYAMA Nagaoka University of Tehnology, Nagaoka-shi, 94-188 Japan Abstrat- This paper proposes an aquisition method using a ode-orthogonalizing filter (COF) in an ultra wideband -impulse radio (UWB-IR) multiple aess ommuniation system with time hopping / spread spetrum (TH/SS). In the COF of this system, the partiular template signal of the TH system is adopted. By using a COF, multiple aess interferenes are suppressed, and orthogonalization of an own-sequene is ahieved. Then, alulating the orrelation between the oeffiients of a COF and the own-sequene of in-phase, the synhronous phase an be obtained. It is shown that the proposed method an be superior to the onventional sliding orrelator when the number of users inreases. Key words- UWB-IR, TH/SS, aquisition method, ode-orthogonalizing filter, multiple aess interferene, average aquisition time I. INTRODUCTION In reent years, the various demands for radio ommuniation systems have inreased rapidly. Ultra wideband impulse radio (UWB-IR) systems an realize high date rate ommuniations using nano-seond pulse of base-band. In one of UWB-IR systems, spread spetrum (SS) tehnique using time hopping (TH) is adopted. It is modulated by position of pulse and an be avoidable for influene of amplitude flutuation [1][]. In UWB/spread spetrum systems, signal aquisition is important [3][4]. A sliding orrelator is well known as a onventional aquisition method in SS tehniques, however, in a large multiple aess interferene (MAI) environment (e.g., near-far problem), synhronous aquisition using the sliding orrelator will be diffiult to ahieve beause large peaks of ross-orrelation funtions will appear periodially. To overome this problem, an aquisition method with a ode-orthogonalizing filter (COF) in diret sequene/ ode division multiple aess (DS/CDMA) system has been proposed previously [5][6]. In this method, the tap oeffiients are adaptively ontrolled to orthogonalize to all user s sequenes inluding the desired user s spreading sequene for eah phase. By this method, the PN sequene aquisition an be ahieved in a large MAI environment. However, it is neessary to modify for appliation of the UWB-IR (TH/SS) system. In this paper, we propose a modified aquisition method using COF in an UWB-IR (TH/SS) system and ompare the average aquisition time of the proposed method with that of a onventional method. In Set., we present a brief introdution to the system model of UWB-IR (TH/SS type) system. In Set. 3, we explain the basi onstrution of a COF in UWB-IR, and, in Set. 4, we propose an aquisition method using a COF for UWB-IR. Simulation results are disussed in Set. 5. Finally, onlusion remarks are made in Set. 6. II. SYSTEM MODEL An UWB-IR (TH/SS type) system is used nano-seond pulse of base-band, and adopted pulse position modulation (PPM), and its own TH pattern for eah user. The typial transmitter out put signal s (t) is given by NS 1 ( k) ( ( ) ( ( ) ( ) s f j k T s t w tit jt b T a j k u i k )) i j -783-978-/6/$. 6 IEEE 49
1 for t T / where w(t) { otherwise, where t is the transmitter lok time, w(t) represents the transmitted monoyle waveform (see Fig. 1), T s is symbol duration, T f is frame duration, T is hip duration of waveform, T =T f /N f (N f : the number of hips per frame), N s is the number of frame per data, data symbol duration is T s =N s T f, the whole ode length is N s N f, b j is the j-th pulse position of k-th user (TH pattern), a ( j k) ( 1, ) is the PN sequene of the j-th pulse position of k-th user. u i ( k) ( 1, ) is the i-th data of k-th user. means addition of modulo. When the number of users is N u, the reeived signal r(t) is given by Amplitude. 1. 1..5T Amplitude -1. -.5.5 1. 1.5 -.5.5 1. 1.5 Time[s] xt Time[s] xt -1. Fig. 1. Reeived signal. Fig.. Template signal. 1 1 Nu Ns1 ( k) rt Ak w( ( k) ( t tppm k ) ) nt ( ) (3) k1 i j where ( ) ( ) t k tits jtf b j k T (4) t ( ppm k) T ( a ( j k) u ( i k) ) (5) where ( k) is delay time (phase), and n(t) is an additive swhite Gaussian noise. The template waveform in a pulse v(t) (see Fig. ) at the reeiver is given by T vt wt wt ( ) (6) As a summation for duration T s, the k-th user s template signal in a symbol ( v ) t is given by s k s k Ns k v ( ) 1 t v( t ( ) jtf ( j k) T ) j The demodulation is ahieved by the orrelation between the reeived signal and the template signal (see Fig. 3). In ase that data is, the ross-orrelation funtion value is plus value, in ase that data is 1, that is minus value (() in Fig. 3). (7) (a) Reeived signal. (b) Template webform. () Collelation. Fig. 3. Demodulation by PPM. III. CODE-ORTHOGONALIZING FILTER (COF) AND ITS ALGORITHM IN UWB-IR (TH/SS TYPE) Fig. 4 shows blok diagram of COF in UWB-IR (TH/SS) for suppression of MAI. In this setion, the desired user s signal (k=1) is in-phase ( 1 ). The reeived signal vetor r is defined as r ˆ rframe, rframe,..., rframe (Ns 1) (8) where ( ) T denotes transposition of matrix ( ), and r frame (j) is the j-th integral value of T f duration. The tap oeffiient vetor is defined as T, 1 Ns1,..., (9) In Fig. 4, using least mean square (LMS) algorithm by unit of delay time T f, the tap oeffiients are usually ontrolled to minimize the error signal e( i) whih is the differene between the deided (desired) signal d( i) and the transversal filter output yi. This algorithm means the approah of orthogonalizing to reeived all other user s sequenes. In this paper, to approah to orthogonalize is represented as to orthogonalize for onveniene of engineering sense. The signals of the i-th symbol are given by T 5
y( i) T r i i e( i) d( i) y( i) (11) ( i1) ( i) e( i) r( i) (1) where vetor ( i) and vetor r( i) are represented as the tap oeffiient vetor and the reeived signal vetor r of the i-th symbol, respetively. As the initial value of i, desired user s sequene is set. The reeived signal is orrelated the template signal. In every Tf seonds, the tap oeffiient vetor of the (i+1)-th symbol is obtained by (1) where is the step size for LMS algorithm. ( i) and r( i) are represented as Ns-th order vetors, and the alulation of Eq. (1) is ahieved by parallel operation for eah tap oeffiient. Reeived signal r(t) Template signal vs(t) Tf Tf Tf Tf Tf Ns-1 1 LMS Algorithm y(i) Fig. 4. Blok diagram of ode-orthogonalizing filter. IV. A PROPOSED SYNCHRONOUS ACQUISITION METHOD USING A CODE-ORTHOGONALIZING FILTER IN UWB-IR (TH/SS TYPE) To employ the COF for an aquisition method in UWB-IR (TH/SS), the deided signal d( i) is applied for the basi COF of Fig. 4. The priniple of the proposed aquisition method is explained as follows. First, the k-th user s sequene vetor a is defined as ( k) T ( ) ( ) ( k k a ( ), ( ),, ( k) a a 1 a ( N s 1)) (13) where ( ) a k ( 1, 1) (14) - -- e(i) Output d(i) onsider the differene of the reeived signal between in-phase reeption ( 1 ) and out-of-phase reeption ( 1 ) with the desired signal. Fig. 5 shows the state of tap oeffiient vetor both in-phase reeption and out-of-phase reeption. In-phase reeption is the ondition that the phase of the desired user s sequene in the reeived signal oinides with no phase offset of the desired user s sequene prepared at the reeiver ((a) in Fig. 5). Out-of-phase reeption is otherwise ((b) in Fig. 5). In this system, the tap oeffiients are orthogonalized to all user s sequenes inluding desired user s sequene, i.e., di of Fig. 4 is applied. Then, in the ase of in-phase reeption, the tap oeffiient vetor is orthogonalized to the desired user s sequene vetor a whih has no phase offset (i.e., T a =). While in the ase of the out-of-phase reeption, it an be onsidered that the desired user s sequene whih has no phase offset does not exist in the reeived signal. The tap oeffiient vetor is orthogonalized to the desired user s sequene vetors, a (-1), a, whih are partial sequenes divided by phase offset [5]-[7] (i.e., T a (-1)=, T a =), however, in general, the sequene vetor a is not orthogonalized to the tap oeffiient vetor (i.e., T a 1 unknown ). Then, a proposed synhronous aquisition method is shown as follows. The orrelation between the reeived signal and the template signal is alulated. And it is input to a COF. Tap oeffiients of COF are onverged for the inoming signal phase. (3) The ross-orrelation funtion value between the tap oeffiients and the desired user s sequene with no phase offset a is alulated and stored to the memory. (4) Tap oeffiients of COF are onverged for.5 hip sliding of the inoming signal. (5) The ross-orrelation funtion value between and a is alulated and stored. (6) The similar manner is ahieved for next.5 hip sliding in order. The number of ross-orrelation funtion values beomes N f N s. (7) The synhronous phase is deided from the phase, whih takes the minimum ross-orrelation funtion value. As a desired user, k=1 is adopted at the reeiver, we 51
Desired sequene vetor a Orthogonalization Tap oeffiient vetor (a) In ase of in-phase ( 1 ). Desired sequene vetor Tap oeffiient vetor Correlation funtion value (b) In ase of out-of-phase ( 1 ). Fig. 5. State of tap oeffiient vetor for the proposed method. V. COMPUTER SIMULATION RESULTS Table 1 and Table show speifiations of simulations in the proposed aquisition method and the sliding orrelator, respetively. E b is bit energy. Channel model is AWGN with the double-sided power spetral density N /. E b /N is 9.8[dB]. Signal reeption symbol timing and frame timing are asynhronous, and hip timing is synhronous, onveniently. The initial tap oeffiients of COF is set the desired user s PN sequene a. The interval of phase sliding of proposed aquisition method is.5t. Reliability of the phase by the aquisition is more than 9 (The total number of trials is 1). Tolerane level of in-phase detetion is from in-phase to ±.5 hip. The method of moving averages [8] is adapted in the onvergene deision of tap oeffiients. In this method, the mean square error (MSE) is averaged during average bits and a alulated at intervals of shift bits. The MSE value to the previous MSE value ratio, i.e., the onvergene deision parameter, beomes more than.7, the onvergene deision is ahieved. Step size is 1-3. The number of users is 1 to 16. SIR is represented as power ratio of desired signal to interferene per user. In the sliding orrelator (Table ), threshold level is ± 7.. The synhronous phase is deided when the threshold level is exeeded n-times ontinuously. n is deided to satisfy the required reliability 9[%]. The number of users, interval of phase sliding, required reliability and SIR are the same values of Table 1. TABLE 1SPECIFICATION OF SIMULATION (PROPOSED METHOD). Primary modulation Binary PPM with balaned PN sequene Seondary modulation Time Hopping Code length N s,n f 8,8 E b /N [db] 9.8 Channel model AWGN Initial of tap Desired PN sequene oeffiients Interval of phase sliding.5t Required reliability [%] 9 Average time of moving average [bits] Shifted time of moving average [bits] Convergene deision parameter 3 3.7 Step size parameter μ 1-3 Number of users 1~16 SIR [db], -3 TABLE SPECIFICATION OF SIMULATION (SLIDING CORRELATOR). E b /N [db] 9.8 Threshold level ± 7. Deision of in-phase When the threshold level is exeeded n-times ontinuously 5
Fig. 6 shows the average aquisition time vs. the number of users for several SIR and the omparison of sliding orrelator. In the proposed method, it an be ahieved that the average aquisition time is almost onstant in large multiple aess interferene environment, beause it an be suppressed the multiple aess interferene by using a COF. Further, the aquisition time of proposed method of SIR= [db] almost oinides with that of SIR=-3 [db]. They overlap eah other. While, in the sliding orrelator, the aquisition an not be ahieved when the number of users inreases and interferene power inreases, beause the peak values of ross-orrelation funtions beome large. For example, in ase that SIR is [db], the average aquisition time of the proposed method is shorter than that of the sliding orrelator when the number of users is more than 8. In ase that SIR is 3[dB], the superiority of proposed system appears when the number of users is more than 4. Average aquisition time Proposed method(sir=[db]) Proposed method(sir=-3[db]) Sliding orrelator(sir=[db]) Sliding orrelator(sir=-3[db]) T [s] 1.E+1 1.E+9 1.E+8 1.E+7 1.E+6 1.E+5 1.E+4 4 8 1 16 Number of users Fig. 6. Average aquisition time vs. number of users. VI. CONCLUSIONS In this paper, we have proposed an aquisition method using a COF in UWB-IR (TH/SS type) multiple aess. We have presented the priniple and performane evaluation of the proposed method, ompared with the sliding orrelator by omputer simulations. As a result, we have shown that it an be ahieved the synhronous aquisition by using the proposed method even in large power multiple aess interferene environments. For further studies, disussions of a omparison the proposed tehniques with similar MUD tehniques in terms of performane and omplexity, a method of fast onvergene of the tap oeffiients in COF and the appliations for several hannels will be neessary. REFERENCES [1] Moe Z, Win, Robert A. Sholtz, Ultra Wide Bandwidth Time-Hopping Spread-Spetrum Impulse Radio for Wireless Multiple-Aess Communiations, IEEE Transations on ommuniations, Vol.48, No.4, April. [] S. Sumi, S. Tahikawa, On a Traking Method for UWB-IR (TH/SS Type), Tehnial Report of IEICE, WBS3-84, pp. 5-3, Ot. 3. [3] W. Suwansantisuk, M. Z. Win, and L. A. Shepp, On the Performane of Wide-Bandwidth Signal Aquisition in Dense Multipath Channels, IEEE Trans. Veh. Tehnol., vol. 54, no. 5, pp. 1584-1594, Sept. 5. [4] W. Suwansantisuk and M. Z. Win, Optimal Searh Strategies for Ultrawide Bandwidth Signal Aquisition, in IEEE Int. Conf. On Ultra-Wideband, Zurih, SWITZERLAND, pp. 349-354, Sept. 5. [5] J. Murata, S. Tahikawa, A Proposal of Novel Synhronous Aquisition Method with an Adaptive Filter in Asynhronous DS/CDMA, IEICE Trans. Fundamentals, vol.e8-a, no.1, pp. 38-388, De. 1997. [6] C. Nakano, M. Tahara, M. Hamamura, S. Tahikawa, A Modified Aquisition Method Using Code-Orthogonalizing Filters in Asynhronous DS/CDMA, IEICE Trans. Fundamentals., Vol.E83-A, No.11, pp.143-146, Nov.. [7] M. B. Pursley, Performane Evaluation for Phase-Coded Spread Spetrum Multiple Aess Communiation Part I, IEEE Trans. Commun., COM-5, pp.795-799, Aug. 1977. [8] S.Hamada, M.Hamamura, H.Suzuki, S.Tahikawa, A Proposed DS/CDMA System Using Analog PN Sequenes Produed by Adaptive Filters, IEICE Transations E81-A, no.11, pp.61-68, Nov. 1998. 53