Blind Detection of Modulation Type and Number of Active Users in CDMA with both QPSK Data Modulation and Spreading Siavash Ghavami*, Aliasghar Beheshti Shirazi** *Iran University of Science and technology, [email protected] ** Iran University of Science and technology, [email protected]

Abstract: In this paper, a new algorithm is proposed for joint number of active users estimation and recognition of both data and spreading modulation types based on eigen analysis of received signal covariance matrix using a proposed adaptive threshold. The proposed modulation recognition technique can recognize signals with binary phase shift keying (BPSK) and quadrate phase shift keying (QPSK) data and spreading modulations. Moreover, two blind synchronization methods, which are used for BPSK spreading, are analyzed for QPSK spreading modulation without any prior knowledge about spreading sequence in asynchronous multiuser direct sequence spread spectrum (DS-SS) systems. Computer simulations confirm that the proposed adaptive threshold is a strong tool for joint type of modulation and number of active user estimation. Furthermore, simulation results confirm our analysis about two blind synchronization methods for QPSK spreading in low SNR regime.

Keywords: Blind detection, recognition, adaptive thereshold. 1.

DS-SS,

Modulation

Introduction

It is o DS-SS signals have been used for secure communications, command and control for several decades [1]. In conventional multi-user DS-CDMA systems, the spreading sequence is typically known to the receiver for the despreading operation and data detection [2]. However in non cooperative contexts such as spectrum surveillance and eavesdropping, the spreading sequence used by the transmitter is unknown (as well as other transmitter parameters such as bit epochs of active users and duration of the sequences). Hence, it is almost impossible to estimate the data sequence when it is very difficult even to detect the presence of the signal and to synchronize the receiver without knowledge of the spreading sequence. Furthermore, QPSK spreading increases resistance against of signal eavesdropping and enhance system security. In literatures [3-4], different methods including semi blind, have been presented for number of active users estimation. It is assumed that some parameters of signals

716

are known. Traditional methods, for estimation of number of users are based on minimum description length (MDL) and Akike information criteria (AIC) which have high computational complexity [3]. In [4] number of users has been estimated based on fourth order blind identification methods. Also number of detectable users has been increased relative to the MDL and the AIC, in expense of more computational complexity. Recently, in [5] a new method for joint blind user identification and synchronization in non-cooperative CDMA systems at slow flat fading Channels is proposed for BPSK spreading. Synchronization a DS-SS receiver for the single user case, knowing the spreading sequence is addresses well in the literature [6-7]. However, in the case of no knowledge about the spreading sequence, synchronizing the receiver is addressed in [8] for a single user scenario and in [9] for multi-user scenario with perfect power control. In [10] and [11] two blind synchronization methods are proposed in equal power scenario for signals with BPSK spreading. In this paper, a blind algorithm for recognition of both data and spreading modulation types and number of active user estimation is proposed. Moreover, two blind synchronization methods, which proposed by [10] and [11] for BPSK spreading, are analyzed for QPSK spreading and the results of analysis are confirmed by computer simulations. An adaptive threshold for recognition of modulation type and number of active user estimation is introduced. Then maximum eignenvalue behavior (MEVB), which proposed in [10], and FROBENIUS square norm behavior (FSNB), which proposed in [11], are analyzed for signals with QPSK spreading. Computer simulations are shown that both aforementioned methods can use for signals with QPSK spreading in low SNR regime at slow flat fading channels. The remaining of this paper is organized as follows: Section 2 presents the system model and Section 3 proposes a method for joint type of modulation and number of active user estimation. Section 4 analyses blind synchronization techniques for QPSK spreading. In

Section 5 the performances of the proposed algorithms are evaluated via computer simulations and our conclusions will be drawn in Section 6. 2.

component. v in# , k ( v #qu , k ) and v †in , k ( v†qu , k ) are normalized eigenvectors of received signal covariance matrix related to in-phase component (quadrate component) of signal which obtained using eigen-decomposition. Each asynchronous user produces four eigenvalues that are † λin# ,k , λqu# , k , λin† ,k and λqu with four corresponding ,k

System Model

We consider the uplink scenario of the asynchronous CDMA systems with QPSK spreading and data modulations. The eavesdropper receives signals of K active users. The received signal is given by +∞

K

∑∑ A d [i]h (t − iT − τ

r (t ) =

k

k

k

s

k

) + n(t ),

eigenvectors

hk (t ) = N

∑c

in , k

[m]p (t − mTc ) + jcqu , k [ m] p (t − mTc ),

v #qu , k ). α k is the time delay of the k th user with respect to the beginning of the processing window. In the following for the simplicity in the analysis, the bit duration is assumed to be 1. Eigenvalues of the received signal covariance matrix similar to BPSK spreading, which is obtained in [10], are obtained as, λin# , k = σ n2 ( βin, k (1 − α k ) + 1)  #  λqu , k = σ n2 ( βin, k (1 − α k ) + 1)  † 2  λin ,k = σ n ( β qu ,k α k + 1)  † 2  λqu ,k = σ n ( β qu ,k (1 − α k ) + 1)  2  λk = σ n

(2)

m =0

where N is the processing gain, Tc = T/N is the chip time, p (t ) is the convolution of the chip pulse shaping waveform with channel filter (which represents the channel echoes) and receiver filter, with unit energy. cin , k [ m ] and cqu , k [ m ] are respectively, the value of the mth chip of in-phase and quadrate spreading sequences, with cin , k [ m ] = 1 and cqu , k [ m ] = 1. j is equal to −1 and

3.

K −1

(

(

+α k βin, k v

† in , k

where β in , k

(v

) + β qu , k v

† * in , k

† qu , k

in ,k

σ



2 sig t

× γ in ,k 2

and

qu ,k

(3)

) }

) +I ,

σ



(4)

k = 0,1,...,K -1 k = K ,...,M − 1

,

Joint Modulation Type and Number of Active User Estimation

mean ( λ j +1 − λ j ) + µ

j = 0,...M − 2

std ( λ j +1 − λ j )

j = 0,...M − 2

(5)

⇒ λnormalized (i + 1) : signal ,

2 sig t

where λnormalized (i )

is

ith normalized eignenvalue,

where mean ( ⋅ ) is mean of absolute variation of

T σ n2Ts . Ts , T , 2 sig qu ,k

k = 0,1,...,K -1

λnormalized (i + 1) − λnormalized (i ) >

)

and β qu , k are defined respectively, as

2 2 β k = σ sig T σ n2Ts , β qu , k = σ sig 2 sig in ,k

(v

† * qu , k

k = 0,1,...,K -1

In this section modulation type of received signal is recognized and number of active users is estimated jointly. In this section an adaptive threshold is introduced for discriminating signal eigenvalues from those of noise. This adaptive threshold update, based on delay situation relative to the beginning of the processing window. The following criteria are applied for discriminating eigenvalues of signals from those of noise

Covariance matrix of the received signal similar to BPSK spreading, which is obtained in [10], can be calculated as follows



k = 0,1,...,K -1

where M is the dimension of the received signal covariance matrix, and eignenvalue with subscript of 0 to K-1 are called signal eigenvalues.

denote the imaginary part of signal. Data symbols {d k [i ]} of different users are independent with identical distributions (i.i.d.) and have QPSK modulation. Also channel model is assumed slow flat Rayleigh fading, and thus, channel coefficients are likely constant in a processing window duration. More details of this assumption is discussed in Section V.

 R = σ n2  (1 − α k ) βin , k v in# , k ( vin# , k )* + β qu , k v #qu , k ( v #qu , k )* k = K s

v†qu , k , also each

λqu# , k with the corresponding eigenvectors are v in# , k and

(1)

where Ak , d k [i ] , Ts and τ k are respectively, the received amplitude, ith data symbol, symbol period and delay of the kth user. n(t ) is additive white Gaussian noise and hk (t ) is expressed by N −1

and

synchronous user produces two eigenvalues λin# ,k and

k =1 i =−∞

−1/ 2

v in# , k , v #qu , k , v †in , k

j = 0,... M − 2

× γ qu ,k are 2

respectively the symbol duration, sampling period, power of the k th user received signal in in-phase and quadrate 2 component. σ sig is power of transmit signal, γ in2 ,k and t

γ qu2 ,k are power of channel gains in in-phase and quadrate

717

eigenvalue, and i=0,...,M−2 . Since the eigenvalues of noise vary continuously, when the difference of two successive normalized eigenvalues is greater than mid term of equation (4), which is selected as a threshold, λnormalized (i + 1) shows signal eigenvalue. Parameter of µ is regulator of adaptive threshold. It is determined based on the situation of delays relative to the beginning of the

processing window. The initial value of µl =1 is selected as unity and in each step will be updated as follows, Start

µl =1 = 1

If the number of eigenvalues which are greater than the selected threshold for µl +1 , … , µl + 4remains constant.

No

Increase l

Yes

Yes

If equation (6) is satisfied for each value of i

No There is not any signal

Parameter µl is selected as regulator of the threshold

End

Fig. 1. Flow chart of determining of adaptive threshold µl + k +1 = µl + k + 1.

When QPSK spreading is used, since based on (4) the number of eigenvalues of signal in received signal covariance matrix is 4 times of number of active users. Moreover, the number of eigenvalues of signal in inphase component or quadrate phase component of received signal covariance matrix is 2 times of number of active users. Besides if a signal with QPSK data modulation and BPSK spreading has been transmitted, the number of eigenvalues of signal in in-phase or quadrate or total received signal covariance matrix is 2 times of number of active users. The case of BPSK modulation is similar to QPSK modulation except, the number of eigenvalues of signal in quadrate phase of received signal covariance matrix is 0. Therefore, the number of signal eigenvalues which are estimated by (5) can determine whether there are different component in received signal covariance matrix or not. More details about modulation type and number of active user estimation are discussed in Section V. 4. Blind Synchronization for QPSK Spreading Blind synchronization for QPSK signal with BPSK spreading based on FSNB and MEVB have been considered respectively in [10] and [11], in equal power scenario. In this section will be shown the aforementioned algorithms can synchronize the received signal with different spreading sequence in in-phase and quadrate components. The FSNB algorithm [10] is based on the maximization of the covariance matrix of received signals eigenvalues. According to equation (5), the sum of eigenvalues is equal to the power of received signal plus the noise power; therefore it is independent of synchronization time.

(6) M −1

The following algorithm determines the threshold adaptively: µl =1 = 1

2.

If the number of eigenvalues which are greater than the selected threshold for µl +1 , … , µl +4 remains constant, ignore step 3 and go to step 4, where µl + k +1 = µl + k + 1 for 0≤k≤4, otherwise go to step 3.

3.

If (6) is not satisfied for any value of i, go to step 5, otherwise increase l and return to step 2.

4.

Parameter µl is selected as regulator of the threshold and go to step 6. There is not any signal, go to step 6.

6.

End.

k

k =0

K −1 M −1   =  (λin0 , k + λqu0 , k + λin−1, k + λqu−1, k ) + σ n2  = k =0  k = 0 





(7)

  σ ( βin, k + β qu , k  + M ).  k =0  K −1



2 n

1.

5.

∑λ

According to (7), an appropriate criterion for synchronization is the summation over square eigenvalues. Therefore the FROBENIUS square norm of the estimated covariance matrix of the received signal is used for avoiding calculation of covariance matrix eigenvalues. Since the square of eigenvalues summation is equal to the square norm of the covariance matrix, FROBENIUS square norm can be written as follows 2

{

K

(

)

(

)

R = σ n4 ∑ (2 βin,k + βqu,k + βin2 ,k + βqu2 ,k ) + M k =1

K −1

}

(8)

+ 2σ n4 ∑ βin2 ,k + βqu2 ,k (−αk + αk2 ) k =0

Fig. 1 shows flow chart of determining of adaptive threshold. The selected µl depends on the delays and the number of users. The criterion (5) is considered clearly in simulation results. By selecting µl , since the eigenvalues are arranged in ascending order, eigenvalues with greater indexes than i are related to signal.

718

In order to maximize (8) in terms of α k , we can simplify the optimization with concentration on the variable part of the above equation,

K −1

∑(β

F (α 0 , α1 ,..., α K −1 ) =

2 in , k



) (α

2 qu , k

k =0

2 k

− α k ),

(9)

This is similar to the FSNB for QPSK signal with same spreading sequence in in-phase and quadrate components. The beginning of the processing window is shifted for synchronization. d f and τ k are respectively time of beginning of the processing window and delay of kth user since is substituted with αk = d f −τ k .αk mod(( d f − τ k ), Ts ) since the periodic property of αk s with respect to the beginning of the processing window, which is the residual of d f − τ k over Ts. And F can be expressed as, F(d f ) =

K −1

∑(β

+ β qu2 , k

2 in , k

k =0

}

){( mod (d

f

− τ k ), Ts 

)

2

(10)

− mod (d f − τ k ), Ts  . The MEVB which proposed in [10] for BPSK spreading is defined for signal with QPSK spreading as follows

( )

C df =

max

τ k ≤ d f ≤τ k +1



# in , k

)

, λqu# , k , λin† , k +1 , λqu† , k +1 ,

(11)

for k = 0,1,...,K -1 . In equal power scenario and with same power of channel gain in the in-phase and quadrate component λin# , k = λqu# , k and λin† , k +1 , λqu† , k +1 , therefore (11) is simplified to

( )

C df =

max

τ k ≤ d f ≤τ k +1



# in , k

)

, λin† , k +1 , ,

(12)

which is similar to MEVB for signal with BPSK spreading. In the next section the analysis of FSNB and MEVB for QPSK signal with QPSK spreading are evaluated using computer simulations. 5. Simulation Results In this section the performance of the proposed algorithm for joint modulation type and number of active user estimation is evaluated in the asynchronous CDMA system through computer simulations. Moreover, performance of blind synchronization techniques in pervious section is evaluated for signal with QPSK data modulation and QPSK spreading using computer simulations. In simulations, the spreading sequences of the users is random QPSK with processing gain of 18dB (N = 63) (e.g. different spreading sequence in in-phase and quadrate components). Chip time and sampling frequency are 50 n sec , and Fs = 200MHz , respectively. The length of the processing window is assumed 200 symbols with the duration of 315µ sec and the bit duration of 1.575 µ sec . The slow flat fading is considered for channel model. Since, the maximum tolerable Doppler frequency of less than

719

1/ (10 × 315µ sec ) = 316.5Hz in conventional wireless systems, the assumption of constant channel coefficients, in one processing window is valid. Simulations are performed with 3 active users; all of them are asynchronous relative to the beginning of the processing window. Users power are same. Delays of them relative to the beginning of the processing window are 0.2 µ sec , 0.8 µ sec , and 1.4 µ sec , respectively. Received signal SNR is considered -11 dB before despreading. Fig. 2-a shows the eigenvalues of the total received signal covariance matrix with QPSK data modulation and BPSK spreading; the number of signal eigenvalues is estimated 6 using (5). Fig.2-b is similar to (a), except that the covariance matrix is calculated for in-phase component of signal; the number of signal eigenvalues is estimated 6 using (5). Fig.2-c is similar to (b), except that the covariance matrix is calculated for quadrate component of signal; the number of signal eigenvalues is estimated 6 using (5). Fig. 3-a shows eigenvalues of the total received signal covariance matrix with QPSK data modulation and QPSK spreading; the number of signal eigenvalues is estimated 12 using (5). Fig. 3-b is similar to Fig. 3-a, except that the covariance matrix is calculated for inphase component of signal; the number of signal eigenvalues is estimated 6 using (5). Fig. 3-c is similar to Fig. 3-b, except that the covariance matrix is calculated for quadrate component of signal; the number of signal eigenvalues is estimated 6 using (5). It can be seen from Fig. 2, by BPSK spreading, the number of signal eigenvalues of the total received signal covariance matrix is equal to number of signal eigenvalues of the in-phase or quadrate phase components of received signal covariance matrix. Also, by QPSK spreading, the number of signal eigenvalues in the total received signal covariance matrix is twice of number of those in the inphase and quadrate-phase components of received signal covariance matrix. Fig.4-a shows number of eigenvalues of signals with QPSK spreading (which identified using the adaptive threshold in (5)), in terms of the µ parameter for total, in

Fig. 2-a. eigenvalues of the total received signal covariance matrix with QPSK data modulation and same spreading sequence in in-phase and quadrate components, Fig. 2-b is similar to Fig. 2-a, except that the covariance matrix is calculated for in-phase component of signal, Fig. 2c is similar to Fig. 2-b, except that the covariance matrix is calculated for quadrate component of signal.

Fig. 3-a. eigenvalues of the total received signal covariance matrix with QPSK data modulation and different spreading sequence in in-phase and quadrate components, Fig. 3-b is similar to Fig. 3-a, except that the covariance matrix is calculated for in-phase component of signal, Fig. 3c is similar to Fig. 3-a, except that the covariance matrix is calculated for quadrate component of signal.

phase and quadrate phase components of received signal. It can be seen in Fig. 4-a. number of signals eigenvalues for total received signal is identified 12, and µ is obtained 5 using the adaptive threshold in (5). Furthermore number of signals eigenvalues for in phase and quadrate received signal is identified 6, and µ is obtained 7 using the proposed adaptive threshold in (5). Fig.3-b shows number of eigenvalues of signals with BPSK spreading (which identified using the proposed adaptive threshold in (5)) in terms of the µ parameter for total received signal, in phase component and quadrate component of received signal. Fig. 4-b shows number of signals eigenvalues for total received signal is identified6, and µ is obtained 7 using the proposed adaptive threshold in (5). Moreover, the number of signals eigenvalues for in phase and quadrate-phase components of received signal is identified 6, and µ is obtained 7 by using the adaptive threshold in (5). Fig. 5-a. shows the MEVB criterion for signal with QPSK data modulation and QPSK spreading in terms of different shifts of the beginning of the processing window. Fig. 5-b. shows the FSNB criterion for signal with QPSK data modulation and QPSK spreading in

Fig. 4-a. Number of estimated signal eigenvalues as a function of µ with QPSK data modulation and different spreading sequence in in-phase and quadrate components, Fig. 4-b. it is similar to Fig. 4-a. with QPSK data modulation and same spreading sequence in in-phase and quadrate components

720

Fig. 5-a. MEVB in terms shifts of beginning of the processing window, Fig. 5-b. FSNB in terms shifts of beginning of the processing window.

terms of different shifts of the beginning of the processing window. It can be seen both of FSNB and MEVB can estimate delay time of different user relative to the beginning of the processing window with QPSK data modulation and QPSK spreading in SNR of -11 dB. Which is validated the theoretical of synchronization methods, related to signal with QPSK spreading in Section IV. 6. Conclusions In this paper, a new algorithm is proposed for modulation type and number of active users estimation. Moreover, two blind synchronization methods, which used for BPSK spreading by [10] and [11], are analyzed for the case of QPSK spreading without any prior knowledge about spreading sequences in asynchronous multi-user direct sequence spread spectrum (DS-SS) systems. Simulation results show that modulation type and number of active users is estimated using the proposed adaptive threshold in SNR of -11 dB for spreading factor of 18 dB. Moreover, computer simulations confirm that our analysis about both blind synchronization methods for QPSK spreading in SNR of -11 dB. References [1] R. A. Sholtz, “The origins of spread-spectrum communications,” IEEE Trans. Commun., vol. COMM-30, pp. 822–854, May 1982. [2] R. L. Pickholtz, D. L. Schilling, and L. B. Milstein, “Theory of spread spectrum communications,” IEEE Trans. Commun., vol. COMM-30, pp.855–884, May 1982. [3] M. Wax, T. Kailath, “Detection of Signals by Information Theoretic Criteria”, IEEE Trans on Acoustic, Speech, and Signal Processing, Vol. 33, No. 2, 1985. [4] A. Ferréol, L. Albera, and P. Chevalier, “Fourth-Order Blind Identification of Underdetermined Mixtures of Sources (FOBIUM)”, IEEE Trans.on Signal Processing, Vol. 53, No. 5, pp. 1640-1653, 2005. [5] S. Ghavami and V.T. Vakili, "Joint Blind Users Identification and Synchronization in Non-Cooperative CDMA Systems in Slow Flat Fading Channels", appeared to ICEE2008, May 2008. [6] K. K. Chawla and D. V. Sarwate, “Parallel acquisition of PN sequences in DS/SS systems,” IEEE Trans. Commun., vol. 42, pp. 2155–2163, May 1994. [7] U. Cheng, W. J. Hurd, and J. I. Statman, “Spread-spectrum code acquisition in the presence of Doppler shift and data modulation,” IEEE Trans. Commun., vol. 38, pp. 241–249, Feb. 1990. [8] Bouder, C.; Azou, S.; Burel, G, “A robust synchronization procedure for blind estimation of the symbol period and the timing

offset in spread spectrum transmissions”, IEEE conf. on Spread Spectrum Techniques and Applications, vol 1, pp. 238 – 241, Sep 2002. [9] C. Nsiala Nzea, R. Gautier, and G. Burel, “Blind synchronization and sequences identification in CDMA transmissions,” IEEE Military Communication conference, Vol. 3, pp.1384-1390, Nov 2004.

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[10] C. Nsiala Nzéza, R. Gautier, and G. Burel, “Parallel blind multiuser synchronization and sequences estimation in multirate CDMA transmissions,” IEEE conf. on Signals, Systems and Computers, pp. 2157 – 2161, Oct.-Nov. 2006. [11] C. Nsiala Nzéza, R. Gautier, and G. Burel, “Blind Multiuser Identification in Multirate CDMA Transmissions: A New Approach,” IEEE conf. on Signals, Systems and Computers, pp. 2162 – 2166, Oct.-Nov. 2006

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