Design of fast acquisition algorithm for TDRSS burst signal

Design of Fast Acquisition Algorithm for TDRSS Burst Signal Qi-yi Ren, Zheng Yao , Member, IEEE, and Ming-quan Lu Department of Electronic Engineering, Tsinghua University Beijing, P. R. China, 100084 [email protected] burst datas may be lost during the acquisition process, and traditional CBC generally uses a preamble sequence before burst datas [5]. However, under the condition that the duration of burst signal is restricted, the preamble limits the maximum length of burst data frame and restrict the transmission efficiency of CBC.

Abstract— Driven by the problems that the Tracking and Data Relay Satellite System (TDRSS) cannot meet the requirement of cooperative burst communication (CBC), a spreading code structure based on the burst signal pilot channel and a fast acquisition algorithm based on this new signal structure are proposed. The new designed spreading code is composed of short spreading code sequence modulated by overlay code. By utilizing the features of new signal structure, the new algorithm segments the received burst signal into sections and respectively processes these sections by the FFT method . Then it synthetically analyzes of the acquisition results and estimates the burst signal information in the absence of preamble. Theoretical analysis and experiments with simulated data show that the algorithm has higher acquisition and transmission efficiency and can satisfy the TDRSS service requirements.

To solve these problems, a new spreading code structure based on the overlay code is proposed. The new designed spreading code is composed of short spreading code sequence modulated by overlay code. The new algorithm could significantly improve the capture efficiency with segment the burst signal and make synthetically analysis of the acquisition result. Furthermore, the new data structure can also extract the starting position of the burst signal in the absence of preamble, and thus greatly improves the transmission efficiency of CBC.

Keywords; Acquisition; TDRSS; Overlay code; Burst signal;

II.

I. INTRODUCTION The developing TDRSS has a significant impact on the space communication network. It provides a long data communication and continuous tracking for the near-earth space target [1] and plays a significant role in the field of manned space flight, satellite data transmission and emergency communication over the past 30 years [2] [3].

THE COOPERATIVE BURST SIGNAL ACQUISITION MODE

A. Burst singal format A new signal structure of TDRSS burst signal with a signal power split has been proposed by [6] and it is composed of data and pilot channels. The data channels modulate the burst signal data bits while the pilot channels provide accurate ranging information of the TDRSS. The spreading code of data and pilot channels is designed to be orthogonal. Besides, the new fast acquisition algorithm could fully extended coherent integration time since there is no data on the pilot channel. The received pilot signal can be modeled as s(t ) Ap(t  W )sin(2S fc (t  W )  I0 )  n(t ) (1) where W is the code phase delay due to travel time, A is the amplitude of the signal, I0 is the carrier phase of the signal, p(t ) is the spreading code modulated on pilot channel, f c is the intermediate carrier angular frequency of the incoming signal and n(t ) is the thermal noise.

With the expansion of TDRSS applications, some specific users expect that the system could satisfy the requirement of CBC. The burst communication reduces the probability of investigation and intercept by other system because of its random starting point and duration. However, the current TDRSS’s work mode was designed for the non-bursting communication, and it built a stable communication link with the user before the data transmission. Therefore, the current researches of TDRSS are based on the non-bursting communication [4] transfer mode. There are two key factors influencing the implementation of CBC for the TDRSS. First of all, the characteristics of burst signal is discontinuity and short duration, so the acquisition and tracking process of burst signal must be completed in a short time. The CBC is always accompanied by low signal noise ratio (SNR) which increases the difficulty of acquisition. In order to slove this problem, the most universal method is to increase the coherent integration time. However, the burst signal is of short duration and random time slot distribution, which will limits the coherent integration time. Secondly, some

B. Signal acquisition The purpose of signal acquisition is to get carrier frequency and code phase information. The process could be described as a sequential search in a 2-dimensional search space. Figure 1 schematically illustrates the flow chart of signal acquisition using the linear search [7]. As shown in Fig.1, After I/Q mixed with local carrier and correlated with local code, the output of received burst signal are squared and summed to form the

Project supported by the National Key Scientific Instrument and Equipment Development Project (2012YQ140026)

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B. The fast acquisition algorithm The new acquisition algorithm is proposed based on the new long spreading codes. Assume the acquisition coherent integration time Tcoh kTs and Ts is the period of short spreading code sequence. Assume that the processed data contains k chips overlay codes and defined as C [c1 , c2 , ˜˜˜, cm , ˜˜˜ck ] , and the possible value of cm are 1 or -1. At first, the processed burst data will be segmented into k sections. Then, the segmented data will be processed as the flow in Fig.1 ,and the local generated code will be multiplied by sm since the corresponding overlay code cm is unknown. Obviously, sm 1 or  1 . After mixinged with local carrier and correlatinged with local code, the m -th segment data output YI( m ) and YQ( m ) in Fig.1 can be defined as [10]

acquisition-decision variable V followed. If the maximum V is higher than the detection threshold, the locally generated code phase and carrier frequency is considered the same as received signal, which means the signals are acquired. Mixer

Correlator

()2 YI V

Incoherent integration

Singal Mixer

V>Vt ?

Correlator

()2 YQ Sine table

false

Cosine table

Code generator

Acquisition control

NCO

Figure 1 Principles of signal acquisition

Figure 2 schematically illustrates the acquisition process when the fast Fourier transform (FFT) method is used[7]. The correlation across all code offsets can be computed by taking the inverse Fourier transform (IFFT), which greatly improves the computation efficiency. i TDRSS signal

YQ

IFFT

||

q Conjugae Sine table

Cosine table

FFT

NCO

k

rI

Code

Asmcm R(W )sinc( feTcoh ) cos I0

(2)

Asm cm R(W )sinc( feTcoh )sin I0

(3)

rQ

THE ACQUISITION ALGORITHM BASED ON OVERLAY

AR(W e )sinc( f eTcoh ) cos I0 ¦ sm cm

(4)

¦ YQ(m)

AR(W e )sinc( f eTcoh )sin I0 ¦ sm cm

(5)

m 1

CODE MODULATED SIGNAL

k

¦ YI (m)

m 1 k

Figure 2 FFT method of signal acquisition

III.

(m)

while R(˜) is the normalized autocorrelation function of p(t) , W e is the code phase difference between the local code and burst signal, f e is the frequency difference between local carrier and burst signal. Then, the total coherent integration output result of the k sections can be denoted as

Choose max

Multiply

FFT

YI (m)

m 1 k

m 1

And the acquisition-decision variable V can be denoted as

A. Overlay code structure and characteristics Overlay code is also known as secondary code, and it has been used by modernized global navigation satellite system (GNSS) signals such as GPS L1C [8] and Galileo. As shown in Fig.3, the long spreading codes are generated by a tiered code construction, and an overlay code sequence is used to modify successive repetitions of a primary code periods [9]. The new structure improved the correlation property of spreading codes. Furthermore, in this way, the code phase of burst signal could be directly obtained after the acquisition process demodulated a certain length of the overlay code.

V

rI2  rQ2

k

AR(W e ) | sinc( f eTcoh ) || ¦ sm cm |

(6)

m 1

Assume that S [s1 , s2 , ˜˜˜, sm , ˜˜˜sk ] and l 2k , the code sequence S can have l possible values defined as S1 ~ Sl .Moreover, the output V has l possible values defined as V1 ~ Vl . Then, the output V will reach the maximum when code sm is equal to bm or bm , m  [1, k ] . Thus, the output variable Vi will be greater than other l  1 if k

| ¦ sm(i ) cm | k

N chips

(7)

m 1

Primary Code:

Overlay Code:

Epoch i

1st chip

...

Epoch i+1

2st chip

...

Epoch i+Ns-1

Vi kAR(W ) | sinc( feTcoh ) | (8) In this case, the code value of sequence Si is equal to the overlay code sequence C or -C .

Epoch i+Ns

Chip Ns-1

Tiered code period

the proposed algorithm is summarized as follows. Firstly, The algorithm get l possible values of rI(i ) , rQ(i ) and Vi

Overlay Code

/Ns fcs

Clock fc

Primary Code

( m  [1, l ] ). Then, the maximum value can ben found from V1 ~ Vl , the acquisition-decision variable V can defined as

Tiered Ranging code

Clock rate fc

l

V

Figure 3 The usage of overlay code (secondary code)

¦ H mVm

m 1



(9)

while H n is given by

Computational Complexity

Step

­1 if max(V1 ,V2 , ˜˜˜,VL ) Vn (10) Hm ® otherwise ¯0 The overlay code in the received burst data can be defined as C r Si where i arg max Vi . 1i l

Although the value of C is uncertain, the total overlay code sequence that in the burst signal can be determined. Assume the total length of overlay code is M , and the minimum sequence length required to identify the chip position is k0 , which also named as “Characteristic Length (CL)” [14]. If the successful acquisition processes number nc satisfies nc k t k0 , the phase information and overlay code of the received signal can be obtained.

Data length

Addition

data FFT

kN s

kNs log 2 ( Ns )

code FFT

Ns

Ns log 2 ( Ns )

IFFT

kN s

2kNs log2 ( Ns )

Multiplication

kN s log 2 ( N s ) 2 Ns log 2 ( N s ) 2 kNs log 2 ( Ns )

Assuming the N 1024 and M 256 , the calculation amount of the FFT method and new method are two functions of the input sampling rate, which is shown in Figure 4. The new method greatly reduces the amount of calculation and improves efficiency of burst signal acquisition. 8

10

Calculation amount

multiplication(FFT method) multiplication(new method)

Besides, the new algorithm can use the FFT method. The IFFT outputs Zm (1: N ) ( m  [1, l ] ) represent the correlation results across all code offsets. The outputs are multiplied by sm and added together , that is

7

10

6

10

k

Ri (1: N ) | ¦ smi Z m (1: N ) |

(11) 5

m 1

10

Then l possible outputs Ri (1: N ) ( i  [1, l ] ) are obtained. If the maximum value denoted as Ra (b) exceeds the detection threshold, it can be said that the overlay code is rSa and the code phase is b .

IV.

1

data FFT

kN s

kNs log2 (kNs )

code FFT

MN s

MNs log2 (MNs )

IFFT

MN s

2MNs log2 (MNs )

TABLE II.

1

1 3ms

3ms

1

1

1 -1 1 3ms

1ms noise

1

1

3ms

1ms burst data

Figure 5 The structure of burst signal

Table III cites the TDRSS burst signal structure and the acquisition parameters used in the simulation. The period of short spreading code is 1ms and the coherent integration time is 3ms. It means that processed data would be segmented into 3 sections. The length of overlay code sequence is 18, and its CL is 6, which means that two successful acquisitions could obtain enough overlay code to reach the CL. The SNR of TDRSS burst signal is based on the link budget.

Computational Complexity Addition

SIMULATION ANALYSIS

Noise:2.25ms

THE COMPUTATIONAL COMPLEXITY OF FFT METHOD

Data length

6

A. Simulation parameters The length of burst signal used in the simulation is 12ms. The first 2.25ms of burst signal is noise and the last 9.75ms conisit burst data. The burst signal structure is shown in Figure 5.

Assuming thtat the short spreading code sequence length is N , and the total spreading code length is MN , the overlay code length is M and the frequency of spreading code is f c , the sampling rate of burst signal can be defined as f s ts fc . Obviously, one period of short spreading code number after sampling can be defined as N s ts N . Table I and II shows the computation complexity of the two methods.

Step

3 4 5 sampling rate/spreading code speed

Figure 4 the contrast between FFT method and new method

C. Performance evaluation In order to evaluate the acquisition efficiency of the new proposed algorithm, a comparison between the new algorithm and FFT method is presented. The calculation amount of FFT method mainly consist of three parts: the FFT of local replica code and received burst signal and the inverse Fourier transform (IFFT) of the output. The new method segments the received burst signal into k sections and processed separately which greatly reduced the computation complexity.

TABLE I.

2

Multiplication

TABLE III.

kN s log 2 (kN s ) 2 MN s log 2 ( MN s ) 2 MNs log2 (MNs )

THE COMPUTATIONAL COMPLEXITY OF NEW METHOD



SIMULATION PARAMETERS

Parameters

Value

Sample rate

12 MHz

Overlay code

[1,1,1,1,1,1,-1,1,1,1,1,-1,-1,1,-1,1,-1,-1]

Short spreading code period

1ms

Coherent integration time

3ms

B. Results and analysis Three sections IFFT output can be defined as Z1 (1: N ) , Z 2 (1: N ) , Z3 (1: N ) . The three IFFT outputs multiplied by S1 ~ S4 and processed according to (11) can be defined as [1, 1ˈ 1] or [1, 1, 1] R2 | Z1  Z 2  Z 3 |

S3

[1, 1, 1] or [1, 1, 1] R3 | Z1  Z 2  Z 3 |

10000

5000 0

R3(n)

R2(n)

10000

0

15000

15000

10000

10000

5000 0

0

0

5000 10000 code phase R4=|Z1+Z2-Z3|

5000 10000 code phase

R2(n)

5000 0

5000 0

5000 10000 code phase R3=|Z1-Z2+Z3|

15000

15000

10000

10000

R4(n)

R1(n)

10000

0

R3(n)

R2=|Z1-Z2-Z3|

10000

5000 0

0

5000 10000 code phase

0

5000 10000 code phase R4=|Z1+Z2-Z3|

0

5000 10000 code phase

5000 0

10000

5000

0

5000 10000 code phase

5000 10000 code phase R4=|Z1+Z2-Z3|

0

5000 10000 code phase

5000 0

CONCLUSION

REFERENCES

After two successful acquisition the obtained overlay codes reaches the characteristic length, it is obtained that the overlay codes that in the second and third processes are [1, 1, 1, 1, 1, -1] according to table III. Furthermore, the start position can also be obtained according to the code phase. 15000

15000

10000

V.

However, as shown in Fig.7 and Fig.8, the maximum result can exceed the detection threshold in the second and third acquisition. The maximum value are R1 (9000) and R4 (9000) , and it means that the possible overlay code is S1 and S 4 according to (12).

R1=|Z1+Z2+Z3|

15000

0

This paper proposed a signal structure for TDRSS burst signal communication and a fast acquisition algorithm based on this new signal. Theoretical analysis and simulation results have showed that this new algorithm could effectively reduce the amount of calculation and improve the capture efficiency. Besides, this new algorithm could accurately estimates the starting position of burst signal without adding preamble before burst signal, which greatly improves the transmission efficiency. The proposed data structure and algorithm is particularly useful for the TDRSS burst signal transmission.

Figure 6 Result of the first time acquisition (noise 2.25ms and data 0.75ms)

15000

0

5000 10000 code phase R3=|Z1-Z2+Z3|

In the practice, the start position of the burst signal could be obtained by using a received buffer that designed according to the length of overlay code sequence and coherent integration time. When the obtained overlay code reaches the CL, the start position of burst signal could be determined according to the obtained overlay code and code phase.

5000 0

5000 10000 code phase

0

0

5000

Figure 8 Result of the third time acquisition (data 3ms)

5000 0

5000 10000 code phase R3=|Z1-Z2+Z3|

5000

0

R2=|Z1-Z2-Z3| 15000

R4(n)

R1(n)

R1=|Z1+Z2+Z3|

10000

R2(n)

R1(n)

(12)

S4 [1, 1, 1] or [1, 1, 1] R3 | Z1  Z 2  Z 3 | As Fig.6 shows, the first processed 3ms data contains 2.25ms noise and 0.75ms burst data. Hence the maximum acquisition result does not exceed the detection threshold. 15000

10000

R4(n)

[1, 1, 1] or [1, 1, 1] R1 | Z1  Z 2  Z 3 |

S2

R2=|Z1-Z2-Z3| 15000

0

R3(n)

S1

R1=|Z1+Z2+Z3| 15000

Figure 7 Result of the second time acquisition (data 3ms)



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