A Cooperative Transmit Diversity Scheme for ... - Semantic Scholar

6 downloads 48542 Views 410KB Size Report
system is a very effective way to provide these services due to its wide area ... encode signals rather than being simple amplifiers. A user terminal has ..... REFERENCES. [1] Sang-Jin Lee, SangWoon Lee, Kyung-Won Kim, and Jong-Soo Seo,.
A Cooperative Transmit Diversity Scheme for Mobile Satellite Broadcasting Systems Sooyoung Kim

Heewook Kim and Do Seob Ahn

Division of Electronics & Information Engineering Chonbuk National University, Jeonju Visiting research staff, ETRI, Korea [email protected]

Global Area Wireless Technology Research Group, Electronics and Telecommunications Research Institute (ETRI), Korea [email protected], [email protected]

Abstract—In this paper, we propose an efficient cooperative diversity scheme for mobile satellite multimedia broadcasting systems. The proposed scheme is a transmit diversity technique to adapt different channel environments, and thus we do not need any channel quality information from the return link. In the proposed scheme, we utilize space-time block coding (STBC) and rate compatible turbo codes in order to realize the transmit diversity for the mobile satellite system with several repeaters. The satellite and several repeaters operate in unison to send the encoded signals, so that the receiver may realize diversity gain. The simulation results demonstrate that the proposed scheme can provide highly improved performance.

In addition to this STC scheme, we propose an efficient transmit diversity scheme combined with STC and a channel coding scheme for cooperative satellite-terrestrial network. We use a rate compatible punctured turbo code as the channel coding scheme. The receiver first get the diversity gain from the STC encoded signals from the satellite and terrestrial repeaters, and then it also get additional channel coding gain by combing different parity parts from the satellite and terrestrial repeaters.

Keywords- space-time block coding (STBC); turbo codes; rate compatible codes; mobile satellite communications; transmit diversity;

I. INTRODUCTION Multimedia broadcast and multicast services (MBMS) will play an important role in future mobile systems, and a satellite system is a very effective way to provide these services due to its wide area coverage, reconfigurability, and multicast capabilities. A cooperative satellite-terrestrial network can provide high quality MBMS by seamlessly mixing the most powerful aspects of each technology. The unidirectional nature of MBMS prohibits the use of control commands for power control and adaptive modulation and coding (AMC). Therefore, in this situation, the downlink strategies should be focused on improving the performance. Cooperative satellite-terrestrial network concepts have been implemented in several satellite digital multimedia broadcasting (S-DMB) systems including Korean S-DMB and XM Satellite Radio systems [1][2]. Recently, more advanced techniques for these cooperative networks have been proposed using terrestrial repeaters with appropriate signal processing capabilities. For example, an efficient diversity technique using space-time coding (STC) for a satellite-terrestrial network was proposed in [3][4]. Because this is a transmit diversity technique, it does not require any channel quality information (CQI) from return link. In this scenario, the repeaters and satellite may cooperate to transmit space-time coded signals, and the repeaters have the ability to encode signals rather than being simple amplifiers. The satellite and the terrestrial repeaters each with a single antenna operate in unison to achieve STC gain at the receiver.

In section II, we first introduce the system configuration of the cooperative satellite-terrestrial network along with the concept of the transmit diversity using STC. In section III, we describe the proposed scheme in more detail by using a specific application example. In section IV, we demonstrate the simulation results compared to the conventional schemes. Finally, we draw a conclusion in section V. II.

TRANSMIT DIVERSITY FOR THE COOPERATIVE SATELLITE-TERRESTRIAL NETWORK

A. System Model Figure 1 shows the system architecture of the cooperative satellite-terrestrial network to utilize the proposed diversity techniques using STC and channel coding schemes. In the network shown in Fig. 1, a multi-spot beam satellite in the geostationary orbit and an ensemble of terrestrial cell sites with m(N-1) repeaters are deployed. We assume that the satellite transmits data with frequency bands of f1 to user terminals and all m(N-1) repeaters on the ground. Then, each of the repeaters transfers the received signal into a given encoded format, and sends them using an allocated frequency band. For example, repeaters of R1i, R2i, …, Rmi sends the encoded signal using the frequency band of fi . With the system architecture in Fig. 1, the repeaters and satellite may cooperate to transmit space-time and error correction coded signals, and the repeaters have the ability to encode signals rather than being simple amplifiers. A user terminal has the ability to receive signals with different frequency bands, and to decode the STC signals. The system architecture in Fig. 1 can be a future solution of an S-DMB system. The detailed encoding algorithm will be explained in the following sub-sections.

This work was supported by the IT R&D program of IITA. [2008-F01001, Development of satellite radio interface technology for IMT-Advanced]

978-1-4244-2204-3/08/$25.00 ©2008 IEEE

[ s, p1 ]

repeaters are available. In this system, we use a rate compatible code with a rate 1/3 code as the mother code with sequences of [s p1 p2], where s stands for the systematic symbol, and p1 and p2 stand for the parity symbols.

[ s , p1 ]

[ s, p 2 ]1 1 N

[ s, p 2 ] m

[ s, p3 ]1

R2m

R

[ s, p1 ]

[s, pN ]m

[ − p *2 , s * ]

RNm

R 31

[ s, p 3 ]

m

R3m

Figure 1. System model for cooperative satellite-terrestrial network

B. Transmit diversity Referring to Fig. 1, in the transmitter of the satellite gateway, a rate compatible turbo code generates an encoded signal sequence of [s p1 p2, …, pN] with code rate of 1/(N+1). In the encoded sequence, s and pi (1≤i≤N) stand for a systematic and the ith parity parts, respectively. We assume that we can decode the information by using the systematic part and any combination of the parity parts. The satellite, then, passes through the sequence [s p1] to the user terminal and all m(N-1) repeaters. When a repeater Rji (2≤i≤N) receives [s p1], then it first generates another parity part pi by using the same encoder used in the gateway transmitter. Second it generates STC signal format, [s pi ]j, by using the sequence [s pi]. The repeater, now send the encoded sequence [s pi]j to the user terminal using the frequency band of fi. If the user terminal receives multiple signals from the repeaters and also from the satellite, then it first performs STC decoding process using multiple signals with the same frequency band fi and recover the sequence [s pi ]. This STC decoding process is performed for all available frequency bands at the terminal. Next step is to combine [s p1], [s p2], …, and [s pN] to generate the mother code or a higher rate code, and apply the iterative decoding to recover the original information. Although any of the paths is missed, we can still recover the information. This is because the rate compatible code used in the system is self decodable, that is this code can be decoded with the systematic part with any combination of the parity parts. III.

h1

[ s, p N ]1

R 21

APPICATION EXAMPLE USING THE PROPOSED SCHEME

A. Transmit diversity usingboth STC and channel coding Figure 2 shows an example of the system using the proposed scheme, where the satellite and the repeaters transmit signal using frequency band of f1 and f2, respectively. In Fig. 2, Repeater R21 stands for a half of the terrestrial repeaters deployed on the ground, and Repeater R22 stands for the other half of them. We assume both repeaters of R21 and R22 are alternatively deployed so that a user terminal can maximize the diversity gain when signals from the both

[ s, p1 ] [ s, p1 ] [ s, p1 ] [s , p2 ] [ s, p1 ]

h21 R12 h22

[− p *2 , s * ] [ s, p 2 ] [− p2* , s * ] * * [ s, p 2 ] [ − p 2 , s ]

R22

[ s, p 2 ]

Figure 2. An application example of the proposed transmit diversity

In this scheme, the satellite passes through a signal set consisting of [s p1], which is the systematic and half of the parity parts, during the period 2T. Receiving error-free signal of [s p1] from the satellite, each repeater regenerates the other half of the parity symbols p2 using the systematic symbols s. In addition to this parity generation, each repeater applies spacetime encoding to [s p2]. By using these STC results, Repeater R22 transmits a signal set [s p2], whereas R21 transmits [-p*2, s*] during the period 2T, where * represents complex conjugate operation. In this way, as indicated in Fig. 2, each transmitter, i.e. the satellite and the repeaters transmit signals with channel coding scheme with the code rate of 1/2. In the receiver, we may achieve additional coding gain by combining two different parities and thus utilizing the code of rate 1/3 when both signal paths from the satellite and a repeater are available. Because each transmitter sends signal sets punctured from the original mother code, we can use a unique decoder regardless of the received patterns. A user terminal can receive a various combinations of signal sets depending on the signal availability mainly by the location. Let us firstly investigate the first example when the user terminal can only receive a signal from the satellite, then the user terminal can decode the information by decoding the received signal using the rate 1/2 channel coding scheme punctured from the rate 1/3 mother code. This situation mainly happens in open or rural areas. We refer to this case as C1, as indicated in Fig. 2. As the second example, a user terminal can receive two different signals from Repeaters R21 and R22, but no signal from the satellite, then it can get STC gain by decoding [s p2] and [-p*2, s*]. When applying STC scheme, we can utilize the transmit diversity by using an ordinary decoding algorithm for Alamouti scheme [4]. That is,

sˆ = (h21 ) r1 + (h22 )r2* *

pˆ 2 = (h21 ) r2 − (h22 )r1* , *

(1)

where h21 and h22 are complex multiplicative distortion for two different repeaters, R21 and R22, respectively. r1 and r2 are the serially received signals at the terminal with the frequency band of f2 over the period of 2T, and are expressed as

r1 = h21s − h22 p2* + n1 r2 = h21 p2 + h22 s* + n2 ,

(2)

generates a parity symbol with two bits, [W1 Y1], and the second RSC generates the other parity symbol for interleaved version of [A B] with two bits, [W2 Y2].

where n1 and n2 are complex random variable representing receiver noise and interference. Since the terminal can recover [s p2] by using the STC decoding process, it decodes the information by using the rate 1/2 channel coding scheme punctured from the rate 1/3 mother code. This case mainly happens in urban areas where we can hardly get direct signal from the satellite. This case is indicated as C2 in Fig. 2. As the third example, a user terminal can receive a signal from the satellite and the other from one of the repeaters, i.e. the signal [s p1] from the satellite and either [s p2] from R22 or [-p*2, s*] from R21. In this case, the terminal first apply STC decoding scheme to the signal received by frequency f2, as for case C2. After recovering [s p2], the terminal combines [s p1] from the satellite and decode the information by using the rate 1/3 mother code. This case is indicated as C3 in Fig. 2. As the last example, when a user terminal can receive three different signal sets of [s p1] from the satellite, [s p2] from R22, and [-p*2, s*] from R21, then it can utilize STC diversity gain by using [s p2] and [-p*2, s*] as in (1) and (2). In addition to this, it can also achieve additional coding gain by combing p1 p2 and thus applying decoding algorithm for rate 1/3 code. Referring to Fig. 1, this situation may be occurred when a user terminal is located at the border of two different terrestrial cells, and thus it can receive satellite signals and two different signals from Repeaters R21 and R22. This case is indicated as C4 in Fig. 2. IV.

SIMULATION RESULTS

A. Simulation environments In order to assess the performance enhancement of the proposed diversity scheme, we apply a simple simulation model. We assume the signals are modulated using the QPSK scheme. We first consider the performance of various cases when the signal power of each transmitter, i.e. the satellite, Repeater R22, and Repeater R21 is Pt/2, where Pt is the total transmitted power when a single transmitter is employed. For fair comparison, we consider when the signal power of each transmitter is Pt/3. We also assume that the amplitude of fading from the satellite to the receiver are Rician distributed with factor K of 10 dB, and those from the repeaters to the receiver are uncorrelated Rayleigh distributed. As in a normal Alamouti scheme, we assume that fading is constant across two consecutive symbols and the average signal powers at the receiver from each transmitter are the same. The receiver has perfect knowledge of the channel. As the channel coding scheme, we use the duo-binary turbo codes in Fig. 3, which is specified in many wireless systems including Digital Video Broadcasting – Return Channel via Satellite (DVB-RCS) and IEEE 802.16e [6][7]. For the turbo code with information block size of N symbols, the encoder generates 3N symbols with the code rate of 1/3. Referring Fig. 3, for an information symbol with two bits, [A b], the first recursive systematic convolutional (RSC) code

(a) component recursive systematic convolutional (RSC) code

(b) encoder Figure 3. Schematic diagram of duo-binary turbo codes [6]

In our example, the satellite and the repeaters transmit 2N symbols with the code rate of 1/2. Using the notation in Fig. 3, the satellite passes through [A B W1 W2], and the repeaters transmits the space-time encoded version of [A B Y1 Y2]. Assuming QPSK modulation scheme, [A B] can be mapped to the systematic symbol of s in Fig. 2, and [W1 W2] and [Y1 Y2] are mapped to the parity symbols of p1 and p2, respectively. In our simulation, we use the information block size, N of 212, i.e. 212×2 bits. B. Simulation results Figure 4 shows bit error rate (BER) performances of the proposed schemes when the signal power of each transmitter is Pt/2, compared to the coding schemes with rate 1/3 and 1/2 codes but without any diversity scheme. As an iterative decoding algorithm for the turbo code, we used the Max-log-MAP algorithm. The BER performance for the turbo code shown in Fig. 4 is after 8 iterations. As shown in Fig. 4, case C2 of the proposed scheme, where we have two different available signals from the terrestrial repeaters, shows more than 1 dB gain compared to the normal rate 1/2 turbo code without diversity. Case C3 of the proposed scheme, where we can combine a signal from a repeater and the other from the satellite and thus can use rate 1/3 turbo codes, shows about 0.5 dB gain compared to the normal rate 1/3 turbo codes. If we can combine all three signals in case C4, one from the satellite and two from different repeaters, then we can achieve the best performance.

No diversity (tx. power=Pt) R=1/2, Rayleigh R=1/3, Rayleigh R=1/3, Rician

1

0.1

0.01

0.01

1E-3

1E-3 BER

BER

0.1

1E-4 Proposed scheme (tx. power=Pt/2) C2 C3 C4

1E-5

1E-6

No diversity (tx. power=Pt) R=1/2, Rayleigh R=1/3, Rayleigh R=1/3, Rician

1

1E-4 Cooperative diversity (tx power=Pt/3) C3 C2 C4 C4 *

1E-5

1E-6

1E-7

1E-7 0

1

2

3

4

5

Eb/No (dB)

0

1

2

3

4

5

6

Eb/No (dB)

Figure 4. Comparison of BER performance with tx. power, Pt /2

Figure 5. Comparison of BER performance with tx. power, Pt /3

For fair comparison of various schemes, Figure 5 compares BER performances when the signal power of each transmitter is Pt/3. In case C4, the proposed scheme shows approximately 0.3 dB performance gain, comparing to the rate 1/3 scheme without any diversity scheme. Although the performance improvement is little, the proposed scheme achieves 60 % improvement in spectral efficiency. This is because all transmitters in the proposed scheme send data with the code rate of 1/2, and achieves the BER performance even better than the conventional method with code rate of 1/3. We also plotted the BER performance of Case C4, when the path gain of the satellite path is also assumed to be Rayleigh distributed. It is denoted as Case C4 *. The BER of Case C4 * shows almost the same performance as that of the original rate 1/3 code, and achieves 60 % improvement in spectral efficiency.

V. CONCLUSION In this paper, we proposed a novel cooperative transmit diversity scheme for mobile multimedia broadcasting systems. We introduced the basic concept and system configuration of the proposed cooperative diversity scheme. Multiple terrestrial repeaters regenerate signals in the way that the received signals from the repeaters and the satellite can be combined to produce additional coding gain as well as spatial and time diversity gain. We demonstrated an example of the proposed scheme, where each transmitting station sends data with an effective code rate of 1/2. At the receiver we can utilize a rate 1/3 code by combining signals from different transmitters, thus achieving further coding gain and diversity gain. In addition, this scheme achieves high spectral efficiency because we can achieve almost the same coding gain of rate 1/3 code by using rate 1/2 code. REFERENCES [1] Sang-Jin Lee, SangWoon Lee, Kyung-Won Kim, and Jong-Soo Seo, “Personal and Mobile Satellite DMB Services in Korea,” IEEE Transactions on Broadcasting, vol. 53, no. 1, Mar. 2007, pp. 179-187 [2] http://www.xmradio.com/ [3] Hee Wook Kim, Kunseok Kang, and Do-Seob Ahn, “Distributed SpaceTime Coded Transmission for Mobile Satellite Communication Using Ancillary Terrestrial Component,” IEEE ICC 2007, Jun. 2007 [4] Sooyoung Kim, Hee Wook Kim, Kunseok Kang, and Do Seob Ahn, “Performance enhancement in future mobile satellite broadcasting services,” accepted for publication in IEEE Communication Magazine [5] S. M. Alamouti, “A Simple Transmit Diversity Technique for Wireless Communications,” IEEE Journal on Selected Areas in Communications, vol. 16, no. 8, Oct. 1998, pp. 1451-1458. [6] ETSI EN 301-790, “Digital Video Broadcasting (DVB); Interaction channel for satellite distribution systems”, V1.4.1, Sep. 2005 [7] IEEE 802.16 Standard - Local and Metropolitan Area Networks – Part 16, IEEE Std 802.163-2005