mimo applicability to satellite networks - array - UPC

MIMO APPLICABILITY TO SATELLITE NETWORKS A. Pérez-Neira, *

Technical University of Catalonia (UPC) c/ Jordi Girona 1-3, Módulo D5 08034-Barcelona [email protected]

C. Ibars, J. Serra, A. del Coso, J. Gómez, M. Caus* Centre Tecnològic de Telecomunicacions de Catalunya Av. Canal Olimpic s/n 08860-Castelldefels, Barcelona, Spain {cibars,aitor-del-coso,[email protected]}

Abstract—This paper evaluates the applicability of MIMO techniques to satellite networks in order to achieve diversity and multiplexing gain through dual polarized antennas. In single satellite scenarios the proposed STTC and OSTBC techniques proposed offer better BER than plain stream multiplexing along each polarizations and SISO transmissions. By adding a satellite with dual polarization antennas and performing a joint distributed OSTBC, the spectral efficiency increases as the satellites transmit in the same frequency band. Finally the hybrid satellite-terrestrial network has been considered for MIMO transmission. In this case, the spectral efficiency can be multiplied by 4 if a joint encoding of satellite and terrestrial signals is performed. In each scenario the broadcast signal follows the DVB-SH standard.

(Multiple Input Multiple Output) techniques in the satellite scenario. Through the spatial multiplexing and the maximization of the diversity gain we can achieve better spectral efficiencies and bit error rates than SISO systems. However, in order to achieve multiplexing and diversity gains of MIMO systems, large antenna spacings and dense scatter distribution around the transmitters and receivers are needed. In satellite scenario, dual polarization can be seen as an effective and cost efficient means to create a MIMO transmission with only one physical transmit and receive antenna. Indeed, by using dual-polarized antennas at the transmitter and the receiver, the system becomes a MIMO transceiver with two inputs and two outputs. The idea of exploiting different polarizations allows to obtain independent fading profiles in satellite paths, which provides robustness in shadowed and fast-fading scenarios.

I. INTRODUCTION Demands on high quality audio and video broadcasting services are increasing recently. Mostly in the scope where the reception is mobile and portable. In this scenario satellite broadcasting sets up as a perfect solution to satisfy the customers demands and provide services such as mobile digital television. Besides, the satellite transmission is able to provide wide coverage area in a more cost efficient way than terrestrial networks. Since the QoS (Quality of Service) required by the audio and video broadcasting services is very high, the number of repeaters that a terrestrial network requires would not be affordable. In consequence the DVB-H standard has evolved bringing about DVB-SH standard. It proposes a hybrid architecture which consists of GEO satellite and a network of terrestrial repeaters called gap fillers. With a single satellite we can provide service to the whole Europe. Besides, the cost of the satellite is independent from the number of users. However, if the receiver doesn’t have line of sight with the satellite the signal may suffer blockage. It mostly happens in suburban and urban areas. Thank to the gap fillers we can achieve coverage in urban and suburban areas and inside the buildings as well. The reserved band to broadcast the services is the S-band from 2170 MHz to 2200MHz. As S-band is adjacent to the band used in 3G, the antennas of the base stations can be used. In consequence, it is an interesting business model because the terrestrial network can be integrated in an existing infrastructure.

In Section I0, we propose a DVB-SH MIMO satellite transmission so that the satellite and the receiver have dual polarized antennas. Specifically we will address the design of OSTBC (Orthogonal Space Time Block Codes) technique and STTC (Space Time Trellis Codes) technique. In order to asses the performance of the STTC and OSTBC techniques, in section III a comparison is made. The techniques are compared against three cases: i) a SISO system, ii) an independent transmission on two polarizations with independent decoding; iii) a technique which consists of a MIMO transmission characterized for independent parallel transmission and joint decoding stage. In order to improve the original architecture introduced in DVB-SH standard, Section 0 brings the concept of satellite diversity. Instead of a single satellite, the satellite network consists of two satellites with dual polarized antennas. In this section we focus on the design of distributed OSTBC and a system which transmits in separate bands. In section V a comparison between the OSTBC technique described in section II and the techniques described in Section IV is made. Firstly it is compared the technique that transmits in separate bands against the OSTBC technique used in the single satellite scenario. Next all the techniques described in section IV are compared. In Section 0, we address the design of a hybrid satellite-terrestrial network for mobile broadcasting with multiple antenna terminals. Exclusive satellite coverage may not be sufficient for urban or suburban environments, where the satellite path is often blocked by buildings, and where indoor coverage is also desirable. The current solution is based on hybrid coverage from a satellite and from a network of terrestrial gap fillers (GF). Typically, these networks are

Taking into account the architecture proposed in the DVBSH standard, this paper focuses on the design of the MIMO

978-1-4244-2573-0/08/$25.00 ©2008 IEEE

arranged in separate frequency bands and the signal is combined at the receiver by means of selection combining, or maximal ratio combining [9]. An enhancement has been proposed, which uses space-time encoded signals for the terrestrial and satellite components [10], which still reside in separate bands. Also, for the DVB-SH standard for broadcasting to mobile handheld devices, a configuration using OFDM in a single frequency network (SFN) mode is proposed (SH-A) for both the satellite and GF. In this configuration, a single polarization is used at the satellite. In section VII it is analysed the technique that encodes jointly the satellite and the terrestrial gap fillers network. It is compared against a system which satellite and terrestrial components use separate frequency bands. In section VIII we expose the conclusions. SINGLE SATELLITE WITH MULTIPLE RECEIVER ANTENNAS The first scenario proposed consists of a single dual circular polarization satellite where the handset incorporates dual circular polarization as well. Firstly it is presented the system model and then we address the coding and decoding scheme of OSTBC technique and STTC technique.

Figure 1. Block diagram of OSTBC technique.

Figure 2. OSTBC transmitter block diagram.

II.

A. System Model As a result of the multi-antenna and multi-polarization channel we can model the downlink between the satellite and the handset as a MIMO 2x2 communication system ⎡ y1 ⎤ ⎡ h11 ⎢ y ⎥ = ⎢h ⎣ 1 ⎦ ⎣ 21

h12 ⎤ ⎡ s1 ⎤ ⎡ n1 ⎤ + h22 ⎥⎦ ⎢⎣ s2 ⎥⎦ ⎢⎣ n2 ⎥⎦

(1)

where y1,y2 are the samples recovered at each of the polarizations, x1,x2 are the two symbols transmitted over the two polarizations at a particular channel access, hij is the response between jth polarization at the transmitter and the ith polarization at the receiver and finally n1,n2 are noise samples contaminating the reception of two polarizations. In order to describe the narrow-band multi-polarization and multi-antenna channel the land mobile satellite matrix channel is modelled following a Loo distribution [1] which can be decomposed as ⎡h H = ⎢ 11 ⎣ h21

h12 ⎤ ⎡ h11 =⎢ h22 ⎥⎦ ⎣h21

h12 ⎤ ⎡ h11 ⎥+⎢ h22 ⎦ ⎣ h21

h12 ⎤ ⎥ = H + H h22 ⎦

Figure 3. OSTBC receiver block diagram.

In this scheme, the Alamouti encoder is appended to the output of a DVB-SH FEC encoder, generating two spatial streams from the FEC codeword. Regarding to the receiver, the Alamouti decoder is placed in front of the DVB-SH FEC decoder. It consists of two inputs from the two orthogonal polarizations, and one output which is passed to the FEC decoder. Thanks to the Alamouti scheme, the OSTBC decoder provides equivalent channel gains to the FEC decoder. The Alamouti encoder provides the following transmission matrix ⎡ s1 X =⎢ ⎢⎣ s2

* − ( s2 ) ⎤ ⎥ ( s1 )* ⎥⎦

(3)

where the first row is transmited by the first antenna and the second row by the other one. Each column refers to a symbol period. Assuming the channel model considered in the previous section we can write consecutive received samples as y1 ( n ) = h11s1 ( n ) + h12 s2 ( n ) + n1 ( n )

y 2 ( n ) = h21s1 ( n ) + h22 s2 ( n ) + n2 ( n )

(4) (5)

y1 ( n + 1) = − h11 s2 ( n ) + h12 s1 ( n ) + n1 ( n + 1)

(6)

y2 ( n + 1) = − h21 s2 ( n ) + h22 s1 ( n ) + n2 ( n + 1)

(7)

*

(2)

*

*

*

where hij follows a log-normal distribution (to account for shadowing) and hij follows a Rayleigh distribution (to account for multipath).

Combining the samples like the Alamouti decoder the receiver constructs the standard form of the Alamouti STBC coding.

B. OSTBC Technique The dual polarization is an effective and cost efficient mean to create a MIMO transmission. This leads us to consider the space-time encoded signals in order to achieve diversity gain. Since we have created a MIMO transmission, the MIMO technique that will be addresses in this section is the Alamouti encoding scheme [2]. In order to perform a DVB-SH MIMO transmission the scheme depicted in figure 1 is proposed.

⎡ y1 ( n ) ⎤ ⎢ ⎥ ⎡ s1 ( n ) ⎤ ⎢ y2 ( n ) ⎥ ⎢ y ( n + 1)* ⎥ = H eq ⎢ s ( n ) ⎥ + W ⎣ 2 ⎦ ⎢ 1 ⎥ ⎢ y ( n + 1)* ⎥ ⎣ 2 ⎦

(8)

Applying the matched filter to the estimated symbols, Maximum-Likelyhood detection can be performed with linear complexity. Specifically the estimated symbols are

⎡ sˆ1 ( n ) ⎤ ⎡ s1 ( n ) ⎤ H H ⎢ˆ ⎥ = H eq H eq ⎢ ⎥ + H eq W s n s n ⎣ 2 ( )⎦ ⎣ 2 ( )⎦

(9)

C. STTC Technique The STTC design for the single satellite system is based on the 1-dimensional extension of the FEC of the DVB-SH standard. A 1-dimensional STTC is obtained by multiplexing the output of a single FEC into several transmitter antennas [3]. The following figures provide a high-level block diagram as well as a detailed block diagram of the transmitter and receiver. Note that the bit interleavers are placed before the symbol mapper, and are standard-compliant. Besides, the optimization of two iterative loops in the receiver (the Turbo decoder loop and the iterative demapping loop) is difficult and likely to depend on the code.

a suburban scenario with a two states according to Markov chain [5]. Two interleavers have been considered: Interleaver 1 (with depth T=0.3553 s) for 3Km/h and 50 Km/h, Interleaver 2 (with depth T=21.12 s) for 120 Km/h. In the table I are listed the parameters to obtain the simulations presented below. TABLE I.

SIMULATION PARAMETERS Parameter

Value

Modulation

TDM+QPSK(gray mapping)

FEC code rate

¼

Environment

Suburban

Satellite Elevation

40 degrees MIMO satellite defined in Error! Reference source not found. SISO: 0.5 b/s/Hz Indep-Indep: 1 b/s/Hz Benchmark: 1 b/s/Hz Adv. Tech 1: 0.5 b/s/Hz Adv. Tech 2: 1 b/s/Hz

Channel Model

Spectral efficiency

Figure 4. Block diagram of STTC technique

Suburban scenario,2 states according Markov chain,v=3km/h

-1

10

Adv. Tech. 1 Benchmark Indep.-Indep. siso Adv. Tech. 2

-2

10

Figure 5. STTC transmitter block diagram

-3

BER

10

-4

10

-5

10

-6

10

Figure 6. STTC receiver block diagram

III. RESULTS In the single satellite system the following simulation cases have been evaluated:

2

2.5

3

3.5

4 Eb/No [dB]

4.5

5

The plots compare the performance of the 5 studied techniques for different handset speeds. It has been considered

6

Figure 7. BER vs Eb/No. Comparison of different techniques at 3 km/h. Suburban scenario with 2 states. Suburban scenario,2 states according Markov chain,v=50km/h

0

10

•

Adv. Tech. 1 Benchmark Indep.-Indep. siso Adv. Tech. 2

-1

10

-2

10 BER

SISO system with one polarization. It represents the performance of a DVB-SH transmission using one polarization only. • Independent coding – independent decoding. It consists of two DVB-SH SISO transmissions on two circular polarizations. Decoding is performed independently. • Benchmark case: Joint decoding of DVB-SH SISO transmissions on two circular polarizations [5]. • Advanced Technique 1: Alamouti OSTBC. • Advanced Technique 2: one-dimensional STTC. Besides the benchmark and the two advanced techniques two more cases have been considered.

5.5

-3

10

-4

10

-5

10

2

2.5

3

3.5

4 Eb/No [dB]

4.5

5

5.5

6

Figure 8. BER vs Eb/No. Comparison of different techniques at 50 km/h. Suburban scenario with 2 states.

y = [ y1

Suburban scenario,2 states according Markov chain,v=120km/h

0

10

10

Adv. Tech. 1 Benchmark Indep.-Indep. siso Adv. Tech. 2

-3

10

T

(12)

(13) where y1,y2 are the samples recovered at each of the polarizations, s satj ( n ) is the symbol transmitted by the kth j satellite with the polarization of jth antenna at a particular channel access, H is the land mobile satellite MIMO channel n = [ n1

-2

BER

T

s = ⎡⎣ s1sat1 ( n ) s2sat1 ( n ) s1sat 2 ( n − d ) s2sat 2 ( n − d )⎤⎦

-1

10

(11)

y2 ]

-4

10

n2 ]

T

sat 1

response between the ith satellite and the receiver, d is the relative delay between satellites and finally n1,n2 are noise samples contaminating the reception of two polarizations.

-5

10

-6

10

-7

10

2

2.5

3

3.5

4 Eb/No [dB]

4.5

5

5.5

6

Figure 9. BER vs Eb/No. Comparison of different techniques at 120 km/h. Suburban scenario with 2 states.

We can conclude that 2x2 MIMO either with STTC (Adv. Tech 2) or with Alamouti (Adv. Tech 1) offer better BER than plain stream multiplexing along each of the polarizations: • •

•

STTC presents equal or better BER than the SISO case but doubling the spectral efficiency. STTC presents: 1.5 dB gain (3Km/h), 1 dB gain (50 Km/h) and 0.5 dB gain at (120 km/h) when compared with independent coding techniques. Alamouti presents approximately 1 dB gain with respect to plain SISO, for all speeds.

The better BER of Alamouti is due to the fact that its spectral efficiency is one half the spectral efficiency of STTC. IV.

TWO SATELLITES WITH MULTIPLE RECEIVER ANTENNAS For the satellite diversity scenario we consider two satellites with dual polarization each and a unit terminal with dual polarization as well. In contrast to single satellite scenario there is a delay between the paths of different satellites. As a result we can’t assume all the signals arrive at the same time because the antennas aren’t co-located. In order to analyse the delay we first formulate the system model and then we address the transmission at separate bands using the Alamouti technique in each satellite and the joint encoding and decoding scheme of the block-wise Alamouti technique [6]. A. System Model Taking into account the multi-satellite, multi-antenna and multi-polarization channel the system is formulated as a MIMO 4x2 communication system. Since the antennas aren’t colocated there will be a satellite which signals will be received with a delay regarding the signals from the other satellite. As a result the system becomes asynchronous as the signals doesn’t arrive at the same time. The received samples can be expressed as y = ⎡⎣ H sat1

H sat 2 ⎤⎦ s + n

(10)

In turns out that each channel matrix can be decomposed into a log-normal component and a rayleigh component. The lognormal matrix represents the line of sight component and the Rayleigh matrix represents the diffuse component. The whole distribution as in the single satellite scenario follows a Loo distribution [1]. ⎡ hi H sati = ⎢ 11i ⎣ h21

h12i ⎤ ⎡ h11i =⎢ i i ⎥ h22 ⎦ ⎣ h21

h12i ⎤ ⎡ h11i ⎥+⎢ h22i ⎦ ⎣ h21i

H sati = H sati + H sati

h12i ⎤ i ⎥ h22 ⎦

(14) (15)

where hijk follows a log-normal distribution and hijk follows a Rayleigh distribution. B. Transmission in Separate Bands with Selection Combining Since the relative delay between satellites could be large, we propose an independent encoding scheme in order to deal with it. The encoding scheme consists of a transmission at separate bands so that each satellite uses the Alamouti technique described in section II and the satellites in turn transmit the signals in separate bands. With regard to the receiver we employ a selection combining technique. The criteria is based on the power of the channel. We compute the power of both satellite channels and select the one which provides more array gain. Since the receiver only selects one of the two bands, it is necessary that the satellites transmit the same information. On one hand, this technique achieves half spectral efficiency with respect to OSTBC technique described in section II. The reason is that it uses two frequency bands sending the same information in each band. On the other hand, the technique provides more robustness because of the separation between satellites. In other words, if one of the satellites suffers a deep fading, the receiver can select the signal that comes from the other satellite. Therefore, the probability of suffering blockage at the receiver is lower. As a result, the availability of the service is increased mostly in suburban and urban areas. C. Block-Wise Alamouti Technique The main difficulty when both satellites share the frequency band is the large differences in the propagation path length from the two satellites. This may result in the two signals arriving at the handset with hundreds of symbols of relative delay. In addition, since we are considering a broadcast application, the system must be robust to all possible relative

delays, which depend on the mobile position. To deal with the relative delay between satellites exist solutions based on equalization. However, the equalization becomes unfeasible when the relative delay amounts to hundreds of symbols. In order to simplify the receiver, we propose a distributed OSTBC, which has affordable complexity. A challenge is that OSTBC require synchronization among transmitting antennas, and this is not available in the satellite diversity case. One possibility is the design of time-interleaved OSTBC [8], but the code must be designed for a specific delay, which is unfeasible for a broadcast application where each receiver experiences different relative delays. Another possibility is a coarse alignment of the satellite delay in order to reduce the large delays in satellite diversity systems, but, again, such alignment is only possible for a single receiver and is not feasible for a broadcast application. The solution that is proposed is a blockwise Alamouti that is based on the work by Li [6]. Specifically the solution proposed is an extended scheme. In order to avoid inter-block interference a guard interval is added between blocks so that no symbols are transmitted. s1sat1 (1) " s1sat1 ( N ) GI − s1sat 2 ( N )

*

sat1 2

s

(1)

sat 1 2

" s

( N)

GI −s

sat 2 2

( N)

" −s rev

− s sat 2

s1sat 2 (1) " s1sat 2 ( N ) GI s1sat1 ( N )

*

s2sat 2 (1) " s2sat 2 ( N ) GI s2sat1 ( N )

*

(1)

*

*

" s2sat1 (1)

*

rev * sat1

Figure 10. Satellite diversity scheme.

Let d1, d2 be the transmission delays, N be the block length, GI be the guard interval and H1, H2 be the channels between satellites and the receiver which keep constant over the frame and may vary afterwards. In order to compensate the delay, the receiver obtains T samples per slot so that a buffer is required. In the first slot the received signals are gathered in a 2xT matrix. Y

slot 1

⎡ s sat1 ( n − d1 ) ⎤ slot1 = ⎡⎣H 1 H 2 ⎤⎦ ⎢ ⎥ +W ⎣⎢s sat 2 ( n − d2 )⎦⎥

(16)

0≤ n ≤T

⎡ ⎢ h1 2 + h1 2 + h 2 2 + h 2 2 21 11 21 ⎢ 11 ⎢ A=⎢ * * 1 1 1 1 ⎢ ( h12 ) h11 + ( h22 ) h21 + ⎢ * * ⎢ ( h112 ) h122 + ( h212 ) h222 ⎣

Y

slot 2

W

slot 2

= ⎡⎣ H 1

⎡− s ( N − n + d1 + GI ) ⎤ ⎥+ H 2 ⎤⎦ ⎢ rev * ⎢⎣− s sat1 ( N − n + d 2 + GI ) ⎥⎦

(

*

(h ) h + (h ) (h ) h −(h )

2 h22 −⎤ ⎥ * 2 ⎥ h21 ⎥ ⎥ ⎥ ⎥ ⎥ ⎦

1 * 11

2 12

1 * 21

1 * 12

2 11

1 22

0

(h ) h + (h ) h (h ) h + (h ) h

+ ⎤ ⎥ ⎥ 2 * 2 2 * 2 12 11 22 21 ⎥ ⎥ ⎥ 1 2 1 2 2 2 2 2 h12 + h22 + h12 + h22 ⎥ ⎥ ⎦ 1 * 11

1 12

1 * 21

)

−1

H

H

⎡ s sat1 ( n − d1 ) ⎤ Sˆ = H ZF Y = ⎢ ⎥ + H ZF W ⎢⎣ s sat 2 ( n − d 2 ) ⎥⎦ 0≤ n≤T

(17)

0≤ n≤T

⎡ ⎤ Y Y = ⎢ slot 2 ⎥ * ⎢⎣Y ( N − n + d 2 + d1 + GI ) ⎥⎦

H

H ZF = H H

Previously the decoding we construct a 4xT matrix.

(21)

1 22

As a solution it is proposed a zero-forcing to obtain the estimated symbols.

Similarly the received signal in slot 2 is rev sat 2

(20)

B⎤ ⎥ * A ⎥⎦

⎡ ⎢ 0 ⎢ ⎢ B=⎢ 1 * 2 1 * 2 ⎢− ( h11 ) h12 − ( h21 ) h22 + ⎢ * 1 * 2 ⎢( h121 ) h112 + ( h22 ) h21 ⎣

(s )

ssat 2

⎡ s sat1 ( n − d1 ) ⎤ H H Sˆ = H Y = I ⎢ ⎥+H W ⎢⎣ s sat 2 ( n − d 2 ) ⎥⎦ 0 ≤ n ≤T ⎡ A H H H =⎢ H ⎢⎣ B

" s1sat1 (1)

(19)

Notice that thanks to the time reversal operation applied in the samples received in the second slot, we have a similar expression like the one obtained in the Alamouti scheme but with a different equivalent channel. Thanks to the cross polarization discrimination factor of the receiver antennas the value of the cross-polar terms are low. As a result, applying the matched filter algorithm we almost obtain separately the symbols that have been sent through each polarization. We say almost separately, because the cross-polar terms aren’t negligible so that the equivalent channel matrix isn’t diagonal. This expression shows that there is ISI in the estimated symbols.

*

sat 2 2

H 2 ⎤ ⎡ s sat1 ( n − d1 ) ⎤ ⎥ ⎥ +W = H S +W * ⎢ − H 1 ⎦⎥ ⎣⎢ s sat 2 ( n − d 2 )⎦⎥

0≤ n≤T

" −s1sat 2 (1)

( )

ssat1

⎡H1 Y =⎢ * ⎣⎢ H 2

(22)

(23)

In order to assure the performance of the system we must take into account several considerations.

slot 1

(18)

•

In order to avoid inter-block interference the guard interval has to be at least equal to relative delay

abs(d1-d2). Hence, the bigger N is compared to abs(d1-d2) the more efficient will be the system. •

Since the channel remains constant during the transmission of the frame, the length of the frame 2(N+GI) has to be less than the coherence time. V.

RESULTS

We analyse the performance of •

OSTBC technique: single satellite scenario.

•

Transmission in separate bands with selection combining: diversity satellite scenario.

We simulate the BER that is obtained with the simulation parameters of the table II and III.

TABLE II.

SIMULATION PARAMETERS

Parameter

Value

Modulation

TDM+QPSK(gray mapping)

FEC code rate

¼

Environment

Open

Satellite Elevation

40 degrees

Channel Model Spectral efficiency

Figure 11. BER vs Eb/No for OSTBC technique and transmission in separate bands technique. LOS state.

Next we simulate the following techniques.

MIMO satellite defined in Error! Reference source not found. Selection Combining: 0.25 b/s/Hz Block-wise Alamouti: 1 b/s/Hz

TABLE III. Scheme OSTBC Technique. (Single satellite scenario) Selection Combinig (Satellite diversity scenario) Block-wise Alamouti. (Satellite diversity scenario)

•

Transmission in separate bands with selection combining. Diversity satellite scenario.

•

Block-wise Alamouti with Diversity satellite scenario.

•

Block-wise Alamouti with zero-forcing. Diversity satellite scenario.

matched

filter.

RESOURCES Bandwidth

Power

W

P

2W

2P

W

2P

According to the figure 11 we can conclude that both systems have a similar performance in terms of BER. With regard to the availability, having two satellites allow to have better coverage in urban and suburban areas. Nevertheless, it is required an extra satellite transmitting the same power and twice bandwidth. As a result the spectral efficiency is 0.25 b/s/Hz.

Figure 12. BER vs Eb/No for the Block-Wise Alamouti technique and transmission in separate frequency bands. LOS state.

Thanks to the Block-wise Alamouti scheme both satellites share time and frequency. As a result, the spectral efficiency is increased in 4 respect to the system that transmits in separate frequency bands. However, there is a penalty of 4dB.

The matched filter and zero-forcing, have a similar performance because the ISI terms of the expression (20) are very low. This happens because the cross-polarization factor of receiver antennas is 15dB [5]. However, in terms of BER the zero-forcing has better performance.

result of polarization changes introduced by multipath propagation.

R1

R1

VI.

MIMO TECHNIQUES FOR A HYBRID SATELLITETERRESTRIAL NETWORK

The proposed hybrid network uses a single frequency band for both satellite and GF signals and performs joint encoding of satellite and terrestrial signals. We first address the coding scheme of terrestrial signals and then the joint decoding of terrestrial and satellite signals at the mobile receiver. A. Space-time Coded Gap Filler Network The GF network is usually set up as a single frequency network [9]. In an SFN, all transmitters transmit the same OFDM signal, which is able to cope with the artificial multipath propagation that results from the multiple GF to the receiver. The main advantages of an SFN are that it has a frequency reuse factor of 1 and that it can provide very high coverage levels by providing path diversity against shadowing. On the other hand, since an identical signal is transmitted, an SFN does not necessarily provide diversity against multipath fading. In order to provide an additional degree of spatial diversity, the GF system has been designed as a coded SFN, where different transmitters send different parts of a space-time codeword. The system is akin to a space-time coded multiple antenna transmitter, but where different antennas are placed in different geographical locations. Since the OFDM signal structure deals with the resulting propagation delays, a standard space-time code may be used. The proposed space-time coded SFN is shown in Figure 1. Transmitters marked with R1 (yellow-colored cells) transmit a subset of the space-time coded symbols, while transmitters marked with R2 (orange-colored cells) transmit the complementary subset (as in a two-antenna transmitter). Assuming that a full diversity code is used (such as the Alamouti code), this scheme provides a diversity order of two. Note that a different codeword reuse scheme could be used, combining more subsets of space-time coded symbols. Finally, a conventional SFN may be viewed as a codeword reuse factor of 1, which provides no diversity. B. Multiplexing of Satellite and Terrestrial Signals The mobile receiver is equipped with two (left and right) circularly polarized antennas, which make it capable of receiving both circular and linear polarizations. A linear polarization is received at both antennas with a 3dB loss with respect to the antenna gain. The equivalent space-time coded scheme is shown in Figure 1. The satellite signal is dualpolarized, with a space-time encoded signal on each polarization. The gap-filler signal is vertically polarized and consists of an additional two components, due to the spacetime coded SFN. The resulting MIMO system is 4x2, i.e. 4 transmit antennas and 2 receive antennas. Note that the arrows represent the received signal, at each antenna, in the absence of multipath fading. However, in a realistic scenario, the two satellite polarizations will be received at each antenna as the

R2

R2

R1

R1

R2

R2

R1

R1

Figure 13. Space-time coded SFN with codeword reuse of 2.

Figure 14. Space-time encoder and decoder of the hybrid satellite-terrestrial network. The encoder is distributed, with two components in the satellite and the other two distributed in the GF network.

The space-time code consists of spatially multiplexing the standard-compliant (DVB-SH standard code [13],[14] ) code onto the four transmit antennas, which increases the code rate by 4. A distributed implementation is simply carried out by each transmitter encoding the signal with a standard-compliant code followed by puncturing to obtain the desired space-time code component. VII. RESULTS A hybrid satellite-terrestrial network with the parameters defined in TABLE IV. has been evaluated. The proposed technique has been evaluated against a benchmark system consisting of a space-time coded gap filler network that is combined with a dual-polarized satellite that implements joint encoding. In the benchmark system, satellite and terrestrial components use separate frequency bands, and the receiver uses selection combining based on the received SINR of each frame. Both satellite and terrestrial components use an Alamouti encoder after the FEC encoder in order to exploit spatial diversity. The bit error rate of the proposed system and the benchmark system are shown in Figure 1 and Figure 1,

Adv. Technique, Urban Scenario, satellite in NLOS state

0

10

-1

1 Turbo Decoder iteration 2 Turbo Decoder iterations 5 Turbo Decoder iterations 8 Turbo Decoder iterations

10

-2

10

BER

respectively. Up to 8 turbo decoder iterations are shown. From the figures, we notice that the required Eb/N0 is about 2 dB higher for the proposed scheme. However, the proposed scheme operates at a spectral efficiency 4 times higher. The increase in spectral efficiency comes from using a single frequency band and from the fact that a higher space-time code rate is used. Therefore, the proposed scheme misses out on part of the diversity gain of using multiple antennas but provides a much higher multiplexing gain.

-3

10

-4

TABLE IV.

10

SIMULATION PARAMETERS

-5

10

Parameter

Value -6

10

OFDM + QPSK, 8k mode

FEC code rate

¼

Environment

Urban, NLOS

Satellite Elevation

40 degrees

Channel Model

MIMO satellite and terrestrial models, defined in Error! Reference source not found.,Error! Reference source not found.

Speed

3 km/h

Spectral efficiency

Proposed system: 2 b/s/Hz Benchmark system: 0.25 b/s/Hz

VIII. CONCLUSIONS OSTBC technique and STTC technique have been proposed to decrease the BER in the single satellite scenario as the dual polarized antennas allow to increase the diversity order. In terms of BER performance both OSTBC and STTC techniques show better performance than the MIMO systems with independent coding and SISO systems. It has been assumed that the OSTBC technique have half spectral efficiency than the STTC one. As a consequence its BER is lower. In order to improve the spectral efficiency and the availability it has been proposed to add one satellite bringing about MIMO 4x2 system. To solve the problem of large relative delays between satellites, firstly it has proposed transmitting two alamouti codes in separate frequency bands so that each satellite implements the OSTBC technique used in the single satellite scenario but in separate bands. Since each satellite transmits the same information and the receiver applies the selection combining algorithm, there is no interference between satellites and the systems is robust against the relative delay between satellites. Assuming that the whole power transmitted in the satellite diversity scenario is twice much than the single satellite scenario, the transmission in separate bands has better performance in terms of availability. In other words if the satellites are in line of sight state, the proposed selection combining algorithm has the same performance than the OSTBC technique applied in the single satellite scenario. However, the probability of blockage is lower in the case that we use two satellites as it is more likely that one satellite suffers a deep fading than two satellites.

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Figure 15. BER vs Eb/No for the advanced technique. NLOS state. Benchmark, Urban Scenario, satellite in NLOS state

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Modulation

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Figure 16. BER vs Eb/No for the benchmark technique. NLOS state.

Nevertheless, as the same signal is transmitted in each band the technique has half spectral efficiency. The second technique that we addressed in the diversity satellite scenario has been the implementation of distributed OSTBC scheme called block-wise Alamouti scheme. This scheme allows to almost double spectral efficiency compared to the OSTBC technique as the two satellites share time and frequency. We say almost because of the guard interval there is a penalty in the spectral efficiency. Unfortunately the estimated symbols obtained by the original STBC decoding algorithm which consists of applying the matched filter have ISI. The higher the cross-polarization factor of the antennas is the lower the amplitude of the ISI terms is. In order to mitigate the ISI a zero-forcing have been proposed instead of a matched filter. Radiating in the diversity scenario twice the power than in the single satellite scenario we can obtain whether better availability through the transmission at separate bands or higher spectral efficiency through the block-wise Alamouti scheme.

Considering the hybrid architecture contained in the DVBSH standard we have proposed an enhancement. Typically these satellite-terrestrial networks split the band in two separate bands so that the terrestrials gap fillers and satellite transmit in different bands. At the receiver the signals are usually combined with a selection combining or maximal ratio combining. Combining the dual-polarized satellite and terrestrial gap fillers in a single frequency band, through distributed space time coding the diversity gain is maximized and multiplexing gain is achieved. the performance of the systems is enhanced. Specifically, the spectral efficiency can be increased in 4. However, it is required an increase of 2dB respect to the system which uses separate bands for the satellite and terrestrial signal. ACKNOWLEDGEMENTS

This work was partially supported by the Catalan Government under grant SGR2005-00996, the Spanish Government under grant TEC 2006-10459/TCM and European project ICT-2007.1.1 NEWCOM++ 216715. The authors gratefully acknowledge valuable discussions on the topic with Enrico Casini and Konstantinos Liolis from the European Space Agency/Estec. REFERENCES [1] [2]

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