J. Cent. South Univ. Technol. (2010) 17: 117-122
DOI: 10.1007/s11771-010-0019-1 
Enhanced multiple access schemes for future broadband satellite communications
JIA Min(贾敏), GU Xue-mai(顾学迈), GUO Qing(郭庆)
Communication Research Center, Harbin Institute of Technology, Harbin 150001, China
? Central South University Press and Springer-Verlag Berlin Heidelberg 2010
Abstract: A new structure of next generation integrated communication system was proposed, which is composed of space segment based on satellites and terrestrial segment. Moreover, the characteristics of enhanced multiple access schemes based on orthogonal frequency division multiplexing (OFDM) technique were analyzed for satellite links. However, OFDM is a doubtful candidate as its higher peak-to-average power ratio (PAPR) that causes the distortion of high power amplifier (HPA). Furthermore, different schemes were evaluated and compared in terms of the HPA nonlinearity and the link level performance in detail. And the pilot-aided channel estimation and equalization techniques were also considered for analyzing the problem. Simulation results show that the bit error rate (BER) and block error rate (BLER) performance of orthogonal frequency division multiple access (OFDMA) outperforms that of single carrier-frequency division multiple access (SC-FDMA) for the satellite links in the proposed structure, though discrete Fourier transform-spread OFDM DFT-S OFDM has low PAPR, especially the BER performance of OFDMA is 3.6 dB larger than that of SC-FDMA at the target BER.
Key words: satellite; orthogonal frequency division multiplexing; discrete Fourier transform; high power amplifier; channel estimation
1 Introduction
It is known that satellite communication system not only has broadcast, but also has unicast and multicast functions, which is becoming an essential part in the next generation communication system. Satellite is employed to satisfy the multimedia services with high rates and high quality requirements for the next generation communication system. An integrated and layered communication infrastructure is composed of the space segment employed satellites and the terrestrial segment. We proposed a next generation mobile satellite communication system that is composed of one geostationary earth orbit (GEO) satellite and low earth orbit (LEO) satellites with inter-satellite link (ISL), and can support the capacity of 1Gbps and the maximum bit rate of 100 Mbps transmission. And the proposed structure has more advantages than that in Ref.[1] which is composed of one GEO satellite for the space segment.
Although recent satellite systems can offer larger bit rates and capacities by using different bands than the conventional ones, their bit rates are still much smaller than those of 3G or 4G terrestrial systems. More challenges of the air-interface are brought by this kind of service requirements for satisfying the novel space and terrestrial segment integrated requirements. Due to the difference between satellite and terrestrial wireless system, there is currently no ultimate multiple access scheme appropriate for future satellite systems. Thus, one of the main problems regarding the design of a satellite system is the choice of multiple access technique. Frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA) are traditional types in fixed assignment multiple access (FAMA) group [2]. Although these methods are able to provide optimized solutions for fixed traffic patterns, they are probably unable to perform very well for the future broadband satellite communication services [3]. The scheme based on orthogonal frequency division multiplexing (OFDM) that is a promising technology for the efficient transmission over the wireless channels has been applied in several various broadcasting systems [4-6]. The most important feature of OFDM is that it has strong robust of combating the multipath and narrowband interference over frequency selective channels, as the cyclic prefix (CP) that acts as the guard interval is applied. Channel estimation (CE) is necessary for the receiver in practice. It has the ability of improving the performance obviously at the receiver when it is combined with equalization. Frequency domain equalization (FDE) is employed at the receiver as the traditional time domain equalization has higher complexity [7]. OFDM technology just satisfies the high frequency efficiency requirement and provides the solution of limited frequency resource for broadband satellite communication system. Moreover, it can make higher frequency efficiency and be more flexible for different users by reducing the number of subcarriers and guard band. Otherwise, OFDM technology based on DFT can reduce the complexity of base station equipments. Therefore, the technology based on OFDM scheme is a popular solution for the satellite links.
Orthogonal frequency division multiple access (OFDMA) can be taken as a multi-user version of the popular OFDM scheme. It can also be seen as an alternative to combining OFDM with TDMA. Simultaneous low data transmission rate for constant delay and shorter delay from several users are allowed in this scheme. Moreover, slots are assigned along the OFDM symbol index as well as its sub-carrier index. It is considered to be highly suitable for broadband wireless networks due to advantages including scalability and MIMO-friendliness, and ability to take advantage of channel frequency selectivity [8]. However, OFDM is a proper candidate in doubt, due to its higher peak-to- average power ratio (PAPR) that causes the distortion of HPA. Single-carrier (SC)-FDMA based on discrete Fourier transform-spread (DFT-S) OFDM is an alternative to OFDMA, and some researchers take SC-FDMA as a kind of interleaved FDMA [9]. In fact, DFT-S OFDM has the same principle with OFDM. At present, SC-FDMA is employed for the uplink in the LTE of the 3GPP as its inherently low PAPR properties compared to OFDMA, while many benefits can be obtained by using the flexible frequency granularity of OFDMA [10].
It is known that the aim of the future B3G/4G broadband telecommunication systems is seamless roaming of space segment and terrestrial segment. However, it is still not clear for future broadband satellite communication links, especially the downlink.
2 Future broadband satellite communication system
The next generation integrated system based on space segment and terrestrial segment is depicted in Fig.1. The space segment is composed of one GEO satellite and LEO satellites with ISL, and the multimedia traffic and data are transmitted by the uplink/downlinks. The terrestrial segment is based on different networks, especially network. The whole system can satisfy various requirements of future communication system when the terrestrial communication system can be integrated with the satellite communication system well. And this structure can improve the range of coverage, especially for the country that has large area.
Fig.1 Next generation integrated system based on space segment and terrestrial segment
The uplink is defined from the gateway to the satellite, and the traditional multiple access scheme is multiple frequency-TDMA (MF-TDMA). It is a flexible multiple scheme that can adapt to not only the terminals with different RF capacities, but also the varied occupied bandwidth.
However, MF-TDMA is commonly used in a two-way communication mode and adopted for the return links in practical system [11]. OFDM/time division multiple (TDM) can be employed by the downlink that is defined from the satellite to different terminals. As the ATM satellite network employs high frequency, e.g., Ka-frequency band, the increase of attenuation and the loss of atmosphere cause burst errors. For the uplink, increasing the transmission power can compensate for the fading in general, but the satellite system is power-limited. Therefore, it is difficult to compensate for the fading by high power for the downlinks. However, employing the efficient coding scheme can solve this problem well. Moreover, the link scheme based on the OFDM/TDM technique can make the frequency and power more efficient [12].
The on-board switch is also used to route the TDM data coming from a given gateway to different beams; while employing the simple Fourier transform (FFT) equipment may simplify the architecture of the on-board switch as no complex carrier demultiplexing filters are needed to separate the carriers. The on-board processing (OBP) is the core part of broadband satellite systems. Different schemes applied to OBP are considered in Fig.2, where IMUX is the input multiplexer, and OMUX is the output multiplexer, separately.
Fig.2 Structure of OBP with different multiple access schemes
The link channel is considered as frequency selective Rician fading model when the line-of-sight (LOS) exists, and the envelop of channel response follows Rayleigh distribution when there is no LOS. The structure of channel model is described in Fig.3.
Fig.3 Structure of channel model
3 Multiple access with OFDM and DFT-S OFDM
OFDMA is based on the OFDM technique that is an efficient transmission scheme. It is basically the same as FDMA, but it can satisfy different users’ requirement as it has different subcarrier allocations. There are two modes for subcarrier allocation in OFDMA, which are distributed and localized modes. OFDMA is a more efficient ranging scheme for the downlinks of broadcasting or multicasting systems due to the following inherent characteristics. One is that it is more easily implemented by fast FFT with high data transmission rate. N data are transmitted through N subcarriers, thus the data rate is reduced by N times. At the same time, the broadband channel can be separated to narrowband. The other is that it is of strong robust to multi-path channel distortion and inter-symbol interference (ISI) by using CP. CP appended in front of each symbol is a replica of the last portion of the original OFDM symbol, and the length of CP is always longer than the maximum channel length to avoid the ISI and intercarrier interference (ICI).
The signal process of OFDM system with CE and equalization parts is shown in Fig.4 under the premise that perfect synchronization is considered. Fig.4 depicts the structure of a typical OFDM system, which is composed of an encoder to encode and covert the binary bits input data symbols to parallel array. The pilot symbols for CE are inserted after the modulation block.
Fig.4 Block diagram of OFDM system for one user
Then the inverse fast Fourier transform (IFFT) operation converts the modulated symbols from the frequency domain into time domain before they are converted into serial sequences.
The transmitted frequency-domain signal for OFDM system can be expressed as
XOFDM=[X0 X1 … XN-1]T (1)
where Xi is the modulated symbol, and [·]T represents the transpose operation. The time-domain signal after IFFT modulator can be expressed as
(2)
where 
is the mapping matrix, and 
represents the conjugate operation after N-FFT.
The PAPR is defined as the ratio of peak power value of time-domain signal D to average power value. It can be expressed by
(3)
where N is the FFT size, n, k∈{0, 1, …, N-1}.
Rapp’s model is applied for HPA [13], which is given by
g(D)=D(1+|D|2p)-1/2p (4)
where g is the complex output signal of HPA with the knee factor p=2.
The signal after passing the channel can be expressed as
R=HФN,MX+n (5)
where H is the channel frequency response, and n is the additive white Gaussian noise (AWGN).
However, the main drawback of OFDMA scheme is high PAPR caused by the time-domain multiplexed OFDM signal. Therefore, OFDM is hard to be employed by satellite communication systems due to its low power efficiency.
SC-FDMA can also be called DFT-S OFDM. It outperforms because of its low PAPR characteristic. As mentioned above, SC-FDMA scheme has the similar principles to OFDMA. The block diagram of SC-FDMA is shown in Fig.5. The DFT- spreading block is added after the S/P block, and then IFFT block is followed, making low PAPR.
The transmitted time-domain signal of one user with M allocated subcarriers can be expressed as
x=[x0 x1 … xM-1]T (6)
where xi is the modulated symbol.
The transmitted signal after IFFT block can be expressed as
(7)
where FM represents the M-FFT. The signal after passing the channel can be expressed as
R=HФN, MXFM+n (8)
For the next generation satellite communication systems, both the SC-FDMA and OFDMA are the alternative candidates for the satellite links. The SC-FDMA scheme can achieve low PAPR by spreading block. However, the noise which is spread over the subcarriers degrades the BER performance. Moreover, the advantage of SC-FDMA performs only over the time selective fading channel.
4 Performance evaluation of different schemes
As the link multiple assess technique can determine both the frequency reuse scheme and multiplexing scheme, we focus our discussion on the latter which accesses effects on the transmission performance.
Fig.5 Block diagram of SC-FDMA for one user
The simulation results for evaluation of nonlinear link performance and comparison of link BER performance of OFDMA and SC-FDMA schemes are presented in this section. The evaluation was done in terms of out back-off (OBO) and the required Eb/N0 by computer simulations. The parameters considered for the links are as follows: bandwidth l=10 MHz, NFFT=64, Nu=52, CP=16, M=8, and the modulation scheme is 16QAM. The frame structures of OFDMA and SC-FDMA for simulation were considered as those in Ref.[14]. Thus, the pilot symbol has half the length of the data symbol in this scenario.
In order to illustrate the nonlinear performance, the OBO and total degradation (TD) are defined as follows. OBO is the ratio of the HPA output power to the output power when the modulated signal drives the HPA at a given input power backoff (IBO). TD is defined as the sum of OBO and demodulation loss in decibel. The main relevant evaluation criterion is the necessary amount of OBO that is required to reach BER performance of ×10-3 in practical system design. Fig.6 shows the TD and OBO at the target BER. It is shown that the minimum TD of OFDMA is higher than that of SC-FDMA, and SC-FDMA outperforms OFDMA in this scenario as it has the characteristic of low PAPR.
Both CE and equalization are the necessary parts in practical systems, and the pilot-aided CE is employed for OFDMA and SC-FDMA. The pilot symbols are always distributed in the short blocks of continuous frames for SC-FDMA. When the above mentioned channel is frequency-selective and the CE is not ideal its performance will degrade. Moreover, the traditional time domain equalization has high complexity when it is compared with the frequency domain equalization (FDE). Therefore, FDE is employed at the receiver. And the zero-forcing (ZF) or minimal mean square error MMSE- FDE is adopted for OFDM and DFT-S-OFDM, separately. Fig.7 presents the bit error rate (BER) performance of the uncoded system for QPSK with different equalizers when the ideal CE is considered in this scenario. Moreover, it is known that employing Golay code can limit the PAPR effectively [15]. Fig.8 shows the BLER performance of Golay coded system with CE.
Fig.6 TD vs OBO for OFDMA and SC-FDMA
Fig.7 Average uncoded BER performance with different equalizers
Fig.8 Golay coded BLER performance with CE
It is noticed that the BER performance of OFDMA is better than that of SC-FDMA with ZF and MMSE equalizers as OFDM is robust to channel fading, especially to the frequency selective fading. And the performance of the MMSE equalizer obviously outperforms the ZF equalizer for both two schemes though it has the higher complexity in practice. Moreover, the performance of OFDMA is also 3.6 dB larger than that of SC-FDMA at the target BER.
Moreover, it is found that the performance of BER and BLER shows similar trend for the two multiple access schemes, and OFDMA has better BLER performance than SC-FDMA owing to the inherent characteristics of OFDM. Therefore, there is no doubt that the OFDMA scheme is suitable for the satellite uplink, and both OFDM and DFT-S OFDM can be employed for the satellite downlink. Furthermore, the OFDMA and SC-FDMA are comparable schemes. If the same scheme is employed by both downlink and uplink, the equipment of terminals can be simplified.
5 Conclusions
(1) To satisfy the requirements of the next generation integrated and layered communication system, the structure of the next generation communication system that is composed of space segment based on satellites and the terrestrial segment is presented.
(2) The BER and BLER of OFDMA outperform those of SC-FDMA under the Golay coded scheme condition, and the access scheme based on OFDM is suitable for the satellite downlink.
(3) Both DFT-S OFDM and OFDM can be employed for satellite downlink in certain condition. Moreover, the BER performance of OFDM is 3.6 dB larger than that of DFT-S OFDM at the target BER though the DFT-S OFDM has low PAPR.
(4) There exists the problem of trade-off between PAPR and performance for downlink. Which scheme is suitable for the satellite link depends on the practical environment, and how to take advantage of these two schemes in combination for both links are problems to be resolved in the further.
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Foundation item: Project(60532030) supported by the National Natural Science Foundation of China
Received date: 2008-03-15; Accepted date: 2009-06-08
Corresponding author: GU Xue-mai, PhD, Professor; Tel: +86-451-86418071; E-mail: jiamin@hit.edu.cn
(Edited by CHEN Wei-ping)
