Enhanced spread Aloha physical layer design and ... - Semantic Scholar

INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKING

Int. J. Satell. Commun. Network. (2014) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/sat.1078

Enhanced spread Aloha physical layer design and performance Riccardo De Gaudenzi1 ,, , Oscar del Rio Herrero1 and Gennaro Gallinaro2 1

European Space Agency– European Space Research and Technology Centre, Keplerlaan 1, Noordwijk, 2200 AG, The Netherlands 2 Space Engineering, via dei Berio 91, Rome, Italy

SUMMARY This paper describes the key design and performance aspects of the Enhanced Spread Spectrum Aloha (E-SSA) physical layer, which represents an essential component of the S-band Mobile Interactive Multimedia European Telecommunication Institute standard. Thanks to advanced signal processing at the gateway side, the E-SSA random access protocol allows to achieve unprecedented spectral efficiency in a pure asynchronous random access mode with high robustness against received packets power unbalance. The E-SSA physical layer is closely derived from the Third Generation Partnership Wideband Code Division Multiple Access random access channel physical layer with some adaptation to best operate in the Land Mobile Satellite channels. Particular emphasis is devoted to the E-SSA physical layer system design drivers as well to the packet transmission control policies, which are of high relevance for S-band Mobile Interactive Multimedia. The demodulator architectural design is also illustrated jointly with some of the key detection performance. Finally, the E-SSA random access protocol performance are reported for Additive White Gaussian Noise and satellite mobile channels. Copyright © 2014 John Wiley & Sons, Ltd. Received 9 January 2013; Revised 8 November 2013; Accepted 5 February 2014 KEY WORDS:

satellite mobile communications; random access; spread Aloha

1. INTRODUCTION This paper is focusing on a key component of the S-band Mobile Interactive Multimedia (S-MIM) standard, that is, the physical layer design and performance aspects related to the Enhanced Spread Spectrum Aloha (E-SSA) random access (RA) technique exploited for the messaging services. Current satellite systems supporting interactive applications are largely dominated by DAMA type of resource allocation. This means that each User Terminal (UT) has to make a request of capacity to the central hub station prior to transmitting traffic data. This Demand Assignment Multiple Access (DAMA) approach, which was developed for connection-oriented type of fixed networks supporting voice and data transmission, has evolved through time to also support Internet type of traffic. An example of open standard based on DAMA approach is the Digital Video Broadcasting - Return Channel via Satellite [1]. Also satellite mobile networks so far have been exploiting DAMA type of protocols both in case of Time Division Multiple Access (TDMA) (e.g., Inmarsat, Thuraya and Iridium, or in case of Code Division Multiple Access (CDMA) (e.g., Globalstar). Under these operating conditions, the traditionally used Combined Free and Demand Assignment Multiple Access satellite protocol [2] will not perform optimally as it has been shown in [3]. The Combined Free and Demand Assignment Multiple Access weakness for bursty type of packet traffic calls for devising more efficient multiple access techniques able to provide high Medium Access Control (MAC) throughput with low transmission delays and limited complexity on the terminal side. RA techniques are by nature very Correspondence to: Riccardo De Gaudenzi, European Space Agency – European Space Research and Technology Centre, Keplerlaan 1, Noordwijk, 2200 AG, The Netherlands. E-mail: [email protected]

Copyright © 2014 John Wiley & Sons, Ltd

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R. DE GAUDENZI, O. DEL RIO HERRERO AND G. GALLINARO

robust to this type of traffic and to large population of terminals sharing the same channel. RA techniques used in combination with DAMA are certainly good candidates for the less predictive, low duty cycle as well as time sensitive return link traffic. However, classical RA schemes have been widely investigated in the literature and are known not to perform well in the satellite environment [4, 5]. There are only few examples of satellite mobile systems based on a RA protocol such as Spread Spectrum Aloha (SSA) in case of Omnitracs [6] and the Viasat Arc Light system [7]. Both systems typically operate at Ku-band where their low spectral efficiency is compensated by the possible exploitation of Ku-band inclined end-of-life satellites with reduced transponder cost. For the S-MIM messaging applications a technical solution is required that is able to be at the same time power and spectral efficient while keeping the attractive features of SSA RA. For this reason, an evolution of the SSA protocol dubbed E-SSA has been developed and adopted by the S-MIM standard [8] to satisfy the challenging system requirements set forth for the envisaged messaging services. Following this introduction, Section 2 summarizes key system aspects and requirements relevant to the E-SSA design. The key trade-off performed to select the S-MIM RA protocol is reported in Section 3. Section 4 elaborates on key S-MIM RA modulator and demodulator design aspects. Section 5 provides a summary of the E-SSA performance in typical operating conditions. Finally Section 6 outlines the main findings of the paper. 2. SATELLITE SYSTEM MODEL AND KEY MESSAGING SERVICE REQUIREMENTS System model. The S-MIM protocol is to be operated in an integrated satellite/terrestrial mobile system that provides interactive broadcast/multicast, data acquisition and two-way real-time services to subscribers. Figure 1 shows an example of the system reference architecture. The terrestrial infrastructure is composed of Complementary Ground Component (CGC) in the forward link and collectors in the return link and operate in the same frequency band of the satellite. For simplicity in Figure 1, the CGC is used indifferently for the forward and return terrestrial S-band service provision. The S-band payload of a Geostationary (GEO) satellite is assumed to provide the bidirectional communication links to users. The feeder link between the satellite and the hub station is here assumed to take place at Ku-band although other bands such as C-band or Ka-band represent suitable alternatives. Non-GEO satellites are also compatible with this integrated system architecture provided that satellite Doppler precompensation countermeasures are put in place. It is assumed that the satellite has a bent-pipe architecture. Modern mobile GEO satellites for interactive services are characterized by a large number of beams (50–200+) depending on their coverage region (regional or multiregional) and their antenna reflector size. In our case, being the system designed to provide both broadcasting and messaging systems, it is likely that the forward link antenna beam will be optimized for the broadcast services (e.g., few linguistic contoured beams), whereas the reverse link will have a much higher number of beams to increase the messaging service capacity. A possible forward link companion air interface to the S-MIM return link is the Digital Video Broadcasting - Satellite services to Handheld standard (DVB-SH) [9, 10]. In addition to state-ofthe-art mobile broadcasting and multicasting capabilities, the most recent version of the standard is featuring a low latency interactive profile capability. The latter is essential to support the S-MIM messaging system signaling requirements. It should be remarked that the DVB-SH services to Handheld-B Multi-frequency Network profile (Time Division Multiplex for satellite component and Orthogonal Frequency Division Multiplex for the CGC) represents the S-MIM preferred configuration. This is because the use of Orthogonal Frequency Division Multiplex option of the DVB-SH standard Single Frequency Network profile will not be compatible with the use of CGCs as the forward link signaling information in the presence of return link collectors will be CGC dependent. This will violate the Single Frequency Network condition of common downlink data over the whole beam coverage area.

At end-of-life a satellite would have no remaining fuel to counteract the orbital perturbations. Main effects of these uncontrolled perturbation is that of inclining the satellite orbit with respect to the equatorial plane. Due to the inclination, the satellite would be not any more perfectly stationary when seen from Earth but it would appear to move along an eigthshaped figure.


Int. J. Satell. Commun. Network. (2014) DOI: 10.1002/sat

ENHANCED SPREAD ALOHA PHYSICAL LAYER DESIGN AND PERFORMANCE

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Figure 1. S-band Mobile Interactive Multimedia system block diagram.

One of the key system aspects to be considered when designing a reliable yet efficient messaging system are the Land Mobile Satellite (LMS) channel features. The reference channel adopted in this paper is the very classical LMS channel model described in [11] and [12]. The LMS channel is represented by a three-state Markov process where each state represents a different macro-channel condition (Line of Sight (LOS), moderate shadowing and deep shadowing). The LOS signal is affected by a lognormally distributed multiplicative process, whereas the terrain scatters are modeled by an additive component composed by an attenuated version of the LOS signal multiplied by a Rayleigh fading process. The composition of the two effects is represented by a Loo distribution whose parameters are depending on the type of environment considered (rural, tree-shadowed, sub-urban and urban) as well the satellite elevation angle. Considering that the urban environment is going to be covered by the CGC network, the most challenging and system dimensioning satellite environment is the treeshadowed one. Key satellite random access requirements. The key requirements for designing an efficient satellite mobile messaging system can be summarized as the following: (a) high aggregate throughput with very low packet failure transmission probability; (b) efficient support of small packets with very low transmission duty cycle; (c) minimum signaling overhead; (d) low Effective Isotropic Radiated Power (EIRP) requirements; (e) robustness to the received packets power unbalance; and (f) affordable complexity at the gateway demodulator. Requirement a) is justified by the need of maximizing the single satellite beam throughput considering very scarce overall bandwidth available for the service (typically 15 MHz) and the fact that the satellite beam size is much larger than the terrestrial network cells . This is drastically reducing the amount of frequency reuse the satellite component can exploit in the return link. Furthermore, the packets shall be reliably transmitted to avoid waste of capacity resources occurring in the case of their retransmission. Requirement b) is intrinsic to the nature of the messaging type of services, which has to be supported. Requirement c) is pivotal as the system is supposed to support a very large number of users sparse over a wide geographical area with sporadic transmission of small size packets. The S-MIM protocol shall be able to achieve requirement c) jointly with a) and b); thus not only the multiple access protocol shall be intrinsically spectral and power efficient (see requirement e)) but also generating the minimum amount of overhead in the system. Requirement d) is related to the need to minimize the UT EIRP for making the terminal cost attractive. In particular, the reuse of mass-market S-band high power amplifier used for Universal Mobile Telecommunications System terminals is a key for the UT cost reduction. Requirement e) derives from the fact that the messaging system cannot rely

For S-band, the beam/terrestrial cell ratio can be assumed to be up to two orders of magnitudes.



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on closed-loop power control system, thus shall reliably operate with power unbalance due to the UTs dependent satellite antenna gain, path loss and propagation conditions. Requirement f) aims at ensuring that the signal processing required at the gateway is of affordable complexity to ensure its feasibility with today’s technology. Although the previously listed requirements are particularly relevant for the envisaged satellite mobile messaging system, requirements a)–f) are also of general applicability to narrowband satellite mobile communication systems.

3. RANDOM ACCESS SCHEMES FOR SATELLITE MESSAGING SERVICE This section deals with a summary of the trade-offs justifying the S-MIM RA scheme baseline taking into account what already discussed in Sections 1 and 2. Classical high-efficiency terrestrial RA schemes based on channel sensing [13, 14] cannot be exploited in satellite networks because of the large channel propagation delay. Open-loop RA protocols, such as Slotted Aloha (SA) for TDMA [15] and SSA for CDMA [16] in the presence of power unbalance, are characterized by the fact that low packet collision probabilities (e.g., < 103 ) are achieved at very low loads. Operation in the high collision probability region is not practical in a satellite environment due to the high number of retransmissions needed yielding very high latencies. Therefore, their use today is mainly limited to initial network log-in and the transmission of capacity requests in contention mini-slots. As discussed in [3], the SA protocol [15] or enhanced version of the scheme, such as Diversity Slotted Aloha (DSA) [17], is providing low efficiency and reliability. The enhanced version of DSA dubbed Contention Resolution DSA (CRDSA) [18] with two packet replicas within the frame and interference cancellation at the gateway provides a much better performance than the original SA scheme. As noted in [19], further CRDSA performance enhancement (dubbed CRDSA++) is possible by repeating three or four times the same packet in each frame at each transmission instance instead of just two as originally proposed in [18] and by ensuring a certain degree of received packets power unbalance conditions. The CRDSA performance boosters are the time diversity created by multiple packet replicas within the same frame and the successive interference cancellation (SIC) process exploited in the gateway demodulator. The result is a large increase of the achievable CRDSA throughput with low Packet Loss Ratio (PLR) and robustness against the received packet power unbalance occurring in practical systems. Slotted RA systems will however require UTs to keep accurate time-slot synchronization. The resulting synchronization overhead reduces the network efficiency, in particular for networks characterized by a large number of terminals with (very) low transmission duty cycle like it is the case in the envisaged application. Finally, for TDMA-based RA, the terminal EIRP requirement is related to the aggregated data rate of the TDMA multiple access scheme instead of the single terminal bit rate. Thus TDMA-based slotted RA is penalizing low-cost terminal solutions. An alternative family of RA schemes is based on SSA protocol proposed in [16]. SSA has potentially attractive features as it provides a higher throughput capability than SA for the same PLR target under equal power multiple access conditions when adopting powerful physical layer Forward Error Correction (FEC) codes. As shown in [20], SSA, similarly to CRDSA, can be operated with low PLR close to the peak of the MAC throughput, but with a better absolute performance. Additionally, SSA operates in a truly asynchronous mode, with no overhead for terminal burst synchronization. A further advantage of SSA RA is that the terminal EIRP is largely driven by the single-user data rate‘ and not to the multiplex aggregate data rate as for SA exploiting TDMA. However, the SSA Achilles’ heel resides in its high sensitivity to multiple access carrier power unbalance. This phenomenon is disrupting the SSA scheme throughput. As it is shown in [21], SSA throughput is diminished by several orders of magnitude when the received packets power fluctuates by several decibels around its mean value. Thus to achieve its full potential, SSA requires tight power control. Unfortunately return link closed-loop power control over LMS channels shows important performance limitations due to the

‘

Disregarding the extra UT power required to combat the multiple access interference self-noise.




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propagation delay and requires an unwanted large signaling overhead in the forward link [22]. A very large number of techniques aiming at CDMA interference mitigation have been published in the last decades. The reader can refer to [23–25] for a comprehensive review of multiuser detection techniques. To be remarked that there is much less literature related to interference mitigation for RA SS systems. When looking at techniques that can be implemented with affordable complexity and able to cope with power unbalance, SIC is certainly representing an attractive technique for CDMA interference mitigation. According to [26], CDMA ultimate capacity can be approached by using low-order modulation combined with low-rate powerful FEC, optimal power distribution and minimum mean square error linear filter in front of a SIC detector. Along this information theory guidelines an important, yet pragmatic, SSA enhancement dubbed E-SSA has been investigated in [21, 27]. The E-SSA concept combines SSA with Recursive Successive Interference Cancellation (R-SIC). The key idea of E-SSA is related to the exploitation of a sliding window-based R-SIC algorithm optimized for packet mode operation. R-SIC in combination with SSA greatly enhances the SSA performance in particular in the presence of received packet power unbalance. To keep complexity bounded, differently from what suggested in [26], the minimum mean square error filter has not been included in the E-SSA detector still obtaining very good performance through the R-SIC processing. E-SSA exploits spreading sequence time offset as well as difference in power level and carrier frequency to discriminate the incoming packets. This approach largely simplifies the detector implementation at the gateway side compared with a conventional CDMA scheme whereby multiple spreading sequences are used. In an RA application scenario, E-SSA is also considered easier to be implemented compared with other schemes such as interleave-division multiple access [28], which exploits different interleavers to separate users among them. The E-SSA RA scheme has been retained for the S-MIM messaging service because it best satisfies the requirements set forth in Section 2. In particular, among the schemes considered, the E-SSA provides the highest MAC throughput and robustness to power unbalance while allowing totally time asynchronous packet transmission and the lowest EIRP requirements. Furthermore, the proposed RA scheme is largely based on the Third Generation Partnership (3GPP) Wideband CDMA (W-CDMA) Release 99 [29–31] technology for which Intellectual Property cores and key building blocks including radio frequency components are readily available at affordable cost. Compared with the W-CDMA, the main innovative and distinctive features of E-SSA are in the signal processing at the gateway side with no impact on the user terminal. The packet-oriented R-SIC proposed scheme involves a gateway demodulator complexity fully compatible with current real-time signal processing capabilities. 4. RANDOM ACCESS PHYSICAL LAYER KEY DESIGN ASPECTS 4.1. General In this section, we focus on the key E-SSA design principles and performance results. As previously mentioned, the starting point of the E-SSA physical layer is the 3GPP W-CDMA Release 99 Physical Radio Access CHannel [29]. The physical channel structure, coding, interleaving, and spreading and modulation closely follows the terrestrial standard [29, 30]. The main differences compared with the W-CDMA are the following: (1) (2) (3) (4)

the extension of the preamble length up to 24,576 chips; the type of preamble sequence design; the extension of the possible payload sizes from 300 to 600 and 1200 bits; the support of two extra chip rates (1.92 Mcps and 240 kcps) in addition to the standard 3.84 Mcps one; and (5) the modified transmit procedures. The 3GPP adaptations listed previously are justified by the following considerations: (1) The extension of the preamble length up to 24,576 chips is required to allow reliable packet acquisition at the lowest Ec =N0 operational ratio of about 28 dB (see below and also Section IV.E in [21] for further details). Copyright © 2014 John Wiley & Sons, Ltd


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(2) The extension of the payload size from 300 to 600 and 1200 bits to better match the traffic packet size and to compensate for the extra overhead due to the longer preamble length. (3) The support of two extra chip rates (1.92 Mcps and 240 kcps) in addition to the standard 3.84 Mcps is introduced to allow for smaller frequency granularity taking into account the limited S-band satellite bandwidth available and the possible need to share the spectrum with other services. (4) The transmit procedures and transmit/receive states have been designed to best cope with the LMS channel characteristic and the type of service to be provided.

4.2. Preamble design The burst preamble design was driven by the following considerations: (1) Allows a reasonably good detection probability at Signal to Noise and Interference Ratios (SNIRs) as low as 4 C 10 log10 .Nc / dB where Nc is 256 or 128 or 16, respectively for chip rate of 3840, 1920 and 240 kchip/s. Such performance shall be guaranteed for frequency errors as large as 3 kHz (< 1 kHz typical). (2) Minimize the preamble length in order to reduce the impact on system efficiency. (3) Minimize the burst detection hardware complexity. The SNIR threshold for packet detection is justified by the fact that as shown in Figure 2, the 3GPP rate 1/3 turbo code allows successful decoding, in Additive White Gaussian Noise, with probability higher than 90% at Eb =N0 approximately equal to 0.7 dB (exact value depending on codeword size). Given the code rate (1/3), this correspond to a SNIR after despreading of approximately 4 dB. The corresponding target Ec =.N0 C I0 / is equal to 28, 25 and 16 dB for the spreading factor Nc , respectively equal to 256, 128 and 16. The maximum frequency error for which the system is designed is compatible with the Doppler frequency error due to high speed mobile in S-band. To cope with very cheap local clock reference in the terminal, the mobile terminal clock reference is assumed locked to the network clock distributed on the forward link either through the Digital Video Broadcasting - Return Channel via Satellite (DVBRCS) like Network Clock Reference packets or equivalent mechanism [32]. Following this assumption, the Doppler can be considered as the main contributor to the frequency error seen at the Gateway side. To ease the packet detector implementation (Section 4.5.1.), a composite preamble has been designed. The composite preamble sequence can be mathematically described as the following: p.k/ D s1 .bkcNc / s2 .jkjNc / ;

(1)

Figure 2. Performance in Additive White Gaussian Noise (AWGN) of the S-band Mobile Interactive Multimedia Forward Error Correction (FEC) for different info block lengths (1200, 600 and 300 bits). Ideal demodulation. No control channel overhead. FEC rate 1/3, eight decoder iterations. Copyright © 2014 John Wiley & Sons, Ltd



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where s1 .i/ is a complex outer sequence, s2 .i/ is the inner sequence, bkcN D int¹k=N º and jkjM D k modulus M . The length Nc of the inner sequence s2 is such that a single coherent correlation over Nc chips is possible even in the presence of the maximum frequency error (< 3 kHz). The length of s1 .i/ is L1 D 96. The overall preamble length is thus L1 Nc or 24576 for Nc D 256, 12288 for Nc D 128 and 1536 for Nc D 16. The packet preamble detector performance analysis has been performed in [21] where it is shown that probability of missed detection Pmd lower than 103 can be achieved also in the presence of realistic values for the carrier frequency offset. This result is possible with a parallel frequency search engine at the demodulator as the one described in Section 4.5.1., allowing a false alarm probability Pfa in the order of 102 . False alarm will be quickly dismissed by the demodulator by checking the Cyclic Redundancy Check quality after packet decoding. Quaternary sequences were considered for both inner and outer sequences. As multiple preambles may be useful in some application context, multiple preambles and data part spreading codes are allowed by the S-MIM system. However, in order to minimize packet detection complexity in the presence of multiple preamble codes, the same inner sequence s2 is typically used in a given satellite beam. In that case, only the outer sequence correlations have to be replicated for the different preamble codes. Given that a quite significant part of the preamble detector complexity is related to the inner sequence detector, an efficient implementation of such detector is important. Complementary Golay sequences [33] were chosen for this reason, as they lead to an efficient matched filter implementation [34], while providing good aperiodic correlation properties. In fact, the matched filter can be implemented with a number of arithmetic operation, which only grows logarithmically instead than linearly with the length of the filter. Also correlation with both complementary sequences in the pair can be obtained at the same time (although this is not a requirement here). The outer sequence s1 .i/ is instead chosen in a family of complex sequences obtained from a Gold sequence ń .i/ (of length 511 chips) through the following equation: s1 .i/ D ń .i/ C jń .i C 256/ i D 0; 1; 2; : : : ; 95

(2)

4.3. Burst design The transmitted burst is composed by the preamble described in previous section and two multiplex channels, which follows the preamble: the Physical Data CHannel (PDCH) and the Physical Control CHannel (PCCH). The two channels are I-Q multiplexed using two Orthogonal Variable Spreading Factor Walsh sequences in line with the 3GPP W-CDMA approach [31]. The S-MIM PDCH and the associated PCCH have burst lengths, which are multiple of the frame period of the 10 ms 3GPP WCDMA frame. In particular, the following burst lengths are possible according to the current S-MIM specifications: 30, 60, 120 or 240 ms (Figure 3). The purpose of the PCCH is similar to that in 3GPP. In particular, it allows to perform robust channel estimation even in difficult mobile channels including terrestrial one (in case of usage of the technique in terrestrial gap-fillers and provide the possibility of transmitting some physical layer signaling informing the receiver on the actual format of the burst without forcing blind detection of the burst format. At this regard, the four possible lengths of the PDCH/PCCH are associated to two possible data rates for the signal after FEC coding: 15 kBaud (corresponding to 5 kbit/s info bit rate) and 30 KBaudk (10 kbit/s info bit rate) and to three possible message length (300, 600 and 1200 bits). A spreading/scrambling solution similar to that used in 3GPP W-CDMA is adopted for the PDHC/PDCC [31]. Hence, different, orthogonal, Walsh-Hadamard channelization/spreading codes are used for PDCH and PCCH. The two channels are then mapped to the real and imaginary part of a complex signal, which is finally scrambled by a complex scrambling code as shown in Figure 4. Differently from 3GPP, only a long scrambling code option can be used. Moreover, the scrambling code period is not limited to 38,400 chips corresponding to the number of chips in a 3GPP W-CDMA frame. In fact, a period equal to the entire PDCH burst length is selected to minimize code collision probability between RA users. Although in principle the standard allows to use more then one spreading

k

30 kBaud is not allowed at chip rate 240 kchip/s.



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Figure 3. Structure of the Physical Data CHannel and Physical Control CHannel.

Figure 4. Physical Data CHannel and Physical Control CHannel spreading/scrambling strategy.

sequence selected at random to keep low the probability of two users arriving at the gateway with an offset less than a chip (see Figure 6 in [21]), in practice it has been found that a single spreading sequence is sufficient. This is because even if there is a non-negligible probability (e.g., 103 ) of two distinct packets arriving chip aligned at the demodulator input, the randomness of the carrier frequency offset and of the received packet power makes the packets detectable through the iterative E-SSA signal processing.

4.4. Transmission control It is important to elaborate on the transmit procedures, which are very specific to the S-MIM. The procedure described in [35] represents an evolution of initial concepts, which were described in [21]. As explained in the latter reference, Packet Transmission Control (PTC) is important in a satellite mobile RA system whereby the limited UT to gateway link margins do not allow to counteract severe shadowing conditions occurring in certain LMS channel environments. Three PTC options (in addition Copyright © 2014 John Wiley & Sons, Ltd



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to the do not transmit one) are available in the S-MIM standard. PTC avoids the UT (options A and B) to transmit packets when the LMS channel shadowing level exceeds the link margin available. Option A also features a feed-forward power control to counteract the LMS uplink channel shadowing by observing the downlink signal strength under the fairly accurate assumption of full correlation between the forward and return link shadowing. Instead option B implements a simpler on–off transmit control algorithm. In option C, the transmission of packets is delayed in case of no acknowledge (ACK) reception. More specifically, the PTC algorithms are described as follows: Option A. The initial setting of the UT transmit power exploits on an open-loop approach based on a reverse link budget calculation exploiting the UT Signal to Noise Ratio (SNR) local estimation, the satellite EIRP which is broadcasted in the forward link in the Service Description Table (SDT). More UT specifically the UT transmit power PTx is regulated by the following equation: SAT C UT SAT ŒdBm D ŒdB GEOC ŒdBi C N0SAT ŒdBm C b LUP ŒdB Rrand ŒdB; (3) PTx N0 req

where ŒC =N0 SAT req represents the target value used for the desired C =N0 at the satellite transponder SAT input, GEOC is the satellite receive antenna gain at edge of coverage (EOC), N0SAT is the noise power level at the satellite receiver input, b LUP is the path loss for the h current i UT location estimated from the SNR measured at the UT on the forward link (FL) carrier SNR using the value of the FL EIRP

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UT

EIRP PSAT , which is also broadcasted through the SDT, and Rrand is a random component in the UT transmit power, which is a uniform random variable distributed in the range Œ0; Rmax ŒdB, being Rmax another SDT broadcasted system parameter. The synthetic random power generation at the UT side allows to further enhance the E-SSA throughput performance as it has been analyzed in [21] and shown later in Section 5. LUP and LDWN are the uplink and downlink attenuation, respectively which can be estimated as the following: fUP b b LUP ŒdB ' LDWN ŒdB C 20 log10 (4) fDWN h i G EIRP b ŒdBW C ŒdB/K SNR ŒdB C 228:6; (5) LDWN ŒdB ' PSAT UT T UT

b

where fUP and fDW N are respectively the uplink and downlink carrier frequencies known to the UT. Another important element in the packet transmit control mechanism resides in the maximization of the probability that the transmitted packet is successfully received. If the value of the UT transmitted UT power PTx computed through (3) exceeds the terminal capability, no transmission shall take place as this is an indication of excessive channel attenuation. In this case if no CGC is available, packet UT transmission shall be postponed until the estimated PTx comes back to an affordable value. Option h i B. The terminal transmits with maximum EIRP if the measured SNR from the FL carrier SNR is greater than the value SNRLOS Rmax , being Rmax a parameter broadcasted through the

1

UT

1

b

SDT and SNRLOS the estimated LOS SNR for the current UT location. To adaptively estimate at the UT, the LOS SNR level for each mobile terminal, one can simply store as LOS SNR estimate the average of the best Nb UT SNR estimates SNR.tk / at time tk over a sufficiently large observation time window Tobs . In analytical form, this corresponds to the following: 8 9 Nb < ° ±= X N 1 b ref SNRLOS .t/ D max SNRmin ; max SNR.tk / ; (6) : ; Nb tk 2Œt;t Tobs

1

1

1

lD1

ref where SNRmin is a preconfigured minimum system guaranteed LOS received signal power value, and Nb D 10 is recommended value. The use of this minimum system value avoids SNRLOS estimation errors when the mobile terminal remains for a too long time under satellite link obstruction conditions (e.g., car parked in a garage or passing through a long tunnel). Compared with option A, this approach

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closely derived from [21], has the advantage of not requiring transmit power control. This makes the UT terminal potentially cheaper to produce. Option C. In this case, the terminal always transmit with maximum EIRP. Once a packet has been transmitted, the transmission time of next packet depends whether an acknowledgment is required or not, and, if required, if cumulative acknowledgments are allowed. If no acknowledgment is required, the next packet transmission may not be carried out before a time interval computed by using the given persistence value and back-off time, which are signaled for the terminal user group and for the given congestion status on the satellite table. If an ACK per packet is required, no further packet transmission may happen before the reception of the ACK. If no ACK is received before the associated message time-out, a random draw using the current dynamic persistence value and the actual backoff time is performed for computing the packet retransmission time. If the retransmitted packet is not acknowledged and the number of allowed retransmission is less than the allowed retransmission number, the dynamic persistence value is halved. This approach is the simplest but is less robust to possible link shadowing or interruptions.

4.5. Demodulator design The core of the S-MIM messaging system is represented by the E-SSA packet demodulator. The very high MAC throughput with low packet loss ratio is obtained by means of a careful design of the spreadspectrum demodulator with R-SIC. A high-level demodulator functional block diagram is provided in Figure 5. After baseband down-conversion and digital conversion, the signal is going through a SquareRoot Raised-Cosine Chip Matched Filter. The Square-Root Raised-Cosine Chip Matched Filter output is typically down-sampled at 2 or 4 samples/chip, and then samples are stored on memory buffer having the sliding window size of W S-MIM frames (see Section IV-A of [21]). Typically, W should be three times the physical layer packet length in symbols. The sliding window is shifted in time in discrete steps allowing some overlap of packets on each window step. Normally, the window step W shall be between 1=3 and 1=2 of the window length W depending on its size. At each window step, the following iterative detection process takes place:

Figure 5. High-level Enhanced Spread Spectrum Aloha demodulator block diagram. Copyright © 2014 John Wiley & Sons, Ltd



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(1) store in the detector memory the new baseband signal samples corresponding to the current window step; (2) perform packets preamble detection and select the packet with highest SNIR value; (3) perform channel estimation for the selected packet; (4) perform FEC decoding of the selected packet; (5) if the decoded FEC frame is considered correct after Cyclic Redundancy Check then: a. perform enhanced data-aided channel estimation over the whole recovered packet (carrier frequency, phase, amplitude and timing); b. reconstruct at baseband the detected packet; and c. cancellation from the E-SSA demodulator sliding window memory of the decoded packet. (6) repeat from step 2 until either no new packet can be detected or NiW t max iterations are performed. When one of the previous iteration’s stop condition is reached, advance the observation window by the step W frames. 4.5.1. Preamble detection. The first critical aspect in the demodulator design is the packet preamble detection unit. Looking at the possible burst detection strategies for the reverse link, it shall be observed that only a threshold crossing strategy is applicable [36]. A constant false alarm probability criterion can be used for setting the threshold, which has to be chosen as a compromise between the miss-detection probability and the false alarm probability [37]. A too high false alarm probability may have an impact on the receiver complexity as it implies the need of processing fake packets. Several variants of threshold crossing detection strategies are possible for spread-spectrum signals. The preamble length has been sized assuming a full coherent integration strategy. Although this approach may lead to higher complexity for the detection with respect to alternative strategies, it was chosen as it minimizes the preamble length. However, to cope with the resulting detector complexity, a hierarchical preamble construction was adopted. At this regard, in order to cope with frequency error, the integration period may be divided into subintervals with subcorrelations combining through swivelers [38]. In particular, dividing the preamble sequence of length L into Np subsequences, the results of the Np subcorrelations can be coherently combined for different frequency error hypotheses through an Fast Fourier Transform as depicted in Figure 6. In Figure 6, the sequences s1 .i/ and s2 .i/ are respectively the outer and inner sequences composing the hierarchical preamble p.k/ described by (1).

Figure 6. Coherent integration with parallel frequency search through swivelers. The block labeled CMF is the chip matched filter. Copyright © 2014 John Wiley & Sons, Ltd


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4.5.2. Recursive packet detection process. Once the packets in the current window have been detected and the strongest one identified, then this packet goes through a standard spread Aloha packet detection processing with preamble data-aided channel estimation. The relative power of the PCCH with respect the PDCH can be set by the network and is signaled to the terminals on the FL. At this regard, the optimal PCCH relative power level, in most environments, was optimized through extensive simulations and found closed to 8 dB. At this regard, Figures 7 and 8 show the performances in two different Ricean fading environment (C =M D 10 dB with fading bandwidth 75 Hz in Figure 7 and C =M D 15 dB with fading bandwidth 5 Hz in Figure 8) in the presence of channel estimation through the PCCH. Results with ideal channel estimation are also reported (blue curves). The approximate loss with respect to ideal channel estimation is about 1 dB of which approximately 0.64 dB is due to the power overhead represented by the PCCH. The critical step is to enhance the channel estimation prior cancellation exploiting the detected payload bits in a decision directed mode. This symbol reconstruction process is anyway required to cancel the detected packet from the window memory. The data modulation affecting the payload ontime samples can be wiped-out using the reconstructed baseband symbols. In this way, a more accurate packet amplitude, carrier frequency and phase can be reconstructed for canceling the packet from memory. The algorithmic details about the decision directed channel estimation algorithm derived for CRDSA in [39, 40] are also largely applicable to our case. The number of SIC iterations required depends on the MAC load. For very high load close to 2 b/s/Hz, five iterations are recommended; whereas for medium/high loads, for example, 1.5 b/s/Hz, three iterations may be sufficient.

Figure 7. Performance of Enhanced Spread Spectrum Aloha packet transmission (info packet length 1200 bits) in Ricean fading with C =M D 10 dB and Doppler spread D 75 Hz. Information bit rate equal to 5 kbit/s. The Eb =N0 values take into account the power required for the control channel.

Figure 8. Performance of Enhanced Spread Spectrum Aloha packet transmission (info packet length 600 bits) in slow Ricean fading with C =M D 15 dB and Doppler spread D 5 Hz. Information bit rate equal to 5 kbit/s. The Eb =N0 values take into account the power required for the control channel. Copyright © 2014 John Wiley & Sons, Ltd



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5. RANDOM ACCESS PHYSICAL LAYER KEY PERFORMANCE ASPECTS Only a few physical layer performance results are reported in the following section. The reader can refer to [21] for a complete analytical E-SSA performance model as well more extensive simulation results. System level S-MIM multiple access capacity results are reported in a companion paper in this same special issue [41]. The following results are assuming that the power unbalance of the packets received at the gateway is following a lognormal distribution. This approximation has been derived for an LMS channel through detailed simulations described in [21] whereby the terminal exploits a packet transmission control algorithm similar to the one described in Option B of Section 4.4. As explained in Section 4.4, the packet transmission control procedure avoids transmitting packets when the LMS channel attenuation (shadowing) is too large for allowing reliable reception at the gateway. For this reason, as shown in [21] (Table IV), even in harsh Intermediate Tree Shadowed LMS channel environment characterized by large shadowing fluctuations [11], the received packet power standard deviation is reduced to D 2 dB. In a more benign LMS channels such as the open environment [11], simulations results showed a standard deviation further reduced to D 1 dB. This is possible because the packet transmission control algorithm typically avoids transmitting in the more shadowed states LGN power unbalance with std. dev σ=1 dB

1.4

E−SSA Ana E−SSA Sim SSA Ana SSA Sim SA Ana SA Sim

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Figure 9. Performance of Slotted Aloha, Spread Spectrum Aloha (SSA) and Enhanced-SSA packet transmission (info packet length 100 bits, Third Generation Partnership rate 1/3 Forward Error Correction) in Additive White Gaussian Noise channel when the packet power is lognormally distributed with standard deviation D 1 dB. The Eb =N0 D 13:7 dB in absence of lognormal power fluctuations. Copyright © 2014 John Wiley & Sons, Ltd


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R. DE GAUDENZI, O. DEL RIO HERRERO AND G. GALLINARO LGN power unbalance with std. dev σ=2 dB

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Figure 10. Performance of Slotted Aloha, Spread Spectrum Aloha (SSA) and Enhanced-SSA packet transmission (info packet length 100 bits, Third Generation Partnership rate 1/3 Forward Error Correction) in Additive White Gaussian Noise channel when the packet power is lognormally distributed with standard deviation D 2 dB. The Eb =N0 D 13:7 dB in absence of lognormal power fluctuations.

2 and 3 of the LMS channel. Results reported in [21] indicate that E-SSA by far outperforms SSA being able to achieve throughputs in the order of 1.2 bps/Hz with a PLRD 105 , which represents a fourfold enhancement compared with standard SSA when incoming packets power is the same. But the E-SSA SIC strength is magnified in the presence of received packets power unbalance. In Figures 9 and 10, it is shown that with lognormally distributed packets power and Eb =N0 D 13:7 dB in absence of lognormal power fluctuations, the throughput is about 1.4–1.6 bps/Hz with PLRD 104 for D 1 and 2 dB, respectively. Even higher throughput (about 2 bps/Hz) can be achieved by E-SSA with D 3 dB. In this case, conventional SSA throughput for the same PLRD 104 is limited to less than 0.05 bps/Hz, so E-SSA performance advantage versus SSA is truly remarkable. To be remarked that E-SSA when operating with lognormally distributed received packet power is also affected by the same PLR “floor” effect observed for CRDSA(++). Also in this case, the floor level can be easily analytically predicted [21] and mitigated increasing the average SNR at the gateway or by truncating the lognormal distribution tails occurrence. Figure 11 provides some performance results in terms of

Not including the Square-Root Raised-Cosine filter roll-off factor and assuming a packet size of about 100 information bits.




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Figure 11. Performance of Enhanced Spread Spectrum Aloha packet transmission (info packet length 100 bits, bit rate 5 kbps, Third Generation Partnership rate 1/3 Forward Error Correction, spreading factor 256) in Ricean fading K D 10 dB with Doppler spread of 75 Hz, chip rate 3.84 Mchip/s, C =N0 DEIRP(dBW)+45.8 dBHz for different terminal Effective Isotropic Radiated Power and lognormal randomization of the packet power (open-loop power control) with standard deviation D 2 dB.

throughput and packet loss rate for E-SSA in the case of correlated Ricean fading at 3.84 Mchip/s with different terminal EIRP and lognormal randomization of the transmitted packet power. 6. CONCLUSIONS This paper is summarizing the design drivers and the key performance of the Enhanced-SSA physical layer adopted by the European Telecommunication Standards Institute S-MIM standard. Stemming from the S-MIM mobile messaging service requirements, the RA the trade-offs leading to the ESSA design have been discussed. Commonalities and peculiarities of E-SSA compared with the 3GPP W-CDMA RA channel physical layer have been described with particular emphasis on the burst preamble and the satellite mobile specific packet transmission control algorithms. The advanced signal processing required at the demodulator side to exploit the full E-SSA potential has been described and key demodulator performance results reported. Finally the simulated E-SSA RA performance at MAC level under Additive White Gaussian Noise and also under correlated Ricean fading type of channel conditions has been illustrated. The E-SSA demonstrates to be a mature high-performance physical layer very suitable to satellite mobile messaging and telematic applications. REFERENCES 1. ETSI EN 301 790 v1.2.2. Digital Video Broadcasting (DVB); Interaction Channel for Satellite Distribution Systems. 2. Le-Ngoc T, Jahangir IM. Performance analysis of CFDAMA-PB protocol for packet satellite communications, IEEE Trans. Commun. September 1998; 46(9):1206–1214. 3. De Gaudenzi R, Del Rio Herrero O. Advances in Random Access protocols for satellite networks, International Workshop on Satellite and Space Communications, 2009, IWSSC: Siena, Italy, 9–11 September 2009; pp. 331–336.



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AUTHORS’ BIOGRAPHIES

Riccardo De Gaudenzi was born in Italy in 1960. He received his doctoral degree (cum laude) in Electronic Engineering from the University of Pisa, Italy, in 1985 and his PhD degree from the Technical University of Delft, the Netherlands, in 1999. From 1986 to 1988, he was with the European Space Agency (ESA), Stations and Communications Engineering Department, Darmstadt (Germany), where he was involved in satellite Telemetry, Tracking and Control (TT&C) ground systems design and testing. In 1988, he joined ESA’s Research and Technology Centre (ESTEC), Noordwijk, The Netherlands, where since 2005, he has been the Head of the Radio Frequency Systems, Payload and Technology Division. The division is responsible for supporting the definition and development of advanced satellite system, subsystems and related technologies for telecommunications, navigation and earth observation applications. He has been responsible for a large number of R&D activities for TT&C, telecom and navigation applications. In 1996, he spent one year with Qualcomm Inc., San Diego, USA, in the Globalstar LEO project system group under an ESA fellowship. His current interest is mainly related with efficient digital modulation and multiple access techniques for fixed and mobile satellite services, synchronization topics, adaptive interference mitigation techniques and communication systems simulation techniques. He actively contributed to the development and the demonstration of the ETSI S-UMTS Family A, S-MIM, DVB-S2, DVBS2X, DVB-RCS2 and DVB-SH standards. From 2001 to 2005, he has been serving as an Associate Editor for CDMA and Synchronization for IEEE Transactions on Communications. He is currently an Associate Editor for the Journal of Communications and Networks. He is a co-recipient of the 2003 and 2008 Jack Neubauer Memorial Award Best Paper from the IEEE Vehicular Technology Society.

Oscar del Rio Herrero was born in Barcelona, Spain, in 1971. He received a BE degree in Telecommunications and an ME degree in Electronics from the University Ramon Llull, Barcelona, Spain, in 1992 and 1994, respectively. He received a post-graduate degree in Space Science and Technology with emphasis on Satellite Communications from the Space Studies Institute of Catalonia, Barcelona, Spain, in 1995. He joined ESA’s Research and Technology Center, Noordwijk, The Netherlands, in 1996. In 1996 and 1997, he worked as a Radio-navigation System Engineer in the preparation of the Galileo programme. From 1998 to 2009, he has worked as a Communications Systems Engineer in the Electrical Systems Department. His research interests include packet access, resource management schemes and IP inter-working for future broadband satellite systems. Since 2010, he has been working for the Iris project in the ESA’s Telecommunication Directorate, aiming at the development of a new satellite-based Air-Ground Communication system for Air Traffic Management.

Gennaro Gallinaro received his doctoral degree in Electronic Engineering (magna cum laude) from the University of Rome in 1979. He has worked in Fondazione Bordoni and Telespazio before joining Space Engineering. He has in-depth experience in the analysis, computer-aided design and simulation of transmission systems (modulation, coding, etc.) and digital signal processing hardware (on-board multi-carrier demodulators, digital beam forming, etc.). He is a co-author of several papers on signal processing and satellite communication techniques and was a co-recipient of the 2003 and 2009 IEEE Vehicular Technology Society Jack Neubauer Memorial Awards and of the Best Paper Award at the IEEE ASMS/SPSC 2012 conference.

