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Advances in Random Access Protocols for Satellite Networks (Invited Paper) Riccardo De Gaudenzi and Oscar del R´ıo Herrero European Space Agency, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands e-mail: {Riccardo.De.Gaudenzi}{Oscar.del.Rio.Herrero}@esa.int Abstract—In this paper we review key properties of recently proposed high performance protocols for Random Access (RA) satellite channels for both Time Division and Code Division Multiple Access (TDMA/CDMA) techniques. The proposed protocols by far outperform traditional satellite random access techniques without the need for quick feedback from the gateway. This makes possible to avoid the utilization of demand assigned capacity for the transmission of small/medium size bursts of packets. A fair comparative performance of state-of-the-art TDMA and CDMA RA schemes is provided together with a summary of their key performance results. It is shown that the proposed enhanced RA protocols, although different between TDMA and CDMA, share commonalities as they exploit iterative interference cancellation at the demodulator side and demonstrate to provide even better performance in the presence of received carrier power unbalance. Typical application scenarios of these enhanced random access protocols are then illustrated, as well as possible ways to combine random access and demand assigned protocols.

I. I NTRODUCTION AND P ROBLEM S TATEMENT The maturing of low-cost interactive satellite terminals for the fixed broadband consumer market and mobile applications calls for the development of efficient multiple access protocols able to cope with large network sizes and very dynamic traffic conditions. In particular, in case of satellite broadband access networks, residential users are likely to generate a large amount of low duty cycle bursty traffic with frequent inactivity periods in the return link. A similar situation may occur in satellite mobile networks whereby a large number of user terminals may generate infrequent packets for signalling transmission as well for position reporting or other messaging applications. Under these operating conditions, the traditionally used Combined Free and Demand Assignment Multiple Access (CF-DAMA) satellite protocol [1] will not perform optimally as it is shown in Fig. 1. For the generation of the bursty traffic source, a modified version of the Web traffic model described in [2] has been used, with a packet size cut-off of 100 bytes, which is more representative of a Web client type of traffic source. We can see that the CF-DAMA protocol behaves as a pure DAMA protocol in front of bursty traffic sources with the consequent increase in the end-to-end delay (typically > 750 ms) which corresponds to a three-hop delay. This CF-DAMA weakness for bursty type of packet traffic calls for the search of new more efficient multiple access techniques able to provide high MAC throughput with low transmission delays and limited complexity on the terminal side. Random Access (RA) techniques are by nature, very robust to this type of traffic and to large population of terminals sharing

the same capacity. 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 very well in the satellite environment. RA techniques based on channel sensing [3], [4], commonly used in terrestrial networks, cannot be exploited in satellite networks because of the large channel propagation delay. Open loop RA protocols such as Slotted Aloha (SA) for TDMA [5] and Spread Spectrum Aloha (SSA) for CDMA [6], are characterized by the fact that low packet collision probabilities (e.g. < 10−3 ) 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 login and the transmission of capacity requests in contention mini-slots. In some cases, they are also used for the transmission of very small volumes of data, and CF-DAMA is usually the adopted multiple access scheme for larger volumes or periodic transmissions of data. Example systems where RA is used for terminals login and short data transmissions are the DVB-RCS standard [7], the IPoS standard [8] and CDMA type of VSAT systems [9], [10]. Therefore, there is a need to develop new more efficient, highly reliable and low-complexity RA protocols to be integrated with DAMA for the new type of scenarios described above. The availability of the enhanced RA protocols will make possible to reduce the exploitation of DAMA techniques to cases where the size of the information packet is quite large and/or quite regular. In the following, it is shown that new RA protocols recently published by the authors dubbed CRDSA++ and E-SSA will make satellite networks able to very efficiently cope with bursty traffic generated by a very large population of terminals. Typical applications scenarios for these new RA protocols will be illustrated together with MAC protocol performance. II. S ATELLITE RA FOR TDMA SA protocol [5] or enhanced version of the scheme, such as Diversity Slotted Aloha (DSA) [11], are used in TDMA systems with low efficiency and reliability typically for logging into the network. As shown in Fig. 2, the MAC throughput is pretty poor for SA (normalized throughput T=10−3 packets/slot for a packet loss ratio (PLR) of 10−3 ). Recently an

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enhanced version of DSA dubbed Contention Resolution Diversity Slotted Aloha (CRDSA) has been introduced [12]. The CRDSA key idea is to transmit two replicas of the same packet at random locations within the same frame as in DSA but with a little extra signalling to point to the ”twin” packet location. In case of successful packet reception at the gateway this extra signalling information allows to locate the twin packet within the frame and to accurately cancel it in addition to the one successfully decoded. To be remarked that using a powerful Forward Error Correcting (FEC) code and an adequate signal to noise ratio, there is non zero probability of correctly detecting the packet even in presence of a collision. By iterating the process a number of times some of the initially lost packets can be recovered and the DSA performance enhanced. For the same PLR target DSA gets a higher yet modest throughput of T = 1.7 · 10−2 packets/slot while CRDSA throughput is T=5.5 · 10−2 packets/slot for a packet loss ratio of 10−3 (see Fig. 2). Although CRDSA represents a 50 and three fold improvement compared to SA and DSA respectively at PLR=10−3 , the performance is still not very attractive for

applications requiring higher RA throughput with low PLR to avoid packet retransmission. Very recently an enhanced version of the CRDSA protocol dubbed CRDSA++ has been presented in [13]. CRDSA++ provides two main enhancements compared to the original version of the protocol: a) increased (optimized) number of packet repetitions (2 in CRDSA 3-5 in CRDSA++); b) exploitation of the received packets power unbalance to further boost the RA performance1 . As shown in Fig. 3 the CRDSA++ throughput can be as high as T=5 · 10−1 packets/slot for a packet loss ratio of 10−5 . This much better performance than the original CRDSA scheme is obtained by repeating four times the same packet in each frame at each transmission instance instead of just 2 as presented in [12] under equal power conditions. This quite remarkable and unexpected result represents 500 (!!!) and 30-fold throughput improvement compared to SA and DSA for a two order of magnitude lower PLR. This counterintuitive result can be explained by the fact that although more packet 1 This was not the case in [12] whereby in case of packet collision no decoding attempt was performed.


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repetitions within the same frame increase the probability of collision, the increased diversity allow to better ”cleanup” the collisions through iterative processing. So overall the RA performance protocol is improved until a MAC load breakdown point is reached for which the RA performance is rapidly decaying. In a practical RA satellite network the packets power received at the gateway will fluctuate because of differences in the terminal EIRP (due to antenna mis-pointing or transmit RF power fluctuations) and satellite antenna gain variation within the coverage region. For this reason it is important to assess CRDSA++ performance when received packets are affected by power unbalance. It is observed that CRDSA++ performance are further enhanced in the presence of power unbalance. This is because the successive interference cancellation process embedded in CRDSA++ is inherently enhanced by the received packet power unbalance allowing to resolve collisions that were destructive otherwise. In Fig. 4 (from [13]) it is shown that with lognormally distributed

(σ = 2 dB) packets power the throughput can be higher than 1 packet/slot at PLR=10−5 . Clearly allowing lognormal received packet power fluctuations increase the probability of erroneous packet detection just because of the insufficient signal-to-noise ratio of the received packet even in absence of collisions. This cause a PLR floor which has been reported in [13] and which can be easily analytically predicted as reported in the same publication. The PLR floor can be kept low either by increasing the average SNR received at the gateway or by limiting the lognormal power standard deviation2 . It should be remarked that recent unpublished results by the authors indicate that the PLR floor can be further mitigated by using a lower FEC coding rate (e.g. 1/3 instead of 1/2) providing the same or even better MAC throughput and PLR but with lower PLR floor in case of packet power fluctuation. This new set of results 2 To be remarked that in practice the assumption of lognormal received packet power distribution is pessimistic and shall be better modelled by a truncated tail distribution for which the PLR floor effect is mitigated.


is opening up new perspectives in the exploitation of RA techniques in satellite networks. The price to pay is a gateway baseband burst demodulator processing increase which can be assumed to linearly increase with the number of CRDSA replicas. According to Fig. 3 the optimum number of replicas appears to be between 3 and 4, approximatively involving a corresponding gateway baseband burst demodulator processing complexity increase. Being RA by nature distributed, it is also important that a congestion control policy is put in place. This can be easily achieved by means of a gateway-based MAC channel load measurement. When the gateway detects that the MAC load is approaching the maximum operating CRDSA++ throughput it starts to offload the RA channel. This can be simply achieved by indicating trough a busy tone on the forward link [14] that the system is approaching an overload situation. In this case terminals will start to divert time critical traffic towards DAMA or will start to back-off packet transmission until the busy tone indication disappears. The detailed way to best combine CRDSA++ with DAMA is still an open area of research. The concept is that once the user terminal buffer indicates that there is sustained traffic the packet transmission will be diverted to DAMA. Instead the bursty traffic can be handled by non demand assigned capacity with high efficiency and unprecedented low latency. Slotted RA systems will however require terminals to keep the time slot synchronization. The resulting synchronization overhead (slightly) 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. III. S ATELLITE RA FOR CDMA SSA protocol proposed in [6] 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 FEC. In [15] through simplified analysis it is shown that SSA throughput is critically dependent on the demodulator signal-to-noise plus interference (SNIR) threshold. Results reported in [15] indicate that differently from SA, SSA shows a steep PLR increase with MAC load3 . Thus SSA, similarly to CRDSA++, can be operated with low PLR close to the peak of the MAC throughput. As an example, using turbo codes and relative small packet sizes, SSA can achieve throughput in the order of T = 0.5 b/s/Hz for a packet loss ratio of 10−3 which is a much better performance than DSA and CRDSA (see Fig. 5). Additionally, SSA operates in a truly asynchronous mode, with no overhead for terminal burst synchronization. Furthermore, SSA throughput is enhanced by using low FEC coding rates [15] and low order modulations. Instead, for 3 To be remarked that these results were obtained under a very simplified e.g. on-off FEC model and are thus pessimistic.

slotted RA schemes there is an optimum coding rate which maximizes the throughput and which is also lower with most advanced schemes such as CRDSA++ compared to plain SA. 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 shown in Fig. 5, SSA throughput is diminished by several orders of magnitude when the received packets power is lognormally distributed with standard deviation of 2-3 dB. Thus to achieve its full potential, SSA requires either tight power control or interference cancellation. To be remarked that return link closed loop power control over LMS channels is having important performance limitations due to the propagation delay and requires an important signalling overhead in the forward link [16]. Finally, SSA terminal EIRP is in principle linked to the single user data rate and not to the multiplex aggregate data rate for SA/TDMA. Although in SSA some extra user terminal power is required to combat the CDMA multiple access interference, also from this point SSA is advantageous compared to TDMA SA. Interference cancellation for SSA has been proposed in the past in [6], [17], [18], [19], [20] and [21]. However, the detailed approach for implementing, analyzing and optimizing the performance of successive interference cancellation in an unslotted packet system where not clearly reported. The combination of SSA with Successive Interference Cancellation processing optimized for packet type of transmission dubbed Enhanced Spread Spectrum Aloha (ESSA) has been investigated by the Authors in [22] and [23]. The key idea of E-SSA is related to the exploitation of a recursive sliding window successive interference cancellation (SIC) algorithm . SIC in combination with SSA greatly enhances the SSA performance in particular in the presence of received packet power unbalance. In [23] it is shown that by using 3GPP-like spreading sequences and physical layer structure, 16 different spreading codes are sufficient to keep the code collision probability negligible also when the MAC throughput is in excess of 1 b/s/Hz. To allow good probability of packet detection and low false alarm probabilities in [23] it is shown that the 3GPP RA return link preamble shall be extended to 32768 chips. To lower the corresponding overhead it is recommended to extend the packet duration to about 1000 information bits. By using two-stage channel estimation (decision aided) for packet initial detection/demodulation and decision-directed for cancellation, the interference cancellation error can be kept low thus allowing remarkably good successive interference cancellation performances. Results reported in [23] and duplicated here in Fig. 6 indicate that E-SSA by far outperforms SSA being able to achieve throughputs in the order of 1.2 bps/Hz4 with a PLR=10−5 which represents a four fold enhancement compared to 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 Fig. 6 it is shown 4 Not including the Square-Root Raised-Cosine filter roll-off factor and assuming a packet size of about 1000 information bits.


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that with lognormally distributed (σ = 2 dB) packets power, the throughput can be higher than 1.6 bps/Hz with PLR=10−4 . Even higher throughput (about 2 bps/Hz) can be achieved by ESSA with σ = 3 dB. In this case conventional SSA throughput for the same PLR=10−3 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 [23] and mitigated increasing the average SNR at the gateway. IV. RA T YPICAL A PPLICATION S CENARIOS Having presented RA techniques capable to achieve RA throughput in excess of 1 b/s/Hz with PLR lower than 10−4 one can consider to extend the applicability of RA in satellite networks for: a) SCADA applications; b) Mobile messaging applications; c) Transmission of sparse traffic/signalling packets of limited size in broadband access networks; d) Support of

unpredictable real-time variable bit rate applications (e.g. VoIP, video conferencing). SCADA and mobile messaging applications multiple access can be purely based on a RA scheme. The application of the advanced RA schemes described in this paper will allow a substantial increase in the utilization of the resources (1000 times more terminals supported for the same bandwidth) with a reduced collision probability (< 10−5 ). RA can also boost the performance of DAMA protocols in the return link of a residential scenario for Internet access. DAMA has performance limitations in this scenario for the transmission of low volumes of data, that are quite frequent in the return link. In addition, DAMA cannot cope well with the transmission of Real-Time traffic, such as VoIP or video conferencing, when variable bit rate codecs are used. These flows are characterized by very tight delay and delay jitter constraints. Thus, real-time flows with variable bit rate (VBR) codecs are normally not performing well under a CFDAMA scheme due to the long capacity reservation delays,


and constant bit rate (CBR) traffic sources are preferred for real-time services with the subsequent waste of return link resources. The integration of DAMA with efficient and highly reliable RA schemes can solve all the above limitations and improve the overall system efficiency and user experience. A reliable RA scheme can replace the Free Capacity Assignment (FCA) mechanism, that is less efficient in a consumer scenario (as we have seen in Fig. 1), and be seamlessly integrated with a DAMA protocol resulting in a very efficient utilization of the resources with delay and delay jitter performances comparable to those on the forward link, despite being a multiple access channel shared by a very large number of terminals. For instance, for the transmission of low volumes of data, the terminal can make use of the random access channel, thus reducing the end-to-end delay for the delivery of a small burst of data. For the transmission of larger volumes of data, where the propagation delay represents only a fraction of the transmission delay, a traditional volume based capacity request (VBDC) under DAMA scheme can be issued. For the transmission of real-time traffic flows, RA can complement the traditional rate based capacity requests (RBDC) under DAMA. While the DAMA scheme can well estimate and serve the realtime flows average aggregate data rate, the RA scheme can well serve the flow variations, thus avoiding queueing of real-time packets on the terminal side. V. S UMMARY AND C ONCLUSIONS We have been showing that exploiting state-of-the-art RA protocols and iterative signal processing at the gateway station, it is possible to obtain unprecedented performance for what concerns the RA throughput keeping a very low PLR even in the presence of received packets power unbalance. The results presented are extremely attractive for both TDMA and CDMA type of access schemes and opens up new perspectives in the RA exploitation for satellite networks particularly when traffic is by nature bursty. DAMA protocols can still be exploited on top of RA to cope with more predictive and heavy traffic. Simple ways to integrate RA protocols with DAMA have been discussed. The enhanced RA schemes reviewed in this paper applicable to TDMA and CDMA networks can reduce the transmission time (single hop) with very low need for retransmission and low overhead compared to conventional techniques. Thanks to the adoption of state-of-the-art physical layer forward error correction and iterative signal processing at the gateway station, very large throughput can be achieved with very low packet loss ratio. The proposed RA schemes showed to provide very high robustness vis-a-vis received packet power unbalance which in fact helps to boost RA performance. In addition RA can be optimally coupled with DAMA protocols to serve with high efficiency a low delay a very wide class of applications. R EFERENCES [1] T. Le-Ngoc, I.M. Jahangir, ”Performance analysis of CFDAMA-PB protocol for packet satellite communications”, IEEE Transactions on Communications, Vol. 46, Issue 9, Sep 1998, pp. 1206-1214. [2] Universal Mobile Telecommunications System (UMTS): Selection procedures for the choice of radio transmission technologies of the UMTS, Technical Report TR 101 112 v3.2.0, ETSI, Aug. 1998.

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