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Medium Access Control Protocols of the PRMA Type in Non-Geostationary Satellites

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Medium Access Control Protocols of the PRMA Type in Non-Geostationary Satellites Medium Access Control Protocols of the PRMA Type in non-Geostationary Satellites Giovanni Giambene [email protected] Dipartimento di Ingegneria dell’Informazione Università degli Studi di Siena Via Roma, 53 53100 Siena, Italy Abstract The challenge of future mobile multimedia networks is to provide worldwide tetherless communication services. Low Earth Orbit-Mobile Satellite Systems (LEO-MSSs) will play a significant role by filling the coverage gaps of future generation terrestrial cellular networks. This lecture presents research results on demand-assignment Medium Access Control (MAC) schemes able to share efficiently LEO satellite resources among users and to support isochronous traffics and the ubiquitous access to the Internet. 1 Introduction Future generation mobile communication systems will achieve a global coverage by integrating a terrestrial cellular component and a satellite one [1],[2]. The satellite system will play a complementary role with respect to its terrestrial counterpart; typical operational environments for satellite systems are regions where the provision of the terrestrial coverage is either technically or economically unfeasible. The role of mobile satellite systems is: (i) to allow the global roaming of users; (ii) to provide Quality of Service (QoS) levels comparable with those of terrestrial systems; (iii) to permit the rapid deployment of mobile services in underdeveloped regions. The satellite component of future mobile communication systems will be based (partly or totally) on non-geostationary constellations. In particular, this study focuses on Low Earth Orbit – Mobile Satellite Systems (LEO-MSSs), since they are close to the earth and allow the use of low-power lightweight mobile terminals [3]. In what follows, an earth-fixed cell system [4] will be assumed where antenna beams are steered so as to point towards a given cell on the earth during the satellite visibility time. Satellites are power and bandwidth limited. Therefore, it is essential that satellite resources are efficiently utilized. Hence, suitable Medium Access Control (MAC) protocols must be identified for the management of resources in a cell. MAC schemes contain a set of rules according to which the power-bandwidth resource is assigned to the different communications. In particular, real-time traffic (i.e., isochronous voice traffic) and Available Bit Rate (ABR) data traffic are considered. Several types of Medium Access Control (MAC) schemes have been proposed for satellite systems [5], but the identification of an efficient MAC protocol, able to guarantee suitable QoS for different traffics, is still an open research issue. A MAC protocol taxonomy can be envisaged as described below. 1. Fixed access protocols that grant permission to transmit only to one terminal at once, avoiding collisions of messages on the shared medium. Access rights are statically defined for the terminals. 1 2. Contention-based protocols that may give transmission rights to several terminals at the same time. These policies may cause two or more terminals to transmit simultaneously and their messages to collide on the shared medium. This class encompasses pure Aloha, Slotted-Aloha and Reservation-Aloha [6]. 3. Demand-assignment protocols that grant the access to the network on the basis of requests made by the terminals. The reason for the presence of many different MAC protocols is that they are suitable for some applications, but often do not meet the requirements for other applications. For instance, fixed access schemes are not efficient with bursty traffics, because they can not adapt to varying traffic conditions. This lecture presents research results concerning new demand- assignment MAC schemes that are evolutions of the classical Packet Reservation Multiple Access (PRMA) protocol. These novel protocols are able to integrate the management of isochronous and data bursty traffics and, therefore, can be useful in the Satellite- Asynchronous Transfer Mode (S-ATM) scenario. 2 The classical PRMA protocol in LEO-MSSs The PRMA protocol was originally proposed for terrestrial microcellular systems [6]: it is based on Time Division Multiple Access (TDMA) and combines random access with slot reservation. The efficiency of PRMA relies on managing voice sources with Speech Activity Detection (SAD): only during a talkspurt, a voice source has reserved one slot per frame to transmit its packets. A feedback channel broadcast by the cell controller informs the terminals about the state of each slot (i.e., idle or reserved) in a frame. As soon as a new talkspurt is revealed, the terminal tries to transmit a packet in the first idle slot (contending state), according to a permission probability scheme [6]. When the transmission attempt of a terminal is successful on a slot, the terminal obtains the reservation of this slot. The main limiting factor for the use of PRMA in LEO-MSSs is the high Round Trip propagation Delay (RTD) value that prevents the mobile terminals on the earth to know immediately the outcome of their transmission attempts. In LEO-MSSs, RTD values vary from 5 ms to 30 ms, depending on the satellite constellation altitude and the minimum elevation angle from mobile terminals to the satellite. The satellite recognizes the request made by a terminal by decoding the header of the received packet on an unreserved slot. For a conservative study, RTD is assumed always equal to its maximum value, RTDmax , for a given LEO-MSS. For the correct protocol behavior, Tf must be greater than or equal to RTDmax + e, where e is the packet header transmission time. Consequently, when a terminal attempts to transmit on a given slot, it receives the outcome of its attempt before the beginning of the same slot in the next frame. Of course, the selection of the Tf value must also account for both the requirements on the voice end-to-end delay, the voice codec and the packetization process. It is considered here Tf » RTDmax (e is negligible). If more terminals attempt to send their packets on the same slot, there is a collision (unless capture phenomena occur [6]): these terminals know that they must reschedule new transmissions only after RTD. This delay is particularly significant for the real-time voice service. In order to relax these problems, the following Section presents several solutions that are also suitable for other scenarios where RTD is greater than the packet transmission time (e.g., high altitude aeronautical platforms recently proposed to provide high bit-rate transmissions in heavy traffic urban areas [7]). 3 Novel schemes based on PRMA This Section surveys three novel MAC schemes that derive from the modification of PRMA in order to make it more suitable for the LEO satellite scenario. Let us consider Mv Voice Terminals (VTs) and Mw Data Terminals (DTs) per carrier per cell. 2 3.1 PRMA with hindering states In the classical PRMA scheme, assuming a typical LEO system where RTDmax is about equal to 16 ms, a terminal could perform at most two access attempts before dropping the first packet (as shown in Section 4.1, the voice packet deadline is 32 ms). To remove this constraint, a modified PRMA protocol is presented here, where a terminal is allowed to attempt transmissions (according to the permission probability scheme) also while it is waiting for the outcome of a previous attempt. If the previous attempt has been unsuccessful, this modification permits a faster access scheme. Otherwise, these further attempts are useless and may hinder the accesses to other terminals. Accordingly, this scheme has been called PRMA with Hindering States (PRMA-HS). It has been shown in [8] that the advantages of this fast retransmission scheme overcome the problems due to useless attempts. In order to integrate VT and DT traffics, different permission probabilities values have been considered; pv and pd, respectively, where pv > pd to prioritize the voice real-time traffic. 3.2 Modified PRMA scheme In the Modified PRMA (MPRMA) protocol, a given field in the packet header is devoted to notify the satellite if a transmission request (i.e., the first packet of a terminal that must acquire transmission rights) comes from a DT or from a VT; accordingly, different algorithms are used. In particular, the management of VTs is as in the classical PRMA scheme. Whereas, DT requests are served as described below. If a message is generated by a DT when its buffer is idle, its first packet (= request packet) is transmitted on an available slot, according to a permission probability, pd. Collisions may occur with the attempts of other terminals. If this first packet does not experience a collision, its header contains a DT transmission request that is stored into a buffer on the satellite in order to form a queue of DTs that need to transmit. The controller on board of the satellite manages a queue of uplink transmission requests for each MPRMA carrier of a cell and decides allocations of slots (not used by VTs) to DTs [9]. In order to guarantee a certain number of idle slots for new accesses of DTs and VTs, the controller assigns an idle slot to a given DT (with its request at the head of the satellite queue) in the next frame, according to an access probability pa. Let us consider a DT that is allowed to transmit the last packet of its presently served message and that has in its buffer other messages arrived in the meanwhile. A suitable flag is set in the header of this last packet so that the DT requests the transmission of another message to the satellite (piggybacked request). Both the random access packet and the packet used for the piggybacked request use a suitable field in the header to notify the message length to the satellite. 3.3 DRAMA protocol In the Dynamic Resource Assignment Multiple Access (DRAMA) scheme, the frame contains first access slots and, then, information slots.
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