Cooperative ARQ: A Medium Access Control (MAC) Layer Perspective
Jesús Alonso-Zárate, Elli Kartsakli, Luis Alonso, Christos V. Verikoukis · InTech eBooks · 2010
Cooperative Automatic Retransmission reQuest (C-ARQ) schemes have become a very active research topic over the last years.C-ARQ schemes constitute a practical way of executing cooperation in wireless networks with already existing equipment.C-ARQ schemes exploit feedback from the receiver, i.e. cooperation is only executed when needed, and thus are sometimes referred to as cooperation on-demand cooperative schemes.In short, the idea of C-ARQ is to exploit the fact that, due to the broadcast nature of the wireless channel, any transmission can be received by any of the stations in the transmission range of the transmitter.What has been traditionally considered as interference, is exploited in C-ARQ schemes to attain spatial diversity.Upon a transmission error, a retransmission can be requested from any (or some) of the stations which overheard the original transmission, which can act as spontaneous helpers (or relays).The result is that the destination of a packet can receive different copies of the same information arriving via statistically independent transmission paths, i.e., space diversity.C-ARQ schemes have been already studied in the literature from a theoretical point of view and there is no doubt that, under some conditions, they can dramatically boost the performance of wireless communications compared to traditional ARQ, where retransmissions are performed only from the source.However, involving a number of users in a communication link requires coordination.To this end, efficient Medium Access Control (MAC) protocols are necessary to get the maximum efficiency of the communications.In this chapter we emphasize the important role of the MAC layer in this context of C-ARQ.Along the chapter, we first review in Section 2 the motivation and operation of C-ARQ schemes into detail.We go through the parameters that affect the performance of these schemes and we point out the role of the MAC layer.Taking into account the specific requirements of the MAC layer in this kind of schemes, we present in Section 3 a novel highperformance MAC protocol specifically tailored for this purpose.Computer-based simulations are presented to evaluate the performance of the protocol.Finally, Section 4 concludes the chapter.12 www.intechopen.comRadio Communications 228 Cooperative ARQ (C-ARQ) Background and MotivationTraditionally, ARQ schemes have been used in communication networks to guarantee the reliable delivery of data packets.Upon the reception of a packet with errors, retransmissions are requested from the source (and along the same channel) until either the packet can be properly decoded or it is discarded for the benefit of the backlogged data.Several variations of ARQ schemes have been proposed in the past to improve the performance of communications.These schemes perform well in wired networks where there is no correlation between consecutive packet error probabilities, i.e., packet errors are random and sparse.However, their performance in wireless networks is compromised by phenomena such as the shadowing and fading of the radio channel.In wireless channels, packet errors might come into bursts, and thus if a packet is received with errors, the immediate retransmissions will be also received with errors with high probability if they are performed through the same channel (Zorzi et al., 1997).C-ARQ schemes constitute a practical solution to combat this fading nature of the wireless channel.Their operation is described in the following section. Description of C-ARQConsider a wireless network formed by an arbitrary number of stations equipped with halfduplex radio frequency transceivers.In order to be able to execute a C-ARQ scheme, all the stations must listen to (overhear) every ongoing transmission in order to be able to cooperate if required.In addition, they should keep a copy of any received data packet (regardless of its destination address) until it is acknowledged (positively or negatively) by the destination.This packet is discarded whenever the destination successfully decodes the original packet.It is assumed that, although both error detection and Forward Error Correction (FEC) bits are attached to all the transmitted data packets, errors can still occur due to the severe wireless channel impairments.Whenever a destination receives a data packet with unrecoverable errors, it broadcasts a retransmission request in the form of a control packet.This packet is referred to as the Call for Cooperation (CFC) packet.A cooperation phase is then initiated.A subset of the stations which overheard both the original transmission from the source and the CFC from the destination, become active relays or helpers.As it will be further discussed later, some relay selection criteria can be attached to the CFC in order to activate the most appropriate subset of stations to act as helpers.Orthogonally in time (TDMA), frequency (FDMA or OFDMA), or code (CDMA), these active relays attempt to retransmit a copy of the original packet to assist in the failed transmission.For the sake of clarity in the explanation and without loss of generality, the data packets retransmitted by the relays will be referred to as cooperative packets.Eventually, the destination might either receive a correct copy of the original packet from a relay or may be able to properly combine the different retransmissions from the relays to successfully decode the original packet.Otherwise, if the destination is not able to recover the data packet after some predefined time (cooperation time-out), it discards it.In any of the two cases, the cooperation phase is finished.