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1、Efficient scheduling for PLC networksT. Chiras, P. Koutsakis and M. PaterakisPowerline communications (PLC) are currently being considered as analternative for high-speed data communications and Internet access.Presented is an efficient bandwidth allocation scheme which signifi-cantly excels in comp
2、arison to the Extended Aloha Medium AccessControl (MAC) protocol for the last mile access PLC networks.Introduction: The unparalleled growth of the Internet, combined withsignificant technological advance ments of VLSI and digital signalprocessing, and with the telecommunications market deregulation
3、around the world, have made pow erline communications (PLC) aviable technology for next generation telecommunications. Withmultiple outlets in almost every room, everywhere, power lines arealready the most pervasive network in the home or small office;therefore, they would be the preferred medium fo
4、r providing broad-band connection to rural or remote areas where telephone and cableconnections may not exist. The market for PLC is twofold: to thehome, or last mile access, and in the home, or last inch access 1.Relevant research on the MAC layer for PLC has focused more onin-home networking 2, 3.
5、 The work presented in this Letter focuseson the last mile problem, and in troduces scheduling ideas whichlead to significant improvements in network performance and userQuality of Service (QoS) compared to the Extended ALOHA 4, 5protocol, for powerline communication networks.Proposed scheduling sch
6、eme:Orthogonal frequency division modula-tion (OFDM) has been outlined as one of the best candidates forapplication in PLC systems with higher data rates, because of itsexcellent bandwidth efficiency 46. We consider an OFDM transmis-sion system which uses a number of subcarriers distributed in afreq
7、uency spectrum. The work presented in 4, 5 proposed threeextensions of the basic ALOHA protocol in order to improve itsperformance on the PLC network: (a) piggybacking, which leads to adecrease in thesignalling delay ;thisisdefinedasthetimeneededforthe realisation of the requesting procedure for the
8、 transmission of apacket and includes the transmission of a request message to the basestation and the reception of its response regarding the access rights;(b) use of data channe ls for signalling (from 7); (c) application of anadaptive backoff mechanism for use r access to the signalling channel,a
9、s well as for user access to the data channels for signalling purposes.In our work, we also adopt the ideas of piggybacking and using datachannels for signalling. However, we do not use the adaptive backoffmechanism proposed in 4, 5 for users to select the slot in which theywill transmit= retransmit
10、 their requests; instead, we propose three newideas, two regarding the slot selection mechanism and one regarding thechannel selection mechanism for a PLC access network.A. Channel selection: We use and compare two mechanisms forchannel selection in our study. The first mechanism is similar tothat u
11、sed in 4, 5 and is named uniform channel selection in thisLetter. With the use of this mechanism, each terminal which needsto access the medium selects uniformly one of the 15 channels (onefor signalling and 14 for data transmissions); the only constraint isthat selection is made among channels whic
12、h have at least one idleslot in the current channel frame (no transmission is scheduled inthat slot from previous channel frame s). If the channel is congested,it is not taken under consideration in the channel selection processfor the current frame.Our proposal for a second channel selection mechan
13、ism is namedweighted channel selection . At the beginning of each channel frame thebase station has full knowledge of the total number of idle slots in allthe data channels and the signalling channel. Let this total number ofidle slots be S . The probability for a terminal to choose channel Y,which
14、has three idle slots in the current channel frame, in order to sendits request is 3 =S. The respective probability for the signalling channelis equal to the total number of slots of the signalling channel (the slotsof the signalling channel are by nature always idle at the beginning of achannel fram
15、e, as no information transmission takes place in them)divided byS . The weighted channel selection mechanism is designed ina way as to push requesting users to choose, in every channel frame,with greater probability the channels with the larger number of idleslots, in order to decrease the probabili
16、ty of collisions in the system.B. Slot selection: After selecting a channel, a terminal needs tochoose the slot in which it will transmit its request. We propose twodifferent mechanisms for slot selection in our study. The firstmechanism is named uniform slot selection: after selecting achannel with M idle slots
