Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

288 ApplicationsTable 5-49 Minimum receiver sensitivitythresholds for IEEE 802.11aNominal bit rate (Mbit/s) Minimum sensitivity6 −85 dBm9 −83 dBm12 −81 dBm18 −79 dBm24 −76 dBm36 −73 dBm48 −70 dBm54 −68 dBmfor transmitting messages, uplink channel access control commands, and other data. Thedownlink interactive information can be either embedded in the broadcast channels ordefined as a standalone channel specifically devoted to interaction with subscribers.5.6.1 Network TopologyA DVB-T interactive network architecture is illustrated in Figure 5-52 [17]. This couldbe seen as a point to multi-point (PMP) network topology consisting of a base stationand several subscribers (terminal stations). Like other digital access networks, DVB-RCT networks are also intended to offer a variety of services requiring different datarates. Therefore, the multiple access scheme needs to be flexible in terms of data rateassignment to each subscriber. Furthermore, another major constraint for the choice ofthe parameters of the DVB-RCT specification is to employ the existing infrastructurealready used for broadcast DVB-T services.As shown in Figure 5-53, the interactive downlink path is embedded in the broadcastchannel, exploiting the existing DVB-T infrastructure [18]. The access for the uplinkinteractive channels carrying the return interaction path data is based on a combinationof OFDMA and TDMA type of multiple access scheme [17].The downlink interactive information data are made up of MPEG-2 transport streampackets with a specific header that carries the medium access control (MAC) managementdata. The MAC messages control the access of the subscribers, i.e. terminal stations, tothe shared medium. These embedded MPEG-2 transport stream packets are carried in theDVB-T broadcast channel (see Figure 5-53).The uplink interactive information is mainly made up of ATM cells mapped on tophysical bursts. ATM cells include application data messages and MAC managementdata.To allow access by multiple users, the VHF/UHF radio frequency return channel ispartitioned both in the frequency and time domain, using frequency division and timedivision. Each subscriber can transmit his or her data for a given period of time on a